Deposition, Diagenesi5 and Mineralization of Sediments in the Fal Estuary, Cornwall

DEPOSITION, DIAGENESI5 AND MINERALIZATION OF SEDIMENTS IN THE FAL ESTUARY, CORNWALL

MARTYN GEOFFREY THORNE

Submitted for the degree of Ph.D. Imperial Collage, University of London. November 1582 DEDICATION

To my mother, Joan and

in memory of my father

Frederick. ACKNOWLEDGEMENTS

The valuable assistance of my supervisor Prof. D. Shearman, and Mr. J. McM Moore and Dr. P. Bush who were involved in the initial-planning of this research is gratefully acknowledged. My thanks are also due to former colleagues at Wheal Jane Tin Mine, Nr. Truro uho performed some'chemical analyses and many colleagues at the Royal School of Mines for their helpful advice. I uould also like to thank my uife, Pauline for assistance throughout the lengthy course of this research and much support and encouragement along the way. Finally, I am deeply grateful for the wonderful practical support of my parents over several years.

Falmouth, November 1982. ABSTRACT

The Fal Estuary of S.U. Cornwall has six major tributaries, three of which contain sediments consisting of various admixtures of marine derived quartz sand and polymineralic fine sand and silt which is partially derived from historical mining activity in the locality. The Tresillian River and Restronguet Creek have been studied partly to determine the sedimentological characteristics of these materials but also to investigate the very high concentrations of particulate and dissolved copper, zinc, tin, arsenic and iron which are introduced into the estuary along with the mine waste and are further concentrated in the sediments. As a second stage of this study samples of these mineralized sediments have been heated and compressed in order to assess whether simulated burial diagenesis can yield evidence on the formation of ore in siliciclastic rocks.

Results from the field areas demonstrate that a continuous series of sedimentary environments from creek mouth complex to saltmarsh are developed from the mouth of Restronguet Creek to the head of the Tresillian River. Uithin these sediments a zoned arrangement of iron oxide, iron sulphide and iron rich silicate minerals is developed on the creek mouth complex and upstream many of the finer grained sediments are rich in organic materials and have been mineralized by assemblages of metal sulphides including bornite (Cu^FeS^) and chalcopyrite (CuFeS2). Sulphide mineral concentrations, mineral species and even mineral textures vary widely and depend to a great extent on very local environmental conditions. Sediment pore waters contain high concentrations of heavy metals and are greatly influenced by tidal flux through more permeable sediments. Major ion concentrations are much less variable.

In the laboratory experimental studies demonstrate that heating and compressing mineralized sediment from the estuary causes major recrystallization of sulphide mineral 3 phases and releases significant quantities of zinc, copper, and other heavy metals into expelled pore waters. This result provides good evidence that early diagenetic sulphide mineralization in siliciclastic sediments is largely consolidated uithin the sedimentary unit by burial diagenesis but that metal ' enriched pore waters may also be generated and could act as mineralizing fluids elsewhere in the sedimentary sequence. Furthermore the experimental technique employed is sufficiently proven to offer the possibility of evaluating the effects of burial condition and source material on metal provenance and potential mineralizing capacity• 4

TABLE OF CONTENTS Page Acknowledgements Abs tract 2 Table of Contents 4 List of Figures 9 List of Plates 14

INTRODUCTION THE FORMATION OF ORES IN SI LI CICLASTIC SEDIMENTS 16 THE SCOPE OF THE RESEARCH AND THE PROCEDURES ADOPTED 19 AREA SELECTED FOR STUDY 19 PHYSICAL AND CHEMICAL METHODS 20 REVIEW OF PREVIOUS WORK 21

PART I THE FIELD AREAS

CHAPTER 1 GEOLOGICAL, PHYSIOGRAPHICAL AND HISTORICAL SETTING OF THE FIELD AREA 27 CHAPTER 2 THE STRUCTURE AND SEDIMENTARY ENVIRONMENTS OF THE FIELD AREA 33 The Creek Mouth Complex 33 The Creek Mouth Complex: flood channels and flood ramp 34 The Creek Mouth Complex: ebb shield 38 The Intertidal Flats 42 The Intertidal Flats: lower division 42 The Intertidal Flats: upper division 45 The Channel Slopes 48 The Point Bars 48 The Tributary Creeks and Streams 51 The Marginal Environments 54 Artificial Environments 62 CHAPTER 3 THE PHYSICAL CHARACTERISTICS OF THE SEDIMENTS AND BULK MINERALOGY 64 3.1 GRAIN SIZE, NATURAL MOISTURE CONTENTS AND BULK SAMPLE MINERALOGY 68 The Creek Mouth Complex 68 5

Page

The Intertidal Flats 71 \ Intertidal Flats: lower division 71 Intertidal Flats: upper division 72 Channel Slopes 75 Point Bars 76 The Streams and Creeks 77 Marginal Environments • 77 Artificial Environments 73 3.2 SULPHIDE MINERALOGY 79 Restronguet Creek: The Creek Mouth Complex 80 Restronguet Creek: The Intertidal Flats • 81 Restronguet Creek: The Marginal Environments 84 Tresillian River Environments 92 Summary gg CHAPTER 4 SOME CHEMICAL CHARACTERISTICS OF THE SEDIMENTS 103 Determination of Metals arrd Sulphur Species - general 103 Determination of Sulphur Species 108 Determination of Sulphur Species: Total Sulphur 109 Determination of Sulphur Species: Acid extractable hydrogen sulphide 111 Determination of Sulphur Species: Pore water sulphate 113 Determination of Total Carbon 113 Determination of Tin and Arsenic 114 4.1 METAL CONCENTRATIONS 112 IRON 118 MANGANESE 123 COPPER 126 ZINC 126 LEAD 131 TIN AND ARSENIC 131 METAL CONCENTRATIONS IN THE SEDIMENTARY ENVIRONMENTS 139 Restronguet Creek Mouth Complex: Flood Ramp and Channel 139 Restronguet Creek Mouth Complex: Ebb Shield 1 Rastronguat Creek: Intertidal Flats 146 Restronguet Creek: Marginal Environments 153 Restronguet Creek: Artificial Environments 157 Tresillian River 157 6 Page Tresillian River: Intertidal Flats 164 Tresillian River: The Point Bars 164 4.2 SULPHUR'DISTRIBUTION AND MODE OF OCCURRENCE OF TOTAL SULPHUR CONCENTRATIONS 165 Restronguet Creek: Creek Mouth Complex 165 Restronguet Creek: Intertidal Flats 165 Restronguet Creek: Marginal Environments 168 Tresillian River 170 MODE OF OCCURRENCE OF SULPHUR 170 Pore Water Sulphate 171 Sulphur as authigenic monosulphide 174 Detrital sulphur 182 Authigenic polysulphide mineralization 183 4.3 TOTAL CARBON CONCENTRATIONS 194 CHAPTER 5 SOME CHEMICAL CHARACTERISTICS OF THE SURFACE WATERS AND SEDIMENT PORE WATERS 200 Determination of the Chemistry of the Pore Waters 202 Determination of the Chemistry of the Surface Waters 203 5.1 MA30R ION CHARACTERISTICS OF SURFACE AND SEDIMENT PORE WATERS 208 THE SURFACE WATERS 208 THE PORE WATERS 217 Restronguet Creek: The Creek Mouth Complex 217 Restronguet Creek: The Intertidal Flats 217 Restronguet Creek: Marginal Environments 221 Tresillian River: Intertidal Flats 225 Tresillian River: Point Bars 227 BEHAVIOUR OF MAJOR IONS 227 Chloride 227 Calcium 227 Potassium 231 Sodium 231 Magnesium 231 5.2 TRACE METAL AMD pH CHARACTERISTICS OF SURFACE WATERS RESTRONGUET CREEK 232 Iron 232 Manganese 235 7 Page Copper 238 Zinc 241 Lead 241 TRESILLIAN RIVER 243 Iron 243 Manganese 244 Copper 244 Zinc 245 Lead 245 CHAPTER 6

PART 1 - CONCLUSIONS 246

PART II THE EXPERIMENTAL STUDIES 251

CHAPTER 7 EXPERIMENTAL APPARATUS, PROCEDURES AND RESULTS 7.1 APPARATUS EQUIPMENT AND EXPERIMENTAL PROCEDURES 251 The Big Cell 251 The "Small Cell" 254 Experimental difficulties 254 Experimental difficulties: Pore fluid pressure control 256 Experimental dif ficu-lties : Contamination 258 Experimental difficulties: Hydraulic Oil Contamination 260 Experimental difficulties: Sample Collection 261 Experimental difficulties: Thermal Expansion Effects 261 Experimental Procedures 261 7.2 THE TESTUORK 262 7.3 CHEMICAL CHARACTERISTICS OF THE EXPELLED PORE FLUIDS 264 TRACE METAL CONCENTRATIONS 264 MAJOR ION CONCENTRATIONS 275 OTHER OBSERVATIONS 287

7.4 CHEMICAL CHARACTERISTICS OF THE SOLID RESIDUES 288 TRACE METAL CONCENTRATIONS 288 SULPHUR CONCENTRATIONS 291 7.5 SULPHIDE MINERALOGY OF THE SGLID RESIDUES 294 Restronguet Creek: Creek Mouth Complex 295 Restronguet Creek: Intertidal Flats 297 Tresillian River: Lower Point Bar 300 8 Page Tresillian River: Intertidal Flats 302 Tresillian River: Upper Point 8ar 302 CHAPTER 8 PART II - CONCLUSIONS 305

Appendix 308 References 318 LIST OF FIGURES Page

Fig. 1 Location of field areas 28 Fig. 2 Restronguet Creek; Structure of the Creek Mouth Area 31 Fig. 3 T.resillian River; Environmental Divisions 32 Fig. 4 Restronguet Creek; Structure of the Creek Mouth Complex 35 Fig. 5 Restronguet Creek Mouth Complex 39 Fig. 6 Some commonly recorded organisms from the Fal Estuary and their traces. 43 Fig. 7 Lower Tresillian River; Section 49 Fig. 8 Upper Tresillian River; Section 57 Fig. 9 Restronguet Creek; Semiquantitative XRD Results 70 Fig. 10 Tresillian River; Semiquantitative XRD Results 74 Fig. 11 Diagrammatic cross section of creek mouth complex showing sulphide mineralogy 100 Fig. 12 Diagrammatic cross section of upper Restronguet Creek showing sulphide mineralogy 101 Fig. 13 Analytical scheme employed for solid sediment samples 104 Fig. 14 Mean Metal Concentrations 115 Fig. 15 A Metal Values 116 Fig. 16 Tin and Arsenic Concentrations 117 Fig. 17 Fe vs Cu Correlation Diagram 119 Fig. 18 Anomalies in Metal Concentration Trends 120 Fig. 19 Restronguet Creek; HiMO- extractable Fe 121 Fig. 20 Restronguet Creek; HC1 extractabla Fe 122 Fig. 21 Restronguet Creek; HNO^ sxtractabla Mn 124 Fig. 22 Restronguet Creek; HC1 extractable Mn 125 Fig. 23 Restronguet Creek; HiMO^ axtractable Cu 1 27 Fig. 24 Rastronguat Creek; HC1 extractable Cu 1 28 Fig. 25 Rastronguet Creek; HIMO^ extractable Zn 129 Fig. 26 Restronguet Creek; HC1 axtractable Zn 130 Fig. 27 Restronguet Creek; Location of Core S am pi es 132 Fig. 28 Trasillian River; Location of Core S amples 133 Fig. 29 Core 35; Mstal Concentration 135 Fig. 30 Core 13; Metal Concentration 136 Fig. 31 Core 11; Mstal Concentration 137 Fig. 32 Core 12; Metal Concentration 1 38 Fig. 33 Core 31 Metal Concentration 140 10 Page Fig. 34 Cora 34 Metal Concentration 141 Fig. 35 Core 35 Metal Concentration 142 Fig. 36 Core 42 Metal Concentration 143 Fig. 37 Core 32 Metal Concentration 144. Fig. 38 Core 30 Metal Concentration 145 Fig. 39 Restrongust Creek - acid extractable metals; Lithology 147 Fig. 40 Restronguet Creek - acid extractable metals: Fe 148 Fig. 41 Restronguet Creek - acid extractable metals: Mn 149 Fig. 42 Restronguet Creek - acid extractabla metals; Cu 150 Fig. 43 Restronguet Creek - acid extractable metals; Pb 151 Fig. 44 Restronguet Creek - acid extractable metals; Zn 152 Fig. 45 Core 9; Metal Concentrations 154 Fig. 46 Core 10; Metal Concantrations 155 Fig. 47 Core 26; Matal Concentrations 156 Fig. 48 Core 5; Metal Concentrations 158 Fig. 49 Core 40 Metal Concentrations 15g Fig. 50 Core 48 Metal Concentrations 160 Fig. 51 Core 45 Metal Concentrations 161 Fig. 52 Core 46 Metal Concentrations 162 Fig. 53 Core 6; Metal Concentrations 163 Fig. 54 Restronguet Creek Lower Intertidal Flats; Distribution of total sulphur values 166 Fig. 55 Restronguet Craek Upper Intertidal Flats; Distribution of total sulphur values 167 Fig. 56 Tresillian River; Distribution of total sulphur .values 169 Fig. 57 Pore water sulphate depletion profiles 172 Fig. 58 Arithmetic mean monosulphide concentrations 173

Fig. 59 Restronguet Creek; Extractable H2S at 10 cms depth 175

Fig. 60 Restronguet Creek; Extractable H2S at 25 cms depth 176

Fig. 61 Restronguet Creek; Extractable H25 at 35 cms depth 177

Fig. 62 Restronguet Creek; Extractable H2S at 50 cms depth 178

Fig. 63 Tresillian River; Extractable H2S at 35 cms depth 179

Fig. 64 Generalized extractable H2S profiles 180 11 Page Fig. 65 Detrital sulphur concentrations 183 Fig. 66 Restronguet Creek; Authigenic polysulphide at 25 cms depth ' 134 Fig. 67 Restrongust Creek; Authigenic polysulphide at 50 cms depth 185 Fig. 68 Restronguet Creek; Authigenic polysulohide at 60+ cms depth 186 Fig. 69 Core 11 Sulphur 188 Fig. 70 Core 12 Sulphur 188 Fig. 71 Core 13 Sulphur 189 Fig. 72 Core 31 Sulphur 190 Fig. 73 Core 32 Sulphur 190 Fig. 74 Core 34 Sulphur 190 Fig. 75 Core 35 Sulphur 190 Fig. 76 Core 30 Sulphur 191 Fig. 77 Core 9 Sulphur 1g2 Fig. 78 Core 10 Sulphur • 192 Fig. 79 Core 5 Sulphur 193 Fig. 80 Core 6 Sulphur 193 Fig. 81 Organic Carbon Concentrations 195 Fig. 82 Organic Carbon vs. total Sulphur 197 Fig. 83 Distribution of S/C Atom Ratio 198 Fig. 84 Comparison of surface water samples and concentrations obtained by sediment leachi ng 207 Fig. 85 Restronguet Creek; Surface Waters - Chloride Concentrations 209 Fig. 86 Restronguat Creek; Surface Waters - Calcium Concentrations 210 Fig. 87 Restronguet Creek; Surface Waters - Potassium Concentrations 211 Fig. 88 Tresillian River; Surface waters - Calcium concentrations 212 Fig. 89 Surface waters; Histograms of concentration ratio with Chloride ion 213 Fig. 90 Surface waters; Histograms of concentration ratio with Chloride ion 214 Fig. 91 Core 34 Pore water chemistry 215 Fig. 92 Core 35 Pore water chemistry 216 Fig. 93 Core 31; pore water chemistry 218 Fig. 94 Core 32; Pore water chemistry 219 Fig. 95 Core 21; Pore water chemistry 220 Fig. 96 Core 30; Pore water chemistry 222 12 Page Fig. 97 Core 2c; Pore water chemistry 223 Fig. 98 Core 24; Pore water chemistry 224 Fig. 99 Core 42; Pore water chemistry 226 Fig. 100 Core 40; Pore water chemistry 228 Fig. 101 Core 45; Pore water chemistry 229 Fig. 102 Core 46; Pore water chemistry 230 Fig, 103 Restronguet Creek; Surface waters - Fe concentrations 233 Fig. 104 Tallscks Saltmarsh; Metal Concentrations in Porewater 234 Fig. 105 Restronguet Creek; Surface waters - Mn concentration 235 Fig, 106 Surface waters; Mn vs. acid extractable b^S 237 Fig, 107 Restronguet Creek; Surface waters - Cu concentrations 239 Fig. 108 Surface waters; Zn vs. Cu 240 Fig. 105 Restronguet Creek; Surface waters - Zn concentrations 242 Fig. 110 Cell general assembly 252 Fig. 111 "Small cell" apparatus employed 255 Fig. 112 Test blank contamination survey 259 Fig. 113 Expelled pore waters; Sample 8R1; Normal P/T regime 265 Fig. 114 Expelled pore waters; Sample BR2; Extreme thermal regime 266 Fig. 115 Expelled pore waters; Sample BR3; Normal P/T regime 267 Fig. 116 Expelled pore waters; Sample BR4; Normal P/T regime 268 Fig. 117 Expelled pore waters; Sample SR5; Extreme thermal regime 269 Fig, 118 Expelled pore waters; Sample BR6; Normal P/T regime 270 Fig. 119 Expelled pore waters; Sample BR7; Extreme thermal regime 271 Fig. 120 Expelled pore waters; Sample SR9; Geothermal gradient X5 Normal 272 Fig. 121 Expelled pore waters; Sample BR2 276 Fig. 122 Expelled pore waters; Sample 3R3 277 Fig. 123 Expelled pore waters; Sample 3R4 278 Fig. 124 Expelled pore waters; Sample 3R5 279 Fig. 125 Expelled pore waters; Sample BR5 280 Fig, 126 Expelled pore waters; Sample BR6 281 Fig, 127 Expelled pore waters; Sample BR7 282. 13 Page Fig. 128 Expellsd pora uatars; Sampls 3R9 283 Fig. 129 A Metal values for selected samples 289 Fig. 130 Total sulphur and acid extractable sulphide concentrations 291

Fig. 131 Comparison of dilute acid extractable H?S in uat and air dried sediment samples 293 14

LIST OF PLATES

Page Plate 1 Parasitic ebb oriented asymmetric ripples on the upper flood ramp 36 Plate 2 Fine grained sediment in the trough zone of a megaripple, Restronguet flood ramp 36 Plate 3 Box core of flood ramp sediment 37 Plate 4 Box core of flood ramp sediment 37 Plate 5 Crest of ebb shield looking north 40 Plate 6 Lower intertidal flats colonized by filamentous green algae 46 Plate 7 Upper intertidal flats sediments 46 Plate 8 Erosion of saltmarsh at head of Tallacks Creek 53 Plate 9 Mudcracked algal flats 60 Plate 10 Colonized mudcracked algal flats 61 Plate 11 Destruction of the saltmarsh 61 Plate 12 Encrusting chalcopyrite associated with poorly ordered hematite 82 Plate 13 Atoll textured chalcopyrite 82 Plate 14 Stellate aggregates of bladed chalcopyrite 82 Plate 15 Diagenetic pink bornite overgrowth on chalcopyrite 85 Plate 16 Chalcopyrita partially replaced by purple bornite 86 Plat.e 17 Atoll textured pink bornite 89 Plate 13 Garlands, rims and overgrowths of pink bornite on chalcopyrite 89 Plate 19 S.E.M. electron image of Plate 18 90 Plate 20 Copper assay for Plate 19 90 Plate 21 Iron assay for Plate 19 90 Plate 22 Pink bornite with atoll texture 92 Plate23 Pink bornite as garlands 92 Plate 24 Chalcopyrite and associated earthy hematite marginal to permeable root zone 93 Plate 25 Sphalerite enclosing pyrite 93 Plate 26 Elliptical pyrits framboids 95 Plate 27 Rosettes and garlands of chalcopyrite 95 Plate 23 Crustiform overgrowths of orange pink bornite 96 Plate 29 Complex grain of chalcopyrita, bornite and pyrite 96 Plate 30 Chalcopyrite flake 99 Plats 31 Pyrite overgrowths on detrital grains 296 15

Paoe Plate 32 Atoll textured pyrite 296 Plata 33 Encrusting bornite 296 Plate 34 Crustiform chalcopyrite and pinkish bornita 298 Plate 35 Crustiform chalcopyrite and pinkish bornite 298 Plate 36 Sulphidation front forming pinkish bornite overgrowths 298 Plate 37 Two phases of bornite overgrowth 299 Plate 38 Gas bubbles developed in compacting sediment? 299 Plate 39 Pyrite overgrowths on framboids 301 Plate 40 Pyrite overgrowths on framboids 301 Plate 41 Pyrite encrusting a woody fragment 303 Plate 42 Chalcopyrite encrusting detrital hematite 303

< 16

INTRODUCTION

\4 This thesis^presanted as a contribution to the study of syngenetic sulphide ore genesis in siliciclastic sedimentary environments. Much of the scope of the research presented lies in the overlap between the fields of study of sedimentology, ore genesis and geochemistry and the aim of the work is to show how techniques from each discipline might be jointly applied and thus act as a stimulus to further detailed studies. The research philosophy as con- ceived in the summer of 1975 was essentially to apply current sedimentological and geochemical techniques to the investigation of estuarine sediments which receive abnormal heavy metal inputs, and by experiment to maturate samples of these sediments at high temperatures and pressures and, if possible, to extrapolate the results to postulated ore forming environments.

