A MINERAGRAPHIC STUDY OF THE PINNACLES LODE HORIZON, BROKEN HILL

A thesis

submitted for the degree of

Master of Science

by

S. VEDCHAKANCHANA

School of Applied Geology

University of New South Wales

Sydney

Supervisor : Professor L. J. Lawrence u.iiv:;i

03538 -S.nAR.77 LIBRARY This work has not been submitted for a degree or similar award to any other University or Institution. iii

ACKNOWLEDGEMENTS

The author wishes to express his sincere gratitude to Professor L.J. Lawrence for his supervision, assistance and time spent discussing various aspects of the work throughout the project.

Acknowledgement is also due to various people especially

Dr. Ian Plimer for his assistance in visiting the deposit;

Dr. M.B. Katz, Mrs. M. Krysko, Mr. R. Haren, Mr. J.. Chisholm and Mr. H. Le Couteur for their invaluable assistance with various aspects of the project. Mr. G.F. Small and Mr. J.

Newbourne for their assistance in the preparation of photographic work and Mrs. C. Hutton for typing the thesis. The writer benefited from the reading of an M.Sc. Thesis by Dr. D.E. Ayres.

Thanks go also to the Chulalongkorn-Amoco Fund for financial assistance to undertake the study. iv

ABSTRACT

The Pinnacles base-metal sulphide deposit occurs in the

Willyama Complex which consists of intensely folded and moderate to highly metamorphosed sediments of lower Proterozoic

Age. Despite the lack of unanimity as to the origin of the sulphide mineralization, recent investigations have given support to the view that the orebody has been metamorphosed contemporaneously with the enclosing rocks.

The basis for the interpretation of textures of ore and gangue relationships in mineral assemblages has been the study of similar textures in deformed and annealed metals. The textures of some deformed sulphides are summarized with reference to recent studies of both natural occurrences and experimental work. In this investigation of the ore of the Pinnacles mine, the textures seen in the sulphides and in ore-gangue intergrowths have been interpreted as being due to the effects of prograde metamorphism at conditions approximating the amphibolite- granulite facies boundary, and during retrograde metamorphism at much lower pressure-temperature conditions. The major sulphide phases, and their textural relationships with each other and with gangue minerals, are described in detail and classified according to their formation. V

INDEX TO FIGURES

Page

Figure 1 Regional map showing Pb-Ag-Zn Broken Hill- type mines in the Willyama Complex with three progressive metamorphic zone established by Binns (1964). 6 Figure 2 Regional geological map of the Pinnacles area (From Aust. Inst. Min. Met., 1968). 8 Figure 3a Quartz (Q) - Garnet (G) - Hedenbergite (Hd) granulite with medium to coarse-grained predominantly equigranular granoblastic microstructure defined by grains which possess straight-slightly curved boundaries. (Drawn from thin section, x 30 magnification) 13

Figure 3b Garnet (G) - Biotite (B) - gneiss with medium to coarse grained granoblastic elongate microstructure defined by xenoblastic quartz (Q) with curved boundaries and biotite with sutured grain boundaries, surrounding elongated grains of gahnite (Gh) and garnet. (Drawn from thin section, x 30 magnification.) 13

Figure 4 Geological map of the Pinnacles mine (after Burns, 1965.) 14 Figure 5a Dislocation model of grain boundaries with low angle of misfit (after Shewmon, 1966.) 21 Figure 5b Coincidence related type of structure of high angle grain boundaries (after Liicke et al., 1972). 21 Figure 6 Schematic diagram showing the balance of surface free energies (A ) and grain boundary free energy (Ag) (after Void and Glicksman, 1972). 22

Figure 7a Equilibrium existing in a single-phase system where 120° 23

Figure 7b Equilibrium existing in a two-phase system where £ Au = 2 A12 C0S 2 23 Figure 7c Equilibrium existing in a three-phase system where , , , A 2 3 _ A13 _ 12 sm 0^ sin 0^ sin 0^ 23

Figure 8 Diagram of polygons with curved sides, except a hexagon; meeting at 120u (after Smith, 1952). 26 vi

Pa ge

9 Diagram of soap films in a regular tetrahedron wire-frame showing mutual angles of cosine 1/3 defined by the angle subtened by straight lines' joining corners of the tetrahedron to a point equidistant from them all (after Smith, 1952). 26

10 Some examples of typical grains in annealed Al-Sn alloy (after William, 1952). 28

11 A stress - strain curve showing: a - elastic strain region, b - plastic strain region, c - rupture region (after Honeycombe, 1968). 30

12 A creep curve showing: a - primary creep, b - secondary or steady-state creep, c - tertiary creep (after Honeycombe, 1968). 30

13a Schematic diagram of nucleation by sub-grain growth (polygonization) (after Cahn, 1966). 36

13b Schematic diagram of bulge-nucleation model (after Beck, 1954). 36

14a Diagram showing the movement of grain boundaries with the growth of a septagon and the shrinkage of a pentagon (after Nielsen, 1966). 38

14b Schematic diagram showing the adjustment of grain boundaries during grain growth (after Smith, 1952). 40

15 Schematic diagram showing shapes of a minor phase at triple-junction point corresponding to different dihedral angles (after Smith, 1948). 42

16 Banded ore (a hand specimen). 50

17 Banded ore - showing foliation defined by preferred orientation of platy mineral (biotite) and lenticular aggregates of garnet (G) (polished section x 60). 52

18 Banded ore - showing lineation defined by preferred orientation of prismatic brown hornblende (Hb) (polished section x 60). 52

19 Banded ore - showing a sulphide "spur" plastically injected across layering (polished section x 50). 53

20 Banded ore - showing flattened garnet (G) grains, and galena (Gn) plastically migrating along microfractures within garnet grains (polished section x 50). 53 vi i

Page

21 Banded ore - pyrhotite (Po) showing polygonal pattern with smooth arcuate boundaries but abruptly .terminated at hedenbergite (Hd) grain faces (polished section x 150 oil). 55

22 Banded ore - sphalerite (Sp), galena (Gn) and chalcopyrite (Cpy) showing arcuate grain boundaries abruptly terminated against biotite (B) grains (polished section x 100). 55

23 Banded ore - discontinuous lenticles of chalcopyrite (Cpy) occurring at triple­ junction points along grain boundaries of sphalerite (Sp) grains (polished section x 400 oil). 56

24 Banded ore - triangular composite grains between spharerite (Sp) and gudmundite (Gd) or pyrrhotite (Po) and gudmundite (Gd) with small galena (Gn) lenticles occurring at triple-junction points and grain boundaries between garnets (G) or between garnet and hedenbergite (Hd) (polished section x 100 oil) . 56

25 Banded ore - the euhedral crystal of arsenopyrite (Asp) contains inclusions of galena (Gn) and pyrrhotite (Po) (polished section x 150 oil). 57

26 Banded ore - showing sulphide minerals wrapped around fragments of silicate gangues (biotite and quartz) and having smooth grain boundaries. Quartz sometimes shows internal polygonal patterns (thin section x 30 partial X-nicols). 57

27 Brecciated ore (a hand specimen). 59

28 Brecciated ore - showing fragments of quartz (black) with smooth grain-boundaries embedded in recrystallized sulphide matrix (polished section x 50) . 59

29 Brecciated ore - showing ilmenite (II) and other sulphide minerals occurring interstitially to rounded garnet grains with zcnally arranged inclusion (polished section x 60). Note also fractures trending top to bottom and possibly related to retrograde stresses. 60

30 Brecciated ore - fragments of single grain quartz showing smooth arcuate boundaries whilst garnet shows internal polygonal pattern (polished section x 50). 50

31 Brecciated ore - Quartz (Q) and Garnet (G) showing ,?disruptedn polygonal aggregate resulting from imperfect recrystallization or subsequent disruption (polished section x 80) . 62 vi i i

Page

32 Brecciated ore - Galena (Gn), sphalerite (Sp) and pyrite (Py) showing mutual arcuate grain boundaries. Note pyrite appears to be an alteration product of pyrrhotite having, as it does, a typical pyrrhotite lenticular shape. (Polished section x 50). 62

53 Brecciated ore - showing sphalerite (Sp) grains demarcated by the arrangement of small triangular or globular bodies of chalcopyrite (Cpy) , galena (Gn) and pyrrhotite (Po). (Polished section x 100 oil) . 63

34 Brecciated ore - showing exsolution intergrowth between sphalerite (Sp) and chalcopyrite (Cpy) (polished section x 50). 63

35 Brecciated ore - showing graphic intergrowTth between galena (Gn) and tetrahedrite (Tt) (polished section x 60 partial x-nicols). 64

36 Brecciated ore - showing sphalerite (Sp) plastically injected into biotite (B) flakes wrapping around rounded fragments of garnet and recrystallized quarts (polished section x 50). 64

37 Brecciated ore - showing sphalerite (Sp) metamorphically corroded by silicate gangue (polished section x 50). 66

38 Brecciated ore - etching of galena brings out a fine polygonal recrystallized pattern (polished section x 100). 66

39 Brecciated ore - etching of sphalerite brings out a polygonal pattern. Grains possess recrystallization twinning. Numerous lenticular bodies of chalcopyrite occur along twin-boundaries, grain boundaries, and at triple junctions (polished section x 100) . 67

40 Brecciated ore - etching of sphalerite brings out polygonal patterns with grains possessing recrystallization twinning superimposed by fine-subgrains (polished section x 125).’ 67

41 Brecciated ore - etched sphalerite showing branched deformational twinning (A) cutting across an earlier twinning (B). (Polished section x 160). 68

42 Brecciated ore - etching of chalcopyrite brings out a complex twinning (polished section x 100) . 68

43 Brecciated ore - after etching,fragments of sphalerite show slightly bent deformation twinning, whilst galena shows fine subgrain texture (polished section x 60). 71 ix

Page

Figure 44 Etching of pyrrhotite brings out a complex twinning overprinted on the polygonal pattern (polished section x 100). 71

Figure 45 Arsenopyrite grain-shape is governed by garnet aggregate (polished section x 80). 73

Figure 46 Ilmenite (11) showing co-recrystallization with garnet (G) (polished, section x 100). 73

Figure 47 Gudmundite (Gd) showing an intergrowth with galena (Gn). (Polished section x 200 oil). 76 Figure 48 Gudmundite (Gd) showing an intergrowth with pyrrhotite (Po) (polished section x 200 oil) . 76

Figure 49 Graphite flakes (Gp) enclosed within a garnet grain. Note arsenopyrite (Asp) having core of loellingite and showing stress twinning (polished section x 50 X-nicols). 77

Figure 50 Fragments of quartz, showing internal polygonal pattern and co-recrystallized sphalerite, all embedded within galena- sphalerite matrix. (Etched polished section x 60 . ) 80 Figure 51a Co - recrystal1ization of quartz (Q) , with garnet (G) showing zoned inclusions, and galena (Gn). Note: minute globules of galena and quartz at garnet grain- boundaries (polished section x 50). 82 Figure 51b Co-recrystallization of hedenbergite (Hd) with quartz (Q) showing polygonal pattern, ilmenite (II), and pyrrhotite which'*partially altered to marcasite or formed a composite grain with gudmundite (Gd). Note (i) minute globules of film of galena occurring at ilmenite-quartz or quartz boundaries; (ii) spheroidized pyrrhotite enclosed within hedenbergite (polished section x 60). 82 Figure 52a Co-recrystallization of garnet (G) , having spherical inclusions of hedenbergite, quartz and pyrrhotite, with hedenbergite (Hd). Note: galena occurring as a film or minute lenticles along garnet boundaries (polished section x 50) . Figure 52b Co-recrystallization of garnet (having inclusions of quartz, hedenbergite and sphalerite) with hedenbergite. Note: small lenticles of galena or composite grain between pyrrhotite and gudmundite and hedenbergite occurring along garnet grain-boundaries (polished section x 150). 84 Page

Figure 53 Co-recrystal1ization of galena (Gn) with sphalerite (Sp) showing arcuate grain boundaries (polished section x 100). Figure 54a Recrystallized garnet showing a foam texture (polished section x 50).

Figure 54b Recrystallized quartz showing a foam texture. Note: sphalerite and siderite grains occurring at triple-junction points (polished section x 60).

