Ci)
AN ANALYSIS OF SULPHIDE DEFORMATION
IN LOW GRADE METAMORPHIC
ENVIRONMENTS
A Thesis submitted in fulfilment of
the requirements for the degree of Ph.D.
of the University of London.
by
Kenneth R. McClay BSc. (Hons.), MSc., DIC.
Geology Department, Imperial College,
London. August 1978. LIST OF CONTENTS Page No.
ABSTRACT 1
CHAPTER 1 AN ANALYSIS OF SULPHIDE DEFORMATION IN LOW 3 GRADE METAMORPHIC ENVIRONMENTS. 1.1 AIM AND SCOPE OF THIS STUDY. 3 1.2 METHODS USED. 4 1.3 OUTLINE OF CONTENTS. 6 1.4 SUMMARY OF RESULTS. 7 1.5 ACKNOWLEDGEMENTS. 9
CHAPTER 2 11 2.1 CONDITIONS IN THE CRUST. 11 2.2 SULPHIDE MINERAL EQUILIBRIA. 18 2.3 METAMORPHISM OF SULPHIDES - A BRIEF SUMMARY. 21
CHAPTER 3 DEFORMATION MECHANISMS AND MICROSTRUCTURES. 25 INTRODUCTION 25 3.1 DEFORMATION MECHANISMS. 26 3.1(i) DISLOCATION GLIDE : SLIP-TWINNING-KINKING. 28 3.1(ii) THE DISLOCATION CREEP REGIME. 32 3.1(iii) DIFFUSIONAL FLOW. 36 3.1(iv) RECOVERY PROCESSES. 44 3.1(v) RECRYSTALLIZATION. 45 3.1(vi) GRAIN GROWTH. 50
3.2 MICROSTRUCTURES. 53 3.2(i) INTRODUCTION. 53 3.2(ii) DISLOCATION GLIDE. 54 3.2(iii) DISLOCATION CREEP. 55 3.2(iv) DIFFUSIONAL CREEP. 60 3.2(v) RECRYSTALLIZATION. 61
3.3 GALENA (PbS). 64 3.4 SPHALERITE (ZnS). 71 3.5 CHALCOPYRITE (CuFeS2). 74 3.6 PYRRHOTITE (Fel_x S - Fe7S8). 75 3.7 PYRITE (FeS2). 77 3.8 CONCLUSIONS. 79 Page No.
CHAPTER 4 DEFORMATION OF SINGLE CRYSTALS OF GALENA. 82 4.1 INTRODUCTION. 82 4.2 EXPERIMENTAL PROCEDURE. 84 4.3 SERIES 1 EXPERIMENTS. 89 4.4 SERIES 2 EXPERIMENTS. 108 4.5 SERIES 3 EXPERIMENTS. 123 4.6 SERIES 4 EXPERIMENTS. 131 4.7 SERIES 5 EXPERIMENTS. 135 4.8 CONCLUSIONS. 136
CHAPTER 5 CRYSTALLOGRAPHIC PREFERRED ORIENTATIONS. 145 5.1 INTRODUCTION. 145 5.2 THE THEORY OF TEXTURE DEVELOPMENT AND THE 148 BASIS OF TEXTURE SIMULATIONS. 5.3 MEASUREMENT OF PREFERRED ORIENTATIONS. 149 5.4 SIMULATIONS OF TEXTURES IN GALENA. 150 5.5 DISCUSSION AND CONCLUSIONS. 158
CHAPTER 6 THE RAMMELSBERG MINE, WEST GERMANY. 162 6.1 INTRODUCTION. 162 6.2 THE GEOLOGICAL SETTING OF THE OREBODIES. 163 6.3 MACROSTRUCTURES IN THE ORES. 173 6.4 MICROSTRUCTURES IN THE ORES. 176 6.5 PREFERRED ORIENTATIONS IN THE RAMMELSBERG ORES. 197 6.6 DISCUSSION AND CONCLUSIONS. 205
CHAPTER 7 MOUNT ISA MINE, QUEENSLAND, AUSTRALIA. 213 7.1 INTRODUCTION. 213 7.2 GEOLOGICAL BACKGROUND. 215 7.3 STRUCTURES IN THE SILVER-LEAD-ZINC OREBODIES. 225 7.3(i) FOLDS OF AMPLITUDE 2 - 30 METRES. 225 7.3(ii) FOLDS OF AMPLITUDE 1 - 100 CM. 230 7.3(iii) ANALYSIS AND CLASSIFICATION OF FOLDS. 238 7.4 MICROSTRUCTURES. 241 7.4(i) INTRODUCTION. 241 7.4(ii) TECHNIQUES. 241 7.4(iii) LITHOLOGIES. 241 7.4(iv) SEDIMENTARY AND DIAGENETIC MICROSTRUCTURES. 242 7.4(v) DEFORMATION MICROSTRUCTURES. 257 ( iv)
Page No.
7.4(vi) SULPHIDE MICROSTRUCTURES. 260 7.5 CRYSTALLOGRAPHIC PREFERRED ORIENTATIONS. 279 7.6 DISCUSSIONS AND CONCLUSIONS. 289
CHAPTER 8 SHEARED GALENA. 295 8.1 INTRODUCTION. 295 8.2 SHEARED GALENA Pibram DP. 296 8.2(i) INTRODUCTION. 296 8.2(ii) MICROSTRUCTURES. 297 8.2(iii) PREFERRED ORIENTATIONS. 301 8.2(iv) CONCLUSIONS. 301 8.3 RUTH HOPE MINE, BRITISH COLUMBIA. 303 8.3(i) INTRODUCTION. 303 8.3(ii) MICROSTRUCTURES. 304 8.3(iii) PREFERRED ORIENTATIONS. 308 8.3(iv) CONCLUSIONS. 308
8.4 SAMPLE SW1 (SOUTH WEST AFRICA). 308 8.4(i) INTRODUCTION. 308 8.4(ii) MICROSTRUCTURES. 310 8.4(iii) PREFERRED ORIENTATIONS. 314 8.4(iv) CONCLUSIONS. 317
8.5 BRAUBACH, WEST GERMANY 318 8.5(i) INTRODUCTION. 318 8.5(ii) MICROSTRUCTURES. 318 8.5(iii) PREFERRED ORIENTATIONS. 325 8.5(iv) CONCLUSIONS. 325
8.6 YERRANDERIE, N.S.W. 325 8.6(i) INTRODUCTION. 325 8.6(ii) MICROSTRUCTURES. 329 8.6(iii) PREFERRED ORIENTATIONS. 337 8.6(iv) CONCLUSIONS. 337
8.7 HALKYN, NORTH WALES. 341 8.7(i) INTRODUCTION. 341 8.7(ii) MICROSTRUCTURES. 343 8.7(iii) PREFERRED ORIENTATIONS. 348 8.7(iv) CONCLUSIONS. 351
8.8 DISCUSSION AND CONCLUSIONS 351 (v)
Page No.
CHAPTER 9 CONCLUSIONS. 354
APPENDIX A ETCH TECHNIQUES. 358
APPENDIX B PUBLISHED WORK. 375
REFERENCES 376
(vi)
LIST OF FIGURES Page No. CHAPTER 2
Figure 2.1 Metamorphic facies and metamorphic 14 reactions.
2.2 Sulphide mineral equilibria. 20
2.2 Atomic ratios of Kerogens from coals and 24 ores.
CHAPTER 3
Figure 3.1 Idealized stress-strain curves for various 27 deformation regimes.
3.2 Elements of slip, twinning and kinking. 31
3.3 Schematic diagram of glide polygonization 33 and kink band formation.
3.4 Models of diffusion controlled creep and 35 of superplastic flow with grain neighbour switching.
3.5 Diffusional creep and grain boundary 38 sliding.
3.6 Deformation mechanism plots for quartz, 42 calcite and galena.
3.7 Microstructures of diffusional creep. 56
3.8 Recrystallization - core and mantle 58 microstructures.
3.9 Static recrystallization microstructures. 63
3.10 Slip systems in galena. 66
3.11 Deformation mechanism maps for galena. 68 - 69
3.12 Structures in sphalerite. 72
CHAPTER 4
Figure 4.1 Inverse pole figures for resolved shear 90 stress in galena.
4.2 Lattice rotations for (110)<110> slip. 93
(vii) Page No.
Figure 4.3 Stress strain curves for series 1 96 experiments.
4.4 Sketches of deformed single crystals from 98 series 1 experiments.
4.5 Deformation microstructures, series 1 99 experiments.
4.6 Deformation microstructures, series 1 102 experiments.
4.7 Partial pole figures, series 1 experiments. 107
4.8 Stress stain curves for series 2 111 experiments.
4.9 Stress strain curves for series 2 112 experiments.
4.10 Stress strain curves for series 2 113 experiments.
4.11 Stress strain curves for series 2 114 experiments.
4.12 Microstructures, series 2 experiments. 116
4.13 Microstructures, series 2 experiments. 119
4.14 Schmid factors and lattice rotations for 124 {100}<110> slip.
4.15 Stress strain curves for series 3 126 experiments.
4.16 Microstructures, series 3 experiments. 128
4.17 Microstructures, series 4 experiments. 132
4.18 CRSS for {110} and {100} slip versus 138 temperature.
4.19 Synopsis of microstructures. 142
CHAPTER 5
Figure 5.1 Diffraction peaks for the common sulphides 151 with relative intensities.
5.2 Inverse rotation diagram for compression 152 of galena. (viii) Page No. Figure 5.3 Texture simulations. Flattening, 154 22.62% strain.
5.4 Texture simulations. Flattening, 155 64.15% strain.
5.5 Texture simulations. Plane strain, 156 22.62% strain.
5.6 Texture simulations. Plane strain, 157 64.15% strain.
5.7 Texture simulations. Simple shear 0.5 Y. 159
5.8 Texture simulations. Simple shear 3.0 Y. 160
CHAPTER 6
Figure 6.1 Hercynian Geosyncline of Central Europe. 164
6.2 Local geology. 165
6.3 Cross section of New Orebody. 168
6.4 Longitudinal section, Rammelsberg mine. 169
6.5 Macrostructures in the ores. 174
6.6 Microstructures in the ores. 177
6.7 Barite Microstructures. 180
6.8 Barite Microstructures. 183
6.9 Pyrite Microstructures. 186
6.10 Chalcopyrite Microstructures. 189
6.11 Chalcopyrite, Galena microstructures. 191
6.12 Sphalerite microstructures. 195
6.13 X-ray diffraction traces, Rammelsberg 198 ores.
6.14 Pole figures, Rammelsberg ores. 200
6.15 Pole figures, Rammelsberg ores. 201
6.16 Pole figures, Rammelsberg ores. 202
6.17 Pole figures, Rammelsberg ores. 204
6.18 Pole figures, Rammelsberg ores. 206
6.19 Pole figures, Rammelsberg ores. 207 (ix) Page No.
Figure 6.20 Hypothetical conditions of deposition of 210 the Rammelsberg ores.
CHAPTER 7
Figure 7.1 Locality and local geology of Mount Isa. 214
7.2 Stratigraphic column, Mount Isa Group. 2.7
7.3a Mount Isa Mine - cross section. 220
7.3b Mount Isa Mine - Level plan 14 level. 221
7.4 Illite crystallinities - Mount Isa shales. 223
7.5 Bedding trends, 5 Orebody, 14-C sub-level. 226
7.6 Fold profile sections. 227
7.7 Detailed section of 72 stope 14 C sub- 228 level.
7.8 Orientation data 14 C sub-level. 229
7.9 Folds in the silver-lead-zinc ores. 231
7.10 Folds in laminated pyrite rich siltstones. 233
7.11 Folds and microstructures in pyritic - 236 silver-lead-zinc ores.
7.12 Dip isogons - Mount Isa folds. 239
7.13 ti/a plots Mount Isa folds. 240
7.14 3000N cross section. Mount Isa Mine. 243
7.15 Silica Dolomite Microstructures. 245
7.16 Diagenetic Microstructures in the Silica 247 Dolomite.
7.17 Silica Dolomite Microstructures. 250
7.18 Silica Dolomite Microstructures. 252
7.19 Microstructures Urquhart Shales. 254
7.20 Details of folds 525 S cross cut. 258
7.21 Minor folds and cleavage development 259 Urquhart Shales.
7.22 Chalcopyrite Microstructures. 261
7.23 Pyrrhotite Microstructures. 264 (x) Page No.
Figure 7.24 Pyrite Microstructures. 267
7.25 Sphalerite Microstructures. 270
7.26 Galena Microstructures. 273
7.27 Galena Microstructures. 275
7.28 Detail of sample M64. 278
7.29 Partial pole figures Mount Isa Galena. 280
7.30 Partial pole figures Mount Isa Galena. 281
7.31 Partial pole figures Mount Isa Galena. 282
7.32 Partial pole figures Mount Isa Galena. 284
7.33 Partial pole figures Mount Isa Galena. 285
7.34 Partial pole figures Mount Isa Galena. 287
7.35 Partial pole figures Mount Isa Galena. 288
7.36 Relative competencies of sulphide minerals 293 at Mount Isa.
CHAPTER 8
Figure 8.1 Specimen DP. 298
8.2 Microstructures sample DP. 299
8.3 Pole figures sample DP. 302
8.4 Sheared galena - Ruth Hope. 305
8.5 Microstructures - Ruth Hope. 306
8.6 Pole figures - Ruth Hope. 309
8.7 SW1 - South West Africa. 311
8.8 Microstructures SW1. 312
8.9 Pole figures SW1. 315
8.10 Pole figures SW1. 316
8.11 Braubach, West Germany. 319
8.12 Microstructures, Braubach. 321
8.13 Microstructures, Braubach. 323
8.14 Pole figures, Braubach. 326
8.15 Sample Yi, Yerranderie. 328 (xi) Page No.
Figure 8.16 Microstructures, Yerranderie. 330
8.17 Microstructures, Yerranderie. 333
8.18 Microstructures, Yerranderie. 335
8.19 Pole figures. 338
8.20 Pole figures. 339
8.21 Pole figures. 340
8.22 Halkyn galena. 342
8.23 Microstructures Halkyn. 344
8.24 Microstructures Halkyn. 346
8.25 Pole figures, Halkyn. 349
8.26 Pole figures, Halkyn. 350
APPENDIX A
Figure A.1 Etch pit shapes in galena. 361
A.2 Scanning electron micrographs of etch pits 363 in galena.
A.3 Etch pits in galena. 366
A.4 Etching features of polished sections. 369
LIST OF TABLES Page No. CHAPTER 2
Table 2.1 Metamorphism of Sulphides - a summary. 24a
CHAPTER 3
Table 3.1 Synopsis of the Physical properties of 81 the Sulphides.
CHAPTER 4
Table 4.1 Summary of Experimental conditions. 85
4.2 Chemical analyses of single crystals of 87 galena.
4.3a Slip Systems in galena. 91
4.3b Critical Resolved Shear stresses for 92 Series 1 Experiments.
4.4 Summary of Results, Series 1 Experiments. 95
4.5 Vickers Hardness, Series 1 Experiments. 106
4.6 Summary of Results, Series 2 and Series 4 110 Experiments.
4.7 Summary of Results, Series 3 Experiments. 125
CHAPTER 6
Table 6.1 Regional events. 166
6.2 Stratigraphy of the Rammelsberg mine. 170
6.3 Metal contents, Rammelsberg ores. 171
6.4 Mineralogy of the Rammelsberg ores. 172
CHAPTER 7
Table 7.1 Stratigraphy at Mount Isa. 216
7.2 Structural History of the Mount Isa 219 Region.
APPENDIX A
Table A.1 Details of Etch Techniques. 372 1
ABSTRACT.
This thesis is a study of the microstructures and textures of sulphide deposits deformed under low greenschist facies metamorphic conditions at Rammelsberg, West Germany, and at Mount Isa, Australia. The textures of these deformed stratiform ores are compared with those from unmetamorphosed deposits. The microstructures of samples of monomineralic sheared 'steel' galena are compared with those from single crystals of galena which have been deformed experimentally under conditions similar to those experienced by the naturally deformed sulphides. The results are discussed in terms of metallurgical theories of deformation and recrystal lisation. X-ray texture goniometry was used to measure the development of preferred crystallographic orientation in naturally and experimentally deformed sulphides. The Rammelsberg ore deposit exhibits a sequence of dynamic recrystallisation, dislocation creep microstructures in the barite, sphalerite and chalcopyrite ores whereas the galena and pyrite have micro- structures indicative of diffusion controlled deformation mechanisms. X-ray texture goniometry on these mixed ores gave unreliable results. Folds in the silver-lead-zinc orebodies at Mount Isa, Australia, were mapped and described. The microstructures in the silver-lead-zinc ores, in the copper ores and in their host rocks are described in detail. The galena microstructures and the lack of preferred crystallographic orientations indicate that diffusion controlled deformation mechanisms operated during the main deformation event. Evaporite pseudomorphs and relict evaporite minerals in the 'silica dolomite' rocks at Mount Isa have been established. This important discovery permits one to infer that the Mount Isa ores may have been formed from sabkha type brines rather than from volcanically derived hydrothermal solutions. The microstructures of sheared 'steel' galena from various localities are remarkably similar and are dominated by dynamic recrystallisation along kink and deformation bands. Very strong preferred orientations are found. Deformation experiments on single crystals of galena at temperatures from 20°C to 400°C showed a dramatic drop in strength both with an increase in temperature and with a decrease in strain rate. Dynamic recrystallisation was found to occur at 200°C while stress relaxation experiments showed recovery effects at temperatures as low as 100°C. The microstructures in the experimentally deformed galena can be correlated directly with those found in naturally deformed galena. 2
The detailed examination of microstructures and preferred orientations of naturally and experimentally deformed sulphides, in particular galena, has enabled a direct correlation of texture, microstructure, and deformation mechanisms with the environment of deformation and metamorphism. CHAPTER 1 AN ANALYSIS OF SULPHIDE DEFORMATION IN LOW GRADE METAMORPHIC ENVIRONMENTS
1.1 AIM AND SCOPE OF THIS STUDY
Many large lead-zinc and copper sulphide ore deposits are located in low grade greenschist metamorphic terrains. Included in this large group of deposits (see reviews in Stanton 1972 and Wolf 1976) are the important mining areas of Mount Isa and McArthur River (Australia), the Copper Belt (Zaire and Zambia), the Sullivan mine (British Columbia), the Appalachian sulphide deposits (eastern USA), the New Brunswick deposits (Canada) and the Rammelsberg mine (West Germany). Detailed mineralogical, chemical and isotopic studies have been conducted on many of these deposits (Stanton 1972, Wolf 1976) but, in general, structural and microstructural studies are comparatively rare. Naturally deformed sulphides have been studied principally by Ramdohr (1953a, b, 1969) and Stanton (1964, 1972). They were able to recognise basic deformation features including slip, twinning and recrystallisation. Systematic experimental studies of the deformation of sulphides were first carried out by Siemes and co-workers (Siemes 1970) followed by Atkinson (1972, 1974, 1975a, b, and 1976a), Clark and Kelly (1973), Salmon et al. (1974), Clark et al. (1977) and by Siemes (1976, 1977). These experimentS have yielded much detailed information on sulphide rheology. The research described in this thesis is a detailed study of the characteristics of sulphides which have been subjected to low grade metamorphism and deformation both in natural environment and under laboratory conditions. The aim of this thesis is to describe, and to compare and contrast the behaviour and the microstructures of sulphides, particularly galena, which have been naturally deformed with those features found in (a) experimentally deformed sulphides (previous studies and also in this thesis) and in (b) naturally occurring but relatively undeformed sulphide ores. To this end, field studies have been combined with optical and scanning electron microscope studies and with x-ray texture goniometry. In recent years, structural geologists have increasingly used metal- lurgical techniques and results in the study of deformation microstructures in rocks and minerals (White 1976, 1977, and Nicholas and Poirier 1976). The simple crystallography of many of the common sulphides, galena, sphalerite, pyrite, chalcopyrite and pyrrhotite and their similarities to metals and ceramic materials makes the study of deformed sulphides amenable to techniques and interpretations used'in metallurgical and materials science studies. In 4
this thesis, reference is frequently made to the metallurgical literature, and strong analogies are drawn between sulphide and ceramic behaviour. One may ask - 'What information is to be gained from a study of deformed sulphides?' This question may be answered in two parts. Firstly, such studies will provide evidence of the mechanical behaviour of the ores during metamorphism; i.e. deformation mechanisms, recrystallisation and preferred orientations. Once recognised, these features may, in turn, be interpreted to indicate the environment of deformation, particularly temp- erature, confining pressure, differential stress and strain rate to which the rocks under study were subjected. Such studies will result in.a more detailed understanding of the behaviour of sulphide ore bodies during deformation and metamorphism, and also of the structures which result from deformation. Secondly, and perhaps more importantly such studies will enable the geologist to distinguish between depositional and deformational micro- structures and also provide evidence for deformation of the ores with refer- ence to the host rocks. Such evidence is extremely important when consider- ing whether the ores were formed prior to or post the deformation and metamorphism and also whether the ores are synsedimentary or epigenetic in origin. These considerations are likely to influence exploration policy for further orebodies in the same district. As will be demonstrated in the following chapters of this thesis, detailed structural and microstructural studies of naturally deformed sul- phides in comparison with experimentally deformed sulphides has enabled estimates of the deformation conditions to be made; and has also been used to explain the macrostructures of the ores and in particular has provided new evidence for the genesis of the Mount Isa ores.
1.2 METHODS USED
This project began with a detailed survey of the literature on copper and lead-zinc ore deposits which have been deformed under low greenschist facies metamorphic conditions. The deposits suitable for study were chosen using the following criteria:
(1) the ores are massive, fine grained,* preferably with monomineralic
X-ray texture goniometry is best applied to fine grained polycrystalline aggregates in order that the largest possible number of grains can con- tribute to the x-ray measurements (Siddans 1971, 1976). 5
bands of galena, sphalerite, chalcopyrite or pyrrhotite, but in which pyrite is not a major constituent**;
(2) the deformation history and the geometry of the ore deposit is well documented; and
(3) accessibility to the deposit and/or a good collection of material from the deposit.
The ore deposits which were considered for detailed studies were Rammelsberg, West Germany; Mount Isa, Australia; Sullivan, B.C., Canada; Flin Flon, Canada; and Siera Celia, Spain. Where possible, polished sec- tions of type material from these deposits were examined to see if suitable deformation microstructures were developed. Of these ore deposits, Mount Isa and Rammelsberg were chosen for detailed studies. These deposits are stratiform - sedimentary lead-zinc and copper ores and fulfill the requirements listed above. Field work was carried out only at Mount Isa, Australia, for it was not possible to obtain permission to visit Rammelsberg. However, detailed studies were carried out on samples collected by the author from Mount Isa and on samples collected by J. McM. Moore from Rammelsberg. Samples of coarse grained galena which have been subjected to shearing deformation from Yarranderie N.S.W., Roth Hope British Columbia, Pibram Czechoslovakia, Halkyn North Wales, South West Africa and from Braubach West Germany, were studied in detail. Detailed comparisons are made between the deformation microstructures observed in the naturally deformed ores listed above and with those found in experimentally deformed single crystals of galena (Chapter 4) and previous experimental studies of sulphide deformation (Atkinson 1972, 1974, Clark and Kelly 1973, Clark et al. 1977). The stress-strain results from the experiments on single crystals of galena are used for computer simulations of preferred orientations in galena. These theoretical simulations are compared with those crystallographic pre- ferred orientations measured in naturally deformed ores. The geometry of the macro structures in the Mount Isa lead-zinc ores are analysed in detail. The microstructures found in the naturally and experimentally deformed sulphides are examined using optical microscopy of polished sections, scanning electron microscopy of cleavage fragments and
** Pyrite has x-ray diffraction peaks at 29 angles similar to those from other sulphides (e.g. pyrrhotite, sphalerite and galena) and as such leads to interference problems in x-ray texture goniometry (see Chapter 5). 6
by x-ray texture goniometry of polished slabs.
1.3 OUTLINE OF CONTENTS
This thesis is divided into two sections. The first half begins with a literature review of the crustal environment and a discussion on sulphide phase equilibria. A synopsis of published work on deformed sul- phide ores is given at the end of Chapter 2. Chapter 3 summarises deform- ation mechanisms in crystalline materials and presents a review of previous studies on the deformation of galena, sphalerite, chalcopyrite, pyrrhotite and pyrite. The results of a series of deformation and annealing experiments on single crystals of galena are given in Chapter 4. The stress-strain characteristics of specimens prepared from single crystals of galena deformed at temperatures from 20°c to 4000c are analysed. The variations in the critical resolved shear stress (CRSS) for both (1001 <110> and 0101 <110> slip with temperature and strain rate are also discussed. This is followed by a detailed description of the deformation microstructures of the experimentally deformed single crystals of galena. In Chapter 5, there is a brief review of the development of crystallographic preferred orientations in crystalline aggregates. The results of computer simulations of preferred orientations in galena are presented. Whereas the first part of the thesis is concerned with the character- istics of sulphides deformed under known conditions, the second part of this thesis relates to detailed studies of naturally deformed sulphides. In Chapter 6, the results of detailed microstructural and x-ray texture gonio- metry studies on samples from the Rammelsberg mine are discussed. The bulk of the research for this thesis was carried out on material and data from the Mount Isa Mine Australia. A summary of investigations on the silver- lead-zinc orebodies at Mount Isa, is presented in Chapter 7. This study includes an analysis of macroscopic folding in the silver-lead-zinc ore- bodies in the mine and is followed by a detailed summary of the microstructure of both the silver-lead-zinc ores and of the copper ores. The development of crystallographic preferred orientations in the Mount Isa galena is discussed in the final part of Chapter 7. In Chapter 8, the microstructures and crystallographic preferred orientations of six samples of sheared coarse grained galena are analysed. The results of this study are compared with those from Chapters 4 and 5. 7
Finally, a synopsis of all the studies presented in this thesis and the implications of these investigations for the study of naturally deformed ore deposits are discussed in detail in Chapter 9. Appendix A is a summary of etch techniques and etch results. Papers published during the course of this research are included in Appendix B.
1.4 SUMMARY OF RESULTS
The results of studies on naturally and experimentally deformed sulphides are briefly summarised below. Single crystals of galena which are compressed parallel to the <001> axis or at 10° to the (001) plane show high flow strengths ( 1-3 Kbars) at low temperatures (20-100°c) but show a rapid decrease in strength in experi- ments conducted at higher temperatures (>200°c). In contrast, single crystals of galena compressed parallel to the <111> axis have a yield point at approxi- mately 50 bars CRSS for temperatures of 20° to 400°c. The rate of work hardening, however, decreases rapidly with an increase in temperature, from being high at 200c to almost insignificant at 3000c and 400°c. Kinking was found in all three orientations tested and was most marked at low temperatures (up to 200°c). {110} <110> and {100} <110> slip were found in most experiments. The CRSS for {110} <110> slip decreased both with an increase in temperature and with a decrease in strain rate whereas the CRSS for {100} <110> slip was approximately constant at 50 bars for all temperatures tested. Dynamic recrystallisation was found to occur at temper- atures as low as 2000c in the laboratory. Static recrystallisation was also found to occur at 2000c. These temperatures are lower than those previously reported for recrystallisation in galena. Stress relaxation tests show significant recovery in single crystals of galena at temperatures as low as 100°c. Static and dynamic recrystallisation microstructures are clearly distinguishable and it is argued that these can be used to determine the deformation conditions of naturally deformed galena. Computer simulations of the development of preferred orientations in galena using only {110} <110> and {100} <110> slip show that in general the textures for plane strain and flattening deformations develop slowly and are weakly developed. For single shear deformations, however, the initial rotations are relatively large. Flattening produces a texture with [10] poles approximately parallel to the axis of compression whereas simple shear produces a texture with [100] poles normal to the shear plane. 8
Studies of Rammelsberg ores have revealed microstructural sequences in the barite and chalcopyrite ores. Preferred orientations are poorly developed and it is concluded that x-ray texture goniometry is not a useful tool for the study of mixed sulphide ores. From the investigations carried out at the Mount Isa Mine, it is argued that most of the folds which are observed in the silver-lead-zinc orebodies are of tectonic origin rather than sedimentary slump folds as other authors have suggested. The signi- ficant discovery of pseudomorphed sulphate evaporites at the Mount Isa Mine permits one to postulate that the Mount Isa ore deposits may originate from sabkha type brines rather than from volcanic-hydrothermal solutions exhaled into the depositional basin. The microstructures and lack of pre- ferred orientations in the galena rich ores permit one to suggest that Coble creep was the main deformation mechanism in the galena at Mount Isa. Coarse grained galena ores which have undergone shear deformation exhibit strong preferred orientations except in the case of the Halkyn galena. The fine grained nature of the recrystallised grains in the Halkyn samples indicates that the probable deformation mechanism in these ores was Coble creep. In the other samples, the microstructures indicate deformation by dislocation glide and dislocation creep with dynamic recrystallisation along kink bands. It is suggested that the textures of naturally sheared galena ('steel' galena) are the result of dynamic recrystallisation along kink bands rather than grain growth as suggested by Stanton and Willey (1972). Kinking is also thought to contribute to the rapid development of preferred orientation during low temperature deformation of galena. The microstructures found in the naturally deformed coarse grained galena are similar to those in the experimentally deformed single crystals of galena. The ease with which most of the common sulphide minerals (except pyrite) deform, permits one to infer that most of the evidence of early deformation episodes in the sulphides will be destroyed by later events. However this study has shown that detailed microstructural studies enable part of the deformation history to be unravelled. However, the crystallo- graphic preferred orientations will, in the main, strongly reflect the later deformation history of the ore deposit. Finally, the studies of naturally deformed sulphides indicate that galena, sphalerite, chalcopyrite and pyrrhotite all exhibit microscopic plasticity during deformation whereas pyrite deforms cataclastically. Fluid assisted diffusion, pressure solution, is a possible deformation mechanism in pyrite. Under geological conditions of strain-rate and temp- erature galena is expected to be extremely weak and this may account for 9 many of the structures found in the Mount Isa silver-lead-zinc ores. These results are similar to those found in experimental studies of poly- crystalline sulphide rheology (Atkinson 1972, 1974).
1.5 ACKNOWLEDGEMENTS
This project was supervised by Dr. N.J. Price and Mr. J. McM. Moore. Their advice, encouragement and criticism are gratefully acknowledged. The first year of study was supported by a Beit Fellowship from Imperial College, and in the following years the author was financed by a George Murray Scholarship from the University of Adelaide. Professor D.M. Boyd (University of Adelaide) gave assistance and encouragement throughout this study. Field work was made possible by generous grants from the Institution of Mining and Metallurgy (G. Vernon Hobson Bequest) and the Central Research Fund, University of London. Mike Fabjanczyk is heartily thanked for his generous hospitality at Mount Isa. Permission to visit Mount Isa Mine and for access to material by Mount Isa Mines is gratefully acknowledged. Mr. P. Stoker and W. Perkins are thanked for their cooperation and discussions on the geology of Mount Isa. Barry Atkinson gave generously of his time to assist with the deformation experiments and the equipment used for these experiments was pro- vided by a NERC grant. R. Holloway maintained the apparatus with his usual consumate skill. Liz Morris and Barry Foster skilfully made the polished sections used in this study. P. Grant, C. Peat and R. Giddens gave generous assistance with scanning electron microscopy during the course of this research. Dr. F.J. Humphreys kindly provided access to the spark erosion equipment. Professor H. Siemes at R.W.T.H. Aachen is gratefully thanked for measuring some x-ray texture goniometer pole figures and for stimulating discussions on the deformation of galena.. C. Hennig-Micheli provided generous hospitality at Aachen. M. Casey, A.W.B. Siddans and J. Whalley at Leeds University kindly measured and computed some x-ray texture results. G. Lister (Leiden) contributed to many stimulating discussions on the deformation galena and computed the theoretical textures. H. Richardson and L. Wright gave generous help with the many photo- graphic problems encountered during the course of this research, while Chris Norris and Sarah Cowen patiently typed the final manuscript. 10
Many discussions with friends and colleagues gave me great stimulation and encouragement. In particular, I would like to thank the following: B. Atkinson, E.H. Rutter, N. Shaw, M. Muir, W.L. Diver, R. Knipe, S. White and G. Lister. The patience and humour of many other friends throughout the years is very much appreciated. Finally, my parents have given support and encouragement over many years and are thanked for their generous assistance in the production of this thesis. 11
CHAPTER 2
2.1 CONDITIONS IN THE CRUST
In order that the geologist may more fully understand the deformation of orebodies, it is essential to know the mechanical properties of the rocks and minerals which constitute the orebodies. To this end, laboratory studies of the rheology of sulphide minerals have been carried out by Atkinson (1972, 1974, 1975a, b, 1976a, 1978), Siemes (1970, 1976), Clark and Kelly (1973), and in this thesis the results of these studies are utilized in the analysis of sulphide deformation in low grade metamorphic environments. It is, therefore, apposite at this point to consider the state of the crustal environment. Heard (1968) and Price (1970) have argued that the mechanical properties of rocks and minerals in the earths' crust will be influenced by five main physical parameters: (1) temperature; (2) confining pressure; (3) differential stress; (4) pore fluid pressure; (5) strain rate. To these five physical properties we can add a sixth parameter, chemistry, and in particular that of pore fluid composition, a factor which may be extremely important in sulphide deformation. Detailed reviews of the importance of these parameters during crustal deformation are given by Atkinson (1972) and by Price in Fyfe et al. (1978). In addition, Atkinson (1972) has discussed the effects of shear stress on phase equilibria and he concluded that the effect of shear stress is probably a general phenomenon in that deformation can accelerate reaction rates but that shear stress does not affect the position of equilibrium phase boundaries. In a similar fashion, Barton and Skinner (1967) argue that the presence of fluid phases (other than S2) in sulphide systems will accelerate reactions but not alter the position of the phase boundaries. The presence of fluids during low grade metamorphism may therefore be an important factor in enabling sulphide systems more readily to attain equilibrium. Let us now consider the variations of the six aformentioned parameters within the crust. The first two parameters, temperature and confining pressure are independent but both are related to depth within the crust. The other parameters are however, frequently dependent upon each other.
TEMPERATURE
Temperature in the crust, except in special cases, increases with depth. The rate of increase, that is the geothermal gradient, however, 12
varies from place to place depending upon the rate of heat generation within the crust and upper mantle and upon the thermal conductibity of the crustal rocks. Geothermal gradients measured today have values typically between 10°C - 40°C/Km (Smith 1973) but values as high as 100°C/Km can be found in active volcanic areas. Price in Fyfe et al. (1978) discusses the available data on geothermal gradients and concludes that although gradients of the order of 30 - 50°C/Km have been inferred from current data, it is unlikely that such gradients are likely to contrive throughout the full thickness of the crust and that gradients of the order of 20°C/Km are likely at depths between 10 and 20 Km. A correlation of geothermal gradients with metamorphic facies can be seen in figure 2.1. Low geothermal gradients 10°C/Km correspond with trench subduction zones and zeolite - glaucophane schists. An average geothermal gradient of 20 - 30°C/Km is often associated with continental crust or oceanic basins and corresponds to increasing metamorphism zeolite - greenschist - amphibolite facies (fig. 2.1). In general an average geothermal gradient of 30°C/Km is often assumed (Price 1970, Atkinson 1972) but it must be pointed out that the heat generated by radioactive decay in the early life of the earth was significantly higher than that today and hence PreCambrian geothermal gradients were probably in excess of 50°C/Km (Fyfe et al. 1978).
CONFINING PRESSURE
In most crustal situations the vertical or confining pressure a increases with depth z so that z
a = p.g.z. 2.1 z
where p is the density of the rock and g is the gravitational constant. For a rock density of p = 2.5gms/cc, the geobarometric gradient will be - 250 bars/Km. In general geobarometric gradients of the crust are estimated to be 250 - 280 bars/Km. It must be pointed out, however, that rock densities are highly variable from 1.8gms/cc for sediments at the surface to 3.4gms/cc for ultrabasic rocks at depth. Price in Fyfe et al. (1978) discusses the estimation of confining pressure in some detail.
ESTIMATIONS OF TEMPERATURE AND CONFINING PRESSURE
Geologists usually approach the problem of temperature and confining pressure estimates by relating one or the other to the depth 13 of burial. The most widely used method is to study the metamorphic mineral assemblages and to relate these to the experimentally determined phase boundaries. Typical phase boundaries are shown in the bottom half of figure 2.1. Estimates based on metamorphic reactions such as these suffer from two main disadvantages. Firstly the phase boundaries are experimentally determined and are often not highly accurate because of difficulties in achieving equilibrium and in quenching equilibrium assemblages. Secondly, distinctive metamorphic assemblages are not always developed and the lack of indicator reactions makes estimations of confining pressure and temperature very difficult. This problem is particularly prevalent for low grade metamorphism for conditions other than the zeolite facies. Phase boundaries for sulphide systems are considered separately in section 2.2. Alternate methods for estimating temperature and confining pressure (and depth of burial) can be briefly summarized as follows. The study of fluid inclusions in vein minerals has seen widespread use for formation temperatures of ore deposits (Roedder 1967, 1976). The technique can also be utilized for the study of tectonic veins. Kerrich (1974) however, has pointed out the pitfalls of this method and in particular the use of deformed veins. In some circumstances, the bubble technique may be used in 16 18 conjunction with 0 /0 geothermometry to estimate both temperature and confining pressure (Kerrich 1974). Oxygen isotope geothermometry is an extremely useful technique provided there has been no significant isotope exchange with the interstitial water (Taylor 1977). In sulphide systems, sulphur isotope fractionation between co-existing sulphides may be used to determine equilibrium temperatures (Coleman 1977). In the absence of material suitable for isotope studies other methods may be used to estimate depth of burial. Porosity determinations may be used to estimate the depth burial as outlined by Price in Fyfe et al. (1978). Alternatively one may use the volatile content of organic material to estimate depth of burial (Price 1963). Other methods which may also be used are illite crystallinity index (Frey 1970), and also graphite crystallinity. Carbonaceous materials are particularly sensitive to low grade metamorphism and as such the vitrinite reflectance (Dutche (Dutcher et al. 1974) and the atomic ratios of kerogens (Saxby 1976) can be used to estimate metamorphic temperatures. Calibration, however, of these methods is often difficult due to the influence of lithology and pore fluid composition on chemical reactions with carbonaceous material. Hara and Nishimura (1977) and Hopwood (1976) have demonstrated that the quartz c - axis fabric can be related to the metamorphic grade 14 /1 12 WET GABBRO ECLOGITE 'MELTING —40 ■ •• P
1 -13 .5/ —30 z
"cr, GLAUCOPHANE 25 ul LAWSONITE II 01
zl —209 e-sr vPf —15 I —10 (ZEOLITE 2 —5
0 200 400 600 TEMPERATURE °C
I I / I I I / 12 Qtz = Quartz An = Anor thite WET GRANITE WET GABBRO Zos=Zoisite MELTING MELTING Pre=Prehnite Gro=Grossular 10 War.Wairakite Law=Lawsonite Lau.Laumontite cfi
8
In 8
6
FQ 'R' a. 11 O ="4 a_
2
0 200 400 600 800 TEMPERATURE °C FIGURE 2.1 Metamorphic facies and mineral equilibria (after Fyfe et al. 1978) 15 and hence temperature.
PORE FLUID PRESSURE
Price (1975) and Fyfe et al. (1978) have discussed the importance of pore fluid pressures during rock deformation. It is often assumed that for rocks in the crust, the pore fluids are connected to the surface by a host of interconnecting pores and hence that the pore fluid pressure increases with depth in a simple linear fashion
P = pw•g.z 2.2 where P = pore fluid pressure, pw = density of water (1.0 - 1.03) and g and z are as in equation 2.1. This gives a pore fluid pressure approximately 40% of az with depth. There is, however, ample evidence that this simple relationship is often not obeyed. Price (1975) and in Fyfe et al. (1978) discusses the generation of high pore fluid pressures in the crust and concludes that at depth (>2000m) the pore fluid pressure during pro-grade metamorphism will, at some periods, approach the value of the overburden pressure az. Price (1975, 1977) has argued that many geological structures are influenced by the formation and maintenance of high pore fluid pressures. At this point it is pertinent to consider the mechanisms by which high pore fluid pressures are generated particularly as sulphides exhibit great potential for remobilization by pore fluids. High pore fluid pressures may be generated by the trapping of connate water within the sediments as they undergo compaction. Shales often have an initial porosity of 60 - 70 per cent which is reduced to less than 3 per cent upon full compaction (Price 1975) (Provided that the depth of burial exceeds 8,000 ft.). If the water from the shales is trapped in underlying sands during burial, high pore fluid pressures will be developed. Details of the compaction story for the generation of high fluid pressure are presented by Price (1975) and Fyfe et al. (1978). High pore fluid pressures may also be generated by dehydration reactions during metamorphism. These dehydration reactions have been discussed in some detail by Fyfe (1976) and Fyfe et al. (1978). In some cases the percentage by volume of water produced in these reactions, over a small temperature range, is considerable. One reaction of particular interest is the dehydration of gypsum to anhydrite which occurs at temperatures of 40° - 125°C and is accompanied by the generation of 50% by volume of water. The widespread association of evaporites with stratiform copper and lead-zinc orebodies (Renfro 1974, Bowen and 16
Gunatilaka 1977), makes this dehydration reaction important during the early stages of deformation of these ores.
DIFFERENTIAL STRESSES
One of the most difficult problems in this synthesis is to estimate the differential stresses which obtain in the crust. Detailed discussions on this topic are to be found in Atkinson (1972) and Fyfe et al. (1978). The maximum differential stress which may be induced by relief (maximum = 4Kms) is of the order of 1 Kbar. The large differential stresses produced by relief will be limited to the upper layers of the crust and will be considerably reduced at depth (Fyfe et al. 1978). Sibson (1975) estimated that cold dry quartzo-feldspathic gneisses could support differential stresses of the order of 3.2 Kbars during the generation of pseudo-tachylites. This is, however, a relatively unusual environment in which anhydrous rocks are subjected to stick-slip faulting in the upper levels of the crust in conditions of retrograde metamorphism. Differential stress estimates from stress drops during stick-slip faulting vary from 50 - 500 bars (Chinnery 1964). A similar result was obtained by Ambraseys (1970) with an average magnitude of differential stress on active faults of the order of 200 bars. It must be emphasized that these values refer to the residual strength of the rock and the stresses required to initiate failure will be somewhat higher than the figures quoted. It must be pointed out that the above discussions refer specifically to conditions pertaining to brittle behaviour. For ductile behaviour it is apparent that the differential stresses during deformation would be much lower than that for brittle failure. From the results of experimental tests, it can be demonstrated (sections 3.3 - 3.6) that the common sulphide minerals galena, sphalerite, chalcopyrite, and pyrrhotite would support differential stresses usually much less than 500 bars under geological conditions. Stress determinations may also be made using deformation microstructures (Twiss 1977, Nicholas and Poirier 1976, Nicholas 1978). Potential indicators are, free dislocation densities, dislocation radius of curvature, tilt wall spacings, optically visible subgrain size, and recrystallized grain size. These methods are discussed in more detail in section 3.2 (iii) but it is pertinent to point out that sub-grain size are recrystallized grain size have given reasonable estimates of differential stresses within the crust (Nicholas and Poirier 1976). 17
For example basalt and kimberlite nodules have deformation microstructures which indicate deformation under crustal conditions and these give stress estimates between 0.30 and 1 kbar (Gueguen 1977). As discussed in detail in section 3.2 (iii) great care must be applied in using microstructures as palaeo stress indicators.
STRAIN RATES
Detailed reviews of strain rate estimates are given by Price (1975) and Fyfe et al. (1978). On geometric grounds Price (1975) argues that _18 _1 geological strain rates may vary from 10 S for slow subsidence and _13 - _ 1 _9 _ 1 uplift to 10 - 10 9 s for tectonic folding (10 s for small folds). White (1975) uses quartz microstructures to deduce that quartz 1 mylonites may have formed at strain rates as fast as 10_9 s whereas normal quartz tectonites may have formed at strain rates of the order of _14 _1 _14 _1 10 s . A strain rate of - 10 s (Heard 1963) is commonly quoted as being a representative 'geological' strain rate (cf. plate movement rates Tchalenko 1975) but it is likely that many ductile (cf. fold and mylonite structures) are formed at faster strain rates - of the order of _12 -9 _1 10 - 10 s (Price 1975).
PORE FLUID COMPOSITION
Pore fluids may not only play a mechanical role during deformation but may also be important chemically. The importance of fluids associated with ores has long been recognised. As stated earlier, the presence of a fluid phase may enhance a metamorphic phase transformation. In sulphide systems, however, many minerals are susceptible to solution or hydrothermal alteration and in these cases the composition of the pore fluid in question can either enhance or buffer a particular reaction. Atkinson (1972), Fyfe et al. (1978) and Mookherjee (1976) have reviewed the composition of fluids in the crust in some detail. It is beyond the scope of this brief review to go into lengthy detail but it suffices to point out that many pore fluids in contactor associated with mineralization are commonly saline brines, rich in such ions as Na+, Cl, ++ ++ + Ca , Mg , K and SO4 . The composition of the pore fluid may not only influence the metamorphic reaction if aqueous phases are involved but may also affect the solubility of a particular sulphide (see Barnes and Czamanske 1967). This latter effect may be important during diffusional creep and is discussed in chapter 3. 18
DISCUSSION
From the above brief review it can be seen that reasonable estimates of the crustal environment can be obtained. For any particular ore deposit, however, a number of methods need to be applied and estimates made in order that the environments of metamorphism and deformation can be ascertained. In this thesis, the effects of the crustal environment on the deformation mechanisms, microstructures and textures of some common sulphide minerals will be examined in detail.
2.2 SULPHIDE MINERAL EQUILIBRIA
At this juncture, it is pertinent to discuss the stabilities of sulphide mineral assemblages under crustal conditions. Detailed syntheses of sulphide stability have been given by Barton and Skinner (1967) Barton (1970) and Craig and Scott (1974). Only a brief synopsis will be given here and the reader's attention is directed to the aforementioned papers for details regarding specific assemblages. Barton (1970) raises the following important points to be considered in any discussion on sulphide mineral assemblages. Firstly, despite an- overall mineralogical variety many sulphide ore deposits exhibit a simple mineralogy at any one stage of mineralization. Secondly, the depositional pattern of many sulphide ores represents a wide variety of superposed chemical systems. Finally, Barton (1970) emphasises that sulphide ores are very susceptible to retrograde metamorphism. Although Barton's first two points were related specifically to hydrothermal ore deposits, they are equally applicable to sulphide ores during metamorphism particularly as the environmental parameters (Temperature, Confining Pressure, Fluid Pressure, Differential Stress and Strain Rate) are likely to have varied significantly throughout the tectonism of any stratabound sulphide deposit. In general, sulphide stabilities are controlled by the activity of sulphur (measured as a (S2) in terms of the fugacity of sulphur vapour in atmospheres f S2). Sulphide systems are commonly open in that a vapour phase (f S2) is generally involved and there are commonly too few sulphide phases for the number of components in the system (Barton 1970). Sulphide minerals attain equilibrium with respect to their environment much more rapidly than silicates or oxides (eg. fig. 7.1 Barton and Skinner 1967). This is because the rates of reactions for sulphides are much faster than those for oxides or silicates. Broadly speaking the silver bearing sulphides attain equilibrium most rapidly, followed by the copper sulphides; the copper iron sulphides; pyrrhotite and pyrite, 19 sphalerite, arsenopyrite in order of decreasing rapidity of attainment of equilibrium (Barton and Skinner 1967). Hence the sulphide minerals are susceptible to early metamorphism and to subsequent retrograde metamorphism. Examples of experimentally determined univariant sulphide reactions are given in 'figure 2.2. These reactions do not involve sulphur vapour and it can be seen that total pressure has little effect.. Consequently, these reactions are potentially useful geothermometers. It must be noted however, that, apart from the pyrrhotite group which have transitions above 450°C, most of the sulphides in the top half of figure 2.2 are not common, beyond the limits of low grade metamorphism. The effect of sulphur activity is illustrated in the bottom portion of figure 2.2 where the phase relationships are plotted against sulphur activity (a(S2)) and temperature. The 'main-line ore forming environment', (considered to be the range of a(S2) in the crustal environment (Barton 1970)) is also plotted in figure 2.2. It can be seen that the important transition pyrite-pyrrhotite is strongly dependent upon sulphur vapour partial pressure and that pyrite is stable at low metamorphic temperatures except when the sulphur partial pressure is low (fig. 2.2). During metamorphism, the sulphur activity may be controlled in several ways. Sulphur may be complexed in the form of H2S, HS, (K, Na, H) SO4 (as discussed by Helgeson 1969) by reactions with pore fluids. Sulphur may be produced by several reactions of pyrite with silicates and carbonates, eg.
4FeCO + 5FeS = 3Fe 0 + 4C + 2.5S 3 2 3 4 2
(Barton 1970). Buffering of sulphur by other sulphide reactions may also be important. In the Fe - Zn - S system, the amount of FeS in sphalerite coexisting with pyrite and pyrrhotite can be used as a geobarometer at temperatures below 550°C (Craig and Scott 1974). Of the more common sulphides in stratiform sulphide ores (pyrite, pyrrhotite, sphalerite, galena, chalcopyrite) pyrrhotite and chalcopyrite have the complicated phase relationships at low temperatures. These are discussed in detail in sections 3.5 and 3.6 but it suffices at this point to say that, in general, low temperature phases and assemblages of pyrrhotite and chalcopyrite are preserved in ores which have been metamorphosed. For example the reaction
chalcopyrite + pyrrhotite.# pyrite + high temperature cubanite ▪ 20 TEMPERATURE °C 200 400 600 1000
13 --1 •••C 7."..; 3. -, eo fp o 3- a -0,„ 0 9.:,-...... a u, • .....: -o :4 o a :::▪ c ID a) ti rt -- 2- 3
•
3-
15-
f
PRESSURE P vs TEMPERATURE FOR VAPOUR FREE UNIVARIANT EQUILIBRIUM
REACTIONS (after BARTON 1970)
main line ore forming environment
LOG a(S2 ) vs 103/ TEMPERATURE °K SHOWING PHASE RELATIONSHIPS
AND THE MAIN LINE ORE FORMING ENVIRONMENT (after BARTON 1970 )
FIGURE 2.2 Sulphide mineral equilibria. 21
occurs at 334°C but in most ore deposits which have been metamorphosed at high temperatures, the low temperature assemblage of chalcopyrite 4. pyrrhotite is found. Detailed lists of the stabilities of many sulphide minerals are given in Craig and Scott (1974) and Barton and Skinner (1967). If the more unusual sulphides are found in the ore deposit, they may possibly serve as a guide to the conditions of metamorphism.
2.3 METAMORPHISM OF SULPHIDES - A BRIEF SUMMARY
In the past two decades, considerable attention has been paid to strata-bound and stratiform sulphide orebodies. Many of these deposits occur in regionally metamorphosed terrains (see reviews in Wolf 1976) and the recognition and understanding of metamorphic effects is most important in order to evaluate the genesis of these deposits. Detailed, although not comprehensive reviews of sulphide metamorphism are given by McDonald (1967), Vokes (1969), Stanton (1972) and Mookherjee (1976). Detailed lists of references to particular metamorphosed deposits are given in these reviews and in Wolf (1976). In general, metamorphic effects have been classified according to those due to contact metamorphism, dislocation metamorphism and to regional metamorphism (Vokes 1969, Mookherjee 1976). Previous reviewers have discussed the textural changes, the chemical effects and the isotopic changes due to metamorphism. The latter two effects are usually correlated with experimental studies of sulphide phase equilibria (section 2.2). In this thesiS, the common sulphide minerals galena, sphalerite, pyrite, pyrrhotite and chalcopyrite are studied and it can be seen that there are few chemical changes in these minerals during low grade metamorphism (section 2.2) Contact metamorphism of sulphide orebodies produces local microstructural and mineralogical changes. Mookherjee and Suffel (1967, 1968) investigated the contact relationships between massive sulphide ores and intrusive diabase bodies at various localities in the Canadian Shield. They found intensive local redistribution of the sulphides and re- equilibration among what were originally stable mineral assemblages. Lawrence (1972) found that thermally metamorphosed pyrite exhibited grain growth and an equilibrium (foam) microstructure. Mookherjee (1976) recognises that the local heating caused by contact metamorphism releases sulphur in the ores. The most common mineral transformation is that of pyrite into pyrrhotite. Table 2.1 summarizes contact metamorphic effects. 22
Regional metamorphism is responsible for the following effects in sulphides: (1) changes in mineralogy; (2) changes in fabric; (3) mobilization of minerals and of elements. Although previous reviewers have classified dislocation metamorphism as a special case, here it is treated as a fast strain rate event (section 2.1) during regional metamorphism (cf. mylonitization). Mineralogical changes during regional metamorphism may include the initiation of exsolution phenomena (cf. chalcopyrite in sphalerite, Mookherjee (1976)) or re-equilibration of solid solution phases (eg. FeS in Zn, Scott and Barnes 1974). Changes in fabric (ie. microstructures and textures) are perhaps the most important feature of regionally metamorphosed sulphide deposits as it is these changes in microstructures and textures which allow the geologist to recognise deformation and metamorphism in sulphides. Broad aspects of structures in sulphide ores such as foliations, augen structures and porphyroblasts have been discussed by many authors (see reviews by McDonald 1967, Vokes 1969, and Mookherjee 1976). The microstructural aspects such as slip, twinning, kinking, recovery and recrystallization have, however, received little attention apart from the important work of Ramdohr (1953a, b, 1969) and Stanton (1964, 1972). Detailed studies of microstructures involve careful etching of polished sections as discussed in Appendix A. This procedure is not often used and only a few recent studies (McDonald 1970, Richards 1966, Davis 1972) have used etching. In the past decade detailed rheological tests have been carried out on galena, sphalerite, pyrite, pyrrhotite and chalcopyrite (Atkinson 1972, 1974, 1975a, b, 1976a, Clark and Kelly 1973, Siemes 1970, 1976, and others - details in chapter 3) and these experiments have allowed the rheology and microstructures of experimentally deformed sulphides to be compared with naturally deformed sulphides. Such an approach is adopted in this thesis. Detailed studies of microstructures have important ramifications in - studies of the genesis of these deformed ores (particularly sedimentary deposits) in that it may be possible to separate depositional, diagenetic and deformational microstructures, and thus provide information on the genesis and deformation history of the ore deposit. In general, the grain size of sulphides in regionally metamorphosed sulphides shows a concommitant increase with an increased metamorphic grade (cf. Vokes 1969, Templeman - Kluit 1970). Thus we see an increase in grain size from 5 - lOpm for the McArthur River sulphides, to 100 - 200 m at Mount Isa and up to several cms for the lead-zinc sulphides at Broken Hill - which are in a sequence of increasing metamorphic grade from 23 a) unmetamorphosed, b) low greenschist, and c) to upper amphibolite facies, respectively. Vokes (1969) discusses the observation of relict microstructures which indicate primary deposition features and Stanton (1972) emphasises that the so called paragenetic sequence for many deposits may be the result of deformation and metamorphism. Finally, it is pertinent to discuss briefly the remobilization and redistribution of sulphide minerals during metamorphism. Taylor (1971) first pointed out the importance of carbonaceous material during ore genesis. Many stratiform ores are associated with organic material and carbon is an integral part of the ore mineralogy. Carbonaceous material is extremely sensitive to low temperature metamorphism and its nature permits one to infer the conditions of metamorphism (fig. 2.3, Saxby 1976). In addition carbon films frequently form around mineral grains and may be responsible for preserving primary microstructural features such as pyrite framboids (chapter 7) and pyrite nodules as found in many massive sulphide deposits (Sangster 1976). Hewett and Solomon (1964) have demonstrated remobilization of galena ores at Mount Isa and McDonald (1970) argued that galena moved preferentially to the hinges of folds, in the same deposit. Similar effects were found in the Skarnesdalen deposits of Norway by Juve (1967). Chalcopyrite, pyrrhotite and galena are commonly remobilized into fractures and fissures during deformation (Sangster 1976, Vokes 1969). Mobilization of elements due to stress directed diffusion has been demonstrated in boudinaged (Berglund and Ekstrom 1974) and folded ores (Stephansson et al. 1977). Mookherjee (1976) has given a detailed exposition of the generation of fluids during the metamorphism of ore deposits and in particular of the potential of these fluids to form new ore deposits. The reader is refered to this paper for greater detail. A summary of the broad aspects of metamorphism on sulphides is given in table 2.1. The rheological properties of the sulphide minerals studied in this thesis are discussed in sections 3.3 - 3.7.
JULIA CREEK ALGINITE • (oil shale)
EXINITE RED SEA • 12 2.0 Waxes • oFals
1 Bacteria Carbohydrate ALGINITE • Animals Fungi • (Cellulose) (average) 1.0
1.5 • Protein • Wood
• Plants EXINITE ,Lignin (average) 0.8 VITRINITE McARTHUR RIVER •
1.0 VITRINITE
-■ •-■ MICRINITE • Humic Acids MICRINITE FUSINITE
0-5 Loss of CO 0.4 FUSINI TE 2
Loss of CO HILTON 2 Loss of 1120 MOUNT ISA 02 (1100) Loss of H I I 1 i 2 02 04 06 (BLACK STAR) --**-(BLACK ROCK) 0/C Loss of .CH4 BROKEN HILL ATOMIC RATIOS OF KEROGENS IN COAL MACERALS
I 0.65 1 010 015 0-120 0.25 0/c (after Saxby 1976) ATOMIC RATIOS OF KEROGENS FROM MOUNT ISA AND SIMILAR ORES FIGURE 2.3 Atomic ratios of kerogens in coal and ore macerals (after Saxby 1976). 24a
TABLE 2.1 A SUMMARY OF METAMORPHIC EFFECTS ON SULPHIDE ORES
METAMORPHIC GRADE OBSERVABLE EFFECTS EFFECTS OF FLUIDS REMARKS/REFERENCES
drastic change in mineralogy minor veins and release of Lawrence (1972) texture and composition of sulphur only. Mookherjee (1976) sulphides, foam equilibrium Mookherjee and microstructures; incipient Suffel (1967, 1 968) CONTACT melting; volatile transport and diffusion of sulphides; METAMORPHISM all reactions generally confined to within a few alms. on contact zones. Typical
reaction:pyrite -iopyrrhotite.
Textural - microstructural local remobilization only Richards (1966) REGIONAL changes - shear textures influence of fluids Lawrence (1965) METAMORPHISM 'Bleischwief' galena mineral- generally minor - local Stanton and Willey ogical changes limited - transport only. (1972) 1) LOW TEMPERATURE located in fault, shear or Yokes (1969) HIGH STRAIN RATE mylonite zones. Slemes and ('dynamic metamorphism') Schachner-Korn (1965)
Initiation of textural, Expulsion of water at Stanton (1972) microstructural and mineral- early stages solution McDonald (1967, 1970) ogical changes. Slip and redeposition local Vokes (1969) 2) LOW GRADE twinning kinking and transport of ore minerals Braithwaite (1974) beginning of recrystall- probably by pressure soln., Davis (1972) GREENSCHIST ization. Some primary features Wall rock for particular FACIES. may be recognisable. Grain alteration by fluids deposits such as size variable - generally probably common. Mount Isa, Roseberry fine. etc.
Medium grade - coarse remobilization by physical Mookherjee (1976) grained size intense processes - diffusion. Stanton (1972) 3) MEDIUM GRADE recrystallization and grain Fluids possibly have Vokes (1969) AMPHIBOLITE growth. Deformation micro- retrograde effects. Juve (1967) FACIES. structures not common. Physical remobilization. Increase in Co/Ni ratio in pyrite.
Very coarse grain size localized hydrothermal 4) HIGH GRADE Juve (1967) partial melting and sulphide fluids with vein Lawrence (1967) UPPER AMPHIBOLITE neomagmas : pyrite less precipitates. Sulphide Mookherjee (1976) - GRANULITE abundant than pyrrhotite. neomagmas - long distance Stanton (1972) FACIES. isotopic compositions narrow. chemical transport? Vokes (1969) 25
CHAPTER 3 DEFORMATION MECHANISMS AND MICROSTRUCTURES
INTRODUCTION
Rock deformation mechanisms may be divided into three broad categories; (1) cataclastic processes; (2) intra-crystalline processes involving dislocation movements; (3) diffusive mass transfer processes. The term plasticity is commonly used to describe the flow of material at constant stress (see Price 1966) but in the context of this thesis, plasticity refers to the deformation of crystalline materials which flow by dislocation or diffusion processes without undergoing brittle failure i.e. microscopic plasticity. In this chapter, we describe the deformation mechanisms exhibited by crystalline materials which have been strained under different environ- mental conditions with particular emphasis being paid to plastic deform- ation mechanisms and the resulting microstructures. Here we take plastic strain to be the permanent shear strain that is obtained for shear stresses greater than the elastic limit of the material. Since most sul- phide minerals, except pyrite (Chapter 2 and Section 3.7 this chapter), exhibit microscopic plasticity during deformation in the earth's crust, pseudo-macroscopic plasticity, which is achieved by microscopic cataclastic processes, is not discussed in this summary of deformation mechanisms. The environmental parameters which may affect crystalline plasti- city include; temperature, differential stress, confining pressure, strain rate, pore fluid pressure and chemistry, phase transformations and chemical variations of the solid components in the deforming system. The first part of this chapter describes plastic deformation mech- anisms such as dislocation glide, twinning, kinking, climb, recovery, dynamic recrystallisation and diffusional flow. Static recovery, static recrystallisation and grain growth are also described. The microstructures which result from the above processes are documented. The second part of this chapter summarises the physical properties of galena, sphalerite, chalcopyrite, pyrrhotite and pyrite. 26
3.1 DEFORMATION MECHANISMS
In this summary of deformation mechanisms and microstructures extensive reference is made to the metallurgical and materials science literature. It is in the context that the term 'creep' is used for the mechanism of dislocation creep - dislocation glide, climb and polygo.ni- sation. This usage is different from that in the rock mechanics liter- ature where creep is used purely to describe time-strain behaviour resulting from constant stress or constant load testing. For a parti- cular material, single crystal or polycrystal deformation behaviour can be illustrated using stress-strain curves (fig. 3.1). The deformation regimes (fig. 3.1) are characterised by differences in hardening rates (h) and recovery rates (i.) (reviewed by Nicholas and Poirier 1976). The values of the environmental parameters, stress, strain rate and temperature for each deformation regime will vary from material to material. For many compounds and metals, the cold-work regime (fig. 3.1a) is found at low temperatures 3 and at high strain rates, (T40. Tmelting) where, an initial period of elastic or plastic strain is followed by rapid work hardening which may ultimately result in brittle failure (fig. 3.1a). The stress-strain curves, however, often level out first as the dislocation density saturates. The dislocation creep regime (fig. 3.1b) is characterised by higher T/Tm or slower strain rates. After an initial period of hardening (primary dislocation creep - 1, fig. 3.1b), the recovery rate ( .)7 equals the hardening rate (h), and the material flows at constant stress (steady- state flow, secondary creep - 2, fig. 3.1b). At higher strains, the hardening rate again increases until failure occurs (tertiary creep - 3, fig. 3.1b). Unstable flow occurs if, after yield, the recovery rate (i') is greater than the hardening rate (h) and constant strain rates are accom- panied by a decrease in the applied stress (fig. 3.1c). This type of stress-strain behaviour is also found when geometric softening occurs (section 3.1(i) ). At high temperatures T/Tm >0.5, and at high strain rates, the hot working regime is characterised by cyclic stress variations (fig. 3.1d), where cyclic recrystallisation generates new grains which rapidly harden and then recrystallise producing stress oscillations (Jonas et al. 1969, Sellars 1978).
27
a) COLO WORK b) DISLOCATION CREEP
failure
work hardening failure 4.) U)
strain strain
c) UNSTABLE FLOW d) HOT WORK
U) U) U) U) a, in
softening cyclic recrystallisation
strain strain
(after Nicholas & Poirier 1976)
FIGURE 3.1 Idealized stress-strain curves for cold work, dislocation creep, unstable flow and hot work. 28
Detailed reviews of deformation mechanisms are given by Nicholas and Poirier (1976), Evans and Langdon (1976), Takeuchi and Argon (1976), Ashby and Verrall (1978), Edington et al. (1976) and Knipe (1977). The mechanisms are briefly reviewed in terms of constitutive equations, with reference to a) dislocation glide (cold work regime) b) dislocation creep (hot work - high stresses) c) diffusional creep (hot work - low stresses) - including fluid assisted diffusion (pressure solution). Finally, recovery,recrystallisation and grain growth are discussed.
3.1(i) DISLOCATION GLIDE : SLIP - TWINNING - KINKING
SLIP
Slip in crystals occurs by the movement of line defects - dis- locations, through the lattice (see Hull 1975, Nicholas and Poirier 1976 for details). If the dislocation line is associated with an extra half plane of atoms it is known as an edge dislocation whereas if the atomic planes are distorted helicoidally but with no extra half planes of atoms the dislocation is known as a screw dislocation (Hull 1975, Nicholas and Poirier 1976). Dislocation glide is characterised by the conservative motion of dislocations, i.e. edge dislocations are confined to their slip planes (Hull 1975, Nicholas and Poirier 1976). Screw dislocations, however, move perpendicular to their length and to the direction of slip and are free to move on a cylindrical surface with the slip direction its axis. Screw dislocations can change their glide planes (usually in the form of dislocation loops, i.e. mixed edge and screw dislocations,Hull 1975, p.67) and such a process is termed cross slip. Conservative dislocation motion is characterised by the glide of a dislocation under the action of a shear stress and involves no transport of matter by diffusion., The dislocations can however, seek alternative but energentically more favourable arrangements by splitting into partial (half) dislocations separated by stacking faults in the lattice (Nicholas and Poirier 1976). Splitting of dislocations is an important factor in the case of glide in all crystals and in many cases the easy glide planes are those where dissociation is possible (Nicholas and Poirier 1976). The motion of a dislocation along its glide plane in a crystal lattice is resisted by forces arising from the breaking of bonds as the dislocation moves through the lattice (Peierls force - see reviews by 29
Nicholas and Poirier 1976, Ashby and Verrall 1978). Obstacles such as impurity ions, precipitates and other dislocations (e.g. dipoles) also oppose the glide of dislocations. In ceramic materials (e.g. MgO, NaC1, PbS and other sulphides), the motion of dislocations is further complicated by the fact that the dislocations are often charged and the impurity ions are also charged. As the dislocations move, the cores drag along a charged cloud which interacts with other dislocations and impurity ions (Gordon 1973, 1975). The impurity ions pin dislocation lines and act as sources of dislocations (Frank-Read sources - Nicholas and Poirier 1976) during dislocation glide. The interactions of dis- locations with obstacles along the slip planes are most important in the dislocation glide regime and are largely responsible for dislocation multiplication and work hardening (Kocks et al. 1974, Whitworth et al. 1976). Knipe (1977), Kocks et al. (1974), Nicholas and Poirier (1976) and Ashby and Verrall (1978) have given detailed reviews of the mechanics and constitutive equations of dislocation glide. Kocks et al. (1974) give a theoretical analysis of dislocation glide. In brief, they found an exponential dependence of strain rate upon the applied stress and upon the activation energy of glide. This was confirmed for the experi- mental deformation of galena by Atkinson (1976a) -
e = K exp (-Q/RT) exp (B a ) 3.1
where K and B are empirical constants, Q is the activation enthalpy, R is the gas constant and T is the absolute temperature. The upper stress limit for the dislocation glide regime is the ideal shear strength of the material for, at applied differential stress levels above the ideal shear strength, catastrophic failure occurs. For some compounds, an empirical relationship of the form, ideal shear strength = G/10, has been found where G is the shear modulus (Frost and Ashby 1973, Ashby and Verrall 1978). As first pointed out by von Mises (1928), a crystal requires five independent slip systems in order to undergo a general homogeneous strain by slip processes. In many ceramic materials the easy glide system pro- vides only two or three independent slip systems (Evans and Langdon 1976). This is also the case for some sulphide minerals - galena and pyrrhotite (Atkinson 1975a). The other necessary slip systems may be provided by harder glide planes e.g. 0101 <110> in PbS, by lattice bending, by 30 kinking (e.g. Paterson 1969) or by deformation twinning. Lister et al. (1978) have shown in their theoretical study of fabric development in quartzites that the fourth and fifth slip systems contribute less than 15% and 1% respectively to the deformation. Micro- cracking and grain boundary sliding during dislocation glide can some- times accommodate these low strains in order to satisfy von Mises criterion. Evans and Langdon (1976) have discussed the formation of microcracks by dislocation processes; e.g. Stroh cracks in crystals with a rock salt structure are formed by the coalescence of dislocations on two intersecting slip planes. Microcracking and cleavage cracks have been observed by Atkinson (1975a) in the experimental deformation of pyrrhotite.
DEFORMATION TWINNING
At low temperatures (-0.3 Tm), in the dislocation glide regime, deformation twinning may be an important mechanism. Twinning is commonly associated with stacking faults and partial dislocations (Nicholas and Poirier 1976), but the maximum shear strain that can be achieved by deformation twinning is limited by the magnitude of the twin shear (fig. 3.2). Twinning is an important deformation mechanism in sphalerite, pyr- rhotite and chalcoRyrite. The geometry of deformation twinning is illustrated in figure 3.2 where K and n are the twin plane and the twin direction respectively, 1 1 and K2 is the undistorted plane with n2 also being in the K2 plane. Frost and Ashby (1973) have suggested a constitutive equation for twinning which has an exponential relationship of strain rate and stress similar to that for dislocation glide (equation 3.1).
DEFORMATION KINKING
Deformation kinking is commonly found in crystals of low symmetry or where there are insufficient independent slip systems to satisfy von Mises' criterion. Geometrical softening of active slip systems is some- times achieved by deformation kinking (Flewitt and Crocker 1976) whereby the rotations generated by kinking increases the resolved shear stress on the slip system. 31 SLIP IN A SINGLE CRYSTAL DURING EXTENSION
I
slip plane normal
burgers vector
ELEMENTS OF DEFORMATION TWINNING 112 undistorted plane
/1 distortion of a circle into an ellipse twin twin direction plane
DEFORMATION KINKING DURING COMPRESSION 1 2
slip planes
kink band boundary '...\.... 1 1
K = shear plane of kink 1 =shear direction of kink 111 K2 slip plane = 12=slip direction
FIGURE 3.2 Elements of slip, kinking and twinning. 32
Starkey (1968) has erected a model for kinking in crystals in which the angle of kinking is related to geometrically necessary dis- locations in the kink band. This model predicts specific kink band angles whereas, in practice, a wide range of kink band rotation angles are found. Broadly speaking kink band boundaries are commonly perpen- dicular to the active slip plane and also perpendicular to the glide direction (Clareborough and Hargreaves 1959, Weiss and Turner 1972, Crocker and Abell 1976). In detail, however, kink band boundaries are complex, commonly comprising of bent lattice planes and tilt boundaries. Kink bands are also formed by glide polygonization (Livingston 1960, 1962) where dislocations of opposite signs pile up along slip planes (possibly due to interactions with dipoles Muller, 1959) and produce zones of reverse lattice curvature (fig. 3.3). The crystallography of deformation kinking has been described by Crocker and Abell (1976) in a similar fashion to that for deformation twinning (Mahan and Williams 1973), where K is the shear plane of the kink n is the shear direction 1 , 1 of kinking, K2 is the slip plane outside of the kink band and n2 is the slip direction in the K2 slip plane (fig. 3.2). It must be emphasised that these geometric relationships are valid only for the onset of kinking and that, as the deformation proceeds, geometrical softening of the slip plane by the kinking process leads to variations in the angular relationships of the kink band boundaries to the slip planes. Deformation kinking may be treated as a slip system and as such may contribute to crystalline plasticity by helping to satisfy von Mises' criterion (Paterson 1969).
3.1 (ii) THE DISLOCATION CREEP REGIME
In this regime, edge dislocations are no longer confined to their slip planes and are able to climb over obstacles and to climb into arrays and form walls. Dislocation climb is accomplished by the diffusion of vacancies to the dislocation line or the removal of atoms from the extra half plane. Because of the statistical nature of thermal activation, during dislocation climb only a part of the dislocation line changes slip planes and consequently a step or jog forms in the dislocation line. The jogs become sources or sinks for vacancies (Nicholas and Poirier 1976). The dislocation creep (power law creep) regime is characterised by a combination of dislocation glide, dislocation climb (controlled by diffusion of vacancies) and a non uniform deformation caused by grain 33
GLIDE WITH DISLOCATIONS OF OPPOSITE SIGN _L 1_ 1 A. IT T _L J L -T" -r J_ -r _L L 1 -r- T -r -1- -r- -r -r obstacle (dipole - inclusion)
GLIDE POLYGONISATION
-r -L _L -r _L. -r -r T T T dislocations of like sign lock up into walls
KINK BAND DEVELOPS C. -L. t -L. _L I ..L... i I _l_ ...."...( I _l_ ■._ 1 -r / -74 1 ..T., -r -7- _L. ....r.-. _L. '..."..::.4. ` 1T -r -r ---,z4I s. :,:, -1- 7- 77 I "7" 1 -r- -r- -r // "7" "1- -r
SCHEMATIC KINK BAND FORMATION
FIGURE 3.3 Schematic diagram of glide polygonisation and kink band formation. 34
boundary sliding. One model for power law creep is shown in figure 3.4. Dislocation glide and climb are involved in the formation of sub-grain cells (misorientation of the order of 2°) and grain boundary sliding occurs between the grains (Ashby and Verrall 1978). In the model in figure 3.4, the cells act as sources and sinks for vacancies and the cell size itself is dependent upon the applied stress in a manner
d cell / b / a 3.2
where d cell = cell diameter, b = burgers vector, p = shear modulus of the material, a = applied stress. An equation of the form
= A (Du b/kT) (a/p)n 3.3
(where é = strain rate, A = constant, n = 3 - 10, b = burgers vector, D is the effective diffusion coefficient, k = Boltzman's constant and T = absolute temperature) is characteristic of power law creep (Ashby and Verrall 1978). A and n are not independent and are usually determined experimentally. Stocker and Ashby (1973) found empirically that the relationship
n 3 + 0.3 lg A 3.4 applied for most materials for which data was available. In galena, Atkinson (1977) has shown that two types of dislocation creep can be distinguished: high temperature dislocation creep in which the rate of dislocation climb is controlled by the diffusion of atomic species through the bulk of the crystal, and low temperature dislocation creep, in which the rate of dislocation climb is controlled by diffusion along dislocation cores (Frost and Ashby 1973). In this case the effective diffusion coefficient D can be written
2 = Dv ( 1 + 10ac/b2) (a/ G ) (Dc/Dv) 3.5 where Dv is the volume diffusion coefficient, Dc is the core diffusion co- efficient and a c is the cross sectional area of the dislocation core. D is approximately equal to Db, the grain boundary diffusion coefficient, 2 if a is taken to be about 5 b c (Frost and Ashby 1973). It is important 35
MODEL OF DIFFUSION CONTROLLED POWER LAW CREEP
* P 4 4 * 4
GRAIN BOUNDARY SLIDING
::DYNAMIC RECRYSTALLISATION
tt t t t t c-• (after Ashby& Verrall 1978)
MODEL OF SUPERPLASTIC FLOW AND GRAIN NEIGHBOUR SWITCHING
boundary sliding
relative translation of grains
a) initial state b) intermediate state c) final state (after Ashby & Verrall 1973)
FIGURE 3.4 Models for Power law Creep and for Superplastic Flow. 36
to note that for core diffusion controlled dislocation creep the strain rate varies as an whereas for volume diffusion controlled dislocation n+2 creep the strain rate varies as a . Various other sophisticated models of dislocation creep have been proposed and these are reviewed in detail by Evans and Langdon (1976). The most important of these is sub-grain creep (Nabarro 1967, Ashby 1972) where sub-grain walls can act as sources and sinks for vacancies and can also act as high diffusivity paths. The stress exponent for sub-grain creep varies between 3 and 6 (see Knipe 1977 and Evans and Langdon 1976 for detailed discussions of sub-grain creep). For many materials, as the stress is raised above approximately -3 10 p (shear strength), the power law relationship breaks down because of an increased contribution of dislocation glide to the strain rate. At high temperatures, a second complication arises in that the material may recrystallise as deformation proceeds (Sellars 1978). Waves of recrystal- lised material sweep out the cells and repeated surges of primary creep occur.(Ashby and Verrall 1978). In the dislocation creep regime, single and polycrystal behaviour are essentially similar (Takeuchi and Argon 1976). Dislocation climb, how- ever, may be important particularly in polycrystalline plasticity where the von Mises (1928) criterion cannot be met by slip processes alone. Groves and Kelly (1969) discussed the change of shape during dis- location climb and Durham and Goetze (1977) and Goetze (1978) have argued that for olivine, climb processes may enable von Mises criterion to be relaxed and thus allow polycrystalline olivine to deform without cavitation. Ball and White (1978) have put forward a similar argument for quartz deform- ation. Detailed reviews of diffusion-controlled power law creep are given by Takeuchi and Argon (1976), Evans and Langdon (1976), Knipe (1977) and Ashby and Verrall (1978). The reader is referred to these papers for greater detail on mechanisms and constitutive equations.
3.1 (iii) DIFFUSIONAL FLOW
At high temperatures ( >0.6 Tm) and at low stresses metals and ceramics can deform by diffusion alone. Lattice diffusion, grain boundary diffusion and fluid assisted diffusion are reviewed. As we shall see, diffusional flow processes are grain size dependent and this has important ramifications in fine grained polycrystalline material. 37
If a grain boundary is subjected to deviatoric stresses, then a chemical potential gradient for vacancy flow is established (see review by Burton 1977, Nabarro 1948 and Herring 1950). The flux of matter, which is equal and opposite to the vacancy flux, leads to depletion of material at the relatively compressed grain boundaries and deposition at relatively extended boundaries. The grains can thus change shape. If the diffusion of the matter is essentially through the grains, i.e. lattic diffusion, the process is termed Nabarro-Herring creep, whereas if it is predominantly around the grain boundaries, the flow is called Coble creep (Coble 1963). Lattice diffusion(Nabarro-Herring creep) is important at high temperatures in metals (see review by Burton 1977) and perhaps - in the Earth's mantle (Stocker and Ashby 1973). At lower temperatures, however, lattice diffusivities and solid state boundary diffusivities are expected to be too slow to account for the observed strains in low grade metamorphic rocks (McClay 1977c). Many textures in low grade metamorphic rocks (T = 200 - 350°C) are interpreted as indicating deformation by diffusive mass transfer (McClay 1977c). It is therefore inferred that the presence of a fluid phase in the grain boundaries enhances diffusive mass transfer in low-grade metamorphic rocks - hence the term 'pressure solution'. Atkinson (1977) has suggested on theoretical grounds, that Coble creep is an important deformation mechanism in fine grained galena ores. Pyrite may deform by pressure solution (Atkinson 1972) as discussed in section 3.7. The presence of fluids in the grain boundaries of sulphide ores could further accelerate grain boundary diffusion particularly in view of the high solubilities of sulphide minerals (see sections 3.3 - 3.7). Diffusional creep in polycrystals whether Nabarro-Herring creep, Coble creep, or 'pressure solution' is accompanied by grain boundary sliding, (Lifshitz 1963, Gifkins 1967, 1976, Raj and Ashby 1971, Ashby and Verrall 1973, 1978) if voids are not to form (i.e. constant volume deform- ation, fig. 3.5). Detailed reviews of diffusional flow have been given by Knipe (1977), Evans and Langdon (1976), Nicholas and Poirier (1976), and Ashby and Verrall (1978). In this section the mechanisms and rate equations for Nabarro-Herring creep, Coble creep and 'pressure-solution' flow are dis- cussed.