In the remainder of this chapter a review is presented of the hypotheses which formed the background to this work. The scope of the research is discussed and a summary of relevant previous work is given.

THE FORMATION OF ORES IN SILICICLASTIC SEDIMENTS

Uolf (1976) presented a comprehensive review of the development of the syngenetic ore formation hypotheses from 1922 until recently. The reader is referred to this work (Vol. 1, Chapter 6) and only the development of the major concepts and additional research published since 1976 are mentioned here.

The term 'syngenesis' appears to have been first mentioned by Fersman (1922), who used it to mean the formation of unconsolidated^sediment including the alteration of detrital particles still in movement in the waters of a depositional basin. In his study of ore deposits Lindgren (1933) observed that there was little evidence for large scale deposition of sulphides other than pyrite or marcasite. In perhaps the first 17 hypothesis of syngenetic ore formation Bastin (1933) proposed that the Mansfeld metalliferous shales (Kupferschiefer) uere formed by copper—rich groundwater which entered sulphide rich pore water and black muds of stagnant lagoons. Thus by 1933 a general syngenetic concept of mineralisation of reduced muds by sulphidic water had been proposed. The concept was broadened in the 1950's when Garlick and Brummer (1951), Davis (1954) and White and Wright (1954) explained mineral zonations in copper-bearing sediment in terms of palaeo- geographically controlled Eh, pH variations. Knigh't ( 1957) was perhaps the first to realise that time was important in syngenetic ore formation and that mineralisation might not be completed within the depositional phase of sediment formation. Thus by 1950 metalliferous sediments were being considered as geological entities in terms of depositional history, metamorphism and tectonism. The significance of diagenetic processes was not recognised until the experimental work of Baas-Becking and Moore (1961) and studies by Love and Zimmerman (1961) demonstrated growth of pyrite and marcasite in unconsolidated Recent sediment. These works also showed that sulphide ions were produced by the reduction of sulphate ions by the activities of the bacterium Desulphovibrio desulphuricans, and it follows that given a supply of nutrients and ions of appropriate metals heavy metal sulphides are virtually certain to be precipitated in natural anaerobic sediments. Davidson (1962) extended the syngenetic concept to include the replacement of early formed iron sulphides by heavy metal ions, although he would not allow the latter to be circulating in the near surface groundwaters. Lovering (1963) formalised this two-stage concept and suggested the term "diplogenetic" although it is not in currant general use. Strahkov (1962) presented useful field data concerning the geochemistry of supposed metallogenic environments such as the Black Sea. Noble (1963) suggested that water expelled from a compacting sedimentary sequence might be sufficiently mineralised to be able to generate certain types of deposits, thus eliminating the need for an exogenous mineralising fluid. Davidson (1965) considered that such waters might obtain the high salinities found in fluid inclusions by leaching of evaporites in the stratigraphic column. These brines were also thought to be capable of leaching heavy metals from the 18 sediments during their ascent to the surface. Papers by Roedder (1968) and D. E. White (1968) reached broadly similar conclusions .

By 1968, ore formation hypotheses were beginning to be enriched by contributions from other geological disciplines. Berner (1969) demonstrated the production of macroscopic banded pyritic sulphides by precipitation and diffusion mechanisms. Hallberg (1972) developed diagenetic concepts by considering the sulphidation of metal oxides and absorbed metal ions in the sediment, and postulated a degree of mobility of metals as organo-metallic complexes. He recognised that in the vast majority of sedimentary systems most heavy metals must pass through an "oxide phase" before they are sulphidized. Works such as those by Presley and Nissenbau.m ( 1972) and Krom (1976) showed that natural sediments from a variety of present day environments do not accumulate any significant concentration of heavy metals. These studies gave added credence to earlier ideas of mineralization of sediments by compactional formation waters as this seemed to be the most likely mechanism for intro- ducing sufficient quantities of metals. Actual field evidence for the existence of these waters was given by Carpenter (1974) and Hitchon (1977). A similar approach was taken by Renfro (1974) for arid environments and the concept was employed by Van Eden (1974) in discussion of the Zambian copperbelt.

Since 1976 research attention has become less centred on advancement of theoretical or conceptual modelling of particular syngenetic ore forming environments. Increasingly studies have been devoted to documentation of diagenetic processes within present day mineralized sediments. For example a number of notable study programmes described in the Annual Reports of the Baas Becking Geobiological laboratory, Canberra, Australia have provided field and experimental data on hydrogen sulphide productivity in sediments (1978), heavy metal distribution in polluted arid estuarine environments (1973) and significant iron-manganese mineralization in intertidal flat sediments. (1979, 1980). 19

It is anticipated that this type of study uill continue to be very important in the near future and a wider variety of potentially mineralized environments will be investigated.

THE SCOPE OF THE RESEARCH AND THE PROCEDURES ADOPTED

It is clear from the works referred to above that ore minerals are not normally concentrated in the majority of siliciclastic environments. However it is still not known whether it is the source of the metals, the mechanism for their fixation as sulphides, or, the post depositional mobilisation and concentration processes which are inadequate. Thus, a metal-rich estuarine environment was selected in which the input of metals and their fixation within the sediments could be studied in an attempt to evaluate the various possibilities outlined above. A second stage feasibility study was to assess the possible mobilisation of metal species at diagenetic and metamorphic temperatures and pressures.

AREA SELECTED FOR STUDY

There are studies by Strahkov (1960) and Samama (1973) which suggest that arid climates favour the accumulation of metals in sediments. Uhen the field area was chosen in 1975 there were no known examples of present day arid metalliferous environments and the available funds would not have supported overseas visits. Thus the study had to be located within the British Isles. In order to obtain a sufficiently high metal input a polluted nearshore sedimentary environment was required. Locations along the north and south coasts of Uales, parts of the Cornish coast and certain Irish estuaries, appeared to be likely areas. However Cornwall was eventually selected because of the advantages of the ease of transport of samples to the London laboratories, prior local knowledge, satisfactory known heavy metal input and the diversity of sediment types. Accord- ing to local opinion the Red River (O.S. 1" map reference 605 417) is reputed to contain nodules of chalcocite "growing in place" but it could not be considered for study due to active tin-streaming and the limited range of sediments at 20 its mouth. The Fal Estuary (O.S. 1" map reference 830 370) was chosen because of the knoun major heavy metal pollution from its tributary the Carnon River: fig. 1. The 15 m deep central channel of the Carrick Roads seems to impede deposition of marine-derived coarse sediment in the upper estuary so that large areas of silty sediments occur in Restronguet Creek and the Tresillian River. These two tributaries were chosen for study as they are easily accessible, relatively safe and are exposed for at least six hours at low Spring tide.

PHYSICAL AND CHEMICAL METHODS

The investigation of the sediments was designed, first to reveal the major sedimentological characteristics of the various estuarine environments. The distribution of ions of Fe, Mn, Cu, Pb, Zn and their speciation within the various sedimentary facies were to be studied together with the distributions of organic carbon and the simple sulphur species. The input of "dissolved" heavy metals from the river waters was to be determined, and the response of these metal species to the tidal hydrogeological system assessed. The experimental methods were not selected to measure the overall concentration levels with great accuracy but with sufficient accuracy to establish whether variations exist between different lithologies and to indicate the underlying trends. It was also decided that limited studies of the pore-water sediment system beneath the water-table should be carried out to provide orientation data for the experiments on diagenesis. With the exception of unsafe areas in Restronguet Creek, sediment samples were to be collected from both the near surface and sub water-table zones by manual boring on a regular but non-gridded pattern.

In order to investigate potential changes in samples of the sediments, experiments at increasing temperatures of from 20°C to 200°C and pressures of up to 5000 psi (34.47 MPa) it was intended to use existing apparatus in the Engineering Geology section of the Department of Geology at Imperial College. However certain limitations were found in this 21 equipment, but in the time available they could not all be rectified. Thus only a preliminary feasibility study and limited experimental objectives at temperatures below 120°C were possible.

REVIEW OF PREVIOUS WORK

The more deeply buried sediments in the Fal Estuary have been briefly discussed by Dines (1956) but he does not describe the superficial sediments considered in this thesis. In general, the sedimentological terms summarized by Evans (1965) and by American workers at the University of Massachussetts Coastal Research Group, see Lauff (1967) are employed in the present study and many of their environmental divisions are present in the Fal. Alternative terms, such as those proposed by Oertel (1972), were found not to be as appropriate. Hosking and Obial (1966) determined the approximate levels of heavy metals in sediments and waters from the Fal Estuary, but no sediment profiles were examined and the sediments were not described.

Previous reports of aspects of siliciclastic sediment geochemistry were found to be generally useful in the present work but are not referred to in the main text unless directly relevant to the method or argument. Thus a general review of such work is presented here.

Many of the earlier studies showed that virtually all marine and brackish waters have very low trace element concentrations (Schultz and Turekian (1965), Durum and Haffty (1963). Further studies confirmed this conclusion (Brewers et al. (1976)) although subsequently much of the analytical work has been criticised. Normal marine sediments e.g. Hirst (1962) and Clague (1974) are not enriched in heavy metals through contact with the dissolved metal in seawatsr. For enrichment to occur abnormal conditions must exist. For example, very slow sedimentation allowed high metal concentrations to develop in the deepest North west Pacific sediments discussed by Volkov et al. (1975). The Red Sea sediments, as reported elsewhere, are very clearly abnormal. Finally, the higher 22 metal contents of marine sediments off Southern Africa were attributed by Calvert and Price (1970) to increased deposition of organic debris. The geochemistry of nearshore and fluviatile environments is less well knoun although many studies have dealt uith limited pollution problems, (e.g. Foster et al. (1971)). Mobilization of metals into a temperate nearshore environment was demonstrated by Foster and Hunt (1975) but similar evidence for an arid area uas not obtained by Reinson (1975). The temperate environment uas discussed in more detail by Grieve et al. (1976), Loring (1976) and ' Presley and Nissenbaum (1972). Grieve showed that deltaic sedimentation is too swift to allow accumulation of significant mineralisation although Loring demonstrated that sulphides are present in the Saguenay Fjord. Presley and Nissenbaum described the geochemistry of a fjord with a high sill and stagnant bottom waters. It is clear from that comprehensive study that although anomalous concentrations of heavy metals were present in the pore waters, the sediments were not significantly mineralised. Krom (1976) followed a similar course and explained the processes acting in Loch Duich. His study was particularly successful in relating certain aspects of the sedimentology and the geochemistry. More recently B. Bubela (1978) and G. U. Skyring (1978) reported enhanced concentrations of Pb, Zn and Fe in polluted sediments from the Spencer Gulf, S. Australia.

The literature contains many examples of the types of sulphide minerals found in Recent environments. However, very few papers discuss the environmental conditions or regional geochemistry of the mineralized area. An exception is the work by Strahkov (1962) who confirmed earlier work by Zobell, that, given a minimum of decomposable organic carbin and sulphate ions, sulphate reducing bacteria will promote the formation of pyrite in the diagenetic zone. Further evidence was given by Rickard (1973) and field examples were described by Volkov (1974) and Trudinger et al. (1980). There is continuing debate concerning the unstable iron monosulphide precursors of pyrite and the .mode of sulphidation of detrital iron oxide particles. The reader is referred 23 directly to the work of Berner (1964 a, b, c, 1967 a, b, 1969, 1970 and 1974), Doyle (1968), Rickard (1974, 1975), Sweeney and Kaplan (1973) and Volkov (1961) as these considera- tions are peripheral to the present study. Pyrite was recorded in many natural environments and occurred in a variety of forms, e.g. Hallberg (1966), Hein (1972), Rickard (1970) and Volkov (1958). Although Curtis (1967), Cabri (1973) and Rickard (1970, 1972) demonstrated the possibility of the present day formation of other metal sulphides in these natural sedimentary environments only one documented example is known to the author, Luther et al. (1980). Thus, it seems very likely that metal sulphides other than pyrite are formed by natural processes but are mostly too fine grained for simple isolation and study. The role of organic materials in the formation of metal sulphides has received more recent attention. Thus Saxby (1973) showed that metal sulphides are formed from metal ions and sulphur containing amino acids at between 100°C and 200°C. The sequestrating effect of organic humate compounds was demonstrated by Pauli (1975) who reported metal sulphides on the surfaces of complex metal-humate droplets after treatment with hydrogen sulphide. A natural example was described by Degens et al. (1972) from East Africa. However, the most useful studies relating mineralogical phenomena to the overall geochemical and sedimentary environment were found to be the works by Perel'man (1967), Curtis (1969) and Rickard (1973). The best summary of these concepts was given by Hallberg in 1972 although that work is not ideal as the "energy circuit system approach" is obscure.

Also of importance in the formation of sedimentary sulphides is the formation and distribution of hydrogen sulphide. The bacterial processes in the upper sediment layers are well known and are described elsewhere. Both the concentration and spaciation of sulphur in the sediments are ur important in this study. In this context, studies by Cj?dtis et al. (1962), Goldhaber et al. (1975), Saxby (1972) and more recent studies at the Baas Becking Geobiological Labora- tory, Australia related the rate of formation of diagenetic hydrogen sulphide to the chemistry of the organic substrate 2.4 and other factors such as sediment temperature and pore water salinity. Berner (1967) has shown how in deep stable sediments, sulphate ion supply could be a limiting factor. Bella et al. (1972) showed that hydrogen sulphide, native sulphur and sulphide .ions can all be present in the tidal flats environment. They have also made the important point that local anisotropics within the environment e.g. a tidal scour, can produce considerable variation from the expected speciation as predicted from the general thermodynamic calculations of Garrels et al. (1959). Although high temperature abiological formation of sulphur species has been postulated, Martin et al. (1973) established a satisfactory link between the diagenesis of sulphur in some proteins and the formation of oil. Hydrogen sulphide was found to be an important by-product of these reactions.

In the estuarine environment, the interaction of local continentally derived groundwater with the tidal hydrological system is of paramount importance in consideration of the diagenetic history and possible mineralisation of the sedimentary unit. Unfortunately, the available studies do not seem to deal with the whole of this geochemical cycle. For example, and although there are several very detailed accounts of the input of metal species into certain Cornish estuaries there are no data on the fate of these materials in the tidal system. Most general accounts have dealt with the concentration distribution of either major or minor constituents of the pore waters: e.g. the major constituents by Krom (1976), Manheim et al. (1974), Nissenbaum et al. (1972), Emerson (1976) and Berner, Scott et al. (1970) and the minor constituents by Brooks et al. (1968), Presley and Nissenbaum (1972), Duchart et al. (1973) and Elderfield et al. (1975), (1981 a) (1981 b) (1981 c) . However, the complexities of the reactions involving aqueous colloidal, suspended and dissolved species, fine grained sediments and the tidal recharge mechanism are such that the findings of these authors in a number of ways conflict. In many cases the different environments studied are not even broadly comparable. The methods employed have been called into question by such studies as those of Manheim 25

(1974) and Murthy and Ferrell (1972). Thus, the only general conclusions which can be drawn from these works are that pore water sulphate ion concentrations decline with depth and carbonate concentrations increase with depth in the sediment column. Chloride ion concentrations remain fairly constant. 2+ There is no overall consensus about the behaviour of the Ca , 2+ + + Mg , Na , K ions although calcite or aragonite is supposed to precipitate in some pore waters. In addition, Edzwald and O'Melia (1975) presented evidence to show that clay o minerals are diagenetically altered to the more potassic 10A species in deeper water environments. Finally, Bischoff 2+ (1975) attributed the progressive depletion of Mg in some pore water columns to the diagenetic removal of organic and ferric oxide coatings around clay minerals with a subsequent decrease in cation exchange capacity. Thus the processes acting in pore waters are poorly understood at present.

A similar situation exists in the study of the heavy metal trace elements although more relevant generalisations can be deduced. Published studies have indicated that the heavy metals, Cu, Fe, Zn, Pb, exceed their expected solubility in almost all pore waters. This is especially true in reducing, sulphide-rich diagenetic environments. The metal concentration profiles obtained from sediment cores are thus irregular and show no overall obvious trends. Manganese is an exception, as its concentration profile has been shown to be asymptotic indicating mineralisation of Mn ions at depth as albandite (MnS) or manganiferous calcite: Elderfield (1981 b). It has also been suggested that in some profiles, there is diffusion 2+ of Mn to the surface where an iron oxide - manganese oxide complex "crust" is formed. The calculations presented by Gardner (1974) have indicated that the other heavy metals do not behave similarly either because of organic chelation reactions, or the formation of inorganic complex ions. Most workers have attributed these effects to the humic and fulvic fractions of dissolved or colloidal organic matter, e.g. Elderfield (1975), (1981 a, b, c), Cooper and Harris (1974), Nissenbaum and Swaine (1976), and Rashid and Leonard (1973). In an early publication Rashid (1972) demonstrated that the 26- adsorption process is selective and could give concentrations of certain heavy metals in this phase. A significant contribution uas made by Pauli (1975) who showed that hydrogen sulphide is capable of reducing these heavy-metal humate complexes to kerogen and metal sulphides. Natural pore waters also contain colloidal mineral particles which are known to adsorb many heavy metal ions. Steger (1973) has shown that smectites will adsorb copper ions and O'Connor and Kester (1975) have reached similar conclusions for the adsorption of copper and cobalt by "illite". Unfortunately, in the latter study the layer silicate' mineral was not adequately described. The behaviour of other metal ions in colloidal-mineral systems is not well documented, although Hem (1976) (1977) offered some evidence which suggests that lead behaves in a similar manner to copper. This field of study is suitable for further investigation as there is some possibility that preferential sorption of different metal ions could occur in different estuarine and pore water systems. If this is so, then the process could be important in the formation of some virtually monometallic mineral deposits in sedimentary rocks. 27

PART I THE FIELD AREAS

CHAPTER 1

GEOLOGICAL, PHYSIOGRAPHICAL AND HISTORICAL SETTING OF THE FIELD AREA

As may be seen from Fig. (1), the Fai Estuary has several tributary estuaries. Of these, Restronguet Creek and the Tresillian River are discussed in this thesis. The influence of man is strong in Restronguet Creek which receives the waters of the Carnon River at tha town of Devoran via a complex of largely derelict sluices, settling ponds and entraining walls of old mine workings. Probably a significant proportion of the flow is from the dormant Gwennap copper mining area (1" 0 ,S . 740410) and Bissoe Valley tin mines (763420) via the main County Adit and othsr subsidiary adits. In the period 1810 to 1910, Devoran was an important port with extensive wharves, basins and desilt- ing sluices. At its zenith the wharves stretched from Devoran to Yard and the river was virtually canalized as far as Tallacks Creek (fig. 2). In the period 1830 to 1840, a lead smelter operated at Penpoll and a tin smelter was built in 1842. The works were extended to Point (see fig. 2), and an arsenic flue was erected in 1880. The smelter was eventually removed to Liverpool in 1921. These works must have caused a great deal of pollution of the estuary. In particular, quantities of smelter slags were used to build the entraining walls at Tallacks Creek and on the opposite bank of th-e River Kennal. Further disturbance of the natural environment was caused by tin streaming works in the Carnon River and upper Restronguet Creek. The Creek itself was worked many times until 1320 at lsast as far as Tallacks Creek. The stanniferous bed is just above the bedrock so the original sedimentary sequencs is virtually obliterated in thase areas. However, from 1820 onwards, all workings downstream of Tallacks Creek ware by direct mining of the tin gravels from shafts either on the bank, (Tallacks Creek 1322-7 and Point Quay 1371), or from artificial 28 29 islands in mid-Creek, (Yard or Old Carnon Mine 1843). Thus downstream of the west bank of Tallacks Creek the surface sediments have largely reverted to the natural state though areas of very thinly bedded tailings and artificial spoil dumps may occasionally be recognised. At the present time the estuary drains partly oak-wooded somewhat hilly country- side which is naturally well drained. Deep soils are developed in this part of Cornwall though the input of organic material to the estuary is probably only moderate. The geology of the hinterland is typically Lower Devonian slates and shales ("killas") traversed by altered quartz- porphyry dykes ("elvans") and quartz-chlorite veins with significant copper, zinc and iron sulphides, cassiterite and arsenopyrite. In some areas tourmaline, wolframite and hematite are important in these veins. Granite does not crop out in the immediate hinterland. The sedimentary input might be expected to be unusually enriched in chlorite, kaolin, tourmaline, metal sulphides and cassiterite which were derived from the tailings of the various mines and • quarries,

The Tresillian River estuary receives the waters of the Tresillian River which does not drain any areas of significant mineralisation. Thus the only likely pollution is from Tresillian and other villages along its course, and from ancient china clay waste tips.