Figure 55 Co-recrystallization of quartz (Q) with ilmenite (II), hedenbergite (Hd) (showing exsolution lamellae), and garnet (G) having zoned inclusions. Note: minute globules of galena and quartz at garnet grain-boundaries (polished section x 100 oil) . 88 Figure 56 Brecciation of sphalerite-galena grains, possessing prograde annealing texture and healed by migratory siderite (Sd) (polished section x 50). XI

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS iii

ABSTRACT . iv

INDEX TO FIGURES v

CHAPTER 1 - INTRODUCTION 1

1.1 PREVIOUS WORK 1

1.2 METHOD OF INVESTIGATION 3

CHAPTER 2 - GEOLOGICAL SETTING 5

2.1 SUMMARY OF BROKEN HILL GEOLOGY 5 2.2 GEOLOGY OF PINNACLES AREA 7 2.3 GEOLOGY OF THE PINNACLES MINE AREA 11

CHAPTER 3 - BASIC PRINCIPLES "THE METALLURGICAL APPROACH" 18 3.1 GRAIN BOUNDARIES AND THEIR SURFACE FREE ENERGY 18 3.2 GRAIN SHAPE IN POLYCRYSTALLINE AGGREGATES 24 3.3 STRUCTURES DUE TO PLASTIC DEFORMATION 27

3.4 STRUCTURES DUE TO ANNEALING RECRYSTALLIZATION AND GRAIN GROWTH 33

CHAPTER 4 - THE EFFECTS OF METAMORPHISM ON SULPHIDE ORES 43 4.1 SUMMARY OF RECENT EXPERIMENTAL DEFORMATION ANDANNEALING OF SOME SULPHIDE ORES 43

4.2 SUMMARY OF THE EFFECTS OF METAMORPHISM ON SULPHIDE ORES 46

CHAPTER 5 - THE PINNACLES ORES 49

5.1 GENERAL CHARACTERISTICS 49 5.2 ORE MINERALOGY 69 xii

Page

CHAPTER 6 - EFFECTS OF REGIONAL METAMORPHISM ON THE PINNACLES OREBOTY 78 6.1 EFFECTS DUE TO PROGRADE METAMORPHISM 78 6.2 MINERALOGICAL CHANGE DURING PROGRADE METAMORPHISM 87

6.3 EFFECTS DUE TO RETROGRADE METAMORPHISM 89

CHAPTER 7 - DISCUSSION AND CONCLUSIONS 94

REFERENCES 98 1

CHAPTER 1 INTRODUCTION

1,1 PREVIOUS WORK

The Pinnacles Mine, situated in the Barrier Ranges (lat.

141°20’E, long. 32°3's) about 16 km south-west of the Broken

Hill city and with a production and ore expectancy of about 200,000 tonnes, is the second largest known lead-zinc-silver sulphide mineralization (apart from the main lode Broken Hill) in the district. It produced ore with a characteristic metal content of 11% Pb, 481.95 gm/tonne Ag and 2.5% Zn mainly from the "Lead Lode".

The Pinnacles area has been previously examined and mapped by Jaquet (1894) , Andrews (1921) , Turner (1927) and Burrell (1936). Subsequent detailed mapping of the mine area has been carried out by King (1950, 1953), Burns (1965) and Johnson (1966). Mineragraphic study of the Pinnacles ore has been made by Smith (1922), Stillwell (1926, 1953), Garretty (1943), Ramdohr (1950) and Ayres (1962).

Since it is recognized that the Pinnacles deposit occurs in a similar geological environment to the main Broken Hill lode the deposit has been examined in the light of knowledge gained from the main Broken Hill lode. The deposit has been previously regarded as a result of extensive replacement of country rocks by hydrothermal solution presumably derived from the magmatic source (see Stillwell, 1926; Turner, 1927;

Garretty, 1943). The hydrothermal origin was vigorously opposed by Ramdohr (1950) and King (1953). After extensive microscopic examination of the Broken Hill and Pinnacles ores. Ramdohr has 2 indicated that the Broken Hill type mineralized layers are composed of high temperature gangue minerals, such as roepperite, bustamite, rhodonite, manganhedenbergite, spessartine and others, and low temperature ore minerals. In addition, the ore-gangue intergrowths are indicative of their simultaneous crystallization.

Consequently Ramdohr has suggested that the ore textures, as existing at present, are entirely due to metamorphism.

One of the most striking features of the Broken Hill type mineralization is the spatial relationship between sulphide mineralized layers and stratigraphy. The sulphide concentration consists of a series of conformable layers or lenses enclosed in the so called "lode horizon" which comprises mica-garnet- sillimanite schists, gneisses and quartzite. Even though there are some local irregularities, the interface between ore lenses and enclosing rocks is generally concordant with the foliation of the wall rocks. In accounting for the distribution and abundance of the base-metals in the ore layers, it is clearly indicated that each mineralized lens possesses distinctive silver to lead ratios and zinc to lead ratios with distinctive gangue minerals. This is another remarkable feature of the Broken Hill type deposit.

Subsequent regional mapping has not established the presence of an igneous source rock to which the mineralization may be ascribed. Instead most parts of the Broken Hill-type mine area comprise high grade metamorphic rocks presumably having a sedimentary origin. On the basis of the preceding observation King (1953) suggested that some sort of sedimentary process would be most likely as a mechanism for the original deposition of the lode constituents, that is, the sulphide ores have been formed at the same time as the enclosing rocks and 3

both subsequently modified by regional metamorphism.

In attempting to understand the origin and post-depositional history of the Broken Hill-type deposits, subsequent investigations have been made with special emphasis on the following aspects:

(i) Studies of the regional metamorphic geology.

(ii) Studies of isotopic compositions and geochronology. (iii) Studies of regional structural geology of the Broken

Hill area and those of the Broken Hill orebody; and

(iv) Studies on mineralogy and geochemistry of the Broken

Hill ores. The result of these studies have been extensively reviewed by Hobbs et al. (1968). However, it is noteworthy to conclude here that the main lode Broken Hill and, unquestionably, the Pinnacles have been involved in two main and separate metamorphic events, that is, the 1,700 m.y. prograde Willyama metamorphism and the 500 m.y. retrograde metamorphism. To date, despite the uncertainty as to whether the base- metal sulphide ores, in their primitive state, resulted from surficial processes or from submarine volcanic exhalations, most geologists have agreed that the Broken Hill-type ores in their original state were deposited syngenetically with the enclosing rocks.

1.2 METHOD OF INVESTIGATION Although an enormous amount of investigation on the effect of metamorphism on the Broken Hill orebodies has been recently carried out by many workers such as Richards (1966) , Lawrence

(1967, 1973), Both (1970, 1973), Maiden (1972, 1975) and Boots

(1972), there has been very little direct investigation on 4

metamorphic features of the Pinnacles ore (see Ramdohr 1950).

Subsequent mineralogical study has been carried out only by

Ayres (1962) . Ayres has provided a sound knowledge of the mineralogy of the deposit and has concluded that the ore was likely to be of a syngenetic type. He did not, however, deal with the metamorphic features of the ore though it should be pointed out that such studies were not in vogue at that time.

The aim of this thesis is to outline the post-depositional history due to the effect of metamorphism of the Pinnacles ores.

Emphasis is placed on the study of textural features and textural changes in the ores on microscopic scales. Because of the abandonment of the Pinnacles Mine, the laboratory investigation has involved microscopic study of some 200 polished sections and 50 thin sections made from systematically collected dump specimens. Mineralogical determinations of some gangue minerals have been supplemented by x-ray diffraction studies. Textural characteristics of the major sulphide minerals such as galena, sphalerite, pyrrhotite and chalcopyrite have been revealed by etching 25 per cent of the total polished sections used. Etchants used for particular minerals are:

Galena and sphalerite:- Mixture of 5 parts of thiourea solution (100 gm/1) and 1 part of

cone. HC1. Etch at about 60°C for

5 minutes.

Chalcopyrite and Pyrrhotite:- Mixture of 1 part of 3% ^2^2 aRd 1 part of 5N NH^OH. Etch for 30-60

seconds. 5

CHAPTER 2 GEOLOGICAL SETTING

2.1 SUMMARY OF BROKEN HILL GEOLOGY

Lead-silver-zinc sulphides of the Broken Hill-type mineralization occur widely in the highly metamorphosed Lower Proterozoic rocks known as "the Willyama Complex" in western

New South Wales. The Willyama Complex occupies about 4,000 square kilometres (Figure 1). The Willyama rocks range from slate, phyllite and andalusite schists in the north-western part of the Complex to sillimanite schist and gneiss, granite gneiss, amphibolite, quartzite and pyroxene granulite in the south­ eastern part. Three zones of progressive metamorphic grades across the Complex have been established by Binns (1964) as shown in Figure 1. On the basis of petrological studies, Binns has also suggested that temperatures ranging between 650°-850°C were attained during the initial metamorphism of the country rocks in the immediate vicinity of the main lode at Broken Hill.

Sr/Rb measurements have dated this high grade metamorphic event, referred to as the "Willyama metamorphism", at about 1,700 m.y.

(Shaw, 1968).

Anderson (1965) recognized three subsequent retrograde events having a complex relationship to folding and pegmatite intrusions. Vernon (1965) has indicated that the retrograde schists are mainly localized in planar zones where new schistose textures have developed. Most of the retrograde schist zones consist of rocks having mineral constituents corresponding to the greenschist or low amphibolite facies of metamorphism

(Turner and Verhoogen, 1960). It is also recognized that 6

-31* 30

CHAMPION ZONE A PARNELL

ZONE B' CENTENNIAL. .NINE MILE

WOLSELEY • GREAT WESTERN

SILVER PEAK

ZONE C -3? 00

/ ^PINNACLES

LITTLE BROKEN HILL SOUTH BOULDER LAUREL *

GALENA HILL

ANGUS

SCALE CP KILOMETRES

TORROWANGEE SERIES MINES BROKEN HILL TYPE

MUNDi MUMDI GRANITE BOUNDARIES BETWEEN METAMORPHIC ZONES

t AF TER BfNNS 1S64)

WILLYAMA COMPLEX

FIGURE 1 Regional map showing Pb-Ag-Zn Broken Hill-type mines in the Willyama Complex with three progressive metamorphic zone established by Binns (1964). 7

retrogression in the southern part of the block is relatively simpler than in the northern part, hence at present unambiguous correlation of the retrograde events of the Willyama Complex is impractical. However, Sr/Rb measurements on biotite samples from the retrograde schist zone have dated this metamorphic event at 500 m.y. (Pidgeon, 1967).

Subsequently Hobbs et al. (1968) have suggested that there are two distinct metamorphic events which have affected the

Willyama Complex rocks. The first even at amphibolite-granulite grade corresponding to two stages of intense regional folding occurred at about 1,700 m.y. ago. The second event at lower amphibolite-greenschist grade, correlated with a stage of retrogression of discrete shear zones, occurred at about 500 m.y. ago.

2.2 GEOLOGY OF PINNACLES AREA

The geology of Pinnacles area has been previously mapped by Jaquet (1894), Andrews (1922) , Turner (1927) and Burrell (1936). Subsequent detailed mapping and interpretation have been carried out by King (1950, 1953) and Burns (1965) (Figure

2).

Stratigraphic Succession Rocks of the Pinnacles area consist of high grade metamorphic sediments, presumably having ferrugenous pelitic-psammitic origins

(Ayres, 1962), are represented by biotite-sillimanite schist and gneiss, aplitic gneiss, quartz-magnetite rocks, and amphibolites

(Table 1). However, on the basis of geochemical investigation of amphibolites, Binns (1964) and Plimer (1975) have suggested that they may have originally been theoliitic sills or tuffs.

The Pinnacles ore occurrence consisting of several thin horizons 8

r ■") ..BROKEN HILL

2120C00CN .