NABARRO-HERRING CREEP
Nabarro-Herring creep (Nabarro 1948, Herring 1950) involves the diffusion of vacancies through the lattice. An equilibrium number of
A B C EXTENSION
FIGURE 3.5 A. Undeformed hexagonal array of grains with marker line. The compressive stress axis is horizontal. B. If diffusional creep occurs without grain boundary sliding (i.e. the centres of the hexagonal grains and the marker line are not displaced) then there would be a volume increase and voids (black areas) would form against the compressive stress. This is an unlikely mechanism and the diffusional creep is accompanied by grain boundary sliding. C. Diffusional creep with grain boundary sliding resulting in offset of the marker line and no voids forming (constant volume deformation). 39
vacancies is always present in order to maintain a minimum free enthalpy (Kelly and Groves 1970, Nicholas and Poirier 1976). The mobility of vac- ancies can be defined in terms of a diffusion coefficient, Dv, which is related to the absolute temperature T (°K) by
Dv = D exp ( -Qv/RT) 3.6 ov
where D is the absolute diffusivity, Qv is the activation energy, ov and R is the gas constant. In two component systems, such as ionic solids and sulphides, the diffusing ions and vacancies carry a charge and it is the slower moving component which determines the diffusion coefficient, and so limits the creep rate (Ashby and Verrall 1978). The rate equation for Nabarro-Herring creep can be written as
e = B( aa VDv/kTD2) 3.7
where é = strain rate, V = molar volume, k = Boltzman's constant, d = grain diameter, a a = applied stress, and B is a constant depending upon the assumed grain shape and diffusion path geometry. It is implicit in the derivation of equation 3.7 that the grains suffer the same shape change as the specimen (Ashby and Verrall 1978). In order to maintain continuity of the specimen, however, grain boundary sliding occurs (fig. 3.5).
COBLE CREEP
If, during diffusional flow, vacancy diffusion occurs around the grain boundaries, the process is termed Coble creep (Coble 1963) and the rate equation can be written as
= B a Va D / k T d3 3.8 a b
with a = the grain boundary width, Db = grain boundary diffusivity, and 1 B a constant dependent upon the grain shape and the diffusion path geo- metry. The grain boundary diffusivity Db is related to temperature via an equation similar to 3.6. In general the activation energy for grain boundary diffusion is not known and it is often taken as 2/3 of that for volume diffusion (Mistler and Coble 1974, Stocker and Ashby 1973). 40
The results of Rutter (1976) and Schmidt (1976) indicate that activation enthalpy for grain boundary diffusion in calcite is approximately 2/3 of that for volume diffusion. The effective grain boundary width 6 cannot be determined experi- mentally. The value of 6 has important effects on the flow rate (equa- tion 3.8). Mistier and Coble (1974) have discussed the effective grain boundary widths for metallic and ionic compounds. For metals 6 is taken as 2 x b where b = burgers vector, whereas for ionic materials it may be as large as 100 (Mistier and Coble 1974). In ionic or partially ionic materials (cf. sulphides), the grain boundaries may have a surface charge with a balancing charge inside the crystal. These charges will affect the vacancy concentrations and hence the volume and grain boundary diffusivities. Atkinson (1977) has shown how the choice of 6 signifi- cantly affects ore rheology.
SUPERPLASTIC FLOW - GRAIN NEIGHBOUR SWITCHING
Superplasticity was first coined for the large strain tensile deformation of binary alloys (e.g. Pb - Sn eutectic alloys) which are fine grained and have a stable microstructure. Superplasticity has been reviewed in detail by Edington et al. 1976. The term superplasticity has been extended to more general use to describe the high temperature, low stress and high strain deformation of materials other than binary eutectic alloys. This may be modelled by the mechanism in figure 3.4 where the grain slide past each other changing their neighbours and altering their shape by diffusion only where it is necessary for continuity to be main- tained. Ashby and Verrall (1973) have modelled superplastic flow as grain boundary sliding accommodated by diffusion and have obtained a rate equation similar to those for Nabarro-Herring and Coble creep. At a satisfactory level of approximation, superplasticity has a strain rate 5 times faster than that for diffusional flow (Ashby and Verrall 1978). Edington et al. (1976) have reviewed other models for superplasticity. One such model has been proposed by Gifkins (1976) in which grain boundary sliding is accommodated by (i) dislocation climb and glide, (ii) by folding at triple points, and (iii) by grain boundary diffusion. Superplasticity has been proposed as an important deformation mechanism in some mylonites (Gueguen and Bouiliier 1976, White 1976). The microstructures found in superplastic materials are discussed in section 3.2. 41
PRESSURE SOLUTION
Pressure solution may be considered to be a special case of Coble creep in which the grain boundary diffusivity is enhanced by the presence of an intergranular fluid film. Pressure solution in rocks and minerals has recently been reviewed (Paterson 1973, McClay 1977c - Appendix B). Evidence for pressure solution in rocks and minerals is readily found in the form of stylolites in lime- stones, spaced cleavages, truncated fossils and pitted pebbles (cf McClay 1977c). Good textural evidence for pressure solution has been found in experiments by De Boer (1975), De Boer et al. (1977) and Sprunt and Nurr (1976). Atkinson (1972, 1975a)has suggested that pressure solution may be important in the deformation of pyrite. Rutter (1976) has erected a theoretical model for pressure solution at low stresses ( as <300 bars). The strain rate is given by
DbO /RTp d3 3.9 = 32 Cra V Co where e, as V, Db, 6, R, T, and d are as before and Co is the concentration of a saturated solution in equilibrium with the unstressed solid and P is the density of the solid. Rutter and Mainprice (1978) have proposed a mechanism of grain boundary sliding accommodated by pressure solution to account for their experimental results on the stress relaxation of wet Tennessee sandstone. The inferred low heat of activation for pressure sol- ution (Rutter 1976) prevents significant speeding up of the pressure sol- ution process by substituting higher temperatures for slower strain rates in experiments. The activation energy for pressure solution is much lower k cals/mole) than those for Nabarro-Herring and Coble creep. The rate equations for Nabarro-Herring, Coble and Pressure Solution creep have been evaluated for galena, quartz and calcite (cf. McClay 1977c) and are presented as grain-size/strain rate plots for two temperatures i.e. 200°C and 350°C (fig. 3.6). These two temperatures are considered, on geological grounds (McClay 1977c) to be the limits for pressure solution -11 processes in the crust. Geologically feasible strain rates; 10 -14 -1 10 s for ductile structures in folds (Price 1975); are achieved by pressure solution in quartz and calcite and by Coble creep in galena (fig. 3.6, see McClay 1977cfor a full discussion). Strain rates for pres- sure solution deformation in sulphides have yet to be evaluated. - 4
CALCITE STRESS LEVEL 100BARS (107Pa) STOICHIOMETRIC STRESS LEVEL 100BARS (107Pa) GALEN%__A T.35cfc 106- 10 6- 106- T=200°C 168- 10 8- •`.‘0, /1=350°C 10 8- ••■•-■ -10 * -10 •N 0 16 10 6■•■ 10 - N. •■ ..1. 16s, 1 2 12 '■ • -12 10 - 10 10 - •%0 `TO 1 4 -14 1 10 10 - ■ 10 - LI ‘• •Z — _ 1=350°C ‘• -16 ; 10 ‘ON.N t10 ♦ -•,_•%. — - -18 18- •,•■ ILI -18 • 10 1-cc 1010 N. N \ I- 10 cr •N.•■ cc - z ♦ •■• :a -20 z -20_ cc10 • — 10 tn ♦ ct -2 2 (r) -22 10 - 10 - 2 4 24 10 - -24 10 " ■ 10 _o _•- PRESSURE 2 6 -26 SOLUTION -26 10 10 - 10 1MP — — COBLE CREEP — -- COBLE CREEP 2 8 -28 -28 10 - 10 NABARRO-HERRING CREEP 10 - NABARRO-HERRING CREEP
6 5 4 3 2 6 -5 -4 3 -2 5 4 -3 -2 10 10 10 10 10 10 16 16 10 10 10-6 10 10 10 10 GRAIN SIZE (metres) GRAIN SIZE (metres) GRAIN SIZE (metres)
1pm 10tim 1004m 1mm lcm 1tim 10tim 100pm 1mm 1cm 10iun 1004m 1mm 1cm
FIGURE 3.6 Deformation mechanism plots of strain rate against grain size for quartz, calcite and galena. The data used for quartz and calcite creep rates were taken from Rutter (1976) and the data for galena from Atkinson (1976b, 1977). Note, that a strain rate of 10-20 s-1 is effectively stop. 43
Impurity ions and stoichiometric defects will have marked effects on the creep and pressure solution behaviour of minerals. In particular the complex nature of sulphide solubility and of the stability of sul- phide ions in complexing solutions (Barnes 1967) will make any theoretical evaluation of pressure solution of sulphide minerals very difficult. Rutter (1978) emphasises that the nature and chemical behaviour of the intergranular fluid film involved in pressure solution processes may be markedly different from that in bulk solutions. Further detailed research on the nature and chemistry of the intergranular fluid film and on the grain boundaries involved in pressure solution processes is needed.
FACTORS AFFECTING DIFFUSIONAL CREEP
It is generally accepted that the rate of diffusion is the rate controlling step in diffusional flow (Elliott 1973, Rutter 1976, McClay 1977c). In ceramic materials (e.g. MgO, NaCl_ both anion and cation dif- fusion must occur but not necessarily at the same rate. A detailed review of diffusional creep in ceramic materials is given by Evans and Langdon (1976). The observed creep rate is determined by the movement of the slower diffusing species along the fastest dif- fusion path. Evans and Langdon (1976) stress the difference between diffusional creep for ceramic materials and that for monatomic systems, and in particular specific cases where,for intermediate grain sizes,there is a transition from Nabarro-Herring to Coble creep with increasing temp- erature or,at constant temperature,the transition occurs with increasing grain size. These transitions are opposite to those found in monatomic materials (Evans and Langdon 1976). For all materials, there is a temperature above which thermally produced defects (intrinsic diffusion) swamp the environmental - non stoichiometric defects (extrinsic diffusion) (Gordon 1973, 1975). This results in an upper limit for Coble creep. Burton (1977) has indicated that, at low homologous temperatures (-,d0.4 Tm), the stresses needed to create sources and sinks for vacancy diffusion may be sufficiently high to inhibit Coble creep in favour of dislocation mechanisms. Harris et al. (1969), Harris (1973) and Burton (1977) have shown that diffusional flow is inhibited by second phase particles which col- lect or form on grain boundaries. These particles decrease the ability of these boundaries to act as sources and sinks for vacancies (Harris 1973). 44
Ashby and Verrall (1978) have discussed the effects of variations in confining pressure on diffusional flow. Pressure will affect the acti- vation volume for diffusional flow (Ashby and Verrall 1978). In some materials, the creation of a vacancy in a solid leads to a volume increase and thus an increase in the confining pressure will tend to oppose vacancy creation. In multi-component systems, complications arise when vacancies of one species are stabilized by aliovalent impurities or by vacancies of the other species. In these cases, pressure will not then change the vacancy concentration. Some ionic solids are expected to have a large expansion of the lattice when vacancies are formed. Thus, for materials which suffer expansion on vacancy formation and provided the diffusion is not affected by impurities (intrinsic diffusion), an increase in con- fining pressure is expected to decrease the rate of diffusional creep.
3.1 (iv) RECOVERY PROCESSES
Static and dynamic recovery processes both lead to a reduction in the stored strain energy resulting from deformation. Sellars (1978), Stuwe and Ortner (1974), Honeycombe (1968) and Honeycombe and Pethen (1972) have given recent reviews of recovery processes. Static recovery occurs when the deformed material is raised to or maintained at elevated temperatures (> 0.6 Tm) after deformation. The recovery process involves the climb of edge dislocations into walls and the annihilation of dislocations of opposite sign. The stored strain energy is reduced by decreasing the dislocation density and a polygonal microstructure is formed. The rate of recovery decreases with time. Static recovery often preceeds static recrystallisation but at low strains the stored strain energy may be insufficient to cause static recrystalli- sation (Sellars 1978). During steady state dislocation creep,dynamic recovery opposes work hardening (fig. 3.1b). Dynamic recovery may be achieved by three processes all of which may operate at the same time - a) Above a certain stress level, cross slip of screw dislocations may occur thus enabling annihilation of screw dislocations of opposite signs. The stress above which cross slip is favoured is lowered by increasing the temperature or by lowering the strain rate (Stuwe and Ortner 1974). b) Edge dislocations can leave their slip planes by climbing and then are able to annihilate with dislocations of opposite sign. 45
Climb is facilitated by the generation of vacancies during deformation (Stuwe and Ortner 1974). c) Under high temperature deformation conditions, dynamic recovery involves subgrain boundary migration - 'repolygonisation' which maintains an equant subgrain structure (Takeuchi and Argon 1976) with misorientations that reach a steady state value of only a few degrees (Sellars 1978). The movement of subgrain boundaries to maintain a con- stant microstructure during high temperature creep contrasts with sub- structures formed during cold work where subgrain size and shape change with strain and where misorientations increase with increasing strain (Doherty 1974). Both static and dynamic recovery processes lead to a decrease in the dislocation density and to a softening of the material to further deformation. Static recovery is controlled by the stored strain energy and the annealing temperature whereas dynamic recovery is controlled by the stored strain energy, the temperature, the strain rate and the applied stress. Dynamic recovery, during creep deformation, is balanced by the continued dislocation generation and pile up during work hardening processes.
3.1 (v) RECRYSTALLISATION
Recrystallisation is the process during which new grains with high angle boundaries ( > 80 misorientation across the boundary) are formed from a deformed or deforming matrix of old grains. Nicholas and Poirier (1976) have distinguished two main types of recrystallisation phenomena, a) stress induced recrystallisation (piezocrystallisation) and b) strain induced recrystallisation (static or annealing recrystallisation and dynamic recrystallisation). Piezo- crystallisation is based principally on Kamb's models of crystallisation in a non-hydrostatic stress field (Kamb 1959) where the driving force for crystallisation is determined by the energy difference between the recry- stallised and unrecrystallised states. This energy difference is attributable only to the difference in elastic strain energy between elastically anisotropic crystals in a favourable orientation and those in an unfavourable orientation-in a non-hydrostatic stress field. In strain induced recrystallisation, the difference in energy between the recrystallised and unrecrystallised states is due to the 46
difference in the stored plastic strain energy arising from dislocations and strain induced defects in the crystals. The driving force for recry- stallisation is attributed to this energy difference.
STRESS INDUCED RECRYSTALLISATION
Kamb (1959, 1961), McLellan (1966, 1968, 1970) Paterson (1973)• and others (see McClay 1977c) have presented detailed accounts of the thermodynamics of non hydrostatically stressed solids. Kamb (1959, 1961) uses his models to predict preferred orientations of minerals which crystallise in non hydrostatic stress fields. Preferred growth in a non hydrostatic stress field may account for crystallisation during diffusive mass transfer and, it is worth noting, perhaps during pressure solution. The basic weakness of the theory, how- ever, is that it neglects the role of grain boundary energy and stored strain energy due to plastic deformation. The magnitude of these latter two energies may be at least comparable with the variation in elastic strain energy even. under high stresses (Paterson 1973). Kamb's theories are based on elastically anisotropic crystals and ignore the internal defects and microstructures in the crystals and, as such, may not be sig- nificant for the recrystallisation of cubic minerals such as PbS and ZnS. They may be important if pressure solution occurs, e.g. pyrite (section 3.7). Nicholas and Poirier (1976) give a detailed review of stress- induced recrystallisation. It can be concluded that piezo-crystallisation may only be an important recrystallisation mechanism during diffusional creep of anisotropic minerals and that strain induced recrystallisation is the most likely mechanism during most recrystallisation events.
STRAIN INDUCED RECRYSTALLISATION
Strain induced recrystallisation can be divided into two categories a) syntectonic or dynamic recrystallisation and b) static or annealing recrystallisation. In materials science problems, the differences between the above processes are fairly clear cut in that static recrystallisation is induced by elevating the temperature of previously cold worked material and dynamic recrystallisation occurs during hot work and is usually followed by rapid quenching. Cyclic processing of alternately hot working and annealing a sample is sometimes carried out. In geological deformation, 47 the differentiation between dynamic and static processes is often not as clear cut because of the long times and slow strain rates which are involved. Consequently dynamic recrystallisation may be altered by very slow strain rates and concomitant annealing as slow uplift occurs or alternatively, annealing recrystallisation may be affected by sub- sequent slow strain rate, lower temperature deformations which may give rise to dynamic recrystallisation. The differences between the mechanical processing of construction materials and geological deformation must be borne in mind when comparing the mechanisms and micro-structures which are described in the metallurgi- cal literature with those found in geological materials.
STRAIN INDUCED RECRYSTALLISATION - STATIC RECRYSTALLISATION
Static or annealing recrystallisation is the nucleation and growth of new grains in a cold worked matrix. Static recrystallisation can be divided into two problems, (a) the problem of nucleation of the strain free grain and (b) the problem of growth of the nucleus into a new grain. Doherty (1974) has reviewed the nature of cold worked material and the nucleation of recrystallisation. The driving force for recrystal- lisation is generally accepted to be the stored energy of cold work where the temperature of deformation is < 0.5 Tm (Doherty 1974, Doherty and Cahn 1972). Recrystallisation lowers the energy of the deformed material. This can also be achieved by polygonisation when,during recovery,disloca- tions climb from their slip planes and form small cells or polygons. Polygonisation, however, may not necessarily give rise to recrystallisation (Honeycombe 1968) and the nucleation of recrystallised grains need not necessarily be related to recovery structures. There are four suggested nucleation mechanisms for the nucleation of new grains in a cold worked matrix (Doherty 1974). (1) The classical fluctuation theory of the random nucleation of small new grains in the deformed matrix. (2) The growth of a polygoned subgrain - Cahn Cottrell model. (3) Sub-grain coalescence in which the successful subgrain has achieved an increased size and possibly a larger misorientation by coal- escence of two or more sub-grains. (4) Strain induced boundary migration in which a sub-grain of one grain adjacent to a high angle boundary grows by migration of that boundary into the neighbouring grain. 48
Homogeneous nucleation is almost impossible owing to the small driving force and the high surface free energy of a high angle grain boundary. If nucleation occurs on a pre-existing grain boundary, the energy required for nucleation is reduced but even in the most favourable case the reduction of energy is insufficient to promote nucleation by the fluctuation model. Models (2), (3) and (4) are dependent upon the migration of low and high angle grain boundaries. Doherty (1974) summarises the evidence for boundary mobility. High angle boundaries migrate more rapidly than low angle boundaries because migration can occur by single atom jumps across the rather open boundary structure whereas low angle boundary migration necessitates the coordinated movement of atoms in the disloc- ation networks which constitute the low angle boundary. Models (2), (3) and (4) are all based upon the development of regions already present after deformation - i.e. preformed nucleii. Development of recrystallised grains by model (2) involves the gradual movement of low angle boundaries until sufficient misorientation is achieved to allow rapid boundary migration. Only a few sub-grains in the deformed matrix will achieve this status. Model (3) is a modific- ation of model (2). An alternative and perhaps more promising hypothesis is model (4) in which strain induced boundary migration of high angle boundaries produces recrystallised grains, Doherty (1974), Doherty and Cahn (1972), and Cahn (1978) have shown that small areas of high mis- orientation (up to 50° at 60% strain) occur in the cold worked state in transition bands (Dillamore et al. 1972). They document examples where the new grains are formed only in highly misoriented regions. Doherty (1974) also cites evidence for sub-grain coalescence within the highly misoriented region of a transition band. Sellars (1978) has reviewed both static and dynamic recrystalli- sation in terms of critical strain for nucleation of new grains. The critical strain for static recrystallisation is lower than that for dyn- amic recrystallisation (Sellars 1978). Once the new grains have been nucleated, recrystallisation pro- ceeds by grain growth until the new grains impinge upon each other. The mechanisms of grain growth are discussed in detail in section 3.1(vi). 49
DYNAMIC RECRYSTALLISATION
The characteristic effect of dynamic recrystallisation during creep deformation is to produce one or more periods of accelerated creep. In constant strain rate tests, analogous fluctuations in the flow stress are observed at low strain rates (fig. 3.1d) (Jonas et al. 1969, Sellars 1978). Recent studies have shown that the fluctuating stress strain curves (fig. 3.1d) can be successfully described by combinations of pheno- menological equations involving work hardening, recovery and dynamic recrystallisation (Sellars 1978, Honeycombe and Pethen 1972, Stuwe and Ortner 1974). Dynamic recrystallisation involves the nucleation of new grains followed by their growth and concomitant deformation and finally recrystal- lisation of these new grains. It appears probable that at high strain rates the concurrent deformation destroys the driving force for grain growth after an initial period of growth from the nucleus (Sellars 1978). A dynamic recrystallisation model which can be used to explain the high temperature deformation stress-strain curves (fig. 3.1d) is presented below. Nucleation of new grains occurs after a critical shear strain is reached and with increasing strain recrystallisation proceeds leading to strain softening - decrease in stress (fig. 3.1d). As work hardening of the new grains occurs, recrystallisation ceases and the stress builds up until, at a critical strain for the new grains further nucleation occurs. This process is repeated each time this critical strain is reached, until the process becomes sufficiently out of phase in different regions of the material to make the recrystallisation effectively continuous (Sellars 1978). The nucleation of new grains during dynamic recrystallisation is achieved mainly by grain boundary migration, but may also occur by sub- grain coalescence and sub-grain rotation (White 1976). The recrystallised grain size is independent of the starting material grain size but dependent upon the applied shear stress. These two factors are related by an equation of the form
T = K d-n 3.10 50 where T is the shear stress, K = constant, d is the grain size and n is a constant of value between and 1. The original grain size may, however, influence the dynamics of the first cycle of recrystallisation (Sellars 1978). In an area where recrystallisation occurs, the deformation tends to be heterogeneous. The softer recrystallised grains formed first cause concentration of strain within them, and this leads to favourable conditions for nucleation in adjacent regions where a strain rate grad- ient exists. This gives rise to instability and localisation of strain as recrystallisation proceeds. Sellars (1978) has shown that dynamic recrystallisation occurs principally in the high temperature creep field but it also extends into the low temperature creep field (section 3.1 (ii)). A special form of dynamic recrystallisation is termed in situ recrystallisation (Nicholas and Poirier 1976, Sellars 1978) whereby sub- grains or parts of deformation bands become so misoriented with increasing strain that they constitute new grains. This can be achieved by sub-grain rotation and this has been proposed as an important mechanism in the deformation of quartzites (White 1976). Dynamic recrystallisation microstructures are discussed in section 3.2.
3.1 (vi) GRAIN GROWTH
Once nucleation of a new grain is achieved by the processes dis- cussed in section 3.1(v), growth of the strain-free new grain is accompl- ished by grain boundary migration. In this section, the kinetics and mechanisms of grain boundary migration and grain growth are briefly reviewed. During dynamic recrystallisation, grain growth is usually inhibited by deformation of the new grains (section 3.1(v)) and consequently most of this discussion applies principally to grain growth during static - annealing recrystallisation. Two types of grain growth have been recognised, normal grain growth and abnormal grain growth (secondary recrystallisation).
NORMAL GRAIN GROWTH
Once annealing recrystallisation has produced strain free grains from a matrix of deformed grains, grain growth will occur as the boundaries of the new grains migrate until they impinge upon each other. Further 51 grain boundary migration (normal grain growth) occurs as these boundaries readjust themselves into minimum energy configurations. This process gives rise to a gradual coarsening of the grain structure (Honeycombe 1968). The driving force for grain growth is the reduction in the surface energy of the grain boundaries achieved by reducing the grain boundary area per unit volume (Smith 1964). Migration of the grain boundaries typically occurs towards the centre of curvature of the moving boundary, leading to an increase in grain size as the smaller grains disappear (Smith 1964). Smith (1964) has identified a body centred cubic array of Kelvin tetrakaidecahedra as the most appropriate model of a 3-D grain structure. In 2-D sections, this structure is seen as a group of poly- gons with the average number of sides being 5 3/7 and with the edges of the polygons meeting at triple points with dihedral angles approaching 1200. For polygons with six sides, the theoretical grain boundaries should be straight but as the equilibrium structure contains polygons with 4, 5, 7, 8 etc. sides, surface tension requires that the grain bound- aries are curved (Smith 1964). During grain growth, polygons with less than six sides tend to disappear and polygons with more than six sides tend to grow (Smith 1964)
THE KINETICS OF GRAIN GROWTH
Grain boundary migration is envisaged as a transfer of atoms across the boundary from one grain to another under the stimulus of thermal activation. The rate of grain boundary migration is dependent upon the driving force. Grain growth with time is frequently expressed by an equation of the form
Dg = Ktn 3.12 where Dg is the grain diameter (Dinitial `< Dg), t = time, n is the time exponent, usually less than the ideal value of 0.5, and K is the temper- ature dependent mobility term where
K = Ko e -Q/RT 3.13
Ko = mobility constant, Q = activation energy for grain growth, R is the ideal gas constant and T is the absolute temperature (Higgins 1974, Simpson et al. 1971, Honeycombe 1968). 52
Higgins (1974) points out that the driving force for grain boundary migration can be considerably reduced by, a) the interaction of the grain boundary with a free surface, b) the interaction with second phase particles in the material, c) orientation characteristics of the boundary.
As grain boundaries approach a free surface, they tend to straighten and reduce their curvature thus decreasing the driving force for grain boundary migration. Second phase particles will pin grain boundaries and retard grain growth so that eventually grain growth may cease altogether. This condition is reached when
R = 4r / 3f 3.14 where R = effective grain boundary curvature, r = particle radius and f = volume fraction of second phase particles (Higgins 1974). In addition to the drag exerted on grain boundary migration by second phase particles, grain boundary migration is inhibited by the intrinsic lattice drag, and by the drag exerted by solute atoms in 'the lattice(Higgins 1974). Grain boundaries with a high coincidence of lattice sites for grains either side of the boundary have a high mobility (Honeycombe 1968). Such high coincidence boundaries, however, have decreased grain boundary diffu- sivity (Higgins 1974) which would retard grain boundary migration. It is found that boundaries which are nearly coincident have the highest grain boundary mobility (Higgins 1974). Grain boundary mobility and hence grain growth will be inhibited in certain materials with a preferred orientation (texture) which gives rise to a large number of relatively immobile bound- aries (Higgins 1974).
ABNORMAL GRAIN GROWTH
Abnormal grain growth occurs where only a small fraction of grains grow in an otherwise stable grain structure. The resultant microstructure consists of large irregular grains in a uniform matrix. Abnormal grain growth may result from surface effects where differences in surface energy promote grain growth. Certain grain boundaries will be oriented for high mobility and may be able to coalesce and migrate irrespective of second phase particles which pin the other grain boundaries. 53
In a structure that has strong preferred orientation, normal grain growth is inhibited by the low energy of the boundaries and a low mobility. Consequently, any grains that are larger than the average size and have a different orientation from the primary preferred orientation may undergo preferential grain growth. The pronounced matrix texture means that any grain that starts initially with a boundary of particularly high mobility will maintain this position throughout its development (Higgins 1974). This can lead to enhanced textures such as the cube tex- ture of recrystallised copper. The heterogeneous nature of deformation and recrystallisation will give rise to pronounced differences in recrystallised grain size. The distribution of second phase particles will also affect abnormal grain growth and textures by pinning boundaries of specific orientations. Detailed examples of this latter process are given by Higgins (1974).
3.2 MICROSTRUCTURES
3.2 (i) INTRODUCTION
In this section the microstructures developed during plastic deform- ation are briefly described. Other reviews of microstructures are given by Spry (1969), Evans and Langdon (1976), Takeuchi and Argon (1976), Nicholas and Poirier (1976) and Knipe (1977). The microstructures which result from dislocation glide, dislocation creep, diffusional flow and recrystallisation processes are described. Close parallels are drawn between metallurgical results and geological studies (reviewed by Nicholas and Poirier 1976). In certain cases, microstructures may be used to identify deform- ation mechanisms. It is pertinent to note however, that the slow strain rates and long times typical of geological deformations (Price 1975) allow scope for the microstructures resulting from deformation, to be substanti- ally altered during uplift. In addition, several deformation processes may operate simultaneously although one may contribute most of the strain (see Atkinson 1977 for mechanism activity plots). These considerations must be borne in mind when interpreting the microstructures from geologi- cal materials. 54
3.2 (ii) DISLOCATION GLIDE
The dislocation glide regime is characterised by the following microstructural features: slip lines; deformation bands; kink bands; deformation twins; microfracturing (Stroh cracks); straight dislocation 10 -2 lines; dislocation tangles; high dislocation densities ( >10 cm ); crude cellular arrangements of dislocations in poorly defined walls and hedges; glide polygonisation. In general, extensive grain elong- ation is absent. During dislocation glide the dislocations tend to be straight and confined to their slip planes. With increasing strain the dislocation 10 2 density rises rapidly to 10 cm (see reviews by Friedel 1967, Nicholas and Poirier 1976). Etch pit techniques (Appendix A) can be used to observe slip lines, deformation bands, kink bands, deformation twins, the crude cellular arrangements of dislocations and glide polygonisation. For example, Livingston (1960, 1962) used etch pit techniques to study glide polygonisation and kink bands in copper. The crude cellular arrangements of dislocations during dislocation glide are generally smaller and less well defined than those produced by dislocation creep (Takeuchi and Argon 1976). When deformation is concentrated in narrow regions, deformation bands are formed. Deformation bands commonly have clusters of parallel slip lines which can be revealed by etching. These deformation bands and transition bands (Dillamore et al. 1972) may develop large misorientations with the host grains. Large misorientations are also found in kink bands particularly where glide polygonisation forms crude dislocation walls per- pendicular to the glide plane (see fig. 3.3). Dislocation glide may also lead to the development of crystallo- graphic preferred orientations (see chapter 5) and these can often be recognised during microstructural studies by characteristic etch reactions or similarity of extinction positions. The microstructural features of dislocation glide in geological materials can be modified by stress changes during uplift or by static recovery and recrystallisation. Stress changes may introduce new dis- locations and alter the arrangement of the existing dislocations. The microstructures characteristic of static recovery and recrystallisation are described in section 3.2 (v). 55
3.2 (iii) DISLOCATION CREEP
The microstructures formed during dislocation creep may be listed as; elongate grains; deformation bands; sub-grains - with elongate or equant shapes; dislocation walls - tilt walls and complex walls, dis- location loops and spirals and irregularly shaped dislocations within sub-grains; relatively low dislocation densities within sub-grains com- pared with dislocation densities found in dislocation glide; grain boundary migration; nucleation of new grains; dynamic recrystallisation forming new grains; preferred crystallographic orientations. The important microstructures developed during dislocation creep are, (a) sub-grains, (b) recrystallised grains, (c) preferred crystal- lographic orientations. Takeuchi and Argon (1976) outline four stages in the development of the sub-grain microstructure during the dislocation creep of metals and ceramics. These four stages are -
(1) The development of dislocation cells (fig. 3.7a) which are bounded by tangles of high dislocation density and containing a lower internal dislocation density;
(2) The development of elongate sub-grains with well defined boundaries, kink bands, deformation bands and tilt walls. The sub-grains are thought to have developed from (1) by recovery (fig. 3.7b);
(3) The development of an heterogeneous sub-grain structure of banded sub-grains (fig. 3.7c);
(4) The final stage is the development of an homogeneous sub-grain micro- structure with both equant and elongate grains (fig. 3.7d). This sub- structure is in dynamic equilibrium as the boundaries are moving during deformation but the sub-grain size is a constant over a time average.
The above features are best observed in the transmission electron microscope but may also be observed using etch pit studies, particularly in coarse grained materials. Although sub-grains are good indicators of recovery, it is not always easy to distinguish between statically and dynamically formed sub- grains in rocks (White 1976, 1977). Both climb and glide polygonisation can give rise to sub-grains. White (1977) suggested that these two pro- cesses can be distinguished as sub-grains due to climb have a more definite polygonal outline and have evenly spaced dislocations in their walls. 56 DISLOCATION SUB-STRUCTURE IN SINGLE CRYSTALS DURING CREEP
A is se Ase is11, 's < <
A A 'e ,C •Cs< A A ›• X):/,;( A< < XYA AY - • se ./ is•e' e .se is< < < • se ,,< NC sese se- < ),/N: \ ,„).> •ec.es( • x/. „
<,, ?A1' i'e ),A" Ps,c1. se'e •e XN AP YA ' , ` y t ,g`is>.s,is'' 4? ),),i, is ..,•< )s sese'c'e.e-e ).'' < A A,s A..,),) s sese < , st y ). is A)sw*- A Ne< - A se <- ›. se ,.),, 'e x's ,<.e-5 ,sc ..,), i- -e A 5,, X AAA yse < ,.( lsY )' e -e se A AYX'\ <`e X < se i',.. 2.)''' -<‘e .? , ), A •C /s• ,?• ), Ary ase i- A A "c'eNeA/ 'is 5( ).• •ie ),>)"e`x.., ie` A ,<,,,<"•e-ce se ke.e ..,/ .), > _, 'y < i- A 'e ), I...? )..),X e1 s ). 'f. si< Xt xIkeke 1.• ..." \ ...‘ ,>•.,...4, ).)%S.. ,,-)s .< „, is Y X I. •A 'C 'Cy >.).-Y1.Y"9,x x V..( '5: )s 'e se sAi>.i".'1• se -1"Y'A4e ,e A is)s A), se ).)—Y,V,,, .e,1( ,,se C Transient Creep D Steady State Creep
banded sub-grains homogeneous sub-structure
(AFTER TAKEUCHI & ARGON 1976)
FIGURE 3.7 Four stages in the microstructural development in single crystals during creep. 57
Glide polygonisation tends to produce short wall segments which have an uneven distribution of dislocations. This distinction, however, does not permit a separation of static or dynamically produced sub-grains as climb is operative in both processes. In general, within dynamically produced sub-grains there is a high dislocation density and dislocation features indicative of glide (section 3.2(ii) ) whereas these are gener- ally absent in sub-grains produced by static recovery. In geological materials, however, dislocation features within the sub-grains may be introduced during uplift (Nicholas and Poirier 1976). White (1977) has suggested that the sequential development of slip features, deformation bands and sub-grains with increasing strain indicates dynamic recovery processes. In addition, the inverse relationship between stress and sub- grain size (White 1977, Twiss 1977, Mercier et al. 1977) during dynamic recovery may enable differentiation between static and dynamic sub-grains. The stress dependence of dislocation density, sub-grain size and recrystal- lised grain size during dislocation creep is discussed in detail at the end of this section. Dynamically recrystallised grains generally have irregular, ser- rated or lobate boundaries which indicate grain boundary migration, and are commonly flattened perpendicular to the al , (or El) direction and elongate parallel to a3 (or £3). Dynamic recrystallisation may be distinguished from static recry- stallisation (section 3.2(v) ) by the presence of dislocations and sub- grains within the new grains (Jonas et al. 1969, Sellars 1978). These features, however, may be introduced during uplift (White 1977). White (1976), however, points out that the core-mantle microstructure (Gifkins 1967, 1976), in which a sharp contact exists between the new grains and the old deformed grains with progressive deformation into the old grains (fig. 3.8), is a good indication of dynamic recrystallisation. The new grains are equant and are formed by progressive sub-grain rotation (White 1976). As we shall see, this microstructure is found in some dyn- amically recrystallised ores (chapter 8). Crystallographic preferred orientations are commonly developed in materials undergoing dislocation creep. The mechanisms of preferred orien- tation development are discussed in detail in chapter 5. In this section, it suffices to say that it may be possible, using preferred orientation measurements, to determine the glide systems and the temperature during deformation, (Lister 1974, 1977, Lister et al. 1978). Climb, however,
58
RECRYSTALLISATION CORE — MANTLE MICROSTRUCTURE
Ilm'Ilikl 111a 1
••• OOOOO • • • • • • •••• •• •••• 04. • • • • • • • • • • III* • • • • • • • • • • • • • • • • • • • g • • • • •••• ••• •• • • • • e • • •••.•• •••• ••• .... • • • • • • • • • • • • • • • ••••••• • _ • • • • • • • • • •••••••.•••••...... • • • • • • • • • • • • • • •'• • • OOOO ••••••••••••• •••••••••••••.. • • • • • • • • • • • • • • • • • • • • a • • . • • • • • • • • • • • • • • • • • • • • • • • • • • •..- • • RECRYSTALLISED GRAINS •...... • • . • • T • ..• • :t . ..._•_•i •• •• •• • •• •• \ •,s ,t•. •• •• ,...\.: • • • • ..• • • • • 2. ,. • . . • .. , . . . . . • . • • • • •_•_ •. ,- • • • • • • • • • • • • • • • 1...• • . •. • ...• . • ..• •. • ..*::::: , (after White1976)
FIGURE 3.8 Dynamic recrystallization microstructures. 59 although allowing deformation to proceed by satisfying von Mises (1928) criterion, is not expected to contribute to preferred orientation develop- ment (Lister 1978 pers. com.) as Ball and White (1978) would suggest.
STRESS DEPENDENCE OF - DISLOCATION DENSITY, SUB-GRAIN SIZE, RECRYSTALLISED GRAIN SIZE
In recent years, there has been considerable emphasis on palaeo- stress determinations in rocks (Nicholas and Poirier 1976, Nicholas 1978, Twiss 1977, Mercier et al. 1977). This work has mainly applied to the flow of olivine in the crust and upper mantle but stress determinations are now being applied to quartz mylonites (White pers. com.). Stress estimates may be related to a) the dislocation curvature, b) the dislocation density, c) sub-grain size, d) recrystallised grain size. Nicholas (1978) and Nicholas and Poirier (1976) have discussed in detail stress determinations in rocks. Techniques a) and b) have been used successfully in experimental studies (Dunham et al. 1977) but their applicability to naturally deformed rocks has been questioned (Nicholas 1978, Nicholas and Poirier 1976) largely because of the ease with which the dislocation microstructure is altered and new dislocations are intro- duced during uplift. Stress estimates using these methods are generally high and reflect stresses operating during uplift (perhaps only causing 0.5-1.0% strain) rather than the stresses responsible for the plastic deformation of the material. Stress estimates using sub-grain size and recrystallised grain size have met with more success (Nicholas 1978). The relationship between the sub-grain size ( in pM), and the maximum stress (c in Kbars) is expected to have the following form
0. = K p b d-1 3.15 where K is a constant, p = shear modulus and b = burgers vector (Nicholas 1978, Nicholas and Poirier 1976). Reasonable agreement is achieved between experiment and theory has been achieved (Nicholas 1978 and reviewed by Nicholas and Poirier 1976) but difficulties are encountered as to what sub-grain size is to be measured. In practice, the optically visible sub-grains are measured and not the low angle tilt cells observed in electron microscopic studies (Nicholas and Poirier 1976). 60
Post deformation annealing may rearrange the sub-grain structure and hence in geological materials the stresses measured may not be those applied during deformation. The most successfully used palaeopiezometer is that of recrystal- lised grain size (Twiss 1977, Nicholas 1978). An equation similar to 3.10 has been used by Nicholas (1978) to measure stresses from mantle peridotites. For stresses below 2 Kbars, Nicholas (1978) argues that equant new grains would form by progressive sub-grain rotation (fig. 3.8). He uses these recrystallised grains to measure palaeostresses. The con- stants for equation 3.10 are either calculated from experimental studies (Nicholas 1978, Mercier et al. 1977) or assumed by analogy with metall- urgical results (Twiss 1977). Nicholas (1978) has argued that strains of the order of 1% at high stresses and low temperatures are not likely to alter the recrystal- lised grain size. Post tectonic annealing under low differential stresses may induce grain boundary migration and secondary grain growth but these microstructures can usually be distinguished from dynamic recrystallisation microstructures (Nicholas 1978). Twiss (1977) also suggests that dynami- cally recrystallised grain sizes may be preserved in two'phase systems where any potential grain growth is inhibited by second phase grains.
3.2 (iv) DIFFUSIONAL CREEP
The microstructures which may be developed during diffusional creep include; grain elongation by the removal of material from sites of com- pression and deposition at sites of tension (fig. 3.5); accumulation of initially dispersed inter or intra crystalline phases at grain boundaries or discontinuities; low dislocation density although initial dislocation structures may be preserved or annealed; no crystallographic preferred orientation unless initially present or the diffusion is extremely aniso- tropic - i.e. preferred growth directions. The recognition of diffusional creep microstructures in metals and ceramics is relatively easy when there are inert phases distributed throughout the grains. These become concentrated at compressed grain boundaries and are absent from the material added to tensile boundaries (see Plate 1D, McClay 1977c Appendix. B). In geological materials, the recognition of diffusional creep microstructures is often more difficult because there are often no con- veniently distributed second phase particles. As we shall see, second 61
phase particles do occur in some sulphide ores and can be used to identify diffusional creep. In addition, overgrowth structures related to possible diffusional creep, have been recognised in pyrite rich ores (chapter 7). The microstructures associated with pressure solution processes have been discussed by McClay (1977c). Knipe (1977) argues that mutual penetration of fossil fragments and development of pitted pebbles as being the only reliable evidence for pressure solution processes. Evidence of this nature is not often forthcoming in the study of sulphide ores. In prograde metamorphic rocks, particularly phyllosilicates, meta- morphic reactions and transformations may give rise to second phases and fluids. Knipe (1977) argues that some quartz pressure shadows may not be the result of pressure solution processes as is often assumed (Mitra 1976), but rather produced during metamorphic reactions. In the sulphides con- sidered in this thesis, galena, sphalerite, chalcopyrite, pyrrhotite and. pyrite, phase transformations only occur at very high temperatures and are considered to be above the range of low grade metamorphism (200 - 400°C). The lack of crystallographic preferred orientation in materials which have undergone diffusional creep may be an important factor in recog- nising this deformation mechanism. As discussed in section 3.1 (iii), diffusional creep is accompanied by grain boundary sliding. Diffusional creep and grain boundary sliding are favoured by small grain size and the initial grain shape (often equi- dimensional) can be maintained to large strains (i.e. superplasticity). Dislocations and voids may be found at grain junctions and triple points where they may be necessary to accommodate grain boundary sliding. Within the grains, the dislocation density is generally low. Grain boundary sliding may be indicated by square or rectangular grain shapes with large areas of grain boundaries parallel to the dominant shear direction (Nix 1972). Second phase particles or voids may stabilise these planar bound- aries (White 1977). In geological environments, the dislocations in materials which have undergone diffusional creep may be introduced or altered by processes operating during uplift.
3.2 (v) RECRYSTALLISATION
The microstructures formed during recrystallisation may be divided into two categories, those formed during static recovery and recrystalli- sation and those formed during dynamic recovery and recrystallisation. 62
Static recovery produces a polygonal sub-grain structure with a low dislocation density within the sub-grains. More commonly, however, the critical strain for recrystallisation is exceeded (Sellars 1978) and static recrystallisation occurs. Statically recrystallised grains are generally equant with straight grain boundaries and 120° triple junctions. Slip lines and deformation bands within the grains are generally absent. Tilt walls and sub-grain boundaries are not often found within the recrystallised grains. Inside the recrystallised grains the dislocation densities are generally low. Two types of static recrystallisation microstructure are shown in figure 3.9. In deformation bands, the boundaries attempt to relax towards equilibrium angles after annealing (Dillamore et al. 1972) (fig. 3.9). This produces elongate grains with straight grain boundary segments normal to the original deformation band. Where sub-grain coalescence occurs (fig3.9) an equant to square shaped microstructure with curved boundaries is formed. During static recrystallisation, secondary grain growth by grain boundary migration produces irregular grain shapes commonly with lobate and dentate grain boundaries. In addition the grain boundaries may be pinned by inclusions or may leave a trail of inclusions indicating the original grain size. The preferred orientations of annealed grains are related to those of the host grains and are generally weaker than.the host preferred orientations. New preferred orientations may be developed during second- ary and tertiary grain growth e.g. the cube texture (001) [100] in metals (Cahn 1970). Syntectonic - dynamic recovery and recrystallisation microstructures have been described in section 3.2 (iv) and are presented here in order to compare them with the static recrystallisation microstructures which are discussed above. As described in section 3.2 (iv), dynamic recovery produces both equant and elongate sub-grains with appreciable dislocation densities within the sub-grains. In general, dynamically recrystallised grains have irregular bound- aries and are flattened perpendicular to 01 and elongate parallel to 03. Mercier and Nicholas (1975) have shown that equant grains may be produced during natural syntectonic recrystallisation of olivine. Dynamically recrystallised grains contain sub-grains, deformation bands, kink bands and generally have a high dislocation density. Mercier et al. (1977) 63
RECRYSTALLISATION MICROSTRUCTURES
I ,.
*NM
,■■
DEFORMATION BAND AFTER ANNEAUNG — RELAXATION
(TRANSITION BAND) TOWARDS EQUILIBRIUM ANGLES
Grain Boundary
GRAIN BOUNDARY WITH POLYGONAL AFTER ANNEALING — SUB-GRAIN
SUB-GRAINS COALESENCE AND BOUNDARY
MIGRATION
FIGURE 3.9 Static recrystallisation microstructures in deformation- transition bands (Dillamore et al. 1972) and sub-grain coalescence (Doherty 1974). 64 and White (1976, 1977) have described similar recrystallisation micro- structures in olivine and quartz respectively. Finally, dynamically recrystallised grains are commonly related to the principal stress or strain axes and generally have strong crystal- lographic preferred orientations (Mercier et al. 1977 and see chapter 5).
3.3 GALENA (PbS)
Galena (PbS) is a IV - VI semiconductor with a rock salt (NaCl) structure - face centred cubic (fig. 3.10a). The lead and sulphur atoms occupy the positions of sodium and chlorine respectively with unit cell dimensions of a = 5 936 A° (Dalven 1969). The bonding in galena is part ionic and part covalent and according to Pauling 1970) has the following components - 51% (S+ ) (Pb ) , 42% (S°) Pb°), 7% (S) (Pio+ ) and 0.2% 2+ (S2 ) (Pb ). The partial ionic character of the bond balances the formal positive change on the sulphur (see Pauling 1970 for details). The stoichiometry of galena is markedly affected by the sulphur fugacity f , particularly at temperatures above 400°C. Non stoichiometry s2 in galena galena has (Pb : Sg 1) changes the conductivity. At high fs Pb vacancies resulting in conductivity by holes (p - type) whbreas at low f it hasS vacancies resulting in conductivity by electrons (n - type), s, (Stanlon 1953). This extrinsic behaviour (i.e. controlled by impurities and stoichiometric defects) in galena is found up to approximately 800°C. Above this temperature thermally induced defects (intrinsic) swamp the stoichiometry and impurity defects (Seltzer 1968, a,b). Atkinson (1976b) has demonstrated that stoichiometry deviations 18 (up to 10 atoms/cc) have marked effects on the diffusion controlled mech- anical properties of galena. In the light of hardness testing during experimental 'deformation and annealing of galena (Siemes 1970, Stanton and Willey 1970, 1971), it is significant to note that stoichiometric variations in galena may give rise to wide variations in hardness, reflectivity and conductivity. The possible point defects which can occur in galena are listed in Craig and Scott (1974). The sulphur ion vacancies are likely to give rise to vacancy collapse structures, stacking faults, and negative crystals which may be revealed by etch pitting (see Appendix A and chapter 8). In addition the point defects and interstitial impurity ions will have sig- nificant affects on dislocation mobility (Evans and Langdon 1976). Possible impurity ion interstitials and substitutions for Pb are Ag, Co, Sn, Bi, and 65
Sb (Craig and Scott 1974). The slip systems in galena were first investigated by Mugge (1898, 1914) and Buerger (1928) and they both observed slip on the. {001} <110> system (fig. 3.10a). In addition their interpretation of prismatic punching experiments on cleavage fragments of galena (producing the 'Tarrico' percussion figure) suggested {001} <010> prismatic slip. Sub- sequent studies by Davisson et al. (1955), Franklin and Wagner (1963), Mathews and Isebeck (1963), Urusovskaya et al. (1964), and Lyall and Paterson (1966) verified that {001} <110> is the primary glidesystem in lead sulphide. Davisson et al. (1955), and Franklin and Wagner (1963) found evidence for {001} <010> prismatic slip while Mathews and Isebeck (1963) found sessile {001} <010> dislocations. Lyall and Paterson (1966) found evidence for {110} <110> (fig. 3.10b) slip in suitably oriented single crystals. {110} <110> slip was found in the experiments of McClay and Atkinson (1977) which are reported in detail in Chapter 4. The primary glide system {001} <110> in galena contrasts with 0101 <1I-0 which is usually found in NaC1 - structure compounds such as NaC1 and Mg0 (Kelly and Groves 1970), Evans and Langdon (1976). Buerger (1928) and Gilman (1959) concluded that high ionic polari- sability (partly covalent bonds) favours {001} <110> slip. Mechanical twinning (principally in large grains > 2-3mm) in galena has been observed by Sadebeck (1874), Siefert (1928), Grigor'yev (1961), Lyall (1966) and Lyall and Paterson (1966). The most common twin composition plane is (441) with a (110) plane of shear and shear magni- tude of y = 0.354. Other mechanical twins have been found with (113), (332), (112) and (221) composition planes. Lyall (1966), however, relates (441) twinning to shock deformation and considers it to be an unlikely mechanism at the slow strain rates occurring in the earth's crust (Price 1975). Deformation kinking as described in section 3.1 of this chapter, has been observed in experimentally deformed single crystals of galena (Lyall and Paterson 1966 , McClay and Atkinson 1977, and Chapter 4, this thesis). Kinking has been found in naturally deformed coarse grained galena (Stanton 1972, and Chapter 8 this thesis). Polycrystalline aggregates have been experimentally deformed in compression at room temperatures and at relatively fast strain rates by Osborne and Adams (1931), Lyall and Paterson (1966), Siemes (1967, 1970). At room temperature, polycrystalline galena is relatively strong (flow stresses ti 2-3 Kbars Siemes 1970). Atkinson (1972, 1974, 1976a) 66
SLIP SYSTEMS IN GALENA PbS
(001) <110> SLIP
OS b= Burgers vector = 0.707a, in both cases
• Pb a.= unit cell dimension
(110) <110) SLIP
4 FIGURE 3.10 Slip systems in galena. 67 deformed polycrystalline (of 70 pm grain size) galena at temperatures up -4 -8 -1 to 400°C and at strain rates from 10 to 10 s while Salmon et al. (1974) deformed somewhat coarser grained polycrystalline galena (up to -5 -1 2mm grain size) at a constant strain rate (7.2 x 10 s ) at temper- atures up to 500°C. At low temperatures 20-200°C {001} <110> slip and kinking were found in coarse grained galena (Salmon et al. 1974) whereas Atkinson (1976a) found only rare kinks at temperatures up to 200°C in finer grained galena. At 300°C and above, polygonisation and recrystallisation were found in Atkinson's (1976a) and Salmon et al. (1974) experiments. Atkinson (1976a) established flow laws for polycrystalline galena and these are summarised in Table (3.1). Atkinson followed his initial work with the calculation of deformation mechanism maps for polycrystalline gal- 3 ena (Atkinson 1976b, 1977). Those for lacm and 10 pm grain size are reproduced in fig. 3.11, (after Atkinson 1977) and the deformation mechanism map for 10011m grain size has been calculated from the data of Atkinson (pers. com. 1978). Rudimentary high temperature tests on polycrystalline galena have been carried out by Davies (reported in Gill 1969). Seltzer (1966, 1967, 1968a,b ) has conducted a series of experiments on single crystals of galena in the temperature range 600 - 750°C. He found a stress depend- ence, e a a n where n varies between 4 and 7. Seltzer concluded that, in the extrinsic range, diffusion in galena was achieved by the movement of singly charged lead and sulphur vacancies. Above 800°C, the diffusion becomes intrinsic with thermally produced defects swamping environmentally produced defects. At low temperatures, however, at 300°C - 400°C and below, the composition of solid galena becomes virtually fixed and independent of the surrounding vapour pressure (Scanlon 1963). High temperature stress relaxation tests on synthetic polycrystalline galena (Atkinson 1978) strongly support Atkinson's (1976a, 1977) earlier contention that high temperature dislocation creep (stress exponent n = 5) gives way to diffusional creep (n = 1) on lowering the applied stress. Annealing and recrystallisation experiments on naturally deformed galena have been carried out by Siemes (1961, 1964), Stanton (1964), Stanton and Gorman (1968), Stanton and Willey (1970, 1971, 1972). Lyall and Paterson (1966) report the results of heating of previously deformed single crystals of galena; Siemes (1976, 1977) studied the annealing behaviour of deformed polycrystalline galena and Clark et al. (1977) also studied the annealing of experimentally deformed polycrystalline galena. 68 HOMOLOGOUS TEMPERATURE ( T I Tm ) 06 0 1.0 2 #.2 PbS 103 pm DISLOCATION GLIDE 7.01mo,aw. 10,, riiirivilimmimmtimummlimmimumumumma 3 HIGH-TEMPERATURE DISLOCATION 3 CREEP 2
2
4 FE
1 6 0 5 LOW-TEMPERATURE DISLOCATION CREEP 0 o b b 6 COBLE 0 rn CREEP 0 -1
7 NABARRO -HERRING CREEP -2
8 0 100 200 300 400 500 600 700 800 900 1000 1100 Tm TEMPERATURE °C
HOMOLOGOUS TEMPERATURE ( T/ ) 0- 0.4 0.6 0.8 1-0 DISLOCATION GLIDE PbS 100 pm
HIGH - TEMPERATURE DISLOCATION CREEP
3 4
a w w M W M M W
a ON ATI
C -0 O 10 0 b DISL 11 - COBLE CREEP 12 0 0 13 0 14 16 7-
NABARRO - HERRING CREEP --2
100 200 300 400 500 600 700 800 900 1000 1100 Tm
TEMPERATURE °C
FIGURE 3.11 Deformation mechanism maps for Galena (after
Atkinson 1976b). HOMOLOGOUS TEMPERATURE T / Tm 0.4 0.6 0.8 1.0 2 0.2 PbS 10pm DISLOCATION GLIDE
V2a
0 rn 0 cn 0
COBLE CREEP NABARRO - HERRING CREEP 2 13 -2
100 200 300 400 500 600 700 800 900 1000 1100 Tm TEMPERATURE °C FIGURE 3.11 Deformation mechanism map for galena calculated from the data of Atkinson(pers. corn. ). 70
McClay and Atkinson (1977) studied the annealing of single crystals of galena under confining pressure in sealed jackets of very small volume (in addition to the specimen). All but the experiments of McClay and Atkinson (1977) were carried out either in vacuo or were in some way vented continuously or periodically to the atmosphere. Under these con- ditions, particularly at temperatures above 400°C, significant sulphur loss is expected, which will, in turn, introduce vacancy defects and col- lapse structures. These introduced defects are expected to dramatically effect the recovery and recrystallisation of galena.- Annealing experi- ments conducted under confining pressure, in sealed, small volume jack- ets, (McClay and Atkinson, 1977) are expected to rapidly establish an equilibrium sulphur vapour partial pressure around the specimen and although the stoichiometry of the galena may change according to the annealing temperature, it is not likely to be subjected to the significant sulphur losses as experienced in the other annealing experiments cited above. Bearing in mind these limitations, the results of annealing experiments carried out on galena show that dynamic recrystallisation occurs in the laboratory at temperatures of 200°C at strain rates -7 -1 3 x 10 sec (McClay and Atkinson 1977) whereas static annealing shows incipient recrystallisation at 200°C (Clark et al. 1977) and marked recry- stallisation at 400°C and above (Clark et al. 1977, Siemes 1976, 1977, Stanton and Willey 1972). Stanton and Gorman (1968) and Stanton and Willey (1970, 1971) noted changes in dihedral angles between grains (even a low temperature -,,100°C) and also studied variations in indentation hardness with annealing but offer no textural - microstructural evidence for low temperature recrystallisation. Clark et al. (1977) show that dynamic recrystallisation microstructures in galena are extremely stable to annealing. They found that cold worked galena anneals with mosaics of new grains with a microstructure different to that found in dynamically recrystallised galena. The presence of a fluid phase in the grain boundaries of natural polycrystalline ores may enhance the grain boundary diffusivity and thus promote Coble creep - pressure solution deformation (section 3.1(iii)). In the light of widespread studies on sulphide solubilities and sulphide complexing (see Barnes 1967 for a review), it is appropriate to discuss the solubility of sulphides with reference to possible diffusive mass transfer processes. 71
The solubility of galena in chloride or bisulphide solutions has been extensively investigated (Czamanske 1959, Anderson 1962, Barnes 1967, Hemley et al. 1967). Below 200°C Pb C142 complexes are stable and the chloride content of the solution strongly affect the solubility (Helgeson 1964). The solubility is affected by the pH, the f02, the fS2, the activity of chloride, and bisulphide ions and temperature and pressure. For example, Hemley et al. (1967) found a solubility of approxi- mately 4.5 gms/litre of galena at 400°C in a solution of 2m KC1 buffered with K-feldspar-muscovite-quartz. Depending upon the composition of the intergranular fluids, galena may have a greater solubility than quartz under the same conditions and one may infer that, at low temperatures (200-300°C) pressure solution type processes may possibly operate in galena ores.
3..4 SPHALERITE (ZnS)
Sphalerite has a cubic close packed structure (f.c.c. diamond type) in which every other tetrahedral site is occupied by zinc. The remaining tetrahedral and octahedral sites are empty. Iron (also cadmium and mang- anese) enters the structure by replacing zinc and results in an increase in the cell volume as shown by Barton and Toulmin (19.66). Above 1-2% Fe the lattice becomes distorted. In the Fe-Zn-S system the iron content in sphalerite coexisting with pyrite and pyrrhotite is used as a geobarometer (see summary by Craig and Scott 1974). ZnS is a semiconductor displaying both p- and n-type conductivity. For this reason the structures and defects of ZnS polymorphs have been investigated in detail. Scott and Barnes (1972) have examined the sphal- erite (cubic) - wurtzite (hexagonal) inversion as a function of sulphur fugacity fs and temperature. Sphalerite and wurtzite can coexist at temperature below 400°C but only at very low sulphur fugacities - below the main line ore forming environment of Barton (1970). The most important defects in sphalerite are stacking faults in the layering of the sphalerite lattic (fig. 3.12a). These can occur along the {111} planes but a special kind of stacking fault forms {ill} <112> type glide or annealing twins (fig. 3.12b). The normal stacking sequence (fig. 3.12a) is disrupted by a stacking-fault along the twin plane
72 STACKING SEQUENCE OF (111) PLANES IN SPHALERITE
[1io]
• • • • • — B a
— A Y
C
—B a
—A Y
A projected onto the (112) plane •Zn OS
[111 <112> GLIDE TWINNING IN SPHALERITE
K2(111) ---l• shear direction 012) \ magnitude y = 0.707 - -2 1 twinned ■• .• 2 A • , g, ID 3 , ,,,..• a...
twin plane— stacking fault V 3
untwinned 2 1 0 (001) 0 Zn o S • Zn in twin • 5 in twin B
ANNEALING TWINS IN SPHALERITE (after Richards 1966)
at triple junctions cp feather twins with chalcopyrite precipitates
propagation across grain at gain boundaries boundaries propagation across a twin
C displacement of twin boundaries
FIGURE 3.12 73
(fig. 3.12b). The sense of shear is indicated in fig. 3.12b with a shear magnitude of y = 0.707. These twins can either be deformation or anneal- ing-growth twins (fig. 3.12c). Veit (1922) and Buerger (1928) deformed sphalerite at room temperature and under confining pressure. Veit (1922) attributed deform- ation to glide on 0111 planes in the<112> directions whereas Buerger found Mil <112> twinning. Saynisch (1970) deformed polycrystalline sphalerite at room temperature up to confining pressures of 5 Kbars and Siemes et al. (1973) deformed single crystals of sphalerite under the same conditions. Recently, experiments on single crystals of sphalerite (Siemes and Borges 1978) and on polycrystals (Clark and Kelly 1973) have been carried out at elevated temperatures (up to 500°C). Twin gliding on {111} <112> was reported by Saynisch (1970) and Clark and Kelly (1973). Saynisch (1970) also attributed the development of pre- ferred orientations in polycrystalline sphalerite to the operation of 0111 <112> dislocation glide. Ramdohr (1969) points out the difficulty in dis- tinguishing between 0111 <112> twinning and 0111 <112> glide as the same plane is used in both mechanisms. Siemes et al. (1973) and Siemes and Borges (1978) report translation gliding on {111} <110> planes in single crystals of sphalerite deformed at temperatures up to 450°C. In addition, Siemes and Borges (1978) found a strong dependence of twinning upon the iron content of sphalerite. Single crystals of sphalerite with a high iron content (12.9%) only twinned at high strains. They concluded that for sphalerites with a low iron content (<4%) the critical resolved shear stress for twin gliding was smaller than that for translation gliding whereas for iron contents above 4% the reverse occurred. Dislocations in sphalerite can easily split into partial dislocations and several types of stacking faults can be formed (Kelly and Groves 1970 p.255). Dislocation and twin interactions are therefore complex in sphalerite, a fact which makes interpretation of slip features very difficult. Clark and Kelly (1973) and Ramdohr (1969) have described microfractures in sphalerite principally associated with twin intersections. These zones of intense deformation at twin intersections may be sites for recrystallisation nucleii or sites for precipitation of a second phase - e.g. chalocopyrite. Both deformation twins and annealing or growth twins have been reported from studies of naturally deformed sphalerite (Stanton 1972, Ramdohr 1969, Richards 1966). Annealing twins are usually broad and coherent extending across the whole of the grain whereas deformation twins are usually 74
(1) thin, (2) lenticulate at the ends, (3) end within the grain, and (4) often show the development of more than one set of twin lamellae resulting in bent and kinked twins (fig. 3.12c). Sphalerite does not appear to recrystallise easily under laboratory conditions, although Stanton and Gorman (1968) report grain boundary mig- ration at 400° and nucleation of new grains at 600°C (Stanton and Willey 1971). These experiments, however, were not carried out in controlled sulphur fugacities and the expected sulphur losses would seriously affect recrystallisation and grain boundary migration, nucleation sites and hard- ness values obtained. The solubility of sphalerite, like that of galena (Section 3.3) is strongly dependent upon the nature of the solution. According to Hemley et al. (1967) solubilities up to 3 gms/litre at 300°C and 9 gms/litre at 400°C can be achieved.
3.5 CHALCOPYRITE (CuFeS2)
Chalcopyrite, CuFeS2, is the most common of the ternary copper iron sulphides and has an ordered tetragonal structure which is stable up to 557°C. Above 557°C chalcopyrite decomposes to pyrite + an intermediate solid solution (Craig and Scott 1974). Although tetragonal in symmetry and structure it is very similar to sphalerite (ZnS) and is considered to have a derived sphalerite structure (Stanton 1972) with a = 5.28A° and c = 10.40 (c= 2a of the sphalerite cell). Other minerals of similar composition to chalcopyrite and which are often found in intimate associ- ation with it are Talnakhite (Cu and its high temperature poly- 9Fe8S16) morphs I and II, Mooihoekite (Cu9Fe9S16 ) and intermediate phase A, and Haycockite (Cu4Fe5S8). Talnakhite has a cubic structure, Mooihoekite is tetragonal and Haycockite is orthorhombic but all these structures are derivatives of the sphalerite structure. Putnis and McConnell (1976), using electron microscopic techniques, have demonstrated the phase trans- formations between these structures. It is likely, therefore, that in natural chalcopyrite, sulphur deficient phases such as those described above, will be found and these may play an important role in the behaviour of chalcopyrite during deformation. Early experiments on chalcopyrite single crystals by Mugge (1898) and Buerger (1928) at room temperature and at high (but unknown) confining pressures showed that chalcopyrite deformed by slip on {112} planes in the <110> direction (Buerger 1928) but much of the deformation was accomplished by cataclasis. Later studies by Newhouse and Flaherty (1930), Shadlun (1953), Krishnamurthy (1967) and Gill (1969) showed that ductile 75
behaviour in chalcopyrite was achieved by twinning. The most detailed studies of chalcopyrite have been carried out by Lang (1968), Atkinson (1972, 1974), Kelly and Clark (1975) and Roscoe (1975). Kelly and Clark (1975) report that deformation twinning occurred above 100°C at confining pressure whereas Atkinson (1972) reports deformation twinning in room temperature experiments at confining pressures of 3 Kbars. Kelly and Clark (1975) describe lenslike {110} and {102} twins which are either growth twins or more likely tetragonal - cubic inversion phases. Thin parallel sided polysynthetic deformation twins (similar to those in sphalerite) on {112} planes have been described from the experiments of Kelly and Clark (1975), Atkinson (1972, 1974), and Roscoe (1975). Translation gliding on {112} <110> and {112} <021> (the closest packed directions in the lat- tice) was found in the experiments of Atkinson (1972), Roscoe (1975) and Clark and Kelly (1975). Gill (1969) reports recrystallisation in chalcopyrite above 565°C and Roscoe (1975) found subgrain development above 200° - 300°C. Stanton and Gorman (1968) and Roberts (1965) describe static annealing experi- ments on chalcopyrite. Chalcopyrite recrystallises fairly rapidly into polygonal mosaics but this behaviour is most likely enhanced by the phase transformations found in the Cu-Fe-S system (Putnis and McConnell 1976). Non stoichiometry in natural chalcopyrite (Pridmore and Shuey 1976) is related to metal excess and is probably related to the phase transformations discussed above. Atkinson (1972) has shown that the experimental observations on the stress/strain rate behaviour of dry polycrystalline chalcopyrite (at 300 - 400°C) can be equally well explained by an exponential flow law of the form of equation 3.1 or by a power law relationship of the form of equation 3.3 Similar results were obtained by Roscoe (1975) and the rheological constants are summarised in Table 3.1. The solubility of chalcopyrite in water has not been extensively studied. Vukotic (1961) reports solubilities up to 1.9 mg/litre at 50°C in H2S.saturated water. The solubility is lower than that for galena, sphalerite and pyrite.
3.6 PYRRHOTITE (Fei_ x S - Fe7S8)
Pyrrhotite occurs principally as two phases - hexagonal pyrrhotite (Fel_xS) and monoclinic pyrrhotite (Te7S81 ). Between its maximum melt- ing temperature of 1190°C and 308°C, the full width of the pyrrhotite 76 phase field is occupied by a single solid solution Fel_xS in which iron and vacancies are randomly distributed in the cation sites of the NiAs (1 C) structure (Craig and Scott 1974). Below 308°C the phase relationships are very complex (Craig and Scott 1974, fig. CS 3) where slow reaction kinketics obscure the phase relationships. At temperatures below 308°C superlattice structures of MC, NA, NC and 5C hexagonal pyr- rhotites, 11C and 6C orthorhombic pyrrhotites and the 4C monoclinic pyr- rhotite are found (Craig and Scott 1974). As the temperature decreases there is an increasing ordering of vacancies across the pyrrhotite phase field and this gives rise to various superstructures (Craig and Scott 1974). The upper stability limit for monoclinic pyrrhotite is now con- sidered to be 254°C (Craig and Scott 1974). Early experimental studies on the deformation of pyrrhotite by Buerger (1928), Osborne and Adams (1931), Bridgman (1937) all found that at 20°C pyrrhotite deforms mainly by cataclasis. Experiments performed under confining pressure and at elevated temperatures by Gill (1969), Graf and Skinner (1970), Atkinson (1972, 1975b) and Clark and Kelly (1973) demonstrated that above 100°C pyrrhotite will deform by slip, twin- ning and kinking. The most detailed experiments were conducted by Atkinson (1972,1975b) and Clark and Kelly (1973) at temperatures up to 400°C and 500°C respectively. They found that at low confining pressures and at temperatures below 200 - 100°C cataclasis was the main deformation mechanism. At higher temperatures 100 - 200°C to 250°C moderate kinking was found while at 250 - 300°C twinning began to occur and intense kink- ing twinning is found above 300°C (Clark and Kelly 1973, Atkinson 1975b). Dislocation glide on the (0001) plane in the <1120> direction is the pre- dicted slip mechanism for pyrrhotite (Buerger 1928, Graf and Skinner 1970, Clark and Kelly 1973, Atkinson 1975b). Slip lines on the (0001) plane were observed in the experiments of Clark and Kelly (1973) and Atkinson (1975b). Twinning in pyrrhotite occurs on the (1012) plane in the <7011> direction (Clark and Kelly 1973). Experimentally deformed pyrrhotites contain abundant fractures (Atkinson 1975b Clark and Kelly 1973). Some of these are undoubtedly produced by stress relief on unloading the speci- mens but others are Stroh cracks (Atkinson 1975b) due to pile up of dis- locations. Further cracks are produced by inhomogeneous deformation because the {0001} <1120> slip planes cannot provide five independent slip planes necessary for macroscopic ductility (von Mises 1928, Taylor 1938). At low temperatures cataclasis probably plays an important role in achieving macroscopic ductility in pyrrhotite (Atkinson 1975b) whereas at higher 77 temperatures (250° - 300°C) kinking, twinning, and dislocation climb con- tribute to macroscopic flow. Atkinson (1975b) has shown that the differential stress supported by polycrystalline pyrrhotite varies from 5-6 Kbars at 20°C to 0.4 - 1 Kbar at 400°C (depending upon the confining pressure). Gill (1969) reports that recrystallisation in pyrrhotite would begin at 525°C - 666°C. It is probable, however, that syntectonic recry- stallisation would occur at much lower temperatures during natural deform- ation at slow strain rates ( 10-12 -10-14 s-1 ). The complicated phase relationships in the pyrrhotite phase field result in many lamellar intergrowths (Putnis 1975) in naturally occurring pyrrhotites (see Appendix A). In many cases these are confused with twins and kinks. Sub-grain structures are not often observed in experi- mentally or naturally deformed pyrrhotite (Atkinson 1975b). This is most likely a result of the fact that in hexagonal materials dislocations in the basal plane usually pile up only at the grain boundaries or in kink bands (see Atkinson 1975b). Atkinson (1972,1975b) was able to describe his results on the deformation of polycrystalline pyrrhotite by a modified version of the flow law for dislocation glide (equation 3.1) i.e. an exponential stress/ strain rate relationship. The best approximations to the exponential flow law were achieved above 200°C and variations of the constants between 200°C and 300°C are accounted for by the onset of deformation twinning (Table 3.1). The rheological constants are listed in Table 3.1. As far as the author is aware, there are no reliable data on the solubility of pyrrhotite. The solubility is likely to be complex parti- cularly because of the stoichiometry of the Fei_xS system.
3.7 PYRITE (FeS2)
The crystal structure of pyrite is based on a face-centred cubic 2+ array of ions with the NaC1 type structure. The cations (Fe ) are located + in Na positions. The anions are present as S - S units with the centre of each S - S band located on a Cl position. The long axes of the S - S units are oriented parallel to non-intersecting three fold axes. Each sulphur is bonded to three metal ions and one sulphur (in the S - S unit). In pyrite the bonding is dominantly covalent (Pauling 1970) in contrast to the largely ionic bonding of sodium chloride. The S - S bond is parti- cularly strong and strongly covalent whereas the Fe - S bond is less strong but still exhibits considerable covalency (Mariano and Beger 1971). 78
Pyrite is stable to 743°C where it undergoes a peritectic break- down to hexagonal IC pyrrhotite and sulphur (Kullerud and Yoder 1959). The stability of pyrite with variations in the sulphur fugacity (fs ) and confining pressure (Pc) is discussed in detail by Craig and Scott (1974). For the pyrite-pyrrhotite solvus Craig and Scott (1974, fig. CS 10) show that the transformation temperature of the reaction pyrite to pyrrhotite is lowered by lowering fs whereas an increase in Pc also lowers the transformation temperaturg. The use of this pyrite-pyrrhotite geothermometer is generally unsatisfactory because of the variation introduced by f and Pc and o '2 because the phase relationships below 300 C are not well known. In the range of sulphur fugacities likely to be encountered in the ore-forming environment (Barton 1970 fig. 11) the pyrite-pyrrhotite inversion would occur in the range 270° - 450° (see Chapter 2). Non stoichiometry in pyrite has been detected (Craig and Scott 1974) and the experiments of Kullerud (1967) on pyrite-marcasite trans- formations suggest that an excess of sulphur is found in pyrite relative to marcasite. The relationship between pyrite and its polymorph marcasite remains unclear despite intensive investigation. Rising (1973, as quoted by Craig and Scott (1974)) has demonstrated that the inversion of marcasite to pyrite above 157°C is directly proportional to temperature and inversely proportional to grain size. Craig and Scott (1974) also suggest that marcasite is stable only below 115°C and metastable in the range 115°C- 157°C. The preservation of marcasite in metamorphosed ores possibly indi- cates metamorphic temperatures below 157°C but care must be taken as Kullerud (1967) demonstrated coexisting marcasite and pyrite up to 423°C at 2kb in the presence of water but not in its absence. The solubility of pyrite has been investigated by Masalovich (1977) and other studies are reported by Barton et al. (1963). Ramdohr (1969) reports that Becke investigated the dependence of solubility upon crystal- lographic orientation and showed that the (100) and (210) faces are resistant to attack by acids and that the (111) faces are resistant to attack by alkaline solutions. Barton et al. (1963) show that equilibrium between pyrite and solution is rapidly reached even at low temperatures (98°C) compared to quartz (Barton et al. 1963 fig. 2). Masalovich (1977) found solubilities of pyrite in weak HC1 solutions in the range of 3-15 gms/Kgm o over temperatures 192 - 458°C. He also found a sharp increase in pyrite solubility in the range 300°- 350°C. Thus pyrite is expected to be 79 readily soluble under geological conditions and as Atkinson (1975a) sug- gests, fluid assisted diffusion - 'pressure solution' may be an important deformation mechanism in naturally deformed pyrite ores. Experimental deformation of single crystals of pyrite has been investigated by Robertson (1955) and Graf and Skinner (1970). Polycryst- alline deformation studies have been carried out by Adams (1910), Newhouse and Flaherty (1930), Bridgman (1937), Lang (1968) and Atkinson (1975a) Robertson (1955) and Lang (1968) showed that room temperature ultimate strength increases rapidly with increase in confining pressure. Graf and Skinner (1970) and Atkinson (1972, 1975a) investigated the influence of variations in temperature and strain rate on the behaViour of pyrite. These experiments were carried out on dry pyrite which was found to be brittle and to deform cataclastically in all of the studies cited above. Atkinson (1975a) describes the production of comminution zones of fine grained pyrite fragments which allow cataclastic flow to occur in dry polycrystalline pyrite at temperatures from 20° - 400°C, confining • -4 -7 -1 pressures 1 bar - 3 kbars and at strain rates from 10 - 10 s Dry polycrystalline pyrite is expected to support differential stresses in the range of 4 - 7 kbars during tectonic deformation (depending upon the confining pressure) at temperatures up to 400°C (Atkinson 1975a). The strength and resistance of pyrite to plastic deformation by intracrystalline processes is perhaps essentially due to the influence of the highly covalent bonding on dislocation motion. Mookherjee (1971) reports curved or bent pyrite crystals and cites these as evidence of plastic deformation. It is possible that these features, however, may result from distorted growth (Font-Altaba 1963) during deformation. Lawrence (1972) and Read (1968) describe high temperature (600° - 850°C) annealing recrystallisation in pyrite which produced foam textured pyrite grains with well developed triple junctions.