The lower reaches of the river flow over rather narrow marshlands (see elsewhere the river drains typical slightly hilly Cornish country. However, the river and estuary margins are much more wooded than those along Restronguet Creek and input 'of organic material is very . much greater. Much of the estuary forms the boundary of Lord Falmouth's Tregothnan Estate and has been artificially embanked above the high tide line. Thus, sediment input from lower tributaries is negligible. However, it is believed that 18th and 19th century china clay wastes have been reworked in the upper reaches of the tributary river and hence some pollution by kaolin, muscovite micas and 30

some feldspar has occurred.

Both the Tresillian River and Restronguet Creek flow ^into the Carrick Roads which has a 15m deep central channel and is a typical drowned valley. Thus there are no extensive intertidal deposits outside these tributary astuaries and, as will be discussed below, the deeper water tide-dominated environments extend a short distance into the tributary Restronguet Creek. FIG. 2

RESTRONGUET CREEK

TALLACKS CREEK STRUCTURE OF THE CREEK MOUTH AREA

KEY FLOOD RAMP EBB SHIELD LR. INTERTI DAL FLATS " with nlgcil colonies

" with fossil colonies NSID UR. IN TER TIDAL FLATS HALWYN " mudc racked

11 colonized SALTMARS H MARGINAL ENVIRONMENTS TRIBUTARY AND DELTA N SID H©P WMi ARTIFICIAL SEDIMENTS ABANDONED CHANNEL SECTION LINE

NSID MOT SURVEYED IN DETAIL FtSTR JNGUET PAS SAGE D

SCALE Part Ot sheet SW83NW 32 33

CHAPTER 2

THE STRUCTURE AND SEDIMENTARY ENVIRONMENTS OF THE FIELD AREAS

The field studies show that there is a continuous sequence of sedimentary environments extending from the mouth of Restronguet Creek to the head of the Tresillian River. This series is basically divisibla into seven major types:- i) The creek mouth complex ii) The intertidal flat iii) Channel slopes iv) Point bars v) Tributary creeks and streams vi) Marginal environments vii) Artificial environments

The distribution of the major environmental types is shown in figs. 2 and 3. A number of sub-divisions which are characterized by less important physiographical, floral and faunal features are discussed in detail under each main heading.

The Creek Mouth Complex

The creek mouth complex environment at the mouth of the Restronguet Creek is subject to a daily tidal variation of about 4.7m during Spring Tides and about 1.9m during Neaps. This tidal variation is large although a similar regime occurs along partsof the eastern seabord of the U.S.A. and many of the estuarine features found there can be recognized in Restronguet Creek, Lauff (1967). As may be seen from fig. 2, the Creek mouth complex consists of two rocky headlands which confine the channels and the flood delta. The main central channel is used by both flood and ebb tides and is characterised by megaripples in the intertidal portion and sandwaves in the subtidal part. The 34 study areas were limited by accessibility, to the main central channel, minor flood channels, an ebb channel near the shore to the southwest, the flood ramp and the ebb shield. To the west and north, the flood ramp becomes progressively more modified by the ebb tides and passes into intertidal flats and a partially buried abandoned channel.

The Creek Mouth Complex: flood channels and flood ramp

As may be seen in fig. 4 there are three channels in the creek mouth complex labelled A, 8 and C. The main central channel (A) has a well developed flood ramp and there is a subsidiary flood ramp embayment to the southwest (B). The minor channel (C) is also utilized by the ebb-tide. In the subtidal and intertidal areas near A and B, sand waves of wavelength 10-20 metres and amplitudes greater than 1m are common. On the studied part of the intertidal flood ramp, the most common bedform is the megaripple with wavelengths of about 4-8 metres and amplitudes about 35cms.

The modification of the megaripples varies with position on the flood ramp. The lower flood ramp is characterised by megaripples with parasitic ebb-orientated asymmetric ripples with wavelengths averaging 10-15cms and amplitudes about 4cms. In the central area these are always cuspate and predominantly lunate but to the northwest they become more linear and symmetrical. See Plate 1. As may be seen they are highly sculptured by rill and swash action but the crest lines are commonly sub-parallel for up to 1m. The variation in orientation of these parsitic ripple sets is illustrated in fig. 4. Most of the larger forms and some parasitic ripples accumulate fine grained sediment and organic matter on their lee sides and in some areas a mudflake conglomerate may be formed. The larger forms are characterised by a braided ebb-drainage system along the trough zones. These channels winnow the finer material and organic debris from the sediment and deposit it as a sheet on the lower part of

PLATE 1 Parasitic abb oriented flood ripples on the upper flood ramp

PLATE 2 Fine grained sediment in the trough zones of megaripples, Restronguet flood ramp PLATE 3 Box core of flood ramp sediment PLATE 4 Box core of flood ramp sediment (Right hand scale in inches) 38

the ramp where the drainage is by percolation through the very porous coarse sand, (see Plate 2). The largest of these channels may also form small deltas up to 30cms wide and a ripple set with amplitudes of about 2cms and wavelengths of about 5cms. Plates 3 and 4 show box cores of typical near surface sediment from the upper 30cms of the flood ramp. As may be seen, bi-directional cross bedding is rare and the planar cross bedding is almost always flood dominated. Flaser bedding is very common and the fine grained, organic rich braided channel sheet deposits are well represented in Plate 4. Similar sediments were obtained in the vertical cores 13 and 35 for which pictorial logs are given in the appendix.

Floral and faunal remains are scarce in this environment. Bird tracks and beak marks are common.

The Creek Mouth Complex: ebb shield

Fig. 5 illustrates the steepness of the rampart which has been thrown up at the crest of the flood ramp and is here termed the ebb shield. The rampart is at its widest in the southeast, and the width decreases to the northwest where the feature loses its prominence and is cut through by several runnels and braided channels. As may be expected, the crest of the shield largely directs the ebb run off away from the faca of the flood ramp and is thus responsible for the preservation of the delicate mud flaser and mud • sheet deposits which form in that environment. A complex pattern of ripples is developed on the shield: this is illustrated in Fig. 4. Behind the crest of the rampart and westwards towards the shore, the ripple form varies from cuspate to linear although tha mean wavelength and amplitude are fairly constant at 15cm and 4cm respectively. These ripples are clearly a product of the initial ebb run off from the intertidal flats and, as may be seen from Plate 5, the lunate form is dominant. The approximate area illustrated and viewing direction of the photograph are shown in Fig. 4.

PLATE 5 Crest of ebb shield looking North 41 Where the ebb shield is subjected both to strong ebb and strong flood tidal currents complex interference ripple patterns are common. For example in the central-northwestern area of the ebb shield (middle distance of Plate 5) a flood oriented ripple set (wavelength 1 m, amplitude 10 cms) is overprinted by an early ebb oriented set (wavelength 1 m, amplitude 10 cms) which is itself modified by a later, linguoid, parasitic ebb set, (wavelength 5 cms, amplitude 2 cms) •

Simple, linear ebb oriented ripples are dominant on the upper ebb shield areas and exhibit wavelengths of from 1 m to 10 cms and amplitudes of from 2 cms to less than 1 cm.

Although the ebb shield is regularly cut across by braided channels, these do not accumulate significant amounts of organic debris and thus fine grained materials are rare in this environment. However, some of the larger ripple forms on the upper shield have some silty material on their lee sides and recognisable concentrations of black heavy minerals are common.

The floral and faunal assemblage of the ebb shield is virtually as limited as that of the flood ramp with the addition of rare fragments of the molluscs C erasto derma, Mytilus, Ostrea and Pecten. One single vertical burrow was found in ebb shield facies sediments at depth in a box core from the flood ramp (see Plate 3) and is interpreted as belonging to the worm Pyqospio sp.

Despite the variety of bedforms recorded on the abb shield there is little evidence for them in the sedimentary sequence. Box and vertical cores show a rather homogeneous sedimentary column with some graded bedding, sandy flaser bedding and poorly preserved low angle planar cross bedding. Flaser bedding is better developed in the generally finer grained sediment at the northwestern extremity of the shield. The Pygospio burrow appears to be cemented by iron 42 oxide and cuts the sedimentary structures. In a few pits woody fragments appeared to have acted as sediment traps and were enclosed by fine grained sediment. Detailed logs of vertical cores 11 and 12 are given in Appendix. Both logs, show buried intertidal flats sediments underlying the ebb shield sediments.

The Intertidal Flats

Intertidal flat environments are the most common in the Falmouth Estuary and occur in both Restronguet Creek and the Tresillian River. As may be expected, the sediment grain size and the flora and fauna vary progressively from the mouths to the heads of the tributary estuaries and environmental subdivisions are somewhat arbitrary. In this study the intertidal flats are divided into an area of lower flats on which flood tide phenomena are recognisable and upper flats where there are generally no easily identi- fiable tidal structures.

The Intertidal Flats: lower division a

Lower intertidal flat occurs only in Restronguet Creek (see Fig. (2)) and is the most westerly environment which receives any significant input of sandy sediment from flood tides. The bedforms are generally limited to interference sets of dominantly planar, slightly linguoid ripples with wavelengths of 10-15crns and a mean amplitude of 3cms; although at the transitional boundary with the ebb shield, cuspate ripples with rill and swash marks do occur. In this environment much of the ebb tidal flow is directed along meandering channels which, near the ebb shield, are up to 15cms deep and accumulate leafy organic debris over areas of up to one square metre. A concentration of channels in the central area probably marks the site of a depression which may have been an abandoned flood channel. The organic debris and fine grained sediment which accumulates on the lee side of the ebb shield ripples, becomes mora 43

Fig, (6): Some commonly recorded organisms from the Fal Estuary and their traces. (a) Burrows of Nereis diversicolors, (b) Scrobicularia plana" (c) Burrow Pyqospio sp. (d) Hydrobia ulvagi (e) Crab burrowing, ff^ Crab tide streaked pit marks 44

continuous in this environment and forms linear and even sheet oozes in some western areas. In places only the crest lines of ripples penetrate the ooze and on the western boundary ripples are either absent or obscured. Where the surface deposit is well d-eveloped crab burrows are fairly common and the debris thrown up during the excavation is streaked out in the direction of the flood tidal flow, sse ^g. (4).

Flora and fauna are scarce near the ebb shield but, to the west, eroded disarticulated Pec ten and freshly disarticulated Cerastoderma and Macoma are common. Drifted brown and green algae occur only rarely although leaf and twig debris are common. Nereis diversicolour and Decapoda become increasingly numerous to the west as is evidenced by their burrows. Some of the major features of these fauna are illustrated in fig. (6). .

The sedimentary sequences obtained in this environment are complex. In general they comprise homogeneous sandy units which, near the shield, pass down into thinly bedded sands with mudstreaks and flasers whila bioturbated somewhat sandy sediments with discrete leafy organic lenses up to Scms thick occur to the west. In unbioturbated sediments, bi-directional cross stratification appears to be common.

Cores 31, 32, 34, 41, 42 illustrate the sediments of this environment (see Appendix), although channel deposits occur at the base of core 31 and may record an earlier, now abandoned channel.

An important and easily recognised subdivision of this general environment is an area of fine grained sediment which has been colonised by webs of filamentous green algae. According to local knowledge, the distribution of these areas has changed significantly over the past twenty years but at present there are two areas of colonisation. These are characterised by a hummocky surface relief and innumerable sinuous drainage channels and pools between 45 the hummocks: Plata 6. The surface of the hummocks may be 10cms above the general level of the flats and is indented by minor rill channels, bird tracks and beak marks. Nereid burrows are very common. There is little or no shell debris although much laaf and algal debris accumulates in the pools and in the point bar positions of the 2 meandering channels. These debris sheets may cover 1m 2 4m but are very irregular. The living algae colonise only the hummock, tops, although individual species were not distinguished. The sediment column is highly variable and depends on whether a hummock or channel is cored. In some cases a buried hummock underlies a present day channel. The hummocks develop a very thinly bedded or laminated alternation of algal remains and fine sediment but this is partly destroyed, especially on burial, by Nereid bioturbation. Thus, the sub-surface sediments appear as organic rich highly bioturbated slightly lensoid units with minor evidence of a pre-existing stratification. To the east, near the channel, these algal banks seem to have subsided and are being destroyed. The small area labelled "fossil algal colonies" in Fig. 2 has no living algae on the hummock tops and is slowly being buried as the interhummock channels are filled with leaf debris, algal strands and fine sediment. To the southeast, this process is virtually complete and the bubbly algal ooze of the intertidal flats has covered the fossil algal banks. However, heavily bioturbated, very poorly stratified and organic rich sediments are obtained in cores from this area and old interhummock channels are just detectable. Core 30 (see Appendix) gives an excellent illustration of a typical algal flat sequence which rests on buried coarser grained sediment from the flood delta.

The Intertidal Flats: upper division

The. upper intertidal flats as illustrated (Plate 7) are coversd by a layer of bubbly, bird tracked algal ooze. This may appear to be greenish or brownish depending on the season of the year and possibly, to a limited extent 46

PLATE 6 Lower intertidal flats colonized by filamentous green algae

PLATE 7 Upper intertidal fiats sediments 47 on the mineralogy of the sediment. It is generally less well developed in Restronguet Creek than in the Tresillian River.

The flora and fauna are important features of this environment and include filamentous green algae, Nereid diversicolour and Decapoda on the Restronguet flats and, Macoma (gradually being replaced by Scrobicularia) in the Tresillian River. Both estuaries are colonised by Hydrobia although these are more numerous in the Tresillian flats. Tracks of birds, drifted organic debris and various faecal materials are common in both estuaries.

The surfaces of the upper tidal flats are sculptured by meandering creeks and channels which are very common. The creeks are up to 4m wide and 2m deep, but the most common size is about 0.4m wide and 15cms deep. The banks are characterised by mudflows and mudslips. The creek slopes are always bioturbated except on some point bar 2 areas where sheets of up to 4m of organic debris have accumulated. These are buried to form lenses of up to 10cm thickness, but are partially destroyed by bioturbation. Decapod remains and Nacoma and Scobicularia valves form coarse lags on the creek bed. Slightly elevated levees commonly develop along the margins of the main channel and major tributary creeks and may be colonised by green algae. The levees are seldom bioturbated by Nereids and in time they become colonised by the halophytic plants 5alicornia and Pucinellia. Isolated remnants of these well bedded levee deposits occur sporadically throughout the intertidal flats and are shown in Fig. (3).

The sedimentary sequence in this environment is homogeneous and is illustrated by the detailed logs of cores 5, 40 and 48 given in Appendix. 48

The Channel Slopes

The channel slope environment, which is indicated in Fig. (3), resembles the flats described above but is more unstable. The most prominent features are mudflows.and slumps in the upper reaches of the Tresillian estuary and mudslips in the lower reaches. The slump planes are composed of multiple en echelon planes each about 0.5m long. Elsewhere a thick brown algal ooze covers the slopes themselves and tends to mask the surface features. Near the head of the Tresillian River it is difficult to distinguish the intertidal flats from the channel slopes as the inclination of the surface is fairly constant and there are no algal banks associated with breaks in the slope. Tracks of birds are not common in this environment and gastropods are absent. There is much Scrobicularia debris and abundant evidence of M a c o m a, Mytilus and Cerastoderma in the coarse logs of the runnels and creeks which cut these sediments. Nereid diversicolour is very widely distributed. Leaf and other organic debris appears to be derived from[exposures of organic lenses within the sediments themselves, although a small proportion may be fresh.

The sediments occurring in this environment are very similar to the finer grained intertidal flats sediments although the moisture content is much higher and organic lenses up to 10cms thick are much mora common. Slump structures are rapidly destroyed by bioturbation and no fossil examples were located.

The Point Bars

Point bars are formed in both of the tributary estuaries, although in Restronguet Creek they are confined to minor tributary channels and ars very small scale. Along the Tresillian River the only significant examples of point bars occur at each meander, figure 3, although the best development is in the downstream area near

St. Clement (850440). The description given here is of the latter example. There are three main components to the point bar. They are: the debris slopes at the top, the sheet silts, and the coarse lag which occurs in the channel. The typical distribution of these units is shown in figure 7. The debris slopes are composed of drifted land and marine organic matter with abundant broken molluscs such as Cerostoderma, Mytilus, Macorna, Scrobicularia and various Decapoda. No living flora or fauna were recorded. The surface is undulating but there are no sedimentary structures. The sheet silts partly cover the debris slope and are composed of lozenge shaped hummocks about 3m by 15m on average with a long axis approximately at right angles to the channel direction. These hummocks receive relatively little coarse silt on the flood tides and appear to be starved megaripples of wavelength about 6m and amplitude 0.5m. The margins of the sheet are ragged and in some central areas the underlying units are exposed. The surface of the hummocks is covered with beak marks, bird tracks and Nereid casts and burrows. Between the hummocks, fine silts, muds, shell debris and organic matter has accumulated in standing water. The lower areas and the coarse lag in the main channel could not be studied in detail, but the lag itself appears to rest on an eroded surface of burrowed muds and is composed of cobble-sized mud balls and some shell debris. The surface of the mud balls is commonly armoured by reddish-orange limonitic iron oxide rims, although the more recently derived ball may still be grey-brown. At the boundary with the silt sheet, the eroded surface has a thin covering of fine sediment. The point bar environment is cut by complex drainage systems in most areas. The margins of each division are at breaks in slope, which may permit accumulation of standing water which is then drained by sub-surface flow and minor anastamosing channels. These latter tend to accumulate the finer organic matter and shell debris. In one area at St, Clement the silt sheet is disrupted by a major tributary creek and in consequence its boundary with the downstream channel flats lithology cannot be traced. 51

A complete sedimentary sequence from this environment uas not obtained in any of the cores and the coarse lag uas not penetrated. However, the detailed logs of cores 6, 45 and 46 given in the appendix illustrate quite clearly the relationship between the thick lenses of organic rich debris flat sediment which appear to be randomly repeated throughout the vertical^sequence. The intervening muddy silts contain lenses of shelly debris and pebble con- glomerates .

Upstream from this area near St. Clement, point bars become progressively less well developed, the silt deposits are noticeably thinner and are heavily bioturbated. The subsurface sediments from the upstream bars are less rich in organic materials and resemble the channel slope sediments very closely indeed. This may result from the fact that with reduced sediment supply organic debris is not buried as quickly as in the lower reaches of the estuary and much more is therefore probably destroyed by oxidation.

The Tributary Creeks and Streams

Several types of creeks and streams occur in both tributaries although the deposits formed by them cover only very small areas.

Many of the streams which drain the estates bordering the estuaries are culverted and thus emerge into the estuary to form small deltas with many braided channels. Very little sediment is carried down onto the intertidal flats although some pebbly to coarse sandy sediment is deposited in irregular linear bars which show little evidence of statification and only rare imbrication of the pebbles. The pebbles are from local head deposits and are only rarely even subrounded. Many of tha bars are colonised by filamentous green algae. The larger deltas, located on Fig. 2, occur at Point (801385) and at Halwyn (800385). 52

Buried deltaic sediment is seldom recognisable in the sedimentary column as it is immediately mixed with the overlying oozes by Nereid bioturbation.

Many artificial lakes have been built along the Tresillian River and the stream now enters the estuary via a sluice gate. Thus there is no delta and the periodical force of the flow has enabled small but deep creeks to be cut in the tidal flats as at St. .Clements (85044)) and Kiggon (850455). The course of these creeks changes regularly and erosion is rapid. However, there is virtually no sediment supplied tcr the estuary from the streams. Sedimentary processes in this type of creek are restricted to erosion of bioturbated tidal flats, and redeposition of the fine muds in cross bedded point bars. Small quantities of coarse sands and gravels, locally derived quartz pebbles and a few armoured mud pebbles form channel lag deposits but these are quickly obliterated by Nereid bioturbation. Any remnant mud pebbles disinte- grate as soon as the limonitic iron oxide is dissolved in the reducing point bar environment. There is only rare drifted algal debris but much land derived organic matter is present. A few disarticulated 5crobicularia were recorded. As noted above, these creeks may develop levees which are colonised by filamentous green algae.