> |g

."ICKTH PINNACLE

,E PINNACLE

SOUTH PINNACLE

scole of kilometres

DOLERITE POTOSI GNEISS

PEGMATITE AMPHIBOLITE

GRANITE GNEISS SCHIST t SlLLIMANiTE GNEISS

k +»»4»a R 4 t ■»( ♦ 4 APLITE QUARTZ - MAGNETITE ROCK kt ft*

FIGURE 2 Regional geological map of the Pinnacles area (from Aust. Inst. Min. Met., 1968). 9

TABLE 1

GENERALIZED STRATIGRAPHIC SUCCESSION OF THE PINNACLES AREA, FORMATIONS ARE SEPARATED BY SILLIMANITE-BIOTITE-GARXET GNEISSES (AFTER BURNS, 1965)

FORMATION MAXIMUM THICKNESS (m.) DESCRIPTION

1 450 + Amphibolites with gneiss horizons minor lode horizons Aplitic gneiss

2 90 Amphibolite Quartz magnetite with aplitic gneiss Amphibolite

3 180 Garnet hematite magnetite with lode horizons Aplitic gneiss (local) Amphibo1ite

4 180 Garnet hematite magnetite Aplitic gneiss Amphibolite Garnet hematite magnetite

5 245 Lode horizons (lead and zinc lodes of Pinnacles Mine) Amphibolite Garnet hematite magnetite Aplitic gneiss Amphibolite with minor lode horizons

6 300 Garnet granitic gneiss 10

exists in a sillimanite gneiss horizon- For correlating with the stratigraphic sequence of the main Broken Hill lode, it has been generally agreed that the Pinnacles mine sequence lies stratigraphically above that of the Main Lode area (see King

1950, 1953; Ayres 1962; and Burns 1965). However, on the basis of the study of high grade schistosities in the northern part of the Complex and in areas in the immediate vicinity of

Broken Hill for the purpose of providing a reliable framework for the interpretation of the structure of the lode horizon, Rutland and Etheridge (1975) have concluded that:

"The options still available for regional interpretation allow the principal occurrences of a lode horizon throughout the Willyama Complex to belong to the same stratigraphic interval."

Structure The structure of the Pinnacles area is quite complex. However, because of good exposures and continuity of the marker bed, namely aplite, the whole structure of the Pinnacles area can be readily determined than that of the main Broken Hill lode (King 1950, 1953). The structure has been interpreted as a broad syncline having a north-south trend. The fold axis is undulating and observed to plunge towards the south at a low angle, but north plunging minor folds crop out south-west of the South Pinnacles suggesting a reversal of plunge of the major syncline. The western limb of the major syncline dips consistently to the east at a moderate angle, while the eastern limb dips towards the west at a moderate to steep angle in the northern part of the area and is possibly overturned with an easterly dip at a steep angle in the southern part. This relatively simple structure has been complicated by buckling of beds associated with subsequent retrograde shear zones having 11 two principal trends of WNW-ESE and NW-SE (Figure 2). The buckling increases in intensity towards the south and results in the formation of minor folds plunging in easterly and south­ easterly directions (Burns, 1965).

2.3 GEOLOGY OF THE PINNACLES MINE AREA

The rocks cropping out in the Pinnacles mine area consist of sillimanite gneiss, gahnite-garnet hematite rock or quartz- magnetite rock, aplitic gneiss and amphibolites. Previous petrographic discription of these rock types has been made by

King (1950) and Ayres (1962) . Subsequent detailed petrological study of these high grade metamorphic rocks has been carried out by Binns (1964) and Vernon (1968, 1969). On the basis of textural and mineralogical studies, as illustrated in-Figure 3a and Figure 3b, Binns and Vernon have concluded that these rocks belong to the granulite facies. The Pinnacles mine succession has been established by Burrell (1936) , King (1950) and Ayres (1962) as shown in Table 2.

Four major folds have been recognized in the mine area.

These are the southern syncline, the theta anticline, the Fischer syncline, and the Junction anticline from south to north respectively. Axes of these folds pitch at low to moderate angles towards the east and south-east are almost perpendicular to that of the regional fold; this has been attributed to drag folding of beds by subsequent shearing. The southern limb of the southern syncline and the northern limb of the Junction anticline have been attenuated and sheared off by the Consols and Pine

Creek shears respectively. In addition, there are numerous minor shears occur between these two major shears as some of them are illustrated on the surface geological maps (Figure 4, after Burns, 1965). 12

TABLE 2

THE PINNACLES MINE SEQUENCE (AFTER AYRES, 1962)

THICKNESS MARKER HORIZONS LITHOLOGIC TYPE

25 m Quartz magnetite Biotite Chlorite Gneiss rock

Quartz-Muscovite-Garnet Gneiss with Plagioclase and Potash feldspar rich bands

Potash feldspar-Quartz- Garnet Sandstone Muscovite-Plagioclase Gneiss

110 m

Quartz-Amphibole Pyroxene Rock

Zinc Lode (?)

Zinc Lode

Lead Lode Quartz - garnet-sericite Gneiss with Potash Zinc Lode Feldspar and Sillimanite rich Bands

Zinc Lode (?)

Zinc Lode (?)

65 m Quartz-Sericite-Garnet- Sillimanite Gneiss 13

T"“------

/ ~N.y •

FIGURE 5a Quartz (Q) - Garnet (G) - Hedenbergite (Hd) granulite with medium to coarse-grained predominantly equi- granular granoblastic microstructure defined by grains which possess straight-slightly curved boundaries. (Drawrn from the section, x 30 magnification).

FIGURE 3b Garnet (G) - Biotite (B) - Gneiss with medium to coarse grained granoblastic elongate microstructure defined by xenoblastic quartz (Q) with curved boundaries and biotite wTith sutured grain boundaries, surrounding elongated grains of gahnite (Gh) and garnet. (Drawn from thin section, x 30 magnification). 14

APLITIC GNEISS

AMPHIBOLITE 100 200

SHEAR ZONE SCALE OF METRES

f X ANTICLINAL AXIS SHOWING PITCH

X SYNCLINAL AXIS SHOWING PITCH

FIGURE 4 Geological map of the Pinnacles mine (after Burns , 1965) . 15

Mineralization The base metal deposit at the Pinnacles consists of three conformable mineralized horizons separated by sillimanite gneiss.

They are: the hanging zinc lode; lead lode; and footwall zinc lode. In addition, numerous weakly mineralized lode materials characterized by quartz-gahnite mineralization have been found

in gneisses associated with quartz-magnetite rock and amphibolites. Additionally, it has been demonstrated that each mineralized horizon possesses distinctive metal assays as demonstrated in Table 3 (Ayres, 1962 and Johnson, 1966).

Throughout the Pinnacles mine's history, ore has been produced only from the lead lode (about 1.5m thick). Production and ore reserve have been estimated with a total of about 200,000 tonnes to a depth of 75 metres (Burns, 1965). However, subsequent drilling has resulted in the discovery of high grade ore with economic thickness to a depth of 110 metres. The ore reserves of Pinnacles lode (lead lode), therefore, has been re-estimated with a total of about 240,000 tonnes of 8.91 Pb,

331.70 gm/tonne Ag, and 1.6% Zn (Johnson, 1966). Galena is the principal component of the lead lode with the minor constituents: pyrite, arsenopyrite-lol1ingite, sphalerite, pyrrhotite, chalcopyrite*, jamesonite, ilmenite, marcasite, tetrahedrite, gudmunaite, pyrargyrite, graphite and magnetite (most of these minerals were reported by Ayres, 1962).

Sphalerite and pyrite are the principal components of the zinc

lodes with minor constituents such as arsenopyrite-lollingite,

There are two distinctive generations of chalcopyrite: the more abundant "primary" chalcopyrite, and that developed from the metamorphic breakdown of complex tetrahedrite and usually intergrown with gudmundite. 16

TABLE 3

CHARACTERISTICS OF THE THREE MAJOR MINERALIZED HORIZONS (AFTER JOHNSON 1966)

TYPICAL ASSAY ORE HORIZON Pb (%) Ag Zn {%') (gm/tonne)

Hanging Wall Zinc Lode 4.3 107.73 7.3

Lead Lode 8.9 331.70 1.6

Footwall Zinc Lode 3.3 96.39 14.0 17 galena, pyrrhotite, chalcopyrite, ilmenite and goethite.

Ayres (1962) listed the following differences between the lead and zinc lodes:

(i) Relative amount of galena and sphalerite. (ii) Jamesonite has been found only in lead lode, and

(iii) Goethite has been found only in footwall zinc lodes.

The principal silicate gangue minerals are quartz, green feldspar, spessartine garnet, and manganhedenbergite. In addition, it has been reported that bustamite, calcite and fluorite are absent (King, 1950). 18

CHAPTER 3 BASIC PRINCIPLES "THE METALLURGICAL APPROACH”

Since it has been established already that the Broken Hill- type ore occurrences have been involved in the Willyama and subsequent retrograde metamorphisms (a summary of the effects of metamorphism on ores will be presented in the next chapter) , there have been textural changes of grains and grain-aggregates in response to deformation, annealing recrystallization and secondary grain growth. Despite the increasing amount of literature on the subject of metamorphism of sulphide ores, investigations on the deformation of ore minerals are few. However, it has been agreed generally that ore minerals are analogous to metals to some extent. In addition, the deformation of metals has been elaborately investigated, and therefore, studies of the deformation of ore minerals have been commonly made in the light of knowledge gained from that of metals. A brief outline of the basic principles of grain topology is given.

3.1 GRAIN BOUNDARIES AND THEIR SURFACE FREE ENERGY

Natural substances, such as ore minerals, are usually polycrystalling aggregates. They consist of crystalline particles which adhere to one another along boundaries. In metals, a grain boundary can be considered as a line separating two grains

(crystals), being different in crystallographic orientation, composition and dimensions of the crystal lattice (McLean, 1957).

In accounting for the structure of grain boundaries, two theories have been proposed. The first one, called the

’’amorphous cement” theory, states that grain-boundaries consist 19 of amorphous layers because the boundaries behave as regions of high strength at low temperatures with high rate of deformation, and as regions of weakness at high temperature with low rate of deformation. These characteristics also define amorphous materials. The second theory, called the "transitional region" theory, states that grain-boundaries are transitional regions from the lattice of one grain to the other. The width of a transitional region amounts only to a few atomic diameters.

There is one fundamental difference between the two concepts; that is, in the former concept grain-boundaries are characterized by random atomic arrangement whereas in the latter concept the array of atoms will be dependent on the relative crystallographic orientation of grains meeting at the boundaries. Experimental data on grain boundaries has been, in detail, reviewed quantitatively by Aust and Chalmers (1952). The experimental results have led to the abandonment of the amorphous cement theory and given support to the transitional lattice theory. In addition, it has led to determination of the actual structure* of this transitional region.

Present views on grain-boundary structure have been extensively reviewed by several workers, such as Aust and

Chalmers (1952), McLean (1957), Gordon and Vandermeer (1966), and Shewmon (1966). It is customary to classify grain boundaries as either low-angle or high-angle boundaries. The division between these two structures is indefinitely placed by a relative grain "mismatch" angle. The low-angle boundaries may be generally regarded as discrete dislocations of the array

* The term "structure" is used here as in metallurgy as a synonym to "texture" in the mineralogical sense. 20

(Figure 5a). In contrast, unanimity is still lacking among workers concerning the nature of high-angle grain boundaries even though the coincidence-lattice models, in which the boundaries has been regarded to consist of periodical repetitions of a very small unit of structure, appear promising (Figure 5b).

When a polished specimen of a metal is sufficiently heated, a network of grooves coinciding with the grain-boundary positions is formed. A boundary groove represents an approach to the equilibrium shape of the surface resulting from the balancing of two surface free energies and the grain-boundary free energy

(Figure 6). At equilibrium, assuming that the boundary groove is symmetric and the grain-boundary free energy is independent of the crystallographic orientation, the relationship between the surface free energy, the grain-boundary free energy and the angle of groove can be expressed by the following equation:

L = 2 ) cos B s %2 where Ag = grain-boundary free energy

As = surface free energy

0 = angle of groove In two dimensions (Figure 7a, 7b and 7c), it has been observed that three grain boundaries usually meet at a point, referred to as a triple-junction point. Therefore, if equilibrium can be established, the angles between such boundaries would indicate the relative grain-boundary energy.

In the case of a single-phase aggregate three grain boundaries meeting at a point will make an angle of 120° to each other and possess equal resolved surface tensions. In a two-phase system, the configuration consists of two equal angles and one angle of FIGURE Sa Dislocation model of grain boundaries with low angle of misfit (after Shewmon, 1966).

FIGURE 5b Coincidence related type of structure of high angle grain boundaries (after Liicke et al., 1972) s

FIGURE 6 Schematic diagram showing the balance of surface free energies (A ) and grain boundary free energy (Ag) (after Void and Glicksman, 1972). '* A2

FIGURE 7a Equilibrium existing in a- single-phase system where

®1 = ®2 = ®3 = 120°

FIGURE 7b Equilibrium existing in a two-phase system where

X11 = 2 X12 C0S 2

FIGURE 7c Equilibrium existing in a three-phase system where

13 12 sm 61 sin 02 sin 24

different value. The resolved surface tension parallel to A^ can be expressed as:

A 2 A cos e 11 12 2 where 0 = dihedral angle which is constant for a specific

phase surrounded by two grains of a different phase.