3.8 CONCLUSIONS
Deformation mechanisms and the microstructures produced have been reviewed in detail. The similarities between sulphide minerals and cera- mics such as MgO and NaC1, permit detailed comparisons to be made between the deformation behaviour of these materials. As has been emphasised throughout this chapter, care must be taken in drawing analogies between metallurgical deformations and geologi- cal deformations where the post tectonic history may substantially alter 80 the microstructures observed at the present time. The physical properties of the sulphides galena, sphalerite, chalcopyrite, pyrrhotite and pyrite have been reviewed in detail. A syn- opsis of the properties pertinent to mechanical deformation is presented in Table 3.1. SULPHIDE GALENA PbS SPHALCRITE ZnS CHALCOPYRITE Cufe5 PYRRHOTITE Fe S PYRITE feS 2 1-x 2
STRUCTURE Face centred cubic Cubic - Diamond structure Tetragonal Hexagonal 5 above 254°c { Men Fe1-x - centred cubic Monoclinic below 254°c Fe758
SLIP SYSTEMS 11001 (110) 11111 (112) 1121 ell 1000i (11B) not observed (and Critical Resolved 50 bars at 20°c 11111 (011)420 bare at 20°8 112 021 brittle failure cnly Shear Stress where 11101 (1T0) 210 bars at 450 c (Kelly and Clark 1975) (Clark and Kelly 1973) determined) 500 bars at 20°c (Lyall and (Stomas and Borges 1978) Paterson 1966)
MECHANICAL TWINNING 14411 (110)" (most common 11111 (113)345 bars at 20°8 11121 direction 110521 (T011) interpenetrant (CR55 where measured) 250-320 bare 135 bare at 450 c unknown growth twins only (Lyall and Paterson 1966) (Siemer and Borges 1978) (Clark and Kelly 1973)
'KINKING 11101 (1T0) not observed not observed abundant kinking end not observed common phase transformations
FLOW LAW DISLOCATION GLIDE i m K exp( 4/RT)exp(Bs) 0 = 23.9 Kcal/mole not determined 0 = 32 Kcal/mole Roscoe 0 am 61.4 300 - 400°c not observed (eqn 3.1) B . 12.7 Kbar-1 B . 1.5 Kbar-1 (1975) B ., 16.4 at 10% strain (Atkinson 1976e) (Atkinson 1972) 0 = 50.5 Kcal/mole Atkinson B = 6.0 Kbar-4 (1972) B . 11.5 - 25 (Atkinson 1975b)
FLOW LAW DISLOCATION CREEP i = A(Dpb/KT) WO Q . 22.5 Kcal/mole up to not determined 0 = 30 Kcal/mole Roscoe 0 . 55.3 Kcal/sole not observed (eqn 3.3) n . 7.5 400°c n . 8.6 (1975) n . 14.2 at 10% strain 0 . 76-247 Kcal/mo e 500- Q . 55.5 Kcal/mole Atkinson 300 - 400°c n . 4.4-5.4 800°c n . 11.9 (1972) (Atkinson 1972) (Atkinson 1976a, 1978)
FLOW LAW DIFFUSIONAL CREEP isEs° (n . 1) 0 = 60-117 Kjoules/mole not determined not determined not determined not observed (egne 3.7 A 3.8) n . 1.8-0.7 600° - 800°c (Atkinson 1978)
STATIC RECRYSTALLISATION 300°c 500°c 500°c not determined 600 - 850°c TEMPERATURE (Slemen 1976, 1977) (Clark and Kelly 1973) (Gill 1969) (Reed 1968)
OYNAMIC RECRYSTALLISATION 200°c not observed 200 - 300°c sub-grains only not determined not observed TEMPERATURE (McClay and Atkinson 1977) (Roscoe 1975)
SOLUBILITY up to 4.5 gms/litre up to 3 gms/litre 300°c 1.9 mg/litre at 50°c not determined, but possible 3 - 15 gma/Kgm and comments on the (Hemley at al. 1967) up to 9 gms/litre 400°c (Uukotic 1961) fluid assisted phase T 192°c - 458°c possibilities of pressure (Homley at al. 1967) transformations probable pressure solution solution Pressure solution possible Pressure solution unlikely Pressure solution
TABLE 3.1 SYNOPSIS OF THE PHYSICAL PROPERTIES OF THE SULPHIDES co (Data from the references in the text) 82
CHAPTER 4 DEFORMATION OF SINGLE CRYSTALS OF GALENA
4.1 INTRODUCTION
Previous research on the experimental deformation of galena has either been on polycrystalline samples which have been deformed at temperatures from 20°C to 450°C (Siemes 1970, Atkinson 1972, 1976a, Salmon et al. 1974) or on single crystals deformed at room temperature (Lyall and Paterson 1966). There is, however, a complete lack of information on the stress-strain characteristics of single crystals with variations in temperature and strain rate. Similarly, there are no data on the effect of temperature and strain rate on the critical resolved shear stress (CRSS) for both the {100}<110 and {110}<110>slip systems. In addition, there is a marked absence of detailed microstructural studies of experimentally deformed single crystals and also, to some extent, for polycrystalline galena. In particular, little is known about the formation and preservation of kink bands in galena. This chapter presents the results of a series of deformation and annealing experiments which were designed to investigate the following -
a) the stress-strain behaviour of single crystal galena with variations in temperature and strain rate;
b) the variations in the critical resolved shear stress (CRSS) for both {100}<110> and {110}<110> slip with temperature and strain rate;
c) the microstructures developed in single crystals of galena at different temperatures and strain rates;
d) dynamic recovery and dynamic recrystallisation in single crystal galena;
e) static recrystallisation in cold worked single'crystal galena.
These experiments were developed in parallel with the studies of naturally deformed coarse-grained galena which are reported in Chapter 8. It will be demonstrated in Chapter 8 that naturally deformed coarse-grained galena exhibits microstructures similar to those found in deformed metal crystals (Section 3.2) and, as such, are interpreted to indicate deformation 83
by dislocation glide and dislocation creep processes. These results support the predictions, based on deformation mechanism maps (Atkinson 1976b, 1977), that coarse-grained galena ( >0.5 cm) is expected to deform by dislocation glide and dislocation creep in low grade metamorphic environments. The primary aim of these experiments was therefore to investigate the formation of dislocation glide and dislocation creep microstructures (slip, kinking, twinning, polygonisation and dynamic recyrstallisation - section 3.2) in single crystals of galena and to study their stability during annealing. Dislocation glide and dislocation creep are likely to produce crystal- lographic preferred orientations whereas deformation by grain boundary sliding and diffusion processes is, if anything, likely to destroy preferred orient- ations. In order to interpret the crystallographic preferred orientations found in this study of naturally deformed galena, as will be reported in Chapters 6, 7 and 8, it is necessary to have a knowledge of the variation of the CRSS for each slip system with temperature and strain rate. A secondary aim of these experiments was, therefore, to investigate the stress-strain characteristics and CRSS variations of single crystals of galena with temperature and strain rate. The results of previous experimental studies on the deformation of single crystals of galena may be summarised as follows. The experiments were either restricted to room temperature (Lyall 1966, Lyall and Paterson 1966), or were concerned with establishing activation enthalpies for high temperature flow (Seltzer 1967, 1968). Lyall and Paterson observed both {100}<110> and {110}<110> slip and kink bands developed in only one particular orientation. For {100}<110> slip, the CRSS was found to be approximately 50 bars at room temperature whereas the CRSS for{110}<110> slip was estimated to be greater than 500 bars (Lyall and Paterson 1966). Detailed micro- structural studies were not reported by Lyall and Paterson (1966). Salmon et al. (1974) deformed coarse-grained (average grain diameter 4.6mm) poly- crystalline galena and report slip and kinking microstructures at temper- atures of 2000c and below. They cite evidence for dynamic recrystallisation at deformation temperatures above 3000c. Annealing studies of naturally deformed galena have been made by Stanton (1970), and Stanton and Willey (1970, 1971, 1972). They conclude (Stanton and Willey 1972) that the schistose textures in sheared galena ores are the result of static recrystallisation and grain growth. Siemes (1964, 1976, 1977) has studied annealing in experimentally deformed polycrystalline galena. He found new grains forming at 3000c. In a contemporary study of 84
annealing and dynamic recrystallisation in coarse-grained galena, Clark et al. (1977) have been able to distinguish between dynamic and static recry- stallisation microstructures. In this chapter it will be demonstrated that dynamic recrystallisation, commonly after kink bands, occurs at very low temperatures (200°c) at labor- atory strain rates. These are lower temperatures than have been previously reported. Furthermore the formation of kink bands in galena has been investigated in detail and the microstructures described in this chapter are used in Chapter 8 to demonstrate that naturally deformed coarse-grained galena (sheared galena) exhibits dislocation glide and dislocation creep microstructures. It will be demonstrated that, in these experiments, the formation of recyrstallisation microstructures similar to those found in sheared galena (Chapter 8) permits one to infer that many naturally deformed galena ores exhibit dynamic recrystallisation features rather than static recrystallisation as that proposed by Stanton and Willey (1972). Five series of experiments are reported. The first series of experi- ments was carried out with B.K. Atkinson and the results have been published (McClay and Atkinson 1977, Appendix B). Further series of constant strain rate tests were carried out by the author (Series 2-4). For completeness, a series of stress relaxation experiments (Series 5) which were conducted with B.K. Atkinson are also briefly reported.
4.2 EXPERIMENTAL PROCEDURE
Five series of deformation experiments on single crystals of galena were carried out and the experimental conditions are summarised in Table 4.1.
STARTING MATERIAL
Oven dry cores taken from large, natural single crystals of galena were used in all the experiments. The starting material for Series 1 experi- ments were from Broken Hill, N.S.W., Australia. Starting material for Series 2 and 4 were obtained from the Tri State Lead-Zinc district U.S.A., while the starting crystals in Series 3 and 5 came from the Illinois Lead- Zinc district, U.S.A. Different starting materials were used because only a few complete cores could be drilled from each single crystal. The author is indebted to the following for providing single crystal material suitable for this study: C. Blain, Mining Geology Museum, Imperial College; G. Lister, Leiden University; B.K. Atkinson, Imperial College; P. Gerdeman, St. Joe Minerals - Corporation Missouri; and D.C. Brockie and TABLE 4.1 SUMMARY OF EXPERIMENTAL CONDITIONS
COMPRESSION AXIS DEFORMATION TEMPERATURE ' ANNEALING TEMPERATURE STRAIN RATE RANGE CONFINING PRESSURE ORIENTATION RANGE RANGE
o -5 -1 -7 -1 1 10 to <001> 200c - 400°c 2000c - 3000c 3x10 s - 3x10 s 1.5 Kbar along (110) plane
-5 -1 -7 -1 2 parallel to <001> 200c - 400°c 3x10 s - 3x10 s 1.5 Kbar
-5 -1 3 parallel to <111> 200c - 4000c 3x10 s 1.5 Kbar
-5 -1 4 parallel to <001> 200c 1000c - 4000c 3x10 s 1.5 Kbar
-5 -1 5 parallel to <001> 20°c - 3000c - 3x10 s 1.5 Kbar RELAXATION TESTS 86
M.S. Burdette of Eagle Picher Minerals Division, Illinois. The starting material for Series 1 experiments came from three large single crystals from the New Broken Hill Consolidated Mine, Broken Hill, N.S.W., Australia. For Series 2, 4 and 5 expeMments the specimens were prepared from two large single crystals from the In State Lead-Zinc dis- trict, Oklahoma, U.S.A. For Series 3 experiments specimens were taken from a large single crystal from Galena, Illinois, U.S.A. At least two polished sections were cut from each single crystal. Although they were from different localities, all the single crystals showed essentially similar microstructural features with only occasional large, low angle growth subgrains. Small inclusions (up to 50pm, but usually 5-15pm) of the tetrahedrite - tennantite group sulphosalts were found in most crystals. In the Broken Hill material, inclusions of chalcopyrite and sphalerite were occasionally found. The Tri State crystals (Series 2, 4 and 5) contain only a few inclusions of tetrahedrite - tennantite, and in this respect the crystals from Galena, Illinois are similar. It was not possible to distinguish. between different sulphosalts in polished sections although many of the inclusions had the appearance of tetrahedrite-tennantite. Other sulphosalts of Lead,e.g. Ni-Pb-S series, Pb-Sn-S series, Pb-Zn-S series, Pb-Sb-S series (including boulangerite), Ag-Bi-Pb-S series,Ag-Pb-Sb-S series and As-Bi-Pb-S (Craig and Scott 1974) series commonly occur in galena but could not be positively identified in the crystals used in this study. Direct reading specotrphotometer analyses were conducted on all crystals used as starting material. The results are summarised in Table 4.2. Trace element contents are generally low appart from copper, tin, arsenic and antimony. These elements are most likely associated with the sulphosalts of the tennantite tetrahedrite and polybasite groups (details of these sulphosalts are given in Craig and Scott, 1974). Silver contents are low (several ppm only). It was not possible to analyse for Bismuth. In general, crystals from the same locality exhibited similar trace element contents. Etched cleavage fragments of all starting crystals were examined in the scanning electron microscope. Slip lines were not observed. The dis- location density was low (though variable) and only a few low angle tilt boundaries were found. TABLE 4.2 CHEMICAL ANALYSES OF SINGLE CRYSTALS OF GALENA (using Direct Reading Spectrophotometer)
SAMPLE NO. Mo Mn Fe Co Ni Cu Ag Zn Ga Ge Sn As Sb COMMENTS ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm
Cl, G2 1.9 6.11 37.8 2.25 4.03 8.6 0.96 847.5 4.47 10.35 134.8 377.9 156.36 Source NHBC Mine, Broken Hill NSW Starting Material Expta GSX 1-3
8H 1,2,3 - 11.3 313.9 - 2.43 1462.2 1.88 48.2 - 4.91 199.7 - 141.86 Source NHBC Mine, Broken Hill NSW Starting Material Expte GSX 4-9
GSX 10 1.36 301.6 37.8 1.61 - 672.6 0.99 43.8 - 6.78 177.3 324.4 231.1 Source NHBC Mine, Broken Hill NSW Expt GSX 10
GSX 11 1.90 52.7 1129.5 4.53 8.28 1313.6 2.09 1160.9 3.86 9.33 226.9 671.8 829.66 Source NHBC Mine, Broken Hill NSW Expt GSX 11
GSX 20 1.54 - 19.38 - - 4.66 - 10.36 - 8.31 128.9 270.9 107.5 Source NHBC Mine, Broken Hill NSW Expte GSX 20-23
GSX 30 1.23 3.1 - 2.07 - 9.33 - 7.02 - 7.28 129.9 340.7 82.27 Source Tri State, USA Expt GSX 30
GSX 31 - - - - - 7.37 - 5.22 - 5.25 121.2 - 77.38 Expt GSX 31, Tri State, USA
GSX 32 1.00 3.5 - - - 6.05 - 7.70 - 5.93 124.5 244.3 94.6 Expt GSX 32, Tri State, USA
GSX 36 - 4.81 - 1.80 - 4.66 0.79 5.22 - 9.33 122.3 404.6 107.5 Expta GSX 36, 35, 34, Tri State USA
GSX 39 - 32.04 266.0 2.34 2.43 1052.3 2.23 160.03 - 8.31 222.6 297.7 701.7 Expte 37, 38, 39, Tri State USA
G5X 40 1.35 4.81 303.1 - - 5.7 1.06 3.67 - 4.23 116.9 271.0 99.9 Expte 40, 41, 42a, Tri State USA
GSX 43 1.35 4.81 - 1.61 200.30 6.4 - 7.27 - 9.33 134.3 271.0 107.5 Expta 42, 43, Tri State USA
GSX 45 - 4.81 - 1.80 99.8 4.7 - 5.22 - 5.25 134.3 297.1 115.0 Expta 45 Tri State USA
GSX 45A 1.54 4.81 19.38 1.61 131.15 5.7 0.86 7.79 - 9.33 143.0 377.7 99.9 Expte 45, 46, Tri State USA
M 15 1.90 6.11 - 3.62 4.03 6.38 1.06 5.73 5.08 14.43 145.2 645.0 190.2 Source Galena, Illinois, USA Expta 50-59
M 12 - - 19.38 - - 18.17 - 13.45 - - 101.6 - 197.7 ()cots 50-59
M 13 - - - - - 11.05 - 10.36 - 3.21 101.6 - 190.2 Expts 50-59
T 2 - - - - - 1.23 - 2.64 - - 95.1 - 77.4 Source Tri State USA Expts 60-63
Detection 0.9 2.4 19.0 1.6 2.3 1.2 0.6 1.3 3.5 2.2 3.6 178 25 - indicates below detection limit Limits 88
Specimens used in the deformation experiments were cylinders approxi- mately9mm in diameter and 16-18mm long. All but a few specimens had aspect ratios (length/diameter) very close to 2:1. Specimens were cored in the orientations specified in Table 4.1. Coring was achieved using a Servomet SM II spark erosion machine. Coring was conducted using a low spark energy and low erosion rate (approx. 2.5cms in 14 hours) in order to minimise specimen damage. Prior to experi- mental deformation, Laue x-ray photographs were taken of the galena cores to check the orientation. Sharp diffraction patterns, typical of undeformed single crystals, were observed. Damage inspection of cored crystals revealed only a thin (1-2pm) layer of partially melted material. Mechanical damage features - slip lines and deformation bands, were not observed.
DEFORMATION AND ANNEALING
Specimens were deformed in a fluid-medium triaxial apparatus (Heard 1963) at strain rates of approximately 3 x 10-5 s-1 and 3 x 10-7 s-1 . Detailed descriptions of the apparatus are given by Rutter (1970, 1972) and Atkinson (1972, 1976a). The heart of this apparatus is an externally heated thick-walled steel pressure vessel. Axial loads on the test specimen are measured using an internal steel force gauge in the form of a hollow column actuating a linear variable differential transformer outside the pressure vessel. Oven dry specimens were jacketed in thin walled (0.25mm) copper tubes and swaged to tapered, solid steel loading pistons to prevent contact between the specimen and the silcone oil confining pressure medium. At ele- vated temperatures (all experiments conducted at 200°c and above), the specimens were encased in aluminium foil to prevent chemical reactions between the galena and the copper jackets. Further details of the apparatus, details of the accuracy of measurement and reduction of data are fully described elsewhere (Heard 1963, Rutter 1972, Atkinson 1972, 1976a). All deformation and annealing experiments were carried out at the same confining pressure of 1.5 Kbars. After removing the axial differential load accumulated during the experimental deformation, selected specimens were allowed to anneal for 10 days (240 hours). Several annealing experiments were conducted in 'a hydrothermal "Tempres" unit. These were kindly carried out by N. Shaw. Series 5 experiments involving stress relaxation were carried out using the methods described by Atkinson (1978) and Rutter et al. (1978). 89
DEFORMATION CONDITIONS
Experiments were conducted under the following conditions. -5 -1 -7 -1 Temperatures: 20°c - 400°c; strain rate: 3 x 10 s - 3 x 10 s ; and confining pressure: 1.5 K bars. The temperature range for the experiments covers those expected to prevail during low greenschist facies metamorphism (see Chapter 2). At temperatures below 400°c the stoichiometry of lead sulphide is expected to be reasonably stable during the experimental runs (see Section 3.3). The confining pressure of 1.5 Kbars was chosen to inhibit brittle failure and for comparison with other work (Atkinson 1972, Atkinson 1976a). Although confining pressure is known to influence cross slip in NaC1 (Nicholas and Poirier 1976), this effect has not been investigated in galena.
EXAMINATION OF DEFORMED MATERIAL
After deformation, specimens were impregnated with epoxy resin under vacuum, while still in their metal jackets, and then the galena was cut using either a microtome saw or a spark erosion wire saw. One portion was polished for optical examination and the other was examined by Laue x-ray diffraction and scanning electron microscopy. Some sections were also used for x-ray texture goniometry. Polished sections were etched using the Brebrick and Scanlon (1957) etch reagent (Appendix A) at a temperature of 65°c for times between 30 seconds and 120 seconds, lightly repolished with Al 0 on a cloth lap, and then re-etched. Small cleavage fragments were 2 3 also etched (usually for 20-60 seconds) and then studied, uncoated or coated with gold, in a Cambridge Instruments 600 Stereoscan. For some samples, Vickers hardness tests were conducted using a Lietz 'mini-load' hardness tester with a load of 25gms which gave indentations from 20-30pm across.
4.3 SERIES 1 EXPERIMENTS
Series 1 experiments were designed to investigate kink band formation and preservation in single crystals of galena. Experiments were conducted -5 -1 at temperatures from 20° to 400°c and at strain rates of 3 x 10 s and -7 -1 3 x 10 s , and at a constant confining pressure of 1.5 Kbars (Table 4.1). MAXIMUM RESOLVED SHEAR STRESS FOR SLIP IN GALENA
(100) <011> SYSTEM (110)
cr 000[101] 1 (001) [110] 001 001 (101)[101] 010
(011) [oTi] (011) [oil] (011) [oli] (01 1) [01i]
(100) [oil] (110) Ho] (lfo) [11o] 100 100
cr =initial position of the compressive stress axis 1 (100) [011] etc. are slip combinations with maximum resolved shear stress
FIGURE 4.1 Inverse pole figures contoured for maximum resolved shear stress for both the f100}<100> and {110}<110> slip systems CD 0
i0
TABLE 4.3a SLIP SYSTEMS IN GALENA
SYSTEM {100} <011> (3 Independent Slip Systems)
I
Plane 100 010 001
Direction 001 011 101 101 110 110
SYSTEM {11O} <110> (2 Independent Slip Systems)
I II III IV V VI
Plane 110 110 101 101 011 Oil
Direction 110 110 101 101 011 011 92
TABLE 4.3b RESOLVED SHEAR STRESS - SCHMID FACTOR SERIES 1 EXPERIMENTS
1. SYSTEM (110) <110>
PLANE DIRECTION 0 A COS0 x COSX = M
(110) <110> 90° 80° 0.0 (110) <110> 80° 90° 0.0
(101) <101> 52° 39° 0.4785
(101) <101> 39° 52° 0.4785
(011) <01> 390 52° 0.4785
(011) <011> 52° 39° 0.4785
2. SYSTEM (001) <110>
PLANE _DIRECTION 0 A COS0 x COSX = M
390 (100) <011> 97° -0.0947
(100) <011> 97° 52° -0.0950
(010) <101> 83° 52° 0.0950
(010) <101> 83° 39° 0.0947
(001) <110> 10° 80° 0.1710 (001) <110> 10 90° 0.0
SCHMID FACTOR M IS THE RATIO OF RESOLVED SHEAR STRESS ACTING ON THE SLIP PLANE TO THE MAXIMUM PRINCIPAL STRESS. M = COS® x COSX WHERE 0 IS THE ANGLE BETWEEN THE STRESS AXIS AND THE SLIP PLANE NORMAL AND A IS THE ANGLE BETWEEN THE STRESS AXIS AND THE SLIP DIRECTION. 93
101
111
Duplex Slip on (101) & (011) Planes
Stress Axis
001 011
DIRECTIONS OF LATTICE ROTATIONS
FOR (110) (110) SLIP
FIGURE 4.2 Directions of lattice rotations for (110)<110> slip in series 1 experiments 94
Annealing experiments were carried out on some samples deformed at -5 -1 3 x 10 s by leaving the specimen in the deformation rig, removing the axial load but maintaining the deformation temperature (200°c and 300°c) and confining pressure for a period of ten days. The microstructures of the starting material, the deformed and annealed specimens were studied by optical and scanning electron microscopy as well as by Laue x-ray dif- fraction. The results of this series of experiments have been published (McClay and Atkinson 1977, Appendix B). Single crystals of galena from Broken Hill, N.S.W. were cored at 10° to the <001> direction along the (110) plane. This orientation was chosen to inhibit slip on the {001} planes and favour kink band formation. Figure 4.1 shows the orientation of the initial compression axis and con- tours of the critical resolved shear stress for both the {100} <011> and {110} <110> slip systems. In this particular orientation, the slip systems in galena (Table 4.3a) have the Schmidt factors listed in Table 4.3b. From Table 4.3b it can be seen that four planes of the {110} <110> system are equally stressed with a high Schmid factor while the {001} <110> slip planes have very low Schmid factors. The predicted lattice rotations for compression along this axis (10° to the <001> direction) are shown for 0101 <110> slip in fig. 4.2. It is probable that these rotations are balanced by nearly equal and opposite rotations caused by a small component of {001} <110> slip. Thus this compression axis is likely to be a semi- stable position, with little lattice rotation until considerable kinking has developed.
RESULTS
The results of this series of experiments are summarised in Table 4.4. Several repeat experiments are ommitted from this table as they duplicate the features of the experiments listed.
STRESS STRAIN CURVES
Stress strain curves for series 1 experiments are presented in Figure 4.3. They all show a very steep initial portion up to axial strains of approximately 2 per cent. At higher strains the slopes of the stress- strain curves decrease. At 200c and 100°c an ultimate strength is reached at axial strains of approximately 5 to 8 per cent. At higher temperatures curves flatten much more readily and an ultimate strength is achieved at EXPERIMENT NO. TEMPERATURE STRAIN RATE TOTAL STRAIN YIELD CRITICAL RESOLVED ANNEALING TIME KINKING AND DYNAMIC STATIC % STRESS SHEAR STRESS HOURS TEMPERATURE SLIP FEATURES RECRYSTALLISATION RECRYSTALLISATION °C s. -1 bars (bars) . GSX - 9 20°C 4.9 x 105 18 3250 1555 - Abundant kinks - - 2 sets along 11101 opened cleavages
GSX - 10 100°C 4.2 x 10-5 15 2150 1028 - 2 sets of kinks - - slip lines, glide polygoni nation
GSX - 1 200°C 3.3 a 10 5 27 1300 622 - Kinking confin d Polygonisation - to two zones some grain boundary mig- ration in kinks
GSX - 2 200°C 5.3 x 105 19 1325 634 - As GSX-1, slip As GSX-1 - and glide polygonisation
GSX - 4 200°C 4.5 x 10-5 26 1325 634 240 Hours As GSX-1 8 2 As G5X-1 i 2 Significant grain boundary migration 200°C small equant new grains
GSX - 5 300°C 4.4 x 10-5 15 SOO 239 - Slip features Polygonisation - rare Kink and elongate bands with new grains in polygonisation kinks - serra- ted boundaries ----GSX - 8 300°C 3.7 x 10 5 19 500 239 - As GSX-5 New elongate - grains with serrated boundaries
GSX - 7 300°C 4.4 x 10-5 16 500 239 240 Hours No slip fea- Grain boundary New grains smooth tuns migration in straight boundaries 300°C kinks
GSX - 13 400°C 3.0 x 10-5 22 not ma- - - No slip fea- Recrystallised - sureable tures grains
GSX - 11 100°C 3.4 x 10-7 17 1825 874 - Slip lines, Glide polygoni- - deformation sation only bands, abund- _ ant kinks GSX - 3 200°C 3.6 x 10-7 20 700 335 Kink bands Kink band bound- - with ary migration. polygonisation new elongate serrated grains
GSX - 6 300°C 4.2 x 10-7 20 300 143 - Kink bands - only few slip stallised grains features in kink bands
TABLE 4.4 SERIES 1 EXPERIMENTS. SINGLE CRYSTALS OF GALENA COMPRESSED AT 10° to (001). ALL TESTS WERE CONDUCTED AT A CONFINING PRESSURE OF 1.5 Kbars. 4 96
GSX-9 3
GSX-10
■■••• 2 GSX-11 -0 Y -5 20°C, 10 : GSX-9 100°C, 10 5 : GSX-10 100°C, le: GSX-11 200°C , 1O : GSX-1,2,4 200°C , le: GSX -3
l 5 300°C , 10 : GSX-5,7,8 ia
t 7
n 300°C , 10 : GSX-6 re
iffe 1.5
D GSX-4
GSX-1 GSX-2
GSX -3\
0.5 GSX-7 GSX- 8
GSX-6
II I I I I I I II I I I I I I I I IJ.__ 0 2 4 6 8 10 12 14 16 18 20 26 27 Axial Strain ( Percent )
FIGURE 4.3 Stress strain curves for single crystals of galena • compressed at 10° to <001>. The temperature and -1 strain rate (expressed in s ) are given with the appropriate reference number (see table 4.4 for details). All tests were conducted at a confining pressure of 1.5 Kbar. 97
axial strains from approximately 2 to 4 per cent. Portions of the curves at yet higher strains were either flat or showed small negative slopes. A large drop in ultimate strength occurred on raising the temperature or lowering the strain rate (Fig. 4.3). It was assumed that in these experi- ments, the initial slip occurred on {110}<1TO> planes and the C.R.S.S. for {110}<110> slip was calculated by taking the stress at the change in slope in the stress strain curve in the low strain regime (i.e. the beginning of stage 1 easy glide, Honeycombe 1968) and multiplying this stress by the Schmid factor (Table 4.4).
DEFORMATION MICROSTRUCTURES
The microstructures developed during deformation and annealing experiments were revealed by studies of etched polished sections and etched cleavage fragments. A brief summary of the more important features is given in Table 4.4. At 200c the deformation is characterised by the development of kink bands throughout the crystal (Fig. 4.4). Broad deformation zones are also found (Fig. 4.5a). These zones have abundant interfering kink bands, slip lines and deformation bands (Fig. 4.5a). Broad kink bands (0.1m - 0.4mm) elsewhere in the crystal commonly have opened cleavages (Fig. 4.5b). These kink bands are often observed to be perpendicular to the axis of compression but some are also at a low angle to the axis of compression. The boundaries of both sets of kink bands are approximately parallel to {11O} planes. Details of the deformation zone in Fig. 4.5a are shown in Fig. 4.5c. Clusters of parallel slip lines occur in deformation bands while the kink bands have curved boundaries and cut across these deformation bands. Small second stage kinks (3 in Fig. 4.5c) are found normal to the deformation bands. Zones of strongly developed slip lines with straight but discontinuous boundaries (deformation bands) are also found (Fig. 4.5d). Both (110) [110] and (100) [011] slip were identified from etch pit orientation studies. In addition to broad kink bands (Fig. 4.4b) smaller (10 - 30um) secondary kinks occur in specimens deformed at temperatures up to 200°c. These secondary kinks are found in deformation bands (Fig. 4.5e) and develop from glide polygonisation (Fig. 4.5f, g) where irregular walls of dislocations are formed perpendicular to the slip direction. On further straining these regions of reverse lattice curvature develop into curved kink bands (Fig. 4.5h). 98
20°C 100°C
EXTENSIVE KINKS AND DEFORMATION BANDS
a1 a1 200°C 300° C 4
(110) Slip
• STRONG DEFORMATION IN TWO ZONES POLYGONISATION / RECRYSTALLISATION IN KINKS
SKETCHES OF DEFORMED SINGLE CRYSTALS
FIGURE 4.4 Sketches of deformed single crystals showing the distribution of main deformation features for crystals compressed 100 to the (001) plane - Series 1 experiments. 99
Figure 4.5
Experimentally deformed single crystals of galena. Compression axis 10° to <001> along the (110) plane. Confining pressurec1.5,Kbars. Strain rate approxi- mately 3-4.5 x 10 ' s-I or 3-4.5 x 10-7 s-I. All samples etched with Brebrick and Scanlon (1957) etchant.
a) 200c. Zone of intense deformation showing abundant deformation bands and slip lines (1) and curved second stage kink bands (2). Experiment GSX-9.
b) 200c. Broad kink bands shown by zig-zag cleavage trace (cl). Compression axis horizontal. Experiment GSX-9.
c) 200c. Detail of a zone of (a) showing clusters of slip lines in deformation bands (1), broad early kinks (2) and curved, thin, second stage kinks (3). Experiment GSX-9.
d) 200c. Deformation bands with clusters of slip lines showing different slip directions. Experiment GSX-9.
e) 200c. Kink bands (1) with abundant slip lines developing glide polygonisation and second stage kink bands (2). Experiment GSX-9.
f) 200c. Detail of (e) showing slip lines (near horizontal) developing second stage kinks. Experiment GSX-9.
g) 200c. Deformation band with slightly wavy slip lines (vertical) and the development of second stage kinks. Experiment GSX-9.
h) 2000c. Detail of curved second stage kinks showing rotation of slip lines. Experiment GSX-1. o i) 200 c. SEM photograph of kink bands with {11O} type boundaries. Experiment GSX-2.
i) 200°c. Lath of tetrahedrite cut by slip lines in the host galena crystal. Experiment GSX-l.
100
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The microstructures of specimens deformed at 1000c (Fig. 4.4) were similar to those of specimens deformed at 200c with the exception that opened cleavages were not observed. Deformation at 2000c gives rise to kink and deformation bands which are strongly developed in two zones, at the top and bottom of the crystal (Fig. 4.4). Similar behaviour is shown by single crystals of some metals (Honeycombe 1968). Kink bands with (1101 type boundaries are found (Fig. 4.5i). The other microstructural features of specimens deformed at 200c and 100°c are also well developed in these zones e.g. slip lines, deformation bands and secondary kinks. In addition, wavy slip lines are seen, indicating cross slip of edge dislocations. In the less deformed portions of the crystals (001 ) [110] and (010) [101] duplex slip occurs. Small lath like inclusions of tetrahedrite are broken up by slip in the host galena crystal (Fig. 4.5i). In this case, the applied shear stress has been sufficiently great to allow the dislocations to shear through the obstacle. o -5 -1 At 100 c and 200°c, varying the strain rate from approximately 4 x 10 s -7 -1 to 4 x 10 s had little discernable influence on the deformation micro- structure. At 2000c some kink bands had slightly serrated boundaries which indicate possible grain boundary migration. The microstructures developed during experiments at 300°c are markedly different from those formed at lower temperatures. In the faster strain -5 -1 rate experiments ( 4 x 10 s ) gently curved kink bands form which contain large subgrains (50 - 200pm across) (Fig. 4.6b). Deformation bands and other slip features are poorly developed. Where these subgrains are suffic- iently misoriented (i.e.> 8°) they constitute new grains. Only a few new grains were observed in the kink bands. -7 -1 Deformation at a slower strain rate of 4 x 10 s results in the formation of extensive polygonisation in the kink bands (Fig. 4.6c). The subgrains formed are often large (100 - 400pm), elongate and have lobate boundaries (Fig. 4.6c). In detail, however, these large subgrains are sub- divided into smaller'regions separated by low angle tilt walls. Grain boundary migration is substantial and recrystallisation occurs both within the kinked regions and at kink band boundaries (Fig. 4.6c).
ANNEALING MICROSTRUCTURES
Experiment GSX-4 which was annealed for 10 days at 200°c shows mig- ration of kink band boundaries giving rise to serrated high angle boundaries (Fig. 4.6a). A few small new grains are formed at these migrating 102
Figure 4.6
Experimentally deformed galena-compression axis 10u to <001> along the (110) plane.
a) 2000c. Serrated kink bands showing high angle bound- ary migration. GSX-4, 10 day anneal at 200'c.
b) 3000c. Elongate subgrains in kink band (shown by curved cleavage line. Note the slightly curved bound- aries parallel to the kink band and straight segments perpendicplar,to the kink band. GSX-5, strain rate 4.4 x 10- s-i.
c) 3000c. Well developed recrystallisation in kink bands with new grains forming as a result of grain boundary migration. GSX-6, strain rate 4.2 x 10" s-1.
d) 400°c. Large recrystallised grains with straight grain boundaries. Note the development of elongate grains and triple points. GSX-12, strain rate
e) 200c. SEM photomicrograph of deformation band showing intense development of slip ling on cleavage fragment. GSX-9, strain rate 4.9 x 10- s I .
f) 2000c. SEM photomicrograph showing high density of etch pits. The etch pits show the formation of beaks and grooves along the dislocation cores. GSX-1, strain rate 3.3 x 10-5 s '. Cleavage fragment.
g) 200c. SEM micrograph showing {110} <110> slip traces on cleavage fragmept jyertical array of etch pits). GSX-9, strain rate 10 ." s ' (see also McClay 1977a).
h) 1000c. SEM micrograph of cut and etched surface showing curved kink bands outlined b.y curyed cleavage traces. GSX-10, strain rate 4.2 x 10 's-1 .
All samples etched with Brebrick and Scanlon (1957) etchant as described in Appendix A. ▪ 103 .44
••• 11," ");•0 • ••■••••-;
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', .', i /1//frit'-'/ .' .. . T ' ; • , ' ' ' ' I '/ ' Ii ' ,,,9';4 ,',.."•1'.7 , . • ,, /.• ''.1 • • /,/or. , ,,,,.. ,, / /,,, ,-,.., 0; . •. . ,. 4' . , • I,/ A„,;,..?, ., , , • , ,-(),.:) /roe 1 104 boundaries by strain induced boundary migration (Chapter 3.2). Annealing of a specimen for 10 days at 3000c after deformation at this temperature -5 -1 and at a strain rate of 4 x 10 s (expt. GSX-7) produced less specta- cular results. The kinked regions have large elongate recrystallised grains with straight to slightly curved boundaries (Fig. 4.6d). Large sub- grains are more extensively developed than in the unannealed material and subgrains are also found outside the kinked regions.
SCANNING ELECTRON MICROSCOPE STUDIES
Etched cleavage fragments of specimens deformed at 20°c, 100°c and 2000c show a high etch pit density (Fig. 4.6f). This cannot yet be directly correlated with a high dislocation density, however as a 1:1 correspondence between etch pits and dislocations has not been established for etching of galena (Appendix A). It is probable that the background of smaller etch pits are sessile {001}400> dislocations (Lyall and Paterson 1966) Sulphur vacancies may also etch out. The deformation bands found in specimens def- ormed at 200c, 100° c and 2000c have closely clustered slip lines (Fig. 4.6e). Slip lines of {110} <110> type (Fig. 4.6g) and large kinks with {110} bound- aries (Fig. 4.5i) are most prominent but {100} <110> slip can also be identified. Detailed micrographs of etch pits (Fig. 4.6f) reveal a number of etch pit types - pyramidal, flat-bottomed, and etch pits with grooves and beaks (Fig. 4.6f) which probably indicate decoration of dislocations (Barber 1965). Broad kink bands are also revealed in the scanning electron microscope. The scanning electron microscope studies confirm the slip fea- tures observed by optical microscopy.
LAUE X-RAY DIFFRACTION
Back reflection Laue photographs were taken from several regions of each specimen. Those deformed at 200c, 100°c and 200°c all show regions of continuous asterism (circular streaks). Analogous results have been obtained for heavily cold worked metals (Maddin and Chen 1954). However, these specimens also have regions in which blocky asterism occurs, indicat- ing a polygonal microstructure (the Laue photographs are seen in Appendix B., McClay and Atkinson 1977). All Laue photographs from the specimens deformed at 3000c showed blocky asterism and the specimens subjected to a slow strain rate deformation showed spotty asterism similar to that given by recrystallised metals (Maddin and Chen 1954). 105
VICKERS HARDNESS
In series 1 experiments, the Vickers hardness number (VHN) was meas- ured in order to compare the results with those obtained by Siemes (1970, 1976). The results are presented in Table 4.5. An increase in the VHN is noted after all deformation experiments and this is reduced on annealing (Table 4.5). However, as discussed in Section 3.3, Vickers hardness is an unreliable measure of hardening and recovery because of a) inhomogeneity of deformation in the specimens and b) variations in stoichiometry induced during deformation and annealing.
X-RAY TEXTURE GONIOMETRY
X-ray texture goniometry was carried out on several deformed single crystals. In all of the partial pole figures obtained (Fig. 4.7) there is only a slight spread in the (200) poles. Measurement of the spread of orie- ntations is made difficult because of the intense x-ray reflections caused by the single crystals. The annealed specimen (GSX-4) does however show a wider spread of (200) poles than the unannealed specimen (GSX-1), indicating probable grain boundary migration causing increased misorientation of sub- grains/new grains.
DISCUSSION
As can be seen from Fig. 4.1, the initial orientation of specimens with respect to the direction of maximum compression favours slip on the {110}<110> system. Microstructural studies have shown that this slip system is important in the deformation of all series 1 specimens. A previous study (Lyall and Paterson 1966) has indicated that the {110} slip system has a high critical resolved shear stress at room temperature. This is confirmed by the present observations which also indicate that the CRSS for slip on 0101 planes is reduced dramatically by an increase in temperature or decr- ease in strain rate. Initial slip on the 0101 system causes the crystal lattice to rotate into an orientation which favours kinking and {100} slip. This accounts for the post-yield portions of the stress-strain curves where geometric softening is evident (Fig. 4.3). Contrary to the results of Lyall and Paterson (1966), no 441 twins were observed in these experiments. 106
Table 4.5 Vickers Hardness Determinations
Sample Mean VH Range No
Undeformed 51.5 46.1 - 54.5 70 -5 -1 Deformed at strain rate 4 x 10 s
20°C 80.5 78.1 - 85.4 50
100°C 77.3 74.0 - 84.3 40
200°C 79.5 68.6 - 92.6 48
300°C 66.7 58.9 - 74.2 46 -7 -1 Deformed at strain rate 4 x 10 s
300°C 57.2 51.2 - 61.5 50 -5 -1 Deformed at strain rate 4 x 10 s - Annealed 10 days
200°C 62.3 55.5 - 73.9 52
300°C 54.8 45.0 - 66.0 48
Instrument, Leitz Durimet Pol Miniload Hardness Tester with 25 gram weight to give indentations 20pm to 30pm. 107 GSX1 EXPERIMENTALLY DEFORMED AT 200°C I COMPRESSIVE STRESS AXIS i
GSX 5 EXPERIMENTALLY DEFORMED AT 300°C
GSX 4 EXPERIMENTALLY DEFORMED AT 200°C
PARTIAL POLE FIGURES 200 REFLECTIONS SINGLE CRYSTALS OF GALE NA
CONTOUR LEVELS TOTAL COUNTS- 20 SECS COUNTING TIME
FIGURE 4.7 Partial pole figures of single crystals of galena compressed at 10° to the <001> direction - Series I experiments. 108
Since the {110} <110> slip system can provide only 3 of the 5 inde- pendent slip systems (Table 4.3a) required to satisfy the von Mises crit- erion of ductility (e.g. arbitrary shape changes in polycrystals) (von Mises 1928) {100} <110> slip is likely to operate during deformation. Kinking (e.g. Paterson 1969) may also provide the equivalent of another independent slip system and allow plastic deformation. Recovery and recrystallisation processes are first evident after deformation and annealing at a temperature of 200°c, and they are important in all specimens deformed at 2000c. The increasing importance of polygoni- sation and recrystallisation observed at 3000c at the lower strain rate indicates that, during natural tectonic deformation of galena (which prob- -10 -14 ably occurs at strain rates from approximately 10 - 10 sec-1 (Price 1975)), recovery processes will be important. This conclusion agrees well with the theoretical predictions of Atkinson (1976b, 1977). The initial orientation of the starting material with respect to the axis of maximum compression (Fig. 4.1) gives rise to possible slip on four {110} - type planes and at least one {100} - type plane (these have a low resolved shear stress). In consequence of the loading constraints on the specimen and the deformation environment. (T, 6), kinking soon develops. Broad kinks are formed with boundaries initially in the (110) plane (Fig. 4.5i); a situation similar to that of kinking in single crystals of some metals (Clarebrough and Hargreaves 1959, p.65, 85). Increasing strain, how- ever, produces rotation and interference of both kink bands and deformation bands leading to complex boundary geometry. Glide polygonisation (Livingston 1962) can also produce zones of reverse lattice curvature which rapidly develop into later second stage kink bands (Fig. 4.5f-h). It is possible to infer from the results of series 1 experiments that that preservation of slip and kink features in naturally deformed galena ores indicates that such ores have not been subjected to post-tectonic temp- eratures greater than 2000-3000c. The high strain energy of kink zones enables them to act as preferential sites for polygonisation and recrystalli- sation. The polgonised and recrystallised grains tend to be elongate parallel to the kink band boundary.
4.4 SERIES 2 EXPERIMENTS
This series of experiments was designed to investigate the variation of the critical resolved shear stress (CRSS) for {110} <110> slip with temp- erature and strain rate. The experiments also aimed to investigate the 109
microstructures developed in single crystals deformed at temperatures from 20°c to 4000c. In Series 2 experiments, single crystals of galena were compressed parallel to the <001> axis. Tests were conducted at strain rates of 3 - -5 -1 -7 -1 o 4 x 10 s and 3 - 4 x 10 s , over a temperature range of 20 c to 4000c and at a fixed confining pressure of 1.5 Kbars (Table 4.1). In this <001> orientation, it is the{110} <110> slip system which is subjected to the maximum resolved shear stress (Fig. 4.1) whereas the {100} <110> system experiences zero resolved shear stress (Fig. 4.1).
RESULTS
The results of this series of experiments are summarised in Table 4.6. Series 2 experiments do not include tests GSX 60-63 which are discussed in Section 4.6.
STRESS STRAIN CURVES
Series 2 stress strain curves are presented in Figures 4.8 - 4.11. -5 -1 The results of experiments carried out at strain rates of 3 - 5 x 10 s -7 -1 (Figs. 4.8, 4.9) are discussed before those for strain rates 3 x 10 s (Figs. 4.10, 4.11). The stress-strain curves (Figs. 4.8, 4.9) all have very steep initial portions up to axial strains of approximately 1-2%. Single crystals deformed at 200c (Fig. 4.8) have an ultimate strength between 2900 bars and 4400 bars. Two crystals, GSX-30 and 42A, have ultimate strengths between 4200 and 4400 bars while the other crystals, GSX-42 and Series 4 experiments GSX 60-63, all have ultimate strengths of 2900 - 3400 bars. At strains above 3-5% the stress strain curves peak at an ultimate strength and then rapidly attain a large negative slope. For deformation at 1000c (expts. GSX-31 and GSX-43, Fig. 4.9), flow occurs at lower stress levels than at 200c. The stress-strain curves have similar forms to those at 200c (Fig. 4.8), but the rate of geometric soften- ing is less. At 2000c, deformation occurs at even lower stresses (= 1 Kbar) and the stress-strain curves have only slight negative slopes at high strains. At 3000c and 4000c flow occurs at low stresses (cf. 200c and Table 4.5) and the stress-strain curves are essentially flat at high strains ( 5% axial strain). One stress-strain curve at 3000c and all of those for deformation at 4000c (Fig. 4.9) have double yield points. At 4000c, the stress-strain
TABLE 4.6 SERIES 2 4143 SERIES 4 EXPERIMENTS SINGLE CRYSTALS Of GALENA COMPRESSED PARALLEL TO THE <001> AXIS All experiment. wore carried out at • fixed confining prisons of 1.5 Alit.. , EXPERIMENT NO. TEMPERATURE STRAIN RATE TOTAL STRAIN YIELD STRESS STRESS AT 10% ANNEALING TIRE KINKING DYNAMIC STATIC .. -1 (bars) STRAIN-(brs) HOURS °C SLIP FEATURES RECRYSTALLISATION RECRYSTALLISATION % TEMPERATURE
GSX - 30 20°C 3.73 A 10 5 23.3 3600 4076 ± 382 - Large kinks - 11101.11p 5 GSX - 42. 20°C 5.25 x 10 11.2 3400 2443 = 443 - KInks,11101ellp - GSX - 42b 200C 3.12 A 105 3.8 2900 - - Heavily 'cacti -yid - /1101 and Iloo alip,awall kink. 5 GSX - 31 100°C 3.77 x 10 25.5 1455 1752 = 50 - A You slip linos, minor kink. - - 5 GSX - 43 100°C 2.59 x 10 11.0 1675 1709 5 74,1 A You slip line. - - A kinks. Homo- geneous deform- ation -b GSX - 32 200°C 3.12 A 10 16.2 B90 1023 = 18.7 Slip 4 kinking PolygonlaatIon, - minor grain boundary migration GSX - 44 200°C 3.00 x 10-5 19.7 910 1031 5 19.3 Ae above Am above - GSX - 33 300°C 4.07 x 105 13.3 630 636 = 11.7 Kink blinds - Polygonisation A - slip minor roc aaaaa Illastion In kinks 5 GSX - 45 300°C 3.20 x 10 13.0 425 579 ± 17.8 As above Elongate grains - grain boundary migration GU - 34 400°C 3.19 x 105 10.7 110 149.4= 9.9 Kink bands - Elongate recry- - slip mlnor - otallised grains ebeont CSX - 46e 400°C 3.60 x 105 19.7 210 219 = 6.3 - Kink bands, Rocry•tal tttttt on - • curvod cleaysgsa ln kink bands GU - 46b 400°C 2.90 A 105 12.0 235 196 C 14.6 - As above .18.11101 15-25xas grains In - 411001811p linos kink bands GSX - 41 20°C 3.55 A 10 7 19.0 • 2420 3041 5 349 - (110)slip - few - - Minim mar (root - urom 7 GSX - 35 100°C 3.10 0 10 10.00 1350 1006 = 153 - (110)slip, Pow - - kinks, as GSX-31 ' GU -36 200°C 3.51 • 107 22.4 670 B11 5 19.1 - Paw kink...Drain Small new groins in - boundary migration kinks - boundary minx tion CU - 37 300°C 3.43 x 107 29.0 380 429 5 15.7 - Kinks - few slip Recryatallioetion - llne. In kinks -7 GSX - 40 400°C 3.30 x 10 33.0 10 5 = 10 - Abundant relict Elongate corvine in - kink Panda kink band, CU - 60 20°C 4.16 A 10 5 9.7 2345 .. 240 Hsur. Snell recrystel- - Elongate Arsine, 200 C Used grains in straight boundaries kinks in kink bond* CU - 61 20°C 3.53 x 10 5 13.1 2600 3096 = 3.5 240 Hours Deformation far - . 100°C tures obscured 5 GSX - 62 20°C 5.52 • 10 SA 2420 240 Hours A few mulct kinks - Liege squint recrystel- 400°C no slip lines no Ileed grign.,patcTily deformation bond. day.lop.d,..00t( curvAd boundaries GSX - 63 20°C 5.29 • 10-5 14.4 2610 2671 1 47 340 Hours A few relict kinks - (quint rscrysta1lisod 300°D no .lip lines grains. etohily deve- loped 111 4500 SINGLE CRYSTALS OF GALENA /,...... • – •—•\\* DEFORMED IN COMPRESSION A-4-e (001) ORIENTATION 20°C 4000 /. STRAIN RATE 4.39 x 10-5 sec-1(average) • \ -..... 2 •• f \•
GSX 42 5,_S In / •,_, 3000 • GSX 61 z In VI W 1 )72 4, CC tilI- 4 I z \ f w • cc • GSX 63, 62 w u_ u_ 8 2000 \
GSX 42A GSX 30
•
f=falure 1000 • '4, GSX 60
I I- ITIFTI I I T T l I TlIl 'TITO I 5 10 15 20 25 30 STRAIN %
FIGURE 4.8 Stress-strain curves for single crystals of galena compressed parallel to the <001> axis at 200C and at a fixed confining pressure of 1.5 Kbars. 112 2200—
SINGLE CRYSTALS OF GALENA
DEFORMED IN COMPRESSION 2000— (001) ORIENTATION • 5 -1 STRAIN RATE 3.27 X 10 sec (average) 100°C • 1800 — failure
• 1600— GSX 43
1400 — • failure
cc co 1200 - \\\ z • GSX 31 ti) iiirdi"-4 24::>. • ...... t. 1E1000 - VI 0„...... 200°C —J GSX 32 • Z • GSX 44 z LLI 3 800— Lt. Et • 300°C GSX 33 600— •---410 • GSX 45
400°C !-• — GSX 46 –.2.6_,GSX 46b , • • • • GSX 34
• •
risilltlirriTli. IIIIII,Trti 0 5 10 15 20 25 30 STRAIN
FIGURE 4.9 Stress-strain curves for single crystals of galena compressed parallel to the <001> axis at temperatures from 100°C to 400°C and at a fixed confining pressure of 1.5 Kbars. 113 4000
SINGLE CRYSTALS OF GALENA
DEFORMED IN COMPRESSION
(001) ORIENTATION - STRAIN RATE 3.5 x 10 7sec 1 ...... --•'."--u---"---....„...... 3000 / N..„..... \• failure z in in w 1- in _, 2000 a P z w • w u. w 6 \ GSX 41 20°C
1000
10 15 20 25 30 STRAIN 'I.
FIGURE 4.10 Stress-strain curve for a single crystal of galena deformed in compression at 20°C, at a strain rate of -7 -1 3.5 x 10 s and at a confining pressure of 1.5 Kbars 1800 -
SINGLE CRYSTALS OF GALENA
DEFORMED IN COMPRESSION 1600- Failure (001) ORIENTATION
• STRAIN RATE 3.3 x 107 sec-1 (average)
1400
1200- u) • cc GSX 35 100°C c 0 z 1000— U) w
:,-h( 800 - 1= z w GSX 36 200°C cr • w
600-
• • • 400- AI • • • GSX 37 300°C
JI
200-
GSX 40 400°C 111 _I. ▪ 1 I I WIWI 11 11 0 5 10 15 20 25 30 35 STRAIN 'I. FIGURE 4.11 115
curves show a slight cyclic variation similar to that found in high temper- ature deformation of metals (Sellars 1978). Stress-strain curves for deformation at the slower strain rate -7 -1 3.5 x 10 s are presented in Figures 4.10 and 4.11. The forms of the curves are broadly similar to those at a strain -5 -1 rate 3 x 10 s . Under similar deformation conditions (temperature and confining pressure) ductile deformation occurs at lower stresses than those -5 -1 for deformation at strain rates 3 x 10 s . As the temperature of deformation is increased from 20°c to 400°c, the flow stress at strain rates -7 -1 3 x 10 sec decreases dramatically (Figs. 4.10, 4.11). At strains above 5%, the stress-strain curves for deformation at 2000c and above are essenti- ally flat. The yield stresses for Series 2 experiments are listed in Table 4.6. These have been taken as the point at which the stress-strain curves depart significantly from a straight line ("elastic portion", cf Honeycombe 1968). Steady state flow is only achieved for experiments conducted at 200°c or above. This is reflected in the standard error estimates at 10% strain (Table 4.6). Below 200°c, unstable flow (Nicholas and Poirier 1976, Section 3.1) operates until, in some crystals, failure occurs (Fig. 4.8 - 4.11). For both of the strain rates used in these experiments, an increase in temperature produces a dramatic decrease in the flow stress for single crystals of galena in the <001> orientation.
MICROSTRUCTURES
The microstructures found in single crystals of galena compressed parallel to the <001> axis are very similar to those found in the Series 1 experiments, provided that the deformation was conducted. under the same con- ditions. -5 -1 After deformation at a strain rate of 3 x 10 s and at 20°c, broad kinks (-0.5mm) were found throughout the specimens. {110} <110> slip -7 -1 and deformation bands were also found. At a strain rate of 3.5 x 10 s the deformation is concentrated into two deformation zones at the top and bottom of the crystal. Complex small second stage kinks are strongly deve- loped in this deformation zone (Fig. 4.12a). The scanning electron micro- graphs in Figure 4.12b and 4.12c show the development of these second stage kinks from glide polygonisation. In figure 4.12b polygonisation walls (vertical) can be seen developing perpendicular to {100} <110> slip in a deformation band. As the deformation increases, rotation in the kink band produces zones of reverse lattice curvature (Fig. 4.12c). 116
Figure 4.12
Experimentally deformed single crystals of galena. Compression axis parallel to <001>. Confining pressure of 1.5 Kbars. All samples were etched with Brebrick and Scanlon (1957) etchant.
a) 200c. Complex deformation bands and kinks. Note second stage small curved kinks. Experiment GSX-41.
b) 200c. SEM photograph of part of an area similar to (a). Horizontal slip lines show the development of small second stage kinks probably as a result of glide polygonisation. Experiment GSX-42. Cleavage fragment.
c) 20°c. SEM photograph showing well developed second stage kinks as in (a) and (b). Note rotation of hori- zontal trending slip lines. Experiment GSX-42b. Cleavage fragment.
d) 200c. SEM photograph of deformation band showing cluster of slip lines. Experiment GSX-42b. Cleavage fragment.
e) 200c. SEM photograph of slip lines with the develop- ment of micro-cracks. Experiment GSX-42b. Cleavage fragment.
f) 1000c. Cracking associated with deformation bands and slip lines.. Microshears 90' to the deformation bands show fragmentation of galena. Experiment GSX-43.
g) 2000c. Boundary migration - probably a relict kink band. Experiment GSX-44.
h) 300°c. Kink band shown by curved cleavage line. Well formed polygonal subgrains are found in the kink band. Experiment GSX-45.
i) 3000c. Recrystallisation with elongate new grains in a kink band. Experiment GSX-45.
j) 300°c. SEM photograph of recrystallisation in a kink band. Small new grains have formed at the kink band boundaries. Experiment GSX-45. Cleavage fragment.
116
Evidence for the rapid strain softening of single crystals at 200c can be seen in the microstructures. The deformation becomes concentrated in a single narrow zone (Fig. 4.12a) with intense development of slip lines (Fig. 4.12d), deformation bands and second stage kinks. Intense development of slip along only one or two slip systems necessitates the production of microcracks (e.g. Stroh cracks, Evans and Langdon 1976) to satisfy von Mises criterion (Fig. 4.12e). Ultimately failure occurs along a shear zones in the region of intense deformation (Fig. 4.12f). Similar features are found in single crystals of galena compressed in the <001> direction at 200c and at -7 -1 the slower strain rate (-3 x 10 sec ) and also in experiments conducted at 1000c. For deformation at 2000c, similar features as described above for 20° and 100°c are found for experiments carried out at both strain rates. Grain boundary migration, however, is also observed (Fig. 4.12g) and this permits one to infer that climb and recrystallisation processes are operating. Simi- lar features also occur at 2000c in Series 1 experiments (Section 4.3). At 3000c kink bands perpendicular to the compression direction are developed in experiments conducted at both strain rates (Fig. 4.12h). Poly- gonisation is found within the kink bands (Fig. 4.12h) with elongate sub- grains (-20pm x 40-50pm). Outside the kink band, however, the subgrains are larger and generally equant. Recrystallisation in these kink bands follows elongate deformation bands (Fig. 4.12i). Small new.grains are nucleated in the kink band bound- aries (Fig. 4.12j), In detail the kink band boundaries are serrated with the new grains growing by grain boundary migration (Fig. 4.12j). Single crystals deformed. at 400°c all underwent dynamic recrystalli sation. Relict kink bands (Fig. 4.13a) were found in crystals deformed at both strain rates. Parallel planar kinks are formed at high angles to the compression direction (Fig. 4.13b). Within these kinks slightly elongate recrystallised grains are observed. Micro-cracks also occur in curved cleavage traces kink bands (Fig. 4.13d). Other microstructures found in deformation experiments at 400°c include parallel deformation bands (Fig. 4.13e), polygonisation with rectangular subgrains (Fig. 4.13f). The larger new grains, particularly outside kink bands, have highly serrated boundaries (Fig. 4.13g). The recrystallised grains have straight boundaries parallel to the kink band boundary (Fig. 4.13h). Shorter boundaries are dominantly at high angles to the kink band boundary. Scanning electron microscopy reveals small elongate subgrains in the kink bands of crystals deformed at 400°c, (Fig. 4.131). Very small 119
Figure 4.13
Experimentally deformed single crystals of galena. Compression axis parallel to <001>. Confining pressure 1.5 -5 -1 Kbars. Strain rates either approximately 3 x 10 s or -7 -1 3 x 10 s . All samples were etched with Brebrick and Scanlon (1957) etchant.
a) 4000c. Curved cleavage traces and bent cleavage pits indicating strong kinking. Experiment GSX-40.
b) 4000c. Parallel planar kink bands strongly developed. Experiment GSX-40.
c) 4000c. Kink band (shown by curved cleavage trace) with abundant recrystallised grains. Note dominant planar boundaries parallel to the kink band (horizontal) with shorter grain boundaries perpendicular to the kink band. Experiment GSX-40.
d) 4000c. Micro-cracks in curved cleavage traces outlining kink bands. Experiment GSX-34.
e) 400°c. Planar deformation bands parallel to (001) plane. Experiment GSX-46b.
f) 4000c. Rectangular subgrains with boundaries parallel to the <001> directions. Experiment GSX-46.
g) 400°c. Migrating kink band boundary. Experiment GSX-46b.
h) 400°c. Elongate recrystallised grains in old kink band. Experiment GSX-46.
4000c. SEM photograph of kink band with elongate sub- grains. Dislocation density is low. Experiment GSX-46. Cleavage fragment.
j) 400°c. SEM photograph of small recrystallised grains in kink band. Dislocation density increases towards triple points. Experiment GSX-46. Cleavage fragment.
120
•
H 220pH,
\ \+.
‘1'4Sk\ H 300p H
a ?Zs* • —\1.-
• t - • ., .6'•-• . .A,1411fii61 MIPPOSIMMININOBIONIR . 0" 7: • • ' • • . ;I),. ' .:„...... 1.: . -,.0.6.7.11.:.L....: • ,C• - ... 7..: 2 1.. : 1•::lavt .•41: 7..." ,;,....66:7, -...... : 1 . , : 4.; . . ... 1 . . . ..,' :: . .4 • : , ...... r.-••....1.V: • 1 . • ' 1 ... I, :.„ . . . . •••• - ,.,,,.... E F . 'a `'-`1'''.de•s‘ 121
(4 - lOpm) new grains are found to nucleate in these kink bands (Fig. 4.13j). o In some experiments at 300°c and 400 c, small patches of lead (10 - 50pm) were found along (100) cleavage planes. This indicates sulphur loss during the experiments. For this reason, Vickers Hardness measurements were not made on this series of experiments.
SCANNING ELECTRON MICROSCOPE STUDIES
Scanning electron microscopy of deformed specimens of Series 2 experiments revealed similar features to those found in Series 1 experiments. A detailed study was made of second-stage kinks which arise from glide poly- gonisation (Fig. 4.12b, c). Of considerable importance is the observation of tiny cracks in deformation bands (Fig. 4.12e). These cracks would allow relaxation of the von Mises criterion and enable plastic deformation to be achieved with less than five independent slip systems.
DISCUSSION
The stress-strain curves for Series 2 experiments show a dramatic decrease in CRSS for {110} <110> slip with both an increase in temperature and a decrease in strain rate. These results are discussed in greater detail in Section 4.8. The post yield portions of the stress-strain curves at low temperatures (20° - 100°c) show a dramatic decrease in stress with strain. This may be interpreted as geometric softening associated with {110} <110> slip and with kinking. As {110}<110> slip occurs, the crystal lattice rotates in the dir- ections indicated in Figures 4.1 and 4.2. As a consequence of this, the resolved shear stress on the softer {100} <110> slip system increases which allows 100 110 slip to operate at lower differential stresses. Kinking will also increase the resolved shear stress on the softer {100}410> system. Microstructural studies indicate that the geometric softening with strain is associated with the development of intense deformation zones with abundant kinks and deformation bands. Failure may occur along these zones. These zones constitute approximately 20% of the crystal and give rise to local strain rates which are approximately five times that of the bulk strain rate. At higher temperatures and lower strain rates, the amount of geometric softening is less. This is attributed to more homogeneous deformation at 122 these higher temperatures (200°c and above) and this is confirmed by the microstructural studies. At 300°c and 400°c a double yield point is observed. This may indicate a change in deformation mechanism and is dis- cussed in more detail in Section 4.8. Steady state flow is achieved at o -5 -1 300 c and above for strain rates of the order of 10 sec and at 200°c -7 -1 for strain rates of the order of 10 sec . The microstructures for Series 2 experiments' are similar to those observed in Series 1 experiments. {110}<1.10> was found to be the dominant slip system although in high strain zones {100} <110> slip also occurs as lattice rotation increased the resolved shear stress on this system. Kink- ing was found in all experiments up to 4000c but it was minor in some speci- mens (Table 4.6). Deformation bands were commonly found within broad kink bands. Small second stage kinks which result from glide polygonisation in deformation bands (Livingston 1962) were seen in single crystals deformed at temperatures up to 200°c. Dynamic recovery and dynamic recrystallisation microstructures were found at 200°c and are increasingly important at 300°c and at 400°c. Poly- gonisation and recrystallisation are initiated in kink bands. Elongate sub- grains and new grains commonly occur along old deformation bands within kink zones. Subgrain and grain boundaries are often observed to be sutured indicating grain boundary migration. New grains are nucleated at kink band boundaries (Fig. 4.13j). Similar dynamic recrystallisation features are found in hot-worked metals (Honeycombe 1968, Sellars 1978). Scanning electron microscope studies of deformed single crystals con- firmed the microstructural features observed in the optical microscope. Dislocation densities could not be accurately determined because of the pre- sence of sulphur vacancy etch pits (which were increasingly important at 3000c and above) and because sessile {100} <001> dislocations (Lyall and Paterson 1966, Mathews and Isebeck 1963) most likely contribute to the etch pit density. Detailed scanning electron microscope studies revealed small sub- grains and small new grains (Fig. 4.13i, j) in dynamically recrystallised material. These features were not observed during the optical studies. 441 twins were not observed in this series of experiments. Vickers Hardness testing was not carried out in Series 2 experiments because of the inhomogeneity of deformation at low - temperatures and because of the sulphur losses at 300°c and 4000c. 123
4.5 SERIES 3 EXPERIMENTS
This series of experiments was carried out to investigate the varia- tion of CRSS for {100}<110> slip with temperature. The experiments were also designed to study the development of microstructures in single crystals compressed parallel to the <111> axis. Cores of a single crystal of galena from Galena, Illinois (Table 4.2) -5 -1 were compressed parallel to the <111> axis at a strain rate 3.5 x 10 s and at . temperatures from 20°c to 4000c. As in all other experiments des- cribed in this thesis, the confining pressure was 1.5 Kbars (Table 4.1). For compression parallel to the <111>axis, the {110}<110> slip system experiences zero resolved shear stress (Fig. 4.1) whereas the {100}<110> system experiences a high resolved shear stress (Fig. 4.14). In this orient- ation slip will occur on six planes of the {100}<110> slip system (Fig. 4.14) which are all equally stressed.
RESULTS
The results for this series of experiments are given in Table 4.7.
STRESS-STRAIN CURVES
The stress-strain curves for compression parallel to the <111> axis are given in Figure 4.15. At low strains, the curves are markedly different from those for< 001). compression - (Fig. 4.8 - 4.11). With the exception of GSX-58, yielding occurred in all experiments at differential stresses below 212 bars. Examination of the Laue diffraction pattern for GSX-58 showed that the starting material was misoriented away from the <111> axis which thus accounts for its higher yield stress. All stress-strain curves exhibit double yield points in the region of 0.3% to 1.0% axial strain. Stresses measured at the upper yield point vary from 80 to 212 bars (excluding GSX-58, Table 4.6) and showed no discernable pattern of variation with temperature. For deformation at 20°c, the stress-strain curves exhibit rapid work hardening at axial strains greater than 1% (Fig. 4.15). The rate of work hardening decreases with increase in temperature with the result that, at 300°c and 4000c, the rate of work hardening is low and large strains are achieved at flow stresses below 200 bars.
124 SCHMID FACTOR FOR (001) <110> SLIP SYSTEM
Slip on (001) 001
100
LATTICE ROTATIONS FOR (001) (110) SLIP SYSTEM
001 010
100 FIGURE 4.14 Inverse pole figures showing Schmid factors and lattice rotations for {100}<110> slip. EXPERIMENT NO. TEMPERATURE STRAIN RATE TOTAL STRAIN YIELD STRESS AT 10% ANNEALING KINKING DYNAMIC STATIC o -1 STRESS STRAIN bars TIME (hours) SLIP FEATURES RECRYSTALLISATION RECRYSTALLISATION C s. % bars
GSX - 50 20°C 3.51 x 10-5 15.8 80 699 - 6.8 Abundant kinks - 2 sets one parallel to compression axis - other perpendicular -5 GSX - 54 20°C 3.04 x 10 16.3 98 705 - 6.2 - As above, also - - deformation . bands slip lines + GSX - 52 100°C 3.14 x 10 5 10.53 212 452 - 8.3 - Abundant broad - - kinks as 20°C
GSX - 58 100°C 3.33 x 10-5 25.7 331 587 - 64 - As above - + GSX - 59 100°C 4.34 x 10-5 19.7 102 464 - 44 - As above -
-5 GSX - 53 200°C 3.29 x 10 6.8 143 - - {100} slip lines Not recognised - in deformation slight polygoni- bands,kinks less sation obvious -5 GSX - 57 200°C 3.30 x 10 25.4 104 319 I 5.1 - As above Some boundary - migration 5 GSX - 55 300°C 3.27 x 10 7.2 150 - - Relict vertica Elongate grains - kinks Elongate in kink bands sub-grains Few perpendicular slip lines to compression 5 + GSX - 56 300°C 4.26 x 10 12.2 170 212 - 7.6 - Relict kinks As above -
-5 GSX - 51 400°C 3.81 x 10 8.8 102 93.9 - 4.4 - Slip lines at Large recrystal- - grain boundaries lised grains - and triple subgrains,sutured points boundaries
TABLE 4.7 SERIES 3 EXPERIMENTS SINGLE CRYSTALS OF GALENA COMPRESSED PARALLEL TO THE (Ill) AXIS ALL EXPERIMENTS WERE CONDUCTED AT A CONFINING PRESSURE OF 1.5 Kbars 126 1200
SINGLE CRYSTALS OF GALENA
DEFORMED IN COMPRESSION
1000 (111) ORIENTATION
STRAIN RATE 3.5 x 10-5 seel (average)
800 ARS
B GSX 58 251. N strain I ESS
TR 600 AL S GSX 59 • GSX 52 100°C 25% strain FFERENTI '-- GSX 57 DI
400
•
•GSX 56
,.--r•GSX 55 200
.--• GSX 51 . 400°C
15 20 STRAIN */.
FIGURE 4.15 Stress-strain curves for single crystals of galena compressed parallel to the <111> axis at temperatures from 20°C to 400°C and at a fixed confining pressure of 1.5 Kbars. 127
MICROSTRUCTURES
Crystals deformed at 200c have uniform barrel shapes which permit one to infer that three {100} slip planes operated during deformation. Abundant kink bands are observed in all specimens (Fig. 4.16a). Two sets of flame-like kinks are developed. One set is at a low angle to the com- pression direction and the other set occurs at a high angle to the compress- ion axis (Fig. 4.16a). {100}<110b slip features were observed in all specimens in this series of experiments. Deformation bands are found para- llel to (100) planes (Fig. 4.16b). Features similar to those described above for deformation at 20°c are also observed in specimens deformed at 1000c. For deformation at 2000c, {100}<110> slip lines were observed. A few flame-like kinks similar to those described above were also found but the kink boundaries were poorly defined. Several incomplete subgrains occur but, in general, polygonisation in these experiments (GSX-53 and GSX-57) was minor compared with Series 1 and 2 experiments at the same temperature. o o At 300 c and 400 c, recrystallisation occurred in relict kink bands in all specimens. Relict kink bands at a low angle to the compression dir- ection (cf. Fig. 4.16a) are developed at both 300°c and 4000c (Fig. 4.16c, d, e). Recrystallised grains with irregular boundaries (Fig. 4.16c) occur in broad kink bands. The new grains follow smaller kinks and deformation bands within the broad kinks (Fig. 4.16d). The older relict kinks are out- lined either by different shades of etching (Fig. 4.16c, d) or by clusters of slip lines (Fig. 4.16e). Small new grains are nucleated on kink band boundaries (Fig. 4.16f). Grain boundary migration (Fig. 4.16g) produces irregular and sutured grain boundaries. Slip lines are also found at grain boundaries (Fig. 4.16g). The new grains at 3000c are elongate with variable grain size from 50pm to 500pm. At 4000c the new grains can be up to 3-4 mm in size (Fig. 4.16i). Triple points are observed with slip lines concentrated at the triple junction (Fig. 4.16j). In general, the recrystallised grains at 4000c are larger and have more regular boundaries than at 3000c. Opened cleavages and microcracking were not observed in any experiments in this series.
SCANNING ELECTRON MICROSCOPE STUDIES
Studies of cleavage fragments in the scanning electron microscope confirmed that {100}410> was the main slip mechanism. Kinks similar to those in Section 4.3 were also observed. 128
Figure 4.16
Experimentally deformed single crystals of galena. Compression axis parallel to <111>. Confining pressure -5 -1 1.5 Kbars and strain rate approximately 3 x 10 sec . Photographs of polished sections etched 30 secs. 20% HBr.
a) 200c. Abundant thin kink bands. The near vertical bands are approximately parallel to the compression axis. Experiment GSX-50.
b) 200c. Deformation bands parallel to (100) direction. Slip lines are parallel to the deformation band. Experiment GSX-58.
c) 3000c. Recrystallised grains with irregular boundaries. Faint subgrains can also be seen. Vertical banding results from relict early kink bands (cf (a) ). Com- pression axis vertical. Experiment GSX-55.
d) 3000c. Recrystallised grains along horizontal kink bands - note lobate boundaries. Inclined banding effect is relict kinks sub-parallel to compression axis. Experiment GSX-56.
e) 300°c. Relict slip lines outlining vertical kink bands (sub-parallel to the compression axis). Note the irregular development of subgrain boundaries. Experiment GSX-56.
f) 3000c. Grain boundary with small new grains forming by grain boundary migration (lobate boundaries). Broad diffuse subgrains occur towards base of photograph. Experiment GSX-55.
g) 3000c. Migrating grain boundary. Note slip line traces around upper edge of boundary. Experiment GSX-56.
h) 3000c. Migrating grain boundary in relict kink band. Elongate grain shape 90' to compression axis. Experiment GSX-53.
i) 400°c. Elongate recrystallised grains approximately 80-90° to compression axis. Experiment GSX-51.
j) 4000c. Triple point with folded boundary and concen- tration of slip lines at triple point. Experiment GSX-51.
130
DISCUSSION
The stress-strain curves for single crystals of galena compressed parallel to the <111> axis are markedly different from those for crystals compressed parallel to the <001> axis. In all but one experiment (GSX-58), double yield points occurred below 220 bars differential stress. Above approximately 1% axial strain, rapid work hardening occurs for crystals deformed at 20°c. The rate of work hardening decreases with an increase in temperature. This can be interpreted to indicate that the dislocations no longer pile up in their slip planes (work hardening) but are able to climb over obstacles and out of their slip planes (i.e. recovery). The yield stresses show no recognisable pattern of variation with temperature and allowing for variations in initial crystals and in experimental conditions, it is likely that the CRSS for {100} <110> is not markedly affected by an increase in temperature. This is discussed in greater detail in Section 4.8. The microstructures found in Series 3 experiments are different from those in Series 1 and 2 experiments in that slip features and deformation bands are less obvious. In all experiments, the deformation was homogene- ously distributed in contrast to Series 1 and 2 experiments. Abundant kink bands were found in all crystals deformed at temperatures from 20° to 400°c. Two sets of kink bands are commonly developed, one at a low angle to the compression direction and the other at a high angle. The kinks are flame- like rather than the broad type in Series 1 and 2 experiments. Small second stage kinks were not observed. Deformation bands parallel to the (001) plane were found in the kink bands. Recovery and recrystallisation microstructures were not significant until deformation at 3000c. Irregular subgrains and new grains were developed at 3000c and 4000c. The grain boundaries are sutured indicating grain bound- ary migration. At 4000c the grain size is larger than at 3000c although the grain sizes are highly variable in both cases. At 4000c slip features are observed at grain boundaries. As in Series 1 and 2 experiments, recrystalli- sation is principally along kinks or along deformation bands within kink bands. The polgonisation and recrystallisation microstructures in this series of experiments are similar to those described from dynamically recrystallised metals (Honeycombe 1968, Higgins 1974). The precipitation of small patches of lead along cleavage traces was interpreted to indicate loss of sulphur from the galena at 3000c and 400°c. Therefore Vickers Hardness was not used as an indicator of recovery as it has been used by Stanton (1970) and Siemes (1976) (see Chapter 3.3 for a dis- cussion of non stoichiometry in galena). 131
4.6 SERIES 4 EXPERIMENTS
This series of experiments was designed to investigate the micro- structural changes on the annealing of cold worked single crystals of galena. In experiments GSX-60-63 (Table 4.6), single crystals of galena were com- pressed parallel to the <001> direction at 20°c and at strain rates between -5 -1 3.5 and 5.5 x 10 s , and at a fixed confining pressure of 1.5 Kbars. Experiments GSX-60 and GSX-61 were then annealed at temperature under con- fining pressure in the deformation rig. Experiments GSX-62 and GSX-63 were annealed in sealed jackets at 1.5 Kbars in a 'Tempres' hydrothermal system.
STRESS STRAIN CURVES
These are shown with those of Series 2 experiments in Fig. 4.8. The stress-strain curves cluster close together with an ultimate strength between 2.95 Kbar and 3.12 Kbar (Fig. 4.8) at 2-4% axial strain. Beyond the ultimate strength, the stress strain curves decrease rapidly until at strains between 6-10% brittle failure occurs.
MICROSTRUCTURES
The deformation microstructures of single crystals deformed at 20°c are similar to those described for Series 1 and Series 2 experiments at 20°c (Fig. 4.5 a-f and Fig. 4.9 a, b). Annealing at 1000c observes the deformation microstructures. Kink - bands are only revealed by an increase in etch pit density (Fig. 4.17a). The etch pits are larger and more prominent in the annealed material than in the unannealed material and this probably indicates sulphur loss during annealing. For deformation at 200c, deformation zones similar to those in Series 1 and 2 experiments are formed. After annealing at 2000c, slip lines, deformation bands and kinks within these zones are less well defined. Small recrystallised grains are found within the kink bands (Fig. 4.17b). These grains are found within the kink bands (Fig. 4.17b). These grains have either straight or slightly curved boundaries (Fig. 4.17b). The microstructures of cold worked single crystals of galena which have been annealed at 300°c and 400°c (expts GSX-62,63) show patchy develop- ment of recrystallised grains throughout the crystal (Fig. 4.17c, d). The new grains are larger than those found at 200°c and have either straight or smoothly curved boundaries commonly with precipitates of lead pinning the 132
Figure 4.17
Experimentally deformed galena. Single crystals deformed along <001> axis at 20E and at a strain rate of -5 -1 10 sec . Samples were then annealed for 10 days under a confining pressure of 1.5 Kbars and at the temperature specified. Series GSX 60-63.
a) 1000c. Curved kink bands revealed by an increase in etch pits (horizontal bands). The increase in etch pit density is also probably a result of an increase in sulphur vacancies.
b) 2000c. Deformation bands and slip lines preserved. Elongate recrystallised grains in kink band have straight - slightly curved grain boundaries.
c) 400°c. Large recrystallised grains with straight grain boundaries. Note patchy development of new grains with precipitates (probably lead) on the grain boundaries. There is a strong crystallo- graphic control of new grain boundaries - often parallel-sub-parallel to <001 >directions.
d) 4000c. New grains in old kink band. Note the pre- cipitates which inhibit growth. The grain boundaries are smooth and straight or slightly curved.
e) 4000c. A single new grain with straight-smoothly varying grain boundary. A polygonal network is seen within the new grain.
f) 4000c. Undeformed single crystal subjected to an annealing treatment as a control sample. Photomicro- graph of edge of sample where filing has induced parallel deformation bands (d) which are consumed by the new grain (n). Note the absence of deformation features in the new grain.
g) 4000c Scanning photomicrograph showing abundance of etch pits with (100) boundaries. A large number of these pits are probably sulphur vacancies as a result of annealing.
h) 400°c. Scanning photomicrograph showing triple points between new grains of differing orientations as revealed by the etch pit morphology.
All samples etched with Brebrick and Scanlon (1957) etchant as discussed in Appendix A.
134
grain boundaries (Fig. 4.17d, e). Some recrystallised grains occur along relict kink bands (Fig. 4.17d). Some new grains have an internal cellular substructure (Fig. 4.17e) which are probably vacancy collapse structures due to sulphur loss on annealing. One undeformed specimen was also annealed at 400°c. No new crystalli- sed grains were found in the undeformed part of the specimen. Several new * grains were found at the edges of the sample where filing had introduced slip lines and deformation bands (Fig. 4.17f). These deformation features are consumed by the new deformation free grains growing inwards from the specimen edges (Fig. 4.17f). Scanning electron microscope studies of the annealed specimens show abundant large etch pits (compare with those in Appendix A) which are most likely vacancies and vacancy collapse structures caused by sulphur loss during annealing (Fig. 4.17g). Triple points and grain boundaries are also revealed (Fig. 4.17h). The misorientation of the new grains can be seen from the dif- ferent etch pit structures in Figure 4.17h.