A second type of stream is associated with extensive salt marshes: they are locally called creeks in both the tributaries. The most important examples are Tallacks Creek (800390), and the creeks at Tresemple (855455) and near Kiggon (860455). An extensive study was conducted of the Tallacks Creek and it is fairly certain that much of the present saltmarsh was originally formed under artificial conditions with a very high input of tailings from the mining operations. Since that time the sediment load has been reduced and much of tha upper saltmarsh is now being eroded. Plate 8 illustrates the destruction of saltmarsh adjacent to the upper reaches of the Creek. The resulting sedimentary column is of homogeneous fine-grained silty mud. 53

PLATE 8 Erosion of ssltmarsh at head of Tallacks Creek 54

Along the lower reaches of the Creek, however, there is sufficient sediment load to form point bars. These consist of a coarse lag of gravel, iron oxide coated sands with rare imbricate pebbles, cross stratified silty muds, lenses of organic debris and bioturbated muds. The upper stratum has recently been colonised by filamentous green algae and by a limited salt marsh flora: figure 2. This indicates that at least one area of active salt marsh construction occurs at Tallacks Creek. Downstream the channel slopes flatten and as the Creek splits into distributaries the slopes pass into tidal flats. In all these environments there is very little evidence of life apart from Nereids and sparse drifted, disarticulated Macoma and Decapod remains. Filamentous green elgae, brown Fucus sp. and the drifted remains of marsh plants are much more common. The creek bottom, channel slopes and point bar areas are extensively bird tracked. Minor meandering channels and runnels may accumulate small lenses of organic debris and coarser sediment in the point bar positions but these are obliterated by Nereid bioturbation. The sedimentary columns developed in these channel areas are diverse although, in general, poorly bedded to very thinly bedded root penetrated organic rich silts overlie organic rich muddy silts, which in turn rest on the unconformity at the top of older artificial silts and sands. However in one area the unconformity is overlain by sands and pebble conglomerate with a significant admixture of organic rich muds and some evidence of cross bedding.

The Marginal Environments

In the upper reaches of both the Tresillian River and the Restronguet Creek, colonisation of the marginal environments by halcphytes has progressively formed quite large areas of saltmarsh which are now intermittently submerged. Elsewhere shingle banks, banks and lagoons and simple colonised intertidal flats form the estuarine margins. The simple marginal environments are discussed 55

first and the saltmarsh is dealt with in more detail.

The shingle banks are seldom more than 35m wide except where they a-re cut by braided -stream channels. In the upper areas they consist of poorly sorted subangular quartz pebblss of local derivation and in the lower-areas gravelly to coarse sandy sediment is more common. There are no surface structures apart from the poorly zoned flora. At Halwyn (800385), the highest floral zone is of drifted algae (Fucus and Enteromorpha). Between 2m and 5m from the shore, the green filamentous Enteromorpha may cover up to 95% of the bank. Between 5m and 16m from the shore the bank is colonised by Fucus. Below 16m gravel and coarse sand forms much of the sediment and shelly debris is very common in this zone. Broken and disarticulated fragments of the lamellibranchs Cerastoderma, Medulis, Scrobicularia and the gastropod Littorina were all recorded. The flora is.mostly drifted and decomposing. The transitional boundary with the intertidal flats environment is reached at about 18m from the shoreline. Because of the recent artificial embankment of the estuary and the ria configuration itself, the shingle banks are largely static and sediments of this type do not occur in the subsurface except in their present day locations. The only obvious modification caused by burial is an increase in the finer grained component which is given by rotting plant -debris.

Near St. Clement on the Tresillian River (850 440) the shingle bank environment is better developed and consists of banks up to 15m wide which, in one area, extend to form a spit behind which is a lagoon. Floral colonisation of these banks and the spit is more advanced than at Halwyn, Near shore there is about 60% cover given by Salicornia which does not occur at Halwyn. On the spit top filamentous green algae bind the finer sediment to form thinly bedded silts which rest on poorly sorted pebbles and cobbles. The lagoon itself appears to be accumulating organic debris and may be silting up. The 56

lower zones of the Tresillian shingle banks are similar to those at Halwyn although the cover given by Fucus and Pelvetia is reduced to perhaps 30^ and Hydrobia occur more commonly in the drifted debris.

Upstream from St. Clements in the Tresillian River the occurrence of the various wracks decline further and the majority of shingle banks are colonised by very thin bedded algal mats which entrap silts, muds and sporadic drifted organic materials.

/ At Tresemple and Kiggon, colonisation is well advanced and the original shingle banks are only found in the sub- surface, Extensive artificial embankments have been constructed at Tresemple (855447) and at Kiggon (866455). In this latter area the highest division is a fully established saltmarsh with a majority of land-plants and grasses. Surface depressions are infilled remnants of former drainage channels and will probably be preserved as unbioturbated organic-rich lenses of very fine grained t sediment in the root penetrated marsh muds. Extending for between 8 and 10m from the edge of the marsh, the mudflat is colonised by Pucinellia' and greenish brown algal patches. At the margin there is a narrow discontinuous Salicornia zone in which filamentous green Algae occur. These increase channelwards to cover a 1 - 2m wide zone almost completely. Lower down on the mudflat, extensive polygonal and elongated stellate mudcracks break up the algal mat, and a mudflake conglomerate is accumulating in a zone about 10m wide. The boundary with the tidal flats is marked by a slightly coarser zone with common mudflakes, drifted Hydrobia and Scrobicularia debris and displaced green and brown algae. An anomalous environment occurs at Kiggon as at that locality ornamental reeds have been introduced and now form areas of saltmarsh without any development of Pucinellia or Salicornia. The sedimentary column varies with the morphology of the underlying shincle bank and with location within the estuary. Figure 8 is typical.

In general, near the shore the fine root penetrated marsh muds pass down into an iron rich zone of coarse gravels and iron oxide coated mudflakes which overlie a coarse gravelley to pebbly conglomerate. On the mudcracked flats very thinly bedded, mudcracked and algal root penetrated muddy silts pass down through an iron oxide coated pebble and mudflake conglomerate into the shingle conglomerate. However, towards the channel, the bedding structures are progressively though somewhat inhomogeneously obliterated by bioturbation ,

Near the village of Tresillian the saltmarshes gradually widen to reach a maximum of 40m about 300m downstream of the road bridge. The cultivated reeds become very important in the formation of these marshes and cover large areas on both banks. As can be seen from figure 3, the channel slopes discussed above gradually pass up into a narrow zone covered with brown and green algal ooze which is colonised by reeds and coarse grasses. Broken reed stems are bound together by webs of algal filaments and act as efficient traps for sediment, so that the succession of halophytes is not present and the land-plants are rapidly established. These marshes have not been studied in detail as they are considerably modified by human activity.

The marginal environments at the head of Restronguet Creek are somewhat similar to those of the Tresillian estuary in that a lower mudcracked, algal flats environment passes up into a zone of mudflats which have been colonised firstly by Salicornia then Pucinellia and .finally by a typical saltmarsh flora with extensive development of Halimione. These major floral and environmental divisions are shown on Fig. (3). Ornamental reeds are not present in Restronguet Creek.

The mudcracked algal flats are characterised by drifted plant debris, polygonal and stellate mudcracks, bird tracks and beak prints, rare shell fragments, and, 59 a mat-like development of green, filamentous algae, especially towards the contact with the upper intertidal flats: plate 9. There are traces of fossil Nereid burrows but none appear to be recent. In places the surfaces of the mudcracks are coatad with reddish-orange iron oxides which also appear to have impregnated the surrounding sediment. This iron oxide cemented sediment is resistant to erosion and accumulates in limited pockets and embay- ments to form a mudflake conglomerate: Plate 9. The subsurface sediments which occur beneath this environment are thought to have been substantially influenced by man and are typically thinly bedded and mudcracked muddy silts with some evidence of slight Nereid bioturbation and rather more common root bioturbation. Detailed logs of Cores 1, 2, 3, 4, 21, 22, 23 and 24 from this environment may be found in the' appendix.

The Salicornia and Pucinellia environments which lie at higher levels in the estuary differ from the mudcracked algal flats only by the absence of the mudflake conglomerate and traces of bioturbation and the presence of the colonising plants. The subsurface sediments are very thinly to thinly bedded and are commonly penetrated by roots . Some units contain tiny iron oxide cemented ellipsoids which are commonly in the size range 1mm - 4mm end are of unknown provenance. A s illustrated by Plate 10, extensive pools of standing water may occur and these commonly accumulate fine grained sediment and organic debris. In most of the studied examples, this fine sediment was thoroughly bioturbated by Nereid diversicolour.

Towards the head of the estuary extensive tracts of saltmarsh are developed, although the only known example of the present day formation of this facies has been described above from the lower point bar in the Tallacks Creek, Plate 11 demonstrates that at the present day erosion of the saltmarsh is probably the dominant process. The surface features of the marsh are limited to a linear development of Halimione along the banks of the creeks PLATE 9 Mudcracked algal flats 61

PLATE 11 Destruction of the seltmarsh 62 and their tributaries and blanket cover by Salicornia. Pucinellia and filamentous green algae elsewhere. The subsurface sediments from most areas of saltmarsh are blackened and highly sulphidic and consist of root penetrated thinly bedded muds and silts which rest on a basal iron oxide cemented mudflake conglomerate. As indicated by the detailed core logs given in the appendix the underlying sediment is thinly bedded mining waste with traces of Nereid bioturbation and common mudcracks.

Artificial Environments

Near the head of Restronguet Creek, large areas of thinly bedded silty and sandy sediment are overlain by only a very thin veneer of muddy silt and have been progressively colonised by the sequence of saltmarsh plants described above. The remains of tailings lagoons, dams, launders and entraining walls are common and it is evident that this sediment was artificially deposited mining waste similar to that which is found in the lower zones of the sediment cores from Tallacks Creek. The Ordnance Survey plans of 1908 indicate quite clearly that deposition of tailings continued well into this century and may have been partly responsible for the silting up of the port of Devoran. Thus, these higher areas are excluded from the present study and, the author is also aware that certain lower areas near Tallacks Creak have only been subject to natural sedimentary processes for about the past 60 years.

Elsewhere in Restronguet Creek the creek mouth complex itself may have been formed as a result of human activity. Local information is that about 25 years ago a wreck blocked a main flood channel beneath the present creek mouth complex and diverted the tidal flow into its present central channel. Evidence was given earlier for the presence of such an abandoned channel and its course is roughly platted on figure (2). However, it is also possible that this example of channel reversion occurred as a result of diminishing sediment supply from the 63.

tributaries and headwaters. The effect upon the distribution of sedimentary environments is considerable. Algal banks which were probably formed on levees or on an ebb shield are now being eroded and the.living algal banks are retreat- ing towards the shore. The old channel is filled with fine silts and organic rich muds which rsst directly on coarse channel sand. The tidal flats associated with the defunct channel are now being eroded by flood tides at the base of the flood ramp and are also bsing modified by flood tidal action behind the ebb shield. There is now a predominance of marine derived coarse sand in the lower estuary whereas the older flats seem to have been of fine sand and silt which was arguably tailings sand. Thus, it appears that an earlier rather low energy tidal flat sedimentary environment is being replaced by a higher energy flood delta environment with a marked marine aspect. 64

CHAPTER 3

THE PHYSICAL CHARACTERISTICS OF THE SEDIMENTS AND BULK MINERALOGY

In this chapter, the grain size distribution, mineralogy, natural moisture content and physical appearance of sediments from the major environmental divisions are described. The sulphide mineralogy is of such major importance that it is dealt with separately at the end of the chapter.

The data recorded belou were all obtained from a systematic sampling and recording programme in both the Tresillian River and Restronguet Creek.

Sample points were pre-selected according to a simple random stratified system which was adapted as necessary in the field. Areas which were not accessible due to safety or ownership difficulties were omitted from the study. Some sample points were duplicated up to five times within an 2 approximate area of 100 m in order to provide some indication of sampling variability. At each point as many of the following samples as possible were obtained:-

(a) Grab samples of 500g of sediment occurring in the depth intervals Q-10cm, 1Q-20cm and 20-30 cms. These were sealed in airtight polythene and stored at 4°C. (b) A vertical undisturbed 6.5cm diameter sediment core which was completely sealed within its plastic core barrel and stored at 4°C. (c) Duplicate 30cm diameter canisters of undisturbed sediment from the 10-30cm depth interval which were also completely sealed and stored at 4°C. (d) A surface slick sample of the 0-1cm depth interval collected from an arsa 1m 2 . (e) A box core from all sandy environments. (f) A 21 sample of the water draining into a pit excavated up to 40cms but excluding simple surface run off. Collected in clean polythene 65

ware and stored at 4°C after filtration through a 0.membrane and acidification to pH2 with a known amount of nitric acid. (g) A water sample collected in the same way but filtered through a 0 .4&U/ membrane • and stabilise-d with alkaline zinc chloride solution at the sample point itself, (h) Where possible, a water sample was obtained from all creeks and channels at high and low tide and treated as for (f) above.

In addition, the sediment colour and pH was recorded on site at each point using a U.S.G.S. Colour Chart and pH paper sensitive to about 0.4 pH units. All other sediment characteristics were recorded in a notebook at the site.

In the laboratory the following initial procedure was adapted for the sediment cores:-

Core into Laboratory

Cut and split into two halves

Split into two Two channels cut in halves centre of core. Remaining material is sub sampled for grain size analysis

Stored for Thinned for (D (2) reference X Radiograph

X Radiograph Grain size analysis Subsampled and impregnated with G eochemical G eochemic AY 18 Araldite analysis analysis resin or Carbowax (solid) (liquid) (D (2) Polished and thin sections

Grain size analyses were carried out on air dried grab 66 and core sample materials using 8ritish Standard sieves for sand sized grains and a Stokes' Law settling tube for silt and clay sized grains.

Pretreatments such as washing or digestion of organic matter with hydrogen peroxide were .not undertaken although after Carver (1971) the silt and clay fraction was dispersed in the tube with 10^ sodium hexameta phosphate solution. No attempt was made to analyse the clay fraction (-2/iC ) in detail as in all but a few samples it appeared to be a negligible, proportion of the total.

The results were plotted on probability graph paper using the Phi (0) scale proposed by Krumbein (1934) as a modification of Uentworths (1922) grade scale. The median grain size (050), sorting QD0) and skewness (Skq0) statistics were derived from these graphs. Simple measures QD0 = (075 - 025) 1.35 and Skq0 = (025 + 075 - 205O)/2 were employed, after Krumbein (1934) and Krumbein and Pettijohn (1938). These somewhat crude statistical computations are included here purely as descriptive aids for it was quickly established that detailed grain size analyses and comparisons would not be required to distinguish either surface sedimentary environment or subsurface facies variations.

Polished and thin sections were routinely produced using two impregnating media:-

(i) AY 18 araldite resin in the ratio 100:75 with hardener, vacuum impregnated at room temperature and cured at 50°C for three weeks. (ii) Carbowax applied to the sediment at 60°C continuously over a period of one month and allowed to cool slowly.

Both were equally successful although the carbowax procedure is more straightforward. Problems ware only encountered with the finest grained sediment and with a tarnish which developed rapidly on all the polished surfaces. Unfortunately, some of the sulphides are so fine grained that this could not be entirely eliminated in some cases. Most of the 67

impregnated sediments were so delicate that very careful polishing was required and this also avoided any risk of polishing alteration of the finest grained material.

The bulk silicate mineralogy of the sediment sample was determined in order to assess the results of trace element studies and to assist in the delineation of the various environments. The X ray diffraction method adopted proved to be sensitive to all physically recognisable environments and in addition gave an approximate estimate of the relative volumes of the silicate minerals in the sample and indicated their degree of alteration. The sediment sample was carefully ground to pass the 400^ sieve avoiding significant bias from the platy or harder minerals. i

A 50g subsample was cavity mounted and irradiated with CoKoc radiation in Phillips X.R.D. equipment. The sample was rotated at 1°/min to give a graphical trace from the recorder which was set at 400 counts/min. A series of standard samples was prepared by separately spiking clean quartz with known amounts of illite/muscovite and kaolin. From the output diffractogram a number of prominent and non-interfering lines were selected for each minerals-

Muscovite 9.990, 1.990 K aolin 3.580, 2.340, 2.370 Quartz 4.250, 2.4550, 2.2820, 1.8170 Chlorite 140, 4.2^0, 3.540 Tourmaline 2.970, 3.990, 6.360

The relative peak height intensity of these lines was then averaged to give a mean index value for the mineral in each of the spiked standards. It was found that this index varied regularly for up to a 50% mineral concentration in the spiked standard and enabled calibration curves to be drawn. By comparison of the index value from the sample with the calibration curves a semi-quantitivs estimate of the concentrations of quartz, illite/muscovite and kaolin 68 was obtained. The chlorite concentration was then deduced by subtraction. The index values were found to vary regularly with sedimentary environment and gave an excellent discrim- ination of the various sediment types. Standards for the determination of tourmaline, feldspar, calcite and sulphides were nob prepared and only qualitative estimates of the concentration of these minerals were made.

A number of representative samples of (-2/ju ) clay sized materials from the pipette analyses were vacuum mounted on ceramic discs and irradiated at a rotation rate • f 2CI/Iv1inute. These samples were then treated with ethylene glycol and irradiated once more. Finally they were heated for an hour at 500°C and irradiated again. A simple comparison of the three diffractograms for each sample satisfactorily distinguished the expanding layer clay minerals (smectite, swelling chlorite, vermiculite and mixed layer minerals) which were thought likely to occur in the study area,Thorez (1975). A detailed study of the ratio of major peak intervals of chlorite was also undertaken in order to assess whether more than one chlorite variety was present in the samples.

3.1 GRAIN SIZE, NATURAL MOISTURE CONTENTS AND BULK SAMPLE MINERALOGY

The Creek Mouth Complex

The sediments of the flood channels and ramp form the topographically lowest part of the creek mouth complex and are medium sands with a median grain size of 1.50. They are well sorted with QD0 and Skq0 values of 0.74 and 0.20 units respectively. The sediments on the northern and western margin are generally of finer grain size with median 0 values falling in the range + 1.30 to 2.20.

The sands are generally dark yellowish brown (G.S.A. Rock Colour Chart Committee Pubcn. standard 10 YR 4/2) although the upper few centimetres may be dark reddish 69

brown (10R 3/4). The thin organic rich beds and rnud flasers are significantly darker and may be moderate or dusky brown to black. The sediment is free draining and thus the water table in the flood ramp responds rapidly to the state of the tide. As the samples inevitably drain very rapidly during excavation, the recorded mean natural moisture content of 22% is probably rather low.

The silicate mineralogy of these sediments is dominated by quartz but muscovite, kaolin, chlorite and feldspar are also present. Semi-quantitative X.R.D. studies, indicate that quartz forms 70%, muscovite less than 20% and kaolin less than 10% of the sediment. The remaining few percent comprises chlorite, feldspar and tourmaline. Detailed X.R.D. studies of the silt and clay fraction also revealed the presence of sphalerite, chalcopyrite, cassiterite and kesterite, (Fe (Cu, Sn) S2). Attempts were made to obtain a concentrate containing the latter mineral for X.R.D. fibre identification but were unsuccessful. It was shown to be very nearly as soluble as sphalerite in warm 2m HC1. These detailed studies also demonstrated that: the tourmaline is dravitic in composition, complex clay minerals are not present, and, the chlorite exhibits a constant ratio of peak intervals. Fig. 9 shows the locations of the X.R.D. samples taken in this environmental sub-division. The bar graphs illustrate the relative proportions of the major silicate minerals present in these samples.

The ebb-shield sediments have a median grain size of + 2.75 JZl and are fine sands. They exhibit a unimodal distribution of grain sizes with QD0 and Skq# of 0.55 and 0.125 JZl units respectively. They are thus better sorted than the flood ramp sands. There is some variation of mean grain size along the ebb shield with finer grained sediments occurring on the western and north western margins. The fresh sediment is rather homogeneous light olive grey (5Y 5/2) to olive grey (5Y 3/2) in colour and is free draining. However, the grab samples gave an average moisture content of 27%.

71 The mineralogy is virtually identical to that of the flood ramp although the peak intensity averages indicate that quartz is a little less abundant and chlorite and kaolin are more abundant. The semi-quantitative data indicate that on the ebb shield, kaolin forms about 5% more of the. silicates than •on the flood ramp. The minor species are also virtually the same as on the flood ramp with feldspar and tourmaline dominant. These minerals are concentrated in the silt sized fraction along with major kesterite, cassiterite and minor hematite, pyrite, chalcopyrite and sphalerite. Only a very poor clay fraction was available for XRD analysis, but the thermal and ethylene glycol treatments suggested the presence of very minor amounts of smectite which appeared to be calcium monmor- illonite. As may be seen from Figure 9 areal variation of the major species is limited to a slight snrichment of muscovite in the more sheltered and higher southern and western ebb shield areas (see sample 1*1150).

The Intertidal Flats

The intertidal flat environment is sub-divided into lower and upper flats which differ considerably in surface sedimentary features. These differences are emphasized by the grain size data presented below.

Intertidal Flats: lower division

The lower intertidal flats receive coarse marine derived sediment on the flood tides and finer sediment at slack water which accumulates as mudflasers and sheet oozes. A north north-westerly traverse along the boundary with the ebb shield gave the following grain size analyses. The samples may be located in Figure 9.