In a three-phase system, the relative free energies of the boundary surfaces can be calculated using the following equation

X23 _ X13 _ X12 sm 0^ sm ©2 sm 0^

The above equations are valid under equilibrium conditions with the assumption that the interphase free energies are independent of the crystallographic orientation. In addition, the angles between interphase boundary traces must be measured on the surface which is perpendicular to the grain-boundary surfaces.

3.2 GRAIN SHAPE IN POLYCRYSTALLINE AGGREGATES

In considering the shape of grains (crystals) in polycrystalline substances, Smith (1948, 1952, 1964) has firstly drawn attention to the analogy between the configuration of soap bubbles and those of polycrystalline aggregates. Because the

individual grains in an aggregate must adhere to one another in such a way that there are no interstitial voids and all grain boundaries must be interfacial, the shape of each grain will then be governed by the interaction of the following two factors

(a) the requirement to minimize the interfacial free energies

at the triple junction points, and

(b) the requirement to fill up the available space. In two dimensional configuration, it has been observed that the interfacial grain-boundaries must meet in groups of three at annual angles of 120°, and additionally, only grains possessing six sides can have regular straight sides. In other cases each grain boundary must form an arc of a circle with its curvature proportional to the surface free energy operating across it, as illustrated by Figure 8.

In three dimensions, Smith has demonstrated that the space must be filled up by polyhedra whereby at each vertex six faces meet each other; the lines of intersection of the faces meet in groups of four. Additionally, to minimize the surface energy and surface area, it requires that three surfaces must meet each other at mutual angle of 120°; the lines of intersection of the faces meet in groups of four mutually, at an angle of

109O28’26". This angle can be illustrated by soap foam in a tetrahedral wire frame (Figure 9). The ideal polyhedron, therefore, must consist of 22.86 vertices and 13.43 faces with 5.106 sides per face. Obviously, there is no regular polyhedron with plane sides that will attain this. It is found that the pentagonal-dodecahedron consisting of 12 faces with 5 sides per face and 20 vertices closely approaches the ideal one, but it has been demonstrated that these regular pentagon-dodecahedra alone are not able to fill up the available space. Since space must be filled, it is essential to introduce curvature to the grain boundaries. In addition, on the basis of experimental observations on shapes of bubbles, metals and biological cells Smith has indicated that there is no shape can possibly be regarded as that of the typical grain.

In an aggregate of uniform grain size, the average number of sides to each polyhedron will approach fourteen and grains in 26

FIGURE 8 Diagram of polygons with curved sides, except a hexagon; meeting at 120° (after Smith, 1952).

FIGURE 9 Diagram of soap films in a regular tetrahedron wire­ frame showing mutual angles of cosine 1/3 defined by the angle subtened by straight lines joining corners of the tetrahedron to a point equidistant from them all (after Smith, 1952). 27

section with other four to six sides will be rare. Some typical grains in annealed Al-Sn alloy are shown in Figure 10.

These may be compared with the illustrations of Pinnacles ore contained in this dissertation.

5.3 STRUCTURE DUE TO PLASTIC DEFORMATION

Many ore deposits have been deformed during regional metamorphism; the textures of such ores appear to be the result of various mechanisms including deformation, annealing recrystallization and grain growth which have operated during metamorphism. An outline of the principles of deformation are given below.

It is generally agreed that the response of natural substances to stress varies widely. Likewise, strain behaviour of a specific material varies greatly with changes in conditions such as temperature, confining pressure and duration of stress. However, quantitative experimental stress-strain and strain-time data provide a great deal of information as to the behaviour of a specific substance that has undergone prolonged stress under such experimental conditions. Unfortunately, there has been little work done on the deformation of ore minerals, and therefore it is necessary to consider this in the light of its analogy with metals.

When a metal wire suspended from a fix point is subjected to an increasing load, extension will occur. If the extension is instantaneously and completely recoverable when the load is removed, the metal is said to be deformed elastically. Beyond a certain load complete recovery of the strain will not occur on unloading, since the metal has been deformed permanently or plastically. In general the total elastic strain is extremely 28

FIGURE 10 Some examples of typical grains in annealed Al-Sn alloy (after William, 1952). 29

small and the plastic strain has accounted almost for the whole deformation. Prolonged plastic deformation will ultimately result in fracture 'of the metal wire (Figure 11).

Because most crystalline substances are anisotropic with respect to physical and mechanical properties, deformation phenomena of polycrystalline aggregates are very complicated to interpret. A comprehensive account of plastic deformation of single crystals must be given prior to discussion of polycrystalline substances.

Plastic Deformation in a Single Crystal

In the foregoing, it has been demonstrated that if a crystalline substance is stressed beyond the threshold value, namely the elastic limit, it will either deform plastically or fracture.

Plastic deformation in single crystals can take place by means of two mechanisms: (a) by intragranular failure, and (b) by diffusional creep.

The intragranular failure in deformed crystals is revealed by the presence of slip bands, deformational twinning and lattice distortion. It has already been established that in ductile failure the bonds within a crystal will not be broken permanently.

In the case of crystals yielding by development of slip and twinning it has generally been assumed that the deformation has been homogeneous throughout the crystals and that the changes in orientation have occurred uniformly. The development of slip and twinning has been attributed to relative movement of contiguous parts of the crystal along glide planes. Slip and twinning development and direction of movement are governed by the geometry of the crystal-lattice and charge distribution 30

STRAIN

FIGURE 11 A stress - strain curve showing: a - elastic strain region, b - plastic strain region, c - rupture region (after Honeycombe, 1968).

FIGURE 12 A creep curve showing: a - primary creep, b - secondary or steady-state creep, c - tertiary creep (after Honeycombe, 1968). 31

of the crystal concerned. Aggregation of closely spaced slips makes them appear as broader structures usually referred to as slip bands. Slip bands can readily be distinguished optically from twinning by etching the specimen with apprpriate reagents, the boundaries of slip bands are usually incoherent and cut across the grain, while the boundaries of twinning are characteristically tapered within the crystal.

When stress is heterogeneous, the crystal may also yield by lattice distortion resulting from inhomogeneous lattice collapse, in which the unique orientation of the crystal has been replaced by a range of orientations. The variation in orientation of the crystal may be revealed by: undulose extinction in anisotropic mineral, bending of cleavages or kink bands, variation in intensity of etching and staining, and formation of subgrains. In the preceding view, the strain so far has been studied as a function of stress. However, it has been known for a long time that under a constant stress there is a time-dependent component of the plastic deformation usually referred to as creep. Moreover, due to the fact that a regional metamorphic period is extended for a great length of time, unquestionably plastic deformation by diffusional creep must be an important process involved in the metamorphism of ores. In metals, the experimental strain-time curves are usually referred to as creep curves. At elevated temperature, the creep curve can be divided into three well-defined stages, as illustrated by Figure 12. The first part of the curve is referred to as primary creep or logarithmic creep and can occur in the absence of thermal activation. The second part, usually referred to as secondary or steady-state creep, occurs as a 32

linear function of the time. It represents the equilibrium between work hardening and recovery processes occurring at relatively high temperature (usually about half of the melting- point temperature of the substance concerned). The third part, known as tertiary creep, represents the region of rapidly increasing strain rate and finally results in creep fracture.

Deformation of single crystals under creep condition occurs during the stage of decreasing rate of work hardening. By supplementing the slow strain rate it will lead to the acceleration of the recovery process which finally eliminates work hardening. This development has been revealed by the coarsening of slip bands and the formation of sub-grains.

Plastic Deformation in Polycrystalline Aggregates

After considering the plastic deformation in single crystals, an attempt is here made to elucidate the behaviour of plastic deformation in polycrystalline aggregates. It has been demonstrated that the aggregates themselves impose a number of important conditions on deformation processes, including grain boundary, grain size, variation in orientation of the components and the distribution and quantity of other phases in the aggregates. Consequently the behaviour of the aggregate during deformation is extremely complex. Besides the development of slip bands, deformational twinning and lattice distortion in separated components, it has been indicated, on the basis of experimental observations, that elongation of grains and grain­ boundary sliding are the most common mechanisms of the deformation in polycrystalline aggregates. In the case of multi-phase systems, the deformation phenomena will be greatly dependent on the disposition of the comparatively hard and soft components. If the brittle phase 33

forms a continuous film around the grain-boundaries of the softer phase, the aggregate will frequently yield by fracture.

On the contrary, if the brittle component is completely enclosed by the softer phase, the system will yield by plastic strain.

In addition, in the case of random arrangements of the hard and soft components, it has been observed that the former will commonly yield by fracture while the latter will flow around the fragments of the former. Moreover, since the softer component is able to migrate more easily further than the relatively hard constituents, this course of action could conceivably lead to the separation of both components.

In addition, when an aggregate with the minor phase occurring at the low energy regions of the major phase, such as Widmanstatten precipitation, is subjected to light cold­ working, the minor-phase grains will reduce their surface area and become spherical. In metals this phenomenon is referred to "spheroidationM.

3.4 STRUCTURES DUE TO ANNEALING RECRYSTALLIZATION AND GRAIN GROWTH

When a crystalline substance is deformed a fraction of the mechanical energy required for the deformation is stored in the substance. Therefore the deformed substance has usually been, in this regard, considered to be thermodynamically unstable with respect to well-annealed substances. Upon increasing the temperature the material will lower its free energy by the reduction and rearrangement of lattice defects and eventually attain a more stable state, viz. annealed state.

In metals, the annealing process has frequently been divided into three fundamental stages which consist of recovery 34 process,recrystallization and grain growth. It has been experimentally demonstrated that a deformed polycrystalline metal attains an annealed state by the interaction of these three processes of which the first two processes may either have taken place successively or have been overlapping.

Recovery Process

The recovery process has usually been defined as a return of physical and mechanical properties, such as electrical conductivity, density, hardness and strength, of deformed substances towards the values of the annealed materials without the replacement of the deformed grains by new strain-free grains.

This process has involved reduction of point defects and dislocation loops within the grains. Additionally, at elevated temperature, the reduction of strain energy may have been accompanied by the migration and re-arrangement of dislocations resulting, eventually, in the formation of sub-boundaries. This phenomenon has usually been referred to as "polygonization" first recognized by Cahn (1949). The sub-grains have characteristically been slightly misoriented from one another and that of parent grains.

Recrystallization Process

In contrast, the recrystallization process has usually been defined as the stage of the absorption of the deformed grains by new strain-free grains. It is evident that the recrystallization process has involved the nucleation and the growth of new strain-free grains. To date, two different concepts of nucleation have been developed. They are the following: (a) the sub-grain coarsening models, and (b) the bulging mechanism or strain-induced boundary migration. 35

The sub-grain coarsening models are based on the assumption that on the formation, presumably by polygon!zation of subgrains with small angle-misoriented grain-boundaries, the new strain-free grains or nucleii may have formed either by a growth mechanism or by a sub-grain coalescence process. In the former mechanism it has been assumed that there are some places where marked misorientations have existed in the configuration of sub-grains and these sub-grains, which possess high-angle boundaries, are able to grow at the expense of their immediate neighbours. In contrast, another process has been proposed that one sub-grain is able to merge with its neighbour by the rotation of one of the subgrains until its lattice is parallel to that of its neighbour. This process will produce not only large subgrains but also high-angle boundaries. The schematic diagram of nucleation by subgrain growth is sho;vn in

Figure 13a. The bulging mechanism has been postulated on the assumption that the unstable grains have already possessed high-angle misoriented subgrains. This mechanism is best explained by reference to Figure 13b. On the basis of experimental observations, it has been demonstrated that subgrains in the protuberant grain are always larger and have a larger dislocation density than those of the "victim” grains. Sub-boundaries tend to bulge as shown in the illustration. The sub-boundary migration has been attributed to lowering of free energy of the system as a result of the reduction of the total surface area of sub-boundaries per unit volume. With respect to the above view, the primary recrystallization may be summarized thus: (a) Nucleii preferentially form in places where the local degree of deformation is highest. Such places include FIGURE 13a Schematic diagram of nucleation by sub-grain growth (Polygonization) (after Cann, 1966).

FIGURE 13b Schematic diagram of bulge-nucleation model (after Beck, 1954). 57

grain-boundaries with high-angle mis orientation and deformation bands.

(b) The boundaries of the growing grain always move away

from their centres of curvature, and (c) it has been indicated, on experimental evidence, that

the rate of growth is dependent on various factors

including the presence of impurities, the variation of

orientation between the growing grains and that of the

deformed matrix, the degree of deformation, the

temperature at which the recrystallization takes place

and the grain size and composition of the substances.