DISCUSSION
Previous studies of annealing in galena have been carried out by Lyall and Paterson (1966), Siemes (1976, 1977) and Clark et al. (1977). As discussed in Section 3.3, these experiments probably had significant sulphur losses during annealing. Indeed, even with the sealed small volume (i.e. small volume in addition to the specimen) jackets which were used in these experiments, sig- nificant sulphur loss can be inferred from the etch pit studies (Fig. 4.17g, h). The sulphur losses are expected to produce variations in Vickers Hardness which may not correspond with softening due to recovery (cf. measured by Siemes 1976, 1977) and to inhibit diffusion, particularly by producing lead precipitates which may pin grain boundaries and inhibit grain boundary migration. Annealing of cold worked (i.e. deformed at 20°c) galena single crystals produces microstructures and recrystallised grains which are markedly different from dynamic recovery and dynamic recrystallisation microstructures found in single crystals of galena deformed at 3000c and 400°c (Sections 4.3, 4.4). Slip lines and deformation bands become less well defined on annealing as dislocations climb out of their slip plane. Well developed subgrains are not found in the annealed single crystals. At 3000c and 4000c large equant new grains are patchily developed. These have smoothly curved or straight grain boundaries sometimes parallel to <001> directions and well developed triple points. Grain growth and grain boundary migration are inhibited by lead precipitates.
* in order to fit the specimen into the Tempres unit. 135
In contrast to these microstructures, dynamically recrystallised single crystals (Sections 4.3, 4.4.) have elongate grains with sutured grain boundaries. Clark et al. (1977) have found that these dynamically recrystal- lised grains are stable when subjected to further high temperature annealing. Annealing at 3000c after deformation at 3000c (Section 4.3) did not produce dramatic changes in the microstructure. Elongate grains with slightly sutured boundaries were found in the kink bands (Section 4.3). These contrast with the equant grain shapes found in the statically recrystal- lised cold worked samples used in this series of experiments.
4.7 SERIES 5 EXPERIMENTS
This series of experiments was designed to investigate the rate of recovery in single crystals of galena. The results of previous experiments described in this thesis (Sections 4.3 - 4.5) permit one to infer that recov- ery processes can operate at low temperatures ( 200°c) in galena. Stress relaxation tests may allow the study of recovery processes (i.e. climb of -9 -1 dislocations) at strain rates 10 s over shorter time periods than by other more conventional techniques. A series of stress relaxation experiments were conducted on single crystals compressed parallel to the <001> axis. The specimens were -5 -1 deformed at a constant strain rate of 4 x 10 s . After an arbitrary amount of strain is imposed, the specimen length is held constant and the applied stress is allowed to relax with time. Ideally, using an infinitely stiff testing machine, the elastic strain energy in the specimen is dissi- pated through permanent deformation of the specimen. The rheological chara- cteristics of the material determine the rate of stress relaxation. Details of the stress relaxation techniques are given by Rutter et al. (1978). Although only small strains are achieved during stress-relaxation tests (length held constant - small internal adjustments hence small strains), the -9 -1 tests have the advantage that slow strain rates 10 s can be achieved in a shorter space of time than by more conventional techniques (e.g.- constant strain rate tests). Provided that the structure of the material remains con- stant, stress relaxation tests can be used to obtain information about flow laws and dislocation dynamics (Rutter et al. 1978). The stress relaxation tests on single crystals of galena were designed to investigate low temperature, low strain rate recovery. The crystals were deformed to a specific strain (,.1%), the piston movement stopped and the applied stress allowed to relax over a period of approximately 1 week. The 136
load was then reapplied and the crystals were deformed a further small amount and then another relaxation test carried out. Relaxation tests were conducted after strains of 1, 2, 4, 6, 8, 10 and 15% strain at temperatures of 20°, 100°, 2000c and 300°c. Due to cali- bration difficulties the results have not been fully processed and the results are only briefly summarised. For all tests at 200c, a high residual stress ( 1.5 - 2 Kbars) remained even after stress relaxation for a week. For relaxation tests above 1% strain at 100°c, the residual stress decreased after relaxation to o lower values ( 0.7 - 1.0 Kbar). At 2000c and 300 c, and at strains greater than 1%, the stresses decrease very rapidly, upon relaxation, to values between 50 and 100 bars. These are estimates only because machine stiffness calibrations need to be taken into account. Examination of the deformed samples in polished sections showed only minor kinking and slip lines. These results permit one to infer that, at low temperatures (of about 2000c and perhaps below), recovery through dislocation climb occurs very rapidly in galena. This recovery is most likely achieved by core diffusion along the dislocation lines allowing edge dislocations to climb from their slip planes. These results are in agreement with those from Series 1 and 2 and 3 experi- ments.
4.8 CONCLUSIONS
In this section, the results of the experiments described in this thesis are compared with previous work on single crystals of galena (Lyall and Paterson 1966) and with studies of ceramic single crystals e.g., NaC1, Mg0 and LiF (Evans and Langdon 1976). In particular, detailed comparison is made with the properties of NaCl and MgO, both of which have a face centred cubic structure and have similar slip systems to galena. Lyall and Paterson (1966) have carried out the only other extensive study of single crystals of galena. They investigated the effects of orientation, confining pressure and aspect ratio on the compression of cubes of galena single crystals. All of their experiments were conducted at room temperature. The CRSS for f1001 <110> slip was found to be approximately 50 bars at room temperature whereas the CRSS for {110} <110> slip was greater than 500 bars. The results and conclusions from this study are sum- marised below. 137
PLASTIC FLOW OF SINGLE CRYSTALS OF LEAD SULPHIDE EFFECTS OF TEMPERATURE AND STRAIN RATE
The CRSS for {110} <110> and {100} <110> slip have been calculated from the results of Series 1, 2, 3 and 4 experiments on galena single crystals and have been plotted as log10 CRSS versus reciprocal temperature in Figure 4.18. It is seen from Figure 4.18 that {100} <110> slip shows little varia- tion of CRSS with temperature. All of the data points lie within 50 bars of each other and their scatter is most likely due to variations in the starting materials and to experimental errors. Lyall and Paterson (1966) estimated that the CRSS for {100} <110> slip in galena was approximately 50 bars at room temperature. The essentially athermal behaviour of {100} <110> CRSS found in this study can be interpreted to mean that dislocation motion is impeded by long range stress fields due to either large precipi- tate particles or to the presence of other dislocations on parallel slip planes (Evan and Langdon 1976). Similar behaviour is exhibited by NaC1 with a <100> loading axis (i.e. {110}410> slip) in the temperature range 200°K to 400°K (Evans and Langdon 1976). Although in this study, compression tests for {100} <110> -5 -1 slip have been conducted at one strain rate ( 3 x 10 s ), the athermal behaviour of the CRSS for other materials is found to be independent of strain rate (Fig. 21"Evans and Langdon 1976). In contrast to the above, the CRSS for {110} <110> slip in galena is markedly affected by an increase in temperature and by a decrease in strain rate (Fig. 4.18). Two regimes are evident from Figure 4.18, both of which are sensitive to thermal fluctuations. At temperatures up to 3000c log10 CRSS/temperature data can be fitted by a straight line. Above 3000c the date became less reliable but there is a marked drop in the CRSS for {110} o <110> slip with increased temperature above 300 c. For temperatures below 3000c a decrease in strain rate causes a decrease in CRSS (Fig. 4.18) although this effect is not as marked as that due to temperature. The strong influence of thermal fluctuations on CRSS in this region can be interpreted to mean that these fluctuations augment the effects of the external stress field and thus assist the dislocations in overcoming obstacles with short-range stress fields in their glide planes (e.g. solute atoms or the Peierls barrier; Evans and Langdon 1976). Similar effects have been found.for deformation of single crystals of NaC1 (Carter and Heard 1970)* and for Mg0 single crystals (Evans and Langdon 1976). In NaC1 and MgO, however, this behaviour occurs for the {100} <110> slip system 138
5- T 200 300 400 5000 6001 '800 00 11000101500 ' TEMPERATURE °K
FIGURE 4.18 CRSS for {110} and {100} slip plotted against temperature. 139 which is the easy slip system in these crystals. The difference between PbS and NaC1, Mg0 can be accounted for by the more covalent nature of the bonding in PbS (see Section 3.3). At temperatures of --3000c and above, there is a rapid decrease in the CRSS with increasing temperature. Although the data are not very reli- able at 4000c, it is possible-to infer from the trends of the data in Figure 4.18 that there is a change of mechanism around 300°c (or slightly below). In this regime diffusion processes are probably rate controlling.
YIELD POINTS
Lyall and Paterson (1966) found that the aspect ratios of the crystals deformed in the <001> orientation had a marked effect on the stress-strain curves for {110} <110> slip. In their experiments the length/area ratio varied from 0.35 - 1.0. In the experiments reported in this thesis the length/diameter ratios were in most cases 2.0. Small variations in this ratio could account for the differences in the yield portions of stress-strain curves for deformation at 200c (Fig. 4.8). Variability in stress-strain curves (particularly around the yield point) may also be introduced by varia- tions of favourable glide path lengths depending upon the crystal shape. In the tests reported in this thesis, however, most of the glide planes which may be activated should have similar glide path lengths. For <001> deformation an increase in temperature results in the yield points being better defined and at 300° and 400°c double yield points are observed (Fig. 4.9). Stress strain curves for <111> deformation have well defined double yield points (Fig. 4.15). The double yield points arise from the necessity to generate sources for mobile dislocations in the galena single crystals. It has been demonstrated for MgO, that in single crystals which contain mobile sources of dislocations (e.g. slip bands, dislocation half loops, introduced at the surface), yielding occurs without double yield points (Evans and Langdon 1976). For crystals which contain no mobile sources of dislocations yielding typically occurs with pronounced double yield points (Fig. 25, Evans and Langdon 1976). In these experiments, it appears that the impurity particles may act as the source of dislocations. The high shear stress needed to over- come the stress concentration around the impurity particle and generate dislocation loops, is relaxed once mobile dislocations are formed because these then allow rapid dislocation multiplication and flow to occur at a lower stress (Evans and Langdon 1976). Hence the double yield points. 140
The presence of impurities such as lead sulphosalts in the single crystals of galena used in these experiments has already been noted (Section 4.2). In addition the chemical analyses (Table 4.2) indicate the presence of copper, zinc, tin, arsenic and antimony in the galena lattice probably both as solute atoms and precipitates.
WORK SOFTENING
After yield, the stress-strain curves for <001> deformation at 20°c and 100°c show an initial period of work hardening which decreases with increased strain, Stage 1 parabolic hardening (Honeycombe 1968), until an ultimate strength is reached. Beyond this point geometric softening counter- acts work hardening and allows deformation to occur at lower stresses.
WORK HARDENING
Work hardening was most significant for compression parallel to the <111> axis (Fig. 4.15). Beyond the yield point, the stress-strain curves exhibit rapid work hardening, the rate of which decreases with increasing temperature. This type of behaviour, in which stage 1 easy glide (Honeycombe 1968) is suppressed, can be correlated.with a relatively high impurity con- tent. Solute or impurity atoms within the lattice impede the motion of dis- locations - solute hardening. Precipitation of second phase particles also impedes the motion of dislocations - precipitation hardening. The impurity levels in the single crystals of galena (Table 4.2) are interpreted to be sufficient to cause either precipitation or solute hardening or both. Simi- lar effects are noted for deformation of NaC1 and Mg0 single crystals (Evans and Langdon 1976). Lead sulphosalt precipitates (tetrahedrite group and others, Section 4.2) were found in the starting material but these were com- paratively rare. Some of the impurities may also be accounted for by solute atoms in the lattice.
MICROSTRUCTURES
SLIP
Both {11O} <110> and {100} <110> slip systems have been found to operate during experimental deformation of galena. From etch pit studies, it was not possible to distinguish between edge or screw dislocations. Nor 141
was it possible to obtain estimates of mobile dislocation densities because sessile {100} <001> dislocations also produce etch pits. Similar features were also observed by Lyall and Paterson (1966).
DEFORMATION BANDS
Deformation bands were found with boundaries commonly parallel to <001> directions. Deformation bands had either single or duplex slip (Fig. 4.19) with cross slip. There are both {100} 4110> and 0101 <110> slip lines in the deformation bands.
KINKING
Kink bands are found in all experiments. At low temperatures (below 200°c), broad kink bands with straight boundaries were found in single crystals compressed parallel to the <001> axis. Deformation bands are found within the. kink bands. In <001> compression two sets of interfering kink bands are common (Fig. 4.19). Two sets of flame like kink bands were found during com- pression of single crystals parallel to the <111> axis. A third type of kink band is one which results from glide polygonisation within intense deformation bands (Fig. 4.19). These kinks are small and only found in Series 1 and 2 experiments. All types of kink band initially have {l1O} boundaries but become misoriented with increasing strain. Detailed SEM studies show that the kink band boundaries can be either simple tilt walls of dislocations or a complex of elongate subgrains. No general angular relation- ship similar to that proposed by Starkey (1968) was found in these studies.
TWINNING
In contrast to the experiments of Lyall and Paterson (1966), no 441 or any other deformation twinning was found in the experiments reported in this study.
DYNAMIC RECOVERY AND RECRYSTALLISATION
Evidence was found for dynamic recovery and recrystallisation at 2000c -5 -1 o at strain rate approximately 10 s . At 200 c and at slower strain -7 -1 rates of 10 s , dynamic recrystallisation and recovery became more pro- minent, and was found in all experiments at 3000c and 4000c. These results
142
SYNOPSIS OF MICROTEXTURES
20°C 100 °C INTERFERING KINKS DEFORMATION BANDS
(110) Duplex Slip
50p,
200°C 200°C
GLIDE POLYGONISATION CURVED KINK BANDS
45ti
IIA _l_ _t. --:--_ 1 I ---t- I 1 _1---r 1\-k--, I fr— — — — _____--.. 1 „---__ 1 —' ,____i •----:-_...—1-. LI 1_7i—- -7:1 1— I-1 1------_. , — , ' . 7 1 --I 1 k.—.4cl- 1(10)1 ---T 1 __ I --11 (1 0 13)
300°C 300°C POLYGONISATION IN RECRYSTALLISATION IN KINKS KINKS
1504 Cleavage trace
•
FIGURE 4.19 Synopsis of microstructures found in experimental deformation of single crystals of galena. 143
are similar to those obtained by Salmon et al. (1974) for experimental deform- ation of coarse-grained polycrystalline galena. Dynamic recovery and recrystallisation in single crystals of galena produces elongate subgrains and elongate new grains. At 200° and 3000c these have sutured and serrated grain boundaries and commonly follow either deform- ation bands or kink bands (Fig. 4.19). New grains nucleate at kink band boundaries and grow by grain boundary migration. At 4000c, dynamic recry- stallisation occurs in kink bands. The new grains are elongate but tend to have more planar boundaries than at 300° and 2000c. Although a steady state stable grain size was not reached in these experiments, there is a broad co- rrelation of increasing recrystallised grain size with decreasing differential stress. A similar correlation was noted for subgrain sizes. In experiments described in this chapter, dynamic recrystallisation occurred in zones of intense deformation (e.g. kink bands and deformation bands) - high local strains, and this effect is particularly notable at 2000c and 3000c. One can infer that thare is a strain dependence for nucleation and recrystallisation in galena in a manner similar to that found in metals (Sellars 1978 and Chapter 3.1 and 3.2). At 400°c, fewer new grains are nuc- leated because the strain is more homogeneously distributed in the crystals than at lower temperatures (Sections 4.3 and 4.4). This also perhaps corre- lates with the proposed change in mechanism --300°c to diffusion controlled glide as discussed earlier. At high temperatures (>300°c) diffusion would tend to lower the stored strain energy due to deformation thus making it more difficult to nucleate and grow new grains. Consequently large grains and grain boundary migration will tend to be found at these temperatures.
STATIC RECOVERY AND RECRYSTALLISATION
Static recovery and recrystallisation were found at annealing temper- atures of 2000c and above. Similar results have been obtained by Clark et al. (1977), Siemes (1976, 1977) on polycrystalline galena but in their experiments convincing evidence for recrystallisation was usually found only at 3000c and above. Lyall and Paterson (1966) found recrystallisation in experimentally deformed single crystals of galena after annealing at 4000c. Equant sub-grains due to static recovery were only poorly developed in these experiments. The static recrystallised grains are-markedly different to those formed by dynamic recrystallisation. Annealing produced equant grains with straight or curved smooth boundaries commonly with well developed triple points. Foam type mosaics were formed at high temperatures. Nucleation and 144 grain growth, although occurring dominantly in kink bands, also occurred outside highly deformed areas indicating a lower critical strain for anneal- ing recrystallisation than for dynamic recrystallisation (cf. metals, Sellars 1978). Some elongate grains with straight grain boundaries are formed at 2000c but at higher temperatures equant grains dominate.
GEOLOGICAL SIGNIFICANCE OF EXPERIMENTAL DEFORMATION OF GALENA
The values for CRSS for the slip systems in galena obtained in these experiments have been used in Chapter 5 as starting data for simulation of crystallographic preferred orientations in galena. The stress-strain data confirm the theoretical predictions of Atkinson (1976b, 1977). The import- ance of diffusion (probably diffusion along dislocation cores, cf Atkinson 1977) during deformation and recovery at 200°c and above at laboratory strain rates, may be extrapolated to lower temperatures under geological strain rates. As described above the microstructures resulting from the various com- binations of deformation and annealing processes in this study are notably different and can easily be distinguished. Low temperature deformation microstructures are annealed although the new grains do follow deformation features. Dynamic recrystallisation microstructures are relatively stable to annealing although some relaxation and straightening of grain boundaries is found. High temperature deformation microstructures can be distinguished from lower temperature microstructures. Clark et al. (1977) obtained similar results for the behaviour of polycrystalline galena. From the results of these experiments, it is possible to infer that galena may preserve features indicative of the deformation conditions even after post deformation annealing. It is possible, therefore, to use the microstructures found in naturally deformed galena from low grade metamorphic environments to establish the deformation and post deformation conditions. In Chapters 6, 7 and particularly Chapter 8, the results of these studies are used to interpret the microstructures found in naturally deformed galena from the low grade metamorphic environments. 145
CHAPTER 5 CRYSTALLOGRAPHIC PREFERRED ORIENTATIONS
5.1 INTRODUCTION
This chapter briefly reviews the development of crystallographic preferred orientations in polycrystalline aggregates. It is beyond the scope of this thesis to give a detailed analysis of the development of preferred orientations and the reader is referred to reviews by Turner and Weiss (1963), Barrett and Massalski (1966), Dillamore and Roberts (1965), Hu et al. (1970), Lister (1974), Spiers (1975), Nicholas and Poirier (1976) and Lister et al. (1978) for greater detail. In the petrological context the term 'fabric' denotes both preferred crystallographic orientations in populations of crystal grains (metallurgists call this 'texture') and also the shapes and mutual dispositions of matrix grains and subsidary phases (what metallurgists call morphology). In this thesis, the term texture will be restricted to crystallographic preferred orientations and the terms microstructure and shape fabric will be used to describe the morphology of the mineral phases. The metallurgical interest in textures stems primarily from the necessity to describe the material anisotropy arising from processing operations. This anisotropy may have important strength or magnetic characteristics and, as such, needs to be understood and adequately described. In metallurgical deformations, the deformation history (temperature, strain rate, strain path) are known, whereas in geological situations this is not generally the case. Lister (1974) has discussed the development of textures in great detail and it would appear that deformation textures in rocks and minerals may hold important clues as to the deformation history of a particular rock mass. The study of deformation textures may permit one to make inferences about: (1) temperature and strain rate of deformation; (2) nature of the total deformation and the characteristics of the principal strain axes and (3) the deformation path. The influence of the deformation path on texture development has only recently been recognised because of the research of Lister (1974) and is likely to be an extremely important factor in textures of rock masses. In this thesis, crystallographic preferred orientations are used principally as a tool to investigate the deformation conditions and deformation mechanisms that operated during the plastic deformation of sulphides. 146
Usually, insufficient information is available to enable the deformation paths to be investigated.
DESCRIPTION AND REPRESENTATION OF TEXTURES
Metallurgists have traditionally described textures in metals in terms of pole figures of an ideal orientation usually related to stress directions associated with the texture forming process (eg. wire drawing and sheet rolling)(Dillamore and Roberts 1965). In uniaxial textures such as those found in extension and compression, metallurgists specify which crystallographic axes lie parallel to the principal stress directions or directions of flow (Barrett and Massalski 1966). For sheet textures, the ideal orientation is given by the rolling plane and a line parallel to the rolling direction (Dillamore and Roberts 1965). This type of description does not completely specify all components of a texture and becomes extremely cumbersome for textures of low symmetry minerals. It is also inadequate to describe textures which result from complex deformation geometries. A more powerful method of describing textures in aggregates is that of the orientation distribution function (ODF); (gunge 1969, Siemes 1977b, Baker et al. 1969), in which the specimen axes are related to the crystallite axes by three Euler angles thus allowing the three representative crystallographic axes of a given crystal to be represented by a single point in space. Lister (1974) and Nicholas and Poirier (1976) have discussed the advantages and disadvantages of representing textures by ODF's. In particular, for low symmetry minerals a large amount of data is necessary to compute an ODF and even then there is difficulty in visualizing the preferred orientation. In this thesis, textures of sulphide minerals will be represented by conventional pole figures and partial pole figures, and in general, the intensity maxima are related to principal stress/strain directions.
PREFERRED ORIENTATIONS IN GEOLOGICAL MATERIALS
The geometric relationships between textures, shape fabrics and geological structures have long been recognised (see reviews by Turner and Weiss 1963, Friedman and Sowers 1970, Spiers 1975, Nicholas and Poirier 1976). Sander (1930) established the basis for modern geological texture studies and in particular he emphasized the importance of symmetry relationships between the deformation system and the resultant textures. It is beyond the scope of this chapter to give a detailed review of crystallographic preferred orientations in geological materials but it is pertinent to mention several relatively new and important developments. 147
Improved techniques for x-ray measurement of textures (Siddans 1971, 1976, Siemes 1977b) have greatly facilitated texture measurements. Computer handling of data and improved mathematical descriptions of preferred orientations (inverse pole figures, orientation distribution functions, Baker et al. 1969, Bunge 1969) has greatly improved the production of meaningful texture data. These techniques have been successfully applied to the study of deformed metals but care must be taken in their application to low symmetry minerals (Nicholas and Poirier 1976, Lister 1974). In the 1970's advances in experimental deformation have increased our understanding of the development of preferred orientations. For example the work of Tullis (1977) and Schmidt (1976) has provided much information on the development of textures in quartzites and calcite rocks respectively. In the main, these experiments have been either axisymmetric extension or compression but Kern (1977) has conducted triaxial experimental tests. Rutter and Rusbridge (1977) have made a most valuable contribution by demonstrating the effects of non coaxial strain paths on texture development in marbles. Studies of naturally deformed rocks (Carreras et al. 1977, Bouchez 1977) have advanced since the early work of Sander (1930) particularly through the use of High Voltage Transmission Electron Microscopy (eg. White 1976, 1977). The development of preferred orientations are usually related to the deformation mechanisms and the environment of deformation. Natural tectonites are usually compared with experimentally produced textures. One particularly significant development in the understanding of textures in rocks has been the extension of the Taylor-Bishop-Hill analysis for textures in metals by Lister (1974) to the study of deformation textures in low symmetry minerals. This has allowed the simulation of deformation textures of quartzites, calcite rocks and (in this thesis) polycrystalline galena. Lister's work has made an important impact on the study of textures in rocks. In particular, for the first time, the complex effects of deformation paths and non coaxial strain paths can be demonstrated. It may therefore be possible to deduce the strain paths as well as deformation mechanisms and environment of deformation from the textures of naturally deformed rocks. In spite of these significant advances, problems remain. Computer simulations cannot yet adequately handle twinning and kinking nor do they take climb recovery and recrystallization into account. Furthermore, the simulations also do not allow for strain and stress inhomogeneity during the 148 development of textures, but the most important lack is that there is no satisfactory theoretical treatment of the chemical reactions which occur during geological deformations. These reactions are particularly important during the prograde metamorphism of phyllosilicates and hence the development of preferred orientations in phyllosilicates has yet to be fully explained.
PREFERRED ORIENTATIONS IN SULPHIDES.
Systematic studies of preferred orientations in experimentally and naturally deformed ore minerals have been carried out by Siemes (1970) and his co-workers (see review by Siemes 1977b). This research group has mainly used X-ray texture goniometry to determine preferred orientations. Larson (1973) used the reflectance anisotropy of pyrrhotite to determine preferred orientations in pollycrystalline pyrrhotite. Siemes (1977b) has presented a detailed review of the determination of preferred orientations in sulphides. Apart from the development of preferred orientations in sheared sulphides and in axisymmetric compression experiments (Siemes 1977b) texture studies of deformed ores have not been very fruitful. In this study, textures of naturally deformed sulphides are compared with computer simulations which have been generated using the programmes of Lister (1974).
5.2 THE THEORY OF TEXTURE DEVELOPMENT AND THE BASIS OF TEXTURE SIMULATIONS
An understanding of the physical basis of the development of deformation textures during dislocation glide may be gained by considering the behaviour of a single crystal during deformation. During extension, the slip plane normal rotates away from the extension axis whereas during compression the slip plane normal rotates towards the compression axis (fig. 3.2). Lister (1974) has discussed the theories of texture development in detail. In brief two groups of texture theories have developed. The first analyses the behaviour of isolated grains without considering grain to grain interactions (lower bound theory, Dillamore and Katoh 1971) and the second assumes homogeneous deformation to maintain material continuity (upper bound theory, Dillamore and Katoh 1971, 1974). The theories which consider the deformation of an isolated grain calculate the slip rotations arising from slip on one or more glide systems assuming an imposed stress field. Various models have been proposed (reviewed by Lister 1974) the most widely used being the 149
Calnan and Clews model (1950, 1951). In all of these models deformation takes place with a heterogeneous distribution of strain from grain to grain. The Calnan and Clews model is useful for predicting slip rotations in experimental deformation of single crystals (cf. fig. 4.14). The most widely used model for textures in poly-crystalline aggregates is the Taylor-Bishop-Hill analysis (Taylor 1938, Bishop and Hill 1951). This theory considers that each crystal suffers the same shape change as all other crystals, thus satisfying the material continuity condition. This model has been examined in detail by Lister (1974) and Lister et al. (1978). The model makes the following assumptions, 1) The deformation takes place solely by dislocation glide and that thiscan be treated in terms of the simultaneous operation of a number of discrete glide systems; 2) The deformation is homogeneous throughout the polycrystalline mass and each grain undergoes the same strain; 3) The material obeys a rigid-plastic flow low in terms of the resolved shear stresses and the strains in each glide system (Lister et al. 1978). For rigid-plastic deformation, the deformation is controlled by a yield surface which determines the allowed stress-states (Lister et al. 1978). The combinations of glide systems on the yield surface (usually 5 to satisfy the von Mises criterion) a calculated by the extremum principles (minimum internal work, Taylor (1938); maximum external work, Bishop and Hill (1951)). With these assumptions, it is possible to compute theoretical textures using many combinations of slip systems and imposed strain states. This has been successfully applied to the study of deformation textures in metals (Dillamore and Katoh 1974) and has been successfully extended to studies of deformation textures in low symmetry minerals (Lister 1974, Lister et al. 1978). In section 5.4 the results of Taylor-Bishop-Hill simulations of galena textures are presented. The limitations of texture simulations are discussed in section 5.5.
5.3 MEASUREMENT OF PREFERRED ORIENTATIONS
X-ray texture pole figures presented in this thesis were measured either on the texture goniometer at Leeds University (Siddans 1971, 1976) or at RWTH Aachen on the Lucke automatic pole figure goniometer (Siemes 1977b). Details of the methods and the correction factors for X-ray texture goniometry are given by von Gehlen (1960) and Siddans (1971). For pole figures measured by both instruments mentioned above, data was collected at 5° intervals along small circles. The reader is directed to 150
the above papers for greater detail but it is appropriate to mention some of the problems associated with the pole figure determinations for sulphides. Reflection mode pole figures were usually measured out to the 60 or 70° small circle. Beyond this, the high absorption factor for sulphide minerals (von Gehlen 1960) makes the X-ray intensity fall off dramatically away from the centre of the pole figures. Similarly, the high linear absorption of the sulphide minerals (von Gehlen 1960) makes it almost impossible to use the transmission mode of X-ray texture goniometry (Siddans 1976) except for ultra thin (10pm) sections. This difficulty with transmission mode may be illustrated by the distorted pole figures found in the study of the Rammelsberg ores (figs: 6.15, 17), particularly when transmission mode and reflection mode scans are matched together (fig. 6.17). This can only be satisfactorily achieved by matching over the same preferred orientation peaks (Dillamore and Roberts 1965). Many sulphide minerals have interfering diffraction peaks as illustrated in figure 5.1. This makes preferred orientation studies of mixed sulphide ores very difficult. Finally there is the problem of grain size inhomogeneity. Large grains may reflect a disproportionaly large number of X-rays so that the final pole figures have many isolated and sharp maxima (eg. fig. 6.14). These peaks may swamp the preferred orientations of the many small grains.
5.4 SIMULATIONS OF TEXTURES IN GALENA
Theoretical textures for flattening, plane strain and simple shear deformation of galena were calculated by Lister (Leiden) using a Taylor- Bishop-Hill analysis (Lister 1974, Lister et al. 1978). Although the ratios of CRSS for C1101 <110> and {100} <110> slip vary from approximately 25:1 at 20°C to 1:1 at approximately 300°C, it was only possible to obtain results for simulations using a 1:1 ratio of CRSS for slip in galena. Variations in the ratios for CRSS in galena are unlikely to affect the final textures but may affect the rates of rotations of the grains towards the final texture. The inverse rotation diagram for slip in galena (fig. 5.2) calculated using the Taylor-Bishop-Hill analysis is the same as that calculated by Siemes (1970, 1974). The diagram indicates the directions of lattice rotations. All rotations are away from the [111] direction but no end position is indicated.
151
C11 IN
0 0
O
O0
3 O CV 0 1,4 ■ .4 O ■ O 0 ■ O 0
I 1 1 0 60 55 I I t I F 1 1 1.5 40 35 30 25 29 Cu Ka. RADIATION
GALENA CHALCOPYR I TE PYRRHOTITE
SPHALERITE PYRITE
FIGURE 5.1 Theoretical diffraction peaks with relative intensities for some common sulphide minerals. CRITICAL SHEAR STRESSES .GALENA (1001)011 1.000 (11011110 1.000
FIGURE 5.2 Inverse Rotation Diagram for axisymmetric compression of galena. 153
TEXTURE SIMULATIONS
The results of the simulations of textures in galena are given in figures 5.3 - 5.8. The flattening and plane strain deformations are coaxial and irrotational: with the principal stress axes corresponding to the principal strain axes (X is the axis of extension; Y is the axis of intermediate strain; Z is the axis of shortening). One hundred grains with a random orientation distribution are used in each simulation and the deformation gradients (Means 1976, Lister 1974) are given in the figures. The pole figures are plots of the relevant crystal axes with the lines attached to each point indicating the directions of rotation. In the inverse pole figures, the relevant strain axes and their rotation directions are plotted relative to the standard triangle of the projection of a cubic crystal. For each deformation path two simulations, one at 22% strain and the other at 64% strain are given.
FLATTENING
The texture simulations for flattening deformations of galena (axisymmetric shortening) are presented in figures 5.3 and 5.4. At 22.62% strain, the preferred orientations are poorly developed. The inverse pole figure (fig. 5.3) of the shortening axes shows a tendency towards a [110] concentration. The rates of rotation indicated by lengths of the lines attached to each point are low (short lines). At 64.15% strain the preferred orientations are stronger than at 22% strain although they are still weak relative to those for quartz and calcite (Lister 1974, Lister et al. 1978). There is a tendency for the [110] axes to line up parallel to the axis of compression (Z axis).
PLANE STRAIN
After 22% plane strain deformation of galena there is no marked preferred orientation developed (fig. 5.5). At 64% strain (fig. 5.6) preferred orientations are moderately developed. Symmetrical point maxima concentrations are produced. There is no singular alignment of any one crystallographic axis with any one strain axis. The [001] axes tend to concentrate towards the X (extension) axis whereas the [110] axes align towards the Z (shortening) axis. From the inverse pole figures (fig. 5.6) one can see that no single maxima are developed.
• 154 TEXTURE SIMULATIONS GALENA PbS FLATTENING
111 4. 4,_. • ... ' .. • • • SLIP SYSTEMS {100i <110) • • ..„ 4 •• . .• a . . '4 .,••• 1. •. .... i1 <110> ..• _ . •4 • ..- 4 0-. • • • s • • • • • • : •Ie. 4 • V*. ' OW ... !it . DEFORMATION GRADIENT • •• • • • Sop • it, . .. • • Ili •,„ I.• •.**6 •• ••• •• ••• • 1025 0.0 00 --. ••• Lb C • .#'0% ; 4 . ••• ••••• 0 • II 4fdle:• • ..064 . I•• 0.0 1025 0 0 t • . • •• • • • I It lib a . e. :, .• • •••• - • » 0 el 4 . t 0.o 0 0 095 . 4. , • • • •.• • - •a, .- ' . • • •• • 100 GRAINS 5 INCREMENTS ♦ V/ lio .• • TOTAL STRAIN 22 62% 4 •••
POLE FIGURES
110
INVERSE POLE FIGURES 001 110 001 110 0e1 110
• • •
• • lei,'
X 11
FIGURE 5.3 155
TEXTURE SIMULATIONS GALENA PbS
FLATTENING SLIP SYSTEMS {100 <110> {110} do> DEFORMATION GRADIENT 1025 0.0 001 0.0 1025 0.0 0.0 0.0 0.95 100 GRAINS 20 INCREMENTS TOTAL STRAIN 64.15% POLE FIGURES
INVERSE POLE FIGURES 001 110 001 110 001 110
X 111 111 11
FIGURE 5.4 156
TEXTURE SIMULATIONS GALENA PbS PLANE STRAIN
SLIP SYSTEMS {100} <110> 1110k1I0>
DEFORMATION GRADIENT 1.05 0.0 0.0 0.0 1-0 0.0 0-0 0.0 0.95
100 GRAINS 5 INCREMENTS
TOTAL STRAIN 22.62 °l° POLE FIGURES
y
INVERSE POLE FIGURES
001 110 001 110 001 110
FIGURE 5.5
157 TEXTURE SIMULATIONS GALENA PbS PLANE STRAIN
SLIP SYSTEMS {100} <110>
{110} <110>
DEFORMATION GRADIENT
1.05 0.0 0.0 0-0 1.0 0.0 0.0 0.0 0.95
100 GRAINS 20 INCREMENTS TOTAL STRAIN 64.15% POLE FIGURES
tig • Por#,..1.:. .:1 lf •••le - g Ii, •Par ;#• • .•, ..4 3%414.11 % . a% t11 /4 4 • i •V ' 1 te,„," l• .•• • 4 " •• t. j •4 , C b Iv b.: • . ..i .e Niw:lieek i•,.... ee a e - 4 • • 4•1• ..,.. ...e. •. 11% •legrY. .. . %,.1: 1..-- t:i wr• • 0 4 sv PP I `41 - f* ti• .1 g',..4 .. to •••7••• 1 " 4 a. :4 PP•ia, Volt :44■1 • • • # 4 •1 4 .. • Ilt de° o
INVERSE POLE FIGURES
FIGURE 5.6 158
SIMPLE SHEAR
The simple shear simulations are plotted relative to the flow direction and the flow plane because the deformation is rotational and the stress axes do not coincide with the strain axes. In contrast to the small rotations associated with the flattening and plane strain deformations (figs. 5.3 - 5.6) simple shear has large circular rotations (figs. 5.7, 5.8). At low strains (0.5 Y fig. 5.7) no strong preferred orientations are indicated. All of the rotation is concentrated in the flow direction or normal to the flow plane (fig. 5.7). At higher strains (3.0 Y fig. 5.8). Slight preferred orientations are developed notably with the [001] axis normal to the flow plane (fig. 5.8).
5.5 DISCUSSION AND CONCLUSIONS
In this chapter the deformation textures produced by dislocation glide have been discussed. Insufficient detail is known about the effects of recrystallization on texture development in sulphides and as such cannot be evaluated. The Taylor-Bishop-Hill model has been successfully used to simulate deformation textures in metals (Dillamore and Katoh 1974) and has been applied to textures in quartzites (Lister et al. 1978). This model, however, suffers from a number of limitations in that it cannot adequately handle twinning nor does it take into account kinking, inhomogeneous strain or dislocation climb. Ball and White (1978) argue that climb is an important factor in the development of textures but Lister (pers. com.) maintains that climb will only make the fabrics more diffuse. Taylor-Bishop-Hill simulations of texture development in polycrystalline galena show that deformation textures arising from dislocation glide alone develop slowly (cf. more slowly than those in metals (Dillamore and Katoh 1971) and more slowly than those developed in marble (Rutter and Rusbridge 1977)). For flattening of galena the [110] axis is aligned parallel to the compression axis. This texture is found in axial compression experiments on galena (Siemes 1970) and on halite (Kern 1977). Plane strain deformation simulations show point maxima textures which are not the same as the girdle fabrics found in the plane strain experiments on halite (Kern 1977)(nb. halite has the same slip systems as galena and as such should develop the same deformation textures). Siffiple shear simulations or galena give a texture with [110] axes normal to the shear plane. Siemes and Schachner-Korn (1965) found 100 axes nearly parallel to the shear direction or [100] axes normal to the shear plane. 159
TEXTURE SIMULATIONS GALENA PbS
111 SIMPLE SHEAR SLIP SYSTEMS {100}<110> {110}<110>
DEFORMATION GRADIENT 1.0 0.1 0.0 1 L 0 0 1.0 0.0 [0 0 00 t0 100 GRAINS 5 INCREMENTS TOTAL STRAIN 0-5 'Y POLE FIGURES 110 1 1
INVERSE POLE FIGURES 001 110 001 110 001 110
111 FLOW DIRECTION 111 NEUTRAL AXIS 111 NORMAL TO FLOW PLANE
FIGURE 5.7
160
TEXTURE SIMULATIONS GALENA PbS
111 SIMPLE SHEAR SLIP SYSTEMS {100}<110) {110} DEFORMATION GRADIENT 1-0 0.1 0.0 1 0.0 1-0 0.0 0.0 0-0 1.0 100 GRAINS 30 INCREMENTS FLOW PLANE TOTAL STRAIN 30 Y POLE FIGURES 110 .• 4.• 14 • • ka a • • • •.• •• a • • • r. I • •• •• • • • b t • No .1. • •• • • • I' • • % • 1 • • • b. • • •• CO ,•• 1. • %.**.t.• • •• •• • •-• •• FLOW PLANE FLOW PLANE INVERSE POLE FIGURES 001 110 001 110 001 110 111 FLOW DIRECTION 111 NEUTRAL AXIS 111 NORMAL TO FLOW PLANE FIGURE 5.8 161 The development of deformation textures in galena with increasing strain has important implications for the preservation of textures in naturally deformed galena. Siemes (1970) found that compression textures developed after 15% strain and were well developed at 40% strain. The textures simulated by computer only develop at higher strains. It is proposed that the large and rapid rotations associated with kinking make a significant contribution to deformation textures in naturally deformed galena. This postulate is examined in greater detail in chapter 8 where the textures of naturally sheared galena are examined. From the computer simulations it is possible to infer that the deformation textures in naturally deformed galena ores (grain size <500pm and where kinking is minor) need at least 22% strain (most likely 50 - 60% strain) to develop and as such are likely to reflect the last stages of the strain history although it is unlikely that earlier deformation textures would be completely destroyed by subsequent deformations (the effect of deformation path as discussed by Lister 1974). 162 CHAPTER 6 THE RAMMELSBERG MINE, WEST GERMANY 6.1 INTRODUCTION The Rammelsberg mine, West Germany (fig. 6.1, 6.2) has a record of mining operations dating from AD 968 to the present day, making it Europe's oldest operating mine. The sulphide and barite deposit is stratiform and has been deformed under lower greenschist facies meta- morphic conditions (Ramdohr 1953a, Kraume 1955). The ores are characteristically massive fine-grained sulphides and they show abundant deformation textures. For these two reasons, they were chosen as a first study of microscopic deformation textures and also as a subject for an evaluation of the use of X-ray texture goniometry on a fine-grained sulphide ore. Specimens were obtained from Mr. J. McM. Moore and the Kraume collection of the Mining Geology Museum, Imperial College. Dr. T. Hopwood also provided several specimens from his visit to the mine. The samples studied came from the 9th, 10th and 11th levels in the New Ore-body. In spite of numerous attempts, it was not possible to visit the mine and therefore this chapter relies upon the published work of Kraume (1955), Ramdohr (1953a), Moore (1971), and Schot (1971, 1973) for field and macroscopic observations of deformation and metamorphism. Detailed mineralogical studies of the Rammelsberg ores have been carried out by Ramdohr (1953a) and Schot (1973). Little attention, however, has been paid to the study of the deformation textures in the ores and recent studies of experimental deformation of sulphides (Atkinson 1974,1976a and McClay and Atkinson 1977) permit a more detailed analysis of sulphide deformation than was previously possible. Moore (1971) and Ramdohr (1953a) have both described deformation textures in the Rammelsberg ores and they recognised that the ores have been recrystallised. Moore (1971) ascribed the recrystallisation to a post deformational annealing event. Moore (1971) discussed the folding in the orebodies in terms of flexural flow folding followed by homogeneous flattening. Anger et al. (1966) investigated the isotopic composition of the ores and concluded that the copper, lead and zinc sulphides and some of the pyrite were formed from submarine magmatic hydrothermal springs. The remainder of the pyrite (approx. 50%) and the barite were thought to have formed by bacterial reduction of sea water sulphate. 163 A detailed microstructural and textural study of the ores have been carried out. X-ray texture goniometry, scanning electron microscopy, X-ray diffraction and optical microscopical techniques have been used. 6.2 THE GEOLOGICAL SETTING OF THE OREBODIES The Rammelsberg copper-lead-zinc sulphide deposit is one of a number of stratabound deposits found in the Hercynian geosynclinal belt of Central and Eastern Europe (fig. 6.1) (Krautner 1970). The Rammelsberg orebodies occur in a sequence of Devonian shales, greywackes and volcanics near the Harz mountains, West Germany (fig. 6.2). The geology of the mine area has been described in detail by Kraume (1955) and summarised by Anger et al. (1966), Moore (1971), and Schot (1971, 1973), and therefore only a brief resume will be given here. The deposit consists of three independent and isolated lenticular ore- bodies - the Old Orebody in the south west - the New Orebody in the north east - and the Grey Orebody (the barite orebody) lying between the Old and the New orebodies. The sulphide orebodies lie towards the base of a Middle Devonian sequence of shales, calcareous shales and sandstones. Subsequent to deposition, the ores have been deformed and metamorphosed under lower greenschist facies conditions (Ramdohr 1953a, Kraume 1955). STRATIGRAPHY Stratigraphically, the ore-bodies lie in the Middle Devonian Wissenbach Shale about 250m. above the base of the Middle Devonian (fig. 6.2). Table 6.1 summarises the stratigraphy and the post depositional events in the mine sequence. The orebodies parallel the strike of the host rocks and are largely concordant with them. Schot (1971, 1973) states that the observed discordancies are tectonic in origin. Devonian fossils and microfossils have been found in the ores (Kraume 1955, and Schot 1973). STRUCTURE The regional structure has been described by Kraume (1955) and summarised by Schot (1971, 1973). The orebodies lie in the overturned limb of a nearly isoclinal and overturned syncline, the axial plane of which dips south east and strikes south west - north east (fig. 6.2b). ~ -. -:'-o~,~""':- ...., ,..l.- • 0 0 0: 'V. . " Q~I' N .,..s.-- 0 •• •• o· • 0 0 0 • 0 : o~ YO· '0 0 • • • • • • • • • • • 0 • • • • ~ •• •• 0 O· 0 • 0 .' ••• •• '. 0 0 0 0 0 : 0 0 • 0.' • • y.. 0 ~ 0 0 O. ~ • _ ..:.-:....J.- _ _"_~ ~ ••• Vro .,...... · ...... '. .. 60. -,:::~-- -- -,~,~"" ~. . . A'. • :. :A) __ "", ,\. • .•.~r. : ...... · ·:~\"\ ." ..--~~.... ~ ,\ ,. '. • · • ..• • 0 ....• • -yi"R8 O? - _~~L\.(\. ,1-\ // \\\ \ '"'".. • • .• ~" . ' 0.:.... "i_"" .. ~ /.~ II "" ...... J1. • ° • • • 1/, "" J JI' '':...... ~/, • ••••or /I~",,,, ~ ~ ~II ~ ...• ,"'.:/ • W(.- v.,.,. /~ C!5 , I 0, 0' .:; Bohemian mts. I,.:,:.:: _ ~ '~'Oil" -_-:::.-:..-::..-~ __ ...... _ .... -=- --...... ~ ...... ' ...... , b ~""'~O ~~, o, 200, 400, Carpathian mts. \\', \ km ) I , ,-..... /, '-, ,_- ..... ~ I I-I Fe oxide,carbonate, c'..--,.,..- .....'l,.-_ ...... // sulphide ores ..... '" mPb,Zn,Cu sulphides .>' >' Southern limit Old Red Facies I:~·:.I Hercynian Miogeosyncline 1=--:.' Hercynian Eugeosyncline HERCYNIAN GEOSYNCLINE & STRATABOUND ORES OF CENTRAL EUROPE (after Krautner,1970) L=Lahn Dill, M=Meggen, R=Rammelsberg, GK=Gesenke, RO= Rodna, G=Gemeriden , P= Poiana Rusca. .-.. en FIGURE 601 -I=' 165 RAMMELSBERG REGIONAL GEOLOGY MITTEL-SCHULENBERG LOWER DEV3HLAN MIDDLE DEVONIAN UPPER DEVONIAN +++ OILER GRANITE (¢11.4. Kroare 1911) RAMMELSBERG CROSS SECTION SCHAUE • RAMMELSBERG ANTICLINE „ - RAMMELSBERG `, OILER VALLEY 600- SYNCLINE - 600 \ , 400 .. ••\.s 114 . 400 t% • :1a. ± 0 1 \ Ill 200-°0 0 . s.,s. .„0 l 1441 ,: +++ - 200 O o 01* 0 N. oil m 0 - oo 0 o or.10.8 0.. 7 \' 11 -.' *12 TH LEVEL -O m 200- ° ° ° ° ° -.01 k - 1 1. +1-+ - i-+ -I -÷ i O 0 0 0 0 0 0 0 0 - 200 'en o 0 °coo I +++A - LOWER DEVONIAN MIDDLE DEVONIAN WISSENBACH SHALE 1 UPPER DEVONIAN I, LOWER CARBOMFEROUS : 101B * . 0 1 : --=-=-1 I EMI ii i CALCEOLA SHALE CLAY SHALE SAND BANDED 1 ; OILER GRANITE 8 ORE DEPOSIT SHALE 0 501 0m 1000m (atter SCHOT 1971) FIGURE 6.2 Regional geology and cross-section of the Rammelsberg mine, Harz Mountains, West Germany. 166 TABLE 6.1 STRATIGRAPHY AND GEOLOGICAL HISTORY OF THE RAMMELSBERG DEPOSIT. A. REGIONAL EVENTS (after ANGER et al. 1966). STRATIGRAPHY EVENTS Intrusion of Oker granite Folding and metamorphism CARBONIFEROUS of Rammelsberg ores Diabase intrusions - extrusions. UPPER DEVONIAN ENY Stringocephalus Diabase intrusion Limestone N OROO CA IS MIDDLE Wissenbacher AR V Formation of Rammelsberg DEVONIAN Schiefer (slate) ores Calceola schiefer Tuffs of initial Volcanism limy slate Specious Sandstone LOWER DEVONIAN B. MINE STRATIGRAPHY 1. Ore Body Horizon with Old, New and Gray (20 - 25m thick). Ore bodies. 2. Wissenbach Shale - clay shale - slate. (55 - 60m thick). 3. Wissenbach Sand Banded Shale. (70 - 140m thick). 4. Calceola Shale - fossilifrous carbonate (65m thick). shale. 5. Speciosus Sandstone (with Corbis Bank). (30m thick). 6. Kahleberg Sandstone (Sommersprossen Sandstone). 167 At the 11 and 12 levels in the mine the New Orebody occupies the core of this overturned syncline (fig. 6.3). All three orebodies are thus over- turned. Faulting occurs within the mine sequence and is described in detail by Kraume (1955) and Schot (1973). The western and eastern ends of the Old and the New Orebodies respectively are bounded by faults with the ores showing considerable drag effects Schot (1971, 1973). The small scale folds and boudin structures found in the ores are discussed later in this chapter. THE ORES There are two main types of ore in the Rammelsberg mine - rich ore and poor ore (table 6.2, 6.3). In general the ores are fine grained, compact and often finely banded. The poorer ore types are either those with sulphide bands intercalcated with unmineralised shale bands or the mineralised 'Kneist'. The banded ores are a transitional facies between the rich ore types and the Wissenbach shales whereas the mineralised Kneist is a hard, brecciated and siliceous rock in which the sulphides occur essentially as fracture fillings and veins. The metal content of the ores is given in Table 6.3 and Table 6.4 summarises the sulphide and gangue mineralogy. The three orebodies, the Old Orebody, the New Orebody and the Grey Orebody are generally concordant with thehost rocks and their spatial distribution is shown in the longitudirial section of figure 6.4. The samples used in this study come from the Grey Orebody and the New Orebody. The New Orebody shows a stratigraphic distribution of ore types as dep- icted in figure 6.3 and summarised in table 6.2. On a macroscopic scale the ores are finely banded (fig. 6.5) and they often show either minor fold structures (fig. 6.5 e,f) or boudin fractures (fig. 6.5 e). Ramdohr (1953a) and Kraume (1955) have given detailed descriptions of the minor folds and boudin structures in the ores. Diagenetic concretions of pyrite often developed around small fos- sils (Schot 1973) are found both in the ores and also in the unmineralised shales (fig. 6.5 f). Detailed mineralogical descriptions of the ores have been given by Ramdohr (1953a), Kraume (1955) and Schot (1973). RAMMELSBERG NEW OREBODY 9TH LEVEL (cross-section at ord.1540/1550) .11■10■...... ■•■•■:. ■ •■,....m..... ■ ••■.. M.M.Mh'•■=mon AM. ■ INIMIMI'l•b. • ,...... 11=11■11•7■ 10TH LEVEL LEGEND NIMM•Marm.vh. ■ ...... NIENO7,...... ■.- . , SHALE — . ., i. . . .1 . i...... b . . . - ...... • BANDED ORE - - - : - -k•-, ,'` MAMA BARITE RICH LEAD ZINC ORE Ott•S.44h:1 \NimmI'mmmil.mLCl=IIII.h.‘ m1 I milI I Iu •. .I .• . I I . ■••■ ■ II, ■ IIMI• BARITE, CARBONATE, PYRITE, WM•11■■•.1111.MIL LEAD ZINC ORE • 02■,.IffanINN.1.I:1 OE 11 TH LEVEL BROWN ORE 4 ■•■• ••■ \ \\\ ... "w ...... KIESIGES ORE PYRITE ORE r N... ■Imr-....arso '%.,..... annii ll MEIMIMM.u1■All■mmh. • • • COPPER RICH PARTS ■ •••-...... morJ•Mir MIM...■71•••••■ • • • • 11•LAMilill ■■• 11■. m•-.■••■ 0 50m .A::. VEINW 1=1••■ : : • • —x—X—AXIAL PLANE OF SYNCLINE . • : . (after SCHOT 1971 ) FIGURE 6.3 Detailed cross-section of the New Orebody. LONGITUDINAL SECTION RAMMELSBERG MINE RAMMELSBERG SHAFT LEVELS -STOLLEN -1 -3 -5 -7 r 8 - 9 /1 -10 FAULT -11 (SCHEMATIC) -12 FIGURE 6.4 Longitudinal section of the Rammelsberg mine. (schematic, after Kraume 1955) 170 Table 6.2 a STRATIGRAPHICAL SEQUENCE OF ORE TYPES RAMMELSBERG NEW OREBODY (after Moore 1971) ORE TYPE MAJOR MINERALS Top Banded Ore (Banderz) Sulphides, banded and dispersed in slate. Grey Ore (Grauerz) Barite. Lead-Zinc Ore (Bleizinkerz) Galena, sphalerite with some barite. Brown Ore (Braunerz) Sphalerite, some galena and occas- ional barite. Mixed Ore (Melierterz) Banded mixture of various massive ore types. Pyritic Ore (Kiesigeserz) Pyrite, sphalerite, some chalco- pyrite. Copper Ore (Kupfererz) Chalcopyrite as lenses in pyritic ore and sulphur ore. Sulphur Ore (Schwefelerz) Pyrite and chalcopyrite. Footwall (Liegendes) Pyritic slates (the Kneist ore also occurs in this position). Bottom. Table 6.2 b IDEALISED STRATIGRAPHIC SUCCESSION IN A KUROKO ORE DEPOSIT (Matsuhuma and Horikoshi 1972) hanging wall upper volcanic and sedimentary formations ferruginous chiefly hematite, quartz and minor pyrite quartz zone barite ore zone massive barite ore Kuroko zone sphalerite - galena - barite Oko zone cupiferous pyrite ores Sekkoko zone anhydrite/gypsum - pyrite ore Keiko zone copper bearing siliceous disseminated and/or stock work ore footwall silicified rhyolite and pyroclastic rocks - disseminated and veined with sulphides. Table 6.3 MEDAL CONTENT OF THE RAMMELSBERG ORES (after Schot 1973). Zn Cu Fa Ca0 Ag Au ORE TYPE Pb Bas°4 6 - 10% 12 - 20 % 0.5 - 1.9% 8 - 12% 20 - 2594 75 - 200g/t (0.4 - 1.2g/t) g (8.5%) (10%) 23% 4% (120 - 160g/t) (0.6 - 1.0g/t) U (18.9%) (1.3%) wy Banded 3.5 - 4.5% 6 - 9.5% 0.4 - 0.6% 7 - 6.6% 2 - 5% 60 - 70g/t 0.3 - 0.5g/t Ore 9% (4%) (8%) (0.5%) (8%) E R O R Mineralized 1.3 - 2% 1.3 - 1.5% 6% POO 1% 28g/t 0.2g/t Kneist (1.4%) 3% (1.4%) (as sulphide) 0'46 Old 10% 18% 2% 14% 12% 73% 160g/t 0.8g/t Orebody N New 9 - 12.5% 18 - 25% 70 - 194g/t A 2% 10% 26% 1.48/t Orebody (12)% (21%) 3% (160g/t) i 0 Gray 2 - 3.7% 2.6 - 4.0% 0.1 0 0.196 1.5 - 2% 76.9 - 87.8% 69 - 156g/t Orebody (2.8%) (3.6%) (0.1%) (1.0) (79%) 296 (140g/t) 0.6g/t 172 Table 6.4 MINERALOGY OF THE RAMMELSBERG ORES MOST COMMON MINERALS (after Ramdohr 1953a , Kraume 1955, Schot 1973) . RICH ORE BANDED ORE KNEIST (Sulphide and Sulphate mineralogy) sphalerite pyrite pyrite galena sphalerite chalcopyrite pyrite (chalcopyrite) (galena) chalcopyrite marcasite pyrrhoti te magneti te tetrahedri te bournoni te jamesoni te boul angeri te bari to (Gangue Mineralogy) Quartz quartz quartz calcite calcite calcite dolomite dolomite dolomite sericite sericite sericite chlorite chlorite chlorite 173 METAMORPHISM The Wissenbach shales and the enclosed Rammelsberg ores have under- gone low greenschist facies metamorphism during the Variscan orogeny (Ramdohr 1953a, Kraume 1955). The grade of metamorphism is probably chlor- ite sub-facies or below this as the shale mineral assemblage is not a distinctive one (Winkler, 1974). The shales now have a well developed slaty cleavage and essentially a quartz - sericite - chlorite - carbonate - sulphide mineralogy. Graphite occurs in the more carbonaceous shales and ore types. Carbonaceous and graphitic rings are preserved around some microfossils which are found in the sulphide ores (Schot 1973). Anger et al. (1966) described the metamorphism as a weak epizonal metamorphism but the overall metamorphic temperature is thought to have been low. Schot (1971) states that the presence of mackinawite indicates that temperatures were below 200°C while Ramdohr (1953a) wrote that the presence of cubanite and vallerrite exsolution in chalcopyrite indicates maximum metamorphic temperatures between 200°C - 250°C. Marcasite is also found in the ores (Schot 1973, Ramdohr 1953a, and this also indicates low temperatures of metamorphism. The use of the stabilities of these sulphide assemblages in order to pin point specific metamorphic temper- atures is difficult because their stability ranges are not well defined (Craig and Scott, 1974). Textural evidence from the sulphides also suggests low grade meta- morphism. Primary depositional textures such as colloform and framboidal textures, are thought to indicate relatively low temperatures of form- ation and these are found preserved in the Rammelsberg ores (Ramdohr 1953a). The deformation textures which are discussed in detail later in this chapter, also are indicative of generally low metamorphic and deformation temperatures. 6.3 MACROSTRUCTURES IN THE ORES Folding, fracturing and boudinage are well developed in the Rammelsberg ores. Ramdohr (1953a), Kraume (1955) and Schot (1973) have described the macrostructures in some detail. The New Orebody has been tightly folded into an isoclinal syn- cline (fig.6.3) which Moore (1971) ascribed to flexural flow folding followed by homogeneous flattening. Many of the minor folds (as 174 FIGURE 6.5 MACROSTRUCTURES IN THE ORES a) Brown ore consisting of finely banded sphalerite and galena (dark grey) and banded pyrite rich ore (light grey) below. b) Banded copper rich pyrite ore consisting of dark bands of chalcopyrite which appear to flow around light coloured pyrite augen. c) Mixed ore consisting of finely banded sphalerite, galena and pyrite layers. d) Pyrite rich Banded Ore with pyrite rich layers and pyrite augen with fine laminations of shales. Thick siltstone layer shows boudin fractures infilled with quartz. e) Folded pyrite rich banded ore with fractures deve- loped parallel to the axial plane. Coarse secondary growth of pyrite porphyroblasts can be seen. f) Tightly folded shale layer in mixed ore with well developed radial fractures infilled with quartz. g) Hanging wall slate with coarse grained pyrite porphyroblasts which appear to be developed syntecto- nically. Complexly folded banded ore showing possible inter- ference fold patterns. All specimens are from the Kraume Collection, Mining Geology Museum, Imperial College. They were collected from 9 and 10 levels, New Orebody. A B . - ----;.■■■■■11■1..••••."1"01■■• I- 2.5c m 176 described by Ramdohr (1953a), Kraume (1955) and Schot (1973)) appear to be almost isoclinal with an axial planar slaty cleavage. Decollement folding and disharmonic folding occurs particularly where there is a large contrast in mechanical properties between the sulphides and the slates. Transposition of the layering (Hobbs et al. 1976) may occur in regions of isoclinal folding. The fine scale banding in the ores is probably in part primary lithological layering and is also probably in part a metamorphic folia- tion generated by recrystallisation and grain growth during deformation and metamorphism. Some hand specimens (fig. 6.5h) show complex fold patterns which suggest interference patterns of more than one fold phase (Ramsay 1967). Figure 6.5f shows a class 3 type fold (Ramsay 1967) with extension frac- tures around the fold hinges. Some folds show sheared out limbs (fig. 6.5e) while boudinage is commonly developed on all scales and has been described in detail by Kraume (1955). Boudin fractures are often filled with quartz and calcite gangue (fig. 6.5d). Pressure shadows are developed around pyrite augen (fig. 6.5g) and concretions of gangue minerals (Schot 1973). In zones of intense deformation these pyrite augen are fractured and pulled apart (Ramdohr 1953a, Schot 1973). The macrostructures found in the Rammelsberg ores (folds, frac- tures and boudins) are very similar to those found in other stratiform sulphide deposits such as Mount Isa (chapter 7), Roseberry (Braithwaite, 1974), Caribou (Davis 1972) and the Besshi type deposits of the Sanbagawa metamorphic belt in Japan (Kanhira and Tatsumi 1970), all of which have been deformed and metamorphosed at or below low greenschist facies con- ditions. 6.4 MICROSTRUCTURES IN THE RAMMELSBERG ORES INTRODUCTION Polished and thin sections have been studied in detail. A number of fracture specimens and polished sections have been examined in the scanning electron microscope. Detailed mineralogical studies have been reported by Ramdohr (1953a) and Schot (1973). In this investigation 177 Figure 6.6 MICROSTRUCTURES IN THE ORES a) Small fold in banded ore showing redistribution of galena and chalcopyrite axial planar to the fold. Specimen R111C unetched. b) Pyritised fossils - lamellibranch shells in banded ore matrix of quartz and sericite. Specimen R112 unetched. c) Augen texture of an aggregate of pyrite grains with a matrix flow texture of sphalerite around the augen. Speci- men R004. d) Band of foliated galena in sphalerite (grey) rich brown ore. Small laths of pyrrhotite (white) in the sphalerite are sub-parallel to the banding. Specimen R007, etched 60 secs 65°C Brebrick and Scanlon (1957) etchant. e) Small pyrite framboids (white) in fine grained mosaic tex- tured sphalerite. Specimen R006, oil immersion. f) Elongate laths of pyrrhotite in matrix of equigranular sphalerite. Pyrrhotites show a preferred orientation para- llel to the bedding/foliation. Specimen R004 oil immersion. g) Large fractured porphyroblast of pyrite in fine grained sphalerite with laths of pyrrhotite which show a flow pat- tern around the porphyroblast. Specimen R005. h) Colloform texture of pyrite and galena in centre surrounded by an overgrowth of pyrite (white) then marcasite (mottled grey). Specimen R006. 179 the deformation microstructures highlighted by careful etching (Appendix A) are reported. The Rammelsberg ores are strongly banded, exhibiting a well deve- loped foliation which is axial planar to fold hinges (fig. 6.6a). Pyritised microfossils are often found in shale units within the banded ores (fig. 6.6b): as are flow and augen textures, particularly where the softer sulphides have been deformed around hard quartz or pyrite grains (fig. 6.6c). Most of the ores are finely banded (fig. 6.6d) with thin monomineralic laminations of sulphides often intercalcated with shale and silty units. Pyrite framboid layers (fig. 6.6e) are found in all ore types. Pyrrhotite grains are particularly common in the sphalerite rich zones often forming elongate tabular grains which outline a metamorphic foliation (fig. 6.6f). Large subhedral - euhedral pyrite grains and aggregates are found in all ore types. Where these have undergone deformation they are commonly fractured and boudinaged (fig. 6.6g). Ramdohr (1953a) and Schot (1973) have given detailed descriptions and micrographs of primary growth textures such as framboids, colloform textures and atoll structures (fig. 6.6h). Many of these features were also observed in the specimens studied by the author. NON SULPHIDE MICROSTRUCTURES BARITE MICROSTRUCTURES Ramdohr (1953a) and Schot (1973) recognised that most of the barite in the Rammelsberg ores was recrystallised. Figures 6.7 and 6.8 show details of various barite deformation and recrystallisation micro- structures. Most of the barite from the Grey Orebody and the New Orebody exhibits an eqoigranular mosaic texture (fig. 6.7a) with straight or slightly curved grain boundaries. The grain size is com- monly 150 - 250 TIM and the grains show little or no undulose extinction. However, relict old grains are found in the eyes of augen (fig. 6.7b). These large old grains are often outlined by rims of fine grained sulphides, (pyrite and sphalerite) and show undulose extinction and deformation bands (fig. 6.7c). The deformation bands are often recrystallisation nucleation sites (fig. 6.7c) where complex lobate and sutured boundaries (fig. 6.7d) indicate that strain induced grain boundary migration has occurred. In the first instance subgrains form and then as these misorient, small new, strain free grains are formed at the boundary. 180 Figure 6.7 BARITE MICROSTRUCTURES All photographs are from a suite of specimens R009A - D and are taken with crossed polars. a) Equiaxed mosaic (foam) texture of recrystallised barite with no shape orientation. Section perpendicular to foliation and parallel to lineation. b) Same section as (a) but showing coarse grained augen texture of old partly recrystallised grains sheathed in sulphides. c) Deformation bands in large old grain of barite with recrystallisation occurring at the grain boundaries. d) Grain boundary showing highly lobate form indicating grain boundary migration and recrystallisation. Sub- grains form at the boundary and then new recrystallised grains are developed. e) Large old grain with sutured simple twin boundary in matrix of recrystallised barite. f) Grain boundary mantle texture showing recrystallised grains developing at the grain boundary. g) Partially recrystallised old grain sheathed in pyrite grains. h) Augen texture of recrystallised old grains sheathed by pyrite layers (crossed polars with tint plate). T CV 182 Only a few deformation twin features were observed in the barite studied. Schot (1973) has reported deformation twins in the Rammelsberg barite and Deer et al (1966) state that barite undergoes glide twinning on (110) but the twins found in this study were mainly large growth twins (fig. 6.7e) with sutured twin boundaries. The deformation microstructures exhibited by the barite are strikingly similar to those in quartz (White 1976). Relict old grains are often mantled with small equigranular recrystallised grains (fig. 6.7f). Partially recrystallised old grains are often found (fig. 6.7g) but more often the old grain shapes are outlined only by sulphides as the barite has completely recrystallised to a mosaic of equant grains (fig. 6.7h). Primary depositional textures are sometimes found in the barite ore, particularly where they have been protected from deformation by large sulphide aggregates (fig. 6.8a and b). Radiating barite crystals (fig. 6.8a) and coarse rosette features (6.8b) are found. Small new grains, however, have formed at grain boundaries and growth interfaces (fig. 6.8a). These deposition textures have been compared with the rosette (fig. 6.8c) and plumose structures (fig. 6.8d) from an undeformed barite deposit (Ballynoe, Eire (Barrett 1975)). Kraume (1955) and Ramdohr (1953a) have reported stylolites in deformed Rammelsberg barite but they were not observed in this study. MICROSTRUCTURES IN THE SLATES AND SILTSTONES Bedding is often defined by small bands of pyrite cubes and fram- boids (fig. 6.8e) or silty layers in the ores (fig. 6.8f). Small folds are common and the slaty type cleavage is outlined by pressure solution seams (fig. 6.8f) nearly axial planar to the folds. In detail, the slaty cleavage (fig. 6.8g) is outlined by an alignment of phyllosilicate minerals - notably sericite and chlorite and also occasionally graphite. Detrital quartz grains often have overgrowths in the direction of the cleavage. Fibrous quartz overgrowths around euhedral pyrite grains (fig. 6.8h) indicate solution transport of quartz probably as a result of pressure solution. Dolomite and calcite occur in the ores and in the slates but were not common in the material studied by the author. In the ores dolomite occurred as porphyroblastic untwinned grains whereas the cal- cite exhibited deformation twins. In the slaty lithologies, the carbon- ates were too fine grained to study in detail. 183 Figure 6.8 BARITE MICROSTRUCTURES a) Coarse grained relict plumose and radiating structure in barite ore. Recrystallisation is occurring at the grain boundaries. Specimen R009A, crossed polars. b) Well developed radiating texture in Rammelsberg barite. Specimen R21 crossed polars. c) Rosette texture of undeformed (unfolded) barite from Ballynose (Eire). Crossed polars. d) Plumose texture truncated by stylolite in undeformed barite. Ballynose (Eire), crossed polars. e) Banded barite - pyrite ore showing euhedral pyrite grains. Specimen R22 plane polarized light. MICROSTRUCTURES IN THE SLATES AND SILTSTONES f) Folded siltstone layer in Banded Ores with the development of pressure solution type cleavage. Speci- men R111c, plane polarized light. (p.p.l.) g) Slaty cleavage in banded ores showing the alignment of phyllosilicates and undeformed detrital quartz grains. Specimen Milo, (p.p.1.). h) Fibrous overgrowths of quartz on euhedral pyrite grains. The quartz is only slightly deformed (some undulose extinction). Specimen R111c (crossed polars). 184 185 In some thin sections, refolded folds were found in the slates, possibly indicating two phases of deformation, but many more samples and fieldstudies are needed to confirm this hypothesis. SULPHIDE MICROSTRUCTURES PYRITE Pyrite is found in all ore types in the Rammelsberg mine. It occurs as either framboidal aggregates (fig. 6.6a) or as subhedral to euhedral recrystallised grains (fig. 6.9a and b). The recrystallised pyrite often shows growth zoning (fig. 6.9c) indicating deposition on an original framboidal nucleus. Other sulphides and silicate minerals are often included in the large anhedral pyrite grains (fig. 6.9b). Growth bands (fig. 6.9d) and subgrains (fig. 6.9c) are revealed by strong etching. Massive pyrite nodules and aggregates show abundant overgrowths (fig. 6.9f) possibly indicating remobilization of pyrite by pressure solution. Primary deposition textures and growth textures are often pre- served in the pyrite grains. Framboidal pyrite often has a colloform overgrowth of sphalerite and marcasite (fig. 6.9g). Large colloform overgrowths of galena and marcasite/pyrite are also frequently found (fig. 6.6h). Carbon or graphitic envelopes around microfossils and framboidal pyrites appear to have preserved these structures in spite of the metamorphism. Similar features have been reported from meta- morphosed ore deposits from Timmins area (Canada) and the Cyprus deposit, Manitoba (Sangster and Scott 1976). The Rammelsberg pyrite deforms mainly by cataclastic processes. Fractured pyrite crystals (fig. 6.6g) are commonly found in a matrix of 'softer' sulphides. Pyrite framboids are often disaggregated in zones of intense deformation. Spectacular shatter features (fig. 6.9h) are found in crush zones in pyrite rich ore. The abundant pyrite over- growths and euhedral crystals of pyrite in the ores, however, suggests that at least some remobilization by solution processes has occurred during deformation. CHALCOPYRITE Chalcopyrite is the principal copper mineral in the Rammelsberg ores. It occurs either as massive monomineralic bands often associated 186 Figure 6.9 PYRITE MICROSTRUCTURES a) Typical pyrite rich ore showing well developed euhedral pyrite grains in a matrix of quartz and barite. Speci- men R008 unetched. b) S E M (Scanning electron microscope) photograph of euhedral pyrite grains in pyrite rich ore with growth inclusions of gangue minerals. Specimen R001, fracture surface. c) Pyrite rich ore with large well developed growth struc- tures - colloform type banding. Specimen R10, electro- lytic etch (Appendix A). d) Etch pits in pyrite showing probably dislocation distri- bution in growth bands. Specimen R10 - very heavily electrolytically etched. e) Growth substructure in coarse grained pyrite ore. Specimen R14 electrolytic etch. f) Pisolitic texture in pyrite showing secondary overgrowths probably caused by redistribution during deformation. Specimen R16, electrolytic etch. g) Small pyrite framboids with sphalerite and marcasite colloform overgrowths. Specimen R003, oil immersion. h) Intensely shattered pyrite grains in strongly deformed pyrite rich ore. Matrix (black) is quartz. Specimen R15 unetched. 188 with pyrite in the lead zinc ores; the brown ores and also in the pyrite ore (fig. 6.5) or as segregated grains and exsolution blebs in sphaler- ite. This latter type of chalcopyrite is discussed with the sphalerite textures. Most of the massive banded chalcopyrite is revealed by careful etching to consist of a mosaic of small fairly equigranular grains (fig. 6.10a). An annealing - growth microstructure is common with gently curved grain boundaries and broad growth twins (fig. 6.10b). A few of these grains show fine slip lines (fig. 6.10b) and deformation twins. However, a relict coarse grained microstructure is often revealed by careful etching. These large (often 500pm - 2 cm) grains show abund- ant deformation twins (fig. 6.10c) with kink bands, slip lines and sub- grains developed. The large old grains can be identified by their orientation (fig. 6.10d) even though new recrystallised grains are deve- loped within them. The large old grains had broad growth/annealing twins which acted as sites for preferential nucleation and recrystalli- sation (fig. 6.10d). The large old grains have abundant deformation twins (fig. 6.10c). Recrystallisation occurs around the grain boundaries with the new grains relatively free from deformation features (fig. 6.10e). The annealing/growth twins show development of subgrains and strain induced grain boundary migration (fig. 6.10f). Subgrains also develop within the old grain with progressive misorientation leading to new recrystallised grains at the grain boundaries (fig. 6.10g) and this gives rise to a core and mantle microstructure similar to that developed in quartz (White 1976). The final recrystallisation texture is a mosaic of near equiaxed grains (fig. 6.10h). These grains show some annealing twins (fig. 6.10b) and also a few deformation twins (fig. 6.10h). Polished sections etched with the NH OH/H 0 etch (Appendix A) 4 2 2 show old/new grain relationships particularly well (fig. 6.11a,b,c,d). Old twins and new grains are shown in figure 6.11a. Pyrite porphryo- blasts are fractured and pulled apart whereas the matrix chalcopyrite grains show no grain shape orientation (fig. 6.11b). The old grains, however, tend to be lenticular probably as a result of plastic deform- ation (fig. 6.11c and d). GALENA Two principal microstructures were found in the Rammelsberg galena studied by the author. The galena occurs either as thin mono- mineralic layers intercalcated between layers of other sulphides or as 189 Figure 6.10 CHALCOPYRITE MICROSTRUCTURES a) S E M photograph of fracture surface showing fine equi- granular chalcopyrite grains. Specimen R001. B) Etched polished section of mosaic textured chalcopyrite showing gently curved grain boundaries. New recrystallised grains are growing inwards towards older deformed grains (showing etch pits and slip lines). Annealing twins and lack of etch pits are seen in the new recry- stallised grains. Specimen R001, electrolytic etch (Appendix A). c) Kink bands in large relict old chalcopyrite grain with curved deformation twins. Specimen R17, electrolytic etch. d) Recrystallisation by grain boundary migration along broad twin boundaries in a large grain of chalcopyrite. The host old grain is distinguished by the common orientation of thin deformation twins whereas the new recrystallised grains are small and equigranular. Specimen R001 eletrolytic etch. e) Similar to (d) but the recrystallisation occurs at the grain boundaries of a large old grain with abundant deformation twinning. f) Detail of broad twin boundary showing serrated high angle boundary with migration - sub grain development and recrystallisation. Specimen R006 electrolytic etch. g) The development of a fine grain equiaxed mosaic texture from a large old grain. Subgrains and then new grains are developed at the grain boundaries. Specimen R001, electrolytic etch. h) Heavily etched equiaxed mosaic of recrystallised chalco- pyrite showing some later deformation twins. Specimen R001, electrolytic etch. - • • • r I- 220u -I 191 Figure 6.11 CHALCOPYRITE MICROSTRUCTURES a) Coarse grained chalcopyrite with large simple twin showing boundary migratioh. Small new grains are forming in the host old grain but with a similar orientation to the old grain. Specimen R006, NH OH/H 0 etch. 4 2 2 b) Mosaic of near equiaxed grains with curved grain boundaries and deformation twins. There is no marked shape orientation although the pyrite grain (white) is fractured and pulled apart. Specimen R11, NH OH/H 0 etch. 4 2 2 c) Recrystallised chalcopyrite showing mosaic texture around an elongate old grain. Specimen R001, NH OH/H 0 etch. 4 2 2 d) Elongate old grains showing progressive recrystalli- sation along twin boundaries and along grain bound- aries. Specimen R008, NH4/OH/H202 etch. GALENA MICROSTRUCTURES e) S E M photograph of mosaic textured galena showing curved grain boundaries and fairly heavy dislocation etch pit density. Cleavage fragment of Specimen R003 etched 70 secs 65°C. Brebrick and Scanlon etchant. f) Detailed S E M photograph of same specimen (e) showing {110}<110> type slip lines. g) Optical micrograph of mosaic texture galena with straight grain boundaries - some are curved where other sulphides have pinned the boundaries. Specimen R003, etched 60 secs 65°C. Brebrick and Scanlon etchant. h) Same specimen as (g) but showing band of foliated galena with a distinct shape fabric. 192 193 isolated lenses. Some of the galena layers and the lenses have a fine to coarse grained mosaic texture while other galena layers exhibit a schis- tose texture with the long axes of the grains aligned along a foliation at an angle to the bedding (fig. 6.6d). Etching of the galena in the Rammelsberg ores proved extremely difficult principally because of the small grain size and heavy staining caused by adjacent minerals. Scanning electron microscopy of the galena rich ores show the curved grain boundaries of the mosaic texture .galena (fig. 6.11e). Most of the galena cleavage fragments etched very heavily (fig. 6.11e and f). 