Sample No. Median grain size Sorting QD0 Skewness Skq0 M 15 5 , M1 50 +3.250 to +3.50 0.740 0.1 20 407 + 3.60 1 .30 0.5250 384 +4.250 1 .850 0.500 72

As may be seen, finer sediment is progressively admixed along this traverse in a north and westerly direction to give a series of moderate to poorly sorted very fine sands and coarse silts. To the west, this process continues and a median grain size of +4.60, QD0 of 1.85 0 units and Skq0 of 0.4 0 units was obtained in sample 400. These poorly sorted coarse grained silts pass shorewards and laterally into finer but better sorted silts (median +5.250, QD0 1.30 units, Skq0 -0.125 0 units) at the boundary with the upper tidal flats. The sediments are commonly olive grey (5Y 3/2) to greyish brown (5YR 3/2), although in sample 384 an unusual colour variation from brownish black (5YR 2/1) near the surface to greyish red purple (5RP 4/2) at a depth of 12cms was recorded. They are rather poorly drained and thus the recorded mean moisture content of 35% is higher than the ebb shield mean.

Variations in silicate mineralogy correlate well with the grain size variations described above. Quartz decreases progressively in a northern and western direction where kaolin may comprise up to 35% of the sample. Muscovite and chlorite also increase in this direction but less dramatically. An anomalously high quartz content was recorded for sample 421. This is interpreted as being due to its topographically high position (relative to sample 407) which may have allowed coarse detrital muscovite to be winnowed out by tidal action. The bulk variations in mineralogy are illustrated in Figure 9. Feldspar and tourmaline are common in this sub-division but sulphides and cassiterite were not positively identified. Detailed studies reveal only minor smectite and kaolin clay minerals but indicate that two varieties of chlorite are present. In most samples a well crystallized, probably detrital, chlorite can be identified but in some samples a second, slightly different group of characteristically indistinct peak interval ratios provides evidence for the presence of a poorly crystallized chlorite variety which is significantly enriched in iron. In samples from the algal banks which occur in the north west of the area, this variety is of major importance and probably amounts to 73

ten percent of the sample. Detailed clay fraction studies yield the best evidence for this mineral and thus indicate that it is more common in the finest grained materials. This would support the view that it is an authigenic iron- magnesium silicate although it could possibly also be an alteration product of fine grained detrital chlorite.

Intertidal Flats: upper division

Sediments in the upper intertidal flats are much finer grained than on the lower flats, although a continuous sequence towards the head of the Restronguet Creek and then up the Tresillian River may be recognised. The average median" grain size in Restronguet Creek is about 5.50 (QD0 of 0.550 units, Skq0 -0.1250 units) and in the Tresillian River the average is about 5.750 (QD0 of 0.930 and Skq0 of 0.1250 units). The relatively poor degree of sorting recorded for the Tresillian sediments is attributed to the presence of woody, detrital organic materials and the possible admixture of a little coarse silt from the point bar environment described below. The silty sediment from the Restronguet Creek is commonly dusky brown (5YR 2/2) to brownish black (5YR 2/1), although Nereid bioturbation produces a brownish grey (5YR 4/1) to dark yellowish brown (10YR 4/2) colouration in places. The Tresillian River sediments exhibit similar colours although some extensive areas of olive to light olive grey (5Y 5/2) were recorded. The mean natural moisture content for sediments in both tributaries is about 45/£.

The bulk silicate mineralogy of these intertidal flats closely correlates with the grain size variations although in the Tresillian River, quartz is rather less common than might have been expected. In Restronguet Creek, muscovite comprises about 25^ and kaolin about 40^ of the total, whereas in the Tresillian River muscovite has increased to 30 - 40°£ and kaolin may form up to 60% of the sediment. Feldspar is a much more important component in the Tresillian River and its relative peak height parameter is also shown on the bar charts for this area. See figure 10. Sample 74 75

242 has a high muscovite content and 246 a high quartz content which may indicate that tidal winnowing in the area of 246 has displaced coarse detrital muscovite shorewards. Tourmaline is common in both tributaries and appears to be of dravitic composition, Kesterite is present, only in the Restronguet sediments and cassiterite could not be positively identified. Pyrite and sphalerite are ubiquitous but can only be positively identified in gravity concentrates of the sediments. Detailed studies of the clay mineral fraction indicated that well crystallised chlorite and kaolinite are present, calcium montmorillonite may be present in very small-quantities, and mixed layer clays are absent. Poorly crystallized chlorite is present in only very minor amounts,

Channel Slopes

The channel slopes are more affected by tidal flux than the upper intertidal flats and with a median grain size of between 50 and 5.50 they are slightly coarser grained. On the whole, these sbdiments are also better sorted (QD0 0.740 and Skq0 of 0.250 units), and it appears that the flood tide sweeps coarse silt from the point bars onto these slopes and winnows the fine sediment shorewards. The environment is characterised by a high average natural moisture content of 52^ which is probably partly responsible for the instability of the slopes. The sediments are a pale, yellowish brown (10YR 6/2) or yellowish grey (5Y 7/2) at the surface but become olive grey, (5Y 4/1) at depth.

In the central Tresillian areas, the bulk silicate mineralogy is essentially similar to that of the upper intertidal flats sediment although muscovite may amount to about 5% less of the total. However, in Restronguet Creek, and at the mouth of the Tresillian estuary, quartz is much more common and may be of marine provenance. The anomalously high quartz feldspar and tourmaline concentrations at the head of the Tresillian River are of fluviatile origin. See Figures 9, 10. Detailed studies did not reveal 76 significant amounts of smectite. Sulphides and cassiterite were not positively identified.

Point Bars

Significant point bar deposits are restricted to the Tresillian River where organic rich sediments accumulate in three main areas. The grain size distributions are controlled by the organic materials which comprise just over 40% of the sediment and which fall into the 0.2mm to 6mm size range. The silicates are of medium silt grade although they appear to be more poorly sorted than similar tidal flat or channel slope sediment. The abundant organic material is very porous and thus ground water levels vary considerably with the tide. The recorded moisture contents of the grab samples averaged 54%. These sediments are commonly black although bioturbated silts may be light olive grey (5Y 6/1). The surface of this environment is generally mottled black and grey by the presence of black iron monosulphide which is stable at the surface due to high concentrations of dissolved sulphide in both groundwater and standing surface waters. Sulphide productivity is so high in this environment that reducing conditions are maintained up into the surface water column.

It was found that the high organic content of the X.R.D. samples interfered with the silicate peak intensities such that estimates of the muscovite (25-30%) and kaolinite (40-60%) proportions are very crude indeed. Tourmaline, feldspar and pyrite are ubiquitous. Detailed studies show that sphalerite is present in the sediment together with both detrital well crystallised chlorite and a significant amount of disordered, probably authigenic chlorite. The point bars are the only environment in which good evidence for the presence of smectite and vermiculite was obtained although they appear to form mixed layer minerals with illite. Compared with muscovite they do not form a volumetrically significant proportion of the silicates. 77

The Streams and Creeks

Deposits in the streams and creeks do not constitute a significant proportion of the estuarine sediment and were not studied in detail.

Marginal Environments

The marginal environments in the Tresillian River are comprised predominantly of shingle banks which become colonised to form mudcracked algal flats and may pass up into saltmarshes. The shingle banks have a median grain siza of 00 to -20 and are essentially composed of very coarse sand or gravel. They are very poorly sorted sediments. The mudcracked algal flats are commonly very poorly sorted silts with a QD0 value of 3.70 units and an Skq0 of -40 units. The moisture contents of these sediments vary from an average of 62% in the saltmarsh to about 35%, on the shingle banks and about 45/S on the algal flats. In general, the sediment colours are well correlated with the moisture contents. In the saltmarsh sediment there is a transition from the oxidized to the reduced facies at about 10cms depth accompanied by a colour change from moderate brown (5YR 4/4), - to dusky brown (5YR 2/2) or black. The oxidized coarse shingle sands and gravels are commonly light brown (5YR 5/6) to moderate reddish brown (10R 4/6). Offshore, the mudcracked flats are moderate brown (5YR 4/4) although algal laminae may be black. In some areas fossil burrows have permitted deep oxidation of the sediment and a range of brown colours (from 5YR 4/4 to SYR 2/2) occur.

In Restronguet Creek, the major marginal environments are the shingle banks, mudcracked algal flats and saltmarshes. All these deposits overlie artifically deposited tailings. The mudcracked algal flats are composed of very poorly sorted silts (median 5.250, QD0 of 2.60 and Skq of 00 units) and superficially these grain size distributions resemble those from equivalent sediment in the Tresillian River. However, in Restronguet Creek, the sand sized material is 78

not supplied from the shingle banks but is almost entirely composed of mudflakes and, probably gastropod, faecal pellets. Detailed studies indicate that these are absent in similar sediment from beneath the oxidising-reducing interface and this effect is attributed to the dissolution of an iron oxide cement at the interface. The saltmarsh sediments are typically well sorted medium to fine silts with a median grain size of 6.750 and QD0 value of 1.10 and Skq0 of 00 units. The moisture content of the mudcracked flats varies with topographic height but averages about 38^. Similarly the moisture content of the saltmarsh sediment varies from 46^ to 5B%. The mudcracked algal flats are either light brown (5Yr 5/6) or moderate brown (5YR 4/4) but pass laterally offshore into dark yellowish brown (10YR 4/2) sediment. The saltmarsh sediments are commonly pale to dark yellowish brawn (10YR 6/3 to 10YR 4/2) above the oxidizing/reducing interface and black beneath it. This transition occurs between 11 and 16 cms from the surface.

The bulk sample mineralogy of sediments from these marginal environments does not vary consistently across them and as may be seen from Fig. 9 the only obviously significant feature is the rather low chlorite content, of the offshore equivalent of the mudcracked algal flats subdivision. On average, muscovite comprises about 20^ and kaolinite about 30-40?b of the sediment. The overall average chlorite content is thus greater than in adjoining upper intertidal flats sediment. Feldspar and tourmaline are very common although pyrite is rare and no other sulphides could be positively identified. Cassiterite occurs only rarely.

Artificial Environments

As discussed above, artificial environments are widely distributed at the head of Restronguet Creek and in the sub-surface at Tallacks Creek. The sediments are coarse silts and fine sands and are very well bedded with good examples of graded bedding. Thus, the grain size analyses of bulk samples commonly give very wide interquartile 79

ranges of up to 50 units. In some areas of Tallacks Creek a mudflake microconglomerate with iron oxide cement occurs at the unconformable contact with the modern saltmarsh sediments described above. A wide range of colours is exhibited by these sediments although pale to dark yellowish browns (10YR 6/2 to "10YR 4/2) are most* common.

The bulk mineralogy is not significantly different from that of the upper Restronguet marginal environments discussed above although these sediments appear to be tailings which are very rich in cassiterite, pyrite, sphalerite, chalcopyrite, tourmaline and feldspar. Stannite occurs in some samples but is not as well developed as in some samples of the lower intertidal flats sediment. Other copper sulphides could not be positively identified on the X ray diffractograms.

3.2 SULPHIDE MINERALOGY

Small undisturbed samples of the sediments occurring along a longitudinal traverse of Restronguet Creek were vacuum impregnated with an araldite resin and polished for reflecting light microscopical study. In this chapter, the major features of the sulphide mineral assemblage are described. Due attention has been paid to the limited number of sections studied (34) in arriving at general conclusions. Particular difficulties were encountered in identifying sphalerite, the rarer minerals and the Finest grained materials as a result of the unavoidably poor polishing characteristics of some of the subject material. Six sections of sediments from the Tresillian River were examined and very few detrital sulphide grains were found. Thin sections were also studied where possible. In the event samples from cores 1-7 and 13 were studied in detail and supplemented by additional samples prepared from block samples of saltmarsh (Tallacks Creek) and lower intertidal flats sediments. As the sulphide mineralogy of similar environments in the Restronguet Creek and Tresillian River are so different, the two areas are discussed separately. 80 Restronguet Creek: The Creek Mouth Complex

Medium to fine cross-bedded sands are the most common sediment on the flood ramp uhich forms the louer part of the Restronguet Creek mouth complex. There is only sparse organic debris. Sporadic mudstreaks comprise less than" 5% of the total sediments and were probably deposited on the beds of braided channels during slack uater. Polished and thin sections shou that the coarse flood ramp sediment is almost entirely enveloped by both earthy and uell Ordered hematite. Near the mudstreaks, the iron oxide has been partially dissolved and in these zones the matrix includes fine grained organic debris uhich has infiltrated the coarser unit. Commonly the hematite forms platy crystals and stellate assemblages in the pore spaces of the coarsest units. Small detrital fragments and relict boxuork textures indicate that some of the iron is derived from ueathered pyrite. However, a significant proportion is probably derived from the alteration of iron bearing detrital silicates for uhich there is less visible evidence.

In these coarser units pyrite is the major sulphide uith lesser amounts of chalcopyrite (CuFeS2) sphalerite (ZnS) and arsenopyrite (FeAsS). These detrital grains are commonly either altered to hematite or have poorly crystallized hematite rims and overgrouths. Pyrite framboids are very rare and are internally altered to hematite. Flakes of chalcopyrite up to in diameter and 10AL thick uere obtained in gravity concentrates made from the pan fraction obtained in grain size analyses of these coarser sediments. Unfortunately these could not be located in situ. They appear to be spheroidal at their margins and are thought to be authigenic.

A variety of diagenetic sulphides are present in the finer grained organic rich mudfla sers. Framboidal pyrites 2-5//-' in diameter are common in silty sediment and are consistently larger in the most organic rich zones. Diagenetic pyrite cubes tend to form oriented overgrouths on partially altered detrital pyrites and there is a 81

generally higher concentration of diagenetic sulphides near weathered detrial sulphide grains.

No samples were taken of the ebb shield sediment which occurs in a limited area.at the crest of the flood ramp. However, it is probably similar to the lower intertidal flats sediment discussed below.

Restronguet Creek: The Intertidal Flats

The intertidal flat environment has been divided into a lower and an upper unit. The lower unit is confined to the lower Restronguet Creek and is composed of very fine sand and coarse silt with a moderate amount of organic material which commonly forms mucilage envelopes enclosing several detrital grains. Well ordered hematite only occurs where it has replaced pyrite, magnetite or ilmenite. Earthy hematitic iron oxide is uncommon in the fine grained matrix. Detrital minerals include pyrite, chalcopyrite, bornite (Cu^FeS^), cassiterite (SnC^) and arsenopyrite and these may all be rimmed by poorly ordered hematite.

The diagenetic mineral assemblage is dominated by pyrite which occurs as well developed overgrowths to detrital pyrite. These adopt a well defined cubic morphology in some of the finest grained near shore sediments. Framboidal pyrite is less important and is largely restricted to organic rich mucilage envelopes and detrital fragments. Individual framboids are easily disaggregated as there is little pyrite matrix between individual component spheroids.

Diagenetic chalcopyrite is ubiquitous and occurs mostly as minute unresolv/abl e crystallites less than 5 A/- in diameter distributed throughout the sediment. Rather more substantial aggregates of chalcopyrite are present at a number of sites in several areas of the polished sections e.g. adjacent to an abandoned warm tube, near organic debris, or surrounding a detrital pyrite grain. The occurrence of these aggregates appears to be controlled by local concentration and pH, Eh factors which also appear to give PLATE 12 Encrusting Chalcopyrite with poorly ordered hemati te Field 100/^ Oil

PLATE 13 Atoll-textured chalcopyrite

Field 60/uu Oil

PLATE 14 Stellate aggregates of bladed h \ • Chalcopyrite Field 60/v Oil , -to/ ^ 83 rise to a diversity of mineralogical textures. For example Plate 12 illustrates encrusting chalcopyrite associated with poorly ordered hematite. The atoll textured chalcopyrite in Plate 13 has partially replaced detrital pyrite of uhich only a feu remnants can now be recognized amongst the matrix of earthy iron oxide. Finally in Plate 14 stellate aggregates of bladed chalcopyrite have developed as an overgrouth to detrital pyrite in an organic rich matrix. Unfortunately there are insufficient polished sections of these materials available for any attempt at a correlation of textural features uith local sedimentological parameters.

Bornite (Cu^FeS^) is uncommon in this environment and appears to be unstable altering to diagenetic chalcopyrite and overgrouth hematite. A feu of the dusty chalcopyrite crystallites in the finer units appear to be iron poor but even uith S.E.M. analysis this could not be demonstrated satis factorily.

Detrital grains of cassiterite and sphalerite are relatively uncommon in these sediments. As the bulk mineralogical X.R.D. studies indicated that zinc-rich stannite (kesterite) Cu(Fe,Sn,Zn)S2 could be present many grains uere examined by S.E.M. One complex grain of stannite Cu(Fe,Sn)S2 uas positively identified but no grains of kesterite uere found. The stannite uas partially overgrown by crustiform diagenetic chalcopyrite.

The upper intertidal flats in Restronguet Creek are composed of strongly bioturbated silty sediment uith increased organic material. The organic material forms mucilage envelopes uhich enclose the detrital silicate grains and are associated uith diagenetic sulphides. Hematite is rare in this environment and detriai sulphide mineral grains are only moderately altered. Bornite, bornite intergroun uith bluish flame chalcocite (CU2S), pyrite, chalcopyrite (uith blades and flames of bornite), sphalerite and intergroun sphalerite and chalcopyrite are all quite common. 84

The diagenetic minerals exhibit a variety of mineralogical textures, Pyrite is stable in this environment such that some large detrital pyrites are unaltered at their margins and others are overgrown by small irregular masses of diagenetic pyrite with slightly different polishing characteristics. There are many pyrite framboids and cubic and polyhedral crystallites in the organic rich sediment zones. They commonly range from 1-2/^L (barely visible) to about lOyl/Lin diameter. There is evidence that the framboids are metastable as there is no pyrite matrix between the crystallites and several examples appear to be decomposed in the core and overgrown with more homogeneous pyrite in a continuous rim.

The detrital copper minerals are unstable and have partially altered to pinkish bornite. Even early formed bladed chalcopyrite is overgrown with diagenetic pink bornite. Plate 15. Elsewhere a variety of textures are exhibited although crustiform overgrowths up to thick are most common. Sphalerite is rather more common in this environment and occurs as detrital grains, diagenetic overgrowths on chalcopyrite and discrete framboids. Stannite was not identified optically in these sediments.

Restronguet Creek: The Marginal Environments

The marginal environments in Restronguet Creek consist of a lower algal flat unit, algal flats colonised by Salicornia and Pucinellia and an upper salt marsh unit. Sediments in all these subdivisions rest unfonformably upon artificially sedimented tailinos. As none of the IS algal flats sediments a^e* rich in organic materials and all are extensively mudcracked they are generally oxidized. Thus, the predominent metalliferous mineral is hematite which occurs as earthy and well ordered garlands, rims and overgrowth enclosing the detrital sulphides. In some zones, very poorly crystallised iron oxide forms a matrix and a cement to the detrital silicates. Earthy copper oxides, sulphate and probably carbonates 85

ATE 15 Diagenetic pink bornite overgrouth on chalcopyrite Field 60y[^ Oil 86

PLATE 16 Chalcopyrite partially replaced by purple bornite Field 85 A/L Oil 87

are associated uith altered chalcopyrite grains but could not be satisfactorily polished for optical study. The easily visible detrital and diagenetic sulphides are associated uith microscopically unresolvable crystallites of copper and iron sulphides uhich are disseminated throughout the polished sections.

The copper sulphides exhibit a weak zonation uith • depth in these sediments. However it is important to stress that uithin any particular zone there are anomalous chemical micro environments in uhich factors such as oxygen supply, concentration of organic material, pH and porosity vary uith the original depositional characteristics or uith later formation of mudcracks and thus cause departures from the general trend. In general, detrital chalcopyrite and intergroun chalcopyrite and bornite grains are unstable and alter to a moderately uell crystallized hematite assemblage in near surface environments and compact purple bornite uith depth and in near surface reducing microenvironments. Plate 16. Uith increasing depth, more copper appears to be available and grey chalcocite CU2S has partially replaced the purple bornite. Most of the copper sulphide is oxidized to earthy cuprite at the contacts uith more permeable oxygenated zones and reduced to native copper in organic rich, sulphur poor microenvironments. There is no evidence that pyrite is replaced by copper sulphides even in these copper rich environments. Most pyrite grains have either decomposed to hematite or are stable and unaltered in the reduced zones.

The saltmarsh sediments contain significant concentrations of sulphide minerals but as discussed above they are generally being eroded except on one point bar in Tallacks Creek. Immediately beneath the surface of the marsh there is an oxidized zone uhich is pervaded by roots and has a high concentration of earthy hematite. This zone is underlain by similar but reduced sediments uithout significant hematite. However, an iron pan is developed in depth at about the level of the standing uater table 88 uhich generally coincides uith the unconformable contact uith underlying tailings sediments. Thus a detrital sulphide grain uill have been exposed to both oxidizing and reducing conditpns before it is eventually emplaced into the stable sub-uater table reducing environment. This instability is evident in the polished sections for crustiform diagenetic textures are dominant in the saltmarsh sediments and only become less common in the fossil unit beneath the unconformity. The detrital pyrite, chalcopyrite and sphalerite grains in reduced saltmarsh sediments are commonly heavily corroded and tend to be overgrown by disordered hematite.