Grain Growth or Secondary Recrystallization

Following primary recrystallization, if the metal has been continuously annealed at elevated temperature this course of action will lead to gradual growth of grains in a completely recrystallized matrix. This phenomenon is generally referred to as secondary recrystallization or discontinuous grain growth. The driving force for grain-boundary migration of secondary recrystallization has been attributed to the reduction of grain­ boundary area per unit volume. In recrystallized matrix there will be some places where particular grains possess slightly different orientation with respect to their neighbours. The high-angle boundaries possess relatively high mobility and will migrate towards their centres of curvature. An excellent analogy to this process is the growth of soap bubbles demonstrated by

Smith (1948, 1952 and 1964). Smith has demonstrated that during

growth grains with less than six sides will undergo shrinking and finally disappear while grains with more than six sides will have outward movement and grow as illustrated in Figure 14a.

When four boundaries meet at a point, indicating an unstable N--

FIGURE 14a Diagram showing the movement of grain boundaries with the growth of a septagon and the shrinkage of a pentagon (after Nielsen, 1966). 39

condition, the configuration will change instantaneously in order to retain two three-ray corners (as shown in Figure 14b) which is a relatively more stable array. Evidence obtained from various experiments on metals has demonstrated that the movement of grain boundaries is dependent on the variation in the amount of misorientation, the quantity and disposition of impurities, and the temperature. In the case of a high degree preferred orientation, that is the orientations of grains in the aggregate are nearly the same as a result the free energy difference among the grains being very lowr, grain growth is not imminent. Likewise, the disposition of impurities along boundaries will impede the movement of the boundaries. Additionally it has been demonstrated that the grain growth mechanism is a thermally activated process.

Growth Textures Resulting From Annealing The most common phenomenon emerging from the annealing of deformed metals is the growth textures which are analogous to the soap-bubble structures. In the case of two-phase or multi­ phase systems, Smith (1948 and 1964) has extensively demonstrated that the development of the growth textures is greatly dependent on the variation in the amount of dihedral angles, and on the amount and arrangement of the phases presented. In regard to two-phase systems, during the annealing process grains of minor and major phases start to segregate and adjust to one another in order to reduce the free energy of the system. Progressive annealing will lead to the segregation of minor phases at the

triple-junction points and grain boundaries of the major phases.

The development of grain shapes of the minor phase is, therefore, dependent on the dihedral angle; that is, the ratio of grain­ boundary free energy to interphase-boundary free energy. In 40

FIGURE 14b Schematic diagram showing the adjustment of grain boundaries during grain growth (after Smith, 1952). 41

two-dimensions 1 configuration, when the dihedral angle is small

the minor phase will either occur as thin films spreading along

the boundaries of the major component or as cusp-shaped bodies at triple-junction points with concave sides outwards. Kith

increase dihedral angles, the minor phase becomes a tetrahedron with straight sides, then a cusp-shaped body with convex sides

outwards and finally a spherical shape as shown in Figure 15. 4 2

FIGURE 15 Schematic diagram showing shapes of a minor phase at triple-junction point corresponding to different dihedral angles (after Smith, 1948). 43

CHAPTER 4

THE EFFECTS OF METAMORPHISM ON SULPHIDE ORES

4.1 SUMMARY OF RECENT EXPERIMENTAL DEFORMATION AND ANNEALING OF SOME SULPHIDE ORES Metamorphism of sulphide ores, especially those of the stratiform types, has now been generally accepted by most ore- genesis geologists. There is now an increasing number of publications describing metamorphic features in the ores. It is evident that the earlier interpretations of textures of the sulphide ores have been based on the interpretation of textures in similar materials such as metals. This has been due to the lack of experimental data on the behaviour of sulphide minerals under various temperature, confining pressure and strain-rate conditions representative of the earth’s crust. Fortunately, over the last ten years there has been an increase in the quantity of experimental work on the deformation and annealing of some principal sulphide minerals including galena, sphalerite, pyrrhotite, chalcopyrite and pyrite; the aim being to produce deformation and annealing features in original undeformed ores for comparison with natural features of unknown origins.

Lyall and Paterson (1966) have demonstrated that the stress- strain behaviour of galena crystals is dependent on crystallo­ graphic orientation. The predominant plastic deformation mechanisms are intracrystalline gliding on {001} and <110> and associated kinking. After heating the deformed crystal at a temperature between 400°-600°C for about 120 hours, strain-free new grains have nucleated especially in the area of intense deformation bands. Stanton and Gorman (1968) have demonstrated that the rate 44 of annealing of sulphide minerals, including galena, sphalerite and chalcopyrite, is a function of temperature and confining pressure. The deformation features in sulphide ores can be eliminated by heating at appropriate temperature and the progressive stages of annealing can be quantitatively indicated by the movement of grain boundaries and the accompanying adjustment of triple-junction angles towards the equilibrium value, i.e. 120° in the case of single-phase aggregates.

Stanton and Willey (1970) have carried out annealing experiments on deformed galena-sphalerite ores from Broken Hill and demonstrated that, for heating the relevant ores under a given condition, galena can be completely recrystallized whilst the co-existing sphalerite is still in the deformed state. In 1971 they experimentally established that the progressive reduction of hardness of galena, due to work hardening, corresponds to recovery and recrystallization. In addition, following the sharp drop in hardness during the initial stage of primary recrystallization the hardness begins to increase again due to the development of preferred orientations in the aggregate. This phenomenon has been proved by their subsequent

experiment carried out on schistosed galena from Coeur df Alene,

Idaho (Stanton and Willey, 1972). Therefore, it has been shown

that any retention of measurable work hardening in natural

galena is an indication not only of original low temperature deformation but also of mild annealing history. In addition,

Stanton and Willey (1972) have shown that the grain shape and

grain size in the relevant foliated galena can be modified by heating at temperature just above 400°C, and they have concluded that it seems impossible for any galena to retain any deformation

features if it is subjected to temperature above 450°C. Boots (1072) has demonstrated, on heating slugs of deformed

(schistose) galena-sphalerite-pyrrhotite ores from the Browne Shaft orebody, Broken Hill in vacuo at 500°C for 30 days, that the deformed sulphide assemblage has been converted to one having similar features of prograde (i.e. high grade) annealed ore.

Clark and co-workers (Clark and Kelley, 1973; Salmon,

Clark and Kelley, 1974; Kelley and Clark, 1975) have carried out a series of deformation experiments on pyrrhotite, sphalerite, galena and chalcopyrite under laboratory conditions corresponding to those of expected shallow tectonic levels. The result has demonstrated that confining pressure has relatively little effect on the strain behaviour of the ores when compared with the effect of temperature. Collectively, the strength of the sulphide ores decrease consistently with increasing temperature and the dominant plastic deformation mechanisms are translation gliding and twinning. An interesting result emerging from these experiments is that the sulphide ores show remarkable change and even reversal in strength as one variable, i.e. temperature, is changed. A similar result has been reported by Atkinson (1974).

Therefore Clark and his co-workers have concluded that any single ranking of sulphide strengths is unlikely to be applicable to deposits deformed under a wide range of natural conditions.

Roscoe (1975) has demonstrated, on the basis of experimental - 2 deformation of chalcopyrite over the strain-rate range 10 to

10 ^sec \ that at temperatures above 150°C strength of chalcopyrite decreases regularly with decreasing strain-rate.

Similar results obtained from experiments on other geological samples have been reported by Donath and Fruth (1972). In the foregoing summary it is obvious that deformation 46

experiments, so far, have been carried out to investigate the effects of temperature, confining pressure and strain rates on the strength of some sulphide ores and to provide some indication of the plastic deformation mechanism of these ores in the earth’s crust. However the experimental results obtained have provided only a relatively small amount of information toward a better understanding of ore textures because the effect of a cycle of metamorphism on an ore deposit has been said to be also dependent on other factors, for example the initial sulphide assemblage with associated non-sulphide (i.e. gangue) minerals and their mutual intergrowth and chemical nature and the pressure of pore fluids, etc.

4.2 SUMMARY OF THE EFFECTS OF METAMORPHISM ON SULPHIDE ORES

Effects of metamorphism on sulphide deposits can be considered to result from the following processes: textural changes; mineralogical changes; migration of elements and metamorphic differentiation and the possibility of partial melting of the ore. On the basis of field observations and evidence obtained from deformation experiments on some sulphide minerals the sequence of events which a stratiform sulphide deposit undergoes during high grade regional metamorphism may be summarized thus:-

(a) During metamorphism the original non-sulphide constituents will be transformed to a high grade metamorphic mineral

assemblage accompanied by the development of schistosity

or gneissosity presumably by processes incorporating

metamorphic differentiation. Concurrently, similar

transformation of the sulphide fraction is expected, For

example, pyrite may be transformed to pyrrhotite or even

magnetite. 47

(b) Since a metamorphic cycle is always accompanied by

deformation which may consist of several episodes, such

as in the Willyama Complex, the ores will undergo plastic

deformation as indicated by the following: thickening

of ore layers in the hinges of folds; sulphides wrapping around boudins and brecciated fragments of silicate

minerals and injection of sulphide ore either along the

structural elements in the wallrock, e.g. schistosity,

or through the layering in the ore, or wallrocks. (c) Following deformation the ores will undergo recrystallization

and grain growth which may lead to some diversity of ore textural types. In the case of single phase aggregates, the texture will be characteristically polygonal with 120°

triple-junctions and straight to curved boundaries. In the case of two or more phases, the grain shape will be a function of the ratio of interphase grain-boundary free energy, to grain-boundary free energy, and therefore the grain boundaries are characteristically smooth curvilinear to arcuate shapes. Additionally, the minor phases frequently occur either as small lenticles along grain boundaries or as triangular bodies at triple-junction

points. However, the minerals possessing high idioblastic

property, such as arsenopyrite or loellingite, will retain

crystal outline against the lower ones. In addition,

numerous round inclusions of sulphides enclosed in porphyroblastic gangues or reversal arrangement gives rise

to poikiloblastic textures. (d) For very high grade metamorphism with the presence of flux

minerals such as calcite, fluorite and apatite (CO^,F,P2O, there appears to be a possibility of partial melting with 48

the development of metapegmatitic and metahydrothermal

solutions at the peak of the metamorphic cycle as

indicated by the formation of metapegmatite, metahydrothermal and metadeuteric veins containing well-

terminated crystals of gangue minerals in structures

discordant to the trend of surrounding rocks (Lawrence,

1967). (e) During metamorphism differential migration or ore

constituents seems to take place presumably due to

either ionic or atomic diffusion, or migration in response to strain variation, and result in the formation of

metamorphic layering and concentration of more mobile

elements including Pb, Cu, Ag, Co, Ni, As and Sb in low pressure areas, e.g. fracture zones in the wallrock; crests of folds and ore-wallrock contacts. (f) Mineralogical changes may occur at the peak of metamorphism i.e. in response to a change to conditions of high temperature and high pressure. Thus tetrahedrite, initially of complex composition, breaks

down to minerals of simpler composition. Similar changes may occur during the waning stages of metamorphism or

during retrogression - thus pyrrhotite breaks down to

pyrite or marcasite though it is not always certain this

is prograde or retrograde. The development of marcasite

(not stable over 400°C) would surely represent a

retrograde alteration.

In the case of a sulphide deposit which has been subjected to several metamorphic events, such as the main lode at Broken

Hill, it can be envisaged that the deposit may have undergone a repetition of the sequence of processes indicated above. 49

CHAPTER 5

THE PINNACLES ORES

5.1 GENERAL CHARACTERISTICS

From an examination of hand specimens, aided by binocular microscopic, and from polished section study, two main textural patterns can be identified.

(a) Distinctly Banded Ore

This ore-textural type (Figure 16) is best developed in the low grade ore in rocks having pelitic composition, however it can be found also in low-grade ore in rocks of original basaltic composition. In the former instance the ore consists of alternating quartz-garnet - sulphide mineral layers and pelitic layers with thicknesses ranging from a few millimetres up to 20 mm. The most common assemblages in these layers are respectively: quartz-garnet-biotite-arsenopyrite-pyrite, with minor sericite, gahnite, ilmenite, galena, sphalerite and chalcopyrite; and quartz-garnet with minor biotite and sericite.

The amount of sericite is variable and appears to reflect some retrogressive events. In the latter case (the basic rock-type), the ore consists of an alternating garnet-manganhedenbergite- brown hornblende-quartz-pyrrhotite assemblage with minor galena, sphalerite and ilmenite, and a garnet-manganhedenbergite-brown hornblende-quartz assemblage with minor sulphide minerals.