0101 <110>type slip lines were observed in a number of samples (fig. 6.11f). The grain size of the mosaic textured galena varies from 10 - 20 pm to 100pm - 200pm. Coarse grained lenses of galena are com- monly found in sphalerite rich ore types often with straight or gently curved grain boundaries (fig. 6.11g). Secondary grain growth is suggested by the large variation in the grain size of the equigranular galena. In contrast to the galena microstructures described above, the schistose galena layers show a marked grain shape texture (fig. 6.11h) with lobate grain boundaries and highly variable grain size from 511m to 200pm. Zones of highly sheared galena appear to be more fine grained than adjacent grains which do not exhibit a shape fabric (fig. 6.11h). Galena, however, was not abundant in the samples used for this study. Features indicative of deformation by dislocation processes (slip lines, deformation bands and kinks) were not observed in polished section studies. Recrystallisation and grain growth appear to be res- ponsible for the large variation in grain size in the galena. Where galena forms only a minor part of the ore, grain growth is inhibited with duplex textures developed. Grain boundary pinning by small graphite and sericite flakes was observed in some specimens but in general, detailed microstructural features were obscured by the poor etching characteristics of these fine grained and mixed ores. SPHALERITE Sphalerite rich bands are common in many Rammelsberg ore types. Most of the sphalerite occurs as a fine grained (5 - 301.1m) equant mosaic of interlocking grains (fig. 6.12a). The grain boundaries are typically lobate and sutured. The grains have abundant broad annealing twins (Richards 1966 and chapter 3). A few of these small grains show thin lenticulate deformation twins. Clusters of large old grains can be recognised (fig. 6.12b) by the growth of the fine grained mosaic at the 194 grain boundaries. These large (200 - 6004m) grains show abundant internal deformation features - curved deformation twins and slip lines (fig. 6.12b). Broad annealing type twins are not found in these grains. The impinging grain boundaries of these large grains (fig. 6.12b) are highly serrated and sutured indicating strain induced grain boundary migration. Primary depositional textures are particularly well preserved in the large sphalerite grains (Ramdohr 1953a). Large euhedral sphaler- ite grains remain only slightly affected by deformation (fig. 6.12c) with abundant chalcopyrite exsolution blebs particularly along the {111} planes. These exsolution bodies of chalcopyrite are often associated with fine deformation twins and slip lines (fig. 6.12d). The chalcopyrite exsolution appears to be contemporaneous with the deformation because the exsolution blebs are controlled by the deformation twins. In addition the chalcopyrite occurring at the recrystallised margins of large sphal- erite grains is larger in grain size (fig. 6.12d). It is extremely difficult to tell whether the small chalcopyrite exsolution blebs are deformed. Some appear to pinch and swell along twin boundaries and others appear to be boudinaged and displaced by twins. Recrystallisation of the sphalerite occurs particularly at the grain boundaries (fig. 6.12b and d). The mosaic of small grains which results, has grains with curved grain boundaries and broad annealing twins (fig. 6.12e). These twins have a high dislocation density indicated by numerous etch pits (fig. 6.12f). In copper rich parts of sphalerite ore, a chalcopyrite - sphaler- ite duplex texture develops with the chalcopyrite occurring at the grain boundaries and boundary junctions (fig. 6.12g). These grain boundaries are only slightly curved in comparison to the highly curved grain bound- aries of the galena - sphalerite duplex texture (fig. 6.12h). The sphalerite develops highly convex boundaries whereas the galena has strongly concave boundaries. The sphalerite textures in the Rammelsberg ores indicate deform- ation by twinning and slip with dynamic recrystallisation occurring at twin boundaries and at grain boundaries. Relatively large and undeformed old grains can be recognised. Recrystallisation occurring at grain boundaries leads to a fine grained mosaic of only slightly deformed grains with no shape fabric. Annealing and growth twins are found in the recrystallised grains. Evidence for exaggerated grain growth as cited by Stanton (1972) has not been found in this study. 195 Figure 6.12 SPHALERITE MICROSTRUCTURES a) Fine grained mosaic of fairly equiaxed grains with curved and slightly sutured boundaries. Broad anneal- ing twins are developed in a number of grains, Specimen R003. Reflected light, Brebrick and Scanlon etch 150 secs. 70°C. b) Large relict old grains with highly serated grain boundaries - deformation twinning - and recrystalli- sation into a fine grained mosaic occurring at the grain boundaries, Specimen R003. Reflected plane polarised light, Brebrick and Scanlon etch 150 secs. 70°C. c) Large old grain of sphalerite showing abundant chalco- pyrite exsolution blebs,along {111} planes. Matrix is sphalerite with laths of pyrrhotite. Specimen R004, oil immersion, unetched. d) Etched sphalerite grain showing abundant blebs of ex- solved chalcopyrite along gently curved deformation bands (or very thin twins). Specimen R004, oil immersion, Brebrick and Scanlon etch, 150 secs. 65°C. e) S E M photograph of recrystallised sphalerite matrix showing slightly sutured boundaries and broad annealing twins. Cleavage fragment etched Brebrick and Scanlon etchant 70 secs. 70°C. f) S E M photograph showing detail of annealing twins with irregular etch pits. Cleavage fragment etched 70 secs. 700C. Brebrick and Scanlon etchant. g) Duplex texture of sphalerite and galena. The sphalerite develops highly curved convex boundaries while the galena has concave interfaces with the sphalerite and straight internal boundaries. Specimen R009, oil immersion, etched 80 secs. 65°C. Brebrick and Scanlon etchant. h) Duplex texture of sphalerite and chalcopyrite showing chalcopyrite occurring along grain boundaries and at boundary junctions. Specimen R15, oil immersion, etched 90 secs. 70°C. Brebrick and Scanlon etchant. 197 SUMMARY Primary depositional microstructures such as colloform textures, framboids and concretions are preserved in the Rammelsberg ores. Micro- structural sequences from deformed old grains - partly recrystallised grains - to completely recrystallised grains have been recognised in barite, sphalerite and chalcopyrite. Pressure solution type deform- ational processes have probably operated during the deformation of pyrite giving rise to secondary overgrowths and grains of pyrite. Recry- stallised grains of galena, sphalerite and chalcopyrite show some evidence of deformation probably indicating syntectonic recrystallisation rather than a static annealing event after deformation.' 6.5 PREFERRED ORIENTATIONS IN THE RAMMELSBERG ORES INTRODUCTION X-ray texture goniometry was carried out on a number of samples of Rammelsberg ores. Sections were cut either perpendicular to a visible foliation plane and also perpendicular to a visible lineation or they were cut parallel to a visible foliation plane. Where polished section briquettes were used for X-ray studies, the surface was first etched and then lightly repolished by hand on a linen cloth with A1 203 powder. Rock slabs used for X-ray texture studies were ground flat with the final grinding stage using 1400 silicon carbide grit. X-ray diffraction ana- lysis was carried out on powdered samples of all specimens in order to confirm the mineralogy and check on possible interferring diffraction peaks. All ore types were found to be heterogeneous in that many sulphide phases were present (fig. 6.13). Pyrite occurs in varying amounts in all specimens sometimes producing significant interference with sphalerite peaks at low values of 28. Pole figures were measured on the X-ray tex- ture goniometer described by Siddans (1971, 1976). Both reflection and transmission modes were used. Data was reduced by the programmes written by A.W.B. Siddans, J. Whalley and M. Casey at Leeds University. The results are presented in figures 6. 14 - 19, as pole figures contoured in levels times uniform intensity (Siddans 1971). All of the samples used for X-ray texture studies were examined by reflected and transmitted light microscopy. The results of the X-ray texture analysis are dis- cussed in terms of the microstructures and deformation mechanisms infer- red from detailed microscopic studies of the ores. • - h -S O Galena 022 • ■ Galena 022 a Ln., . 6.— a 3NOIJ r-_. in Pyrite 112 Pyrite 112 B co 1:9 n n Dolomite 1120, Pyrite 021 0 0 £ X - P X ct) X X P 00 itt. P CO is., p CD 0' CD ul— C71 CD _ O CD gown. Pyrile 021, Dckende 1120 Pyrite 021, Dolomite 112. 0 Loa CL] - . O 4,_ -s -0 0 Chalcopyrite 020,004 - ct) n t- t-Chalcopyrite 004 y- • cu ..-- Sahalente 002, Pyrite 002 0— ■ Chalcopyrite 020 cr - o Pyrite 002 ,Sphal era, 002 Spholente 002 , Pyrite 002 rs.) o CD - Barite 112 N1 0 CD e..1 -s CD - Dolomte1014 cu ••---• Dolomite 164 Doi ornIt e 1014 Galena 002 4.),„, ''''— Galena 002 ...;„„ Galena 002 Ca _ q. CO •It... Chalcopyrite 112 . l'" Lhol ci22rje. 'A__, ui C Pilee 111 Sphaterite III Sphaterite III , Pyrite 111 - Spholerite Ill _ 0.) Barite 102 i■ Barite 102 LJ Galena M _ 4., f. Barite 002 0 Gal enct 111 O La_ ■ ..- Galena III, Borne 210 0 Baffle 002 NI L., ea tat es co a, r. N K._ or, ej ...1 Cal Ut F11 r- N O CO KJ 0 0 0 0 0 0 0 0 0 UV" CI; ts 1 1 f I I I I I I I 0 o 0 0 cn I I I I I I 1 I I I I i , I 1 INTENSITY INTENSITY INTENSITY 199 GALENA The crystallographic preferred orientations exhibited by galena in the Rammelsberg ores are shown in figures 6.14 and 6.16. In figure 6.14, the pole figures depict a weak preferred orientation of galena from a specimen of sphalerite rich ore (brown ore). The peaks are dis- tributed irregularly over the pole figures (fig. 6.14) with the intensity much reduced for the higher order (311) diffraction peak. In this speci- men (R003), the galena occurs principally as variable size mosaics of equigranular (foam texture) grains (fig. 6.11e and g). Duplex textures of galena and sphalerite (fig. 6.12g) also occur while galena with a shape orientation (foliation) (fig. 6.11h) comprises only a minor fraction of the galena in the specimen. The irregular preferred orientation of this specimen is probably the result of large variations in grain size (50 - 2504m) caused by grain growth in the foam texture component of the galena microstructure. In contrast to the irregular pole figures of galena in sphalerite rich ores, the galena in the barite rich ores shows a reasonably strong crossed girdle pattern for (200) planes (fig. 6.16). In this specimen (R.44) the galena shows a strong shape orientation (figs. 6.11h and 6.6d) at a low angle to the foliation/bedding plane. There is a tendency for these podg poles to cluster at 45° to the pole to the foliation plane - similar to the compression texture for galena (Siemes 1970 and Chapter 5). There is also, however, a distinct girdle pattern and asymmetry possibly related to layer parallel shearing associated with folding. This hypo- thesis, however, would need rigorous testing by sampling in detail around a fold structure of monomineralic galena layer. In the samples of Rammelsberg ores which have been studied, the crystallographic preferred orientations of galena are neither strong nor consistently developed. These results reflect the variation in galena microstructure found in these ores. SPHALERITE Sphalerite, in a manner similar to galena, shows only an irregular development of crystallographic preferred orientations as illustrated in figures 6.15 and 6.17. The pole figures of specimen R003 (fig. 6.15) show an irregular distribution of peaks which may be explained by the extreme variations in the microstructures observed in this specimen. 200 RAMMELSBERG GALENA Specimen R003 (220) (200) COMPLETE POLE FIGURES (REFLECTION & TRANSMISSION) Fr FOLIATION PLANE EQUAL AREA ( UPPER HEMISPHERE ) PROJECTION CONTOUR LEVELS 1 2 222 ,3 4 3 , X UNIFORM FIGURE 6.14 Complete pole figures of galena from sample R003, Rammelsberg. RAMMELSBERG SPHALERITE Specimen R 003 201 (220) (200) COMPLETE POLE FIGURES (REFLECTION & TRANSMISSION) F= FOLIATION PLANE EQUAL AREA ( UPPER HEMISPHERE) PROJECTION CONTOUR LEVELS 1 2 , 3 4 el , X UNIFORM Figure 6.15 Complete pole figures of sphalerite from sample R003, Rammelsberg. 202 RAMMELSBERG BARITE & GALENA Specimen R44 BARITE (002) BARITE (111) BARITE (303) GALENA (200) COMPLETE POLE FIGURES ( REFLECTION & TRANSMISSION) FOLIATION PLANE = EQUATORIAL PLANE EQUAL AREA (UPPER HEMISPHERE) PROJECTION CONTOUR LEVELS 1 , 2 , 3 Li 4 D, x UNIFORM FIGURE 6.16 Complete pole figures from sample R44, Rammelsberg. 203 Several large (presumably old) sphalerite grains (fig. 6.12b) dominate the microstructure. These large old grains lie in a fine grained recry- stallised matrix which appears to have no distinct preferred orientation as indicated by the lack of alignment of the abundant annealing twins (fig. 6.12a). In addition to this grain size heterogeneity, the abundant annealing and deformation twinning in sphalerite (see Chapter 3) is expected to complicate the X-ray pole figures. The pole figures bear no relationship to those obtained by Say nisch (1970) for axisymmetric com- pression of sphalerite. The [200] sphalerite pole figure of figure 6.17 illustrates the problem of matching reflection and transmission mode data particularly in sulphide specimens which have large linear absorption coefficients (von Gehlen 1960 and Chapter 5). In conclusion, the determination of preferred orientations in Rammelsberg sphalerite ores has proved unsuccessful mainly due to extreme grain size heterogeneity. PYRITE X-ray texture measurements were made on pyrite from several speci- mens of mixed ore (Table 6.2) but the results were disappointing in that no distinct pattern of preferred orientation was revealed (R004 fig. 6.17). In addition, it was extremely difficult to make sections thin enough for transmission mode analysis thus producing pole figures with sharp cut offs at the reflection - transmission mode boundary (fig. 6.17). Detailed microscopic analysis of the pyrite in the Rammelsberg ores did not show any obvious crystallographic preferred orientations but rather that the pyrite microstructure is dominated by primary growth features (fig. 6.9). ' CHALCOPYRITE Chalocopyrite pole figures from copper rich ore specimens are shown in figures 6.17 - 19. There are no significant preferred orient- ations in figures 6.17 - 18. A similar result was found by Lang (1968, fig. 62). Microscopic examination of polished sections of chalcopyrite rich ore which have been etched with NH 0H/H 0 solution (Appendix A), 4 2 2 shows that the mosaic textured chalcopyrite (fig. 6.10b) has no discern- able preferred orientations as indicated by the colour of the etched grains. Recrystallisation which has produced this microstructure appears to have occurred by grain boundary migration from large annealing twins 204 RAMMELSBERG Specimen R 004 PYRITE ( 210) 0 • • * V.• 1 6 I \ 0 , SPHALERITE (200) , \24, , 0 1 • II 0 Na C , a a 4 0 2 4 • CHALCOPYRITE (112) COMPLETE POLE FIGURES (REFLECTION & TRANSMISSION) a EQUAL AREA (UPPER HEMISPHERE) PROJECTION CONTOUR LEVELS 1 ' 2, 2 - z 3 4 X UNIFORM F. FOLIATION FIGURE 6.17 Complete pole figures of sample R004, Rammelsberg. 205 (fig. 6.10d). The moderate preferred orientations of the [112] poles shown in figure 6.19 are the result of dynamic recrystallisation of elongate deformed old grains (fig. 6.11c) with the new grains having an orientation close to the old host grains. The distribution of peaks in fig. 6.19 is similar to those found by Lang (1968) for axisym- metric compression of polycrystalline chalcopyrite. The spread of peaks, however, is asymmetric with respect to the pole to the foliation plane and this possibly reflects a component of shear deformation. BARITE The preferred orientations in barite rich ore are shown in figure 6.16. There are a number of sharp peaks distributed over all of the pole figures which do not appear to be random but rather poorly developed small circle and girdle patterns. The abundant recrystallised old grains (fig. 6.7h) are probably responsible for this irregular peak distribution. Barite has not yet been deformed experimentally so there is only limited data on slip systems (Mugge 1898, Buerger 1928) and no data on the development of preferred orientations. SUMMARY The X-ray preferred orientation analysis of the Rammelsberg ores has largely been disappointing. The complex mineralogy coupled with large variations in grain size produces an irregular distribution of peaks over the pole figures. In most ore types, the recrystallised grains show little or no preferred orientation. Moderate preferred orientations were found in some galena and chalcopyrite ores. 6.6 DISCUSSION AND CONCLUSIONS GENESIS OF THE RAMMELSBERG ORES In any evaluation of the post depositional history of an ore body the primary mineralogy and structures must be taken into account. In this context, the problems of the genesis of the Rammelsberg deposit will be briefly discussed. 206 RAMMELSBERG CHALCOPYRITE Specimen R 006 COMPLETE POLE FIGURE (REFLECTION & TRANSMISSION) EQUAL AREA (UPPER HEMISPHERE) PROJECTION CONTOUR LEVELS 1 , 2, X UNIFORM F= FOLIATION PLANE FIGURE 6.18 Complete pole figure sample R006, Rammelsberg. 207 RAMMELSBERG CHALCOPYRITE SPECIMEN R001 1 3 ORTHOGONAL SECTIONS FOLIATION PLANE LINEATION 3 FtA:1/41111111 Reflection Mode 112 DIFFRACTION PEAK ...... 2 Contour Levels X Uniform I I 1 1 IX 2 2 2 2 2X 3X >4X FIGURE 6.19 Partial pole figures of sample R001, Rammelsberg. 208 Early workers favoured a magmatic hydrothermal replacement ori- gin for the ores (see summaries by Kraume 1955 and Schot 1971, 1973). Ramdohr (1953a), Kr'aume (1955), Schot (1971, 1973) and others, however, suggested that the deposit had been formed as a result of submarine mag- matic emanations at the bottom of several small isolated geosynclinal deeps, within a geosynclinal basin. Jenks (1975) suggested that the Rammelsberg deposit was transitional towards the strata bound Mississippi Valley type Pb - Zn deposits while Lambert (1976) classifies the Rammelsberg deposit with the McArthur River type. Gwosdz et al. (1974) showed that the geochemistry of the rocks underlying the ores, chara- cterised the presence of hydrothermal metalliferous emissions. Schot (1971) suggests that deposition occurred in shallow water and that the metals were derived from the exhalative products of ash yielding vol- canoes. The evidence for this hypothesis can be summarised thus:- 1. The concordant strata - bound position of the ore-bodies. 2. The presence of altered tuff layers in the Calceola and Wissenbach Shales. 3. The lack of alteration of the host rock. 4. The interfingering of the Rich-Ores with the Banded Ores and the host rocks in the strike direction and on the foot- wall side. 5. The presence of low temperature ore textures a) framboidal pyrite. b) rhythmic - concentric, pyrite-galena, pyrite-chalcopyrite and pyrite-marcasite. c) colloform and atoll textures. 6. The preservation of microfossils in the Banded Ores (Tere- bratula, Goniatites, Ammonites, Ostracods). 7. The presence of additional smaller separate and isolated ore lenses along st'ri'ke and also in the hanging wall and footwall of the orebodies. 8. The presence of unmineralised carbonate layers in the Banded Ores (points against the replacement of carbonate hypothesis). 209 In recent years the Kuroko ores of Japan have been accepted as type volcanogenic sulphide deposits (Sangster and Scott 1976). The Kuroko ores occur in the Miocene Green Tuff region of Japan. The host rocks are acidic pyroclastic flows and the mineralisation shows a close spatial relationship with lava domes, some of which show evidence of explosive activity, (Matsukuma and Horikoshi, 1970). In an idealised and fully represented Kuroko deposit a distinctive stratigraphic suc- cession is developed (Matsukuma and Horikoshi, 1970) and this can be compared with that developed in the Rammelsberg New Ore-body (Table 6.2a). Schot (1971) is of the opinion that the mineralised Kneist below the Rammelsberg deposit resulted from the fracturing and veining during the Variscan orogeny whereas Ramdohr (1953a) proposed that the Kneist was formed by ascending mineralising fluids as the ore lenses were formed (fig. 6.20). Although there are broad similarities between the Rammelsberg ores and the Kuroko deposits, in detail, the lack of felsic tuffs, tuff breccias and lava footwall rocks in the Rammelsberg deposit together with the abundance of detrital sediments suggests a more distal source for the metalliferous solutions than in the Kuroko ores. MICROSTRUCTURES AND TEXTURES OF THE RAMMELSBERG ORES The Kuroko deposits have only suffered low zeolite facies meta- morphism (clinoptilolite - mordenite zone to analcime - heulandite zone - T. Urabe written comm. 1977) with little deformation. Original depos- itional textures are well preserved (pellet-framboidal pyrite, colloform banding, graded bedding and fine compositional layering - Wantabe (1974)). These microstructures are very similar to those found preserved in the Rammelsberg ores. In addition, many of the primary depositional struc- tures of the Kuroko ores are relatively coarse grained (cf. McArthur River deposit - Lambert 1976) and these may be similar to the relict old grain structures found in the Rammelsberg ores. In contrast to the depositional microstructures, the deformation textures in the Rammelsberg ores can be compared with those of other deposits which have been metamorphosed and deformed. Similar microscopic deformation features and metamorphic foliations have been described from a number of deposits - Roseberry (Braithwaite 1974), Caribou (Davis 1972), - the Besshi type deposits of Japan (Kanehira and Tatsumi 1970) ' and the North American massive sulphide ores as described by Sangster and Scott (1976). In these ores deformation features such as folds, OLD ORE-BODY GREY ORE-BODY NEW ORE-BODY BARITE LAYERS AND LENSES SHALLOW SEA SEDIMENTS ...,..,,I, -..r..11. -1- •+ -1- + +1 -1- -1- -I- KNEIST—v 4" 4" 4\ 4- \4" 4- 4-7 4- 4) I ,44411111111111101■101■ 1 +++-1- + + + + -I- + + I a.m 1- - aew ■•■•■ •••■ ••••• a.= ••■•• ■••■• ■■■ ••■ 1 1 1 1 1 %1 1 1 (\ 4. I TUFF LAYER + + + + 1 I \ 1 1 / 1 ? \ /1 1 -1- +I •*.A.•:•:•:•:-:•:•:±:•:•:+:-:•:+:•:.i-: I \ 1 - - • -1-- • • +• • • + SANDSTONE LAYERS (after Schot 1971 ‘t / + ' ) i & Ramdhor 1953a) + FIGURE 6.20 Probable conditions of deposition of the orebodies in shallow marine basins . The fractures below the orebodies show hypothetical avenues for the ore solutions and formation of the Kneist according to Ramdhor(1953a). 0 211 boudinage, augen textures and remobilization of chalcopyrite into frac- tures and veins have been described. This study however, has also examined in detail the deformation microstructures developed in the Rammelsberg ores, whereas such detailed studies have not been conducted on the other deposits described above. The study of the Rammelsberg ores has shown the following:- a) the sulphide-gangue minerals can be placed in order of increasing ductility (T = 200 - 250°C - and similar grain sizes) - quartz : pyrite : sphalerite (also probably barite) : pyrrhotite : chalcopyrite and galena. This sequence is similar to that deduced from experimental deformation of sulphides (Atkinson 1972, 1974). b) barite, galena, sphalerite and chalcopyrite show recrystal- lisation textures particularly along twin boundaries and also the development of grain boundary mantle textures (cf quartz, White 1976). c) the ores show only poorly developed preferred orientations - often these are dominated by the large relict old grains. The lack of well developed preferred orientations may however indicate that diffusion controlled deformation processes (probably Coble creep - see Chapter 3) have been operating during metamorphism. d) X-ray texture goniometry is not a particularly satisfactory method for analysing the structural history of mixed ores with a complex mineralogy. Some deformation textures, how- ever, are preserved in the Rammelsberg ores. e) detailed optical microscopy and scanning electron microscopy of carefully etched samples does give extremely useful inform- ation on the deformation characteristics and history of the ores. These microstructural results are compatible with the ores having been deformed at low temperatures (probably less than 250°C). The experi- mental work of Siemes (1970, 1976), Atkinson (1972, 1974, 1976a) and McClay and Atkinson (1977) has shown that recognisable recrystallisation of galena occurs at 200 - 300°C in the laboratory and may be expected to occur at even lower temperatures during tectonic deformation. Similar results have been found for chalcopyrite (Clark and Kelly 1976, Roscoe 1976) whereas Stanton and Gorman's (1968) results indicate that sphalerite 212 also anneals at low temperatures. These experimental studies show that recognisable deformation and recrystallisation microstructures in tec- tonically deformed ores may be used to infer their deformational and thermal history. 213 CHAPTER 7 MOUNT ISA MINE, QUEENSLAND, AUSTRALIA. 7.1. INTRODUCTION The Mount Isa Mine, one of the world's largest concordant, base metal deposits, is located in Middle Proterozoic sediments of the PreCambrian of north-west Queensland (lat. 20° 44'S long. 139 29'E). There are two distinct ore types, both of which occur in the pyritic Urquhart Shale Formation (fig. 7.1). The silver-lead-zinc orebodies are well-bedded dolomitic siltstones and shales rich in sulphides (Mathias and Clark 1975) whereas the copper orebodies are located in massive brecciated and recrystallized 'silica-dolomites' (Bennett 1965, 1970 and Mathias and Clark 1975). Although folding is found in both types of ores it is best observed in the silver-lead-zinc orebodies. The grade of metamorphism at the Mount Isa Mine is considered to be low greenschist facies (Mathias and Clark 1975, Wilson 1973). This chapter begins with a review of the geology of the Mount Isa deposit and then presents an analysis of fold styles in the silver-lead- zinc ores. The diagenetic and deformation features of the silver-lead- zinc ores and also of the 'silica-dolomite' - copper ores are documented and described in detail. X-ray texture goniometry has been carried out on samples of galena rich ores. The results of the microstructural and X-ray texture studies are discussed in the light of the experimental deformation of galena (Chapter 4) and the deformation mechanisms which may have operated during folding and metamorphism of the ores. 214 LOCATION OF THE PRECAMBRIAN MINERAL BELT OF NW QUEENSLAND 1 ambrian MUNO CLONCURRY ISA 1 1 1 I_L 1 QUEENSLAND 9 590Km BRISBANE FIGURE 7.1 Locality map and local geology of the Mount Isa area. Symbols on the geology map are given in Table 7.1. 215 7.2. GEOLOGICAL BACKGROUND Detailed descriptions of the geology of the Mount Isa Mine have been given by Bennett (1965, 1970) and Mathias and Clark (1975). This summary is based largely on the above work. STRATIGRAPHY The Lower Proterozoic sequence east of the Mount Isa fault zone has been described in detail by Carter et al. (1961), Bennett (1965, 1970), Robinson (1968) and Mathias and Clark (1975). There are three main units in the sequence with the Tewinga Group at the base, followed by the Haslingden Group and then by the Mount Isa Group (Table 7.1). The Judenan Beds (fig. 7.1) are thought to be the metamorphic equivalents of the Myally Beds (Wilson 1972). The ore bodies occur in the Urguhart Shale Formation (fig. 7.2), which consist of well-bedded carbonaceous, dolomitic, quartzo-feldspathic siltstones, dolomites and tuffites. The silver-lead-zinc mineralization occurs as distinct concordant bands of galena, sphalerite and pyrite throughout this formation. Where there is a sufficient concentration of these bands, these constitute an orebody. Copper mineralization is restricted to areas of 'silica-dolomites' which is the local term used to describe fractured and recrystallized siliceous dolomites, charts and siltstones (Mathias and Clark 1975). In the Urquhart Shale formation, Croxford (1964) found tuff beds (Tuff Marker Beds - TflB's) which are now used for stratigraphic correlation in the mine. STRUCTURE The Mount Isa Group crops out on the western limb of a large North-South anticline, the axis of which lies about 18Km. east of Mount Isa. The beds have a predominant N-S strike and dip uniformly 60 - 65° west except where local folding has developed parallel to the regional pattern. A fold of 200m amplitude and plunging gently northwards, crops out at the northern end of the mine area (fig. 7.1). Elsewhere smaller folds, (which plunge both North and South) occur in the mine sequence, and these have been described by Blanchard and Hall (1942), Darlington (1961) and McDonald (1970). Fold axial planes have a variable strike: individual planes usually dip from 80° west to 80° east. TABLE 7.1 SUMMARY OF MOUNT ISA STRATIGRAPHY (after Plumb and Derrick 1975). LOWER PROTEROZOIC GROUP FORMATION THICKNESS (metres) LITHOLOGY SYMBOL (fig. 7.1) Magazine Fm. 210 Calcareous sericitic shales m Kennedy Fm. 310 Siltstone, Quartzite, Shale k Spear Fm. 170 Dolomitic siltstones s MOUNT ISA GROUP Urquhart Fm. 910 Pyritic dolomitic siltstones, shales u Native Bee Fm. 800 Dolomitic siltstones nb Breakaway Fm. 1040 Interbedded shales and siltstones b Moondarra Fm. 1220 Dolomitic siltstone Warrina Park Quartzite 500 Quartzite Myally Sub-group 4000 Sandstones and Quartzites HASLINGDEN GROUP Eastern Creek Volcanics 6000 Meta-volcanics and sediments ec (greenstones) Mount Guide Quartzite 2950 - 6000m Quartzite and quartz-mica schist TEWINGA GROUP Leichardt Metamorphics Quartz-feldspar-biotite gneiss STRATIGRAPHIC COLUMN MOUNT ISA GROUP DIAGENETIC DIAGENETIC ~.,.VA¥:.t. Top Faulted Out CARBONATES FELDSPARS -~--- MICROCLINE- 'B' Marker Horizon DOLOMITE & CAlCITE T _ : 't. Marker Henzon ALBITE SLTSTONE l ------Current B~d~ SIltstone DOLOMITET - TUFFACEOUS SEQUENCE I SIDERITE COMMON ~.-~~ C.... rft.l Bedd~ Dolomite> STRONG DIAGENESIS ~TSHALEIOr ------.0 Meir. TMB NEAR SI..ICA O 11'1 3000 .-~ NATIVE BEE OOL()w1ITET & CAlCITE ~ + NatIVe a.. Chert Marke>r SILTSTONE ~ (local UncontorlT1t~) z 2500 ~ Bre>akaway (Mrt Marker 1 du u DETRITAl SEOt.£NTATON x DIAGENESIS WEAK BREAJI./;WAYt LEGEND ~ SHALE I .....~ Vl i!""'!I'!i~ Pynhc Shalt's ,",':11I1:~ '.;,1" •••• I1~ ':' • Quartzite ~ ~ m U-!...!...J ~ o o C> Carbonact'O.lS-S.lceous Shale> Eo o oj'Sdlca-OoIomltt" lij (:-=J J: ~tIlARRAt SlLTSTQt£ ~ S.ICI!OUS DoIOmlIlC Shale I·.·. ·.·1 Sedimentary BrecclCl t-:-:-::3 DoIomIllc 5hde - SIltstone l1111I11I1J Marker Horizons alTE local (7Aeg1ona1) Unconformdy RDolomlte> ~ Unconlormlty _ _ _ _ A1b1hc s.tstone (after Mathias & Clark 1975) I- ---I N ~ FIGURE 7.2 Detailed stratigraphic column of the Mount Isa Group. '1 218 Recent structural investigations (MIM staff and Perkins 1976) have established that the sequence east of the Mount Isa fault underwent four deformations (Table 7.2). It was found that the first stage folds did not occur in the Mount Isa Group and that phase 2 folding was the major deformation event in the mine area. During this deformation phase an axial planar slaty cleavage was only poorly developed in the dolomitic Urquhart Shales but was more strongly developed in the more argillaceous Breakaway and Magazine formations (Perkins 1976). Detailed descriptions which are based on my own observations of the folds in the ores and in the Urquhart Shales, are presented in a later section of this chapter. In the mine sequence, both large and small scale faults occur (Bennett 1970, Darlington 1961). Major cross-cutting faults obligue to the bedding, often have up to 30m horizontal displacement. Other faulting occurs in shear zones (Racecourse shear zone, Bennett 1970) where microshearing and slickensiding occur over a wide zone. Thickness variations in the Mount Isa Group have been explained by penecontemporaneous faulting in the Mount Isa Basin (Smith 1969, Bennett 1970, Glikson et al. 1976). The Mount Isa Group is faulted against the basement greenstone sequence(fig. 7.3) but it is uncertain whether this is a penecontemporaneous fault (if as proposed for the major Mount Isa Fault Zone) or a later fault which has been folded. Lack of time and limited access precluded collection of adequate data for an analysis of faulting in the mine. METAMORPHISM Although the Mount Isa Mine is often cited as an example of an ore deposit metamorphosed under low greenschist (chlorite grade) facies conditions (Bennett 1970, McDonald 1970, Wilson 1972, Stanton 1972), the presence of chlorite in the Urquhart Shales is not definitive of greenschist facies conditions (Winkler 1974, p. 71 - 2). The MIM staff (pers. com.) however, maintain that correlation of the schistosity in the underlying greenstones (basement volcanics) with the cleavage in the Urquhart Shales indicates greenschist facies conditions. Fisher (1961) estimated a maximum confining pressure of 3 K bars and temperature of 300°C for the metamorphism of the ore minerals. Hewett and Solomon (1964) suggested that the metamorphic temperature may have been only 150°C. The fine grained dolomitic shales generally show little evidence of recrystallization and abundant carbonaceous material is preserved in the Mount Isa shales (Saxby 1976). TABLE 7.2 CORRELATION OF STRUCTURAL FEATURES IN THE MOUNT ISA REGION WEST OF, AND INCLUDING THE MOUNT ISA FAULT ZONE EAST OF THE MOUNT ISA FAULT ZONE WILSON (1972) CURRENT INTERPRETATION CURRENT INTERPRETATION (W. Perkins 1976) (W. Perkins 1976) SEDIMENTATION SEDIMENTATION SEDIMENTATION Slump folding? Slump folding (MOUNT ISA FAULT ACTIVE?) Minor slump folding PHASE 1 FOLDING PHASE 1 FOLDING PHASE 1 FOLDING not developed SLATY CLEAVAGE SLATY CLEAVAGE in the Mount Isa Group. REGIONAL DUCTILE FAULTING REGIONAL DUCTILE FAULTING MOUNT ISA FAULT MOUNT ISA FAULT PHASE 2 FOLDING PHASE 2 FOLDING PHASE 2 FOLDING Open folds with minor Crenulation cleavage formation, cross-cutting SLATY CLEAVAGE - Well developed in crenulation cleavage, intense mylonitic foliation in quartzites Magazine and Breakaway formations, deformation in Mount Isa Fault less well developed in Urquharts. MAJOR FAULTING - FAULTS 1 - 3 MAJOR FAULTING - FAULTS 1 - 3 WEST OF MOUNT WEST OF MOUNT ISA FAULT ISA FAULT PHASE 3 FOLDING - Kink folds, PHASE 3 FOLDING - Kink folds, axial plane PHASE 3 FOLDING - Minor crenulations lack strong axial plane crenulation cleavage in extreme cases developed only occasionally structure PHASE 4 FOLDING - Open Folds, slight PHASE 4 FOLDING - Open Folds folding crenulation cleavage - dominant in Mount slaty cleavage, good crenulation Isa Fault zone. cleavage in Mount Novit fault zone. MINOR CROSS FAULTING MINOR CROSS FAULTING MINOR CROSS FAULTING W SECTION 6500N E W SECTION 8000N E SURFACE V V V V Ar5.eas0 „ 4;1 P lo io 4/ 0 6° o e 91 4.1 h cy 'I 43 Pd bg 6 14 LEVEL HORIZON k: cd-a ero VVVVVr;?% as, vvvvvvw4 VVVVV rrrvvvvrtvvvvvvvv4l cti vvvvvvvvvd 0 200m. MOUNT ISA MINE CROSS SECTIONS (after Mathias & Clark 1975) COPPER OREBODY 111111 LEAD-ZINC OREBODY 0001 RE 00001 SILICA DOLOMITE BASEMENT VOLCANICS FIGURE 7.3a Cross sections through the Silver-Lead-Zinc Orebodies (with respect to Mine coordinates). 30 40 50 60 70 80 ‘...VVVY.VVIVVVVVVVVVVVVVVVVVVV,VVVVVVVVVV,r,,, VVVr,VVVvVVVVvVVVvVvVVVVVV VV, VV vvvoi,.... NIVVVVoyVVVVVVVVVVVVVVVVVVVVVNVVVVVVVVVVVVVIVVV,WVVVVVVVVVVVVVVVVVVVVV V ,VN dv.VVVV, •VV. J w V1".• VvVVVV , VVVVVVVVVVVVVVVVVVV,,VVVVVVVVVVVVVVVV1.,,VVVVVVVVVVVVVVVVVVVV VVVV..VvvVVvywWv1.! V v ..v•v . vvV... c.o..' NyVV!,, ,VvVVvvVyvVVVVVVVVVV%VVVVVVVvvvvy,vy vvvvv ,:vvvyvvvywyvvyVvvvvy vvvv .. ,.....,,,,,,..--,... ,rdvs, ,,, ,,,evvvvVVVVVVVVVVVVVVVVVvyvvyvvy‘vvv,,....,,,,,,,, ,,,,,, VVVVVVVy VVV.vV4y.VWWVVV vvvvv VVVV•14 I eld• WV V.,, .1.0.__.. _ _ — — _ Ken -Spear Siltstone ".40 ,JfVVVVVVVVVVVVVVVVVVV%VVVVV.Wdo ...... 0.4%y,.V.,VVVVVVVyVVVVVVVVV..V VVVVVVVVVVVVVVVV4.o4 .,"..,...,..,„„,— rec I/ ...... _ _ td.se IvvVVVVVVVVVVVVVVVV,,,,,,,,, -.,,,,,,,,,,,,,,,,,,,,... VVVVV4VVVVV:VVVV./ ,.....vV,1.11.. ...itr •••■ 44X4VVVVWv.vVyvVVVV4V yVVvy,./..VVvv.evyW vvv .... arv.' ,,,,,,,,,,,,,,,,,vvvvvvvvvvvs,,,,, ..---mmr... rat Urquhart Shale • ...,...rVVyvvevvyVVVVvVVVvVVvvvvvyk1 0 ..%1...ye, vvvv,vvvvy 01 0 0 0 063.7)--k 15,0- °Aro o o, /3.4? 9' 650 9.0 o VID0-957e1615-eiP (OS° o0-9- 6157) 0-6-6-6- - 43°. 0-0 0-0 0 o -0-o-o 0'6 o 0,...:zsz 9 o„.0- - 500 3W* DireaM} ggimal minim...0 5/110 Silica Dolomite and Copper Orebodies Atgc i5dT1:: SLQ.1112-913143, 5/200 20 s2 trz$, 7 Lead Zinc creozs , 4e42 VS' 8 ■r"."1".""m° Orebodies sio9_2-499_9_0.9 00.2 00 11 o0Z0,12, 12 9 0 951 ••••••110, 13/80 s51 ----- Main Shaft Area •M54 .. — P77 Fault 1(51 .. 30 J 52 -- /Fault LINES OF SECTIONS MOUNT ISA MINE COPPER BASEMENT OREBOOY VOLCANICS 14 LEVEL PLAN LEAD -ZINC GEOLOGICAL - - 670m below surface OREBOOY BOUNDARY 0 200m 0 0 0 000 SILICA DOLOMITE I FIGURE 7.3b Level plan of No. 14 level, Mount Isa Mine. 222 Illite crystallinities (fig. 7.4) measured by the author indicate anchi - to epi-metamorphic stages (Frey 1970). In all, it is probable that metamorphic temperatures at the Mount Isa Mine reached approximately 200°C. Unfortunately, samples which were collected for fluid inclusion studies, had only scarce primary, unleaked inclusions which were large enough to measure and these gave inconclusive results. GEOCHRONOLOGY The geochronology of the Mount Isa Region has recently been summarized by Wilson and Derrick (1976). The Mount Isa Group (and hence the Ag - Pb - Zn mineralization) was most likely to have been deposited between 1570 and 1500my ago (Wilson and Derrick 1976). This conclusion is supported by the single-stage lead model age of 1500 m.y. (Cooper et al. 1969). MINERALISATION The silver-lead-zinc mineralisation occurs as fine-grained bands of galena, sphalerite and pyrite in the well-bedded, fine-grained dolomitic Urquhart shales and siltstones. Other minerals found in the ores include pyrrhotite (Finlow-Bates et al. 1977), freibergite, tetrahedrite, minor arsenopyrite, chalcopyrite, marcorsite and chlorite. The mineralisation occurs discontinuously throughout a stratigraphic width of 1200m and over a strike length greater than 1600m (Bennett 1970). The known extent of mineralization is limited by current mine workings and drilling programmes and has not been fully determined. Economic concentrations of silver, lead and zinc in a single orebody, have been recorded over strike lengths of 1000m and 700m down dip (Bennett 1970). There are sixteen groups of economic beds - orebodies nos. 1 - 16. Orebodies 1 to 5 are up to 50m wide, fine-grained and contain abundant carbonaceous material (Bennett 1970) whereas orebodies 6 to 16 are generally narower and coarser-grained. The mineralisation is associated with fine-grained framboidal pyrite. Biological activity during deposition of the ores is indicated by the association of microfossils with the mineralization (Love and Zimmerman 1961, M. Muir pers. com.) M. Muir (pers. com. 1977) found an extensive microfossil assemblage - blue- green algae, of probably shallow water origin in the Urquhart Shale. ao- 223 NON - ANCHI- • • ■ • a • ■ to- • EPI -METAMORPHIC 0 0 01 42 03 04 0-5 002/001 INTENSITY RATIO • • Pb! Zn SAMPLES • SILICA DOLOMITE SAMPLES o McARTHUR RIVER SAMPLE • • • 0 • • 15 • in • 0.1 • • oa 9 M° • tn SM I PH • OR INCIPIENT TO WEAK METAMORPHISM TAM o ME s r PI E 41I DIAGENESIS • 3 4 5 6 ILLITE CRYSTALLINITY (QUARTZ INTERNAL STANDARD) FIGURE 7.4 Illite crystallinities measured on shale samples from both silver-lead-zinc orebodies and copper orebodies. 224 Folding in the silver-lead-zinc orebodies (particularly orebodies 2 and 5) has produced some mobilisation and redistribution of the sulphides (McDonald 1970). The copper orebodies are chalcopyrite concentrations in massive, brecciated and recrystallised 'silica dolomites' (Bennett 1965, 1970, Mathias and Clark 1975). The silica-dolomites are a sequence of variable thickness (up to 530m) of massive dolomites, cherts and fractured and silicified dolomitic siltstones. The 'silica-dolomites' interdigitate with the siltstones and shales but in many places sharp transgressive contacts are found. The transgressive contacts may be in part due to inhomogeneous deformation of the siltstones and shales around the more massive silica-dolomite and in part due to remobilization of the silica dolomite during deformation and metamorphism. Bennett (1970) and Mathias and Clark (1975) argue that the copper mineralisation is syngenetic whereas Murray (1961) and Smith and Walker (1971) favour an epigenetic origin for the copper orebodies. The fine-grained sulphides in the silver-lead-zinc orebodies are generally accepted to be sedimentary or early diagenetic in origin (Mathias and Clark 1975, Solomon 1965). Croxford (1962, 1964) and Stanton (1962, 1972) suggest a volcanogenic source for the sulphur and metals. The discovery of pseudomorphed sulphate evaporites in the Urquhart Shale Formation by the author and D. Carlile (McClay and Carlile 1978) enables one to suggest an alternative source for the sulphur and the metals. The genesis of both the copper and the silver-lead-zinc ores is discussed in detail in section 7.6. 225 7.3. STRUCTURES IN THE SILVER-LEAD-ZINC OREBODIES Folds and macroscopic structures in the silver-lead-zinc orebodies of the 11 - 13 M.I.C.A.F. (11 - 13 Level, Mount Isa Cut and Fill), the 11, 13, 14 levels, the 15B and 14C sub-levels together with folds in pyritic Urquhart Shales from 17 level 625S cross cut and 3000N cross section have been studied. Detailed mapping and sampling was carried out in 72 stope 14-C sub-level. Folds of amplitude 2 - 30 m can be readily discerned in the mine workings. Larger folds are difficult to trace because the uniform lithologies make bed to bed correlation difficult. (i) FOLDS OF AMPLITUDE 2 - 30 METRES Major fold zones which trend 10 - 15° west of north can be traced along strike and from level to level for several hundred metres and these cut the silver-lead-zinc orebodies in a number of places. On the 14C sub-level two such zones approximately 500m apart cut the no. 5 orebody. One such fold zone is shown in Figure 7.5. The ore in 5 orebody is folded for approximately 40% of the stoped length on the 14-C sub-level. Figure 7.6 shows a number of profile sections through folds in the silver- lead-zinc orebodies. It was not possible to separate slaty cleavage (phase 2) folds from phase 4 folds because the slaty cleavage is not well developed in the dolomitic Urquhart shales. Fold styles vary from 1C class (Ramsay 1967)(fig. 7.6b,c), to 1B class elastic folds (fig. 7.6 d), to near isoclinal folds (fig. 7.6 e,f). A detailed cross section of no. 5 orebody (fig. 7.7) shows fold relationships across the ore body. The orientation data for 72 stope and 14-C sub-level as a whole are shown in fig. 7.8. Phase 2 folds tend to plunge northwest at 40° - 70° while phase 4 folds are open flexures which gently fold early axial planes and have hinge lines which either lie horizontal or plunge shallowly (up to 10°) northwards. The most competent lithologies are the black dolomitic siltstones which form buckle folds (fig. 7.6, 7.7) that commonly take a form similar to that taken by a buckled elastic strut (elastica folds - Ramsay 1967). The laminated pyritic siltstones (fig. 7.6b) are also very competent but tend to form tight folds (fig. 7.6e). Radial fractures (fig. 7.6c) are often formed during folding of competent layers. These fractures themselves may become curved during subsequent flattening. Many fold limbs are fractured and boudinaged with the boudin fractures infilled with remobilized sulphides, quartz and carbonate. 226 651 I fli Tl \, \ \I m l 7700 ~ I 1 i I .: STRUCTURAL TRENDS a W I- " NO.5 OREBODY ~ ~'l ~O 50 8 ~j~rrhmr ,1 ~8~ ~~ V 14C SUB-LEVEL 1=76--0-0----I-~ of" .L I I ~....: "\j, ~ I dq - ~ ~ V') 0'168 6~ ", '" 7 I 0 -I .: ...... "" ; :fKHt:~/1I/fm lJJ lIn I !} I P77 FAULT ~rh ""' : SYSTEM ,'" 'h 68-1 ! 7500 ~J :I I 50-{ I '~ V') J I~ V') • t- l- N-. ~ I ~ ::> 62" .\ ex: "::> ~I , I I , I . I I I ,I 7100 !h 50 fJ"'~ ) IJ 'f'I ¥5 48-{ ~~6~t" ~~ ~ \, fi11£ ~v .A [( \r IT! QfmT811111 ~ +40 a 30m 551~ ! ! o 8 8 0 ~ 7000 § ~ co ~ N FIGURE 7.5 Trends of bedding in No.5 Orebody 14-C sub-level (from MIM mapping). SLATY CLEAVAGE FOlD FOlDING A SLUMP Fa.D FOLDS ~ PVRITIC SHALES I~y banded pyritic shal~s Hanging wall a Ortbody Surract txposur. GpprolC. coords. 6500N JOOOE 158 Subltve\ lmttre L.~Ocms (skttch 'rom bad! 0' stope) ELASTICAS COMPlEX FOlD ZOt£ 12 OREBOOV 11-13 MJC.A.F. 5 ORfBOOV FOLDS 72 STOPE 11 cnbody n-tJ MI.C.A.F. ~y isoctnal folds I 0 5mttr~ (sk~ch from back of stope) I 05 mttrt I lmttrt 1- FIGURE 7.6 Detailed profile sections of folds in the Silver-lead-Zinc ~ orebodi es . -.....J DETAILED CROSS SECTION 5 OREBODY 72 STOPE 14C SUBLEVEL 201E OF ISOCLINAL FOLDING PYRITIC DOLOMITIC SHALES \\ („\\ \\\ g tik 1'4\4 \44• 'tg HIGH-Y FOLDED GALENA RICH SHALES THTLY BANDED PYRTIC SHALES-HIGHLY FOLDED AND SHEARED E M G ,,tLvE GALENA hiC 30 1 ■ -05 \.47 \-64\0 V° 146 -340 350 AL LOS \2321 X3I4 frD3 15-35 °\4t -320 5440 \ -3O 3r340 35-330 3617 N457:3)3N'r6°295 360 )-1 346 SD 40 30 25 20 15 5 0 1 IS 1 SCALE (FEET) SECTION LOOKING SOUTH HOR SCA , E = VERT SCALE DOLOMITIC SHALES El MARC DCL014mc SHALES 9 9 GALENA RICH SHALES j45 BEDDING FOLD AXIS cl CLEAVAGE tic LIMING Cl F ZIA1f4F NTERSECTION 33- FIGURE 7.7 Detailed section of 72 stope 14-C sub-level (line of section on Fig. 7.5). Horizontal scale = vertical scale. N I XX x Xx x x • X X X x • X xX • • x • • x • • • •• •• 5, 0 X •• • +ie. • • • • • . 0. flit • • + • • • • 11P0 • • 11. •• • I I:** I • a • • S • • • • • as . • • • • • •=pole to bedding x =minor fold axis equal area, southern hemisphere projection 1 72 STOPE 74 - 82 STOPES ( 52 poles to bedding, 29 fold axes ) (155 poles to bedding, 17 fold axes) (from MIM mapping) FIGURE 7.8 Orientation data from No. 5 Orebody 14-C sub-level. 230 ii) FOLDS OF AMPLITUDE 1 - 100cms Hand specimens of the Mount Isa silver-lead-zinc ores show many spectacular minor structures (figures 7.9 - 7.11). These folds are parasitic to larger folds and are often contained between either unfolded layers or layers with only low amplitude flexures. In this size range of folds three main styles have been recognised. These are a) folded dolomitic siltstones in a galena rich matrix, b) folded laminated pyrite rich beds and c) folds in mixed ores - pyrite-sphalerite - galena - dolomitic siltstones. The first type of folding is illustrated in fig. 7.9. Black dolomitic siltstone layers (which contain only minor sulphides) are the most competent units and they form folds which at low amplitudes are simple buckle folds (fig. 7.9(a)) but which at higher amplitudes have shapes which are similar to elastica folds (Ramsay 1967), as shown in figure 7.9(b). Black dolomitic siltstone layers of different thicknesses develop folds of different wavelengths (fig. 7.9 (c,e,f,g)), in a manner similar to that predicted by classical buckling theory (summarized by Ramsay 1967, p.372). Tension fractures associated with the folding of the competent siltstone layers are found in fold limbs and around the outer arcs of fold hinges (fig. 7.9 (b,d,e,f,g)). Most folds of black dolomitic siltstone layers are markedly asymmetric (fig. 7.9 (a,b,e,f,g)). This fact, together with the abundance of shear fractures (fig. 7.9(d)) along fold limbs, indicates a probable component of shearing parallel to the layering, i.e. flexural slip folding (Ramsay 1967), with the layers rich in galena acting as decollement surfaces. From the photographs in figure 7.9 it is possible to infer the development of breccia structures (fig. 7.9(h)) in the matrix galena. Thin layers of siltstone become folded and fractured (fig. 7.9 (c,e,f,g)) and with increasing strain the siltstone shale fragments become separated and 'float' in a matrix of galena, so forming a 'breccia' structure as shown in figure 7.9(h). The second type of fold (fig. 7.10) is markedly different from those described above in that the laminated pyritic siltstones fold as a multilayer packet rather than as an isolated layer in an incompetent matrix (c.f. fig. 7.9). The folds in laminated pyritic siltstones are commonly chevron in style (fig. 7.10 (a,b)). Accommodation structures such as saddle reefs (fig. 7.10(a)) and faulted hinges (fig. 7.10 (a,b)) are also common. Fold limbs show well developed boudins (fig. 7.10 (c,f) which are often rhomboidal in shape. The boudinage on fold limbs and 231 Figure 7.9 FOLDS IN SILVER-LEAD-ZINC ORES. a) Asymmetric fold in dolomitic siltstoneembedded in a galena rich matrix. Smaller wavelength buckle folds have developed in thinner dolomitic siltstone units. Sample T7 15B sublevel, 12 ore-body. b) Elastica folds of black dolomitic siltstone in galena matrix. Note fractures along outer edges of fold limbs which have lead to spalling of siltstone fragments into galena matrix. Sample T10, 15B sub-level. c) Detail of fold in (a). Note buckled and fragmented thin shale- siltstone layers in core of fold. White patches are euhedral pyrite porphyroblasts. d) Sheared fold limb from the same horizon as (a). Note the extension fractures on the outer edges of the thick layer. The matrix of galena has a typical breccia structure with shale fragments (black) and large euhedral pyrite porphyroblasts. Sample T8, 15B sub-level 12 ore-body. e) Folds of thin black dolomitic siltstone showing development of radial fractures. Note that the matrix of galena has zones of fine grained breccia structure. Sample T59, 9 ore-body, 11 - 13 M.I.C.A.F. f) Recumbent folds of black dolomitic siltstones in a matrix of fine grained galena (with laminae of pyrite grains). At the top of the specimen where there is a breccia structure, the galena is coarser grained. Sample M68, 11 ore-body, 15B sub-level. g) Folds in black dolomitic siltstone in a galena rich matrix. These show the development of breccia structures where thin layers become buckled and fractured leaving fragments of shale/siltstone in the galena rich matrix. Sample, T60, 9 ore-body, 11 - 13 M.I.C.A.F. h) Banded breccia structure of galena and shale fragments between slightly folded siltstones and pyritic siltstones. Sample T14, 12 ore-body 11 - 13 M.I.C.A.F. B C 'esetostudest,‘Aidui— 1-20-n-.1 4 E 4 cm --1 H 233 Figure 7.10 FOLDS IN LAMINATED PYRITE-RICH SILTSTONES a) Chevron folds in laminated pyritic siltstones. Note the development of saddle reef structures in the hinge zones and also micro-faulting on the fold limbs. b) Folds in laminated pyritic siltstones with failure at the fold hinges. Fold limbs are boudinaged. c) Laminated pyritic siltstone/shales (lower half of specimen) and black siltstone layers in galena rich matrix (upper half of specimen). Shear boudins are shown in the lower half of the specimen while the black dolomitic siltstone layers are buckled and fractured. d) Isoclinal type fold in finely laminated pyritic siltstones. Pyrite layers are continuous whereas shale layers are boudinaged. A later shear fracture cuts across the beds. Sample F5, 5 ore-body, 72 stope, 14C sub-level. e) Asymmetric class 1C folds in laminated pyritic siltstones. Note the more disrupted style of folding in galena rich beds above and below the thicker siltstone layers. Sample M28. f) Rhomboidal boudins in thick siltstone layer in a finely laminated pyritic matrix. Large pyrrhotite grains are seen in the boudinaged layer. g) Intersecting fractures. The N - S trending fractures are probably parallel to the b tectonic axis whereas the E - W fractures are not penetrative - are filled with quartz - carbonates and sulphides and are probably a - c type joints. Sample T2, footwall of 13 ore- body, 15B sub-level. 235 accommodation structures in the hinges indicate a marked flexural slip component of folding. Isoclinal folds (fig. 7.10d) are found in the laminated pyritic siltstones particularly in fold zones such as that illustrated in figure 7.7. Both radial fractures (parallel to the b tectonic axis) and a - c fractures (Price 1966, p.114) occur in some folded laminated pyritic siltstones (fig. 7.10(g)). Folds in mixed ores (fig. 7.11) are commonly flattened buckle folds with the sulphides - particularly galena remobilized into fractures. Where these folds are broken up only fragments are left in a matrix of remobilized sulphides (fig. 7.11(c)). Isoclinal folds (fig. 7.11(d)) and buckle folds (fig. 7.11(e)) are found in mixed ore zones. Decollement surfaces (fig. 7.11(f)) occur where galena is interbedded with laminated pyrite. Rhomboidal boudins are also developed in mixed ores (fig. 7.11(g)). Graphitic slickensides are commonly found on fold limbs and around fold hinges. These indicate that the beds were lithified and that the folding occurred at elevated temperatures and pressures. 236 Figure 7.11 FOLDS AND MICROSTRUCTURES IN PYRITIC - SILVER-LEAD-ZINC ORES. a) Recumbent folds in sphalerite-galena rich matrix. Fractures are infilled with remobilized galena. Sample M24, 5 ore-body 72 stope 14 - C Sub-level. b) Recumbent folds with remobilized galena in fractures. Where folds have been broken up a breccia structure occurs. Sample T50, 5 ore-body 72 stope 14 - C sub-level. c) Highly folded and fractured ore with wide spread redistribution of galena. Only fold remnants remain. Sample T11, 12 ore-body, 11 - 13 M.I.C.A.F. d) Isoclinal fold in sphalerite-pyrite layers. Sample M67, 5 ore-body 72 stope 14 - C sub-level. e) Contorted buckle fold in galena matrix. Sample T58, 9 ore-body, 11 - 13 M.I.C.A.F. f) Decollement structure along a galena layer with folded and fractured laminated pyritic siltstones above. Sample T40, 5 ore-body. g) Boudinaged pyrite and sphalerite layers in a matrix rich in galena. Note the rotation of the boudin fragments. Sample T10, 11/60 ore-body, 15B sub-level. 238 (iii) ANALYSIS AND CLASSIFICATION OF FOLDS A number of folds from hand specimens and underground exposures were studied for detailed analysis. These folds illustrate the way in which different lithologies in the orebodies behave during deformation. The folds are analysed by the method of dip isogons (Ramsay 1967, p.360) or the t'/a plot (the ratio of orthogonal thickness of the folded layer at an inclination a to the thickness of the layer at the hinge,versus the angle of inclination a of the layer with respect to the axial plane, (Ramsay 1967, p. 365-8)). Figure 7.12 shows profile sections of four selected multilayer folds. Dip isogons have been constructed for various angles of inclination to the axial plane. The more competent units (dolomitic siltstones, laminated pyritic shales and siltstones) generally form buckle folds with convergent dip isogons (class 1B - 1C, Ramsay 1967). In figure 7.13(a) it can be seen that the dolomitic siltstones either plot close to the 1B class or in the 1C field. The pyritic shales are slightly less competent than the dolomitic siltstones and they form buckle folds in the 1C class (fig. 7.12, 13(a)). Galena layers and galena rich shales, however, form either class 2 folds (parallel dip isogons, fig. 7.12), or class 3 folds (fig. 7.12, 13(a)) with appreciable thickening in the hinge and con- commitant thinning of the limbs. Sphalerite layers tend to form 1C or 2 class folds (fig. 7.12). The finely layered sphalerite and pyrite multilayers fold in a similar fold style (fig. 7.12) with near parallel dip isogons. A detailed analysis of a number of single layer folds in dolomitic siltstones shows that these plot along the flattening curves (fig. 7.13(b)) for parallel folds deformed according to Ramsay's model of buckling followed by homogeneous flattening. Dip isogons and t'/a plots illustrate the asymmetry of many of the folds which may have limb ratios up to 1:3. The fold classification and isogon studies enable the various ore types to be placed in decreasing order of competency, assuming all other things, i.e. thickness, grain size being equal. Black carbonaceous dolomitic siltstones - 1B folds - flattened to 1C folds Pyritic dolomitic shales - 1C folds Pyrite and sphalerite - 1C to 2 folds Pyritic shales and galena - 2 to 3 folds Galena - 3 folds. 239 DIP ISOGONS MOUNT ISA FOLDS SAMPLES FROM 5 OREBODY 72 STOPE 14C SUBLEVEL ISOGONS AT 10' Galena Dolomitic Siltstone Pyritic Shale 2 cms Sphalerite Black —Dolomitic Siltstone Galena rich Shales • Pyritic Shales\ Finely Laminated Sphalerite and Pyrite 5 cms o.5metre CLASS 1C FOLDS— SILTSTONES ISOGONS-0',15',30'.45760! SIMILAR FOLDS ISOGONS- CLASS 28.3 FOLDS— GALENA RICH SHALES 12 OREBODY 11-13 M.I.C.A.F. 5 OREBODY 14C SUBLEVEL 72 STOPE FIGURE 7.12 Dip isogons on profile sections of folds in the silver- lead-zinc ores. CLASSIFICATION OF MOUNT ISA FOLDS t'Ia Oa FOR FLATTENED PARALLEL FOLDS 1-2 1.2 CLASS 1A CLASS 1A CLASS 1B CLASS 16 10 1.0 Dolomitic Siltstone 08 Pyritic Shales Plot of Large Fold in 06 06 Dolomitic Siltstone Galena CLASSIC 12 Orebody 11-13 M.I.C.A.F. 0.4 04 CLASS 3 CLASS 3 02- 02 CLASS 2 CLASS 2 0.0 0.0 I I I 1 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 a B a FIGURE 7.13 Fold classification based upon t'/a plots. 241 7.4. MICROSTRUCTURES 7.4. (i) INTRODUCTION In this section, the microstructures of both the ore minerals and the gangue minerals are documented and described. Samples from the silica-dolomite, the silver-lead-zinc ores and from unmineralized UrOhart Shales have been studied. Detailed petrological studies of the Mount Isa Group have been carried out by Van den Heuval (1969) but the results are unpublished except for a summary given by Mathias and Clark (1975). Samples described in this study, were collected by the author and D. Carlile (Carlile 1977). 7.4. (ii) TECHNIQUES In addition to the localities cited in section 7.3 , samples were collected from the 3000N cross section (Carlile 1977), cross section 525S (silica-dolomite-copper orebodies) and from surface exposures at the Mount Isa and Hilton mines. Conventional polished section and thin section petrological examinations were made. Some thin sections were cut to 10 - 15pm thickness in order to be able to see microstructures in the very finegrained dolomitic shales. Polished sections were etched using the techniques described in Appendix A. Thin sections were stained using HCl/potassium ferricyanide in order to determine iron distribution in the dolomites. A potassium permanganate-barium chloride stain (Poole and Thomas 1976) was used to detect the presence of gypsum-anhydrite. Polished fragments and thin sections were examined in a Cambridge 600 and IIA Scanning Electron Microscope equipped with an energy dispersive X-ray analyser. 7.4.. (iii) LITHOLOGIES The results of petrological studies on Urquhart Shale Formation rocks are presented in this section. The Urquhart Shale Formation can be subdivided into three groups - unmineralized Urquhart Shales - mineralized Urquhart Shales - and the 'silica-dolomite' (fig. 7.2). The unmineralized Urquhart Shales are fine - grained (5 - 25pm) dolomitic siltstones and shales. The principal minerals are dolomite, calcite, quartz and pyrite with some detrital mica and feldspar. Authigenic feldspar and clays are also present. Tuff marker beds have a high concentration of volcanic shards, feldspar fragments, and authigenic 242 potassium feldspar. The mineralized Urguhart Shales are similar to the unmineralized shales except that there is an increase in the pyrite content and that galena sphalerite and freibergite are present in economic quantities. Van den Heuval (1969) noted that there was an increase in the amount of potassium feldspar in the mineralized Urguhart Shales. Pyrite is mainly framboidal with some larger euhedral grains commonly growing on framboid cores. Four main 'silica-dolomite' lithologies have been recognised; coarsely crystalline dolomite, marginal siltstone, chert-dolomite and chert (cf. Van den Heuval 1969). The distribution of the silica-dolomite rock types has been described by Van den Heuval (1969) and Mathias and Clark (fig. 11, 1975) but the samples collected in Carlile's study (1977) do not fit a simple pattern. The marginal siltstones occur at the top and at the margins of the silica dolomite mass and the chert occurs towards the basement fault (fig. 7.14). The dominant mineralogy is dolomite, ferroan dolomite, quartz and calcite. Carbonaceous matter is found in bands and along grain boundaries and is particularly abundant in the chert horizons and in the marginal siltstones. Other minerals in the silica-dolomites include chalcopyrite (copper orebodies), pyrrhotite talc, chlorite, sericite and phlogopite. 7.4. (iv) SEDIMENTARY AND DIAGENETIC MICROSTRUCTURES SILICA DOLOMITE The most important microstructures found in the silica-dolomite rocks are sedimentary or diagenetic and are illustrated in figures 7.15 to 7.19. Bedding In the marginal siltstones, laminated cherts are intercalcated with 'chicken wire' nodular dolomite layers (fig. 7.15a). Large poikilotopic dolomite grains occur in bands, between laminae of fine-grained pyrite (fig. 7.15b). Chert layers contain finely laminated carbonaceous material (fig. 7.15c) identical to that found in many silicified stromatolites (W. Diver pers. comm.) and as such are interpreted as cryptalgal (see also Bennett 1970, Stanton 1962). Stylolites parallel to bedding plane traces are found in the coarsely crystalline dolomite layers (fig. 7.15d). 243 GEOLOGY 3000N CROSS - SECTION W E ,, / / I , •• • 7 +1(, , 0' I(, ~ I " ~o, I ~.. ~ I 0., 0, , ~ 9 I q. • • • • • ..q., .. ~, .::,~ ,:., 0 , .::,q. • L E • 12 V " E • • 13 L S 1~ • 1.5 16 m 17 18 Lead-Zinc • F.ult Orebodies • Silica Basement Sample • Locality m;o~1o 0 Dolomite Greenstones 0 100 200 I , metre. FIGURE 7.14 Geological cross-section at 3000N (after Carlile 1977). 244 Diagenetic Microstructures This study resulted in the discovery of pseudomorphed sulphate evaporites at Mount Isa (McClay and Carlile 1978). Two types of pseudomorphed sulphate evaporites have been found in the laminated cherts (fig. 7.15d) and interbedded coarse-grained carbonates (fig. 7.15a). One type of pseudomorph is discoidal or angular dolomite single crystal replacements after gypsum and these are found in the fine-grained dolomi dolomitic siltstones and in the coarse-grained carbonates (fig. 7.15e). The other kind of pseudomorph are lath like replacements after early diagenetic anhydrite (fig. 7.15f). Pyrrhotite is also found, completely replacing anhydrite and gypsum (fig. 7.15g,h). The lath-like pseudomorphs are most easily recognised in the laminated chert layers (fig. 7.15f) where they cut across and distort the bedding (fig. 7.15f,i) indicating that the original sulphate minerals crystallized in the host sediments before compaction - i.e. during early diagenesis (McClay and Carlile 1978). These pseudomorphs have square cross-sections and now consist of a mosaic of quartz grains of a size (50 - 100pm) which is consistently larger than that of the enclosing chert (fig. 7.15i,j; 7.16a). Such textures indicate that the pseudomorphs are void fillings similar to the pseudomorphs after early diagenetic anhydrite from the Pine Point Pb/Zn deposit (Beales and Jackson 1966). The other type of pseudomorphs are large (0.1 - 3cm) single crystal dolomite replacements after gypsum and these are found in the fine-grained (5 - 25pm) dolomitic siltstones and in the coarse grained carbonates. These pseudomorphed crystals have either angular or discoidal terminations. The discoided types have a similar habit to gypsum which has developed interstitially in recent (Park 1977) and ancient sabkha environments (Gill 1977, Shearman 1966). In some of the dolomite pseudomorphs, minute relics (10 - 20Pm) (fig. 7.16b) of anhydrite were found with the petrological microscope thus indicating a complex replacement history of gypsum - anhydrite - dolomite. This indentification was confirmed by X-ray diffraction of carbonate fractions and by scanning electron microscope microanalysis. Staining of several thin sections (Poole and Thomas1976), indicated the presence of minute sulphate inclusions in dolomite grains. The preservation of anhydrite relics; of carbonaceous material (Saxby 1976); the textures and the reduced nature of the sediments (Bennett 1970, Mathias and Clark 1975) indicates that the discoidal gypsum is of evaporitic origin rather that that formed by the degradation of sulphides (Dean et al 1975). 245 Figure 7.15 SILICA DOLOMITE MICROSTRUCTURES a) Laminated fine grained siltstone-chert (dark) and coarse grained carbonate layers which are cut by later veining. Sample 1604, 3000N cross section, 16 level. b) Coarse grained euhedral-anhedral dolomite grains (white) in pyritic (dark) matrix. Thin section plane polarized light; sample 1607, 3000N cross section, 16 level. c) Whispy laminated chert with crypt algal structure. Black spots are pyrite euhedra and black stringers are carbonaceous material. Thin section plane polarized light; sample 1206, 3000N cross section, 12 level. d) Thin section of coarse grained dolimite layers (white-upper half of photo) and finely laminated chert layers (lower half of photo). Chalcopyrite (black) is associated with cross cutting veins and bedding parallel stylolites; sample 1205, 3000N cross section, 12 level. e) Photograph of hand specimen showing fractured pseudomorphs after gypsum. Sample 1204, 3000N cross section, 12 level. Scale in cms. f) Large elongate pseudomorphs (white) after anyhdrite crystals in interbedded chert laminae and coarse grained dolomite. Sample 903, 3000N cross section, 9 level. g) Pyrrhotite (black) with angular and discoidal terminations in fine grained dolomitic siltstone. Plane polarized light. Sample 1405, 3000N cross section, 14 level. h) Pyrrhotite laths (white) in fine grained dolomitic siltstone and chert. Reflected light. Sample M64. i) Mosaic of quartz grains replacing anhydrite laths. Crossed polarizers, tint plate. Sample 903, 3000N cross section, 9 level. j) Detail of (i) showing void filling texture of quartz mosaic in pseudomorph after anhydrite. v, 3 1--WDL-1 WD ---• - • ' +.‘ tr • I • V • .•• • • ,, .t . • . ▪ r . • j', ■ • 7114114' •••;:, , v. • ( V7• , , .. .5 • ' 247 Figure 7.16 DIAGENETIC MICROSTRUCTURES IN THE SILICA-DOLOMITE a) Radiating lath like pseudomorphs after anhydrite in coarse- grained dolomite. Crossed polars with tint plate. Sample 903, 3000N cross section, 9 level. (x12) b Small anhydrite relict in dolomite. Sample 1203, 3000N cross section, 12 level. (x1200) 248 Figure 7.16 SILICA DOLOMITE MICROSTRUCTURES Coarse-grained quartz with carbonaceous material outlining evaporite pseudomorphs or dedolomite microstructures. Sample 1406, 3000N cross section, 14 level. (x12) d) Quartz (dark) replacing dolomite (white) with the original grain boundary outlined by bubble trails. Dark field, sample SC32, 525S cross-cut, 17D sub-level. (x75) 249 Figure 7.16 SILICA DOLOMITE MICROSTRUCTURES WV," '1001111114r •IWO . a i .0, 411t . , -r . , .1 4 fo ' k • -• 4-6. .i"-' 411 0-* " 4 ik,Z° • * ' • .4 • '.1 RI s r /.. , - . *41/, • e) Band of coarse grained carbonate with thin laminae of fine grained chert below. Crossed polars with tint plate. Sample 1408, 3000N cross section, 14 level. (x12) f) Chert with lath like quartz surrounded by fine grained pyrite and carbonaceous material (both black). Crossed polars with tint plates. Sample 1101, 3000N cross section, 11 level. (x120) 250 Figure 7.17 SILICA DOLOMITE MICROSTRUCTURES a) Radiating growth texture of pyrite (white) with abundant included carbon (black). Note texture revealed by feather like inclusions of carbon. Sample SC9, 525 cross-cut, 17D sub-level. b) Similar texture as (a) but of carbon inclusions in dolomite grain (white). Plane polarized light, Sample 907, 3000N cross section, 9 level. o c) Euhedral grain of dolomite with a relict 90 cleavage. Dark field photograph. Sample 1309, 3000N cross section, 13 level. d) Beds of coarse grained dolomite intercalcated with fine grained chert. Crossed polars with tint plate. Sample 1311, 3000N cross section, 13 level. e) Carbonate nodules deflecting the bedding in fine grained chert. Sample 1406, 3000N cross section, 14 level. f) Laminated chert with whispy bands of carbonaceous material-crypt algal laminations? Sample 1309, s000N cross section, 13 level. g) Nodule of chlorite showing bend lattice planes and kink bands. Sample 151, marginal siltstones, 15B sublevel. h) Detrital quartz grains (q) with a fine rim of siderite and with substantial quartz overgrowths. In places these quartz grains are replaced by ferroan dolomite. Sample 1102, 3000N cross section, 11 level. i) Laminated chert (bottom) and fine grained carbonate (top of photo) separated by a stylolite with residue of carbonaceous material. Sample 1309, 3000N cross section, 13 level. j) Coarse grained dolomite cut by stylolite with chalcopyrite (black). Coarse grained carbonate has replaced laminar chert (black carbonaceous laminae left). Sample 905, 3000N cross section, 9 level. 251 Olt; .4,11 4' .yu vo• lit 1 . 1 .11711PI! / lor "i" " ~ ______~252 SILICA DOLOMITE MICROSTRUCTURES coarse euhedral quartz B I I c --bedding o rosette structure I I E rosette structure in dolomite I I I I ...... :..-." : -. '. ...;II: :) : ~ : :, '.~ .u:=:. ".:.. :.:.'.: ~;;: I~" fluid inclusions S .- .:,~:.. l,"~";'.":'''' i . rt .1-'+ ::." .. ~ •• :~ : •••... ,. In qua z .. - .. ,' .~.: . ' ..' .... LJl- -r .. \ ~.:a·:-~i-r.' ~ "'i' • ., • ••..••.• , I •••••••••• ,:s $.. :. :.::. (l.,r\ _ _ & 0'. ,.", I • , ,.,~ 'I.., ':{"/J":'N~ ""::;'--"'''''0'' i} .. -'- ..\-rA-t + -..J - \ ~ :;:-:'-::.~': .... :! '; '::~~'~"!: .L \ \ .... ) -..,I.,.±I _ .±..:l_ ~ )-'J :l: :-C:': • .' .: !", ,' ... ' I • ~!: ~ -: ~'! : :~. ~' .. :.L fIlA --_\ I:' ',' t·:.' ~ . 'tt:::~. 900 re-entrant angles ~:' ~:. ::". I 'l ... :·: :' +- ,; .. ,\.',-:.-' and relict cleavage in dolomite . "'~:"'. G '1#1#':"'", Ie:. I I H I ALL BAR SCALES = 500~m FIGURE 7.18 Silica-dolomite microstructures. 253 Other textural features of the silica-dolomite indicate replacement of sulphate evaporites (fig. 7.16 d - f; 7.17; 7.18). Cockade structures (fig. 7.17a), herring bone structures of included carbonaceous material (fig. 7.17b), relict 90° cleavage and 90° re-entrant angles (fig. 7.17c; 7.18g). Figure 7.16c shows evaporite pseudomorphs'or perhaps dedolomite textures. Some coarse grained carbonate layers (fig. 7.16e; 7.18b,c) have fanning and rosette textures (fig. 7.18d,e). Silicified nodules (fig. 7.17e), and lath like grains (fig. 7.16f) are common in some chert layers. Finely laminated carbonaceous is abundant (fig. 7.17f). The nature of the pseudomorphs, the relict anhydrite, the textures and the petrological characteristics of the sediments at Mount Isa (Van den Heuval 1969, Mathias and Clark 1975) are similar to those found in sabkha-intertidal environments (Park 1977, Kinsman and Park 1976, Gill 1977, Shearman 1966). Evaporitic features have been found over a thickness of 80m and down dip for 500m (levels 9 - 16)(McClay and Carlile 1978). Determination of the full extent of the evaporites is hampered by limited sampling and mine access. The implications of the discovery of pseudomorphed sulphate evaporites are discussed in the conclusions of this chapter. URQUHART SHALES (including the Silver-Lead-Zinc orebodies) ThisieCtion is. a brief resume of the diagenetic changes in the mineralized and unmineralized Urquhart Shales. Van den Heuval (1969) _ noted significant authigenic K-feldspar, chlorite and phlogopite. In this study, K-feldspar was found in a few X-ray diffraction traces but ' was not detected in many samples. Nodules with kinked chlorite grains (fig. 7.17g) were found in some dolomitic shales. Overgrowths on small quartz grains (fig. 7.17g, 19a) are often indicated by dust rims or rims of small siderite grains. Overgrowths of dolomite and calcite are also foudd (fig. 7.19a). Small nodules of quartz, calcite and chlorite occur in some dolomitic siltstones (fig. 7.9c,d). The extremely small grain size (5 - 20pm) of most of the dolomitic shales and siltstones made petrological studies (even with 10 - 15pm thick sections) very difficult. As we shall see in the following sections, most of the microstructures in these rocks can most probably be attributed to deformation features such as pressure solution and recrystallization. 254 Figure 7.19 DOLOMITIC - PYRITIC URQUHART SHALES a) Fine-grained dolomitic siltstone. 15um thin section - crossed polars with tint plate. Overgrowths can be seen on the quartz grains (purple-pink). Fine grained dolomite is yellow. Sample 151, 13 0/B, 15B sub-level. (x1200). b) Cluster of pyrite and carbonaceous material. Pyrite has infilled most of the cellular carbon but thin walled unfilled and partially filled cell-like forms can be found. (x 1200). 255 Figure 7.19 DOLOMITIC - PYRITIC URQUHART SHALES c ) Microboudinage of pyrite layers with a cross-cutting vein of dolomite. Pyritic and pyrrhotitic shales close to silica- dolomite. Sample 901, 9 level, 3000N cross section. (x12) d ) Fine grained dolomitic siltstones with carbonaceous stylolites cross- cutting thin dolomite veins which have produced poikiloblastic dolomite crystals which encompass the fine-grained dolomitic matrix. Sample No. 22, 3000N cross-section. 256 Figure 7.19 DOLOMITIC - PYRITIC URQUHART SHALES e) Slaty cleavage in dolomitic siltstones with thicker layers fractured and boudinaged. Sample 1403, 14 level, 3000N cross-section. (x12). f) Gypsum pseudomorphs in dolomite, Amelia dolomite, McArthur River, NT, Australia. Sample M. Muir. (x25). 257 7.4 (v) DEFORMATION MICROSTRUCTURES SILICA DOLOMITE Deformation in the silica-dolomite results in fracturing, boudinage and veining of the coarse grained carbonates and cherts and folding of the marginal siltstones. Fracturing and microboudinage (fig. 7.19c) associated with strong dolomite veining are common throughout the silica-dolomite (Mathias and Clark 1975). Stylolites parallel to bedding have been found in dolomitic siltstones (fig. 7.17i and 7.19d) and in massive coarse-grained carbonates (fig. 7.17j). Replacement of dolomite by quartz (fig. 7.16d) and the growth of dolomite poikiloblasts (fig. 7.19d) appears to be associated with veining and one of the early deformation episodes (Table 7.2). A close inspection of fig. 7.19 shows the complexities of the veins and poikiloblasts. These features have been affected by later deformation as indicated by undulose extinction and subgrains in the quartz and also by stylotites cutting the dolomite poikiloblasts (fig. 7.19d). Calcite in the silica-dolomite rocks contains abundant broad deformation twins whereas twins are rare in dolomite grains. A slaty cleavage is well developed in some marginal siltstones of the silica dolomite (fig. 7.20). The cleavage is characterized by a dark striping of insoluble material (fig. 7.19e) and is similar to pressure solution cleavage (Cosgrove 1976). Solution and redeposition of soluble material is indicated by the abundance of saddle reef structures and of d olomite and quartz in veins and fracture fillings (fig. 7.19e). In the 525 S cross-cut, the limbs of folds are attenuated and transposition of bedding occurs (fig 7.20)(cf. Hobbs et al. 1976). In some thin sections, a striping parallel to bedding is found, and this has been interpreted (P. Williams pers. com.) as an early cleavage. This striping is folded by phase 2 (Table 7.2) slaty cleavage folds. There is, however, insufficient evidence in the samples used in this study to associate this striping with a pre phase 2 deformation. URQUHART SHALES In most of the Urquhart Shales, a slaty cleavage is developed only in the hinge regions of minor folds (fig. 7.21). Small intrafolial folds have their axial planes parallel to a cleavage which is in turn parallel to the enveloping surface of the bedding (fig. 7.21). These folds were found only in a few thin sections. In a particular case, a strong slaty 258 DEVELOPMENT OF SLATY CLEAVAGE SLATY CLEAVAGE FOLDS , / thick / -- 4? siltstone black dolomitic- siltstone Il i* I i '----at_ pyritic /..------dolomitic --.74...... pyrite siltstones rich bands slaty cleavage /1/ 5cms 5cms TRANSPOSITION OF BEDDING IN HINGE ZONES FOLDS 525S CROSS-CUT 17D SUBLEVEL 1100 OREBODY FIGURE 7.20 Details of folds found in the silica-dolomite 525S cross-cut. 259 MINOR FOLDS AND CLEAVAGE DEVELOPMENT BUCKLE FOLDS INTRA-FOLIAL FOLD dolomitic siltstone dolomitic siltstone pressure '‘( shadows Ik‘" , cre.... _—'__ .....) -----_ ----- strong cleavage 42). pyritic shales .../ intense cleavage 1 cm 500 pm in hinges only FANNING CLEAVAGE AXIAL PLANAR CLEAVAGE pressure pyritic-dolomitic solution seams siltstones \intense cleavage \ laminated dolomitic siltstones 2 cms 2 cms FIGURE 7.21 Details of minor folds and cleavage development in thin sections of Urquhart Shales. 260 cleavage parallel to the bedding was found in the 13 orebody, 15B sub- level. In the southern sections of the mine workings, slaty cleavage (phase 2, Table 7.2) becomes more prominent. Thin layers of laminated dolomitic siltstones are folded with an intense cleavage in the hinge zones. Pressure solution seams which are parallel to fold axial planes, cut the bedding (fig. 7.21). Remobilized quartz and dolomite occur in microboudinage gaps, saddle reefs and pressure shadows (fig. 7.21). Syntectonic quartz pressure fringes are commonly found on pyrite euhedra. The deformation microstructures described above result from the main fold event in the mine area (phase 2, Table 7.2). Subsequent fold phases (Table 7.2) had little effect on the microstructures apart from the development of a slight crenulation cleavage (this was observed only in a few mine openings in the 11 - 13 M.I.C.A.F.), some slight refolding of the main cleavage and the production of undulose extinction and subgrain structures in the quartz pressure shadows. 7.4 (vi) SULPHIDE MICROSTRUCTURES CHALCOPYRITE Fine grained chalcopyrite associated with framboidal pyrite has occasionally been found in the copper ores (Mathias and Clark 1975) and this form of chalcopyrite is cited as evidence of a sedimentary origin for the copper ores. However, in this study, no such fine grained framboidal chalcopyrite was found. Instead, the chalcopyrite occurs as simple and complex veins and is commonly associated with stylolites and fractures (fig. 7.17j). Towards the basement fault in the 1100 orebody (fig. 7.14), massive and banded aggregates of chalcopyrite are found. Mathias and Clark (1975) have described the black carbonaceous 'mylonite' (graphitic schist) which is found at the basement fault contact. This commonly contains streaked out augen of chalcopyrite. The chalcopyrite in veins and irregular segregations is usually associated with pyrite and occasionally with pyrrhotite. The grain boundaries of the chalcopyrite are irregular and the grain size varies from as small as 7Oum to frequently as large as 0.1 - lcms. Banded textures are common towards the contact with the basement fault (fig. 7.22a). A strong crystallographic preferred orientation in the banded-sheared chalcopyrite is indicated by the parallel alignment of polysynthetic deformation twins (fig. 7.22a). Some large grains (interpreted to be relict old grains) are lozenge shaped with strongly developed deformation twins and small recrystallized grains at the boundaries (fig. 7.22b). 261 Figure 7.22 CHALCOPYRITE MICROSTRUCTURES. Polished sections of chalcopyrite etched electrolytically as in Appendix A. a) Band of coarse grained chalcopyrite with a strong preferred orientation indicated by parallel deformation twins. Sample SC9 1100 0/B 17D sub-level 525 S cross-cut. b) Old grain with abundant deformation twins. Small new grains with few deformation twins are found at the margins of the old grain. 1100 0/B 17D sub-level 525 S cross-cut. c) Grains with highly sutured boundaries. Deformation twins are abundant as well as phase exsolution lamellae. Small grains also have deformation lamellae. d) Deformation twins zig-zagging through an earlier set of twins. 1100 0/B 17D sub-level 525 S cross-cut. e) Three sets of polysynthetic deformation twins in a large old grain. Grain boundaries are sutured. 1100 0/B 17D sub-level 525 S cross-cut. f) Highly sutured twin boundaries-grain boundary migration. 1900 0/B 19D sub-level. g) Lobate grain boundaries. Old deformed grains with deformation twins with undeformed recrystallized grains at the margins. 1900 0/B 19D sub-level. h) Lobate grain boundaries with small new grains forming. 1100 0/B 17D sub-level 525 S cross-cut. i)1 Equidimensional grains with lobate grain boundaries.and deformation twins. Sample SC 284. j) Pyrite grains with irregular boundaries - chalcopyrite pressure shadow. 1100 0/B 17D sub-level. 263 Most aggregates of chalcopyrite away from the basement fault, show irregular boundaries, deformation twinning and phase exsolution lamellae (fig. 7.22c). No strong preferred orientation is indicated. Deformed chalcopyrite aggregates typically have lozenge shaped grains (fig. 7.22b) with deformation twins (fig. 7.22d, e). The grain boundaries become sutured and there is a parallel alignment of twin lamellae in adjacent grains. Strain induced high angle boundary migration produces lobate boundaries (fig. 7.22e, f, g) and eventually results in small, relatively undeformed, recrystallized grains at the grain boundaries. This commonly gives rise to a core-mantle microstructure (Gifkins 1976, White 1976) (fig. 7.22h). The final result of deformation and syntectonic re recrystallization is an aggregate of equant grains (fig. 7.22i) with sutured boundaries and polysynthetic deformation twins. Features which could be interpreted as chalcopyrite pressure shadows (fig. 7.22j) around undeformed pyrite augen were found in a few polished sections. The chalcopyrite microstructures described above indicate syntectonic recrystallization at low metamorphic temperatures. Secondary grain growth and the irregularity of the original grain shapes and sizes precludes any useful assesment of strain in the chalcopyrite grains. PYRRHOTITE Pyrrhotite is common in both the silver-lead-zinc and in the copper ores. In the copper orebodies several beds of blade-like grains of pyrrhotite (possibly pseudomorphs of anhydrite) were found (fig. 7.15h). Pyrrhotite in bedding laminae is common in the no. 5 silver-lead-zinc orebody (fig. 7.23a) where it has the appearance of a framboidal structure (fig. 7.23b). These bands of pyrrhotite however have a very strong preferred crystallographic orientation (fig. 7.23a) and minor folds in the pyrrhotite beds have produced kink structures (fig. 7.23a). Finlow-Bates et al. (1977) used geochemical evidence to argue that these bands of pyrrhotite represent a primary depositional phase of pyrrhotite rather than a metamorphic phase (McDonald 1970). However, the strong preferred crystallographic orientations and the rounded nature of the pyrrhotite grains (fig. 7.23b) may possibly be attributed to replacement of framboidal pyrite. Bands of coarser grained pyrrhotite (fig. 7.23c, d) occur parallel to the bedding. These have a strong shape and crystallographic (c - axis) preferred orientation at about 20 - 25° to the bedding (fig. 7.23c, d). This preferred orientation is parallel to the axial plane of the folds in 72 stope (fig. 7.7,8). 