Diagenetic pyrite is ubiquitous in the reduced zone of the marsh and occurs as cubic or polyhedral crystals marginal to remnant detrital pyrites or enclosed by organic debris, Framboids up to 20/^ in diameter are much less common and are associated uith organic materials.

In the reduced zone of the saltmarsh crustiform to granular pink or orange-pink bornite is the most common copper sulphide. Some of the overgrouths on detrital chalcopyrite are up to 12//- thick. Some atoll-textures indicate that early formed diagenetic chalcopyrite from near surface reducing microenvironments has been partially replaced by pink bornite and associated hematite at greater depth. Plates 17,

This process of gradual replacement by overgrouth and eventual central dissolution is illustrated by the sequence of S.E.M. Plates 19,20,21. Comparison of Plates 20 and 21 indicates that the outer rim of pink reflecting copper sulphide is significantly iron deficient. Other sulphides are rare or absent and could not be resolved optically.

Beneath the reduced zone, the majority of the diagenetic minerals are destroyed by oxidation. Most are directly replaced in situ by crustiform hematite. Some remnant detrital sulphide grains may be further preserved by their oxide pellicles but they are much less common 89

PLATE 17 Atoll textured pink bornite Field 60A, Oil

PLATt 18 Garlands, rims and overgrowths of pink bornite on chalcopyrite Field 60/v Gil PLATE 19 SEM Electron image of PLATE 18 Field 250AC

PLATE 20 PLATE 19 SEM Copper assay Field 250,

PLATE 21 PLATE 19 SEM Iron assay Field 250/tv 91 beneath the iron pan.

The diagentic environment beneath the iron pan is probably the most complex in the estuary for there is relatively little organic matter or detrital sulphide but readily available copper and iron. The sediments are generally rather fine grained and thus themselves have lou permeabilities but they are cut by permeable root traces and mudcracks, These various conflicting factors promote the development of a complex sulphide mineralogy which does not approach equilibrium at the present time. Pyrite is common but mainly occurs as crystallites and framboids in the more organic rich intervals away from major mudcracks. Rare detrital grains are marginally overgrown and replaced by purple bornite.

The copper sulphide mineralogy is complex and dependent upon localised conditions. In general, chalcopyrite is unstable and altered to hematite although associated purple bornite is unaffected. One fine example of this phenomenon is given by a complex grain of chalcopyrite and flame-bornite which occurs near an oxidized fossil root structure. The purple bornite flame persists unaltered although the enclosing chalcopyrite is replaced by hematite. Elsewhere in the less permeable, more organic rich units, pinkish bornite is ubiquitous and forms semiframboidal to crustiform rims and overgrowths to organic particles and atoll textures with remnant chalcopyrite cores. Plates 22 and 23. However in those zones with the highest permeability, chalcopyrite has partially replaced both pink and purple bornite to form atoll-textured chalcopyrite assemblages enclosed in an earthy hematite matrix. Plate 24. There is a clear transition into orange-pink bornite away from these permeable zones.

Sphalerite is moderately common and appears to be stable in most environments although it is overgrown by dusty or poorly crystallised pyrite in a few examples. This could be the result of sulphidatio.n of an iron oxide coating acquired during oxidation in the overlying 92

PLATE 22 Pink barnite forming atoll textures Field 60/^ Oil

PLATE 23 Pink bornite forming garlands Field 60/Ct- Oil 93

0 i

• €> * % 0 f

• • * c* t. -V ' * , • w4 ^P -fit* ! • " WSv %

•

isy*-,

PLATE 24 Chalcopyrite and associated earthy hematite marginal to permeable root zone Field 3 rr\m Oil

PLATE 25 Sphalerite enclosing pyrite Field 60/V Oil 94 environments. One section, Plate 25, appears to indicate that in some areas a sphalerite overgrowth is currently developing and is enclosing the diagenetic pyrite.

More recently deposited sediments comprise the con- structional point bar environment and are generally much closer to mineralogical equilibrium. The sedimentary sequence consists of a basal thinly bedded unit, an unconformable coarse channel lag conglomerate.which passes up into generally finer grained bioturbatea silts and silty muds. The uppermost unit is composed of thinly bedded muddy silts which have been thoroughly pervaded by roots.

The sulphide mineral assemblage in the lowest thinly bedded unit is quite different to that described for similar sediments from beneath the saltmarsh. The only common sulphides are pyrite and chalcopyrite which occur chiefly as collomorphic overgrowths on remnant, partially altered detrital pyrite grains or as elliptical framboidal aggregates. Plate 26.

Bornite is rare and only a few grains of collomorphic orange-pink bornite associated with woody fragments are present.

The channel lag conglomerate is a generally well oxidized lithology in which the detrital clasts are commonly cemented by well crystallized plates of hematite. However in some notable blackened zones near organic debris there are many small (40/to) pyrite framboids. In places these are well-aggregated and appear to be replacing the hematitic matrix.

In contrast, the sulphide mineral assemblage in the upper bioturbated silts and muds is diverse. Pyrite occurs as dusty disseminated grains in organic rich zones,as framboids, as individual semi-cubic crystallites and as crustiform overgrowths and replacements of hematite. Chalcopyrite is very common and as may be seen in Plate 27 95

, - #.\VJ l Of _ *r#S>• ^JmFjr., -tt*

PLATE 26 Elliptical pyrite framboids Field 60yCL Oil

v / /'

PLATE 27 Rosettes and garlands of chalcopyrite Field 6Q/tv Oil 96

PLATE 28 Crustiform overgrowths of orange pink diagenetic bornite Field 60/t^ Oil

PLATE 29 Complex grain of chalcopyrite, bornite and pyrite Field 60/v Oil 97

forms elongate to acicular crystals which are aggregated into discrete rosettes and garlands or uhich form radial overgrouths on detrital chalcopyrite grains. Elsewhere crustifarm overgrowths, narrow rims and dusty aggregates of chalcopyrite enclose other detrital grains and exhibit fine a£oll textures in some polished sections. Orange- pink bornite develops similar textures and forms coarse aggregates in the organic rich zones. Plate 28. However, it is less common than in the other saltmarsh environments.

The wide variety of sulphide mineral textures found in the point bar environments reflects the highly variable chemical micro environments present in the sediments themselves. For example in Plate 29 a central grain of crustiform chalcopyrite is marginally overgrown by pink bornite and hematite. The hematite is itself partially replaced and overgrown by dusty and framboidal pyrite. As this grain is situated near an abandoned worm burrow it is argued that the chalcopyrite uas formed first and partially oxidized when the burrow uas excavated. As the burrow became sealed and reducing conditions were re-established a copper rich microenvironment had been created which allowed the crystallization of bornite. Only pyrite has been.formed since depletion of this limited local concentration of copper and continued local production of hydrogen sulphide.

Tresillian River Environments

All the polished sections indicate that the organic rich silty clay sediments on the intertidal flats and point bars are rich in pyrite framboids. The latter range in size from 1C>A-to 100/1/U and about 15% occur as aggregates of framboids. They are composed of cubic or polygonal crystallites. Compared uith the Restronguet Creek environments, there is remarkably little recrystallization to form matrices within the framboids or to attain a cubic external morphology. Several large flakes of diagenetic chalcopyrite were obtained from pan concentrates of rather 98 shingly intertidal flat sediments at the boundary of some small areas of saltmarsh. Unfortunately, these interesting grains could not be obtained in place but they resemble those from the coarse sediments of the Restronguet flood ramp. The scanning electron micro-graph, Plate 30, shows one such flake which has been deliberately bent to give a three dimensional view. The sketch indicates a possible mode of formation by continuous aggregation of rounded crystal aggregates and recrystallisation of the outer surfaces. The S.E.M. semi-quanditative analyses indicate that there is a significant increase in the proportion of copper and sulphur towards the spheroidal margins of the grain and of zinc and iron towards the centre. However, a quantitive analysis would be required to establish whether sphalerite is intergroun uith the chalcopyrite. These types of grains are restricted to the rather coarse-grained near-shore environments where pyrite framboids are very rare indeed. A similar situation exists off the Restronguet flood ramp, (discussed above), although in the absence of definite polished section evidence it cannot be demonstrated that the flaky habit as illustrated is the normal mode of aggregation for microscopic diagenetic sulphide mineral grains in a rather porous, slightly oxidising and only moderately metalliferous sedimentary environment.

Summary

A summary of the results of this investigation of the sulphide mineralogy is given in Figs. (11) and (12) for Restronguet Creek sediments. Although they are considerably simplified these cross-sectional diagrams still show the complexity of mineralogical and textural variations occurring in the near surface environment and illustrate clearly how local chemical microenvironments distort larger scale physico-chemical trends and controls. For example authigenic pyrit.e, sphalerite and pink bornite are the dominant sulphide species in upper intertidal flats sediments and although chalcopyrite appears to be stable it is not forming in any quantity. From simple pH/Eh diagrams (e.g. after Rentzsch (1974)) chalcopyrite should occur as the principal authigenic copper mineral. It seems likely 99

PLATE 30 Chalcopyrite flake (bent in order to show three dimensional morphology) Field 200/LU (SEM image) MOUTH COMPLEX SHOWING SULPHIDE MINERALOGY FIG.U

Intertidal Flats

Lower < , Upper

Detrital pyrite stable with Detrital pyrite and sphalerite authiyenic pyrite overgrowth stable. Chalcopyrite may be stable.

Bornite (Cu^FeS^) locally breaking down to earthy hematite and Chalcopyrite (CuFeS2) Authigenic pyrite as framboids, Authigenic pyrite as large - Euhedral crystallites K^4-) overgrowths and polyhedral and overgrowths to detrital crystallites from (X^t- - 10/^-) pyrite. in diameter. Authigenic chalcopyrite as Authigenic "pink" bornite stellate aggregates (Cu^FeS^) as crustiform associated with detrital overgrowths on chalcopyrite. sulphide grains, crustiform Authigenic sphalerite (ZnS) overgrowths and atoll textured as overgrowths on detrital aggregates up to in sulphide grains and as diameter. polyhedral crystallites.

cpy DIAGRAMMATIC CROSS SECTION OF UPPER RESTRONGUET CREEK SHOWING SULPHIDE MINERALOGY FIG.12

Upper interlidal flats Saltmarsh

Earthy hematite forms Bornite alters to Chalcocite alters to Authigenic chalcopyrite alteration rims and Chaleocite and native Cu which is replaced by pink bornite garlands to all other earthy hematite associated with earthy and hematite. sulphide grains. and more crystalline hematite.

Llnresolvahle authigenic Authigenic minerals Authigenic minerals Authigenic pyrite as Authigenic chalcopyrite Authigenic pyrite as sulphide, probably unresolved unresolved framboids and euhedral and "pink" bornite framboids, crystallites bornitu (Cu. F'e . S2) <5/u crystallites. Authigenic exhibit a variety of and overgrowths. diameter also "pink" bornite(CucFe.S4) crustiform textures and occurs as rims and garlands, atoll structures depending Authigenic chalcopyrite and overgrowing on local chemical as rosettes and authigenic chalcopyrite conditions. garlands. forming atoll textures. Authigenic pyrite and Authigenic "orange- Sphalerite closely pink" bornite is associated in irregular common in organic grains. rich zones forming coarse aggregates and overgrowths to authigenic chalcopyrite

IOM 102 that the deviation from this theoretically prO-dicted result is as a result of the presence of locally enriched copper contents in sediment poreuaters producing what is essentially a metastable mineral assemblage. 103

CHAPTER 4

SOME CHEMICAL CHARACTERISTICS OF THE SEDIMENTS

This chapter describes aspects of the geochemistry of heavy metals, sulphur and organic carbon in the sediments of Restronguet Creek and Tresillian River. The analytical flowsheet employed is given in fig. (13). All the methods are standard within the Geology Department, Imperial College but are not necessarily described in detail in the literature. They are all as economical as possible and as will be pointed out a degree of absolute accuracy in the strict sense has been sacrificed in order to increase the total number of samples analysed. In all cases, however, the geochemical methods employed satisfactorily discriminate different facies types and results are reproducible within the stated interval.

Determination of Metals and Sulphur Species - general

A number of methods for the estimation of these components were considered and the X.R.F. technique for total metal given by Krom (1976) was initially favoured. However, within the constraints of the time available, equipment utilisation and the difficulty of comparing total metal values given by X.R.F. and partial extraction values obtained by A.A.S. analysis of acid leachates, it was decided to adopt an A.A.S. analytical procedure throughout. In this context a number of total and partial dissolution techniques were researched, all of which have well tried but specific applications. The standard method of Chester and Hughes (1967) appears to be satisfactory for deep sea sediments and has bean modified by Presley et al (1972) for sediments for the Saanicn Inlet, British Columbia. However, for the polluted Cornish estuarine materials the oxide and sulphide phases are very much more important and some means of discriminating between them was sought. The literature on this subject was found to be confused and in some cases contradictory, particularly Fig. (13): Analytical Schema Employed for Solid Sediment Samples

Sample I Divided into tuo halves

weighed I wet determination air dried of acid extractable at 80°C H2S I reweighed for moisture content I ground to pass 360// sieve I reground to better dry determination A.A.S. determination than 4GO// for serni of acid of Fe, f"ln, Cu, Pb, quantitive XRD extractable H^S Zn following 2m HC1 decomposi tion

conc. HNQ~ and 6M Hcl | decomposition A.A.S. determination BaCL^ of Fe, Mn, Cu, Pb, Zn turbidimetric determination of S042-

Sainple stored for special analyses etc.

o 105

in descriptions of methods for isolating the iron occurring as oxide coatings to mineral grains. For example, various researchers including Goldberg and Arrhenius (1958) and Gad and Le Riche (1966) have suggested that an acid ammonium oxalate reagent is capable of removing amorphous iron oxides from the sediment uithout appreciably attacking other phases. But McKeague (1967) contended that the oxalate reagent attacked crystalline Fe-Al oxides and even silicate in soil samples. Bullock and Loveland (1975), suggested that a potassium sodium pyrophosphate reagent uas capable of complexing organically bound iron which could then be determined after extraction of the pyrophosphate. However, it was established that pH was an important control, (Sascomb (1968)) and that alkaline conditions favoured a better recovery probably because these conditions would hydrolyse any organic complex present and the eventual determination would represent the metal bound to the hydrolysed species and not the original species. Furthermore, the accounts in the literature give little indication of the behaviour of the heavy metals Cu, Pb, Zn in such systems. Therefore, it was decided to adopt a simple, well known dissolution process and to carry out an orientation programme before selecting the final experimental conditions. Simple initial trials with various standard mixtures and natural sediment showed that warm dilute HCl extracted a proportion of the heavy metals, decomposed most of the oxide and monosulphide phases and destroyed up to 90^ of the chlorite. Some decomposition of the organic material was expected but this could not be verified experimentally. Pyrite, chalcopyrite and stannite were found to be virtually unaffected by this treatment but were readily decomposed by evaporating with concentrated nitric acid and leaching the residue with 6m HCl. ThOs duplicate ground sediment samples were treated with the dilute acid and concentrated nitric acid. In general treatment with dilute acid can be expected to cause the following 'species to go into solution:- 106

Metals contained in pore water salts Metals adsorbed on clays, oxides and some organic materials Iron and manganese oxides Complex iron phosphates Iron, copper, (manganese) lead and zinc monosulphides Manganese (and iron) carbonates Chlorite Some organic materials The concentrated nitric acid treatment decomposes the following species in addition to those aboue:- Pyrite, chalcopyrite and stannite Organic materials

Orientation trials indicated that the best method for producing a dilute acid leachate for A.A.S. analysis was by heating a 0.25g sample of ground sediment to a maximum of 30°C with 10 mis of Analar 2 m HCl over 50 minutes. It was found that the extractable manganese lead and zinc were liberated after only 10 minutes and that copper was liberated progressively throughout the period. Iron was found to be liberated swiftly in the first 10 minutes and progressively after that time. The grain size of the sample and the final temperature attained were not significant factors. Continua- tion of the experiment to higher temperatures (100°C) and more than 50 minutes produced a distinct increase in iron and copper concentrations which were attributed to partial decomposition of pyrite and chalcopyrite. After cooling, the samples were filtered through Whatman No. 42 filter paper and an aliquot was analysed by A.A.S. for the elements Fe, Mn, Cu, Pb, Zn within the Geology Department at Imperial College. It was found that up to 70 samples could be prepared for analysis in a single day and that satisfactory results were obtained for extractable metals in the ranges:-

Fe 40ppm to 10% in the dry sediment sample Cu 25 ppm to 15,000 pom in the dry sediment sample Mn 20 ppm to 5,000 ppm ,T " ,r " " Pb 10 ppm to 5,000 ppm " " " " " Zn 20 ppm to 10,000 ppm " " " " " 107

Replication of selected samples enabled the precision of this method to be estimated. For iron the relative standard deviation of replicate analyses expressed as a percentage of the mean value is 6.7% and for manganese, copper, lead and zinc the percentages are 14.3., 6.5, 6.5, and 9.0 respectively. In all, a total of 66 replications of four standard sampies were performed. An assessment of the total variability of the estimates including natural variability and that introduced by sampling and analysis was obtained 2 from the duplicate samples taken within 100 m in the same sedimentary environment:-

Average relative standard deviation (Ci) Iron 9 ,1% Manganese 13.0% Copper 14.0% L ead 12.6% Zinc 13.0%

The concentrated nitric acid dissolution method was developed from early trials with the specific aim of quantitatively dissolving the pyrite within a ground sediment sample and also producing the maximum possible heavy metal yield. Three samples of natural sediment were repeatedly evaporated with concentrated nitric acid and leached with dilute HCl to establish the optimum dissolution conditions. It was found that if a 0,25g sample of sediment was mixed with 2 mis of concentrated nitric acid and allowed to react at room tamparature for two hours and then evaporated almost to dryness in an air bath at about 110°C, complete dissolution of pyrite occurred. This procedure was adopted as standard. In order to obtain a solution for A.A.S., the residue was leached with 2 mis of 6m HCl at 70°C for one hour and allowed to cool. Ten millilitres of de-ionized water was then added, the sample was thoroughly mixed and filtered through a Whatman Mo. 42 filter paper. One aliquot was then presented for A.A.S. analysis of the Fe, Mn, Cu, Pb, Zn contents, another aliquot was retained for the total sulphur determination. 108

Replication of the standard samples gave the following mean standard deviations expressed as a percentage of the average value recorded.

Relative standard deviations (Ci) Fe 5.4% Fin 5.5% Cu 5.9% Pb 8.4% Zn '5.2% Determination of the duplicate samples taken from within 2 the 100 m area of the same sedimentary environment gave the following indication of total sampling and analytical variability, (also expressed as the mean standard deviation expressed as a percentage of the average value recorded).

Relative standard deviation (Ci) Fe 9.7% Fin 11.2% Cu 12.5% Pb 10.8% Zn 12.5% A total of 47 duplicate samples from nine separate areas were used to calculate these values.

Determination of Sulphur Species

As discussed above, estuarine sediment consists of the pore water, pore gases and the sedimentary mineral 2- - 2- assemblage. Sulphur is present as 504 , HS and S ions in the pore water, ^Sg in the pore gas and as Sc,S*" and polysulphide ions in the mineral assemblage. Various research workers have developed analytical schemes for the estimation of these components with those of Serner (1957a) or Kaplan (1963) being the most comprehensive. In the majority of these reports, the pore water sulphide species were determined by specific ion electrode or by iodimetry after evolution of the equivalent h^S. Pore water sulphate was estimated gravimetrically. The solid sample was leached with organic solvents to extract tha native sulphur 109 which was removed and weighed. Honosulphide phases were estimated by iodimetry after evolution of the equivalent H2S. The polysulphide and organically bound S was determined by oxidation to the sulphate which was then estimated gravimetrically.

It was decided that in this study the distribution of the total amount of sulphur within the estuary and particularly the proportions of the poly and monosulphide species should be determined. The volumetrically insignificant native sulphur and pore-sulphur species were to be neglected.