Boundaries between layers are usually sharp, but can be gradational and are frequently developed parallel to the foliation and lineation. A foliation is defined either by the preferred orientation of platy minerals such as biotite, or by the preferred orientation of lenticular aggregates of minerals 50

FIGURE 16 Banded ore (a hand specimen) 51

such as garnet (Figure 17). A lineation may be defined by the

preferred orientation of prismatic minerals, such as brown

hornblende and manganhedenbergite (Figure 18). In addition,

an individual layer appears to show gradation in grain-size

from coarse at one boundary to fine at the other.

In some polished specimens the ore layers have been deformed by the injection of sulphides from the sulphide-rich

layers across the layering. This leads to brecciation of the

gangue portion in the immediate vicinity of the sulphide "spurs”

and some fragments of the gangue are incorporated into the

sulphide bodies (Figure 19).

Microscopically, with respect to grain-boundary relationships in the banded ore, garnet and gahnite may occur as large idiomorphic crystals showing a poikiloblastic texture.

Additionally, garnet may occur as a single-phase polygonal aggregate or display an annealed polygonal texture, where occurring particularly with manganhedenbergite. On the contrary, manganhedenbergite and brown hornblende rarely display a polygonal texture, even when occurring alone, but rather form

a granular aggregate. Garnet and other gangues have also been drawn out into a series of en-echelon units and these are cut

at 90 degrees by microfractures (Figure 20 and see also Figure

17). Quartz almost always shows a polygonal or equiaxed texture

and biotite always occurs as anhedral flakes, with a preferred

orientation, scattered throughout the ore.

An annealing texture is always present with sulphide

minerals such as galena, sphalerite and pyrrhotite, which occur

as grains in polygonal configurations, or show much smoothing

of boundaries due to the reduction of surface free energy. The

result is the development of club-like, curved and curvilinear

boundaries. In general the grain boundaries between sulphides 52

FIGURE 17 Banded ore - showing foliation defined by preferred orientation of platy mineral (biotite) and lenticular aggregates of garnet (G). (Polished section x 60).

FIGURE 18 Banded ore - showing lineation defined by preferred orientation of prismatic brown hornblende (Hb). (Polished section x 60) . 53

FIGURE 19 Banded ore - showing a sulphide "spur" plastically injected across layering. (Polished section x 50).

FIGURE 20 Banded ore - showing flattened garnet (G) grains, and galena (Gn) plastically migratirg along microfractures within garnet grains. (Polished section x 50). 54 and gangue are observed to be controlled by the shape of the gangue. Therefore, the sulphides may either form polygonal or smoothing arcuate textures in the interstitial areas between the gangue minerals depending upon the number of phases appearing in the aggregates. Sulphide boundaries are commonly terminated abruptly against the grain faces of gangue minerals (Figure 21 and Figure 22).

Lenticular and triangular bodies of minor sulphides or gangues are commonly present respectively along the grain boundaries and triple-junction points between major sulphide and gangue minerals, and between two different sulphide or gangue phases. For instance, small lenticular bodies of galena commonly exist at the grain boundaries and triple-junction points of polygonal garnet and quartz. Similarly, lenticles of chalcopyrite are frequently found at the boundaries between galena and sphalerite. In addition, triangular composite grains of pyrrhotite and sphalerite, with occasionally tetrahedrite, galena and gudmundite, are present along the grain boundaries between garnet grains (Figure 23 and Figure 24). Arsenopyrite always occurs as idiomorphic crystals. It has never been found in arcuate boundary arrangement with other ore minerals. It often forms a composite crystal with a core of loellingite. In other instances, not uncommonly, arsenopyrite crystals are observed to contain inclusions of pyrrhotite and galena (Figure 25). Crystals of arsenopyrite have been occasionally observed to show stress twinning, and to be fractured (or even brecciated) and healed by migratory pyrrhotite.

Another interesting phenomenon often observed in the sulphide rich layering is that the sulphide minerals appear to wrap around

the smoothly shaped fragments of gangue, and additionally, if 55

FIGURE 21 Banded ore - pyrrhotite (Po) showing polygonal pattern with smooth arcuate boundaries but abruptly terminated at hedenbergite (Hd) grain faces. (Polished section x 150 oil) .

FIGURE 22 Banded ore - sphalerite (Sp) , galena (Gn) and ' chalcopyrite (Cpy) showing arcuate grain boundaries abruptly terminated against biotite (B) grains. (Polished section x 100). 56

FIGURE 25 Banded ore - discontinuous lenticles of chalcopyrite (Cpy) occurring at triple-junction points, along grain boundaries of sphalerite (Sp) grains. (Polished section x 400 oil).

FIGURE 24 Banded ore - triangular composite grains between spharerite (Sp) and gudmundite (Gd) or pyrrhotite (Po) and gudmundite (Gd) with small galena (Gn) lenticles occurring at triple-junction points and grain boundaries between garnets (G) or between garnet and hedenbergite (Hd). (Polished section x 100 oil). 57

FIGURE 25 Banded ore - the euhedral crystal of arsenopvrite (Asp) contains inclusions of galena (Gn) and pyrrhotite (Po). (Polished section x 150 oil).

FIGURE 26 Banded ore - showing sulphide minerals wrapped around fragments of silicate gangues (biotite and quartz) and having smooth grain boundaries. Quartz sometimes shows internal polygonal patterns (thin section x 50 partial x-nicols). 58

the fragments are quartz, they often show internal polygonal texture (Figure 26). Moreover, the sulphide minerals, especially

galena and sphalerite, appear ‘to migrate into the microfractures within the gangue grains such as garnet.

(b) Distinctly Brecciated Ore

Brecciated ore (Figures 27 and 28) is coarse grained and consists mainly of fragments of quartz, garnet and gangue-rich material embedded in the massive sulphide matrix. The gangue

fragments frequently display smooth boundaries with curved and

arcuate shapes, but irregular grain shape is not uncommon. The brecciated ore has also apparently lost all trace of primitive

layering. Microscopically, with respect to grain-boundary relationships in the brecciated ore, a number of grain shapes have been observed in the gangue fraction: (i) Fragments of single granets occur frequently as rounded grains, sometimes with numerous zonally arranged quartz (and other) inclusions. The rounded shape may be due to rolling of grains during plastic deformation and hence

compacted between the grains there is a lot of ilmenite

and sulphide minerals. In many of the ore specimens, parallel fracture planes have been superimposed (Figure

29) . (ii) Another intersting feature of brecciated fragments of

single grains is the smoothing out (partial annealing)

of grain boundaries to arcuate shapes (Figure 30). Consequently, brecciated fragments of individual grains

may equally undergo ’’fragment-boundary" migration of

formerly irregular grains, as experimental work by

Boots (1972) has established. 59

FIGURE 27 Brecciated ore (a hand specimen).

FIGURE 28 Brecciated ore - showing fragments of quartz (black) with smooth grain-boundaries embedded in recrystallized sulphide matrix. (Polished section x 50). 60

FIGURE 29 Brecciated ore - showing ilmenite (II) and other sulphide minerals occurring interstitiallv to rounded garnet grains with zonally arranged inclusion. (Polished section x 60). Note also * fractures trending top to bottom and possibly related to retrograde stresses.

FIGURE 30 Brecciated ore - fragments of single grain quartz showing smooth arcuate boundaries whilst garnet shows internal polygonal pattern. (Polished section x 50). 61

(iii) Many gangue fragments, especially quartz and garnet,

have retained remnants of annealed polygonal textures developed during previous recrystallization (Figure

30) . (iv) Quartz and garnet may have formed a new ’’disrupted"

polygonal aggregate resulting from imperfect

recrystallization or subsequent disruption. The breccia

is infilled with sulphides possibly by a filter-press plastic deformation process (Figure 31). Microscopically, the sulphide-rich fraction of the brecciated ore displays a variety of textures. Grain boundaries between galena and other sulphide minerals are commonly arcuate

(Figure 32). Sphalerite grains are frequently demarcated by lenticular bodies of galena, pyrrhotite and chalcopyrite (Figure 33). Exsolution intergrowth between sphalerite and chalcopyrite is occasionally observed (Figure 34). Likewise, graphic intergrowth between galena and tetrahedrite is present, but rare (Figure 35). In some cases the annealed textures of the sulphide ores have been shattered by subsequent microshears and healed by migratory siderite.

Some idiomorphic arsenopyrite crystals have also been observed in the brecciated ore. Composite grains of arsenopyrite with loellingite cores are not uncommon, and in places the arsenopyrite-loellingite crystals display stress twinning or have even been fractured. Galena and sphalerite have been plastically injected into mica (biotite) flakes which in turn wrap around quartz and garnet grains. The whole structure is embedded in an annealed galena-sphalerite matrix (Figure 36). Furthermore, metamorphic

"corrosion" of sphalerite by a silicate gangue has been 62

FIGURE 31 Brecciated ore - Quartz (Q) and Garnet (G) showing "disrupted” polygonal aggregate resulting from imperfect recrvstal1ization or subsequent disruption. (Polished section x 8 0).

FIGURE 32 Brecciated ore - Galena (Gn) , Sphalerite (Sp) and pyrite (Py) showing mutual arcuate grain boundaries. Note pyrite appears to be an alteration product of pyrrhotite having, as it does, a typical pyrrhotite lenticular shape. (Polished section x 50). 65

FIGURE 33 Brecciated ore - showing sphalerite (Sp) grains demarcated by the arrangement of small triangular or globular bodies of chalcopyrite (Cpy) , galena (Gn) and pvrrhotite (Po). (Polished section x 100 oil).

FIGURE 34 Brecciated ore - showing exsolution intergrowth between shpalerite (Sp) and chalcopyrite (Cpy). (Polished section x 50). 64

FIGURE 35 Brecciated ore - showing graphic intergrowth between galena (Gn) and tetrahedrite (Tt). (Polished section x 60 partial x-nicols).

FIGURE 36 Brecciated ore - showing sphalerite (Sp) plastically injected into biotite (B) flakes wrapping around rounded fragments of garnet and recrystallized quartz. (Polished section x 50) . occasionally observed. This resorption process has left a multitude non-replaced sphalerite particles sitting within the corroding "front" (Figure 37). In some areas of the brecciated ore, grain boundaries between galena matrix and fragments of sphalerite and gangue, are not arcuate, but irregular.

In the sulphide matrix, sub-structures in the major sulphide minerals, including galena, sphalerite, and chalcopyrite, have been revealed by treating the polished specimens with appropriate etchants. Galena areas almost always possess rather coarse sub­ grains, showing a polygonal texture with triple-junctions approaching 120°, and irregularly curved sub-boundaries (Figure 38). These would represent new grains derived by way of an initial sub-grain development.

Similarly, many areas of sphalerite show polygonal textures, and sphalerite grains possess annealing twinning with numerous lenticular bodies of chalcopyrite occurring along the grain- boundaries and twin boundaries (Figure 39). Annealing twin lamellae sometimes have very fine sub-grains superimposed on them (Figure 40). In the case of irregular sphalerite fragments, these usually possess curved and wedged-shape deformational twins, sometimes one set of deformational twins transects another set (Figure 41). Likewise chalcopyrite, present in the sulphide matrix or injected with other sulphides into fractures between gangue minerals, may display deformational twinning transecting- the lanceolate growth-twinning (Figure 42). 66

FIGURE 57 Brecciated ore - showing sphalerite (Sp) metamorphically corroded by silicate gangue. (Polished section x 50).

FIGURE 38 Brecciated ore - etching of galena brings out a fine polygonal recrystallized pattern. (Polished section x 100) . 67

FIGURE 39 Brecciated ore - etching of sphalerite brings out a polygonal pattern. Grains possess recrystallization twinning. Numerous lenticular bodies of chalcopyrite occur along twin- boundaries, grain boundaries, and at triple junctions. (Polished section x 100).

FIGURE 40 Brecciated ore - etching of sphalerite brings out polygonal patterns with grains possessing recrystallization twinning superimposed by fine- sub grains. (Polished section x 125). 68

FIGURE 41 Brecciated ore - etched sphalerite showing branched deformational twinning (A) cutting across an earlier twinning (B). (Polished section x 160).