264 Figure 7.23 PYRRHOTITE MICROSTRUCTURES a) Pyrrhotite band in dolomitic siltstones. The high degree of preferred orientation is revealed by the single extinction position. Small kink bands associated with minor folding can be seen (k). Crossed polars, sample M64. Crossed polars, sample from 5 0/B 72 stope, 14C sub-level. b) Detail of pyrrhotite band in (a) showing framboidal pyrrhotite in siltstone/carbonaceous matrix. Crossed polars. c) Elongate pyrrhotite showing preferred shape orientation (bedding is horizontal). Sample 151, crossed polars. d) Elongate pyrrhotite grains showing preferred shape orientation at an angle to the bedding (horizontal). Twins, kink bands and phase boundaries can be clearly seen. 5 0/B 72 stope 14C sub-level. Etched with HI. e) Cross-cutting vein of polygonal pyrrhotite in a matrix of fine- grained framboidal pyrrhotite. Sample 151, crossed polars. f) Columnar pyrrhotite in bands parallel to the bedding. Matrix is siltstone and pyrrhotite framboids? as in (b). Sample 151, crossed polars. g) Small folds in bedded sphalerite and siltstone (dark grey and black). Pyrrhotite is concentrated in fold hinge. 5 0/B, 72 stope 14C sub-level. Etched with HI. h) Siltstone boudins in fine-grained pyrrhotite matrix. Remobilised pyrrhotite fills the scar gaps and also secondary fractures. Sample 151, HI etch. i) Large pyrrhotite porphyroblast. An hedral shape with inclusions of dolomitic siltstone host. Specimen NB, Hilton Mine. j) Large pyrrhotite laths in fine-grained pyrrhotite matrix. The acicular pyrrhotite is possibly replacing anhydrite. Sample 151, HI etch. 266 The coarse-grained pyrrhotite exhibits kinks, deformation twins and phase exsolution lamellae when etched with Hydroiodic acid (fig. 7.23d). Remobilized pyrrhotite is found in polygonal aggregates in cross- cutting veins (fig. 7.23e). The vein polygonal pyrrhotite shows a slight • preferred crystallographic orientation under crossed polars. Pyrrhotite with a columnar microstructure occurs in bands parallel to bedding. Such columnar structures may be either a growth or a deformation microstructure. Where the beds are folded or fractured,pyrrhotite is concentrated in hinge-saddle reef zones (fig. 7.23g) or in boudin gaps and associated feather fractures (fig. 7.23h). Porphyrobastic pyrrhotite is found either as large anhedral grains with inclusions of quartz and carbonaceous material (fig. 7.23i) or as lath like grains possibly replacing diagenetic anhydrite (fig. 7.23j). PYRITE The pyrite in the silver-lead-zinc and copper orebodies is usually found in thin beds, principally as framboids (less than 30um in size) and small pyrite euhedra (fig. 7.24a, b, c, d). The bedding is finely laminated but where diagenetic growth of dolomite and other minerals (evaporite minerals?) has occurred, the bedding laminations are distorted (fig. 7.24b, d). The small pyrite euhedra typically grow on framboid cores (fig. 7.24c). They are similar to the diagenetic pyrite found in the McArthur River deposit (Lambert 1976, Croxford and Jeffcott 1972). Porphyroblastic pyrite euhedra are very common in galena rich bands in the ores (fig. 7.24e) and often include galena, sphalerite and other mineral grains. Coarse-grained (up to 2cms) pyrite porphyroblasts show inclusions of small euhedra and pyrite framboids (fig. 7.24f). Carbonaceous material usually surrounds a pyrite core in the framboid (fig. 7.24c, f). Evidence for remobilization of pyrite during deformation is afforded by pyrite pressure shadows (fig. 7.24g) on pyrite augen. Indentation of one pyrite euhedra on another is seen in figure 7.24h. Other pyrite augen are slightly elongate and fractured during deformation and in the copper orebodies chalcopyrite sometimes infills the fractures (fig. 7.24i, h). Duplex growth, cockcade textures and atoll pyrite textures are commonly found (fig. 7.17a; 24j). The textures described above are evidence for the relative ease with which pyrite can remobilize and recrystallize into large euhedra during diagenesis.Pyrite may also possibly undergo pressure solution (as suggested by Atkinson 1972) during deformation (e.g. indentation fig. 7.24h). 267 Figure 7.24 PYRITE MICROSTRUCTURES a) Typical pyrite bands of small euhedral grains and framboids. Gangue is dolomitic siltstone 15B sub-level 13 0/B b) Finely bedded pyrite with diagenetic growth of crystals disrupting the laminations. Sample MI 111C. c) Pyrite framboids with euhedral pyrite overgrowths. Framboids out- lined by carbonaceous matter. Sample 151, etched dil. HNO3 d) Banding of small grains of euhedral pyrite with porphyroblastic growth of gangue minerals (dolomite (d)). Sample 151. e) Euhedral pyrite porphyroblasts with irregular galena inclusions in a matrix of polygonal galena (light grey) and sphalerite (dark grey). Note zoned texture of pyrite. Sample M4(11) 7 orebody. f) Large pyrite grain enveloping small pyrite framboids and euhedra. Sample 151, etched dil. HNO3 g) Large pyrite porphyroblast with abundant inclusions of dolomitic siltstone. Clear white overgrowth (o) with quartz pressure shadow (q). Sample NB, Hilton mine. h) Large euhedral pyrite cubes with growth zones with possible pressure solution indentation and cracking (arrowed). Fractures infilled with chalcopyrite. Sample 151, etched dil. HNO3. i) Fractured pyrite grain in a siltstone-chalcopyrite matrix. Sample SC9, copper orebodies, 19 level 525S cross-cut. j) Pyrite euhedra growing on pyrite-galena cores. Pyrite white, galena grey, gangue black. Sample T9, 11 0/B 11 - 13 MICAF. 268 269 SPHALERITE Monomineralic bands of sphalerite were not common in the samples collected in this study. However, a few thin (0.5 - 2mm) bands of sphalerite were found (fig. 7.25a) in which the sphalerite has sutured grain boundaries. The most frequently observed microstructure is a duplex structure of equant sphalerite grains with interstitial galena (fig. 7.25b). The sphalerite grains have broad deformation/annealing/ growth twins and a few have thin deformation twins. Some large (i.e. relative to sphalerite at the McArthur River deposit) sphalerite grains (up to 300p) have rounded grain boundaries and thin deformation twins (fig. 7.25c). The twin sub-structure and slip-lines are seen in figure 7.25d. Complex and straight grain boundaries are the result of grain growth (fig. 7.25 e - g). Where chalcopyrite occurs in the silver-lead-zinc orebodies, close to the silica-dolomite, the duplex microstructures of sphalerite grains have small chalcopyrite grains concentrated at the grain boundaries and at triple point junctions (fig. 7.25h). Small pyrite euhedra are commonly found in sphalerite bands (fig. 7.25g, h) and have well developed crystal faces irrespective of the sphalerite microstructures. The grain size (70 - 30014m) of the sphalerite in this study is considerably larger than that found in the McArthur River deposit (Croxford and Jephcott 1972). The abundant broad growth/annealing twins in the Mount Isa sphalerite attest to significant post depositional grain growth. GALENA In the samples of the silver-lead-zinc orebodies used in this study, galena and pyrite are the most common sulphides. A few beds of very fine-grained ( <3011m) galena were found associated with framboidal pyrite and carbonaceous material (fig. 7.26a). Atoll structures consisting of a fine-grained galena core surrounded by euhedral pyrite are frequently found (fig. 7.26b). The amount of grain growth of galena can be estimated by comparing the grain size inside the atoll structures (fig. 7.26b) with that of the galena matrix (fig. 7.26c). The atoll structures also have inclusions of carbonaceous material, pyrrhotite and freibergite. Galena with a columnar structure is found in saddle-reef, boudin gaps and in lenses parallel to the bedding (fig. 7.26d, e). 270 Figure 7.25 SPHALERITE MICROSTRUCTURES a) Banded/bedded sphalerite (medium grey) with galena (white) and a band of pyrrhotite (cream - bottom of photo). Sphalerite has twins and sutured boundaries. Specimen 36. Etched with Brebrick and Scanlon (1957) etchant. b) Duplex microstructure of sphalerite grains with lobate boundaries - etched with deformation and annealing twins with interstitial galena (black). Section 3b. Etched as in (a). c) Single sphalerite grain in galena matrix. Thin deformation twins and broad annealing twins have etched out. Section 3b. Etched as in (a). d) SEM photograph of a cleavage fragment showing twin boundaries and slip lines (grooves). Etched as in (a). e) Small grains of sphalerite with curved grain boundaries and some broad annealing twins and also thin deformation twins have etched out. Section 3a. Etched as in (a). f) Larger sphalerite grains (than in (e)) with irregular boundaries and broad annealing twins - grain growth. Section 3a. Etched in (a). g) Duplex texture of sphalerite (medium grey) and galena (light grey). The sphalerite has a few growth/annealing twins. Sample MI I C (7 orebody) etched with Brebrick and Scanlon (1957) etchant. h) Duplex texture of sphalerite (grey) and chalcopyrite (white). Chalcopyrite concentrated at grain boundaries. A few euhedral pyrite grains occur in bottom left corner of photograph. Sample MI I C (7 orebody) etched sodium hypochlorite. 272 Patches of galena with a polygonal microstructure are common in saddle- reefs (fig. 7.26f). The polygonal galena has a large grain size (up to 3500) with many polygonal sub-grains (fig. 7.26g). The grain boundaries of the polygonal grains are slightly curved, but grain boundary migration is inferred from the pinning of migrating boundaries by freibergite inclusions (fig. 7.26h). Sub-grain boundaries and slip lines in the polygonal galena are also revealed in etched cleavage fragments (fig. 7.261, j). In the zones of breccia structure small relic folds and micro- boudins are found (fig. 7.27a, b). Graphite/carbonaceous material is commonly found in close association with galena (fig. 7.27c). Where galena rich beds are folded, as in 5 orebody (fig. 7.7), two kinds of galena microstructure have been identified . The first kind is that with a dimensional preferred orientation of grains 701.1m to 150A in size (fig. 7.27d). Pyrrhotite grains with irregular grain shapes and laths are found along the galena grain boundaries and at grain junctions (fig. 7.27d, e). The grain boundaries are straight or slightly lobate showing grain boundary migration (fig. 7.27e, f). The shorter-straighter grain boundaries which are approximately 90° to the trend of the dimensional preferred orientation show less evidence of boundary migration (fig. 7.27f). The nature of the etching of samples with a foliation also reveals a crystallographic preferred orientation - in figure 7.27f, h. With an increase of grain boundary migration a slightly more polygonal microstructure develops (fig. 7.27g, h) with either sphalerite or pyrrhotite at triple points. The second kind of microstructure is typified by grain growth, possibly secondary grain growth which causes an increase in polygonal grain size (fig. 7.271) and produces straight grain boundaries with well developed triple points (fig. 7.27j). The relationship between the galena microstructures and the macroscopic fabric elements is illustrated in figure 7.28 which is a particular example from 72 stope (fig. 7.7). On the limbs of fold structures, the polygonal microstructure is found (often in breccia zones) but where there has been intense deformation and flattening of the folds (fig. 7.13b) a dimensional preferred orientation is found - a foliation (fig. 7.28). This foliation is also found in the cores of folded galena bands and is readily visible to the naked eye. On the fold limbs it is at an angle to the bedding and axial planar to the folds. 273 Figure 7.26 GALENA MICROSTRUCTURES Figure 7.26 (a) - (h) are photographs of polished sections etched with Brebrick and Scanlon (1957) etchant. a) Fine grained framboidal? galena (grey - arrowed) associated with framboidal pyrite. The galena appears unmobilised in bedding layers. Sample 51, 5 0/B, 72 stope 14C sub-level. b) Porphyroblastic euhedral pyrite crystal (white) surrounding fine- grained galena core - atoll structure. Section 53, 5 0/B, 72 stope 14C sub-level. c) Atoll structure showing the grain size difference between the included galena and the matrix host galena (grey). Section 51 as in (b). d) Columnar galena perpendicular to bedding planes. Section 22. e) Columnar galena perpendicular to the bedding planes. Matrix is fine-grained galena - equidimensional. Section 22. f) Galena (white) in a saddle reef structure at a fold hinge. Specimen A 3(11). g) Polygonal sub-grains in galena (grey). Section 3a. h) Pinned grain boundaries in galena (grey). Inclusions are probably freibergite (arrowed). Section 3a. i) SEM photograph of etched cleavage fragment showing sub-grain boundaries. j) SEM micro-graph of an etched cleavage fragment showing a high etch pit density and {110} <11.0> type slip traces. i C147-1 ;;11 Ihriariei PLIVve . ord. +ow, • rir4•PM11.'.. ZiShet.... • AL..% • ••.• • ..set",;„ t-...511%ik,S467410;07..!v • :11,11:1•11, ficr - . 1-• snail Ip" JOB . 44)ell ; 647'7';711160 a 4,1 275 Figure 7.27 GALENA MICROSTRUCTURES All photographs of polished sections etched with Brebrick and Scanlon (1957) etchant. a) Relict folds of banded siltstones in a matrix of recrystallized galena. Breccia structure, specimen 22. b) Boudinaged shale layers in a recrystallized galena matrix. Sample T 14. c) Galena (grey) with black fibrous graphite. Section 53, 5 0/B 72 stope, 14C sub-level. d) Preferred shape and crystallographic orientation of galena - sub axial planar. Black pyrrhotite has random shape orientation interstitial to galena. Specimen 55 5 0/B. e) Elongate grains with straight cross boundaries. Black is interstitial pyrrhotite. Section 53 5 0/B. f) Elongate polygonal galena with triple points and boundary migration - lobate boundaries. Sample 51, 5 0/B, 72 stope 14C sub-level. g) Preferred shape/crystallographic orientation of galena grains with some grain boundary migration producing lobate boundaries. Sample T 10 - 2 11 0/B 15B sub-level. h) Stable polygonal microstructure of equant grains. Sample T 10 - 2 11 0/B 15B sub-level. i) Slightly elongate polygonal galena with straight grain boundaries and triple points. Sphalerite (dark grey) concentrated on the grain boundaries and triple points. Specimen A4 (1) 7 orebody. j) Large polygonal grains with tetrahedrite at triple points. Specimen 51, 5 0/B 72 stope 14C sub-level. s"'• ••••■ v '"14 •. • ..4111k Alb .71 . • • • - • • - • . v dt.•. • .16••--. • 4.• -• -- 220p—I ••411i- g iiiiiip' 410,6 '. AGO ve" ior .44 .._ 16.‘ • 12.11 ' •%•3 H 15 Op 150p 277 A poorly developed rodding lineation of galena is sometimes developed sub-parallel to fold plunges (fig. 7.28). Where developed, the pyrrhotite foliation is also axial planar to the folds. Sample M64 from the 5 orebody (fig. 7.28) comes from the central zone of a folded galena layer. Galena is concentrated in the fold hinge zones and the galena layer is attenuated on the limbs (fig. 7.7 and 7.28). Isoclinally folded coarse grained galena bands with a strong dimensional preferred orientation are found in the centre of the specimen (fig. 7.28). Breccia structures are found around the edges of the galena band (fig. 7.28). Zones of particularly high deformation are characterized by bands of fine-grained galena (fig. 7.28). These zones of particular microstructures are correlated with types of crystallographic preferred orientation in section 7.5. SAMPLE M 64 ISOCLINALLY FOLDED COARSE-GRAINED GALENA /steeply plunging coarse grained galena isoclinal folds 50°--... 0000 layers massive galena M 64 vertical A /hod; ' ** ...... - •-•;,../ black dolomitic siltstones dolomitic siltstone SAMPLE LOCATION 5 0/B 72 STOPE 14C SUB-LEVEL band of fine- grained galena foliation breccia texture FIGURE 7.28 Details of foliations in massive galena layers relative to large folds. 279 7.5. CRYSTALLOGRAPHIC PREFERRED ORIENTATIONS X-ray texture goniometry was carried out on nineteen galena samples from Mount Isa, in monosulphide or near monosulphide bands which are very suitable for X-ray texture analysis. Representative X-ray - results are presented in figures 7.29 - 7.35, in the form of (220), (200) and (111) partial pole figures contoured in levels of 20% of mean intensity (mean intensity = 5) (Siemes 1977b). The geometry of the fold structures (fig. 7.7, section 7.3) and the widespread development of breccia structures (section 7.3) indicate significant deformation of the galena layers. There is, however, a notable lack of crystallographic preferred orientation in these layers (fig. 7.29 - 7.35). Three types of preferred orientation are recognised and these are correlated with the macro- and micro- structures in the ores. TYPE 1: Figures 7.29, 7.30 show X-ray partial pole figures from samples with breccia type microstructures (Section 7.3). There is no significant preferred orientation. These pole figures are from samples with a structure similar to that shown in figure 7.9d, h where the bedding is planar or unflexed and any macroscopic foliation in the galena is parallel to the bedding. The microstructures in this kind of ore typically have polygonal grains 70 - 100vm in size (fig. 7.27h). From the lack of preferred orientation; the small 01Acinal grain size; the intensely deformed specimens, one may infer that the dominant deformation mechanism during the formation of the breccia structure zones was probably Coble or Nabarro-Herring creep - i.e. diffusion controlled creep (Atkinson 1976b 1977, McClay 1977c). Similar partial pole figures were obtained from sample 114 (fig. 7.9h) where a traverse across a breccia zone shows no significant changes in preferred orientation (fig. 7.31, 32). The irregularity of the pole figures illustrates the difficulty in finding homogeneous samples. TYPE 2: These samples (e.g. T10 fig. 7.9b) exhibit weak preferred orientations. Specimen T10 has buckled dolomitic shale layers in a matrix of fine-grained galena with trails of shale fragments and a slight dimensional preferred orientation outlining a foliation. The preferred orientation in figure 7.33 shows a weak maximum of (220) poles approximately normal to the foliation plane. The distribution of the other poles are less regular with the (200) pole lying at a low 280 MOUNT ISA 1MA (220) POLE FIGURES (REFLECTION MODE) EQUAL AREA PROJECTION Weir Level 20% of Neal letmitj (5) (200) ...... Ott: ...... 1.4 t!,7,2•2,1,2:5 ••••■ 4.4 ...... .. :21 .... 7.. : ...... 4.4.114.4 SAMPLE ORIENTATION FIGURE 7.29 Partial pole figures of Mount Isa galena. 281 MOUNT ISA GALENA Specimen A4/2 (200) PARTIAL POLE FIGURES(REFLECTION) EQUAL AREA (UPPER HEMISPHERE) PROJECTION CONTOUR LEVELS 1 2 3 I I r ' 2 4 III , 3'42 37 2 '2 2 VI X UNIFORM .... 227 2727 ; .... F= FOLIATION ' ' I 2 3 a 3 a P 2 7 , 2 P 3 2 • + ' ,, „ ' 3 2 II . ' a ) 0 1' 1 1 • 1 a 1 2 a a . e a ' , 2 ° a FIGURE 7.30 Partial pole figures of Mount Isa galena. 282 MOUNT ISA 114 (220) POLE FIGURES (REFLECTION MODE) ... :II • 7I. .... 17 ...... 3,31,1141...... 41,, • • • EQUAL AREA PROTECTION 3 ..... 1a:1:1a. . :41I11,1 lg. Contour Level 20%of Mean Intensity (5) 17.1a ...... 111.11.11 11 ..• 1,111,11.1 ... 1 .. 111.1,117 11...... 1111•5,17171.11 171.11 • ..a. .17 -0,5111.1 . I .17...... f..1••• ...... 1 . 111.1afia11.15 11.11 0, 311111....1.71,11" !It,...... 1 ..... 1 . 11...... et...111011. 1111 .1111.1 .1111111a111.7.,1 ...... felff ...... fall ..... 1,1111151.11...... tp...... 111.17 1a1.1 . ... •••• ...... 1,111.1 ..... •• ea ...... 7.1 1.7 ..... 66•••• 1.1 111.11.71, .12112.1. • 1.11. (200) 11 111.111 2 1.11.1 ••••• 1.1.7111. .111111/1 • .11.17.1. .11.11. aila.1.1. I/I •••••••••• ..... 8.6 ...... • •••■•••••••••• /•.111 f "31 .55,15777 .... 111111.71/11 ..... I.. . • . .I••• f•• • 1.17•111 ...... ••• . . ' • ••• . . . 1:51,".1 11111,1,11•7,11.S.7.71.5171.1751.11 ...... bbs I 5...... ! ...... fa...... •••••• ... t7 ...... 11.11711 .1.17.11111.111.1. ■■ 7.1...... 77.111.71....31 ...... 271,21,1111.11.. f7.1.1/1.11111. a..11111./5115 ...... ^ 11111117 ...... 11,1111.1.11.7 1$111.517 .... .1111 ..... ••• •••• etuaa...... 907 :17777.777777f ,,I,551.71■1f . • . , . ly...... 7.646 1.71.1.1.111 ...... am. fz .. 1 • ...... ff ..... t ...... •• •• •• • • •• ••••• •• Foliation Plane A/ 3111 1. 11 ...... 1.11 .■ Meta ...... • e{c a.. . . 11 . 11111 •••• .1.1 /as, .7 ...... 111 ft. 11.1 .. .. •••• .1 70. 1. al ..... •••• .. 171.11.1. 111 •• .71311,.. ...... 111.11 ...T ...... mrsrr ...... 71 ..... 1..C.77.77 ...... 1...... 1,237,. ..... ,t. 71 ..... 17 ...... t• •••• • .77 1/ I / • • • ...... /7 7, ... ••••■• ...... 7 177 rr 777 1/ •• ., • • ...... 7 ..... 77/ 7 /7 / t., ...... ft, , ...... 177 ,•• 1.77,77/. IIY77,111 ...... (I.. .71.77777. ...a...... ,1,1, SAMPLE ORIENTATION FIGURE 7.31 Partial pole figures of Mount Isa galena. 283 angle to the foliation plane (fig. 7.33 - lineation = fold axis). The distribution of poles is somewhat similar to that found after axisymmetric compression of specimens of polycrystalline galena (Siemes 1970, 1977b)and is consistent with a compression axis normal to the axial plane of the fold. Typically the microstructures associated with this preferred orientation pattern are slightly elongate grains and some polygonal grains (fig. 7.27g). TYPE 3: These are relatively well defined pole distributions although the maxima are not strong. These samples (fig. 7.34, 35) are from the folded galena band in fig. 7.7 and 7.28. There is a strong dimensional preferred orientation of elongate galena grains (fig. 7.27d, e). The operation of recovery and recrystallization processes is shown by the development of grain boundary migration microstructures (fig. 7.27f) in these samples. The maxima in figure 7.34 are streaked out along great circles. In addition, the (220) poles are not normal to the foliation. Section M1 21 (fig. 7.35) is similar to section M1 55 (fig. 7.34) but perpendicular to it. Again there is an asymmetrical distribution of poles about small circles (fig. 7.35). This distribution of maxima may be interpreted to indicate a shear component of deformation - possibly at a low angle to the foliation. PREFERRED ORIENTATIONS, FOLD PHASES AND MICROSTRUCTURES The correlation of fold phases and microstructures with preferred orientation patterns is difficult because the fold phases at Mount Isa appear to be coaxial or nearly so (Table 7.2) and because it is possible to obliterate existing preferred orientations by as little as 30% further strain (Lister 1977, Lister, Paterson and Hobbs 1978). The lack of preferred orientation in type 1 patterns together with a polygonal microstructure and the abundance of this kind of breccia structure with phase 2 folds permits one to infer that the dominant deformation mechanism in galena during the phase 2 fold event was probably Coble creep and grain boundary sliding (McClay 1977c). This is in agreement with the theoretical predictions of Atkinson (1977). Type 2 patterns are interpreted as later modifications of type 1 patterns in that a small component of flattening on the folds has modified the microstructure by dislocation creep (Atkinson 1976b) into slightly elongate grains and produced an irregular orientation pattern. 284 MOUNT ISA 114 2 (220) POLE FIGURES (REFLECTION MODE) .4 ..... S..... • .51.1 vif,••••■• %20 • • EQUAL AREA PROJECTION :12,121, •• . • 5. 11 ••• •• • • 53 a %S..42,141.44,44•1•41,:l2lt.2...14. 1151,2flt ... 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CI ...... f. bf, • ..... ••1 . ■f., •• .7, IS) ...... af, • ,Ifoofaolf. •• • 1111 .17 1.:573 • .• • :, .... • .. 1.. 4:24 1 ...44, • 1%1 ... % . • ,t• . • a: 44 . ... • tt,55S11.41 • .52.1).: • • • ■ • • • ...... • .1.4274.1317.444 • ...." "...... " SAMPLE ORIENTATION FIGURE 7.32 Partial pole figures of Mount Isa galena. 285 MOUNT ISA T10 (220) POLE FIGURES (REFLECTION NODE) •• •• • • •• •.444444444. 4444444 444444 4.•••••••••■••••••••• EQUAL AREA PROJECTION • 46444444.• .44.444.4 •• •• .44.4444 444.4.4 6 •••••••••••• ••••••••••••• M.... 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III NO 11 111 444444 •••••••••••• 3 •••• 11.1,111 II 6. 44444 ••••••• 3311 44444 .444. 33 4444444 ••••• .114. •••• 1.1,1111• 3311 141.11•1• 44444 .444 • • • 1.1.7 11• • 1. ••••••••• • 11.1 •••.•••• 1195111 ••••4 4444444 444 1..4 44444 SS) 444 4 )13 ••••••••••• tilt" 444444444 SS 44444 ••••••■•••••••• 4444444 44444444444• ISMS 4444444444 •••••••• 64••••••• 416 4 644.. ••••••••••••. 444444 4444444 • .• 444•••••••••• SSSSSSSS .. 44.444444444. 44444 • ...... 444444444 44 •• ••••••••••••• SSSSSS •40•4••••••4 44444 ••• •• 44.44444444 1 •• .4..4.64 . •••••••••••••• • 44••••••••••••• N. •• •• 4....4... 4••••••••• SISISSSISS 4444444444 ....., •• .4..44 ISSIIIIIS •••••••••••••• ••••••••••••••••••• •••••••••••••••• -.‘ •:• ••••■•••••••••••••••••••• . 44444 •• • 444. MS./. 44444444 >a : . .1 44444 SS SISI 444444444 .4•44.444.4 444444 SS" -Ss !)1) 414)1 .3" SSISISS 444444444444 ••••••••••••••••••••• ••• 444.44444. t• • 4444.464. 11.111. •••••••• 3,S53113SSII•S! 5),),) ,11”.411,1•SISSI 3 7,11)111,S, '114 IS1 115,>!•11!Ittl!• I/I1 ••••• 1)11 3313 "WI5 1SSI5S 44444444 ) 4444444 44444 $11)>S3),) /1.1411 1334444444 51113,1 4444444444 SS" 4444444 I 44444 • III) ISISSISSI! 444444444 MSS 44444 S 1. • ••••••••• • 44444 4 ISISISMSSIWISS 444444 444444311 VIM) 44444 44444 331> 4444444444 iS13$51,35$1, .44441 S1 SS. 414.4 44)4414 la ISSIfs>>11SSISISt !SI •• •• IS») 44 ••• ••••• SSISsISSIS33$ 4444444444 I •••• •••••• 444444 44444444444 131.1 •••• • ■ .s 44444 ....11.71”. •••• ••Sit!SISS1 11.3 44444 I 131)414513”,)4, 44444 • 134 11. SSSSSSSSSSSS 15441 ff 1)4 if '" .. '.. . SAMPLE ORIENTATION FIGURE 7.33 Partial pole figures of Mount Isa galena. 286 A stronger flattening component, possibly associated with phase 4 folding (Table 7.2 and also section 7.3) is indicated by type 3 orientation patterns. Elongate grain shapes possibly indicate significant dislocation creep. The geometry of the folds where these samples are located, shows a strong flattening component and this has tentatively been ascribed to phase 4 deformation (section 7.3). The pole figures, however, also deviate from an axisymmetric distribution and this perhaps is evidence for the existence of a shear component during phase 4 deformation. An alternative interpretation would be that type 3 and 2 patterns represent early deformation features and that type 1 patterns reflect the destruction of these early patterns by grain-boundary sliding and coble creep during the formation of breccia zones. However, the field evidence is consistent with a late flattening - deformation event, and it is unlikely that a reflection of this would not remain in the preferred orientation distributions Annealing recrystallization and secondary grain growth of galena which has deformed by dislocation glide (see Chapter 3) are unlikely to completely obliterate any earlier preferred orientation (see Chapter 5). It is likely therefore, that type 1 patterns correlate with phase 2 deformation and that types 2 and 3 are caused by phase 4 deformation. 287 MOUNT ISA MI 55 (220) POLE FIGURES (REFLECTION MODE] • ...... EQUAL AREA PROJECTION 5321"1•11%•ZItalaas.laaats% •..... %If . 21,1,•,..a aa•al .... •%•11•Sli•ff %%W. a a•la a ••••% ...... -• •...... >sat ... a• •• • Coeur Level 20% of Meal lotelsity (5) ...... •••••• ...... ••• 1 •1 ••••••••• 444.. ••••••••• a; ats•%!1,1•1! ••••■•••• • lIa?1!1•••••; ll'a WI %If S•• I SZ%•••%, a I . !•%Sala• talt15•10•1% (200) •••••••••• 11•!%•11.%"1,1a •%%15.1%!%3/7%, fa a . S'at•:%”••%•.• a•adaal ,1551.14%:115111%.,Iff SSA% 1••at15,115a1a.,•%-%.11 I%%1:•• ..... • ■•••••••••• Foliation Plane SAMPLE ORIENTATION FIGURE 7.34 Partial pole figures of Mount Isa galena. 288 MOUNT ISA MI 21 (2201 ..... POLE FIGURES (REFLECTION MODE) • 11111 ...... 3111.101111...... 11.11111 11 •• EQUAL AREA PROTECTION Cootoor Level 20% of Meal Mushy (5) 111. • 111) .1.t.1.1 • 111 .. 111211 f1a., ...... 1.11. 711, 11. 111...... Art .' “1""M110.1.,” • • • ..... 1 11..5, .1..1...... •1111115 ■••• .1t.:1,1 ...... 1 . . (200) .101 ... 1 .42)137... ..P.t1 11,11 ... •1111" . ; ...... •• 1 .• a• • t. 51, ••• 1.• • • .. 1 . 15....f.../151 ...... • 11:1•11,5,..o. .. . . 1 ... .51 11,11.1.51.1 ... 44 • ...... ,t, .4 . ... - 11 ,1 4 11114 .■ ", . . .11141401 .4 .• ...... • ....• 1.1.11111. 44., *4,4 111111'01/11 ...... 11. •• 111.11. 11 • ..... • • • • • • SAMPLE ORIENTATION FIGURE 7.35 Partial pole figures of Mount Isa galena. 289 7.6 DISCUSSION AND CONCLUSIONS In this section, the folds, microstructures and preferred orientations in the Mount Isa ores are discussed in the context of the genesis and metamorphic-deformational history of the Mount Isa deposit. GENESIS OF THE ORES Croxford (1962) and Stanton (1962, 1972) proposed that the presence of tuff marker beds (RC's, Croxford 1964) in the Urquhart Shales indicates a volcanogenic origin for the Mount Isa deposit. This has largely been accepted by other authors (Solomon 1965, Bennett 1970, Mathias and Clark 1975, Lambert 1976). Solomon (1965) using sulphur isotope evidence favoured a biogenic origin for the sulphur. Sulphur isotope studies (Smith and Croxford 1975) on the huge, unmetamorphosed McArthur River silver-lead-zinc deposit, which is generally considered to be the unmetamorphosed equivalent of the Mount Isa and Hilton deposits (Croxford and Jephcott 1972, Murray 1975, Lambert 1976), have been interpreted to indicate a dual sulphur-metal source. Lead isotope studies (Gulson 1975) support this view. The isotope evidence at Mount Isa (Lambert 1976) is not reliable or detailed enough to allow such an interpretation. The origin of the copper ores at Mount Isa is, however, disputed (Bennett 1970, Stanton 1972, Smith and Walker 1971). Stanton (1972) and Bennett (1970) consider that the copper ore formed syngenetically in a near-shore algal reef-breccia environment whereas Smith and Walker (1971) support an epigenetic origin for the copper ores. The discovery of pseudomorphed sulphate evaporites at McArthur River (Walker et al 1977) and at Mount Isa (McClay and Carlile 1978) raises on the possibility that these deposits may have formed from evaporitic-sabkha brines (Renfro 1974, Dunsmore and Shearman 1975, Carpenter et al 1974). Microfossils have been described from the McArthur River deposit (Oehler and Logan 1977) and also have been found in the Mount Isa ores (M. Muir, pers. comm). The interpretation that the silica-dolomite was deposited in a marginal sabkha environment (McClay and Carlile 1978) and the abundance of carbonaceous material at Mount Isa (Lambert 1976, Saxby 1976) favours a single stage sabkha brine model (Dunsmore and Shearman 1975) involving bacterial reduction of sulphate for the fixation of the metals (Trudinger and Mendelsohn 1976, Trudinger 1976). Although a sabkha brine model can give rise to lead-zinc mineralization, the source of the copper mineralization at Mount Isa 290 remains enigmatic. Copper is often associated with evaporites (Bowen and Gunatilaka 1977, Renfro 1974) but in this study, there was no evidence for sedimentary or diagenetic copper mineralization. It is probable however, that significant remobilization of chalcopyrite has occurred during deformation and metamorphism. The microstructural evidence and the similarity of the Mount Isa silver-lead-zinc ores to those at McArthur River, leads one to infer that the mineralization at Mount Isa was deposited as fine grained (5 - 20pm) early diagenetic sulphides. DEFORMATION OF THE ORES FOLDING Bennett (1965, 1970) and Stanton (1972) ascribe most of the small folds in the Mount Isa silver-lead-zinc ores to sedimentary slumping. Ramdohr and Amstutz (1964) consider that diagenetic crystallization caused the folding and breaking up of shale layers. McDonald (1970) and Mathias and Clark (1975) however, state that the folds are tectonic in origin but cite no evidence for this. The following observations made during this study support a tectonic origin for most of the folds found in the silver-lead-zinc ores:- a) the minor folds are parasitic and have consistent orientations and plunges in relation to the regional structures. b) in places a fanning axial planar cleavage is developed. c) the fracturing, boudinage and the formation of veins and saddle reef structures are all consistent with the folding of lithified layers. d) graphitic, pyritic slickensides and stretching lineations are found around fold hinges and down fold limbs indicating significant layer parallel slip and competency during folding e) the grain shape textures and microstructures of galena and pyrrhotite indicate solid state deformation of these minerals. Slump folds are occasionally found at Mount Isa (Dunnett 1976) and have been recorded from the McArthur River deposit (Lambert 1976). Woodcock (1976) concluded that slump folds are geometrically similar to tectonic folds but in this case the strong association of the minor folds with major structures, the tectonic veining and the lack of evidence of soft sediment deformation in the Mount Isa ores supports a tectonic origin for the majority of the minor folds. 291 The occurrence of isoclinal folds in the cross-cutting fold zones described in section 7.3 may give rise to local transposition and thickening of the ore beds. Williams (1976) suggests that early isoclinal folding may be more widespread than is recognised to date. If this is so, there is the important possibility of repetition of ore beds by isoclinal folding. Enrichment of galena layers in the hinge zones of folds has been noted by McDonald (1970) and Blanchard and Hall (1942) though doubted by Stanton (op. city. In the sections shown in figures 7.7 and 7.28 enrichment of galena in the hinge zones can be seen. In addition, remobilization of galena whether by solid state diffusion or by solution assisted transfer has been found in fold hinge zones (this study, McDonald 1970 , Hewett and Solomon 1964 ). Thus enrichment of ore grades by folding and repetition of ore beds are possible economic consequences of tectonic deformation of these ores. MICROSTRUCTURES AND PREFERRED ORIENTATIONS The detailed microstructural studies of the Mount Isa ores have allowed the following depositional-diagenetic-deformational sequence be constructed for the silver-lead-zinc ores. 1. Initial deposition as fine-grained (5 - 20pm) sulphides similar to those at the McArthur River deposit (Croxford and Jephcott 1972). The sulphides were probably formed below the sediment - water interface from sabkha type brines (Dunsmore and Shearman 1975, McClay and Carlile 1978). Evidence for this fine-grained depositional phase is found in small spheroidal galena grains and pyrite framboids. 2. Diagenetic grain growth of the sulphide minerals during burial, particularly remobilization of pyrite as evidenced by the overgrowths on pyrite framboids and in the development of atoll structures (pyrite around fine grained galena). 3. Low temperature deformation with probable concommitant grain growth (although there is little evidence of exaggerated grain growth). The majority of the fold structures and deformation features have been correlated (although somewhat tentatively) with phase 2 (slaty cleavage) deformation. The lack of preferred orientations, the grain size and the lack of marked dislocation substructures in galena permits one to infer that Coble creep and grain boundary sliding was the probable deformation mechanism in galena operating during this deformation phase. This result is similar to that predicted by Atkinson (1977) and this can 292 also be inferred from the deformation mechanism maps (fig. 3.3) for galena undergoing low temperature (150 - 200°C) deformation at geological strain rates (section 2.1). The galena grain shapes have been altered from equant to elongate and slight preferred orientations developed by the latter phase 4 deformation event. This deformation event has given rise to flattening of earlier folds and the development of compression textures in galena with the axis of compression normal to the axial plane of the folds. These features and the deformation microstructures observed in the other sulphides, together with the illite crystallinities all support low metamorphic temperatures, probably - 150°C, (of Hewett and Solomon 1964) for the Mount Isa deposit. The microstructures in the silica dolomite indicate a complicated diagenetic - deformation sequence with numerous replacement events. During the burial and deformation of the silica dolomite, the reactions of the evaporite minerals (cf. dehydration of gypsum to anhydrite) and the replacement of evaporites by dolomite will be particularly important. These reactions are likely to involve large volumes of fluids which may have marked effects on the mechanical behaviour of these rocks during deformation (Fyfe et al. 1978). Detailed considerations are given to the following. The breccia structures which consist of shale/siltstone fragments 'floating' in a galena rich matrix may be simply explained by the folding and breaking up of competent layers of lithified shales and siltstones in a matrix of fine grained galena which under geological conditions of deformation is expected to be extremely weak (fig. 3.3). Other authors, Stanton (op. cit.) Bennett (1970), have proposed that these structures are sedimentary slump features. The abundant carbonaceous material in the Mount Isa ores may be used to indicate conditions of deformation (fig. 2.3 and Saxby 1976). It may also help preserve primary depositional grain structures (such as pyrite framboids section 2.3) from grain growth during metamorphism. The comparative rheology of the sulphides during the deformation of the Mount Isa deposit is illustrated by the structures shown in figure 7.36. These structures indicate that the dolomitic shales are the most competent units with the galena rich layers being the most incompetent units. Remobilization of the sulphides (galena in the silver-lead-zinc orebodies and chalcopyrite in the copper orebodies) has been found in this study and also recognised by other authors (Hewett and Solomon 1964, McDonald 1970). BOUDINAGE 12 OREBODY 11-13 M.l.C.A.F MINOR FOLDS 11/60 OREBODY 11-13 M.l.C.A .F ~ :: .---:::...... ~~~~~~ ' ...... ' .. , ' .. ~~.~------~~~-- ~ ~. . ~, .. . .. :~ pyritic = ~ ----- ~ coarse ,. 4 • ~ ,6 • shale massive coarse grained '\ grained dolomitic .. : .- • : II- galena galena ~~~~'~ ~~# .~ shales ~ sphalerite unmineralised shale ~~ aggregates 10cms, 40cms, unmineralised shale N FIGURE 7.36 Sketch diagrams to illustrate the relative competencies of sulphides and shales in the Mount Isa ores . to w 294 In summary, detailed structural and microstructural studies of the Mount Isa ores has enabled the folding to be explained in terms of deformation mechanisms and has enabled deformation features to be recognised in the sulphides. Low temperatures ^,150°C are indicated during deformation and metamorphism. X-ray texture goniometry of monomineralic galena bands has provided information on possible deformation mechanisms in the galena rich ores. Under the conditions of metamorphism proposed for the Mount Isa deposit many of the structures are expected to be controlled by the very weak galena layers. Concentration of sulphides by remobilization'into localities of low mean stress and repetition of ore beds by isoclinal folding may have important economic implications. The recognition of primary depositional features (ie. sulphate evaporite pseudomorphs) as a consequence of the detailed microstructural studies provides new evidence for the genesis of the Mount Isa ores and may influence exploration policy for similar deposits in the Mount Isa district. 295 CHAPTER 8 SHEARED GALENA 8.1 INTRODUCTION In this chapter, the microstructures and crystallographic preferred orientations of coarse-grained galena ores, from various sources, which have all been deformed at low temperatures (approximately 100 - 200°C) are compared with the microstructures of experimentally deformed single crystals of galena (chapter 4) and with the theoretical texture simulations (chapter 5). During the course of this research, dislocation and recrystallization features were found to be best developed in coarse-grained (> 500pm) galena. For this reason, these coarse-grained sheared galena ores were studied in detail in order to investigate the deformation and recrystallization mechanisms in galena. These studies provided the basis for the experimental studies on single crystals of galena which are described in chapter 4. Coarse-grained galena which has undergone shear deformation commonly exhibits a very strong foliation and/or lineation (Ibleischwief' or 'steel' galena). Although Lyall (1966) and Stanton and Willey (1972) recognised that kinking plays an important role in the development of this texture, detailed microstructural and preferred orientation studies.have not been carried out. Stanton and Willey (1972) proposed that grain growth and annealing recrystallization are significant in the development of the sheared galena texture. Detailed studies by the author, however, indicated that dynamic recrystallization at low temperatures was responsible for the foliations observed in sheared coarse-grained galena (McClay 1977b). Six examples of sheared galena from Yerranderie N.S.W., Braubach W. Germany, South West Africa, Ruth Hope British Columbia, Pibram Czechoslovakia, and from Halkyn North Wales, were chosen for this study. The samples are from deformed vein deposits and are essentially monomineralic (galena) with only minor quantities of gangue minerals and other sulphides. The grain sizes vary from 1 - 2 cms down to 5 - 10um but all six samples have a very strongly developed foliation or lineation. From the geology of the deposits and from the structures found in the samples themselves, it is possible to infer that the original galena was deposited as large (1 - 3 cms) grains in open fissures and faults. It is also possible to infer that these deposits have only undergone low temperature shearing late in their geological history apart from the Tsumeb deposit (South West Africa) which has suffered post shearing thermal 296 metamorphism. From each sample polished sections were cut a) parallel to the foliation and parallel to the lineation, b) perpendicular to the foliation and parallel to the lineation and c) perpendicular both to the foliation and to the lineation. Detailed microscopic and scanning electron microscopic studies were made on polished sections and on cleavage fragments. Three mutually perpendicular slabs (cut in the same orientations as the polished sections) were used for X-ray texture goniometry. These slabs were also polished and then etched with Hydrobromic acid (Appendix A). Acetate peels were then made from the etched faces and these peels were photographed and studied under the microscope. Direct reading spectrophotometer analysis and scanning electron microscope micro—analysis were used to identify minor sulphide phases in some of the samples. 8.2 SHEARED GALENA - PRIBRAM DP 8.2. (i) INTRODUCTION This sample of very coarse-grained sheared galena comes from the Rimbaba NW vein, 25th level, Bohutin mine, Pribram ore district, near Prague (Czechoslovakia) and was sent to the author by D. Pertold. The vein deposits occur in folded and faulted lower Palaeozoic rocks. Some of the faults have moved after the deposition of the ore minerals and this has produced the shearing deformation seen in figure 8.1. The ores consist of large grains of galena, sphalerite, pyrite and chalcopyrite with a quartz, calcite and siderite gangue. In the sample studied two bands of foliated galena are bounded by fine grained calcite, quartz, pyrite, sphalerite, and siderite which are parallel to the vein walls. The galena layers consist of strongly foliated, large lozenge shaped grains oriented with their long axes 30 - 40° to the vein walls (fig. 8.1) This gives a shear strain of 1.2 - 1.73 assuming that the banding was initially vertical. The horizontal white bands in fig. 8.1 are fine grained calcite, quartz, siderite, sphalerite, and pyrite. These bands have been fractured and broken up during the deformation. On close inspection of figure 8.1, the banding of the galena grains can be seen to be caused by kink bands with slightly serrated boundaries. The triangular cleavage pits (black triangles in fig. 8.1) show that many grains are oriented with their [Iq axes normal to the plane of section. 297 It is possible that the galena was originally deposited as large elongate grains normal to the vein walls - comb or fibre texture, as often is the case for vein quartz or calcite. It is more likely, however, that the galena was deposited as large equiaxed grains with no shape orientation. This latter texture is typical of galena formed in open fissures and veins eg. as in the Mississippi lead-zinc deposits (reviewed by Stanton 1972). 8.2. (ii) MICROSTRUCTURES The microstructures of the deformed galena in figure 8.1 have been studied in detail. Polished sections perpendicular to the plane of figure 8.1 and perpendicular to the sense of shear, show that the coarse- grained (1 - 2 cm) elongate galena grains have equant cross sections although some flattening parallel to the banding in fig. 8.1 is evident. Many of the microstructures are best observed, however, in polished sections parallel to the plane of fig. 8.1 (ie. parallel to the sense of shear- sinistral, and perpendicular to the foliation). Most of the elongate grains (fig. 8.1) have irregular grain boundaries. The grain size is highly variable from 1 - 2 cm to 200 - 300pm. Some grains show no deformation features while others are traversed by small planar kink bands (fig. 8.2a). The boundaries of these kink bands are very close to {110} planes in the host crystal and the rotations indicated are commonly of the order of 20 - 30° which are larger than the 19° predicted from Starkey's (1968) model of kink bands. Within these kink bands, slip lines are found (fig. 8.2 b, c). The kink bands shown in fig. 8.2 a - c are very similar to those found in (001) compression of single crystals of galena (fig. 4.5 b, d). Although the slip lines are not well developed, duplex slip on {001} planes can be found in kink bands (fig. 8.2c). The kink bands are commonly found at grain boundaries (fig. 8.2b). Opened cleavage traces (fig. 8.2 b, c) and incomplete sub-grains also occur (fig. 8.2b). Scanning electron micrographs of these kink bands reveal considerable micro cracking (fig. 8.2d). Slip lines are occasionaly observed outside kink bands (fig. 8.2b). Figure 8.1. Photograph'of an acetate peel of sheared galena DP (x 2.5). Two layers of foliated galena (grey) are bounded by bands of fine-grained gangue minerals (white). The sense of shear is sinistral, and the plane of the photograph is perpendicular to the foliation in the galena. 298 299 FIG. 8.2 Sheared Galena Microstructures Samples from a sheared vein deposit - sample DP. All samples etched with Brebrick and Scanlon (1957) etchant. a) Kink band with (110) orientation. High etch pit density in kink band. b) Kink bands along a grain boundary. Slip lines occur both inside and outside the kinks. Incomplete sub-grains developed. c) Kink bands with curved cleavage traces. Slip lines are seen in the kink bands but not in the host crystal. Note two sets of slip lines in the kink bands. d) SEM photograph of kinked grain with fractures developed in the kink. Cleavage fragment. e) Kink band (different orientation revealed by different colour and higher etch pit density): The kink band has curved and lobate boundaries. Note the very small tetrahedrite particles on the kink boundaries. f) Small recrystallized grains along grain boundary. g) SEM photograph of cleavage fragment. Grain boundary with slightly curved contact. h) SEM photograph of cleavage fragment. Grain boundary with precipitate (tetrahedrite). Faint slip lines are concentrated at the boundary. Photographs 8.2 a, b, c, e, f, are from polished sections normal to the foliation and parallel to the shear direction. 301 Second phase particles are very common in the galena from this deposit. The inclusions are usually lath-like and are found lying along (100) or (111) planes. Microscopic studies indicate that the inclusions are probably tetrahedrite.. The inclusions are very difficult to photograph but can be seen in figure 8.2e. The broad kink band boundaries in figure 8.2e effectively constitute grain boundaries (misorientations > 80). The sutured nature of the boundaries indicate grain boundary migration and also pinning by tetrahedrite inclusions. The banding in figure 8.1 is mainly composed of broad kink bands (fig. 8.2e) in larger grains. Small recrystallized grains are found along some grain boundaries (fig. 8.2f). Major grain boundaries (not originaly kink bands) are typically curved and smooth rather than sutured (fig. 8.2g). Scanning electron microscopic studies showed both {100} <110> and {110}41-0> slip (fig. 8.2h). The kink band boundaries which were often simple tilt boundaries (fig. 8.2h) with tetrahedrite precipatates. Sub-grains are not often found but where developed, they are generally polygonal and 150 - 300pm in size. Smaller sub-grains are found around inclusions of gangue minerals which act as stress risers. 8.2.(iii) PREFERRED ORIENTATIONS Preferred orientations for Pibram galena are shown in figure 8.3. Strong maxima are shown with no obvious relationships to the shear planes. The maxima are asymmetric and the [001] poles are at a low angle to the shear plane (and hence by symmetry one DO 'D pole will be at a high angle to the shear plane). The low strain 1.215) is probably insufficient to develop a strong texture in this sample. 8.2. (iv) CONCLUSIONS The microstructures of the DP galena indicate deformation by dislocation glide. Deformation is heterogeneous and kink bands are common. Microcracking probably indicates a low confining pressure during the deformation. The absence of significant polygonization and the preservation of slip lines permits one to infer a low deformation temperature (probably --100°C or less). 302 SHEARED GALENA DP (220) POLE FIGURES (REFLECTION MODE) EQUAL AREA PROJECTION Coetser Level 20% of Meal letesity (5, (200) ,....il..11$ ...... 1111.. ...... •• .. , ...... /11/3, ...... slI.J1J 11,1111.1,11111. 1 .1 ..... if ...... 1 ,1111.11,01 • „. '11 ..... ,,1 ...... !, ,,,1,...... ...... ••• ...... lo. 444.40 ...... '3.1431 Lineation 3 ...... Foliation Plane 7 at ! •1418,1 all SAMPLE ORIENTATION FIGURE 8.3 Partial pole figures (back reflection) of sheared galena - DP 303 8.3 RUTH HOPE MINE, BRITISH COLUMBIA 8.3. (i) INTRODUCTION Three samples of massive coarse-grained galena from the Ruth Hope mine, Slocan, British Columbia, were obtained from the Mining Geology Museum, Imperial College. The Ruth Hope mine is a faulted and brecciated vein/lode deposit in the Triassic Slocan series near the town of Sandon, Slocan district, British Columbia, Canada. The mineralization occurs as single or composite veins in fault fissures (Cairnes 1934) which cut the slates, argillites quartzites, tuffs and conglomerates of the Slocan series. The lodes are lens shaped masses of sulphides up to 2i feet thick and 100 feet long,emplaced mainly in the slates of the upper Slocan series (Cairnes 1934). The lodes are highly deformed with boudins and shear zones developed. The mineralization is principally coarse grained galena, sphalerite, quartz and calcite. The silver minerals are freibergite and pyrargyrite. Figure 8.4 is a photograph of an acetate peel of one of the samples from the Ruth Hope mine. At the top of the sample large (2 - 5 cm) grains of galena are seen. These have curved and irregular growth grain boundaries and no preferred shape orientation. Further down the specimen, the galena becomes progressively more deformed, with the cleavage planes of the large grains curving in towards the foliation which is well developed in the bottom two thirds of the sample (fig. 8.4). The bending of the cleavage is accompanied by kink bands which also curve into the foliation plane. In the bottom half of the sample (fig. 8.4) the foliation is strongly developed with lozenge shaped grains 2 - 5 mm in size. The grains are not flattened in the foliation but are more akin to prolate ellipsoids in shape. The grains are typically elongate with length/width ratios of 2 : 1 to 5 : 1. The foliation is commonly marked by parallel alignment of broad kink bands in the larger grains but in places kink bands can be seen to be at an angle to the foliation (fig. 8.4). As we shall see in the description of the micro- structures, the foliation is also outlined by bands of recrystallized galena. In the other samples from this deposit, 2 - 5 cm rhombs of calcite are found floating in the galena. These calcite crystals have been sheared and fragmented by the deformation commonly leaving a single large fragment with a trail of broken off pieces lying in the plane of the foliation. 304 Using the analysis developed by Coward (1976) a length width ratio of 5 : 1 in the old grains gives shear strain values between 1.5 and 3. These values are probably a minimum because of complications arising from kinking,recrystallization, initial grain shape and initial orientation of the grains. 8.3.(ii) MICROSTRUCTURES In the undeformed region of figure 8.4, the galena grains are large (1 - 2 cms) with irregular boundaries. Moving downwards into the shear zone, the cleavage traces bend into the shear direction i.e. sinistral shear. Where the foliation becomes prominent (fig. 8.4) small (50 - 200pm) grains are found (fig. 8.5a). These small grains occur in foliated bands commonly wrapping around larger (1 - 5 mm) augen galena grains (fig. 8.5b). The large grains with bent cleavage planes contain kink bands with (110) boundaries and elongate sub-grains (fig. 8.5c). They have a low density of etch pits and slip lines are not found. The sub-grains are found along the kink bands and they have serrated boundaries (fig. 8.5c). In the more highly strained grains, grain boundary migration occurs in highly misoriented kink bands (fig. 8.5d). Where the foliation becomes more intensely developed in the centre of the specimen, the kink bands are sites of preferential recrystallization with the formation of elongate new grains (fig. 8.5e). The etch pit density in figure 8.5e shows that the kink band is considerably misoriented within the host crystal. The recrystallized grains have a high degree of preferred orientation (fig. 8.5e). Tetrahedrite and perhaps boulangerite precipitates are found distributed throughout the galena with an average density of 1 - 2 per 2 2 10 pm . In figures 8.5e and 8.5f, the small precipitate particles (small light grey specs) accumulate on sub-grain and grain boundaries. Recrystallization is also found along grain boundaries (fig. 8.5f) where a core and mantle structure is developed. Figure 8.4 Photograph of an acetate peel of a specimen of sheared galena from the Ruth Hope Mine, British Columbia (x 3). The sense of shear is sinistral. The photograph is perpendicular to the foliation and parallel to the shear direction. 305 306 FIG. 8.5 Sheared galena from the Ruth Hope Mine, British Columbia. All samples etched with Brebrick and Scanlon (1957) etchant. a) Section perpendicular to foliation showing small recrystallized grains along kink bands giving the appearance of folds. Relict old grains can be seen. b) Relict old grains surrounded by recrystallized grains. Deformation bands cut across the old grain. Section parallel to lineation and perpendicular to foliation. c) Kink bands shown by curved cleavages. Elongate sub-grains form along the kinks. d) Kink band boundary migration with lobate boundaries forming new grains. e) Kink band with recrystallized grains (dark colours). Note that only a few sub-grains have developed outside the kink band. f) Recrystallized grains at the boundary of an old grain. A core- mantle structure is shown from sub-grains at the top of the photograph to larger recrystallized grains at the base. Section perpendicular to the foliation and to the lineation. g) SEM photograph of a kink band boundary etched as a groove with a mosaic microstructure of etch pits either side of the kink band. Etched cleavage fragment. h) SEM photograph of a cleavage fragment showing parallel tilt walls in a kink band. Note tetrahedrite inclusions pinning the tilt walls. i) SEM photograph of a cleavage fragment showing elongate sub-grains in a kink band. j) SEM photograph of a cleavage fragment showing a serrated grain boundary. 307 D 1---150p--1 •' • •:-V-k - E' 1--150p 1-20p-1; J Met 1-10p-1 308 Scanning electron microscope studies show that the kink bands are either simple tilt walls (fig. 8.5g), bands of parallel tilt walls (fig. 8.5h), or bands of elongate sub-grains (fig. 8.5i). Small (2 - 8pm) precipitate particles can be seen concentrated on kink band boundaries, (fig. 8.5h). The distribution of etch pits in figures 8.5g - 8.5j shows a moderate dislocation density. The etch pits are not arranged in slip lines but have a crude cellular arrangement (fig. 8.5g). Etch pits are concentrated in the kink bands. Where recrystallization has occurred, serrated grain boundaries (fig. 8.5j) are found and the etch pit density is lower. The etch pit features indicate climb of dislocations into sub-grain walls and crude cellular arrangements. 8.3.(iii) PREFERRED ORIENTATIONS Figure 8.6 shows the preferred orientations developed in the Ruth Hope galena. Elongate maxima are found. These are very similar to the preferred orientations found in sheared galena by Siemes and Schachner=Korn (1965). The [001] poles are symmetrically disposed around the lineation direction and the [110] poles normal to the shear plane. The textures are well developed and probably indicate significant shear strain. 8.3.(iv) CONCLUSIONS The deformation microstructures in the Ruth Hope galena permits one to infer that the occurred deformation by dislocation creep involving slip, kinking, polygonization and dynamic recrystallization. The deformation textures are strongly developed and are similar to those found by Siemes and Schachner-Korn (1965). These features are consistent with a low deformation temperature (-.-150°C). 8.4 SAMPLE SW1 (SOUTH WEST AFRICA) 8.4.(i) INTRODUCTION This sample of sheared galena comes from the Tsumeb district, Namibia, and was obtained from the Mining Geology Museum, Imperial College. The Tsumeb deposits are considered to be hydrothermal breccia pipes of lead and copper mineralizion which are cut by later dykes and sills (De Kun 1965 p.392). This sample consists of medium (0.5 - 2 mm) grained 309 SHEARED GALENA RUTH HOPE BC (220) POLE FIGURES (REFLECTION MODE) EQUAL AREA PROJECTION Contour Level 20% of Mean Intensity (5) (200) Foliation Plane SAMPLE ORIENTATION FIGURE 8.6 Partial pole figures (back reflection) of sheared galena from the Ruth Hope mine. 310 polygonal galena with a well developed banded structure which has a poorly developed foliation but which has a very strongly developed lineation. Several patches of chalcopyrite grains (1 - 5 mm in size) are rodded parallel to the linear fabric of the galena. Figure 8.7 is a photograph of an acetate peel of a section normal to the foliation plane (horizontal) and normal to the lineation. A large relict old grain can clearly be seen in the centre of the photograph. Relict old grains are outlined by their cleavages. The new grains have formed in orientations that are similar or close to that of the old grains and thus have similar orientations of the (100) cleavage planes. This can also be clearly seen in figure 8.7. Etching of the sample with Hydrobromic acid (HBr)(see Appendix A) revealed what appears to be fold traces in figure 8.7. These were found after each occasion of repeated polishing and etching. The 'fold' axes are parallel to the lineation. It can be seen, however, that the light coloured traces in figure 8 pass through the large grain in the centre of the sample. Without a detailed knowledge of the field environment from which the sample was collected it is not possible to say whether the etch features are folds or not. 8.4. (ii) MICROSTRUCTURES Large relict old grains (1 - 1.5 cm in size) can clearly be identified in acetate peels (fig. 8.7). In peels taken parallel to the lineation, many relict old grains or traces of old grains are clearly revealed by the alignment of cleavage traces. In the centre of figure 8.7 the large old grain is traversed by diagonal kink bands with small polygonal sub-grains. In studies of polished sections and acetate peels the polygonal microstructures are clearly revealed. The specimen SW1 is composed largely of equant grains of galena 200 - 400pm in size. The grain boundaries are straight to gently curved (fig. 8.8a). Triple points are well developed. Tetrahedrite and argentite grains are concentrated on grain boundaries and at triple points (fig. 8.8a). Small calcite grains are also found at triple points (fig. 8.8b). Grain growth is also indicated by the scattering of inclusions within the foam textured galena Figure 8.7 Photograph of an acetate peel of sample SW1 etched with HBr. Sample is perpendicular to the foliation plane (horizontal) and perpendicular to the lineation (x 3.5). 311 312 FIG. 8.8 Sheared galena - specimen SW1 a) Large polygonal grains - equigranular with straight or slightly curved grain boundaries. Boundaries are pinned by grey tetrahedrite inclusions. Etched with 20% HBr. b) Polygonal grains with boundaries pinned by calcite inclusions. Grain growth inhibited by the inclusions. Note small tetrahedrite inclusions within the grains. Etched with Brebrick and Scanlon (1957) etchant. c) Banded microstructure of recrystallized grains with similar orientations along relict kink bands. d) Detail of recrystallization in relict kink band. Elongate grains with straight cross boundaries in a kink band. e) Recrystallization into equigranular polygonal grains between kink bands. Microstructure similar to that produced by sub-grain coalescence. f) Banded microstructure similar to that produced by recrystallization in transition bands where elongate grains retain straight cross boundaries. Photographs c - f are of acetate peels of polished sections etched with 20% HBr. g) SEM photograph of a grain boundary of a polygonal grain. Note the patchy etch pit distribution within the grain. h) SEM photograph of the centre of a polygonal grain showing low etch pit density. i) SEM photograph of slip lines showing banded arrangement of etch pits. j) SEM photograph of concentration of etch pits around a tetrahedrite inclusion. Photographs b, g - j are samples etched with Brebrick and Scanlon (1957) etchant. 313 r, H ;s■ .• • ■ '10 •■•• • • ••• •- . • • •• I 56, • 314 grains (fig. 8.8a and b). Relict kink bands are revealed in acetate peels (fig. 8.8c) as parallel bands of polygonal sub-grains. Note that the sub-grains are slightly elongate with irregular boundaries. In detail the kink bands are composed of bands of similarly oriented sub-grains (fig. 8.8d). The sub-grains are slightly elongate but have fairly smooth boundaries. The sub-grain boundaries perpendicular to the kink bands are commonly straight (fig. 8.8d). This microstructure is very similar to that described by Dillamore et al. (1972) where polygonization- recrystallization follows deformation bands (fig. 3.9 ). Another kind of sub-grain is more equant with curved boundaries (fig. 8.8e). These sub-grains are developed outside the kink bands (fig. 8.8f). The microstructure is similar to that developed by sub-grain coalesence (fig. 3.9 ). The galena grains around the rodded chalcopyrite grains are smaller in grain size (100pm) than those away from chalcopyrite inclusions permitting one to infer that the chalcopyrite grains acted as stress risers during deformation. Scanning electron microscope studies show the smooth grain boundaries of recrystallized grains (fig. 8.8g). A moderate density of etch pits is indicated close to the grain boundaries (fig. 8.8g) whereas in the centre of the recrystallized grains a low etch pit density is found (fig. 8.8h). {001} <110> slip lines are found in kink bands and commonly close to grain boundaries (fig. 8.8i). Tetrahedrite inclusions (fig. 8.8j) in the centres of galena grains have a concentration of etch pits around them. The microstructures indicate annealing of deformed large grains and grain growth (Smith 1964) to an equilibrium foam type microstructure of equant galena grains with smooth grain boundaries and well developed triple junctions. 8.4.(iii) PREFERRED ORIENTATIONS The textures in South West Africa galena are given in figures 8.9 and 8.10. The maxima are well developed and both sets of pole figures show essentially the same features. As in figure 8.6 the [001] poles are not at 90° to the foliation plane but are 60° - 70° to the foliation plane. In figure 8.10 the [110] poles are nearly 90° to the shear plane. Unlike the pole figures for Pibram and Ruth Hope galena, the maxima are not spread out. 315 SHEARED GALENA SW 1 (220) POLE FIGURES (REFLECTION MODE) • • • 1,711,731 ...... 11.1. • • 111 111 ...... EQUAL AREA PROJECTION ..s.1.1.1 .. 4.4 Cutler Level 20% if Mess !Messily (5) 4..4 4. • •• :I . ... i• • •..111 1.1. 111 .s.I.1.1 444 1.1 T. O.1774.57. Off20:1 of • •• .....!loll 1,1 ••■••• •.$111111 (200) Ifio311 • • • 14s:W.3),f • 111...11.. • • • ...." ..... "" Foliation Plane ••• SAMPLE ORIENTATION FIGURE 8.9 Partial pole figures of sample SW] 316 SHEARED GALENA SW1 Reflection Mode Contour Levels X Uniform I I 1 X 2 2 2 2 2 X 3X 4-8 X >8 X PROJECTION PLANE. FOLIATION PLANE FIGURE 8.10 Partial pole figures of sample SW1 317 8.4. (iv) CONCLUSIONS The preservation of deformation bands and relict kink bands in the South West Africa galena permit one to infer that dislocation creep was the deformation mechanism. However, the presence of grain boundary migration and foam microstructures indicates significant grain growth possibly associated with a post deformation annealing event. The inferred deformation and annealing temperatures are somewhat higher (--250°C) than those for the Pibram and Ruth Hope galena. 318 8.5 BRAUBACH, WEST GERMANY 8.5.(i) INTRODUCTION This specimen of sheared galena comes from the Rosenberg mine, Konigsstieler Gangzug, Braubach, West Germany. This particular sample came from the Mining Geology Museum, Imperial College. Similar material was studied by Siemes and Schachner-Korn (1965). The Rosenberg Mine is part of the Braubach-Ems Vein system (south-west of Koblenz, West Germany). The mineralization occurs in a vertical dipping N-S and NNW-SSE vein system which cuts Lower Devonian greywackes and quartzites (details given in personal communication by H. Siemes 1978). The veins have a width of 1 - 2m (sometimes up to 5m). The mineralization was introduced during the Variscan orogeny and consists of siderite, quartz, sphalerite and galena. Post mineralization shearing along the veins produced a schistose texture in the galena. 8.5.(ii) MICROSTRUCTURES In polished section, the well developed foliation (fig. 8.11) can be seen to be composed of bands of equant polygonal grains (50 - 100pm) in size (fig. 8.12a). The grains have 5 - 6 sides, straight to gently curved and smooth boundaries, and well developed triple points. These recrystallized grains can be seen to form after kink bands in larger grains (fig. 8.12b). Traces of relict old grains are seen in figure 8.11. Kink bands with recrystallized grains are seen in detail in figure 8.12c where straight boundaries are seen perpendicular to the kink band whereas the kink band boundaries are highly serrated with small new grains forming. The Braubach specimen has numerous inclusions of tetrahedrite and boulangerite (fig. 8.12c). Boulangerite (Pb Sb S ) was identified 5 4 11 using scanning electron microscope analysis. Traces of gallium were found in the boulangerite. Grain boundary migration is inhibited by these inclusions (fig. 8.12c). A core-mantle microstructure (fig. 3.8) is found at the boundaries of large old grains (fig. 8.12d). Figure 8.11 Photograph of an acetate peel of sample of sheared galena from Braubach, West Germany. Bands of fine polygonal grains can be seen. Relict old grains are revealed by the similarity of cleavages. Note the abundance of curved cleavage traces. Section perpendicular to the lineation and to the foliation (x 3.5). 320 The mosaic of sub-grains in the old grain follow deformation bands. The new grains are equant with well developed triple points (fig. 8.12d). Elongate grains with aspect ratios up to 4 : 1 are also found (fig. 8.12e). Where the tetrahedrite-boulangerite inclusions are particularly abundant, grain growth is inhibited (fig. 8.12f). Polished sections cut normal to both the foliation and the lineation typically show a well developed poly- gonal microstructure with the grain boundaries pinned by inclusions. Sections cut parallel to the lineation and perpendicular to the foliation commonly show elongate grains (possibly after deformation bands) parallel to the foliation (fig. 8.12h). The grain boundaries parallel to the foliation show evidence of grain boundary migration with serrated boundaries (fig. 8.12h). In contrast the boundaries perpendicular to the foliation are commonly straight and show only little evidence of boundary migration (fig. 8.12h). In both figures 8.12g and 8.12h. the etching contrast indicates a high degree of preferred crystallographic orientation. Grain growth is indicated by the abundant evidence of grain boundary migration afforded by pinning of the boundaries by tetrahedrite - boudangerite inclusions (figs. 8.12i, 8.12j). Equilibrium grain growth microstructures (Smith 1964) are illustrated by figures 8.13a and 8.136. Tetrahedrite inclusions are found along grain boundaries and at triple points. The equant grains of figure 8.13a permit one to infer that recrystallization may have occurred by sub-grain coalesence (Chapter 3.2). Scanning electron microscope studies on etched cleavage fragments revealed a number of features. Slip lines {100}<110> were found in many fragments (fig. 8.13c). The high etch pit density in figure 8.13c could be partly attributed to sulphur vacancies or to a background of sessile {100}<001> dislocations. In detail most of the etch pits are pyramidal in shape (fig. A2.a). Tilt boundaries are revealed by an alignment of etch pits (fig. 8.13d). In detail, some etch pits are flat bottomed or asymmetric in shape (cf. fig. Al). The tetrahedrite and boulangerite inclusions are commonly 2 - lOpm in section and of variable length. They lie either along (100) or (111) planes (figs. 8.13 e and f). Dislocations are concentrated around inclusions. In places, the inclusions are intimately associated with the etch pits (fig. 8.13g and fig. A2b). Grain boundaries (fig. A2d) are revealed by deep grooves of etch pits often with precipitate particles along them. {110} tilt walls in kink bands are commonly found (fig. 8.13h) in the larger old grains. Recrystallized grains with a low dislocation density are found with slip 321 FIGURE 8.12 Sheared galena from Braubach, West Germany. All photographs are of polished sections etched with Brebrick and Scanlon (1957) etchant. a) Typical microstructure of Braubach specimen showing polygonal grains in relict kink bands (diagonally across photograph). Section parallel to lineation and perpendicular to the foliation. b) Relict kink bands shown by bands of black grains parallel to the foliation. Note most grains are equant. Section as in (a). c) Kink band with new grains elongate perpendicular to the kink band boundaries. Note the abundant tetrahedrite-boulangerite inclusions in the grains away from the kink band. Section parallel to lineation and perpendicular to foliation. d) Core-mantle microstructure with mosaic of sub-grains (left hand side) and a mantle of polygonal new grains (right hand side). Section perpendicular to lineation and foliation. e) Banded microstructures of elongate grains with straight grain boundaries. Tetrahedrite-boulangerite inclusions concentrated along grain boundaries. Section as in (d). f) Polygonal microstructure showing inhibition of grain growth by inclusions (upper half of photograph). Section as in (d). g) Well developed polygonal microstructure with inclusions concentrated at grain boundaries and at triple points. Section as in (d). h) Banded microstructure in a kink band with elongate grain boundaries undergoing grain boundary migration (often pinned by inclusions) and the shorter cross boundaries remaining stable. Section parallel to lineation and perpendicular to the foliation. i) Grain growth with grain boundary pinned by inclusion (arrowed). ,j) As in (i) showing grain boundary pinning and bulging during grain boundary migration. 7E 323 FIGURE 8.13 Sheared galena from Braubach, West Germany. All samples etch with Brebrick and Scanlon (1957) etchant. a) Equiaxed grains with straight grain boundaries and triple points. Section perpendicular to the lineation and foliation. b) Grain growth illustrated by large grains with tetrahedrite inclusions at triple points. Section as in (a). c) SEM photograph of cleavage fragment showing high etch pit density and {001} slip lines. Inclusions of tetrahedrite-boulangerite are also seen. d) SEM photograph of cleavage fragment showing sub-grain boundary (top) and relatively low etch pit density below the boundary. e) SEM photograph of etch pits (dislocations?) clustered around boulangerite inclusion. f) Slip lines shown by arrays of well developed pyramidal etch pits on a background of less well defined etch pits - probably sulphur vacancies. SEM photograph of a cleavage fragment. g) Boulangerite inclusions associated with pyramidal etch pits. SEM photograph of cleavage fragment. h) Kink band shown by slightly curved cleavage traces with parallel tilt walls running diagonally across photograph. SEM photograph of cleavage fragment. i) Elongate recrystallized grain with slip lines and etch pits at the triple points. SEM photograph of cleavage fragment. j) Sub-grains with irregular boundaries developed in a kink band. SEM photograph of a cleavage fragment. 4 4 ' a • ••• • • • • • • • • A • • a . • • ... ■ , 1 • • : :4% • 1 . •• °a- a- *• • •.. . I. .0. 4.. ••4 . it • • 41* ••A•• iii • • • s .. • . Aril ••■ • ' a t ik I 1110•4!•• . dk . •• ' ,, • . 4 • I" , • .4.. • II, a : . . % a •-•,.* . e 1 * 44 a -' 4` • . • • `a .4 t" . .N ., : • 4 • fa, 2. ..• - ,izi, -C - 41 '4* .0‘ 4i.". ' • 6 • 4. 4 • • ••111! - • •• A 4 4 • .7.: -. OSPIAllii••••• a • di, •• :;. •• '11 . • 325 lines at the grain boundaries (fig. 8.13(1)). Sub-grains are found in kink bands in large old grains (fig. 8.13(j)). The microstructures found in the Braubach galena permit one to infer that large old grains deformed by glide and kinking. Recrystallization occurred along kink bands giving rise to the banded structure of equant grains. Dynamic recrystallization microstructures - core and mantle structures and grain boundary migration are observed. Subsequent grain growth to equilibrium polygonal microstructures is also indicated. Pinning of grain boundaries by inclusions is found in most sections. Detailed scanning electron microscope studies revealed more detail than conventional polished section studies. Tilt walls and kink bands are found in relict old grains. Sub-grains and grain boundaries are also observed. Precipitates of tetrahedrite and boulangerite are associated with etch pits and dislocations. 8.5(iii) PREFERRED ORIENTATIONS Well developed preferred orientations are found in the Braubach galena (fig. 8.14). The DTB poles are nearly parallel to the foliation plane and hence by symmetry also perpendicular to the foliation plane - a texture similar to that indicated by the simulation studies (fig. 5.8). 8.5(iv) CONCLUSIONS Dynamic recrystallization along kink bands, core and mantle microstructures, slip lines and kink bands indicate dislocation creep deformation. Grain boundary migration and foam microstructures permit one to infer that grain growth has occurred, but secondary grain growth due to annealing is not indicated. The strong preferred orientations in the Braubach galena are also typical of dislocation creep, probably at temperatures of 200°C or less. 8.6 YERRANDERIE, N.S.W., AUSTRALIA 8.6.(i) INTRODUCTION The Yerranderie silver-lead deposit is located approximately 95Kms south west of Sydney, N.S.W., Australia. The ores occur in E-W trending veins in Upper Devonian rhyolites. The mineralization appears to be associated with a late Carboniferous granite (Lawrence 1965). The ore mineralogy is principally galena, sphalerite, pyrite, arsenopyrite and tetrahedrite with a gangue of quartz and ankerite (Edwards 1953). 326 SHEARED GALENA BRAUBACH (220) POLE FIGURES [REFLECTION MODE) EQUAL AREA PROJECTION Contour Level 20% of Mean Intensity (5) (200) Ilia: IYllrt;14/119/U 111111111111111/1.41. ill 1.1 .• 7...::::11..i..1 '' ; ' ill:)ii .111 iiiiiiii lh all! lllll III ...1.1 ... 1..." . ... .1171. !Matti:1111. Al. WI hill .. .,!?11...1.2211.11. iiiii 1111111..14.11114 es •,:514-7, 111111111111 '''T.1. ""1 .1t:11 .. 1 ii 1 i a 11/1111 1111 11.1 .11.11. MI.. "It:::4 tf: 11111 •• 11 ...A 11 II 1111111 4. 111111111/41 1.11 11111111.11 11111/4, .1.1)11 11.1 111111, ..... 1111, Its. 1.: .... 311, 11111111111.1. It. "St ••..2111.1.111.11Ial 1.1 151: • .1...... • /1 111 If 1[t 11111)1.11111 Sall 4..11111 121:1111 114111/ •• ee 11) lllll 111124 A(11.111■=thia..... 1 111111 ...Id?! 1 11:1111) 11121 ...... ,1.411 31, .1161/11.1.411.1 It 1..41.11 ll 1.11.4111114 111 4isal. shl .1/11...... 1111 ti.(11. 11141; .11 124.11211i 1 41.1 11 ...... 21/7 A.,3 SA 4/1141...1«.. ll a '11.s1...... 111 .. t.teumelte: .14 11111/7111111/11. 11111111611," II .ttl t., .. 11/1111 11/141111 .1. .., .411.6 1121 III •• •••• .4 Li! "11111. .40 SI...... 1111.1 ..... "4 1' III 11 17as :1/1 11 I 11 0 ...1111.111/1 . .hhh.. 31 ,11I ...... •..... 11...... 11..11 .11)1 s111. lusos al■ tit dAti tez,az .1.4111 1:111111 11.11.1.11 .1111e.11 " .... .11 a1111 • 1.1. ... 111:111111 .11 SAMPLE ORIENTATION FIGURE 8.14 Partial pole figures of Braubach galena. 327 The veins underwent post mineralization shearing along the strike of the veins. Lawrence (1965) considers that this shearing generated sufficient heat and pressure to recrystallize the galena and to impart a sheeting texture to the lode quartz. Edwards (1953) reported that the sphalerite in the ores was commonly fractured, shattered and sheared in a matrix of galena. Three samples were used for this study. Two were obtained from the Mining Geology Museum, Imperial College, and the third was obtained from Dr. G. Lister, Leiden. All of the samples were dominantly galena with minor pyrite, tetrahedrite and other unidentified silver minerals. Minor quartz and iron rich carbonates (ankerite) also occur in the samples. Sample Yi is shown in fig. 8.15 which is a photograph of an acetate peel. At the top of the specimen, large (1 - 2 cms) grains have bent and kinked (001) cleavage planes. The cleavage planes and the kink bands can be seen to curve into the foliation plane where the shearing is more intense towards the centre of the specimen. The foliation is defined by lamellar bands of small recrystallized grains. Large lozenge shaped augen of relatively undeformed grains are common in the lower half of the photograph (fig. 8.15). A high degree of preferred orientation is indicated by the parallel cleavage cracks in the specimen; even in the fine grained galena. In some of the large grains, kink bands at 50 - 70° to the foliation can be seen. The white brecciated grains which occur in clusters (fig. 8.15) are tetrahedrite and pyrite. Dark patches in the photograph are either cleavage pits or holes where ankerite grains have been dissolved by the etchant. This sample clearly shows the transition from large grains at the top of the specimen, to kinked grains and progressively more deformed grains, as the intensity of deformation increases, towards the base. The sense of shear (dextral) is indicated by the bent cleavage planes and cur curved kink bands. Sample YL from the same locality has a strong foliation defined by polygonal galena grains. The microstructures of both specimens are discussed in detail in the next section. Figure 8.15 Photograph of an acetate peel of a sample Yl of sheared galena from Yerranderie, N.S.W. The sense of shear is dextral. The white patches are mainly pyrite augen and the black specs are calcite grains which have been etched out. Section normal to the foliation and parallel to the lineation (horizontal)(x 3). 