Determination of Sulphur Species: Total Sulphur

As discussed in the preceding section, the ccncentrat nitric acid dissolution technique was designed and modified to enable the quantitative determination of pyrite, and by inference all other sulphide species, in an aliquot of the leachate presented for heavy metal analysis. It was found in preliminary trials that the mean recovery from a spiked natural sediment sample in the range 0 - 2.5% S was 101.1% using a 0.25g sample and that more than 2.5% S could be accurately estimated by reducing the sample weight taken to 0.15g. Thus, the total sulphur content of a sediment sample may be obtained by calculation from the determination of sulphate in an aliquot of the solution obtained by the nitric acid digestion and HCl leaching technique described above. The selection of a method for determination of up to 5% sulphate in an HC1 solution proved to be difficult. The traditional gravimetric estimation of a BaSO. precipitate was far too slow for the number of samples required. The alternative methods available for the sulphate were the comparison of turbidity produced by addition of BaC12, by nepheiometry or a wet chemical method in which the sulphate solution is titrated with barium perchlorate in the presence of a 0.2% solution of thorin indicator. As the nephelometric method is subject to instrumental drift and destabilization of the BaSO. 110 suspension with time, the volumetric method was initially favoured. However, preliminary trials with spiked sulphate solution and artificial standards showed that whereas the nephelometric method was sufficiently accurate even in the presence of 30% HCl the volumetric estimation was very imprecise for samples with high acid and heavy metal contents. In a typical nephelometric determination of dissolved sulphate a 0.2 ml to 4 ml aliquot of the leachate from nitric acid digestion was made to 20 mis with de-ionized water 2 mis of NaCl/HCl reagent, (240g NaCl, 20 ml concentrated HCl in 1 litre), and 5 mis of glycerol/Alcohol reagent, (1 glycerol:2 alcohol). A calibration series containing from zero to 1000 mg 50^2- was similarly prepared after taking the required volume of 100 ppm potassium sulphate solution. About 60 mg of SaC^ crystals, (20 mesh, British Standard sieve size) were added to each tube and agitated gently for three minutes. The calibration set was transferred to the measuring tubes and the linear scale reading of the turbidity taken at a standard setting of the Corning Eel Nephelometer instrument. The samples were then determined at the same setting and the quantity of sulphate present was obtained. It was found that the calibration had to be performed after each 25 sampies and that each calibration set must be freshly prepared. No more than 12 samples were measured in a single set as destabilization of the BaSO^ suspension would have occurred in the later samples with resulting error in the measurement. About 30 samples per day could be measured under ideal conditions and as the nephelometric method was found to be suitable for up to 10,000 ppm SQ42~, samples from the estuary containing 10% sulphur were confidently estimated. Analysis of three standard samples indicated that the average standard deviation of the estimates expressed as a percentage of the recorded mean value (relative standard deviation, Ci) was nearly 10%. The total variability deduced from the analysis of duplicate grab samples taken within 100 m of the same environment and expressed in the same way was nearly 17% which indicated a high natural variability in total sulphur content, 111

Determination of Sulphur Species: Acid extractable hydrogen sulphide

Preliminary experiments indicated that a reasonable estimate of the sulphur present as the monosulphide species could be obtained by estimating the hydro gen sulphide evolved after treatment of the sediment uith hot, dilute acid. In addition, a measure of the proportion of the monosulphides uhich are easily oxidised in air uas obtained by air drying selected duplicate samples and repeating the analysis. Finally, an estimate of the concentration of polysulphides in the sample uas obtained by subtracting the monosulphide estimate from the total sulphur concentration obtained as above. Native sulphur and organically bound sulphur concentrations are unlikely to be significant and will be reported as polysulphide in this study.

Orientation studies were performed on samples of sediment from all the lithological divisions and they showed that after a maximum of 12 minutes boiling with 2m HC1, hydrogen sulphide was evolved from the wet sediment at a steady rate. After drying in air and regrinding to pass British Standard sieve size 360/4, the h^S evolution curve differed significantly and a virtually constant evolution rate was maintained. These results suggested that the dilute acid had effectively liberated the H^S from all the monosulphides present in the first twelve minutes of boiling. Pyrite and chalcopyrite were only slowly attacked and by extrapolation of the curves appeared to contribute only a minor quantity of f^S to the total. After air drying, the major proportion of the soluble sulphides was destroyed and h^S was evolved only from any remaining soluble sulphides and the pyrite and chalcopyrite. In addition, the rate of evolution appeared to have been accelerated by the fine grain size of the ground sample. Thus, the simple wet determination was found to give an estimate of the hydrogen sulphide present in sulphides other than pyrite and chalcopyrite, and the difference between wet and dry determinations gave an indication of the proportion contained in easily oxidized moncsulphides. 112

The major sources of error were recognised as being introduced during sub-sampling of uet sediment and by oxidation of fine grained iron, copper and zinc sulphides prior to acidification. However, a satisfactory discrim- ination between sediment types was obtained.

The apparatus employed for the determination of the evolved H2S was simple and consisted of a conical boiling flask, suck-back trap, and collecting flask. The latter contained 20mls of ammoniacal zinc chloride solution in 100mls of de-ionized water. After boiling for 12 minutes the collecting flask was disconnected and 200mg of potassium iodide and a known amount of potassium iodate solution (to excess) was added. After acidification with 2m HCl a straw coloured iodine solution was produced and titrated with a standard sodium thiosulphate solution using a fresh starch solution as an indicator. In practice an iodate solution of which 1 ml I 100mg H23 and an exactly equivalent thiosulphate solution was found to be the most suitable for routine estimation of samples in the range 40 to 10,000 ppm contained H2S. Up to 20 samples per day were routinely estimated using this method. The precision of the technique could not be estimated by replication of the same sample due to storage difficulties but a measure of the total sampling and analytical variability was obtained from the 2 replicate grab samples taken within an area of 100m in the same sedimentary environment. For nine such replications an average relative standard deviation of 40% was obtained. As may be seen from the table, the variability of the more coarse grained sediments from the lower Restrcnguet environments is significantly greater than that of samples from the well bioturbated Tresillian River environments. 113

SAMPLE RELATIVE STANDARD ENVIRONMENTAL DIVISION DEVIATION (Ci)

M102 - 6 (n = 5) 15.5% Tresillian R:Intertidal flats M112 - 115 (n = 4) 27.7% " " » 268 - 272 (n = 5) 36.0% » " " 201 - 205 (n = 4) 22.7% Tresillian R:Point 8ar M107 - 111 (n = 5) 30.4% " » "

322-326 (n = 5) 14.8% Restronguet Creek:Intertidal Flats (upper division)

401 - 406 (n = 4) 81.8% Restronguet Creek:Intertidal Flats (lower division) 383 - 393 (n = 10) 44.6% " » " 362 - 367 (n = 5) 87.2% " » »

Determination of Sulphur Species: Pore Later Sulphate

A very minor proportion of the total sulphur in a dried sediment sample occurs as sulphate, uhich has been produced by evaporation of contained pore water. Estimation of this pore water component was in fact determined for wet sediment by the method described at the beginning of Chapter 5, The reader is referred to this description for an analysis of the effectiveness of the experimental procedure and only the results of these analyses, corrected to dry sediment basis, are employed in this Chapter.

Determination of Total Carbon

Carbon is present in the sediments as organic carbon and rarely as calcium carbonate. The determination of carbon presented problems as a large number of sample determinations were required. The available L.E.C.0. combustion equipment could not be made to give satisfactory results and the published acid decomposition technicue of Sush (1976) was eventually adopted. As this method is slow only a proportion of the intended number of samples were assayed for total organic carbon. In outline, the organic carbon was decomposed by hot chromic acid, after 114 dissolution of any carbonate carbon by orthophosphoric acid. The evolved CO2 was washed and reacted with a known volume of barium hydroxide. The excess barium hydroxide was estimated with standard hydrochloric acid. All reagents were calibrated against a known amount of contained carbon in pure calcium carbonate. In practice, about 12 samples and the duplicates could be performed per day but some difficulties with leakage from the apparatus and in obtaining an even decomposition of the solid sample were not entirely overcome. As a result, the standard deviation of replicate samples expressed as a percentage of the mean recorded value was about 13%. Natural variability of the organic carbon concentrations was also high and the average relative standard deviation of duplicate samples obtained from 100m of the same environment was 45%. However, the method was found to be basically satisfactory and was used to delineate areas of high H^S productivity, to assist in the characterisation of the various sedimentary environments and as an aid in the division of the total monosulphide determinations into iron and copper-zinc monosulphide fractions. '

Determination of Tin and Arsenic

Tin and arsenic were initially assumed to be present in the estuarine sediments and were eventually shown to be present as cassiterite, arsenopyrite and stannite (Fe (Cu, Sn) S2) by X.R.D. analysis. The estimation of these elements was performed by an X.R.F. technique which was specifically developed by the Mine Chief Chemist for alluvial materials containing these components whilst the author was Mines Geologist at the Uheal Jane Mine near Truro, Cornwall. The results quoted in this study were obtained from the Mine Laboratory and independently checked by the standard gallein colorimetric method at Imperial College. The relative standard deviation for both tin and arsenic is 6%. Reconciliation of the X.R.F. and colorimetric determinations was on average to within 10%. 115

n rH 05 IX o X XIo 05 t> 03 M n KrH

X o E<- H a ti a —i au

A METAL VAIDES ppm MAJOR ENVIliONMENT . SUBDIVISION (OONC. HNO-EXTRACTABLj E METAL - dil. HCl EXTRACTABLE METAL)

Fo Mn Cu Pb Zn

RESTRONGUET CREEK

CREEK MOU11I COMPLEX FLOOD RAMI1 6136 73 504 8 144 EBB SHIELD 9924 169 541 1 213

INTERTIDAL FLA'IS LOWER COARSE- GRAINED FLATS 10820 78 586 12 236 LOWER MATS 14753 90 953 20 350 FOSSIL ALGAL BANKS 12840 70 782 13 475 AIjGAI, BANKS 13366 115 1106 14 475 UPPER MA'IS 12458 173 1049 36 423

MARGINAL ENVIRONMENTS OXIDIZED SALT- MARSH 11910 134 152 NIL 76 REDUCED SALT- MARSH 10400 133 200 5 NIL

TRESILLIAN RIVER INTERiTDAL FLATS 7343 46 111 14 67

CHANNEL SLOPES 8967 47 75 4 69

POINT BARS 10931 87 91 15 108 117

4.1 METAL CONCENTRATIONS

Fig, (14) shows the mean concentrations of concentrated nitric acid and dilute hydrochloric acid extractable iron, manganese, copper, lead and zinc in sediments from Restronguet Creek and the Tresillian Rivar. Fig. (15) shows the A metal values which are the simple differences between the results for the nitric acid and hydrochloric acid treatments. As may be seen the geochemical results correlate well with the detailed sedimentological sub divisions of the major environments. Tin and arsenic values are shown separately in Fig. (16).

Fig. (16) Tin and Arsenic Concentrations

MAJOR ENVIRONMENT SUB-DIVISION Sn ppm As ppm

RESTRONGUET CREEK CREEK MOUTH COMPLEX FLOOD RAMP 1833 3422

INTERTIDAL FLATS LOUER COARSE GRAINED FLATS 1759 1027 UPPER FLATS 2262 2680

MARGINAL ENVIRONMENTS SALTMARSH 2776 1898

TRESILLIAN RIVER INTERTIDAL FLATS 396 57

POINT BAR 350 56

MARGINAL ENVIRONMENTS SHINGLE BANK 312 25

Graphical analyse s of thsse data indicate that most of the metals are wall correlated with each other and thus a reasonably consistent variation trend is given by 118 most pairs of metals for a range of sediment types. Fig. (17) for dilute and concentrated acid extractable iron and copper illustrates the typical variation trend given by Restronguet Creek sediments. The overall metal concentrations in the Tresillian River sediments are too low to exhibit a claar correlation. As may be seen from Fig. (17) an idealized line of best fit can be manually fitted to these data and departures from the trend are clearly recognisable. As more than ten actual analyses uere performed for each point plotted, these anomalies have a considerable significance and indicate anomalous metal concentrations uhich cannot be detected in simple studies of the absolute concentration of any one particular element. Fig. (18) shows the positive or negative anomalies which were recognised from correlation graphs of all possible combinations of the metals.

As may ba seen there is a very significant positive anomaly for dilute h'Cl extractable iron in Restronguet flood ramp sediments, major positive anomalies for copper and zinc in the upper Restronguet environments and a negative manganese anomaly in oxidized Restronguet Creek saltmarsh sediments. These features are discussed below in more detail, firstly element by element and secondly in the context of the local sedimentary environments.

IRON

In Restronguet Creek, the more coarse grained sediments of the flood ramp and ebb shield contain the least extractable iron, but there is a steady increase away from the south-eastern part of the ramp towards the algal banks and upper estuarine environments. (See Figs. (19) and (20). As noted above, the Restronguet Creek flood ramp is anomalously enriched in iron, despite the overall low concentrations, and this snrichment is attributed to the widespread occurrence of hematite which coats and encloses the detrital mineral grains in this area. II

FIG 17

Fe vs Cu CORRELATION DIAGRAM

• DiL HCL EXTRACTABLE

• * CONC. HN03

(Each point is one sub -env i r onme ntal mean)

Cu THOUSAND PPM 4 5 J • . , « I Fig. 18

ANOMALIES IN 'HIE METAL CORRELATION TRENDS

+(-) Least significant positive (negative) anomalies, n-f( ) most significant positive (negative anomalies) Cone. HNOg EXTRACTABLE il HCl EXTRACTABLE MAJOR ENVIRONMENT SUBDIVISION Fe Mil Cu Pb Zn Fe Mn Cu Pb Zn

RESTRONUET CREEK CREEK MOUTH COMPLEX ELOOD RAMP ++ +++ + ++

EBB SHIELD ++ ++ - + + + +

INTENT IDAL FLATS LOWER COARSE GRAINED FLATS - + + LOWER FLATS FOSSIL ALGAL BANKS — +++ +++ + - +++ -

ALGAL BANKS + ++ ++ - UPPER FLATS +++ +++ ++ +

MARGINAL ENVIRONMENTS OXIDISED SALT MARSH -- - RE1XJCED SALT MARSH »++

TRESILLIAN RIVER ++ INTERTIDAL FLATS —

CHANNEL SLOPES + —

+ POINT BARS ++ — 1

123

The intertidal flats sediments contain appreciably more iron than creek mouth sediments and tha concentrations are anomalously high in those nearshore areas uith filamentous green algal banks. These anomalous enrichments are restricted to those samples uith evidence of a poorly crystallized chlorite mineral in addition to normal detrital chlorite and they are thus attributed to the development of authigenic iron rich chlorite. As would be expected the anomaly is most evident in the sharply increased dilute hydrochloric acid extractable iron concentrations. Some samples from the Restronguet saltmarsh show similar characteristics,

Elsewhere iron concentrations are normal and reflect the usual detrital and authigenic sulphide mineralogy of tha silty and sandy sediments.

These findings are significant as they confirm that iron silicate mineralization is possible in temperate estuarine environments characterized by high algal productivity and very high iron concentrations, Strahkov (1960), Also the crude zonation of the iron mineralogy from pyrite- iron monosulphide-authigenic iron silicate on the algal banks to pyrite-iron monosulphide on the intertidal flats to hematite on the creek mouth complex very generally resembles that found in ancient iron enriched sedimentary sequences.

Iron concentrations in the less polluted Tresillian River are much lower and there are no obvious concentration trends,

MANGANESE

Figures (21) and (22) indicate that manganese concentrations increase towards the head of the estuaries.

Manganese is depleted in most reduced intertidal flats sediments but in Restronguet Creek an excess amount appears to have been emplaced as a nitric acid extractable

126 mineral in ebb shield sediment. It is possible that in most flats sediment burial and consequent reduction of iron- manganese oxide complexes liberates manganese to the pore waters which migrate and are emplaced updio in the ebb shield possibly as FinCO^.

The apparent strong depletion in Restronguet saltmarsh sediment may be partially attributed to tidal flushing 'of the porous sediment and consequent shorewards displacement of iron-manganese oxide complexes. However in part the anomalies are probably also due to very high levels of other metals giving a biased correlation graph. The nitric acid extractable positive anomaly in Tresillian point bar sediment is thought to be due to the presence of FlnCOg or FlnS but these minerals have not been positively identified in these sediments by any other method.

COPPER

There are very clear copper concentration maxima in upper intertidal flats and buried algal bank sediments. See figs. (23) and (24), The excess copper in these areas is extractable with hydrochloric acid as well as nitric acid and thus cannot be assumed to be present as chalcopyrite or organically complexed copper. The mineralogical evidence would support these data and suggests that widespread authigenic pink bornite mineralization is responsible.

ZINC

There is excellent correlation between the copper and zinc concentration in most area, (see figs, (25) and (26), although there are some differences in detail. In general zinc is at about the expected concentration in the coarse grained lower Restronguet sediments, although a greater proportion than might have been expected is extractable by hydrochloric acid. This tands to indicate a dominant occurrence as sphalerite. Similarly, both the copper and zinc concentrations are anomalously high in the fossil algal banks and upper Restronguet tidal ro CP ro o CJ O 131

flats sedimsnts but a significant proportion of the zinc is only extractable by nitric acid. Thus, it is evident that zinc is preferentially concentrated as an organic complex in the fossil fine grained organic rich upper estuarine sediments. There is a slight depletion in the presently active algal bank sediment. In ths Tresillian River similar processes are.active and there is a zinc maximum associated uith the organic matter in the point bar sediments.

LEAD

The lead concentrations reach a maximum on the upper Restronguet tidal flats and are at a minimum on the coarse grained lower Restronguet tidal flats. The evidence shows that the hydrochloric acid extractable lead is anomalously high in the flood ramp sediment and this would suggest an association with the iron oxide concentrations discussed above. Lead is not associated uith organic matter and the organic rich algal flats appear to be relatively deficient in lead. There is a moderate correlation between zinc and lead but, overall, the major control on lead concentrations appears to be the occurrence of suitable adsorbing fine grained sediment.

TIN AND ARSENIC

From the available data it appears that tin and arsenic concentrations are moderately correlated for the average Restronguet and Tresillian intertidal flats sediment. Samples which are enriched in tin have commonly suffered some form of gravity concentration process.

Arsenic appears to be mobile in reducing estuarine environments but no evidence to indicate the widespread occurrence of stannite in the lower Restronguet environments could be obtained.

1 -\2 134

KEY CORE No O SURFACE 3 SILT

SAND JJLL. CROSS STRATIFIED SAND SANDY FLASER BEDDING FLASER BEDDING o N) MUDSTREAK MUDSTONE o SCOURED SURFACE CJ BEDDING PLANE MUDFLAKES o s WEAKLY BIOTURBATED ss MODERATELY o STRONGLY Cn S5S PYGOSPIO BURROW o ORGANIC MATTER 6* SR ROOT BIOTURBATION

O. SLUMP

MUDCRACKS o 03 M IRON OXIDE CEMENT I f- CONGLOMERATE CL bOA 01 UJ vO Q

LU cC O o

<_J o h- q: LU > ro 3 'FIG 29 CORE 35 METAL CONCENTRATION

O HNO,

O M

O U> « £ « Pb Mn Zn Fe Cu O

O ui

eLAn "FIG 34 CORE 34 METAL CONCENTRATIONS

THOUSANDS PPM 10 20 30 40 50 60 70 BO 90 IOO IIO Fe IO 20 30 40 5J3 60 70 8 0 90 IOO -r • 1—1—1—1— PPM I200 1600 2000 CuZnMnPb 400 800 1200 16 OO 400 8OO O B

HCl HNO> O O

Fe Zn Pb Mn Fe CuZn Pb Mn FIG 31 CORE || METAL CONCENTRATION

Fe THOUSANDS PPM IO 20 30 40 50 60 70 8 0 90 lOO , IO 20 30 40 50 60 7 0 60 90 lOO IIO | 1 1 1 1 4- 1 1 1 1 1 I F—I 1 1 1—4 1 1 1 I-—h- Cu Zn Mn Pb 400 800 I200 I600 PPM 400 800 I200 I600 2000 O O 3

HCl HNO. o O 4 •4-

ft o O ro ft (V>

ft O O « u> ft to

ft O O ft

O ft O ot in ft

O p dv ft Pb Mn Cu Zn 6» Pb Mn Fe Zn Cu FIG 32 CORE 12 METAL CONCENTRATION

Fe THOUSANDS PPM 10 20 30 40 5Q 60 70 8 0 90 IOO 20 30 40 50 60 7 0 80 90 100 HO 4- I 1 1 1 1 -I 1 1 1 1 1 1 1 1 F- Cu Zn Mn Pb 400 800 1200 1600 PPM 400 800 12 OO 1600 2000 O O B

HCl HNO, O

V... p -LL. O O ro ro

O to"te• i UI n jii i j • nuiuiwrH o ft ft ft o

ft O 6v •• ft Pb Mn ZnFe Mn Fe Zn

CD 139

METAL CONCENTRATIONS IN THE SEDIMENTARY ENVIRONMENTS

In this section detailed relationships between the various sediment types and their extractable metal concentrations are examined. Analytical data are presented graphically on a series of vertical sediment core logs taking the surface elevation at the sample point as the plotting reference datum. Locations of core sample points are shown in figures (27 and 28).

Restronguet Creek Mouth Complex: Flood Ramp and Channel

Extractable metal concentrations are quite homogeneous in the upper parts of the creek mouth complex, although the fine grained mud streaks emplaced by the braided channels contain high concentrations of dilute HC1 extractable iron, copper and zinc. See cores 13, 35 in figs. (29) and (30). The fossil sediment which probably represents a buried coarse grained intertidal flat unit at the base of core 35 is also enriched in copper and zinc. However, in that case, the excess metal is extractable only by nitric acid and thus must be present as chalcopyrite and possibly also as organic complexes.