FIGURE 42 Brecciated ore - etching of chalcopyrite brings out a complex twinning. (Polished section x 100) . 69

5.2 ORE MINERALOGY

Previous mineragraphic work on the Pinnacles ores has been given by Stillwell (1926 and 1955) ; Ramdohr (1950) , and Ayres (1962). The interpretation of some replacement textures of the ore minerals by Stillwell is no longer tenable in the light of recent work. In this section emphasis is placed on aspects of particular ore mineral occurrence and on features related to their deformation and recrystallization. Fourteen ore minerals have been identified in the present study and categorised according to order of abundance: two in major amounts, seven in minor amounts and five in trace amounts. The number of ore minerals recorded may increase as additional ore specimens become available.

Major and Minor Components

Galena (PbS) The most abundant lead ore, galena, is the predominant constituent of the lead lode (see Ayres, 1962), and is also common to all other mineralized horizons. Galena has a variety of modes of occurrence. These include irregular patches with other sulphides surrounded by gangue fragments in brecciated ore (Figure 28); as small sulphide "spurs" cutting across the layering (Figure 19); as small lenticular bodies along grain boundaries and at triple-junction points in polygonal aggregates of gangue minerals such as garnet and quartz (Figure 24); as thin films injected along fracture planes in garnet and quartz grains, or along cleavage planes of biotite (Figure 20 and

Figure 22); and as small rounded inclusions with other minerals in garnet porphyroblasts. Polished sections of galena-rich specimens show galena with arcuate and curved grain boundaries 70 intergrown with sphalerite and other minor sulphide minerals

(Figure 32). Intergrowths of galena and tetrahedrite are also present but rare (Figure 35). . After etching with thiourea-HCl solution galena almost always displays a well-developed sub­ structure with irregular to curved sub-boundaries and the sub­ boundary junction angles generally close to characteristic triple-junction (Figure 38).

Sphalerite (ZnS)

Sphalerite is the principal sulphide mineral in the zinc lodes but less abundant in the lead lode. Like galena, it is distributed throughout all mineralized horizons (Ayres, 1962).

Texturally, sphalerite and galena possess similar features. An exsolution texture between sphalerite and chalcopyrite is quite common (Figure 34). Occasionally composite grains of sphalerite- pyrrhotite-galena-tetrahedrite or sphalerite-pyrrhotite- gudmundite are observed along grain boundaries or triple-junction points of major sulphides and silicate gangues (Figure 24). After etching with thiourea-HCl solution, many sphalerite areas show polygonal patterns with the individual grains possessing polysynthetic recrystallization twinning, and sometimes superimposed fine sub-grains (Figure 40). However, curved multiple deformational twinning is frequently observed in sphalerite fragments embedded in galena matrix (Figure 43), and sometimes one set of such deformational twinning cuts across an earlier one (Figure 41).

Pyrrhotite Pyrrhotite occurs as a minor component throughout the ore horizons especially in the layering in garnet-hedenbergite-quartz granulite rocks. Polygonal patterns with curved boundaries and 71

FIGURE 43 Brecciated ore - after etching,fragments of sphalerite show slightly bent deformation twinning, whilst galena shows fine sub grain texture. (Polished section x 160).

FIGURE 44 Etching of pyrrhotite brings out a complex ~ twinning overprinted on the polygonal pattern. (Polished section x 100). 72

triple-junction points are frequently observed in pyrrhotite masses (Figure 21). Pyrrhotite is frequently observed altered to pyrite and marcasite. After etching with 3% l^O^-NH^OH solution pyrrhotite shows complex polysynthetic twinning

(Figure 44) almost certainly induced by stress.

Chalcopyrite (CuFeS^) Chalcopyrite occurs as a minor component and is sporadically distributed throughout the lode horizons. From a textural standpoint chalcopyrite is found to be of two generations. The first generation is found as irregular patches associated with galena and sphalerite injected into fractures in silicate grains such as quartz (Figure 38). Second generation chalcopyrite occurs as composite grains with galena, pyrrhotite and gudmundite at the grain boundaries of polygonal aggregates of silicate gangues such as garnet and quartz. It also forms exsolution intergrowths with sphalerite (Figure 34). After etching with 3% I^C^-NH^OH solution, chalcopyrite frequently shows complex polysynthetic twinning (Figure 42).

Arsenopyrite and Loellingite (FeAsS and FeA$2) Arsenopyrite usually has a core of loellingite and occurs as euhedral crystals distributed throughout the lode horizons but is the predominant component in the low grade layered ore in rocks of pelitic composition (Figure 16). The grain shape of arsenopyrite is sometimes governed by silicate grain shapes, especially garnet (Figure 45). Small inclusions of galena, pyrrhotite and quartz are occasionally observed in euhedral crystals of arsenopyrite. Stress twinning in arsenopyrite crystals is not uncommon. Brecciated arsenopyrite crystals are usually healed by other migratory sulphide minerals (e.g. 73

FIGURE 45 Arsenopyrite grain-shape is governed by garnet aggregate. (Polished section x 80).

FICURE 46 Ilmenite (II) showing co-recrystallization with garnet (G). (Polished section x 100). 74 pyrrhotite).

Pyrite and Marcasite (FeS^)

Pyrite and marcasite are rather abundant in the lode horizons. They are intimately and paragenetically associated with pyrrhotite, as alteration products. Therefore pyrite and marcasite usually simulate characteristics of pyrrhotite and, unlike arsenopyrite, pyrite is never found as euhedral crystals. In addition, concentric fractures are sometimes observed in pyrite masses.

Tetrahedrite (CuFeAgHgZn) ^(SbAs)^ ^_ Tetrahedrite is found as small to trace amounts in some ore specimens. It occasionally forms pseudographic intergrowth with galena, but generally occurs as very small composite grains associated with galena, sphalerite, chalcopyrite and pyrargyrite. The chemical composition of tetrahedrite, written above, is liberally taken from that of tetrahedrite occurring in the main lode Broken Hill and suggested by Lawrence (1968).

Accessory and Trace Minerals

Ilmenite (FeTiO^) Ilmenite is commonly observed in almost all the specimens studied and is fairly abundant among the accessory minerals.

Texturally, ilmenite may have formed in two generations. For the first generation ilmenite appears to occur as zoned minute inclusions associated with biotite and some sulphide minerals in the rounded garnet fragments, and in the second generation, ilmenite occurs as fairly coarse grains, frequently with polygonal texture, in the interstices between garnet grains (Figure 46). 75

Jamesonite (Pb^FeSb^ S-, ^ )_ Jamesonite is sometimes found as an accessory in brecciated ore between galena and sphalerite grains or at the junctions between adjacent galena (Figure 35).

Gudmundite (FeSbS)

Gudmundite (Figures 47 and 48) commonly occurs as composite grains with other sulphide minerals or along grain boundaries of annealed silicate and/or sulphide aggregates. It appears mainly to have been developed from the breakdown of tetrahedrite.

Pyrargyrite (Ag^SbS)

Pyrargyrite is seldom observed in polished sections, however, it is found as very small discrete grains in galena or as small composite grains associated with tetrahedrite and chalcopyrite at the annealed grain boundaries between galena and sphalerite.

Graphite (C) Small flakes of graphite (Figure 49) are sometimes found in the large garnet grains. Also, in some polished sections of brecciated ore, quite a few small graphite flakes are found bent and embedded in the sulphide mass. 76

FIGURE 47 Gudmundite (Gd) showing an intergrowth with galena (Gn). (Polished section x 200 oil).

FIGURE 48 Gudmundite (Gd) showing an intergrowth with pyrrhotite (Po). (Polished section x 200 oil). 77

FIGURE 49 Graphite flakes (Gp) enclosed within a garnet grain. Note arsenopyrite (Asp) having core of loellingite and showing stress twinning. (Polished section x 50 X-nicols). 78

CHAPTER 6

EFFECTS OF REGIONAL METAMORPHISM ON THE PINNACLES OREBODY

It has already been demonstrated, in Chapter 2, that the

Pinnacles orebody lies within an area of hornblende-granulite facies rocks and on the basis of a syngenetic time relation between rocks and ore, in their primitive state, the ore is considered to have undergone prograde metamorphism. The immediate area, however, is also characterized by the presence of major shear zones wherein the prograde rocks have suffered retrogression to lower amphibolite facies, with even lower grades of retrogression evident in places. These major retrograde shear zones, whilst not visibly intersecting the Pinnacles orebody would have transmitted stress at relatively low temperatures into the orebody. It would be expected, therefore, that the Pinnacles ore, having already been retextured by prograde effects, would now show some evidence of a superimposed retexturing of a retrograde nature.

6.1 EFFECTS DUE TO PROGRADE METAMORPHISM

Of two periods of deformation (and possibly an earlier one) all at lower granulite grade only the latest of these deformations (in terms of recovery recrystallization) is presumed to be presently visible in the ore.*

Since in metallurgy an annealed metal can be again deformed whereupon it may again anneal ad_ infinitum. 79

Brecciation and Plastic Flow of Sulphide Minerals

The most common feature ascribed to this process is the occurrence of intensely brecci’ated ore consisting of rounded garnet and quartz grains wrapped by sulphides. The mechanism can be envisaged as a type of rolling and grinding, i.e., the silicate grains may be rolled within a plastic ’’matrix" of a galena-sphalerite rich "fluid" - a "fluidized bed" process. In some instances, it is envisaged, the mass consisted of irregular fragments of harder minerals with deformable sulphides plastically injected into interstices. Furthermore, some gangue minerals, especially quartz fragments, exhibit internal polygonal patterns suggesting an earlier recrystallization prior to the plastic movement of the sulphide components (Figure 50). Another line of evidence for the plastic flow of the sulphide ore is the occurrence of small sulphide spurs and veinlets consisting of galena, chalcopyrite and a minor amount of sphalerite injected across the layering in the low grade banded ore. The foregoing phenomena are considered as taking place towards the end of deformation after the gangue fraction became consolidated, and the sulphides were still mobile (Maiden, 1972 and 1975).

Recovery and Recrystallization of the Ore Minerals

Following or even accompanying the deformation, as the metamorphic intensity waned, the ore minerals underwent recovery and recrystallization. The type of prograde textures that result involve the initiation of new strain-free grains and subsequent grain-boundary migration (i.e., secondary recrystallization or grain growth process) giving rise to a coarsening of grain-size of the ores. They also yield grain shapes indicative of surface 80

FIGURE 50 Fragments of quartz, showing internal polygonal pattern and co-recrystallized sphalerite, all embedded within galena-sphalerite matrix. (Etched polished section x 60). 81 tension adjustment between juxtaposed grains, and reduction of excess surface-energy as manifested by the elimination of irregularly shaped boundaries. Ore-gangue intergrowth textures involving co-recrystallization or sulphides and silicates are most significant. Garnet and quartz frequently display a classic foam structure in which galena and sphalerite as minor phases commonly migrate to triple-junction points, or spread as films or as small lenticular bodies along grain-boundaries (Figure 51a and Figure 51b).

Co-recrystallization of hedenbergite with garnet and galena, and exhibiting a polygonal texture, is most significant. Hedenbergite and garnet possess smoothly curved or curvilinear interphase grain-boundaries whilst galena forms films or discontinuous globules along garnet grain-boundaries (Figure

52a and Figure 52b). In other instances isolated sulphide grains, especially galena, sphalerite and sometimes pyrrhotite, assuming spherical shapes, are found within the garnet, quartz and occasionally hedenbergite grains. This phenomenon may be indicative of a spheriodization process taking place (Figure 52a). In sulphide-rich areas, distinctive recrystallized patterns are observed in intergrowth of galena-sphalerite-pyrrhotite.

Curved and lobate shapes analogous to the typical textures of some annealed metals are exhibited (Figure 53). Commonly minor sulphide phases, such as galena and chalcopyrite, occur lying discontinuously along inter-grain boundaries of major sulphides

(e.g., sphalerite), and at triple-junction points (Figure 23 and

Figure 33). Recrystallization textures in gangue minerals, especially garnet and quartz, are very common and noteworthy (Figure 54a and Figure 54b). Grain-shapes of garnet and quartz with equiaxed 82

FIGURE 51a Co-recrystallization of quartz (Q), with garnet (G) showing zoned inclusions, and galena (Gn). Note: minute globules of galena and quartz at garnet grain-boundaries. (Polished section x 50).