328 329 8.6(ii). MICROSTRUCTURES Typical microstructures found in Yerranderie galena are illustrated in figures 8.16 - 8.18. Sections cut from areas away from the top of figure 8.15 have a typical microstructure as shown in figure 8.16a. Polygonal grains (50 - 100pm) have a strong preferred orientation as indicated by the banding in figure 8.16a. For sections taken parallel to the lineation, the grains are elongate with aspect ratios from 1.5:1 to 3.5:1. Perpendicular to the lineation and the foliation the polygonal grains are equant in shape. Many augen grains of galena (fig. 8.15) are seen in polished sections (fig. 8.16b). These show a well developed core and mantle structure with equant sub-grains developed throughout the relict grain and a mantle of recrystallized grains around the core. At the top of the specimen (fig. 8.15) the large grains are traversed by numerous kink bands (fig. 8.16c). Within the kink bands elongate recrystallized grains form whereas between the kink bands polygonal sub-grains are formed (fig. 8.16c, d). Details of the kink bands in figure 8.16c are shown in figure 8.16d. (001) cleavage traces can be seen to curve through the kink band (fig. 8.16d). New elongate grains are formed along deformation bands which have (001) boundaries. The boundaries of these grains are serrated. Between the kink bands partially formed polygonal sub-grains can be seen (fig. 8.16d). The sub-grains and new grains in sections perpendicular to both the foliation and the lineation are generally equant although some elongate grains are parallel to (001) planes and deformation bands (fig. 8.16e). In contrast, most of the sub-grains and new grains in sections parallel to the lineation and normal to the foliation are rectangular with aspect ratios up to 6:1. The elongate boundaries are parallel to (001) planes (fig. 8.16f) and are similar to the structures found in transition bands in metals (Dillamore et al. 1972). The foliation is strongly developed in the lower portions of the specimen (fig. 8.15) and is marked by bands of elongate grains (fig. 8.16g). The rectangular grains are smaller than the more polygonal grains which in themselves show a preferred shape orientation (fig. 8.16g). Thin bands of fine grained galena are found parallel to the foliation plane (fig. 8.16h). The amount of deformation in the Yerranderie ore can be seen by the development of pull-apart structures in quartz (fig. 8.16i) and also pyrite (fig. 8.16j). Other augen structures are found with completely undeformed galena grains with a pressure shadow of sub-grains (fig. 8.17a). The matrix of all of the microstructures described above is typically a 330 FIGURE 8.16 Sheared galena from Yerranderie, N.S.W. All photographs are of polished sections etched with Brebrick and Scanlon (1957) etchant. a) Strong preferred orientation of galena. Recrystallized grains along relict kink bands. Grains of a similar orientation etch to a similar colour. Elongate grains are parallel to the lineation. Section cut perpendicular to the foliation. b) Relict old grain showing well developed core and mantle structure. Recrystallized grains surround the lozenge shaped old grain. A mottled mosaic of sub-grains can be seen within the old grain. New grains are formed at the margins of the old grain. Section perpendicular to the foliation and parallel to the lineation. c) Recrystallization along kink bands. Relict kink bands are revealed by curved cleavage traces. Small elongate new grains form parallel to deformation bands whereas the host grain remains undeformed and unrecrystallized. Section perpendicular to the foliation and parallel to the lineation. d) Detail of (c) showing undeformed host grain with a few polygonal sub-grains. The kink bands above and below contain many elongate recrystallized grains. e) Polygonal sub-grains with straight boundaries parallel to {001} planes. Section perpendicular to the foliation and lineation. f) Planar elongate new grains parallel to deformation bands - transition bands. Section perpendicular to the foliation and parallel to the lineation. g) Planar foliation revealed by bands of small recrystallized grains - larger grains are mainly polygonal with boundaries at a high angle to the foliation. Some grains show an elongation perpendicular to the foliation and lineation. Section perpendicular to the foliation and to the lineation. h) Section similar to (g) but showing two bands of small recrystallized grains in matrix of large polygonal sub-grains. i), Unetched section showing boudinaged quartz grain in matrix of sheared galena. Section perpendicular to the foliation and parallel to the lineation. j) Fractured pyrite crystals (white) in matrix of fine grained recrystallized galena. Section perpendicular to the foliation and to the lineation. 332 mosaic of recrystallized grains with many boundaries parallel to (001) planes and parallel to the foliation plane (fig. 8.17b). The etching reveals a strong preferred orientation in the recrystallized grains. An annealed sample of Yerranderie ore shows well developed polygonal grains (fig. 8.17c). The foliation is defined by fragmented calcite grains (fig. 8.17c, d). The polygonal grains (fig. 8.17c) are longer than those in the unannealed specimen (fig. 8.17b). The grain boundaries are straight to slightly curved and have well developed triple points. Grain growth is indicated by the irregular grain shapes and the pinning of grain boundaries by small tetrahedrite particles (fig. 8.17d). The calcite grains in figures 8.17c and 8.17d inhibit the grain growth of galena. Small grains are restricted to the layers of calcite fragments (fig. 8.17d). Scanning electron microscope studies of cleavage fragments of Yerranderie ore reveal details of the dislocation substructure. C1001 <110> slip lines are found in many of the augen and slightly deformed galena grains (fig. 8.17e). Details of the etch pits are shown in figures 8.17f and A3b. Both symmetric and asymmetric etch pits occur in arrays along <001> directions. Grooves parallel to <110> directions probably indicate dislocations lying close to the surface of the cleavage fragment. Recovery structures are seen by the arrangement of dislocations into arrays and small polygonal walls (fig. 8.17g, h). (110) kink band walls are revealed as straight lines of etch pits (fig. 8.17h). The Yerranderie galena often etched with irregular etch pits (fig. 8.17g, A3a) which are most likely negative crystals and sulphur vacancy collapse structures. Sub-grain boundaries (fig. 8.18a) etch out as grooves of dislocations. There is commonly a concentration of etch pits at sub- grain boundary triple junctions. Kink band structures are particularly well revealed in SEM studies. Zig-zag cleavage planes (fig. 8.18b) contain abundant elongate sub-grains 10 - 20um in size. In detail, the curved cleavage steps (fig. 8.18c) can be seen to be composed of numerous small sub-grains parallel to the kink boundary. The etch pit density is moderately high in the kink band region. The sub-grains are small and elongate parallel to the kink band boundaries (fig. 8.18d). Recrystallization occurs preferentially in kink bands (fig. 8.18e, f) where small elongate new grains are formed. These new grains have sub-grains and slip lines (fig. 8.18e, f). The etch pit density in these grains is high (fig. 8.18g). These features are characteristic of dynamic recrystallization. 333 FIGURE 8.17 Sheared galena from Yerranderie, N.S.W. All specimens etched with Brebrick and Scanlon (1957) etchant. a) Core and mantle microstructure. Core of undeformed old grain with recrystallization at the margins. Section perpendicular to the foliation and parallel to the lineation. b) Completely recrystallized grains slightly elongate parallel to the lineation. Section perpendicular to the foliation and parallel to the lineation. c) Annealed specimen. Section perpendicular to the foliation and parallel to the lineation. Foliation outlined by fragmented calcite grains. Polygonal grains have straight - slightly curved grain boundaries. Calcite grains are found on the grain boundaries and at triple points. d) Annealed specimen. Grain growth inhibited by small fragmented calcite grains in central band. e) SEM photograph of cleavage fragment showing rows of etch pits revealing slip lines. f) SEM photograph showing detail of etch pits - diagonal running slip lines. Etched out grooves are probably slip lines parallel to the surface. g) SEM photograph showing mosaic structure of etch pits. Unusual etch pits running in a diagonal line are probably negative crystals or sulphur vacancies. h) SEM photograph of kink band boundary. Away from the kink band boundary dislocations are arranged in mosaic structure. • • 334 I - 00 'PS. ' S ••• • I-- 220p • e.,...s....: - . 1--- 220p —I a. at 16,■4;;; t :Jo - • 4.- ,4-47.64f 4 _ 4 .qb .• vil • . .1 'A:. :,;i• - tia .4...._.- - 41. -- a •'''4.* .1: •••• is • .,.<4.. ... lills '''' 4 ‘ 4.1414.4:46.4b- : . • 8'...... 0.. pp. is • "4". AV.. • 2,.._ . le A IL.,....4 • e-. ..,-s'46 *** • VA:04r - —.r••• - 4 0 .."'"km•....1..11:27".. . ,.: -..,....,,,,. .:11P'41%r"h la" q n.....%.0....1,-;.-4,1, •.. 4,,,, . • • dr Alkt; -Nahogf - -.* 4 lit 4 4. .241r:••ib,., •••••, .46 babz...:4, .....472.....(itt, itiiple-_. 41111gZIA . 41„ _7- ;11:41144?. ...4641.% -,_ -4-:' ------00 4 11C"KF- ■•.4..4-1 - 1•r° ...... _ 44e'grt • lb: •tr.' ,10dr a! • A. .7 4* ,•. .-4., .4 - - ■4 N •...... , • tritik: . ' ' ...IP 1...b 44 4 * . . L HI F Ali."111110% '..---....-., 4111 '' Ziliii I-4p —I1 • ..141111r..!• p.,•• . • ••• . • . . — • • . ' ,.••••• r-*• .. • ,- . •• : • • . - . • s.-!. • .• •. • • • - • . 5 5) • .• 4, . ... • • • I' • • 7 •••• • ■•• .1., . • .. ':. • • • -.. IP' . ' Sk"," ••••••,:; :1/44110, 7.■ " • . : • • . . .. • . . •• • • . 'IL • ••••• . •••. #1. ..' . • 'H . _ " .• . • }-10p ' 335 FIGURE 8.18 Sheared galena from Yerranderie, N.S.W. SEM photographs of cleavage fragments etched with Brebrick and Scanlon (1957) etchant. a) Detail of sub-grain walls showing concentration of etch pits. There is a relatively low etch pit density away from the sub- grain boundaries. b) Kink bands in a large grain. Kinked (001) cleavage planes can be seen together with elongate sub-grains parallel to the kink band. c) Kinked cleavage planes with high etch pit density and some sub- grain boundaries in the kink zone. d) Elongate sub-grain boundaries in a kink zone. e) New recrystallized grains in a kink zone. Grain boundaries are parallel to the kink band. Slip line traces can be seen running across the new grains. f) Recrystallization in a kink band. Note the misorientation of the new grains. g) Detail of elongate new grains in a kink band. Etch pit density is high and slip lines can be seen. h) Sub-grains in a kink band showing equiaxed characteristics and triple point development. i) Elongate sub-grains and tilt walls parallel to the kink band - similar to transition bands. j) Triple points in recrystallized grains. Etch pits are concentrated at the grain boundaries and at the triple points. Wavy slip lines can be seen particularly in the left hand side of the photograph. 337 Other microstructures found in the Yerranderie ores are equiaxed sub-grains (fig. 8.18h) within kink bands and parallel tilt walls in <110> directions (fig. 8.18i). The recrystallized grains have well developed triple points (fig. 8.18j). Of particular note is the high etch pit density at the triple points and associated slip lines (fig. 8.18j). The microstructures found in the Yerranderie galena indicate deformation by dislocation glide and dislocation creep with concomitant dynamic recrystallization. Deformation twinning is found in a few grains of chalcopyrite in this specimen. Pyrite and quartz grains are fractured and pulled apart. Augen of largely undeformed galena grains are found throughout-the specimen. Many galena grains are flattened in the plane of the foliation. Static annealing and grain growth textures are found in one sample. 8.6(iii) PREFERRED ORIENTATIONS The preferred orientations in Yerranderie galena are shown in figures 8.19 - 8.21. Figures 8.19 and 8.20 are from samples which show deformation microstructures and dynamic recrystallization. The texture in figure 8.21 is from a sample which shows annealing and grain growth equilibrium microstructures. In figure 8.19 sharp maxima are shown with Ell(] poles normal to the foliation plane. This indicates a component of flattening in the deformation. The [200] poles in figure 8.20 are normal to the foliation plane as suggested for simple shear by the computer simulations (fig. 5.8). For the annealed sample the pole figure maxima are more diffuse (fig. 8.21) than those of the unannealed samples (figs. 8.19, 8.20). The [200] poles in this sample lie in the foliation plane and also normal to it (fig. 8.21). 8.6(iv) CONCLUSIONS The Yerranderie samples exhibit a wide range of deformation microstructures from slip, kinking to dynamic recrystallization and annealing microstructures. It is probable that the deformation temperature was low (100 - 200°C) with a possibly higher temperature for the annealed samples. Strong crystallographic preferred orientations are indicated by the microstructural studies and are found in the X-ray texture studies. The preservation of planar grain boundaries parallel to (001) planes 338 SHEARED GALENA YERRANDERIE 1 (2201 POLE FIGURES (REFLECTION MODE) EQUAL AREA PROJECTION '1312 31,1113 .3 1.011.31,. •• sa , ,,, 1331311,, AAAAAAAAAAA . ) , 1111 Contour Level 20% of Mean Intensity (5) (200) 1fliUf.. i27n 03 :1 11 3,3 311 .33;1.1111 31,132. 11111 31.1..,31:: .,,2 1113 ...... 1.111/ .112111111.. nfl : ..72712.122:2.7,11.12721.7,24272•721,.. .113113.3.4 13..71, , , ,17121 .2.1121:22:01:71117.2“.. 1,1111 ,, ...2:.2.7.11711 .7 1,111111.111:1771 0.712111. 3113 . 22:7141•1,7,,72,1•222i2.22.1. . 2 .. 371.1111a 112.•72•2117•11.2214124 1331313.1313133.313 31•1 22,21121...1,1211111i )311111... .271.2111.1112.stis447 ...... 31113 111111 712112.217.11111t...... 1122:1 •211...1111d 127 722.2 14131 424...... t:i;f1 • 46,- 42/i■ :2 ...... • ...... 112.7...1e12 • •212271.. 111" 1771117. /1111.3 4.),J • . • • • 71211212112101. .' _. • 3.111 ...... 7 ,•Mi.,,,,,,,•"•., . . • • 724, • • • • 111111,1. 22 2722721772.1.2.2•2•2,1111.L"...L" ...... 2 227.212212 •• . .7:22 2, • 2.1227....272,7171 .. •• , ...... 1 ...... Foliation Plane •• ...... SAMPLE ORIENTATION FIGURE 8.19 Partial pole figures of sample Y1 , Yerranderie. 339 SHEARED GALENA YERRANDERIE (220) (200) (311) Reflection Mode Contour Levels X Uniform 1x 2 2 2 2 2x ■•• 3x 4-8 X ...... •••• >8X FIGURE 8.20 Partial pole figures of sample Y1 Yerranderie galena. 340 SHEARED GALENA YERRANDERIE 2 (220) POLE FIGURES (REFLECTION MODE) 1,11/1,1.41 1.1), '''''' ;. ,..tom •• .1171,411. EQUAL AREA PROJECTION 71/11/11 ,1 Colton Level 20% of Meal 'obesity (5) 11111.1111111131 1/11.1 '''''''' 61.1. 1111113 llllll 1111 111s. 11111. . . .1111 1 llllll 1 lllll 512 51 SOO 15 11 1141 1,1 llllll .66 66 ...... 111 1 111/1 11/1 1 • lb PI/ ••• 116 13.11111111. 6:6 (200) 111.34111. ! •• 11 ...... 11111 4111.11 1" 46/.da 4.11.1s Ifte•Ise O:...dd. 4tt i4dt • .411 111111 1,1 ..... d., •• !al. • . ... I.. ItId t. / /, 11.1 l dal tad, "I.. ildt! ra ..... tilt al d;, ... !,;:aata. .... 163 la, 1:V515/1i la,., Ill) tId•aaddIllIsAl[tattad...... I .... ddIdia•atittIttlattlIti, ... ala.11,1s...... •• ,s II/ Its.) sr .1 14 :71 ..... • . .. /I ,o4s 0,11 rlttl ..... rr "::;;::: ;.,. 11 1111.1111.1 .• ..... I II 1, t1s• • •• ...... 6/ ..... aialliaa.Cgaaa sa , 1 , 31.1a:1. .6111,61.11 I I SAMPLE ORIENTATION FIGURE 8.21 Partial pole figures of sample Y2 Yerranderie galena. 341 indicate the stability of these boundaries. This stability may contribute to the preservation of the deformation textures. Layers of small grains (5 - lOum) permit one to infer that superplastic flow has occurred. The annealed sample shows grain growth microstructures but the deformation texture is not destroyed but only made more diffuse. The X-ray textures permit one to infer that the deformation was simple shear with a component of flattening. 8.7 HALKYN, NORTH WALES 8.7.(i) INTRODUCTION The Halkyn lead-zinc:mines are located near Holywell, North Wales. The mineralization occur in E-W veins which occupy normal faults in Carboniferous limestones (Thomas 1961). Fluid inclusion studies on fluorite from the North Wales orefield (Smith 1973) give homogenization temperatures of 93 - 118°C which permits one to infer that the ores have not been subjected to significant post depositional temperatures. The samples of sheared galena used in this study were obtained from the Mining. Geology Museum, Imperial College. The specimen shown in figure 8.22 consists of a matrix of very fine grained 'steel' galena with large brecciated,lozenge shaped augen of galena grains. Other minerals found in this specimen are quartz and calcite and large grains of light brown sphalerite. The sphalerite grains are fractured and strung out in the foliation plane. At the base of the specimen grooving and slickensides are very strongly developed in a layer of very fined grained galena. In figure 8.22 the slickensides and shear direction are into the plane of the photograph. The structure at the top of the sample is very heterogeneous with large augen of galena grains which are traversed by numerous kink bands. These augen are surrounded by a matrix of fine galena grains which have a well developed foliation. Towards the base of the specimen, however, the structure becomes much more homogeneous with only a few augen in a strongly foliated matrix (fig. 8.22). At the base of the specimen there is a thin layer of very fine grained 'steel' galena. Figure 8.22 Photograph of an acetate peel of sheared galena ore from Halkyn, North Wales. Large augen of galena are the lighter grey patches whereas sphalerite grains at the top of the sample are white. Section is normal to the foliation and to the lineation with the shear direction into the page (x 2.5). 342 343 A poor parting is developed parallel to this basal foliation plane. As in the other samples described in this chapter, the galena most likely had a coarse grained (> lcm grain size) structure prior to deformation. 8.7(ii) MICROSTRUCTURES The microstructures observed in the Halkyn ore are illustrated in figures 8.23 and 8.24. The large lozenge shaped augen (fig. 8.22) can be seen to be surrounded by a mantle of small recrystallized grains (fig. 8.23a). The large grains are flattened in the plane of the foliation and are generally elongate parallel to the lineation. The large grains are traversed by numerous kinks (fig. 8.23a). Small new grains nucleate along these kink bands. Smaller relicts of large grains can be seen around the large grain in figure 8.23a. Away from the augen grains there is a strongly foliated matrix of small, slightly elongate galena grains (fig. 8.236). Streaked out relict grains can be seen in figure 8.236. The large galena grains contain many lamellar deformation bands (fig. 8.23c) as well as kink bands. Preferential recrystallization occurs along these deformation bands. Elongate new grains with sutured boundaries are formed within the deformation bands and at their margins (fig. 8.23d). Outside the deformation bands, equant sub-grains are developed. At the margin of the large grains, as shown in figure 8.23a, a core and mantle microstructure is found (fig. 8.23e). This microstructure (fig. 8.23e) is very similar to those observed in naturally deformed quartzites (White 1976) and as illustrated in figure 3.8. Rotation of small sub-grains leads to the development of new grains at the grain margins (bottom left of fig. 8.23e). The importance of deformation bands and kinks as sites for preferential recrystallization can be seen in figure 8.23f where the new grains follow the deformation bands. In the deformation bands, the sub-grains are elongate whereas outside the kinks and deformation bands the sub-grains are equant. Many relict augen grains are largely undeformed (fig. 8.23g) and occur in a very fine grained matrix of elongate grains with irregular boundaries (fig. 8.23h). The etching density of the fine grained matrix galena (fig. 8.23h) permits one to infer that there is no marked preferred orientation. Many microstructural features of the Halkyn galena are best seen in the scanning electron microscope. Slip lines are not observed in the optical studies but are revealed in SEM micrographs of kink bands 344 FIGURE 8.23 Sheared galena from the Halkyn mine, Wales. All photographs are of polished sections etched with Brebrick and Scanlon (1957) etchant. a) Lozenge shaped grain with kinked cleavage traces. Recrystallized small grains surround the large old grain. New grains are forming along kinks and deformation bands. Section parallel to lineation and perpendicular to the foliation. b) Strongly foliated fine-grained galena. Ribbons of old grains can be seen. Section parallel to the lineation and perpendicular to the foliation. c) Laminar deformation bands in slightly deformed old grain. Section perpendicular to the foliation and to the lineation. d) Recrystallization in deformation bands with polygonal sub-grains outside the deformation bands. Section as in (c). e) Core-mantle microstructure. In the upper part of the photograph deformation bands (with recrystallization) and sub-grains can be seen. Progressive misorientation of the sub-grains leads to recrystallization in lower left hand corner of the photograph. Section as in (c). f) Core-mantle microstructure showing completely recrystallized margins of an old grain. Section parallel to the lineation and perpendicular to the foliation. g) Laminated fine-grained recrystallized galena with relict old grain which is largely undeformed. h) Detail of laminated fine-grained galena showing extremely small grain size - also slightly elongate. Section parallel to the lineation and perpendicular to the foliation. 345 346 FIGURE 8.24 Sheared galena from the Halkyn mine, Wales. SEM photographs of cleavage fragments etched with Brebrick and Scanlon (1957) etchant. a) Gently curved kink band with well developed sub-grains. Slip lines can be seen on several sub-grains. b) Kink band with misoriented recrystallized grains at the margins. c) Curved cleavage surface with slip lines and sub-grain boundaries. d) Cleavage surface traversed by several kink bands with small elongate recrystallized grains. e) Kink band with sharp boundaries. Progressive misorientation of sub-grains away from kink band (left hand side of photograph) produces small recrystallized grains. f) Core-mantle microstructure showing progressive misorientation of sub-grains (right hand side) to recrystallized grains (left hand side). g) Polygonal recrystallized grains. h) Small irregular grains with fractures and possibly voids. Sample from most highly sheared section of galena. 348 (fig. 8.24a). Gently curved kink bands (fig. 8.24a) have elongate new grains parallel to the kink band. In some kink bands the new grains are all of a similar orientation (fig. 8.24a) whereas in others, the recrystallized grains show a wide variety of orientations (fig. 8.24b). Note that the recrystallized grains are very small (1 - 4vm) in contrast to those found in other examples of sheared galena (sections 8.2 - 8.5). The large augen grains (fig. 8.23a) have slip lines and numerous small sub-grain boundaries (fig. 8.24c). At the margins of these augen grains, however, kink bands with abundant small recrystallized grains are found (fig. 8.24d). The kink bands may be either simple tilt boundaries (fig. 8.24e) or complex arrays of elongate sub-grains and new grains (fig. 8.24a, b). The development of the core and mantle microstructure is illustrated in figures 8.24e and 8.24f. Sub-grains formed in kink bands and deformation bands show progressive misorientation away from the kink band until a mosaic of small new grains is formed (fig. 8.24g). The small new grains are polygonal with straight grain boundaries. It can be seen that the SEM investigations reveal more detail than the optical studies. From the orientation of the cleavage planes in figure 8.24g it can be deduced that the small recrystallized grains have no marked preferred orientation. At the base of the specimen there is a highly sheared layer of galena grains which have irregular shapes and sizes (fig. 8.24h). It is noteworthy that in the SEM studies of Halkyn galena, the etch pit density is generally low and only a few slip lines, and sub- grain boundaries are observed. 8.7(iii) CRYSTALLOGRAPHIC PREFERRED ORIENTATIONS Two samples were used for X-ray texture goniometry. Both samples were taken perpendicular to the foliation and to the lineation. The sample used for figure 8.25 was measured at Aachen by Prof. H. Siemes while the pole figures in figure 8.26 were measured at Leeds University. Both sets of partial pole figures (figs. 8.25 and 8.26) show no marked preferred orientation with irregular maxima found in figure 8.26. These samples were relatively free from large grains. The lack of a marked preferred orientation in contrast to those found in the other examples described in this chapter permits one to infer that the dominant deformation mechanism is probably diffusional creep with grain boundary sliding. This mechanism is likely to destroy any inherited preferred 349 SHEARED GALENA HALKYN (220) POLE FIGURES (IEflECTIO. lODE) EQUIL IREI PROnCTlO. Clltllf Ln.1 21% If Iial Ilt'lsity (5) ...... (20D) ... .J.~.1~J...... Foliation Plane / Lineation- .. " ...... ,\ .. , ...... SAMPLE ORIENTATION FIGURE 8.25 Partial pole figure of Halkyn galena. 350 SHEARED GALENA HALKYN Reflection Mode • OM Contour Levels X Uniform 1 x 2 2 2 2 2X 3X '4x F • FOLIATION PLANE A. LINEATION FIGURE 8.26 Partial pole figure of Halkyn galena. 351 orientation which may arise from either preferred depositional textures or the early kink bands found in the large grains of this specimen. 8.7(iv) DISCUSSION The large relict grains in the Halkyn ores exhibit kink bands, deformation bands sub-grains and recrystallization in kink bands. Such features are similar to those found in the experiments on single crystals of galena (fig. 4.6 and 4.13) and are typical of dynamic recrystallization (section 3.2(v)). The size of the recrystallized grains (10 - 2pm) in the Halkyn ores is the smallest of the sheared galena studied in this chapter. In the large relict galena grains, the microstructures indicate deformation by dislocation glide and dislocation creep with concomitant recrystallization. The extremely small grain-size of the matrix grains and the lack of preferred crystallographic orientation permits one to infer that the dominant deformation mechanism in the matrix was diffusion controlled. The theoretical predictions of Atkinson (1977) suggest that at low temperatures (< 200°C) fine-grained galena ores would deform by Coble creep (fig. 3.11) under geological conditions. The fine grained equant grain shapes are similar to those found in Coble creep or super- plastic deformation of metals and alloys (section 3.2(iv ))• It is possible to infer that the galena of the Halkyn ores first underwent deformation by dislocation glide and dislocation creep with recrystallization into small grains. Once a small grain size was established the deformation became concentrated in the matrix and any preferred orientation resulting from dislocation glide and creep would have been destroyed by subsequent grain boundary sliding and Coble creep processes. There is no evidence of grain growth in the Halkyn ores and this, together with the low fluid inclusion temperatures (Smith 1973) permits one to infer that the deformation temperature and post deformation temperature were low (probably < 150°C). 8.8 DISCUSSION AND CONCLUSIONS From the microstructures described in this chapter it can be seen that dislocation creep and dislocation glide features are best preserved in sheared coarse grained galena, rather than in the finer grained galena ores described in chapters 6 and 7. The microstructures bear striking similarities to those found in single crystals of galena 352 deformed at 100° - 200°C (chapter 4). Kinking is a significant deformation mechanism in all of the samples studied. Dynamic recrystallization was found to occur along kink bands. Core and mantle microstructures also indicate dynamic recrystallization. Grain boundary migration and lobate grain boundaries were also observed. Many recrystallized grains, and in particular in the Yerranderie galena, have planar boundaries and as such are thought to have formed along deformation bands (cf. transition bands, Dillamore et al. 1972). These planar boundaries are often along (100) planes and such high coincidence boundaries appear to have a low mobility, and thus may significantly contribute to the preservation of deformation textures (Higgins 1974). The fine grained galena in the samples from the Halkyn mine enables one to infer that dislocation creep resulted in recrystallization into very small grains which subsequently deformed by Coble creep. The deformation mechanisms which were inferred from the microstructural data are consistent with the theoretical predictions of Atkinson (1976b, 1977) in which coarse grained galena would deform by dislocation glide and dislocation creep at low temperatures (fig. 3.3). All of the samples studied exhibit microstructures which enables one to conclude that the deformation occurred at temperatures between 100 and 200°C (with reference to the preservation of microstructures, see chapter 4). In addition, the SW, Braubach and Yerranderie samples exhibit grain growth and annealing microstructures. Secondary grain growth (section 3.2) and large new grains were not observed. These features are contrary to those proposed for the formation of sheared galena microstructures by Stanton and Willey (1972). This study indicates that sheared galena microstructures are the result of dislocation creep and dynamic recrystallization and not largely resulting from static annealing and grain growth as proposed by Stanton and Willey (1972). The deformation microstructures may also contain clues as to the stress environment during deformation. The dynamic recrystallization grain size may reflect the differential stress during deformation (Twiss 1977). As such, the highest differential stress level is indicated for the deformation of the Halkyn galena followed by decreasing differential stress levels for Yerranderie, Ruth Hope, Braubach and SW samples respectively. Dynamic recrystallization was not found in the Pibram sample but the opened cleavages are consistent with deformation at high crustal levels and hence relatively high differential stresses. 353 The preferred orientations are similar to those found by Siemes and Schachner-Korn (1965). Pole figures with [001] poles normal to the foliation plane as predicted by the computer simulations (chapter 5) are observed but some samples have [110] poles normal to the foliation plane. It is possible to infer both simple shear and a component of flattening deformation (chapter 5). In all but the Halkyn sample, the deformation textures are very strongly developed in contrast to those textures presented in chapters 6 and 7. Dynamic recrystallization and annealing recrystallization have not destroyed the preferred orientations. In fact, the sharp maxima are similar to single crystal deformation textures (Honeycombe 1968) and this fact indicates a strong host control on the dynamic recrystallization of coarse grained galena. The fact that the textures depart from ideal shear textures may possibly be attributed to the dynamic recrystallization but the new grains have similar orientations controlled primarily by kink band and deformation band orientations. It is proposed that kinking is an extremely important mechanism in the deformation of coarse grained galena in that it 1) allows relaxation of the von Mises (1928) criterion. Grains in unfavourable orientations for {001}<110> easy glide will either remain as large augen (cf. Yerranderie galena) or undergo rapid kinking and dynamic recrystallization. 2) allows rapid development of deformation textures by rapid rotation of crystal axes and 3) exerts a strong host control on the dynamic recrystallization of galena, permitting strong preferred orientations to develop by dynamic recrystallization. In the case of the Halkyn galena, dynamic recrystallization to a small grain size permitted subsequent deformation to occur by Coble creep and grain boundary sliding which destroyed any preferred orientations. From the study outlined in this chapter it is possible to see that direct comparisons can be made between experimental deformation (chapter 4) and natural deformation with the result that useful environmental information can be gained from detailed microstructural and textural studies. 354 CHAPTER 9 CONCLUSIONS In this thesis, detailed microstructural studies, (particularly using etch pitting techniques) together with x-ray texture goniometry have been used to determine the deformation mechanisms and deformation conditions which obtained during the low grade metamorphism of the Rammelsberg, Mount Isa and from sheared galena deposits. The micro- structures in these naturally deformed ores have been compared with those from experimentally deformed sulphides. In addition to the deformation microstructures, this study has revealed primary depositional microstructures and this has lead to a new theory for the genesis of the Mount Isa ore deposit. It has been demonstrated that detailed microstructural and textural studies combined with field studies may be used to understand the deformation and metamorphism of sulphide ore deposits. These studies may also provide new insights as to the genesis of the ore deposits. This latter factor may be of significant economic interest and influence exploration policy for related ore deposits. The results of this investigation are presented as follows. The experiments on single crystals of galena showed a strong control of crystal orientation upon flow strength. Single crystals of galena which are compressed parallel to the <001> axis or at 10° to the (001) plane exhibit high flow strengths (1 - 3 Kbars) at low temperatures (20 - 100°C) but show a rapid decrease in strength for experiments conducted at 200°C or higher. In contrast, single crystals of galena compressed parallel to the <111> axis have a yield point at approximately 50 bars CRSS at temperatures of 20° to 400°C. At 20°cthe rate of work hardening is high but this decreases rapidly with an increase in temperature until at 300° to 400°C the rate of work hardening is insignificant. The most important microstructural feature observed in the deformation experiments was that kinks were found in all three orientations tested and kinks were particularly prominent in most experiments at low temperatures (up to 200°C).{110} <110> and {100} <110> slip features were found in most experiments. The CRSS for {11O} <110> slip in galena is high = 1250 bars at 20°C and decreases rapidly to - 50 bars at 400°C. In contrast the CRSS for {100} <110> slip was found to be approximately 50 bars for temperatures from 20°C to 400°C. This temperature dependent behaviour of the CRSS for {110} <110> slip is interpreted to be due to the presence of solute atoms within the galena lattice. 355 Dynamic recrystallization was found in experiments conducted at 200°C and above. Static recrystallization was also found to occur at 200°C. These temperatures are lower than those previously reported for recrystallization in galena. Stress relaxtion tests show that significant recovery occurs in single crystals of galena at temperatures as low as 100°C. It has been possible to demonstrate that static and dynamic recrystallization microstructures are clearly distinguishable. The implications of these studies for investigations of naturally deformed galena are as follows: (1) Kinking is an important deformation mechanism in coarse grained galena for all orientations: (2) The microstructures may be used to infer the conditions of deformation: (3) The hard orientations for slip in galena will result in undeformed grains - augen occurring in naturally deformed galena: (4) The variations in CRSS for slip in galena may be used in computer simulations of deformation textures. Computer simulations of deformation textures in galena show that the textures develop only at high strains ( -64%). For flattening, the [110] axis aligns parallel to the compression axis whereas for simple shear a texture with [001] poles normal to the foliation plane is produced. It is proposed that in naturally or experimentally deformed galena, kinking contribute significantly to the rate of development of deformation textures. Studies of the Rammelsberg ores have revealed a deformation microstructural sequence in the barite and chalcopyrite ores. Primary depositional microstructures are preserved and these are similar to those found in Kuroko ore deposits. Preferred orientations in the Rammelsberg ores are poorly developed and it is deduced that this is in part due to the deformation mechanisms (Coble creep and grain boundary sliding) and in part due to the mixed nature of the ores. It is concluded that x-ray texture goniometry is not a useful tool for the study of mixed sulphide ores. A detailed study of the folding in the Mount Isa silver-lead-zinc ores was carried out and it is concluded that most of the folds are tectonic in origin rather than sedimentary slump folds as proposed by Bennett (1970) and others. An analysis of the microstructures and of the preferred orientations in the galena in the Mount Isa ores permits one to infer that Coble creep and grain boundary sliding at low temperatures ( -200°C maximum) were the main deformation mechanisms in the Mount Isa galena. 356 In addition to the results on the deformation and metamorphic conditions at Mount Isa, the detailed microstructural studies revealed important depositional features. Of particular significance is the discovery of pseudomorphed sulphate evaporites at the Mount Isa mine. From these observations it is postulated that the Mount Isa ores were formed from metal-rich sabkha type brines of the kind forming today in evaporitic basins (Carpenter et al. 1974), rather than from volcanic- hydrothermal solutions exhaled into the sedimentary basin as proposed by Croxford (1962) and Stanton (1962, 1972). The microstructures and textures of six samples of naturally deformed coarse-grained galena indicate deformation by dislocation glide and dislocation creep. It is suggested that the microstructures and textures of naturally sheared galena ('steel galena') are the result of dynamic recrystallization along kink bands rather than mimetic grain growth as suggested by Stanton and Willey (1972). In the case of the Halkyn galena, dynamic recrystallization resulted in small grains which permitted subsequent deformation to occur by Coble creep and grain boundary sliding. The most important result from the study of naturally deformed coarse-grained is that the microstructures are similar to those found in the experimentally deformed galena single crystals. The ease with which most of the common sulphide minerals (except pyrite) deform, permits one to infer that most of the evidence of early deformation episodes in sulphides will be destroyed by later events. However, this study has shown that detailed microstructural studies enable part of the deformation history to be unravelled. The crystallographic preferred orientations, however, will reflect the latter part of the deformation history of the ore deposit. Finally, the studies of naturally deformed sulphides indicate that galena, sphalerite, chalcopyrite and pyrrhotite all exhibit microscopic plasticity during deformation whereas pyrite deforms cataclastically in low grade metamorphic environments. Fluid assisted diffusion, pressure solution, is a possible deformation mechanism in pyrite. The comparative rheologies of naturally deformed sulphides are similar to those found in experimental studies (Atkinson 1972, 1974). SUGGESTIONS FOR FUTURE WORK This study has shown that detailed microstructural and x-ray texture investigations can be successfully applied to sulphide ones deformed under low grade metamorphic conditions. These techniques should be applied 357 to the study of other deformed ores (eg. Sullivan, British Columbia) particularly where a prograde metamorphic sequence occurs - eg. in the copper ores in Zaire and Zambia. Detailed investigations have been carried out on naturally and experimentally deformed galena and such studies should be extended to include sphalerite, chalcopyrite and pyrrhotite. Particular topics which warrant further investigation include the following subjects. At Mount Isa further detailed mapping of fold structures and analysis of deformation both within the silver-lead-zinc and the copper orebodies is called for. The sedimentology and nature of the Mount Isa ores and of the silica dolomites needs careful research in the light of the discovery of pseudomorphed sulphate evaporites in the silica dolomite. This research may furnish a new understanding of the genesis of the Mount Isa deposit and influence exploration policy in the Mount Isa district. More detailed experimental studies are needed on sphalerite, chalcopyrite and pyrrhotite and in particular the role of pore fluids during deformation should be investigated. The development of preferred orientations in polycrystalline galena needs detailed study in order to investigate the formation of textures with variations in strain, grain size, temperature and - strain rate. These investigations should be compared with 'computer simulations of textures in galena using more sophisticated models involving kinking and climb. Finally, the low temperature - low strain rate experimental deformation of polycrystalline galena should be investigated (possibly using split cylinder and relaxation techniques) in order to determine whether or not Coble creep and grain boundary sliding occur, as the theoretical considerations predict. 358 APPENDIX A ETCH TECHNIQUES INTRODUCTION Mineralogists have long used etch techniques to study growth fea- tures in minerals (Honess 1927) but only a few investigators have carried out etching studies on naturally deformed sulphides, (Ramdohr 1928 , 1953a , 1969 , Stanton 1964 , 1970 , 1972 , Stanton and Willey 1970 , 1972 ). Detailed studies of deformation features in experimentally deformed sulphides have been carried out by Clark and Kelly (1973), Kelly and Clark (1975), Salmon et al. (1974), Roscoe (1975), Atkinson (1972, 1974, 1975a,19764 McClay 1977a, and McClay and Atkinson (1977). Only a few detailed studies of deformation features in naturally deformed sul- phides (Stanton and Wily 1971, 1972, McDonald 1970) have been reported. This appendix summarises a detailed investigation of etch techniques for galena, sphalerite, pyrite, chalcopyrite and pyrrhotite. Two basic types of etchants have been used. The first type attacks dis- location features such as grain boundaries, slip lines, twin boundaries and sub-grain boundaries without leaving an appreciable surface film. The second type of etchant undertakes a more general attack on the grains with the rate of attack dependent upon the crystallite orientation. This latter type of etchant often leaves a surface film or tarnish, the colour or density of which reflects the orientation of the grain beneath. This is particularly useful in assessing the degree of preferred orientation in the sulphide aggregate. Table Al summarises the results of the etch investigations. Dis- location etch pitting is discussed in detail with particular reference to galena (McClay 1977a). DISLOCATION ETCH PITS Dislocation etch pits have been widely used by materials scient- ists to study dislocations in metals, ionic and ceramic materials (see reviews by Johnston (1962), Amelinckx (1964) and Robinson (1968)). To date, only limited studies have been carried out on geological materials. 359 Amelinckx (1954), Moran (1958), Mendelson (1962), Carter and Heard (1970), Gutmanus and Nadgornyi (1970), Heard (1972) and Poirier (1972) have studied dislocation etch pits in sodium chloride. Keith and Gilman (1960) and Bengus (1963) have studied etch pits in calcite while Young (1969) and Wegner and Christie (1974, 1976) have studied etched dislocations in olivine. Ball and White (1977) report on the most recent study of etch pitting in quartz. FORMATION OF ETCH PITS Dislocations in crystals produce elastic stress fields and may also possess an atmosphere of impurity ions, (Hirth and Lothe 1968). Both of these produce local differences in the elastic or chemical energy which thus promotes differential rates of dissolution between the points of emergence of dislocation lines at the surface of the cry- stal and the surrounding lattice. Dislocation etch pits are most readily formed on close packed crystal planes - generally the low index planes. Etchants are often very sensitive to orientation (Johnston 1962) and often will not even produce etch pits on surfaces orientated only a few degrees away from these low index planes. Impurities often segregate along dislocations and these may either enhance or retard etching. Dislocation etch pits are generally pyramidal in shape with the apex of the pyramid lying on the dislocation line and the centre of the base located at the point of emergence of the dislocation line at the surface of the crystal (fig. A.l.a.). Dislocation lines inclined to the surface of the crystal etch to form asymmetric etch pits while those perpendicular to the surface form symmetrical pits (fig. A.l.a.). The use of dislocation etch pits is limited to an examination of the surface expressions of dislocations but it has distinct advantages in that it is quick, inexpensive, non destructive and that large areas of specimens can be examined. It is particularly useful in determining dislocation densities; in determining the orientation of surface features and of deformation features; in the study of dislocation dynamics and of microstructural changes in sequential experimental tests. The main dis- advantages are that not all dislocations etch out, that other defect structures such as vacancies and negative crystals also etch out, and that only surface features can be examined. 360 DISLOCATION ETCH PITS IN GALENA Brebrick and Scanlon (1957) first described etch pits in galena. Franklin and Wagner (1963), Urusovskaya et al. (1964) and Melentyev and Boyarskaya (1971) have described etch pits associated with {100} <110> slip lines in artificially grown galena crystals. Norr (1963, 1966) and Patel and Sangwal (1971) have described dislocation etchants for galena. Salmon et al. (1974) and Atkinson (1976a) have described studies of experimentally deformed galena in which etching was mainly used to reveal subgrain structures. Further, more detailed studies have been carried out by the author (McClay 1977a, McClay and Atkinson 1977). The results of etchant testing on both polished sections and on cleavage fragments of galena are summarised in Table Al. The most reli- able etchant was that used by Brebrick and Scanlon (1957) which was a mixture of a solution of 100 gms thiourea / litre of water and concentr- ated hydrochloric acid in the ratio of 5:1 respectively and which was used at 65-70°C for etching periods of 30 - 120 seconds. The temperature and timing of the etching are critical if consistent results are to be obtained. The best results are obtained from surfaces oriented near or on (100) planes or on (100) cleavage faces. This etchant produces pre- dominantly pyramidal pits with the sides of the bases of the pits parallel to the <100> crystallographic directions. Norr (1963) reports that differ-, ent ratios of thiourea solution to hydrochloric acid produced pits with sides parallel to <110> directions. These changes of pit type with con- centration changes were not observed in the present study. The samples were examined in a reflected light microscope or in a Cambridge 600 or IIA Scanning electron microscope. High resolution photography in the scanning electron microscope proved difficult (probably due to absorption effects) both on coated (carbon or gold) and on uncoated samples. Slip in galena occurs mainly on the {100} <110> and the {110} <110> slip systems (McClay and Atkinson 1977). Dislocations with {100} <001> type burgers vectors have also been reported (Urusovskaya et al. 1964). Screw dislocations of the {100} <110> and the {110} <110> slip systems will have the dislocation line and burgers vector inclined to the face of a cleavage cube and thus will tend to produce asymmetric etch pits (fig. A.1). Similarly on edge dislocation of the {100} <110> system will also produce an asymmetric etch pit (fig. A.1.b.). 361 ETCH PIT SHAPE FOR VERTICAL AND INCLINED DISLOCATION LINES surface dislocation lines A symmetric etch pits asymmetric etch pits 001 (010)(10b (ioi)(1oi) E= edge dislocation line 5= screw dislocation line B b= burgers vector FIGURE A.1. ETCH PIT SHAPE FOR {100)(110> & 11101<110> SLIP SYSTEMS ON A CLEAVAGE CUBE OF GALENA 362 Only edge dislocations in the 0101 <1-1.0> system will have dis- location lines perpendicular to the clearage cube faces and thus produce symmetrical etch pits (fig. A.l.b). If prismatic glide dislocations occur ( {100} <001> system) then they will tend to produce symmetric etch pits. The situations described above are theoretical in the sense that most dislocations in crystals are often combinations of screw and edge components and hence will give rise to many different kinds of etch pit symmetry. The dislocation etch structures found in naturally and experi- mentally deformed galena are discussed elsewhere in detail (McClay 1977a) In general the features illustrated in figures A.2 and A.3 are similar to those found in metals (Amelinckx 1964), ceramics (Johnston 1962) and halite (Carter and Heard 1970, Heard 1972). INDIVIDUAL ETCH PITS The individual etch pits are generally pyramidal in shape and vary in size from 2.5 pm to 0.5 pm (fig. A.2.a). Occasionally, flat bot- tomed pits are found. These indicate that some of the dislocations have moved during etching, probably in response to thermal stresses generated by temperature changes. Although etch pits of different depths are often observed, it has not been possible to distinguish between positive and negative dislocations (Livingston 1962). Although many of the dislocation etch pits appear symmetric many are also slightly asymmetric but the small size of many etch pits makes it very difficult to distinguish between edge and screw dislocations of different slip systems. Very small inclusions (0.5 - 2.0pm) and precipitates of tetrahedrite and boulangerite were found in many samples of naturally deformed galena (fig. A.2.b). These inclusions may be primary or may have precipitated along dislocation lines (Johnston 1962). These particles will have important effects on dislocation movement (Orowan 1948, Burton 1975) and also on diffusional creep (Harris 1973). Surface scratches on cleavage fragments produce arrays of dis- locations shown in fig. A.2.c. which are quite distinct from grain bound- aries (fig. A.2.d) and slip lines (fig. A.2.e). SLIP LINES Slip lines and clusters of slip lines are clearly revealed by etching (fig. A.2.e f). Both {100} <110> (fig. A.2.e) and {110} <110> 363 FIGURE A2 Scanning electron micrographs of etch pits a) Large pyramidal etch pits in a cleavage fragment of sheared galena from Braubach, W. Germany. Etched 90 secs. 65°C. b) Small boulangerite inclusion in the same sample. Etched 90 secs. 65°C. c) A scratch mark across cleavage fragment showing the groove and associated dislocations. Etched 70 secs. 65°C. d) Grain boundary in sheared galena from Braubach, W. Germany with inclusion in grain boundary. Etched 90 secs. 70°C. e) Slip lines in {100} <110> system. Cleavage surface, Yerranderie N.S.W. Etched 60 secs. 65°C. f) {110} <110> slip traces on a cleavage face of a single crystal of galena experimentally deformed at 20°C, at a strain rate of 10-5 sec-1 and at a confining pres- sure of 1.5 Kbar. Etched 80 secs. at 65°C. (Sample from the experimental work of McClay and Atkinson 1977). g) Sub-grain boundary showing strong concentration of dislocations. Ruth Hope Mine, British Columbia. Etched 50 secs. at 70°C. h) Grain boundaries in recrystallisation at kink band. Etched 65 secs. 65°C. Braubach, W. Germany. 364 '410111r 461.911%1,, 41A • 4iiiriaii:2111111111LA - estraja ;ZaT11°r" irv al.r-.4••• Logr Aparsi.P. 4001,4, "•Aget.L.11. Inn ■ - 4a,&11k, 1,114Pervien 4P. 1C44 " r16141P•'44 "4‘11.4646'4 :.41A- " ' 4 17 • ALtr,sj‘.Segvor , 41P.allPre. AP- AF Ara* !LAW. •C ..4 Vallt4 H4 I-) D 11111.1•11,Wr":. 0:, 74.16. vt; r`14, • • -e4 •. 4410. .44102,41te J• ":1;" '•7 E Lig& I-10p -I Pir -1101N•••-' I-10p -1 I 365 (fig. A.2.f) type slip systems were observed in naturally and experi- mentally deformed galena (McClay and Atkinson 1977). GRAIN BOUNDARIES - SUB-GRAIN BOUNDARIES The etching techniques clearly highlight sub-grain and grain boundaries (fig. A.2.g.e.h). Low angle tilt boundaries etch out into grooves which show a strong concentration of dislocations in these bound- aries (fig. A.2.g). Grain boundaries (fig. A.2.h) also etch out strongly. OTHER ETCH FEATURES Many samples show irregular etch features with central beaks and grooves (fig. A.3.a) which probably indicate attack along dislocation lines which have an atmosphere of impurity ions (Johnston 1962). Etching around vacancy loops and negative crystal features may also produce irre- gular etch pit arrangements in fig. A.3.a. Etch pits in Yerranderie galena (chapter 8) show many irregular features. The use of a polishing etchant (Brebrick and Scanlon 1957, Norr 1963) on small growth octahedra of galena (fig. A.3.c) produces round etch pits on (111) crystal faces (fig. A.3.d). KINK BANDS In galena, kink bands often form in an (110) orientation when the easy glide system {100} <110> is not favoured (McClay and Atkinson 1977). Kink bands are commonly found both in experimentally and naturally defor- med galena and are often outlined by bent cleavage steps (fig. A.3.e). The kink band often etches out as a single groove of dislocations (fig. A.3.f,eg) with an array of elongate sub-grains (4 - 30111m) arranged around the kink boundary. Glide polygonisation (Livingston 1960) is thought to initiate some kink bands in galena (McClay and Atkinson 1977) and produces kink bands in crystals with a relatively low dislocation density. Some polished sections in an (111) orientation etch with tri- angular etch pits being formed (fig. A.3.h). 366 FIGURE A3 Etch Pits in Galena a) Scanning electron micrograph of etch pits, grooves and etch pits around precipitates. Some of these etch fea- tures are probably vacancies and stacking faults gener- ated by vacancy collapse. Cleavage fragment of galena from Yerranderie, N.S.W. Etched 50 secs. 70°C. b) Low density of symmetric and asymmetric etch pits, cleavage fragment, Yerranderie N.S.W. Etched 50 secs. 700C. c) Growth octahedra treated with Brebrick and Scanlon's (1957) polishing etch. In Slate galena 50°C 90 secs. d) Detail of (111) face from (c) showing round etch pits and surface precipitate. e) Stepped cleavage in kink bands. Sub-grains in kink band also shown. Yerranderie N.S.W. Etched 70°C 60 secs. f) Detail of kink band boundary with elongate sub-grains developed in the kink. Etched cleavage fragment, Yerranderie N.S.W 45 secs. at 65°C. g) Low angle kink boundary with low dislocation density, Yerranderie N.S.W. Etched 70 secs. 65°C. h) Slip lines revealed by triangular etch pits in single crystal of galena experimentally deformed at 100°C at a strain rate of 3 x 10 sec-1 and confining pres- sure of 1.5 Kbar. Etched 80 secs. 65°C. • . 01 CN 368 ETCH PITS IN GALENA - SUMMARY Cleavage fragments produce the best results particularly for scan- ning electron microscopy. Deformation twinning of (441) type (Lyall 1966) has only been occasionally observed in the etch studies. It has not been possible to establish an exact one to one correspondence between the etch pits and dislocations, but the obvious relationships between etch pits and deformation microstructures (slip lines, kinks, sub-grain boundaries) indi- cate that many of the etch pits correspond to dislocations. Other lattice defects such as vacancies, non stoichiometric defects, impurity ions and inclusions also etch out, but these pits are usually irregular in shape and arrangement within the crystal lattice. THE EFFECT OF OTHER ETCHANTS ON GALENA The results of these investigations are summarised in Table A.1. Other etch techniques are generally unsatisfactory. Hydrobromic acid (HBr) strongly etches grain boundaries (fig. A.4.a, b) but is generally too vigorous to reveal dislocation features. ETCHING OF SPHALERITE The effect of various etchants on sphalerite are summarised in Table A.1. Three etches were found to be satisfactory, Sodium Hypochlorite solution (Ramdohr 1969). Hydroiodic acid (Ramdohr 1969) and the Brebrick and Scanlon (1957) etchant. Photographs of etched polished sections are shown in figure A.4.c, d. Detailed photographs of etch pitting and twin boundaries are shown in figures 6.12a, b, d, f. (Rammelsberg sphalerite) and of Mt. Isa sphalerite (fig. 7.25 ). Etch pits in sphalerite are often triangular with curved boundaries. Etch holes and etch tunnels also appear to form (fig. 7.25d). Both annealing/ growth twins and deformation twins are revealed by the three etchants (figs. A.4.c, d and fig. 6.12). ETCHING OF PYRRHOTITE The most successful etchant for pyrrhotite was Hydroiodic acid (Table A.1.) (Ramdohr 1969). Hexagonal pyrrhotite often etches more strongly than monoclinic pyrrhotite (Vaughan et al. 1971). Deformation 369 FIGURE A4 Photographs of polished sections (plane polarised light) showing etch features. a) Polycrystalline galena (Specimen SW1) etched for 15 secs. in HBr (48%) showing deep etching of grain boundaries and cleavage features. b) Detail of (a) showing strongly etched cleavages and pits parallel to (100) planes. c) Sphalerite from Sullivan, B.C. etched 2 minutes (65°C) with Brebrick and Scanlon (1957) etchant showing grain boundaries and twin boundaries. d) Large sphalerite grain from Sullivan B.C. showing well developed annealing/growth twins. Etched 5 secs. with Sodium Hypochlorite solution at 20°C. e) Monoclinic pyrrhotite from Pickwe mine, Botswana, etched for 10 secs. with 55% HI (20°C). Kink bands and lamellar twinning revealed by etching. f) Etch pits in pyrite. Coarse grained pyrite from Rammelsberg, West Germany. Etched 5 mins., 20°cin chromic acid with electrolytic action. Chalcopyrite etched 40 secs. (20°C) in chromic acid with electrolytic action showing deformation twins and grain boundaries. (Specimen it isa C284). h) Chalcopyrite from copper ore, Rammelsberg which has been etched 1 minute in NH4OH - H202 solution. An irri- descent surface film is produced which reveals twin boundaries and sub-grain features. 371 twins and kink bands are strongly etched by the HI etch (fig. A.4.e). ETCHING OF PYRITE Pyrite is particularly difficult to etch successfully. Dilute nitric acid will etch some growth features but the most successful etch was chromic acid with electrolytic action which etched growth features (fig. 6.9) and also produced etch pits (fig. A.4.f). ETCHING OF CHALCOPYRITE The best results are achieved by chromic acid and electrolytic action which etches twin boundaries, slip lines and grain boundaries (fig. A.4.9). Time has not permitted detailed studies of etch pits in the scanning electron microscope. The other successful etchant for chalcopyrite is the NH4OH/H202 mixture which produces a highly coloured irridescent film which approximately indicates the orientation of the grain beneath. Twin boundaries and subgrains (fig. A.4.h) are also revealed by this etchant. SUMMARY OF ETCHING TECHNIQUES Dislocation etching of galena has been described. Careful etching of galena single and polycrystals produces dislocation etch pits which are often arranged in slip lines, sub-grain and grain boundaries and in kink bands. The disposition of the etch pits can give valuable micro- structural information particularly about deformation mechanisms. Both {100} <110> and {110} <110>slip systems have been found in naturally and experimentally deformed galena. Etching studies of sphalerite, pyrrhotite, chalcopyrite and pyrite have generally been conducted to study twin boundaries; deformation bands and slip lines; and sub-grain and grain boundaries. Detailed etch pit studies of these minerals have yet to be undertaken. 372 TABLE A 1 SULPHIDE ErCHANDS Mineral Etchant and conditions Comments GALENA Hydrobromic Acid HBr 4696 v/v 20°C Very strong rapid etching and ie difficult to control. PbS (ref. Ramdohr 1969). Etches, cleavages, grain boundaries and sub grain boundaries. Often produces black deposit. Depth of etching depends upon grain orientation. Used for acetate peels of large sections. 75 - 80 parts HNO3 (Sg 1.2) Very rapid etch and difficult to control. Brown-black- 20 - 25 parts C2H'OH 5 irridescent precipitate. Strongly affected by presence of other minerals. Some grain boundaries etch out but 20°C 2 - 10 secs. freshly prepared. unreliable. (ref. Ramdohr 1969). 1 1 HC1 + electsolytio action Poor results. Strongly affected by polishing features and 5 volts 50 ml 20 C. Time variable scratches. Often spotty surface. Unreliable. (ref. Ramdohr 1969) HCl + NH C1. 1 part cone HC1 to Spotty etch. Often etches scratches on polished sections. 4 Suitable for small cleavage fragments. 5 parts NH C1 (53 5 gmn/litre) 20°C 4 (ref. Patel and Sangwal 1971). KMn0 + H SO Etches some grain boundaries but generally leaves 4 2 4' irridescent precipitate obscuring all features. (10 gma/litre KMn04 + 1096 H2SO4). 20°C (± electrolytic) HC1 + Thiourea. (either 1 : 3 or Excellent controlled etch of slip lines, sub-grain 1 5 cone HC1 to 100 gra/litre boundaries kinks and twins. Grains orientated in [111] thiourea solution.) orientation generally etch to brown colour. Occasionally 60 - 70°C 30 - 120 secs. spotty and faulty etches develop. (ref. Brebrick and Scanlon 1957). 1 As above but electrolytic at 20°C. Too strong etching. Bubbles and black deposit formed. SPHALERTTE 1094 HNO 20°C. Brown tarnish etch. Some twins but generally poor. ZaS 3 KMn04 + H2SO4 (10 gms/litre Twin boundaries are etched but brown precipitate of manganese hydroxide obscures many features. Electrolytic KMn0 + 1096 H SO ). 20°C 4 2 4 action speeds reaction but also increases precipitate. (± electrolytic 5v 50mA). 1 - 5 minutes. (ref. Ramdohr 1969). KMn0 + KOH. (10 gms/litre each of Etches twin and grain boundaries. Brown tarnish (can be 4 removed by conn HCl). Wiping streaks and scratches give KMn0 and KOH). 20°C. 4 patchy etch. (ref. Ramdohr 1969). Sodium Hypochlorite solution Very good and rapid etch. Generally clean. Etches twin (12% wt available chlorine) boundaries well. Slip features and grain boundaries not Na HC10 . 20°C 5 - 10 secs. as well etched. 2 2 wash 15% HC1. (ref. Ramdohr 1969). 373 TABLE A 1 SULPHIDE ETCHANTS Mineral Etchant and conditions Comments Hydroiodio Acid, HI. Very good and rapid etch. Etches slip, twin and grain 55% like. or 47% w/w. boundary features. Tarnishes adjacent galena. (rgf. Ramdohr 1969). 20 C. 10 - 20 secs. HC1 + Thiourea (either 1 : 3 or Excellent etch without any tarnish. Etches slip, twin 1 : 5 conc. HC1 to 100 gms/litre of and grain boundary features. Etching times longer than thiourea solution). that for galena. 60 - 70°C. 1 - 10 minutes. PYRRHOTITE Aqua Regia vapour. 20°C Little effect on polished sections. Better etching on Fe S 10 - 60 secs. small fragments where twinning and slip features are 1-x (ref. Atkinson 1972). revealed. 15% HC1 + Electrolytic 20°C. Almost no reaction with polished surface. 5 volts 5o mA. (ref. Ramdohr 1969). Chromic acid (conc). Electrolytic Irrideecent film produced, colour of which depends upon 5 volts 5OnA. 20°C. 3o - 90 secs. grain orientation. Reveals twinning and grain boundaries. Slip features difficult to observe. Hydroiodic acid 5936 whit. 20°C. Good rapid etch of kinks, twine and slip features. 5 - 15 secs. Hexagonal pyrrhotite etches more strongly than monoclinic (ref. Ramdohr 1969, pyrrhotite. (film can be removed by gentle rubbing with Vaughan et al 1971). alcohol). PYRITE 106 HNO 70°C. 2 - 4 minutes. Slow etch. Reveals some growth features - often leaves FeS 3 brown stain. Useful for examining framboidal pyrite. 2 15% HC1 and Electrolytic Some growth features are revealed on large grains. 5 volts 5o mA 20C. Results are generally poor. (ref. Ramdohr 1969). Kft0 + . (10 gms/litre Slow, irregular etching found. Not uniform. 4 H2SO4 KMn0 + 106 ). 20°C. 4 H2SO4 (ref. Ramdohr 1969). Chromic acid (conc.) + Electrolytic Variable etch. Generally etches growth features well. 5 volts 5OmA 20°C. 1 - 8 mine. Some dislocation etch pits found in areas of heavy etching. (ref. Atkinson 1972). CHALCOPYRITE NH OH + H 0 (1 : 1 or 1 : 5 Produces highly coloured irridescent surface film - colour 2 2 CuFeS 4 depends on grain orientation. Twinning well revealed. 2 mixture of 3016 H202 + 4896 NH cm). 4 Slip features only poorly etched. 20°C 30 secs - 2 mins. (ref. Kelly and Clark 1975, Roscoe 1975). KC10 + 29}6 HNO 20°C 2 - 5 mine. Poor results. Irregular tarnish. Etches grain boundaries 3 3 and twins. (ref. Ramdohr 1969). 374 TABLE A 1 SULPHIDE EDCHAMS Mineral Etchant and conditions am131412". KM0 + KOH (10 gm/litre of each in Generally streaky etch with tarnish - etches twins. 4 1 : 1 ratio). 20°C 2 - 10 minutes. Chromic acid (conc). Electrolytio Excellent structure etching of grain boundaries, twins 20 C. 5 volts 50mA. 10 - 60 secs. and slip lines. Some topography induced by heavy etching. (ref. Atkinson 1972). 375 APPENDIX B PUBLISHED WORK Copies of the following papers are included in this Appendix. McClay, K.R. 1977a. Dislocation etch pits in galena. Tectonophysics, 40, T1-8. McClay, K.R. 1977b. Cleavage in galena. in Atlas of Rock Cleavage (ed. Bayly et al.), University of Tasmania. Eu 22. McClay, K.R. 1977c. Pressure solution and coble creep in rocks and minerals : a review. Journal Geol. Soc. Lond. 134, 57-70. McClay, K.R. and Atkinson, B.K. 1977. Experimentally induced kinking and annealing of single crystals of galena. Tectonophysics, 39, 175-189. McClay, K.R. and Carlile, D. 1978. Mid-Proterozoic sulphate evaporites at the Mount Isa Mine, Queensland, Australia. Nature, 274, 240 - 241. These publications are the result of work carried out for this thesis. 376 REFERENCES ADAMS, F.D. 1910. An Experimental Investigation into the Flow of Rocks. Int. Geol. Cong. XI, C.R. 911 - 945. AMBRASEYS, N.N. 1970. Some characteristic features of the Anatolian Fault Zone. 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Handbook of stratabound and stratiform ore deposits. Elsevier, Amsterdam. vs 1 - 7. WOODCOCK, N.H. 1976. Structural style in slump sheets: Ludlow Series, Powys, Wales. J. Geol. Soc. Lond. 132, 399 - 416. YOUNG, C. 1969. Dislocations in the deformation of olivine. Am. J. Sci., 267, 841 - 852. Nangrr Vol. 274 20 July 1978 241 or greater and they had square cross-sections. This habit is Carbonaceous material is also found in abundance associated characteristic of diagenetic anhydrite. These pseudomorphs with framboidal pyrite" in the silica-dolomite and in the now consisted of a mosaic of quartz grains of a size (50- Urquhart Shales. The nature of the pseudomorphs, the relict 100 ism) which is consistently larger than that of the enclosing anhydrite, the textures and the petrological characteristics of chert (Fig. le). Such textures indicated that the pscudomorphs the sediments at Mount Isa" are similar to those found in have undergone a void phase similar to the pseudomorphs after sabkha-intertidal environments'""". early diagenetic anhydrite from the Pine Point Pb/Zn In our studies, evaporitic features were found over a thick- deposit". ness of 80 m and down dip for 500 m (from levels 9 to 16 in the The second type of pseudomorphs were large (0.1-3 cm) mine). Determination of the full extent of the evaporites was single crystal dolomite replacements after gypsum and were hampered by limited sampling and because in some places the found in the fine-grained (5-25 Em) dolomitic siltstones and in original sedimentary and diagenetic textures were obscured by the coarse-grained carbonates. These pseudomorphed crystals Later deformation and metamorphism. had either angular or diskoidal terminations. The diskoidal The recognition of a significant volume of sulphate types had a similar habit to gypsum which had developed evaporites at Mount Isa is an important addition to the growing interstitially in recent and ancient sabkha environments"' number of occurrences of such deposits which are at least The angular pseudomorphs after gypsum are six sided, with 1,500 Myr old (ref. 10) or older (refs 21-23). This widespread interfacial angles similar to those of gypsum elongate parallel occurrence of early and mid Proterozoic sulphate evaporites is to the c axis". Many pseudomorphs, however, are commonly a clear contradiction of Cloud's model" of atmosphere- fractured and brecciated. In some of the dolomite hydrosphere evolution in which such deposits are held to be pseudomorphs after gypsum, minute relics (10-20µm) of insignificant before 1,000 Myr BP. anhydrite were found with the petrological microscope, thus Furthermore, the Mount Isa evaporites have significant indicating a complex replacement history of gypsum-anhy- metallogenic implications. Various models for the formation of drite-dolomite. This identification was confirmed by X-ray stratiform lead-zinc and copper ores involving sulphur and diffraction of carbonate fractions and by scanning electron metal rich evaporitic brines have been proposed`'''". Our Mid-proterozoic sulphate evaporites at microscope microanalysis. The preservation of anhydrite relics, interpretation of the deposition of the silica-dolomite in a Mount Isa mine, Queensland, Australia of carbonaceous material", the textures and the reduced marginal sabkha environment and the abundance of carbo- nature of the sediments" indicated that the diskoidal gypsum naceous material in the Mount Isa Wee"' is consistent with a was of evaporitic origin rather than formed by the degradation single stage sabkha brine model' involving bacterial reduction PSEUDOMORPHED sulphate evaporites have been discovered of sulphides". of sulphate for the fixation of the metals. This hypothesis in the Middle Proterozoic (1,600-1,500 Myr DP') silica- contrasts strongly with the view that the Mount Isa deposit is of dolomite sequence" at the Mount Isa Mine, Queensland. volcanogenic origin"'". Although • volcanogenic model Australia (20'44' S, 139'29' E). The silica-dolomites are host cannot be unequivocally refuted, the recognition of replaced to the topper mineralisation at the Mount Isa mine" and evaporites at both McArthur River" and at Mount Ise raises interdigitate with the fine-grained silver-lead-zinc-bearing the possibility that these deposits may have been formed from pyritic and dolomitic Urquhart Shales of the Mount Ira metal rich brines of the kind forming today in evaporitic Group'. A close association of evaporites with copper" and basins'. lead-zinc' has been reported for many ore deposits. The dis- We thank Drs W. Diver, M. Muir, D. Shearman and H. covery of replaced sulphate evaporites at Mount Isa reported Clemmey for stimulating discussion and criticism and Mount here raises the possibility that this large ore deposit may have ha Mines for access to material and for support for D.G.C. been formed from metal-rich brutes of the kind forming today in evaporitic basins' rather than from volcanically derived K. R. MCCLAY hydrothermal solutions". The silica-dolomites comprise a series (of variable thick- D. G. CARLILE ness—up to 530 m) of brecciated and recrystallised siliceous dolomites". Early diagenetic gypsum and anhydrite have been Department of Geology, replaced by quartz, dolomite, calcite and pyrrhotite. Rare Imperial College, minute anhydrite relics have been identified in some dolomite London SW7, UK pseudomorphs after gypsum. Similarly, pseudomorphed sulphate evaporites have recently been reported from the N cross- 44.4..44 24.4.9... eemr.0 IS Aral HOS. McArthur Group" (1,600-1.500 Myr BP) and are also found Ing, I Photographs of samples from the 3,000 section. Mt Isa Mine. A Hand specimen of laminated chert in the Paradise Creek Formation' (1.600-1.500 Myr 0P) (M. I. DIANA A. Y. Devne1, G. 91.. Mime. 1 11 II He. • M. 1. Ar Oct OwerA1A (dark layers) and coarse-grained carbonate layers (light) (14 III-1290971H Muir. personal communication). 14 ow ADM Werld be.. Ma Aiwa lAwd Dee Ids Reed. D. 0 • level, bar scale -2 cm). b. Laminated chert with large lath 1. &..M. E The sequence at Mount Isa has undergone several defor- M.A.., • Ell. 117-1701197.n. pseudomorphs after anhydrite (plane polarised light, 9 level, J. Illadwn. • 0,.t. G / Croalmr 4A...1w wed lewems Now G... mations"' in low (chlorite grade) greenschist facies metamor- bar scale =5 mm). c, Details of pseudomorphs after anhy- C L 1)11-)7I IAlwerelso. Jew ter Meta Melbowee. 11) 11 phic conditions. The deformations were not penetrative and a drite showing quartz mosaic replacements and laths cutting I. Mew... • • Geom.& A A. Gomm: et Gin Aef &mamma Atgewel kwewee Yarn lAseSea.1977). across the bedding (muted polars with tint plate (12 level, slaty cleavage is well developed only in the more argillaceous 3. Itake, A. R Ens Gee/ 441.33-411(1974 beds. Bedding and primary textures were recognisable in bar scale -400 um). d. Pyrrhotite gain with diskoidal Dwomere. H E. • Thearswee, 0 J. oa F. on Oil ant Ow Sodimoom. 119-201 almost all our samples including the coarse grained massive termination in mauls of fine grained dolomitic siltatone Ileowerna1Ca4m Tondo, 19711 (plane polarised light, 16 level, bar scale ...SOO pm). 7. Cu.,....,. A • . T,-A. u I a Pe-ken. E. C. few. Gm/ N. 1 191 097 41, dolomites'. Our samples of the silica-dolomites (collected S. Tower. It 5- De.. lea MA MA./ 71•1-1.41141.23 mainly from the 3,000N cross section") are fine-grained (5- • teweben. t. • • TWA K IM WmOrrm EOM' 112? 1131)1 111. Walter. It N. 14 D. Dm, w. L. 9/01mme. N • VIA., N Nee. kid. 25 um) laminated dolomitic siltstones, laminated cherts inter- 3111-11911,771 bedded with coarse-grained (200 p.m-2 cm) carbonate layers II r.3ta w no, an OA 1. M.* (1,74 Other textural features of the silica-dolomite suggested (Fig. lea and massive coarse-grained carbonates. The carbo- 17 Cadee. D G Impanel Cane, User ',Nal11.1771. replacement of sulphate evaporites. In many dolomitic silt- 13 Seek, f V/ • lacilaw. S A I. M.. Mrml 71. 11 271-213119641 nate minerals are dolomite. ferroan dolomite, calcite, with 14 Sbes•now. (3 1 Dees Ma Wo. Adrmi 73. • JAI 091.61 stones, acicular and diskoidal pyrrhotite grains (Fig. Id) seem minor siderite and ankerite. Two types of pscudomorphed 13 P.O. • K TedhAneteletv 14. 4111-504 (1914 to have completely replaced anhydrite and gypsum. Silicified 16 Mu, W E Dem. G • • Awdenow. • Y Ge.e4ww , 3. J47-772 (197n sulphate evaporites have been found in the laminated cherts 7 rosette structures (1-4 mm) and nodules occurred in the cherts. 17 low,L G • Zuorrarnm. 0 0 Eros 0...1 4 1-10411991/ and interbedded coarse-grained carbonates. IS Yu dew Heeval. H a ihesw.. U... (Nwevenied .1.91 Some coarse-grained carbonate units had fanning and rosette 19 Kwwwwww. 0 I I • P.Y. R K led Visher. 14 R I 431-4111E1..”, The first type of pseudomorph was most easily recognised in textures, diskoidal terminations, 90' reentrant angles and her- Arommdmr. 1974 the laminated chert layers (Fig. lb) where they cut across and 20 0.11. D 1 te4 P. 43. 919-1017 I19771 ring bone textures of included material. Chert layers contain distort the bedding (Fig. I b.c) indicating that the original II SW... I r N • Sum.. h. C W Now Da 319411119M. finely laminated carbonaceous material identical to that found 21 11.1•41.4.5.1.e. 1 C. S.A.+ Ter pel 3.94-102119741 sulphate minerals crystallised in the host sediment before Gael 711w:w.1441 (NM in many silicified stromatolites (W. Diver. personal com- 23 Hunch. r K a ammo. 7 0 Er.. compaction—during diagenesis. The lath-shaped pseudo- 24 Clow,. P reAweedeep 1. 131-117 (1 yAsi munication) and as such were interpreted as cryptatgal". 23 N I w emu.. Ow New. E wowed 014.41 morphs were up to 4 cm long, with length/width ratios of 10: I Morel.11an Pyres,. Ltd 19,4 T1 Tectonophysics, 40 (1977) Tl—T8 © Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands Letter Section Dislocation etch pits in galena K.R. McCLAY Department of Geology, Imperial College, London SW7 2BP (Great Britain) (Received January 31, 1977) ABSTRACT McClay, K.R., 1977. Dislocation etch pits in galena. Tectonophysics, 40: T1—T8. The results of dislocation etch pit studies of naturally and experimentally deformed galena single and polycrystals are presented. The etch pitting is simple, inexpensive, rapid and reveals dislocation substructures similar to those found in metals and ceramics. Both {110 } <110> and {100 } <110> type slip systems have been found in naturally and ex- perimentally deformed galena. INTRODUCTION Dislocation etch pitting is a powerful technique for rapid, non destructive examination of the microstructures of a wide variety of materials. It has been extensively used by material scientists to study dislocations in metals, ionic and ceramic materials (see reviews by Johnston, 1962; Amelinckx, 1964; and Robinson, 1968). Honess (1927) describes etching methods for many crystals but, to date, only limited dislocation etch studies have been carried out on geological materials. For example Heard (1972), Poirier (1972), Moran (1958), and Amelinckx (1954) have studied dislocation etch pits in sodium chloride. Keith and Gilman (1960) studied etch pits in calcite, while Young (1969) and Wegner and Christie (1974) have etched dislocations in olivine. Ball and White (1977) report on the most recent study of etch pit- ting in quartz. Dislocation etch pitting is limited to an examination of the surface expres- sions of dislocations but it has distinct advantages in that it is quick, inex- pensive, non destructive and that large areas of specimens can be examined. It is particularly useful in determining dislocation densities; in determining the orientation of surfaces and deformation features; in the study of dis- location dynamics and in the study of microstructural changes in sequential experimental tests. The main disadvantages are that not all dislocations etch T2 out and that only surface features are examined. The method is particularly suited to the study of opaque minerals such as sulphides where other meth- ods, for example dislocation decoration or transmission electron microscopy, are difficult. This paper presents some of the results obtained in an extensive study of naturally and experimentally deformed galena single and polycrystals. In geological studies, Salmon et al. (1974) and Atkinson (1976) used etching mainly to reveal the arrangement of pits in subgrains, while this study is more concerned with the detailed morphology of the dislocation etch pits. Evidence of {110 } <110> slip has been found in experimentally and natu- rally deformed galena. THE ETCHING TECHNIQUE Dislocations in crystals produce elastic stress fields and may also possess an atmosphere of impurity ions (Ball and White, 1977). These produce local differences in the elastic or chemical energy which thus promotes differential rates of dissolution between the points of emergence of dislocation lines at the surface of the crystal and the surrounding lattice. Johnston (1962), Ame- linckx (1964), and Robinson (1968) have discussed the mechanisms of etch pit formation at dislocations in detail. Brebick and Scanlon (1957) and Franklin and Wagner (1963) in the most comprehensive studies to date, describe chemical etching of artificially grown galena. Electrolytic etching and the etchants described by Honess (1972) did not produce consistent results when tested by the author. The Brebrick and Scanlon etchant used in this study was a solution of one part concentrated HCl to three parts thiourea solution (100 grams/litre). Etching was carried out at 65-70° C for times of 45-120 seconds. The temperature and timing of the etching are critically important if consistent results are to be obtained. Both cleavage fragments and polished sections were etched. The best results are obtained from surfaces oriented near or on (001) planes or on (001) cleavage faces. The etched samples were washed in distilled water (at 60°C) and allowed to cool slowly in order to minimize thermal stresses. Samples were then examined either in a reflected light microscope or in a Cambridge 600 or IIA scanning electron microscope. High-resolution photography in the scanning electron microscope proved difficult (probably due to absorption effects) both on coated (carbon and gold) and uncoated samples. RESULTS AND DISCUSSION The dislocation microstructures in naturally and experimentally deformed galenas which have been subjected to the etching technique are summarized in Figs. 1-3. In general, the features are similar to those found in metals (Amelinckx, 1964) and ceramics (Johnston, 1962). T3 Individual etch pits The individual etch pits are generally pyramidal in shape and vary in size from 2.5 pm to 0.5 pm (Fig. la). Ocassionally, flat-bottomed etch pits are Fig. 1. Scanning electron micrographs of individual etch pits. a. Large pyramidal etch pits in a cleavage fragment of sheared galena from Braubach, W. Germany. Etched 90sec, 65° C. Small boulangerite inclusions in etch pits from the same sample. Etched 90 sec, 65° C. T4 Fig. 2. Details of slip features, a. {100 }<110> type slip traces. Scanning electron micro- graph of cleavage surface, sheared galena from Yerranderie, NSW. Etched 60 sec, 70° C. b. Scanning micrograph of {110 } <110> slip traces on a cleavage face of a single crystal of galena experimentally deformed at 20° C, at a strain rate of 10-6 sec' and at a con- fining pressure of 1.5 kbars. Etched 80 sec, 65° C. (Sample from experiments of McClay and Atkinson, 1977.) T5 found. These indicate that some of the dislocations have moved during etching, probably in response to thermal stresses generated by temperature changes. Although etch pits of different depths are often observed, it has not been possible to distinguish between positive and negative dislocations (Livingston, 1962). Neither has it been possible, at this stage, to distinguish between edge and screw dislocations. Very small inclusions (0.5-2.0 pm) and precipitates of tetrahedrite and boulangerite were found in many sam- ples of naturally deformed galena (Fig. lb). These inclusions may be primary or may have precipitated along dislocation lines (Johnston, 1962). These particles will have important effects on dislocation movement (Orowan, 1948; Burton, 1975) and also on diffusional creep (Harris, 1973). Slip lines Slip lines and clusters of slip lines are clearly revealed by etching (Fig. 2). Both {100 } <110> (Fig. 2a) and {110 } <110> (Fig. 2b) type slip systems were observed in naturally and experimentally deformed galena (McClay and Atkinson, 1977). In Fig. 2a, cleavage fractures etch out as deep channels in contrast to the individual pits along slip-line traces. Concentrations of dislo- cations along (110) slip lines produce the etch traces (Fig. 2b) in a sample of experimentally deformed galena which has a large number of dislocations and which has been heavily etched. Sub-grain boundaries and kink-band boundaries The etching techniques clearly highlight sub-grain boundaries and kink- band boundaries (Fig. 3). Low-angle tilt boundaries etch out into grooves which show a strong concentration of dislocations in these boundaries (Fig. 3a). Figure 3b shows a (110) type kink band revealed by bent cleavage steps. In galena, kink bands often form in an (110) orientation when the easy glide system {100 } <110> is not favoured (McClay and Atkinson, 1977). Small-angle kinks tend to form planar boundaries which etch out as single grooves. As the kink angle increases, small elongate (4-30 pm) sub- grains develop in the kink-band boundary (Fig. 3b). It was found that cleavage fragments produced the most consistent results, but very gentle treatment is needed during preparation in order to avoid in- troducing too many new dislocations. It has not been possible to establish an exact one-to-one correspondence between the etch pits and dislocations, but the obvious relationships between etch pits and deformation microstructures (slip lines, kinks, sub-grain boundaries) indicate that many of the etch pits correspond to dislocations. Other lattice defects, such as vacancies, non stoichiometric defects, impurity ions and inclusions also etch out, but these pits are usually irregular in shape and in arrangement within the crystal lat- tice. Deformation twinning of (441) type (Lyall, 1966) has only occasionally been observed in the etch studies. Many of the deformation features dis- T6 Fig. 3. Details of sub-grain and kink-band boundaries. a. Scanning micrograph of a sub- grain boundary in a cleavage fragment of sheared galena from Ruth Hope mine, British Columbia. Etched 50 sec, 70° C. b. Kink band (revealed by bent cleavage steps) with sub- grains developed in the boundary. Cleavage fragment of galena from Yerranderie, NSW. Etched 45 sec, 65° C. T7 cussed above, are similar to those found in sodium chloride (Carter and Heard, 1970; Heard, 1972). CONCLUSIONS Careful etching of galena single and polycrystals produces dislocation etch pits which are often arranged in slip lines, sub-grain boundaries and in kink bands. The disposition of the etch pits can give valuable microstructural in- formation particularly about deformation mechanisms in minerals. Both {100} <110> and {110} <110> slip systems have been found in naturally and experimentally deformed galena. The dislocation sub-structures which have been observed in the etch pit studies on galena are similar to those found in deformed metals and sodium chloride. It is emphasized that dis- location etch pitting is a simple, inexpensive and rapid technique that can provide valuable microstructural information on deformed minerals which can only otherwise be studied in the transmission electron microscope. ACKNOWLEDGEMENT During the course of this research, the author was supported by a George Murray Fellowship from the University of Adelaide. REFERENCES Amelinckx, S., 1954. Etch pits and dislocations along grain boundaries. Slip lines and polygonization walls. Acta Metall., 2: 848-853. Amelinckx, S., 1964. The Direct Observation of Dislocations. Academic Press, New York. Atkinson, B.K., 1976. The temperature and strain rate dependent mechanical behaviour of a polycrystalline galena ore. Econ. Geol., 71: 513-525. Ball, A. and White, S., 1977. An etching technique for revealing dislocation structures in deformed quartz grains. Tectonophysics, 37: T9—T14. Brebrick, R.F. and Scanlon, W.W., 1957. Chemical etches and etch pit patterns on PbS crystals. J. Chem. Phys., 27: 607-608. Burton, B., 1975. The interaction of dislocations and inclusions during creep. Philos. Mag., 31: 1289-1294. Carter, N.L. and Heard, H.C., 1970. Temperature and rate dependent deformation of halite. Am. J. Sci., 269: 193-249. Franklin, W.M. and Wagner, J.B., 1963. Dislocations and glide in lead sulphide as a function of deviations from stoichiometry and doping additions. J. Appl. Phys., 34: 3121-3126. Harris, J.E., 1973. The inhibition of diffusion creep by precipitates. Met. Sci. J., 7: 1-6. Heard, H.C., 1972. Steady-state flow in polycrystalline halite at pressure of 2 kilobars. p. 191-210 in H.C. Heard, I.Y. Borg, N.L. Carter, and C.B. Rayleigh (editors), Flow and Fracture of Rocks. Am. Geophys. Union, Geophys. Monogr., 16, 352pp. Honess, A.P., 1927. The nature, Origin and Interpretation of Etch Figures on Crystals. Wiley, New York. Johnston, W.G., 1962. Dislocation etch pits in non-metallic crystals. In: J.E. Burke (editor), Progress in Ceramic Science. Pergamon, Oxford, p. 3-75. Keith, R.E. and Gilman, J.J., 1960. Dislocation etch pits and plastic deformation in cal- cite. Acta Metall., 8: 1-10. T8 Livingston, J.D., 1962. The density and distribution of dislocations in deformed copper crystals. Acta Metall., 10: 229-239. Lyall, K.D., 1966. The origin of mechanical twinning in galena. Am. Mineral., 51: 243- 247. McClay, K.R. and Atkinson, B.K., 1977. Experimentally induced kinking and annealing of single crystals of galena. Tectonophysics (in preparation). Moran, P.R., 1958. Dislocation etch techniques for some alkali halide crystals. J. Appl. Phys., 29: 1768-1769. Orowan, E., 1948. Classification and nomenclature of internal stresses. In: Symposium on Internal Stress in Metals and Alloys, London. The Inst. Metals, p. 47-59, Discussion, p. 398-431. Poirier, J.P., 1972. High-temperature creep of single crystalline sodium chloride. Philos. Mag., 26: 701-725. Robinson, W.H., 1968. Dislocation etch pit techniques. In: R.F. Bunshah (editor), Tech- niques of Metals Research, 11, Wiley, New York, p. 291-340. Salmon, B.C., Clark, B.R. and Kelly, W.C., 1974. Sulphide deformation studies: II. Experimental deformation of galena to 2,000 bars and 400° C. Econ. Geol., 69: 1. Wegner, M. and Christie, J.M., 1974. Preferred chemical etching of terrestrial and lunar olivines. Contrib. Mineral Petrol., 43: 195-212. Young, C., 1969. Dislocations in the deformation of olivine. Am. J. Sci., 267: 841-852. Eu 22 CLEAVAGE IN GALENA. Many lead sulphide ores have been tectonically deformed at low temperatures and they now exhibit a cleavage or a schistose (Bleischiefer) texture. This type of galena ore is characterised by a 'steely' appearance, and is usually fine grained with a strong foliation or cleavage. A stretching lineation parallel to the direction of shear may also be present. 'Steel' galena, as it is often called, is commonly found in sheared vein deposits but is also found in deformed orebodies such as Mount Isa, Australia, and Sullivan, British Columbia, where it is often axial planar to the folds. Stanton and Willey (1972) have attributed the formation of this schistose texture to static annealing and exaggerated grain growth at temperatures of 400°C in deformed polycrystalline galena. McClay and Atkinson (1977), however, have shown that dynamic recrystallisation at temperatures about 200°C produces schistose galena in kinked single crystals. Extensive examination of naturally deformed galena has shown that the schistose texture often forms from kinked coarse grained galena by dynamic recrystallisation. The galena first deforms by dislocation glide and kinking. As deformation proceeds, subgrains develop (often by glide polygonisation) and they misorientate to form small new grains in the kink band. Eventually, a fine grained mylonite galena is produced with augen of old grains in a fine grained matrix. Photograph A shows the elongate texture of sheared galena from Yerranderie, NSW, Australia. This is a sheared vein deposit in which coarse grained galena (1-2cm) has been plastically deformed to give a strongly foliated ore with aggregates of grains of similar orientation strung out to produce a cleavage or shistosity. Photograph B is from the same sample showing the development of elongate grains in the kink bands. Photograph Cis of a sample from the same locality showing that although the foliation is defined by strings of dark calcite grains, the galena has annealed to polygonal grains (foam texture). The calcite inhibits grain growth and pins the grain boundaries of the galena. Photograph D is of mylonitic 'steel' galena from the Halkyn mine, Wales. The galena is stongly foliate and relict kinked augen occur in a fine grained re- crystallised matrix. The photographs are all of polished sections etched for 1-2 minutes at 65°C in the Brebrick and Scanlon(1957) etchant. K.R.McCLAY, DEPARTMENT OF GEOLOGY, IMPERIAL COLLEGE,LONDON SW7. 175 EXPERIMENTALLY INDUCED KINKING AND ANNEALING OF SINGLE CRYSTALS OF GALENA K.R. McCLAY and B.K. ATKINSON Department of Geology, Imperial College, London (Great Britain) ABSTRACT Single crystals of galena have been experimentally compressed at 10° to the (001) plane at temperatures from 20°C to 300°C at a fixed confining pressure of 1.5 Kbars. A fluid medium, Heard-type apparatus was used for testing at strain rates of 10 5 and 10 7 sec-1. Stress-strain curves rose steeply until strains of approximately 2 per-cent were reached. At higher strains the curves often became flat or showed a slight negative slope. Deformed specimens were examined by reflected light and scanning electron microscopy, by Laue X-ray diffraction and were tested for Vickers hardness. Deformation at temperatures of 20°C to 200°C resulted in the form- ation of two well developed sets of kink bands, complex slip on the {110} planes and further slip on {100} planes. Deformation at 300°C was chara- cterised by polygonisation and recrystallisation in kink bands, thus indicating that recovery processes (dislocation climb) were operating. Specimens deformed at a strain rate of 10 5 sec 1 at temperatures of 200°C and 300°C were annealed at these temperatures for 10 days under confining pressure. At 200°C kink band boundary migration had begun and at 300°C polygonisation and recrystallisation away from kink zones were observed. Rapid dynamic recrystallisation and annealing recrystallisation of natu- rally deformed galena can be expected to occur even at temperatures below 10 14 200°C during tectonic deformation at low strain rates (10 to10 -1 sec )• INTRODUCTION Kink bands are commonly observed in coarse grained galena ores from deposits which have been affected by faulting or shearing and which have not been subsequently metamorphosed. Yet, very little is known about the mode of formation of kink 176 bands in galena or of the conditions conducive to their pre- servation throughout geological time. The experiments described in this paper were designed to investigate kink band formation in single crystals of galena. Experiments were conducted at temperatures from 20°C to 300°C, -5 -1 -7 -1 at strain rates of 10 sec and 10 sec , and at a con- stant confining pressure of 1.5 Kilobars. Some specimens -5 -1 deformed at a strain rate of 10 sec were subsequently annealed under confining pressure for 10 days. The micro- structures and textures of the starting material, the deformed and annealed specimens were studied by optical and scanning electron microscopy as well as by Laud X-ray diffraction. The specimens were also tested for Vickers hardness. The results of these experiments illustrate the ease with which single crystals of galena deform plastically and the importance of recovery processes at relatively low homologous temperatures (0.35 Tm., where Tm is the melting point in 0K). Previous deformation studies of single crystals of galena were either restricted to room temperature (Lyall and Paterson, 1966), or were concerned with establishing acti- vation enthalpies for high temperature (600 - 750°C) flow (Seltzer, 1967, 1968). Lyall and Paterson found kink bands developed in only one particular orientation for galena single crystals and occasionally developed in aggregates. They suggested that {100} <110> double slip was involved. Salmon et al. (1974) reported that kink bands commonly formed in experimentally deformed coarse grained galena ores (mean grain diameter of 4.6mm.), whereas Atkinson (1976b) remarked on how scarce they were in fine grained galena ores (mean grain diameter of 0.07mm.) experimentally deformed under simi- lar conditions and speculated that kinking in galena may be favoured in some way by coarse grain size. Annealing studies of naturally deformed galena ores have been made by Stanton (1970) and Stanton and Wiley (1970, 1972), while Siemes (1964, 1976) has studied annealing phenomena in experiment- ally deformed galena ores. 177 MAXIMUM RESOLVED SHEAR STRESS FOR SLIP IN GALENA (100) <011> SYSTEM (110)<110> SYSTEM (1011[101] MMH110] 010 001 (701)[101) 010 mm[aq (011) [011] (on) foil] I os (011) [oil] o (010) [101] 0455 0 02 it (100) [011] (no)[11o] nim [no] 100 100 _initial position of the compressive stress axis (100)[011] etc. are slip combinations with maximum resolved shear stress Fig. 1. Inverse pole figures contoured for maximum resolved shear stress for both the (100) 011> slip system, and the {110)<110)slip system. The position of the initial compressive stress axis (al) is shown. EXPERIMENTAL PROCEDURE Starting Material Specimens used in the deformation experiments were cylin- ders (approximately 9mm in diameter and 16.6mm long), cored at 10° to the (001) plane (Fig. 1), from single crystals and large cleavage blocks of galena from Broken Hill, N.S.W., Australia. This orientation was chosen to inhibit slip on the {001) planes and to favour kink band formation, (Fig. 1). The starting material contained only a few large subgrains and occasional (441) twins, as well as minor scattered small incl- usions of tetrahedrite, chalcopyrite and sphalerite (up to 50),m grain size). Coring of specimens was achieved using a Servomet SM spark erosion instrument. Specimen damage was minimised by using a low spark energy and a low erosion rate. Prior to experi- mental deformation the galena cores yielded sharp Laud X-ray diffraction patterns, typical of undeformed single crystals, 178 and a low etch pit density. Deformation and Annealing Specimens were deformed in a fluid - medium, Heard - type (1963) triaxial apparatus at constant strain rates of approxi- -5 -1 -7 -1 mately 4 x 10 sec and 4 x 10 sec . The heart of this apparatus is an externally heated thick-walled steel pressure vessel. Axial loads on the test specimen are measured using an internal steel force gauge actuating a linear variable differential transformer. Specimens were jacketed in thin- walled (0.025mm) copper tubes and swaged to tapered, solid steel loading pistons to prevent contact between the specimen and the silicone oil confining pressure medium. At elevated temperatures the specimens were encased in aluminium foil to prevent chemical reactions between the galena and the copper jackets. Further details of the apparatus, details of the accuracy of measurement and reduction of data are fully des- cribed elsewhere (Heard, 1963, Rutter, 1972, Atkinson, 1976b). Annealing and deformation experiments were conducted at the same confining pressure of 1.5 Kbars. After removing the axial differential load accumulated during the experimental deformation, selected specimens were allowed to anneal for 10 days. Examination of Deformed Material Specimen assemblies were impregnated with epoxy resin under vacuum, and then the galena was cut using either a microtome saw or a spark erosion wire saw. One portion was polished for optical examination and the other was examined by Laud X-ray diffraction and scanning electron microscopy. Polished sections were etched using the Brebrick and Scanlon etch reagent (Brebrick and Scanlon, 1957) at a temperature of 65°C for times between 30 seconds and 120 seconds, lightly re-polished and then re-etched. Small cleavage fragments were also etched for scanning electron microscope studies in a Cambridge Instruments 600 Stereoscan. Vickers Hardness was 179 20°C, 10-5.. GSX-9 100°C, 10 5 : GSX-10 7 100°C, 10 :. GSX-11 -5 in 200°C, 10 : GSX-1.2,4 . 200°C. 107 GSX-3 l 300°C, 165 : GSX -5,7,8 ia t 300°C. : GSX-6 n re iffe D a GSX-6 I it I t t t I I II 0 2 4 6 8 10 12 14 16 18 20 26 27 Axial Strain I Percent Fig. 2. Stress strain curves for single crystals of galena compressed at 10° to (001). The temperature and strain rate (expressed in sec-1) are given with the appropriate reference number. All tests were conducted at a confining pressure of 1.5kb. measured using a Lietz "mini load" hardness tester with a load of 25 gms. to give indentations from 20-30 pm across. RESULTS Stress-strain curves Stress-strain curves for the experiments described here are presented in Figure 2. They all show a very steep initial portion up to axial strains of approximately 2 per cent. At higher strains the slopes of stress-strain curves decrease. Fig. 3. Microstructures of single crystals of galena deformed at 20-C. All micrographs are of etched polished sections. (a)Large kink bands with irregular boundaries (in air) (b)Deformation bands showing strongly developed {11O} slip (oil immersion) (c)Complex deformation features (1)clusters of slip lines in deformation bands, (2) large kinks with irregular boundaries 'containing both smaller kinks and slip lines, (3)small kinks with curved boundaries which develop from glide polygonisation along slip lines, (in air). (d)Detail of small kinks (oil). 181 At 20° and 100°C an ultimate strength is reached at axial strains of approximately 5 to 8 per cent. At higher temper- atures curves flatten much more readily and an ultimate strength is reached at axial strains from approximately 2 to 4 per cent. Portions of the curves at yet higher strains were either flat or showed small negative slopes. A large drop in ultimate strength occurred on raising the temperature or lowering the strain rate (Fig. 2). Deformation Microtextures The microtextures developed during deformation and anneal- ing experiments were revealed by studies of etched polished sections and etched cleavage fragments. o At 20 C the deformation is characterised by strongly deve- loped kink bands (0.1=-0.4mm) distributed throughout the crystal (Fig. 3a). Two sets of such kink bands develop with {110}-type boundaries. Zones of strongly developed slip lines (deformation bands) are also found (Fig. 3b). Complicated multiple slip features are observed both in the kink bands and the deformation bands. Both {110} [110] and {100) [011]slip were identified from etch pit orientation studies on {100} sections. In some parts of the specimen evidence was found to suggest that glide polygonization (Livingston, 1960) occurs (Fig. 3c). A further set of smaller kinks (10-20pm) (Fig. 3 c & d) which cross-cut the major kink bands in the crystal appear to have developed from regions of reverse lattice cur- vature (Fig. 3c) associated with the glide polygonization. Microtextures of specimens deformed at 100°C were similar to those of specimens deformed at 20°C. Deformation at 200°C gives rise to kink and deformation bands which are strongly developed in two zones, at the top and bottom of the specimen (Fig. 4a). Similar behaviour is shown by single crystals of some metals (Honeycombe, 1968). The other microtextural features found in specimens deformed at 20°C and 100°C are also well developed in these zones and, in addition, wavy slip lines are found. In the less deform- ed portions of the crystals (001) [110] and (010) [101] duplex slip occurs. Small inclusions of tetrahedrite (Fig. Fig. 4. Deformation features at 200°C. Figs. 4a,b,c, are of etched polished sections in plane polarised light. (a)Complex interfingering of kink bands and deformation bands in a zone of intense deformation (in air). (b)Lath of tetrahedrite boudinaged by slip in the host galena crystal (oil immersion). (c)Annealed specimen showing ser- rated kink band boundaries indicating boundary migration (oil immersion). (d)Scanning electron micrograph of a cleavage fragment showing kink bands with {110} type boundaries. 183 4b) are strongly boudinaged due to slip in the host galena crystal. At 100°C and 200°C varying the strain rate from -5 -1 -7 -1 4 x 10 sec to 4 x 10 sec had no discernible influ- ence on the deformation microtexture. The microtextures developed during experiments at 300°C are markedly different to those formed at lower temperatures. -5 -1 In the fast strain rate experiments (4 x 10 sec ) gently curved kink bands form which contain large sub-grains (50 - 200pm across). Deformation bands and other features of slip are poorly developed. Deformation at a slower strain rate of -7 -1 4 x 10 sec results in the formation of extensive polygon- ization in the kink bands (Fig. 5a). The sub-grains formed are often large (100-400pm), elongate and have lobate bound- aries. Grain boundary migration is substantial (Fig. 5a, b) and recrystallization occurs both within the kinked regions and at the kink band boundaries (Fig. 5a, b). Annealing Microtextures The specimen deformed at 200°C and a strain rate of 4 x -5 -1 10 sec and then annealed at this temperature for 10 days shows migration of kink band boundaries giving rise to ser- rated high-angle boundaries (Fig. 4c). This allows a con- vincing argument to be made that dislocations can readily climb at this temperature, (which is considerably lower than that previously reported). Annealing of a specimen for 10 days at 300°C after deformation at this temperature and a strain rate of 4 x 10-5 sec-1 produced less spectacular results. The kinked regions show sub-grains that are more extensively developed than in the un-annealed material and sub-grains are also found outside the kinked regions. Scanning Electron Microscope Studies Etched cleavage fragments of specimens deformed at 20°C, o o 100 C and 200 C show a high etch pit density; although this cannot yet be directly correlated with a high dislocation density as a 1:1 correspondence between etch pits and Fig. 5. Deformation features of specimens deformed at 300°C (Fig. 5a,b). (a)Kink band with recrystallization and polygonisation (in air). (b)Recrystgllization at a kink band boundary (oil immersion). (c)Scanning electron micro- graph of a specimen deformed at 200 C. Etched cleavage fragment showing a dense concentration of etch pits. (d) Laue X-ray diffraction patterns. Segments showing (1) continuous streaking(2) blocky asterism (3) recrystallizat- ion spots. 185 dislocations has not been established for the present etchant on galena (Robinson, 1968). Slip lines of {110) <110> type and large kinks with {110} boundaries (Fig. 4d) are most pro- minent but {100} <110> slip can also be identified. Detailed micrographs of etch pits (Fig. 5c) reveal a number of etch pit types - sharp, flat-bottomed etch pits and etch pits with gro- oves and beaks which probably indicate decoration of disloc- ations (Barber, 1965). Scanning electron microscope studies confirm the slip features observed by optical microscopy. Laue X-ray Diffraction Back reflection Laud photographs were taken from several regions of each specimen. Those deformed at 20°C, 100°C and o 200 C all show regions of continuous asterism (circular stre- aks). Analogous results have been obtained for heavily cold- worked metals (Maddin and Chen 1954). These specimens also have regions in which blocky asterism occurs, however, indi- cating a polygonal microstructure (Fig. 5d). All Laud photo- graphs from specimens deformed at 300°C showed blocky asterism and the specimens subjected to a slow strain rate showed spotty asterism similar to that given by recrystallized metals (Maddin and Chen 1954). Vickers Hardness These results are presented in Table 1 and they can be compared with the results of Siemes (1970, 1976). An increase in Vickers Hardness Number (VHN) is noted after all deform- ation experiments and this is reduced on annealing. DISCUSSION AND CONCLUSIONS As can be seen from Fig. 1, the initial orientation of specimens with respect to the direction of maximum compression favours slip on the {110} TABLE 1 Vickers Hardness Determinations No. of Sample Mean VHN Range Measurements Undeformed 51.5 46.1 - 54.5 70 5 1 Deformed at strain rate 4 x 10 sec 20°C 80.5 78.1 - 85.4 50 100°C 77.3 74.0 - 84.3 40 200°C 79.5 68.6 - 92.6 48 3000C 66.7 58.9 - 74.2 46 -7 1 Deformed at strain rate 4 x 10 sec 300°C 57.2 51.2 - 61.5 50 5 -1 Deformed at strain rate 4 x 10 sec - Annealed 10 days 200°C 62.3 55.5 - 73.9 52 300°C 54.8 45.0 - 66.0 48 Instrument, Leitz Durimet Pol Miniload Hardness Tester with 25 gram weight to give indentations 20pm to 30pm. Paterson, 1966) has indicated that the (1101 slip system has a high critical resolved shear stress at room temperature. This is confirmed by the present observations which also sug- gest that the critical resolved shear stress for slip on (1101 planes is reduced dramatically by an increase in temp- erature or decrease in strain rate. Initial slip on the (110) system causes the crystal lattice to rotate into an orientation which favours kinking on (1001 slip (Fig. 1). This accounts for the form of the post-yield portions of the stress-strain curves. Contrary to the results of Lyall and Paterson (1966), no (441) twins were observed in our experi- ments. Further work is required to determine the critical resolved shear stress for the (110) 00> slip system in the light of the fact that the (1001 system can provide only 3 of the 5 independent slip systems required to satisfy the von Mises-Taylor criterion of ductility (e.g. arbitrary shape changes in polycrystals) (von Mises, 1928; Taylor, 1938). Evidence was found in all specimens for slip on both the (110) <110> and the (100) <011> systems. 187 Recovery processes are first evident after deformation and annealing at a temperature of 200°C, but they are important in all specimens deformed at 300°C. The increasing importance of polygonization and recrystallization observed at 300°C on de- creasing the strain rate indicates that during natural tecto- nic deformation of galena (which probably occurs at strain rates from approximately 10 -10to 10-14 sec-1) recovery pro- cesses will be very important. This conclusion agrees well with the theoretical predictions of Atkinson (1976a). The initial orientation of the starting material with res- pect to the axis of maximum compression (Fig. 1) gives rise to slip on four {110} - type planes and two (100) - type planes (these have a low resolved shear stress). In consequence of the loading constraints on the specimen and the deformation environment (T, é), kinking soon develops. Kinks are formed with boundaries initially in the (110) plane (Fig. 4d); a sit- uation similar to that of kinking in single crystals of some metals (Clarebrough and Hargreaves, 1959, p.65,85). Increas- ing strain, however, produces interference of both kink bands and deformation bands leading to complex boundary geometry. Glide polygonization (Livingston, 1952) can also give rise to kinks by producing zones of reverse lattice curvature which rapidly develop into kink bands (Fig. 3c). We infer from the results of our experiments that the pre- servation of slip and kink features in natural galena indica- tes that the ore has not been subjected to post-tectonic temperatures greater than 200°-300°C. The high strain energy of kink zones enables them to act as preferential sites for polygonization and recrystallization. The polygonized and recrystallized grains tend to be elongate parallel to the kink boundary. Stanton (1970) observed that similarly shaped grains developed during experimental recrystallization of a naturally kinked single crystal at 400°C over 23 days, and inferred that natural "schistose" galena textures may form in this way. In both our dynamic and static experiments recry- stallization was observed at lower temperatures than this and over a shorter time. Stanton & Wiley (1973) have proposed that natural schistose galena may form by static annealing 188 and growth at high temperature (400°C). This study shows that schistose galena may also form by dynamic recrystalli- zation at low temperatures (200°C or lower). Hence we con- clude that both dynamic recrystallization and annealing recrystallization will be important in the development of galena microtextures which either form at very low temper- atures or form very rapidly at high temperatures during tect- onic deformation. ACKNOWLEDGEMENTS The deformation apparatus was built and maintained with funds provided by the Natural Environment Research Council of Great Britain. Dr. G. Lister and the curator of the Mining Geology Museum, Imperial College (Dr. C. Blain) generously provided the single crystals of galena. Mr. R.F. Holloway suggested the spark-erosion coring method, and with his usual consumate skill assisted with coring the specimens and main- tained the deformation apparatus. Dr. F.J. Humphreys pro- vided access to the spark-erosion equipment. K.R. McClay is supported by a George Murray Fellowship of the University of Adelaide. REFERENCES Atkinson, B.K., 1976a. Deformation mechanism maps for polycrystalline galena. Earth Planet. Sci. Letters, 29: 210-218. Atkinson, B.K., 1976b. The temperature and strain rate dependent mech- anical behaviour of a polycrystalline galena ore. Econ. Geol., 71; 513-525. Barber, D.J., 1965. Crystal imperfections in magnesium fluoride. J. Appl. Phys. 36, 3342-3349. Brebrick, R.F. and Scanlon, W.W., 1957. Chemical etches and etch pit patterns on PbS crystals. J. Chem. Phys., 27: 607-608. Clarebrough, L.M. and Hargreaves, M.E., 1959. Work hardening of metals. in Chalmers, B. and King, R. (eds.). Prog. Metal Phys. 8, 1-104. Heard, H.C., 1963. Effects of large changes in strain rate in the experi- mental deformation of Yule Marble. J. Geol., 71: 162-195. Honeycombe, R.W.K., 1968. Plastic Deformation of Metals. Arnold, London, 477p. Livingston, J.D., 1960. Etch pits at Dislocations in Copper. J. Appl. Phys. 31; 1071-1076. Livingston, J.D., 1962. The density and distribution of dislocations in deformed copper crystals. Acta. Metall. 10; 229-239. 189 Lyall, K.D. and Paterson, M.S. Plastic deformation of galena (lead sul- phide). Acta. Met., 14: 371-383. Maddin, R. and Chen, N.K., 1954. Geometrical aspects of the plastic deformation of metal single crystals. in Chalmers, B. and King, R. (eds.). Prog. Metal. Phys. 5; 53-95. von Mises, R., 1928. Mechanik der plastischen fcrmanderung von kristallen. Z. angew. Math. Mech., 8: 161-185. Robinson, W.H., 1968. Dislocation etch pit techniques. in Bunshah, R.F. (ed.). Techniques of Metals Research Vol. 11, Wiley, Interscience; 291-340. Rutter, E.H., 1972. The influence of interstitial water on the rheo- logical behaviour of calcite rocks. Tectonophysics, 14: 13-33. Salmon, B.C., Clark, H.R. and Kelly, W.C., 1974. Sulphide deformation studies: II Experimental deformation of galena to 2,000 bars and 400°C. Econ. Geol., 69: 1-16. Seltzer, M.S., 1967. Activation energies for high-temperature steady- state creep in lead sulphide. Metall. Soc. A.I.M.E. Trans., 239: 650-655. Seltzer, M.S., 1968. The influence of stoichiometric defects and foreign atom additions on steady state creep in lead sulphide single crystals. J. Appl. Phys., 39: 2869-2871. Siemes, H., 1964. Experimente and betrachtungen zum rekristallizations- verhalten von naturlich verformten Bleiglanzen. Neues Jahrbuch Mineral. Aby., 102: 1-30. Siemes, H., 1976. Recovery and recrystallization of experimentally de- formed galena. Econ. Geol. 71; 763-771. Stanton, R.L., 1970. Experimental modification of naturally deformed galena crystals and their grain boundaries. Mineral. Mag., 37: 907-923. Stanton, R.L. and Wiley, H.G., 1970. Natural work hardening in galena and its experimental reduction. Econ. Geol., 65: 182-194. Stanton, R.L. and Wiley, H.G., 1972. Experiments on a specimen of galena ore from Coeur d'Alene, Idaho. Econ. Geol., 67: 776-788. Taylor, G.I., 1938. Plastic strain in metals. J. Inst. Metals. 62: 307-324. Pressure solution and Coble creep in rocks and minerals: a review and Conference report Pressure solution and Coble creep in rocks K. R. McCLAY Reprinted from the JOURNAL OF THE GEOLOGICAL SOCIETY VOL 1342 pp. 57-75 (published October 1977) Pressure solution and Coble creep in rocks and minerals: a review K. R. McCLAY SUMMARY A brief review of research on pressure solution grained quartz and calcite rocks may give rise is given. The mechanisms of pressure solution to geological strain rates at temperatures from and Coble creep are discussed. Deformation by 2oo-35co'C. Coble creep is expected to give rise diffusive mass transfer processes is generally to geological strain rates in fine-grained galena accompanied by grain boundary sliding and at low temperatures and also in fine-grained this will have important effects on the textures calcite rocks at temperatures around 350°C. and microstructures produced during deform- The chemistry of natural rock pressure solution ation. Experiments using relaxation testing systems is expected to have significant effects seem promising in allowing access to the slow and needs further detailed study. Further strain rates necessary to observe pressure research is needed to elucidate the nature and solution phenomena. The thermodynamics of role of grain boundaries during diffusive mass non-hydrostatically stressed solids is discussed transfer. Pressure solution phenomena are and an analysis of possible diffusion paths in important in the compaction behaviour of some rocks is presented. Evaluation of theoretical petroleum reservoir rocks and perhaps in some rate equations for pressure solution and Coble faults where sliding is accommodated by creep indicates that pressure solution in fine- pressure solution. MANY ROCKS DEFORMED IN LOW GRADE METAMORPHIC ENVIRONMENTS, particularly in the range of T = I5o°C-350°C and at confining pressures of 2-8 kb, i.e. up so lower greenschist facies (Fyfe 1974, Turner 1968, p. 366) exhibit textures such as tectonic stylolites, truncated fossils, tectonic overgrowths and striped cleavages. These textures suggest deformation involving diffusive mass transfer processes. Since the pioneering work of Sorby (1863, 1865, 1908) and the early research at the turn of the century (e.g. Van Hise 1904, Becke 1903, Adams & Coker 1910), little advance was made until research interest was re- newed in the fifties and sixties (e.g. Heald 1956, Plessman 1964, Weyl 1959, Ramsay 1967) and by the discussion of the thermodynamics of non-hydrostatically stressed solids (Correns 1949, McDonald 1960, Kamb 1961, McLellan 1966, 1968, 1970 and Paterson 1973). A Tectonic Studies Group meeting held at Imperial College on 5 November 1976 (see the Conference Report following this review) aimed to review current research on pressure solution and geological deformation which is accomplished by diffusive mass transfer. This paper gives a brief review of pressure solution and Coble creep (grain boundary diffusion) as deformation mechanisms in rocks and minerals. It has not been possible to make a bomplete survey of the literature or of current research work for this review, which is intended mainly as an introduction to the subject and to the synopses of the papers read at the Tectonic Studies Group meeting which follow. jl geol. Soc. Land. vol. 134, 5977, pp. 57-7o, 4 figs, I plate. Printed in Great Britain. 58 K. R. McClay i . Pressure solution and Coble creep (A) MECHANISMS Rock deformation mechanisms may be divided into three broad categories— (1) cataclastic processes, (2) intracrystalline processes involving dislocation move- ments, (3) diffusive mass transfer processes. If a grain boundary is subjected to compressive and tensile stresses, then a chemical potential gradient for vacancy flow is set up (see review by Burton 1977, Nabarro 1948 and Herring 195o). The flux of matter, which is equal and opposite to the vacancy flux, leads to depletion of material at relatively compressive boundaries and deposition at relatively tensile boundaries. If the diffusion of matter is essentially through the grains, i.e. lattice diffusion, the process is termed Nabarro–Herring creep, whereas if it is predominantly around the grain bound- aries, the flow is called Coble creep (Coble 1963). Grain neighbour switching in superplastic flow (Ashby & Verrall 1973, Edington et al. 1976) is essentially grain boundary sliding with accommodation by diffusional creep. Lattice diffusion (Nabarro–Herring creep) is important at high temperatures in metals (see review by Burton 1977) and perhaps in the Earth's mantle (Stocker & Ashby 1973). At lower temperatures, however, lattice diffusivities and solid state grain boundary diffusivities are expected to be too slow to account for the ob- served strains found in low grade metamorphic rocks (see section on rates of deformation later in this paper). Many textures in low grade metamorphic rocks (T = 20o-350°C) are interpreted as indicating deformation by diffusive mass transfer (Pl. IA, B. C). It is therefore inferred that the presence of a fluid phase in the grain boundaries enhances diffusive mass transfer in low grade metamorphic rocks—hence the term 'pressure solution'. (B) THE EVIDENCE FOR PRESSURE SOLUTION IN ROCKS Evidence for pressure solution in rocks and minerals is readily found, from a macroscopic scale down to the microscopic scale. Since Sorby (1865) first ascribed the pitting of pebbles (Pl. 1B) to pressure solution, many authors have discussed the textures and structures attributed to this form of diffusive mass transfer (e.g. Voll 1960, Ramsay 1967, Plessman 1964, 1972, Spry 1969, Groshong 1975a and b, Trurnit 1968, Ayrton & Ramsay 1974, Kerrich 1975, Logan & Semeniuk 1976, Mitra 1976). Limestones display a wide variety of pressure solution phenomena which occur both during diagenesis and deformation. Stylolites are extensively developed in many limestones (Arthaud & Mattauer 1969, 1972) with an accumulation of quartz and phyllosilicates in the stylolite and clay seams which often truncate fossils (P1. IA). Logan & Semeniuk (1976) have described in detail pressure solution and stylolite development in the limestones of the Canning Basin, Western Australia. The formation of some rock cleavages is often thought to involve pressure solution (Borradaile 1977, Cosgrove 1976, Alvarez et al. 1976, Williams 1972, Groshong 1975a, Siddans 1972, Durney 1972b). Detailed high voltage electron microscopic studies by Knipe & White (1977), however, have shown the com- Pressure solution and Coble creep in rocks 59 plexities of cleavage development and in particular they emphasize the effects of crystallization involving the formation of new phyllosilicates. Striped (or spaced) cleavage in limestones (Alvarez et al. 1976) and in the greywacke rocks of South Devon (P1. IC) has been attributed to pressure solution processes (Ramsay 1967). Beach (1974) and Kerrich (1975) have studied the chemical changes resulting from pressure solution of the Devonian greywackes of SW England and found that at least some of the vein minerals are supplied from the surrounding rock. Differentiation of minerals into low pressure zones during folding (Stephansson 1974) has often been attributed to pressure solution, particularly during the formation of crenulation cleavages (Durney 1972a, Williams 1972, Cosgrove 1976). Tectonic overgrowths (Pl. LA) which often have a fibrous appearance (Durney & Ramsay 1973, Mitra 1976) and pressure shadows (Stromgard 1973) are also considered as evidence of pressure solution processes in low grade metamorphic rocks. Mitra (1976) has presented a very elegant analysis of deformation mechan- isms in quartzites in which he has been able to separate strain components due to dislocation creep and to pressure solution. A very detailed and historical review of research on pressure solution and of pressure solution structures is given by Kerrich (1977) . (C) THE EXPERIMENTAL EVIDENCE Grain boundary diffusional creep was first postulated by Coble (1963) for creep in A1203. Coble creep was later found to operate in pure magnesium (Jones 1965) and in other metals and ceramics such as copper (Burton & Greenwood 1970) and cadmium (Crossland 1974), (for details see Burton 1977). Grain bound- ary creep was found to have a lower activation energy than that for lattice dif- fusion and a strain rate dependence on the reciprocal of the grain size cubed (see later section on rate equations). It has been found to be important in metals at low homologous temperatures o-6 T melting, Burton 1977) whereas Nabarro- Herring creep occurs only at high temperatures 0.9 T melting). FIG. I. A C ExtENSKW A. Undeformed hexagonal array of grains with marker line. The compressive stress axis is horizontal. B. If diffusional creep occurs without grain boundary sliding (i.e. the centres of the hexagonal grains and the marker line are not displaced) then there would be a volume increase and voids (black areas) would form against the compressive stress. This is an unlikely mechanism and the diffusional creep is accompanied by grain boundary sliding—s C. C. Diffusional creep with grain boundary sliding resulting in offset of the marker line and no voids forming (constant volume deformation). 6o K. R. McClay Diffusional creep in polycrystals whether Nabarro—Herring or Coble creep, is generally accompanied by grain boundary sliding (Lifshitz 1963, Gifkins 1967, 1976, Raj & Ashby 1971 and Ashby & Verrall 1973) if voids are not to form (i.e. constant volume deformation) during creep (Fig. 1). Diffusional creep may be inhibited when second phase particles are present in the material (Harris 1973). During diffusional deformation the relatively immobile second phase particles accumulate in compressive stress grain boundaries and become denuded in tensile grain boundaries (Harris 1973). Plate ID shows the microstructure of a magnesium- zirconium alloy which has undergone diffusional creep. Denuded zones free of precipitates can be seen. Grain boundary sliding has occurred resulting in displace- ment and rotation of the grains. In rocks and minerals, experimental evidence for creep by diffusive mass transfer and grain boundary sliding has been found for Solenhofen limestone (Schmid 1976a, b) and in fine grained galena (Atkinson, unpublished work). Good textural evidence for pressure solution has been found in experiments by De Boer (1975), De Boer et al. (1977), and Sprunt & NUT (1976). Rutter & Main- price (pers. corn. 1977) found that the presence of a pore fluid strongly affected the relaxation behaviour of Tennessee sandstone (Fig. 2). They inferred from the observed rheological behaviour that deformation was accomplished by grain boundary sliding accommodated by pressure solution. In salt-mica analogue experiments Means & Williams (1974) have observed mineral segregation in microfolds possibly resulting from pressure solution. Experimental investigations to determine the rheological flow laws of pressure solution have met only with limited success. Plastic deformation by dislocation movement and cracking are difficult to suppress and the inferred low heat of activation (Rutter 1976, Rutter & Mainprice pers. corn. 1977) prevents significant speeding up of the pressure solution process by substituting higher temperatures for slower strain rates. Relaxation experiments, however, may provide access to the low strain rate necessary to observe pressure solution phenomena. 39-8 -7 FIG. 2. wcc s.-%*•• 8-4V.N•q:— co TS30 DRY Relaxation data expressed as log stress (n 31-65. u) wce versus log strain rate (Rutter & Main- price, pars. corn. 1977) for Tennessee 1,7, %17% • TS 33 WET ". sandstone. Both tests were conducted 0 411 at effective confining pressures of 3.5- I" 3 vit kb and Ts 33 had a pore fluid pressure a IV* • zoo bars. The dry test Ts 3o shows no 34-45 T= 300T : ▪ • significant drop in strength over 8 • ▪I • orders of magnitude in strain rate 331 2 3 4 5 6 7 8 9 10 while the wet test Ts 33 shows signi- ficant drop in strength at strain rates -L00 STRAIN RATE 10 less than to-6 sec-1. y. geol. Soc. bond. 134, 1977 MCCLAY PLATE I Truncated Globigerina in calcareous slate. The dark lines on each side of the fosiil (S) are clay seams with accumulations of phyllosilicate minerals. Fibrous over- growths of calcite (C) have formed in a pressure shadow. (Photograph D. Durney (1972a), Imperial College, Department of Geology slide collection). B. Pitted pebbles in the Molassic conglomerates of Carboniferous age, Mieres. Interpenetration of pebbles can clearly be seen. (Photograph J. G. Ramsay). C. Striped cleavage (attributed to pressure solution processes, by Ramsay (1967)) in Devonian greywackes, Torcross, Devon. D. Microstructure of polycrystalline magnesium containing very fine particles of zirconium hydride aligned in stringers which were initially parallel. The speci- men has been crept at 400°C with the tensile axis horizontal in the photograph, at a strain rate of 1•2 x to-4/hour and with a final strain of 42 %. The grains have undergone grain boundary sliding and relative rotation. Denuded zones free of hydride particles can be clearly seen. Photograph Dr R. B. .Jones, CEGB Berkeley Nuclear Laboratories. 6o Pressure solution and Coble creep in rocks 61 2. Thermodyilamics A considerable amount of literature has been devoted to the thermodynamics of non-hydrostatically stressed solids (e.g. Gibbs 1906, Correns 1949, McDonald 196o, Kamb 1961, McLellan 1966, 1968, 197o, Durney 1972b, 1976, Paterson 1973 and De Boer 1975, 1977). The basic principles stem from the work of Thompson (1862), Poynting (1881), Le Chatalier (1892), Riecke (1894) and Gibbs (1906). Thompson (1862) reported that an increase in hydrostatic pressure increased the solubility of a crystal in its saturated solution. Poynting (1881), however, stated that the increase in solubility due to pressure applied simul- taneously to both the solid and the liquid phases is small compared with the solubility change resulting from pressure on the solid alone. Le Chatalier (1892) showed mathematically that a deviatoric stress has a greater effect on solubility than hydrostatic stress. Riecke (1894) stated that if two prisms of the same solid are originally in equilibrium with a common saturated solution, then the application of stress to one of the prisms causes it to go into solution while the unstressed crystal grows at its expense. Gibbs (1906) presented a thermodynamic analysis of non-hydrostatically stressed solids. Paterson (1973) has reviewed and rederived the thermodynamic relationships for non-hydrostatically stressed solids while Durney (pers. com.) has summarized the historical research on non-hydrostatic thermo- dynamics. Following the particularly lucid presentation of Paterson (1973), the equilibrium conditions at the interface between a solid and its solution under non-hydrostatic conditions can be stated thus: (IL = u8 — Ts, + an 1/8 (Paterson 1973, eqn. 8) where III, is the chemical potential of the component of the solid in the solution; us is the molar internal energy of the stressed solid; T is the abolute temperature; ss is the molar entropy of the solid in its stressed state; an is stress component normal to the surface of the solid and vs is the molar volume of the solid in the stressed state. An increase in the equilibrium chemical potential ILL is concommitant with an increase in the solubility of the solid in the solute. The implications of the thermodynamic analysis have been discussed by Paterson (1973) and Durney (1976) and the reader is referred to these papers for greater detail. For pressure solution the following generalizations, following Durney (1976), can be made: (a) a positive increase in the normal stress an always increases the equilibrium solute concentration—i.e. pressure solution occurs—whereas if an de- creases, precipitation occurs. (b) a positive increase in the pore fluid pressure p achieved without changing the normal stress will reduce the effective normal stress (an—p) and hence the solute concentration in the interface—i.e. precipitation occurs. (c) an increase in both the pore fluid pressure and the normal stress most likely produces an increase in equilibrium solute concentration in the interface—i.e. pressure solution occurs. K. R. McClay 3. Diffusion paths The rate of diffusion is generally considered to be the rate controlling factor for deformation by pressure solution (Elliott 1973, Durney 1976, Rutter 1976). In a polycrystalline aggregate five main types of diffusion path may be recognized (Fig. 3). The possible diffusion paths involving lattice and fluid assisted diffusion are: (I) vacancy diffusion through the crystal lattice-Nabarro-Herring creep (Fig·3a). (2) vacancy diffusion along the grain boundaries-Coble creep (Fig. 3a). (3) diffusion along dislocation cores and low angle (sub-grain) boundaries, Ashby 1972, Knipe (peTs. com. 1977), (Fig. 3b). (4) diffusion of matter along an intergranular fluid film and local deposition (Fig·3c). (5) diffusion of matter over large distances through the pores of the material (Fig. 3d). Mass transfer along the diffusion paths outlined above will lead to grain shape changes but paths 4 and 5 may also lead to changes in chemistry and a minera logical differentiation into a new layering (cf. crenulation cleavage, Cosgrove 1976). The recognition of the possible diffusion paths characterized by different dif fusivities is very important when considering the rates for diffusion processes. FIG. 3. Possible diffusion paths in a polycrystalline aggregate. A. Lattice diffusion (Nabarro-Herring creep) and grain boundary diffusion (Coble creep). B. Diffusion along sub grain boundaries (Knipe peTS. com. 1977). C. Pressure solution diffusion along a thin intergranular fluid film and deposition on less stressed crystal faces. D. Pressure solution and stylolite formation but with transport over considerable distance to give rise to fibrous overgrowths. Pressure solution and Coble creep in rocks 63 4. Rate equations Pressure solution models (Weyl 1959, Durney 1972b, 1976 and Rutter 1976) have been generally simplistic in that they consider an isolated grain contact without grain boundary sliding. Elliott (1973) noted that pressure solution is geometrically similar to diffusion creep in metals and ceramics. Rutter (1976) has erected a theoretical model for pressure solution at low stresses (aa < 30o bars). The strain rate 6 is given by é = 32aaV Co Db w/RTpd3 where aa is the applied stress, d the grain diameter, p the density of the solid, w the effective grain boundary width, R the gas constant, T the absolute temperature, V the molar volume of the solid, Co the concentration of a saturated solution in equilibrium with the unstressed solid and Db is the grain boundary diffusivity. Nabarro (1948) and Herring (195o) proposed that the creep rate for lattice diffusion would be B (aaVD„/kTd2) where D, is the volume diffusivity and k the Boltzman's constant. Coble (1963) proposed that, for grain boundary diffusion, the creep rate would be = B' aa VaDbikTd3 where 8 is the effective grain boundary width and Db the grain boundary dif- fusivity. The values of the numerical constants B and B' in these equations depend on the assumed grain shape and diffusion path geometry. These rate equations have been evaluated for quartz, calcite (using the modified forms and the data of Rutter 1976) and galena (using the data of Atkinson 1976a, 1977). The results of strain rate calculations for two temperatures, 200°C and 35o°C, considered on geological grounds to be the range over which pressure solution and fluid assisted diffusive mechanisms are important, are plotted in Fig. 4. Geologically feasible strain rates for the formation of natural ductile structures (e.g. folds) are thought to be in the range 10-9-10-14 sec-1 (Price 1975). In the equation for grain boundary (Coble) creep, the effective grain boundary width was taken as ten times the smallest Burgers vector (a axis for quartz and calcite, -V2 a axis for galena). Although Rutter (1976) and White (1976) have used 2 x b (b = Burgers vector) for the grain boundary width 8 in their analysis of creep in calcite and quartz (i.e. similar to that in metals), Mistier & Coble (1974) have shown that ionic compounds have considerably larger grain boundary widths ( too x b). It is therefore probable that in minerals the effective grain bound- ary widths in minerals would be significantly larger than 2 x b, that taken for metals. Grain boundary widths are also discussed by Elliott (1973). In this theo- retical analysis, to x b was chosen as being a compromise between 3 for metals and 6 for ionic materials (see also Atkinson 1977). Inaccuracies in the diffusion data for quartz (discussed by White 1976 and Rutter 1976) result in these theoretical calculations being no more than order of • CALCITE STRESS LEVEL 100I3ARS (107P0) STOICHIOMETRIC STRESS LEVEL 100 BARS (107120 6 _ - GALENA T.3sec 10 106- 7... ."...... ■T=200°C 10 %%...... 2 T=350•C 108- 10 1610- 10 %:::•4!„).t, T=200°C-.% 1512. •,...„. 4...... ; 1 2 eks. • . .N.- 10 - .•-z.;Toe,. 1014 - -1 4 . ... -,....,9.A. 044_ T=350°C 10 . . •„.:,14, U 7400°C 1616_ 16 T.350°C-"•%, `IZIP, 16 - •■_ 0,:...... ,,...‘•:.„•■ ♦ ti 1618- ■■■ ., ... 5 cr ■ • .. 6. w 1518- N. ■ • • 1. 6520. 11620: 20 T.200•C -S.,. ■ •••• z 10 CC F N. I— - 5 cc - -22 2 2 -22 1 10 - 10 - 24 2 4 24 10 - 10 10 - -•-••-•-• PRESSURE SOLUTION 2 6 -26. SOLUTION 10 - 10 1 -26. ••• ••• COBLE CREEP -.COBLE CREEP - - COBLE CREEP 2 8 10 " NABARRO- HERRING CREEP 10 NABARRO-HERRING CREEP 10 NABARRO-HERRING CREEP . 5 2 I , -2 10 6 10 10 4 10 3 10 10-6 10-5 10-4 1103 110 10-5 10f" 10 10 2 GRAIN SIZE (metres) GRAIN SIZE (metres) GRAIN SIZE (metres) I 1 l ' Opm 18 0pm 1mm 1cm 1gm 100iim 1mm 1cm lOpm 100pm 1mm lcm FIG. 4. Deformation mechanism plots of strain rate against grain size for quartz, calcite and galena. The data used for quartz and calcite creep rates were taken from Rutter (1976) and the data for galena from Atkinson (1976a, 1977). Pressure solution and Coble creep in rocks 65 magnitude estimates. White (op. cit.), however, has shown that electron micro- scopic evidence of naturally deformed quartz supports the theoretical calculations for diffusional creep. For quartz it can be seen that, except for very small grain sizes (t-to p.m), Nabarro-Herring and Coble creep may not give rise to geological strain rates at the two particular temperatures (White 1976). Pressure solution, as formulated by Rutter (1976), may give rise to geological strain rates for grains up to several hundred microns in size. Similarly in calcite Nabarro-Herring creep may only give rise to geological strain rates at very small grain sizes (i vm). There is good experimental evidence that diffusional creep or grain boundary sliding, however, may give rise to geological strain rates in fine-grained calcite rocks at low tem- peratures (cE results of Schmid 1976a, b). Pressure solution may also give rise to geological strain rates in calcite rocks. However, in the region of T = 35o°C, the boundary between pressure solution and Coble creep will be very diffuse because of the large variation in the calculated strain rates which can be caused by small variations in the assumed activation enthalpy. The activation enthalpy is related to the diffusivity and temperature by the relation DT = Do e-H/RT where Do is the extrapolated diffusivity at infinite T, H the activation enthalpy, T the absolute temperature, R the gas constant and DT the diffusivity at a particu- lar temperature T. At low temperatures (200-300°C) small changes in H will give rise to large changes in DT and hence in 6. The activation enthalpy H may also be very sensitive to the presence of alio-valent impurities. The value of H for calcite probably decreases at c. T = 400°C from 25okJ mole-1 to about half this value at lower temperatures (Rutter 1976). Hence, at about 3513°C the strain rates for Coble creep and pressure solution have been calculated to be nearly equivalent. The plots for stoichiometric galena show that geological strain rates will arise from deformation by Nabarro-Herring and Coble creep in fine-grained galena ores (Atkinson 1977). Further complications are liable to arise; for example, Burton (1977) has indicated that at low homologous temperatures (N o•4 T melting), the stresses needed to create sources and sinks for vacancy diffusion may be sufficiently high to inhibit Coble creep in favour of dislocation mechanisms. In geological systems, the presence of clays along the grain boundaries is expected to increase the rate of pressure solution (Heald 1956), whereas in metallurgical systems, the presence of second phase particles may inhibit diffusional creep (Harris 1973). The nature and role of grain boundaries and subgrain boundaries during dif- fusive mass transfer will undoubtedly be extremely important. Only Knipe (pers. com. 1977) has attempted to evaluate this role in quartz and more detailed studies involving chemical analysis and electron microscopic observations are needed in order to evaluate grain boundary behaviour during pressure solution. 66 K. R. McClay 5. Chemical effects Impurity ions and stoichiometry defects will have marked effects on the creep and pressure solution behaviour of geological materials. Atkinson (1976a) has discussed the effects of non-stoichiometry and impurities on the behaviour of polycrystalline galena. However, very little is known about the stoichiometric and chemical effects on the behaviour of quartz and calcite (see section on rate equa- tions). Trace quantities of hydroxyl ions in the quartz structure appear to reduce its strength considerably, enabling plastic deformation and recrystallization to occur below 5oo°C, (Griggs 1967, Blacic 1975, Jones 1975). Little is known, however, about the effects of hydroxyl ions on surface diffusion in quartz and it is expected that this will have important effects on the pressure solution and fluid assisted diffusion around quartz grain boundaries. Fyfe (1976) emphasized that chemical changes during prograde metamorphism (e.g. dehydration reactions, kaolinite pyrophyllite) can have profound effects on mechanical properties of rocks through the provision of free pore water under pressure. He also considers that crystal growth and solution process (i.e. pressure solution) can give rise to strain rates of the order of 10-11 sec-1 (Fyfe 1976) at loo bars differential stress. Knipe & White (1976, 1977) have demonstrated that cleavage lamellae in slates are sites of chemical reactions with new, chemically distinct phyllosilicates developed in kinks and deformation bands. 6. Conclusions This paper has aimed to review current literature and research on pressure solution and Coble creep in rocks and minerals. Pressure solution is a complex problem and although theoretical models have been proposed, care must be taken in applying metallurgical theories to diffusion in rocks where, in particular, the chemistry is undoubtedly more complicated. The nature and role of high and low angle grain boundaries will be very important during diffusive mass transfer processes and these factors are very much unknown quantities in natural rock systems. Very detailed microstructural and microchemical research in natural and experimental rock pressure solution systems is needed, together with electron microscopic studies, in order to determine the nature of grain boundaries in rocks which have undergone pressure solution. To date, only theoretical flow laws have been proposed for pressure solution whereas experiments have established that solid state diffusion processes can give rise to geological strain rates in fine-grained galena and fine-grained calcite rocks. Although pressure solution textural features have been produced experimentally, direct experimental studies to determine the flow laws for pressure solution have had limited success, but increased use of relaxation testing may provide access to the low strain rates necessary to observe the process. Studies of pressure solution phenomena are not only important for understanding rock deformation but they are also important in the compaction behaviour of some petroleum reservoir rocks. In addition, sliding along some faults may be accommodated by pressure solution processes. Pressure solution and Coble creep in rocks 67 ACKNOWLEDGEMENTS. Dr E. Rutter and D. Mainprice are thanked for permission to use un- published data for Fig. 2. R. Knipe is also thanked for permission to refer to unpublished work (Fig. 3b). Dr D. Durney, Professor J. G. Ramsay and Drs B. Burton and R. B. Jones (CEGB) are thanked for providing information and for permission to use the photographs for Plate IA, B and D. Drs E. Rutter, B. Atkinson, S. White, G. Lister, R. Knipe and N. Shaw are thanked for many useful discussions and criticisms for this review. References (including those mentioned in the Conference Report) ADAMS, F. D. & COKER, E. G. 191o. Experimental investigation into the flow of rocks. Am. 3. Sci. (4th series) 29, 465-87. ALLISON, I. & RAMSAY, J. G. 1977. Structural analysis of shear zones in a deformed granite from the Pennine Zone, Swiss Alps. Schweiz. Min. Petrol. Mitt. (in press). ALVAREZ, W., ENGELDER, T. & LOWRIE, W. 1976. Formation of spaced cleavage and folds in brittle limestone by dissolution. Geology 4, 698-701. ARTHAUD, F. & MATTADER, M. 1969. 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McClay, Department of Geology, Imperial College, London SW7 2BP. Conference report Pressure solution and Coble creep in rocks K. R. McCLAY ON 5 NOVEMBER 1976, the Tectonic Studies Group of the Geological Society held a lively discussion meeting at Imperial College on the subject of pressure solution and Coble creep. Deformation mechanisms may be divided into three broad categories: (I) Cataclastic processes (2) Intracrystalline processes involving dislocations (3) Diffusive mass transfer processes. We are concerned here with group (3) processes, flow by diffusive mass transfer of matter from grain boundaries which are subjected to high normal stress to less highly stressed grain boundaries. If the diffusion is predominantly through the grain the process is termed Nabarro-Herring creep whereas if it is predominantly around grain boundaries, the flow is termed Coble creep. In general, grain bound- ary sliding must occur to accommodate the grain shape changes due to diffusion. For these processes to proceed at rates which are detectable, the temperature must be at a large fraction of the absolute melting temperature of the material. Geologists have long recognized in low grade metamorphic rocks, textures in- dicative of deformation by diffusive mass transfer. Even over the relatively long time available for geological deformations, at the low temperatures concerned, solid state diffusivities would be too slow to account for the observed strains. It is therefore inferred that the rate of diffusive mass transfer is enhanced by the presence of an intergranular fluid film. Hence the use of the term 'pressure solu- tion' to describe the process. Thus Coble creep and pressure solution are similar in that they both involve intergranular diffusion. The meeting was intended to review current research on pressure solution and diffusion processes in rocks and to evaluate their importance in geological de- formation. As in previous discussion meetings (McClay 1976, Atkinson 1976b) considerable emphasis was placed on metallurgical-materials science theories and techniques. The meeting was divided into two parts; the first being a review of mechanism and kinetics of intergranular diffusion and the second dealing with current research on natural pressure solution systems. From the abstracts following, it can be seen that over the last decade many data have been collected and many significant advances made, and there has been a convergence towards detailed microstructural, isotopic and chemical studies of natural pressure solution systems. Although only limited success has been achieved so far in experimental work, stress relaxation tests will allow access to the slow strain rates which are difficult to attain in normal triaxial tests. However, careful consideration of the deformation mechanisms, diffusion paths and chemical changes 31 geol. Soc. Lond. vol. 134, 1977, pp. 71-75. Printed in Great Britain. 72 Conference report need to be made in order to evaluate and even to try to quantify pressure solution processes. In particular, the nature and behaviour of grain boundaries and of thin fluid films in these boundaries need further detailed study. Pressure solution—the field data J. G.., analysed. A differential equation for concen- Ramsay, Department of Earth Sciences, Uni- tration is derived. From this equation it is versity of Leeds deduced that matter diffuses from areas of high mean stress to areas of low mean stress, Sorby, in a series of classic papers published (Casey 1976). It is also concluded that long in the late nineteenth century, described range transport is small compared to the observations he had made in the field and with deformation of individual grains. the then newly invented petrological micro- The differential equation governing mass scope which led him to the conclusion that in transport is used to simulate the transport of many rocks deformed by tectonic processes soluble minerals from limb to hinge regions of `mechanical force had been resolved into small scale similar folds. It is found that fold chemical action'. These conclusions have been wavelength, limb dip and fold profile shape fully supported by recent work and the process strongly influence the rate of transport. A of solution and precipitation of carbonates and significant rate of differentiation occurs at un- silicates in deformed rocks is generally termed realistically small wavelengths. This indicates pressure solution. The most important pheno- that a more rapid transport mechanism, such mena illustrating this process are: as diffusion through a pore solution, is required to account for observed differentiation (Cos- 1. The mutual penetration of calcareous fossil grove 1976). fragments in limestones. 2. The development of pits in the pebbles of conglomerates where one pebble enters Coble creep and diffusion in metals and another by solution. ceramics B. Burton, CEGB, Berkeley Nu- 3. The elongation of elastic grains of quartz in clear Laboratories, Berkeley, Glos. slate as a result of silica overgrowths. 4. The migration of silica and carbonates from When a stress is applied to a polycrystalline the limbs to the hinge zone of folds. material at elevated temperatures, creep can 5. The migration of soluble material from the occur mainly by processes involving dislocation sides of stressed objects (e.g. pebbles) facing movement or by the stress-directed diffusion of the principal load directions to the pressure vacancies. The former is characterized by a shadow areas. strong dependence of creep rate upon stress 6. The development of a regular striping in and the latter, known as diffusional creep, slates which cross cuts the lithological depends linearly upon stress. Consequently, layering. since the two processes are independent, diffusional creep tends to be the predominant An analysis of the stress distribution associ- deformation mode at lower stress levels. ated with the development of these features Diffusional creep can be controlled by dif- leads to the suggestion that the mean stress fusion through grains, in which case Nabarro a = (a1 a2 a3)/3 is the critical driving (1948) and Herring (195o) predict: factor. e = Boi2D/d2kT Coble creep and chemical differentiation or by diffusion around grains (i.e. in the grain in response to stress gradients M. Casey, boundaries) in which case the prediction of Department of Earth Sciences, University of Coble (1963) is: Leeds = B'af2wDg/d3kT The theory of creep by surface diffusion where e is the creep rate, a the stress, LI the (Coble 1963) is applied to the problem of the atomic volume, d the grain size, k is Boltz- transport of matter over several grain diameters mann's constant, T the temperature, w the in response to stress gradients. A simplified grain boundary width. D and Dg are the lattice model of a granular aggregate is set up and and grain boundary diffusivities and B, B' are Pressure solution and Coble creep in rocks 73 proportionality constants which take the values pressure-solution phenomena such as over- of to and 150/n respectively. growth and interpenetration of quartz have The stress, temperature and grain size only been reported to occur in experiments dependence of diffusional creep have been above 280°C (Renton, Heald & Cecil 1969, De confirmed in a number of metallic and ceramic Boer 1975, Sprunt & Nur 1976). For carbon- systems. It is to be particularly noted that ates, this temperature is somewhat lower, and because of its stronger grain size dependence visible pressure-solution phenomena are pro- and because boundary diffusion has a lower duced at 200°C. activation energy than lattice diffusion, Coble On the other hand, an influence of pore- creep predominates at lower temperatures and water composition has not been observed. Thus, in fine-grained material. addition of NaOH to pore solutions between Although the parametric agreement be- quartz had no or even a negative effect on tween experiment and theory is good, a compaction rate. Also, an increase in partial discrepancy exists in the absolute magnitudes CO. pressure in carbonate experiments did of predicted and measured creep rates. Experi- not result in any acceleration. mentally measured rates are almost invariably Addition of MgSO4—intended to inhibit faster than predicted. Some of this discrepancy the CaCO3 precipitation—did accelerate com- may be accounted for by an extension to paction as a result of recrystallization of calcite diffusion creep theory by Ashby & Verrall to anhydrite. The absence of the influence of (1973) who consider creep to occur by the pore-water composition on the reaction rate relative movement of adjacent grains with points to the correctness of Weyl's model of diffusional accommodation at triple points. pressure solution (Weyl 1959). This process involves less diffusional transport per unit strain than Coble creep and gives rise Kinetics of pressure solution R. Kerrich, to an increase in the value of the proportionality Department of Geology, University of Western constant by a factor of — o. Ontario, London, Canada A further feature which has recently been noted is that grain boundaries do not always The dominant flow mechanism in tectonic act as perfect sinks and sources for vacancies. processes depends on the rheological properties Consequently all the applied stress may not be of geological materials and the physical con- available to drive the diffusion flux since some ditions prevailing during deformation. The is required to drive the interfacial process relative importance of intercrystalline diffusion responsible for vacancy creation and annihila- (pressure solution) and dislocation creep in tion. This can lead to suppressed creep rates crustal deformation has been evaluated as a and the effect is particularly pronounced in function of temperature (by oxygen isotope materials containing refractory precipitates, thermometry) and grain size. materials of fine grain size and at lower tem- Quartzites with a grain size < 10 1.tm de- peratures. form largely by intercrystalline diffusion at temperatures up to 5oo°C. For grain sizes of Experiments on pressure solution R. B. too ltm and moo pm the dominant deform- de Boer, Koninklijke/Shell Exploratie en ation mechanism changes from intercrystalline Produktie Laboratorium, Rijswijk, Netherlands diffusion to dislocation processes at temperatures of 45o°C and 300°C respectively. The transition In pressure solution experiments on porous temperature in calcite rocks of the grain sizes material the reaction rate must be accelerated quoted is estimated at 360°C, 3oo°C and so that events actually taking place in millions 200°C. of years can be simulated in time spans These data allow recognition of a low- of less than one year. To this end the temper- temperature deformation regime dominated ature, pressure and/or reactivity of the pore by intercrystalline diffusion and a high fluid must be increased drastically. However, temperature regime dominated by dislocation increase of pressure will result in grain break- processes. In non-metamorphic environments, age, thus blurring the pressure-solution pheno- under conditions of very low temperature and mena (Lowry 1956). low effective confining stress, the dominant Temperature has been found to be the most deformation processes are generally grain important factor in pressure solution. Visible fracture and grain boundary sliding, with a 74 Conference report variable component of pressure solution these 'affected volumes' give improbable values depending on lithology. for these two parameters. The shapes of these An abundance of tectonic veining occurs in profiles are determined, in a very complex way, rocks deformed under conditions of low grade by changes in the stress gradients and dif- metamorphism, within the low-temperature fusivities with time, and as yet, no way has deformation regime. The 8180 of vein quartz is been found to use them in calculating a stress- controlled by that of detrital quartz in the strain relationship for pressure solution. country rock. It is concluded that such vein systems form largely by local diffusional mass Pressure solution in shear zones, Yellow- transport over mm to m distances and it is not knife R. Kerrich & I. Allison, Department necessary to appeal to hydrothermal transport of Geology, University of Western Ontario, in solution. Fluids from which the vein minerals London, Canada precipitated are high density aqueous chloride solutions with a small component of CO2, and Metabasalts and granodiorite batholiths at a calculated 8180 of 4.%a to roL. The water/ Yellowknife have deformed in a system of shear rock ratio during deformation and veining is zones, under conditions of greenschist-facies low. metamorphism. The basalt chemistry has been modified by deformation, resulting in a de- Pressure solution in conglomerates T. J. pletion of about 3% SiO2. It is concluded that McEwen, Department of Earth Sciences, The this effect results from pressure solution University, Leeds processes operating during deformation: the silica has diffused over mm to m distances into The pitting of pebbles at their mutal con- extension veins that initiated at about 45° to tacts in deformed conglomerates was first the shear zone walls. In one major structure, ascribed to the process of pressure solution by the Campbell, about 4 x ton kg of silica has been Sorby (1865). A study of the Molasse con- redistributed. Although deformation affects glomerates of France and Switzerland, and the the primary mineralogy and gross chemistry, Carboniferous conglomerates of northern Spain no effects are apparent in trace element or has shown that the conglomerates deform in precious metal abundance, whole rock 8180 or different ways depending on (a) the strength the oxidation state of iron. of the matrix, (b) the deformation rate, (c) the Granodiorites and greenstones deformed into mineralogy of the pebbles and (d) the temper- shear zones experience an initial grain size ature. Strain analysis using the centre-to- reduction from 3 mm to too 1.tm, followed by centre technique and measurements of volume pressure solution of the 'new' grains resulting loss vectors produce inconclusive results. in a depletion of silica. This behaviour con- The stress distributions developed between trasts with that of shear zones described from elastic bodies in contact, calculated from theory the Swiss Alps (Allison & Ramsay 1977) that and from photoclastic experiments, help ex- formed at amphibolite grade, in which the plain why, for pebbles of similar mineralogy, deformation is essentially isochemical, and no the pebbles with the smaller radii of curvature transformation of the primary igneous miner- always pit the pebble with the greater radii of alogy is apparent. The differences in chemical curvature, except in some quartzite pebble behaviour during deformation may be attri- conglomerates where pressure solution and buted to the fact that pressure solution is fracture occur simultaneously. dominant at low temperatures in fine-grained Measurements across pebble contacts using lithologies. the electron microprobe, show that pressure The shear zones at Yellowknife were later solution between pebbles with low contents of `blown apart' under conditions of high fluid insoluble residue occurs only on the pebble pressure. A massive flux of hydrothermal boundary. In contrast, pressure solution be- solutions ( ,---, 2 km3), derived from a source tween pebbles with a high insoluble residue volume of 6o km3, was focussed through content occurs within a large volume of the conduits within the shear zones. These solutions dissolving pebble. precipitated quartz, carbonate, sulphides, Au Calculations of the diffusion constant for and Ag. Major shifts in the oxidation state of pressure solution and the rate of pebble pitting iron and 8180 values are associated with the from the concentration profiles of Ca across hydrothermal alteration. Pressure solution and Coble creep in rocks 75 Some metamorphic aspects of pressure breaking down at such points and that this solution A. Beach, Dept. of Geology, Liver- metamorphic reaction produces illite and pool University quartz. The large accumulation of illite pro- duces the stripe. It is recognized that sediments deformed and The metamorphism of shale to slate is also metamorphosed at very low grades undergo accompanied by large scale pressure solution. extensive pressure solution to produce a spaced Metamorphic reactions have been deduced pressure solution striping in siltstones and which describe the evolution of mixed layer sandstones and a more penetrative cleavage in illite-montmorillonite in a shale to the character- shales. Extensive development of early and istic chlorite, phengite and illite of a slate. Such syntectonic veins commonly show coarse drusy reactions produce large quantities of silica, textures and are filled by quartz and siderite. most of which does not appear to crystallize in The darker pressure solution stripes, seen in the slate, but occupies syntectonic veins else- deformed greywackes, etc., are not strictly the where. Considerable distances of transport are result of accumulation of insoluble residue at often implied. The reactions also character- the solution site. They often contain minerals istically consume H+ and release 01-1— ions and that do not occur in the sedimentary rock and thus have a marked effect on the pH of the are interpreted as being a product of a meta- pore water. Attainment of high pH could morphic reaction. Modal analysis and X-ray account for the commonly observed replace- diffraction studies show that detrital felspar is ment of quartz by siderite in many of the veins. K. R. McCutv, Department of Geology, Imperial College, London SW7 2BP. (The references cited in this conference report are included in the reference list of the preceding review on Pressure solution and Coble creep.) The Journal of the Geological Society Vol. 134, Part I October 1977 Subscription: £20 ($50 U.S.A.); single numbers all issues £8 ($zo U.S.A.) P. E. KENT: The Mesozoic development of aseismic continental margins Presidential Address 1976 J. R. L. ALLEN: The possible mechanics of convolute lamination in graded sand beds 19 R. B. MCCONNELL: East African Rift system dynamics in view of Mesozoic apparent polar wander 33 B. A. STURT, D. M. RAMSAY, I. R. PRINGLE & D. E. TEGGIN: Precambrian gneisses in the Dalradian sequence of NE Scotland 41 K. LAM & R. PORTER: The distribution of palynomorphs in the Jurassic rocks of the Brora Outlier, NE Scotland 45 K. R. MCCLAY: Pressure solution and Coble creep in rocks and minerals: a review 57 Conference Reports Pressure solution and Coble creep in rocks 71 RRS Shackleton in the Indian and south Atlantic oceans 77 Structural style of continental margins: a discussion 8 Proceedings Geological Society 89 Published for THE GEOLOGICAL SOCIETY BURLINGTON HOUSE PICCADILLY, LONDON WIV oJU by SCOTTISH ACADEMIC PRESS LTD 33 MONTGOMERY STREET EDINBURGH, EH7 5JX and printed by WESTERN PRINTING SERVICES LTD BRISTOL and bound at W. H. WARE & SONS LTD CLEVEDON, BRISTOL