FIGURE 51b Co-recrystallization of hedenbergite (Hd) with quartz (Q) showing polygonal pattern, ilmenite (II), and pyrrhotite which’As partially altered to marcasite or formed a composite grain with gudmundite (Gd). Note (i) minute globules or film of galena occurring at ilmenite-quartz or quartz boundaries; (ii) spheroidized pyrrhotite enclosed within hedenbergite (polished section x 60) . 83

FIGURE 52a Co-recrystallization of garnet (G), having spherical inclusions of hedenbergite, quartz and pyrrhotite, with hedenbergite (Hd) . Note: galena occurring as a film or minute lenticles along garnet boundaries. (Polished section x 50) . 84

FIGURE 52b Co-recrystallization of garnet (having inclusions of quartz, hedenbergite and sphalerite) with hedenbergite. Note: small lenticles of galena or composite grain between pyrrhotite and gudmunaite and hedenbergite occurring along garnet grain-boundaries. (Polished section x 150). FIGURE 53 Co-recrystallization of galena (Gn) with sphalerite (Sp) showing arcuate grain boundaries. (Polished section x 100). 86

FIGURE 54a Recrystallized garnet showing a foam texture. (Polished section x 50).

FIGURE 54b Recrystallized quartz showing a foam texture. Note: sphalerite and siderite grains occurring at triple-junction points. (Polished section x 60) . 87

patterns are commonly observed. Co-recrystallization of quartz, garnet and hedenbergite is common with usually straight to slightly curved boundaries exhibited (Figure 55). In other instances, some sizable isolated grains of hedenbergite and quartz of round shape (i.e., spheroidized) are sometimes found within polygonal garnet grains.

6.2 MINERALOGICAL CHANGE DURING PROGRADE METAMORPHISM

The presence of apparently homogeneous ore constituents and the paucity of exsolution intergrowths in the Pinnacles sulphide assemblages, suggests the homogenization of the ores during high grade metamorphism.

During the period of falling temperature following the peak of metamorphic crystallization, the high temperature minerals, no longer stable, underwent decomposition or alteration.

Of most common occurrence is the breakdown of complex tetrahedrite to produce composite grains comprising minerals such as chalcopyrite, gudmundite, pyrargyrite and pyrrhotite. This chemical breakdown was firstly noted by Ramdohr (1950) in his study of the ores from the Main Lode at Broken Hill, and subsequently in more detail by Lawrence (1968). In other instances pyrite and marcasite occur as pseudomorphs after pyrrhotite suggesting that the former minerals are alteration products of the latter. The presence of a loellingite core in the euhedral crystals of arsenopyrite may be attributed to the incomplete alteration of initially formed euhedral crystals of the former mineral with loss of sulphur during a later phase of prograde metamorphism. Furthermore, the presence of numerous inclusions of galena, pyrrhotite and silicate minerals in euhedral crystals of arsenopyrite may be 88

FIGURE 55 Co-recrystallization of quartz (Q) with ilmenite (II), hedenbergite (Hd) (showing exsolution lamellae), and garnet (G) (having zoned inclusions. Note: minute globules of galena and quartz at garnet grain-boundaries. (Polished section x 100 oil.) 89 indicative of earlier formed porphyroblasts (Ramdohr, 1950).

Mineralogical complexity of the ores may also be a result of exsolution and solid state reaction. The most conspicuous exsolution feature* observed is the exsolution of chalcopyrite and pyrrhotite from sphalerite. On the other hand, the formation of jamesonite may be ascribed to the reaction between tetrahedrite and galena (Maiden, 1972). Other solid-state reactions have also been noted (Lawrence, personal communcation).

The foregoing demonstrates that, from a textural stand point, the effects of prograde metamorphism are complex. For example, strain rate was variable: where the strain rate was low, recrystallization took place; where strain rate was high, fragmentation (e.g., of sphalerite) or plastic migration (e.g., of galena) took place in a "fluidized bed" manner. During the waning phase of metamorphism the ores become more mineralogically diverse as the result of exsolution, solid state reaction and decomposition of mineral phases formed at elevated temperatures.

6.3 EFFECTS DUE TO RETROGRADE METAMORPHISM

Since no major retrograde shear zones transect the Pinnacles orebody (as in the main lode Broken Hill, e.g., British Shear etc.), retrograde processes as described by Richards (1966),

Boots (1972) and Lawrence (1973) are not as prominently developed.

The major shears (e.g., Thackaringa - Pinnacles) at some distance from the Pinnacles would, however, have had the effect of mild strain on the orebody at low temperatures. Thus four main effects are observed:

•jjj It should be noted, however (p.87), that exsolution inter­ growths among sulphide minerals are not developed on a substantial scale. 90

( i)____Further Brecciation of Prograde Grain Aggregat.es

Microshears assumed to be of retrograde origin, have frequently been seen in the polished sections of the ores. The microscopic features of the ore within these shears are

characterized sphalerite, which still exhibit prograde grain- boundaries (Figure 56). In some places, where strain rate was high, complete destruction of prograde - textured galena and sphalerite grains resulted. Thus angular fragments of these

sulphides are observed to have been rafted along by migratory

siderite which forms narrow cross-cutting veinlets within the ore.

Brecciation of associated gangue minerals is common. Garnet and quartz are frequently fractured with small displacements.

The fractures are healed by sphalerite or galena, or both. Biotite flakes, with sphalerite occurring in between individual flakes, are microfolded and wrap around fractured garnet grains suggesting that sphalerite and biotite have formed around earlier textured garnet (Figure 36).

(ii) Microshearing or Fracturing in a Specific Direction Amid Previously Retextured Grains^ A most common occurrence is a persistent set of parallel fracture planes developed within the retextured garnet grains of

the low grade banded.ore. The direction of the retrograde

fractures is almost perpendicular to the lineation of the ore defined by the alignment of stretched garnet grains. These

fractures are infilled with migratory galena (Figure 20).

Similar microfractures developed within rounded garnet

grains of the compact brecciated ore are also of common occurrence (Figure 29). 9.1

FIGURE 56 Brecciation of sphalerite-galena grains, possessing prograde annealing texture and healed by migratory siderite (Sd). (Polished section x 50). 92

(iii) Development of Complex Stress-Twinning in Sulphide Minerals

After etching with thiourea-HCl etchant sphalerite-rich areas from the brecciated ore -exhibits twinning of a complex nature. Some retextured sphalerite grains possess slightly bent

polysynthetic recrystallization twins whilst other sphalerite

grains show polysynthetic twin lamallae with branching outlines

indicative of a deformation origin, as noted by Richards (1966)

in ore from the retrograde shear zones transecting the main lode

Broken Hill orebodies. The deformation twinning is occasionally observed cutting across an earlier similar set (Figure 41).

Similarly, chalcopyrite associated with the foregoing

sphalerite shows complex twinning with a set of fine twin lamellae developed across the broad lanceolate twinning. The

former twin set may have been formed by retrograde deformation

(Figure 42). Likewise, a complex twinning similar to that of chalcopyrite is also observed in the polygonal pyrrhotite aggregates (Figure 44).

(iv) Development of Sub-grain Structures Sub-grain structures (Figures 38 and 40) are often super­ imposed or overprinted on prograde grains of sphalerite and galena. After etching galena-rich areas of the brecciated ore,

galena almost always shows well developed subgrains. In general the subgrains are not uniform in grain-size but nevertheless they

possess curved subgrain boundaries which frequently meet in

groups of three at angles approaching those of typical annealing

texture.*

It appears therefore that sub-grains could eventually become new grains if the annealing mechanism persists. 95

On the contrary, sphalerite occasionally shows very fine subgrains developed within broad twin-lamellae of reconstituted sphalerite aggregates. This evidence leads to a suggestion that in some places the retrograde strain rate as well as the strain intensity are sufficiently low to allow sphalerite to undergo lattice strain without fragmentation. 94

CHAPTER 7

DISCUSSION AND CONCLUSIONS

Earlier workers on The Pinnacles geology (King, 1950, 1953;

Ayres, 1962; Burns, 1965; and Johnson, 1966) have recognized that the Pinnacles orebody appears to have a similar genesis to the main lode at Broken Hill and the orebody, on the basis of syngenesis, is believed to have undergone regional metamorphism.

Similarly, Ramdohr (1950) has demonstrated that the ore-gangue intergrowths are the result of metamorphism of the ore.

Subsequent structural, petrological and chronological studies on various parts of the Willyama block (Hobbs, 1966; Pidgeon, 1967; Williams, 1967; Vernon, 1968, 1969; Shaw, 1968; Hobbs et al., 1968; Vernon and Ransom, 1971) show that the orebody has undergone two distinct periods of metamorphism. The first period, at lower granulite grade occurred about 1,700 m.y. ago, was accompanied by at least two separate deformationa1 episodes with possibly an even earlier metamorphism having taken place. The second metamorphic event at lower amphibolite facies or even lower grade closed about 500 m.y. ago; it is generally restricted to narrow shear zones locally transecting the orebody.

Recent work has put new light on the geochronology of Broken

Hill rocks: Dr. Plimer (personal communication) suggests that the retrograde schist zones were initiated during the waning stages of the high metamorphism and become isotopically closed systems about 500 m.y. This idea is based on the field observation that the retrograde shear zones of the Willyama

Complex do not cut across the later Pre-cambrian Adelaidean rocks to the north (Le Couteur, personal communication). Therefore it 95

leaves no doubt that the Pinnacles ore has been reconstituted by the 1,700 m.y. regional metamorphism.

Maiden (1972) and Lawrence (1975) have demonstrated that sulphide ores of the main Broken Hill Lode have undergone

intense brecciation, especially of the more brittle components.

This occurred during high grade metamorphism at a time wThen deformable minerals were yielding by plastic flowage. Following this the ore and the gangue minerals (where amenable) underwent recrystallization including co-recrystallization of sulphides and silicates. If metamorphism is accompanied by several periods of deformation, the plastic flow associated with each stage will virtually obliterate the ore-textures developed during the preceding stage.

Concurrently Boot (1972) and Lawrrence (1973) have indicated that the main Broken Hill lode, outside retrograde shear zones, has not been folded by the "shear zone" event (500 m.y. metamorphic event). Although the retrograde shear has not affected the silicate gangue existing in ores away from the retrograde areas, Lawrence (1973) has suggested that this force could have induced a mild widespread deformation, overprinted on the prograde ore.

From the microscopic observation carried out on some 200 polished sections of The Pinnacles ore, an attempt has been made to characterize a number of ore-textural types that are indicative of effects due to prograde and retrograde metamorphisms.

Ore-textural features, characteristic of the effects of prograde metamorphism are:

(i) Evidence of small sulphide "spurs" cutting across layering

in the low grade ore and showing polygonal texture within

the infilling minerals. The existence of brecciated ore 96

consisting of rounded fragments of silicate gangues

(such as garnet and quartz) embedded in coarse-grained recrystallized sulphide matrix indicates the plastic movement of the ore during the prograde metamorphism.

(ii) Topological patterns resulting from grain-boundary

migration (i.e., recrystallization), yielding shapes

indicative of surface-tension adjustment between

juxtaposed grains, and the attendant reduction of excess

surface free energy manifested by the elimination of irregular-shaped boundaries, suggest equilibrium under

a high-grade metamorphic regime. Co-recrystallization between sulphide minerals and silicate gangues

(including the pyroxene manganhedenbergite) is most significant in this respect.

(iii) High-temperature decomposition of such dynamothermally unstable minerals as tetrahedrite to yield a series of chemically simpler minerals including gudmundite, pyrargyrite, pyrrhotite and chalcopyrite is a typical widespread characteristic indicative of higher metamorphic temperatures having been reached. Partial or even complete alteration of pyrrhotite with the

development of pyrite and marcasite is ubiquitous.

Additionally, exsolution of chalcopyrite and pyrrhotite from sphalerite is not uncommon.

By contrast, retrograde textures include:

(i) Development of a sub-grain structure in readily

deformable minerals such as galena and sphalerite

superimposed on the foregoing sulphide grains that show

remnant prograde textures. (ii) Development of complex low-deformation twinning in

minerals such as chalcopyrite, sphalerite and pyrrhotite, in ore from prograde areas indicative of subsequent retrograde deformation. (iii) Partial brecciation, or even complete brecciation in

some instances, of prograde ore with a small

displacement by microshears of retrograde nature.

(iv) Development of microfractures in a specific direction,

usually normal to the lineation of prograde ore.

The foregoing observations from polished sections of

Pinnacles ore show that it has undergone similar processes of modification as were observed by Lawrence (1973) in ore from the main Lode Horizon. In summary the history of the Pinnacles ore is listed below: (i) Primitive sulphide and silicate sediment.

(ii) Lithification and grain development. (iii) High grade (granulite) metamorphism in three phases

(Mq , M-j , M^) present textures of prograde nature, especially sulphide-silicate grain equilibration, is

considered to relate mainly to . (iv) Low grade (amphibolite) deformation evidenced mainly

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