Restronguet Creek Mouth Complex: Ebb Shield

A wide range of sediment types occurs in the ebb shield, with the coarsest material on the crest directly above the flood ramp and progressively finer sand and silt with mudflasers to the north-west. As may be seen from cores 11 and 12, figs. (31) and (32), the upper coarse grained units contain more concentrated HC1 extractable copper and zinc. As discussed above, lead is probably adsorbed on iron oxide which coats detrital mineral grains in the coarse units. The copper probably occurs as chalcopyrite and the zinc as organic complexes in the finer grained units. The sediments of the fossil intertidal fiats developed in the lower zones of both cores 11 and 12 contain moderate

"FIG 34 CORE 34 METAL CONCENTRATIONS

Fe THOUSANDS PPM IO 20 30 40 5Q 6O 70 80 90 DO . IO 20 30 40 50 6Q 70 8Q 90 IOO HO ! 1 1 1 F- i-fc C Z MP 800 I6OO 2400 3200 PPM 8 OO 1600 2400 3200 4OOO o O 3

HNO3 o HCI O

UU. U1L.

• : •. s/ •

Pb MnZnCu Fe Pb Mn Zn Fe Cu

O O •U p o ui in

O O CA CA "FIG 34 CORE 34 METAL CONCENTRATIONS

F<2 THOUSANDS PPM IO 20 30 40 5p 60 70 8 0 90 IOO , IO 20 30 40 50 60 70 80 90 IOO HO h 1 H f-H P 1 1 1 1 1 I P 1 1 1 (—I 1 1 1 1 CuZnMnPb 800 1600 2400 3200 PPM 4 800 1600 24CO 3200 4000 O

HNO HCL O 3

O ro

\ O is in

Zn F

'FIG 38 CORE 30 METAL CONCENTRATIONS

Fe • THOUSANDS PPM IO 20 30 40 SO 60 70 8 0 90 IOO , IO 20 30 40 50 60 70 SO 90 I 1 1 1 1 F 1 1 1 1 1 I F 1 1 1 1 1 H 1 1 4 1 1 1 146 metal concentrations which are characterised by high copper and zinc values. The excess of nitric acid extractable over hydrochloric acid extractable copper tends to indicate the presence of chalcopyrite or organic complexes. The latter are less likely in these sediments as they contain • so little organic carbon.

Restronguet Creek: Intertidal flats

The intertidal flats in the lower part of Restronguet Creek provide a succession of progressively finer grained sediments from the ebb shield westwards towards the shore- lines and north-westwards towards the upper estuary. The general decline in dilute hydrochloric acid extractable iron, copper and zinc concentrations together with a marked increase in concentrated nitric acid extractable metal recognised in the ebb shield sediment is more marked in this subdivision At and is attributed to increased mineralisation of these metals as pyrite, chalcopyrite, stannite and organic complexes. However, the logs of cores 31, 34, 41, 42 and 32, (figs. (33), (34), (35), (36), (37)) do not indicate any direct correlation with visually estimated organic carbon content. In contrast manganese is preferentially concentrated at several i organic rich horizons.

The algal banks and their fossil equivalents which occur in the north-west of the lower intertidal area are represented in core 30, (fig* (38)), which can be seen to resemble the lowest zones of core 42. Once more iron, copper and zinc occur mainly as nitric acid extractable species. Manganese gives a clear maximum dilute HCl extractable concentration at about the level of the standing water table, where it could be occurring as an oxide or carbonate. Similar maxima do not occur in the more permeable distal environments and this may be due to the more rapid tidal water table fluctuations nearer the main channels. .. „ . , FIG 39 KEY

SURFACE /VRTI F ICIALLY DEPOSITED RESTRONGUET CREEK - V METAL CONTOURS (ppm) 1 BOREHOLE ACID EXTRACT ABLE METALS NEREID 8I0T URBATED SILT MUDCRACKED BEDDED SILT s« [Xl LITHOLOGY MUD FLAKE CONGLOMERATE ROOTED + BEDDED SILT Upper intertidal f la t s / saltmarsh X K X X x ALGAL MAT NEREID BIOT. POOL FACIES

•

24 * o Jr^l/Wis

/* y

U pp er int e r t id a 1 Hats . SCALE 1: tOOO HORIZ.

^ • ARTIFICIALLY DEPOSITED ^ ^ Saltmarsh

,1 T T

153

Restronguet Creek: Marginal Environments

The major marginal environments in the Restronguet Creek are the mudcracked algal flats and saltmarsh at the head of the estuary. A number of vertical cores uere obtained from saltmarsh, algal intertidal flats and upper intertidal flats. Their vertical logs and the recorded metal concentrations have been compiled into cross-sections, figs. (39) to (44). As may be seen, the copper and zinc concentrations are well correlated, the iron and lead concentrations are poorly correlated and the manganese distribution is quite anomalous. In general the metal concentrations in the present day equivalent of the offshore buried units are rather lower than might have bean expected. A comparison of the nitric acid and dilute hydrochloric acid extractable metal concentrations indicates that much of the zinc is present as organically bound and complexed species and much of the copper is present as sulphide, organic complexes or native copper. Iron can be seen to be present as pyrite offshore and as iron oxides in the higher less bioturbated sediments. As in the environments discussed above, the major lead concentrations are associated uith iron oxides. These figures also indicate that manganese oxides are progressively dissolved in the sulphidic offshore units and reprecipitated after mobilisation updip onto nearby mudcracked surfaces or into the higher very thinly bedded algal flats. There is no evidence in this subdivision for the mineralisation of manganese as any minerals other than the oxides.

The saltmarsh consists of an oxidizing zone of about 12 cms thickness and two reducing zones at depth. The lowest reducing zone is at, or just above, the level of the standing water table. In general, the metal concentrations increase as the reducing zone is penetrated. There is XRD evidence to suggest that some iron occurs as authigenic iron rich chlorite in this subdivision. The flood tide which rises swiftly through the permeable saltmarsh sediments carries a red oxide precipitation front ahead FIG 45 CORE 9 METAL CONCENTRATIONS

Fc THOUSANDS PPM CuZnMnPb -HUNDREDS PPM IO 20 30 40 50 6O 70 80 90 IOO . lO 20 30 40 50 60 70 80 90 IOO IIO I 1 1 1 1 (- 1 -i 1 1 1 I f 1 1 1 h—I 1 1 1 1 h-

O3 O R $ R $ HCI H NO; o $ $ s s o $ R O to ro V-fiii'--

O (jj \ o Pb Mn Zn Cu Fe O Pb Mn Zn Cu Fe

o O

O O CA b» I

157 of it. This phenomenon could not be investigated quantitatively as no evidence of its effects could be determined in any of the vertical cores from the saltmarsh. (See core logs 9, 10, 26; figs. (45), (46), (47)). As may be seen, the contact between the saltmarsh sediments and the underlying fossil units is not anomalously mineralised although there is some enrichment at about the level of the standing water table.

Restronguet Creek: Artificial Environments

The artificially deposited sediments beneath the algal banks and saltmarsh in the upper parts of Restronguet Creek consist of bedded sands and silts with only very limited organic material. Cores 9, 10, 26 illustrate the extractable metal distributions within these sediments and a good correlation between the approximate level of the standing water table and iron, manganese and zinc maxima is obtained. The copper and zinc concentrations are poorly correlated in this subdivision and it is thoughtthat this phenomenon is a result of the large amount of detrital copper sulphide in this environment. Zinc appears to be mobilised by the rising flood tides into the iron oxide pan at the standing water table. Unfortunately, the chemical extractions performed were not sensitive enough to distinguish the copper speciation in these units although the concentrated nitric acid results indicate that chalcopyrite is present and native copper is commonly visible to the naked eye.

Tresillian River

The metal concentrations for the three major environmental divisions of the Tresillian River are given in figs. (11), (12) and (13). As may be seen, the absolute magnitude and also the degree of variation is much reduced when compared with similar Restronguet environments. The general lack of variation in Tresillian concentrations is partially attributable to the greater degree of bioturbation of the sediment. F(G 48 CORE 5 METAL CONCENTRATIONS

Fe THOUSANDS PPM Cu TENS PPM 10 20 30 40 5p oO 70 00 90 IOO , IO 20 30 40 50 to 70 BO 90 100 IIO -I—I- I 1 I 1 1 1 I—4-H 1 1 1 1 1 1 1 1 h- 2n Mn Pb AOO BOO 1200 1600 PPM 400 800 1200 1600 o O 2 3 SSS sss O HCI HNOj 0 o 0

J J o o liu % iu % o O Itu' Li

>o O 4*

o o ui ui % sss ;o O % Mn Pb Fe Zn Cu Mn Pb Fe Zn Cu -ric 49 CORE 40 ME TAL CON C[NTRATI ONS Fe THOUSANDS PPM Cu TENS PPM 10 20 30 40 DO 70 BO 90 20 ;p 100 IF :0 50 60 70 BO 90 ICO //0 I I I I I SF I I I I I I I I I I I I I I I I Zn Mn Pb 400 800 1200 1600 PPM 400 800 1200 1600 0 0 - ~ • I' S~ S~ , ~ O£1;> I a Hcr 0 I HNO. - os» - S5> 1 );; );. 1 \1 , . I, 0 S5 I 0 SS I I 1 '" -.d .... -::.~\ 1 '" -.ft'-~\.' I ' 1 : . '- .. I .. I I I " SSS .' SSS I 0 1 () w Iii 1 i~1 1 1 w 1 , 1 ·tt.' , ·1.' "- , I .<'l~i:.-- I <'7:<:0:.- ;~ ;~ I 0 r. 1 0 r. 1 .-.- 1 ;- -i- 11 - , j. 11 - 1 1 I 1 1 1;;'- '::-. 5S I 5S \ \ 9 ~~ 1 0 "'I"""\~ UI 1 UI \ 1 \ , \ . "''' \ , ~~gp.;:---=----= \ , 0 ~~ \ 0 ~~:-~,~ \ e;S~ , , 0- =~. 0. ""'- , . \ ~ l~ 'I <,<,<;: ,-." "%<:-""Z PbMn Fe Zn Cu Pb Mn Fe Zn Cu '!- .

"FIG 51 CORE 45 ME T AL CONCEN TRA T IONS Fez THOU SAN DS PPM Cu TENS PPM 10 30 50 60 70 80 90 100 110 20 40 5F 60 70 8090 100 20 :n 4J J I I I I I I I I I I , If I I I I I 1 I I I I Zn Mn Pb 4CO BOO 1200 1600· PPM 4)0 OCO 1200 1600 . 2:XX) 0 0 !l -~. -= ~ \ ----~. , ~'; '\ _S<,~ \ ~ \ , S. -- \ HCL HNO, 0 ) 0 -s<,~ ~ ;;r I , ~-",,", \, - ~~~ \ ») I, ») I "I 0 I I I\) I I\)~" I SS5 I SSS I I b - I =- ~ . . Pb Mn Fez Zn Cu ~ Mn Pb Fez Zn Cu - - 0 <;:> w w

0 <;:> ~ ""

<;:> ~. lJJ lJJ . 0 0 (), 0> 'FIG 52 CORE 46 METAL CONCENTRATIONS Fe THOUSANDS PPM Cu TENS PPM IO 20 30 40 5Q 6O 70 80 90 IOO lp 2Q 3Q 4Q 5Q 6Q 7Q BO 90 IOO HO

Zn Mn Pb 400 800 £ I2CO 1600 PPM 400 800 I200 1600 2000

HCL HNO3

s Mn Pb Fe Cu Zn Pb Mn Fe Cu Zn

O -t*

O uT

O o FIG 53 CORE 6 METAL CONCENTRATIONS Fe THOUSANDS PPM CU TENS PPM IO 20 30 40 50 OO 70 8 0 90 IOO IP 20 30 40 50 60 70 80 90 lOO HO -I 1 1 1 f—H 1 1 1 1 H F H F Zn Mn Pb 400 800 1200 1600 PPM 400 800 1200 1600 2000 O O a a

HCl H NOj O O

O O

O ui

o o

o 5J •Nl

Mn Pb Fe Cu O Pb Mn Fe Zn Cu U) 164

Tresillian River: Intertidal Flats

The vertical logs for cores 5, 40 and 48 from the intertidal flats are given in figs. (48), (49), (50). There is clearly a general increase in the concentrations of all metals with depth and the differences between the concentrated nitric acid extractable and the dilute hydrochloric acid extractable values indicate that with depth there is more pyrite, chalcopyrite and organically associated zinc.. These trends are unaffected by minor inhomogeneities in the sediments.

Tresillian River: The Point Bars

The point bar sediments at the contact with the intertidal flats are commonly very organic rich and may be perma- nently saturated with water. They are better drained towards the main channel and muddy silt intercalations become more common. Core 45 was taken at the contact with the intertidal flats. Core 46 is from the middle point bar deposits and core 6 is from the lower sediments. As may be seen from the profiles, figs. (51), (52), (53) there is much greater variation of metal concentrations in these sediments than elsewhere in the Tresillian River. The magnitudes are much lower than in Restronguet Creek but it is quite clear that nitric acid extractable iron, copper and zinc concentrations are significantly in excess of the hydrochloric acid extractable concentrations. Thus, there is a tendency for the emplacement of iron as pyrite and copper as chalcopyrite both generally with depth and particularly in the organically enriched beds. The evidence from core 45 is distorted by the great concentration of organic material at the top which has diluted all the metal values. In core 6 there is excellent correlation between the organic partings and hydrochloric acid extractable zinc which indicates mineralisation as sphalerite. This conclusion is supported by XRD results on heavy mineral concentrates from the more organic rich lower point bar sediments. Variation in the 7 165

concentrations of manganese and lead are not significant. It is thought that there is no manganese enrichment at the minimum uater table horizon due to the general permeation of the groundwater by dissolved I^S and the rather rapid tidally induced water table fluctuations themselves.

The A metal values given in fig. (15) indicate quite clearly that a much greater proportion of the acid extractable iron is mineralised as pyrite in the Tresillian point bar sediments than in any other environmental division in the Falmouth Estuary.

4.2 SULPHUR DISTRIBUTION AND MODE OF OCCURRENCE

TOTAL SULPHUR CONCENTRATIONS

The total sylphur concentrations recorded for sediment grab samples are less variable than the equivalent extractable metal concentrations and for some environments a simple statistical approach is appropriate. t

Restronguet Creek: Creek Mouth Complex i The arithmetic mean sulphur concentrations of the flood ramp and ebb shield environmental subdivisions of the creek mouth complex are 1790 ppm and 2000 ppm respectively. The statistical distributions are multimodal and probably reflect the sporadic occurrence of more sulphidic finer grained mudflaser sediment.

Restronguet Creek: Intertidal Flats

Sulphur concentrations in the Restronguet lower intertidal flats display an excellent bimodal lognormai statistical distribution (fig. (54)). As may be seen, the data are divisible into two component populations with modal values of about 4000 ppm and nearly 8000 ppm respectively. On average, each population comprises about 50^ of the overall total number of data. In order to confirm the hypothesis O.Oi O.I O i l)X iO 40 50 bO 70 9* yy 99.yy CUMULATIVE PERCENTAGES (CUM °/o) en -o

ay.aa no jn 4u mi oij vu ua an CUM o/o 168 that the distribution results from the admixture of coarse grained low-sulphur marine-derived material to high sulphur fluviatile materials, a number of grab samples were selected at random and attributed to either the "coarse" or "fine" populations solely on the basis of sedimentological characteristics. When the sulphur concentrations for these classified samples are plotted, (see fig. (54)), they agree very closely with the two statistically derived populations and the hypothesis is validated.

Samples from the present day and fossil algal banks which occur at the margin of the Restronguet lower intertidal flats are considerably enriched in sulphur and give arithmetic mean values of 9400 ppm and 15,500 ppm respectively.

Fig. (55) shows the statistical distribution of sulphur concentrations obtained for samples from the Restronguet upper intertidal flats and, as may be seen, it is virtually identical to the "fine grained population" derived above for the lower intertidal flats. Again, the modal value is about 8000 ppm. As the distribution is approximately lognormal, a higher arithmetic mean value of about 9400 ppm is obtained.

Restronguet Creek: Marginal Environments

As may be expected, the inhomogeneous sediments of the marginal environments at the head of Restronguet Creek exhibit a very broad total sulphur distribution. Most variation occurs within the sediments of the offshore mudcracked algal flat and poorly bioturbated units which are undergoing burial and partial sulphidation. In these units, the mean sulphur concentration is probably about 8000 ppm at the boundary with the upper intertidal flat and about 2000 - 3000 ppm in the oxidized algal bank sediment. The saltmarsh sediments are more homogeneous and are divided into the upper oxidized zone with a mean of about 4300 ppm, the reduced zone with a mean of about 13,200 pom and the i /

CUM °/o 170

lower reduced zone with visible native sulphur which has a mean value of 14,800 ppm. These sediments overlie fossil artificial environments which average about 4400 ppm sulphur.

Tresillian River

The three major environments in the Tresillian River all exhibit similar total sulphur distributions. A distribution curve for the Tresillian intertidal flats as given in fig. (56) has a modal value of about 8000 ppm. The second distribution curve in the diagram is of all the results from the Tresillian point bars, channel slopes and intertidal flats combined, and that also has a modal value of about 8000 ppm. As may be seen from the figure, these distributions are virtually identical with that derived above for the "fine grained" Restronguet intertidal flats sediments and thus the * Tresillian lithologies may be described as the lateral equivalents of the Restronguet intertidal flats at least with respect to the total sulphur concentrations.

t MODE OF OCCURRENCE OF SULPHUR

, The sulphur is present in these sediments in mono and bisulphide metallic minerals, organically bound and native sulphur, pore water sulphates, small quantities of hydrogen sulphide and dissolved sulphide ions. In this study, independent estimates of the monosulphides and pore water sulphates were obtained such that by difference the approximate amount of polysulphide mineralisation may be deduced assuming that organic sulphur and native sulphur are volumetrically negligible. In many sediment cores, the upper parts are thoroughly oxidized such that the total sulphur is almost certainly present as detrital polysulphide. If this concentration is then deducted from the estimated sulphide concentrations lower down in the sequence, a very crude estimate of the diagenetically emolaced authigenic polysulphide is obtained. Thus, for the vertical core samples, estimates can be obtained for: 171

i) Total sulphur concentration ii) Proportion as pore water sulphate iii) Proportion as authigenic monosulphide iv) Proportion as detrital polysulphide and monosulphide v) Proportion as authigenic polysulphide

Pore Water Sulphate

Various authors including Berner (1964b, 1969) illustrated the way in which pore water sulphate is used in metabolism by sulphate reducing bacteria to produce hydrogen sulphide as a waste product. Most of the studied environments have been in deep water, where pore water sulphate can only be partially replaced: by diffusion from seawater at the surface of the sediment, by limited bioturbation in the upper part of the column and probably by diffusion or slow fluid migration up from greater depths in the succession. As a result, dissolved sulphate should be progressively depleted with depth and an asymptotic dissolved sulphate concentration curve should be obtained. According to Berner such curves are of the same form for a variety of environments and tend to be controlled only by the rate of sediment deposition, degree and depth of bioturbation and quantity of metabolizeable organic carbon. However, in the estuarine environment a considerable thickness of sediment is subjected daily to tidal flux and all of the sediments in this study receive at least some sulphate from seawater long after they pass beneath the zone of active bioturbation.

Three main types of relationship between pore water sulphate and depth are developed in the Restronguet and Tresillian estuaries (fig. (57)). The coarse grained sediments from the Restronguet flood ramp show little coherent variation. This is attributed to the very low proportion of metabolizeable organic carbon and the constant tidal flux through the permeable sediment. The ratio of equivalents per million sulphate vs. equivalents per million chloride is found to be the most suitable means for revealing the variation in sulphate concentration with 172

CONCENTRATION RATIO SO4./CI .pi .Q2 ,q3 -OA .05 -Q6 .Q7 -08 -09 -10 -H 12 -13 FIG 57

PORE WATER SULPHATE DEPLETION PROFILES

en O m o3 o X ui RESTRONGUET FLOOO RAMP

CONCENTRATION RATIO SO4. / Cl •Ol .02 .03 .04 .05 .06 .07 .OS .09 O -Ol .02 .03 .04 .05 .06 .07 .08 .09 .IP .1 .12 .B .14

CORE 21

^ O O m u03i "—ui X ALL INTERTIDAL FLATS NCLUDING FOSSIL UNITS FIG. (58)

ARITHMETIC MIAN MONOSULPHIDE CONCENTRATIONS

ARITHMETIC MEAN MQNOSULPHIDE MAJOH ENVIRONMENTAL DIVISION SUBDIVISION CONCENTRATION EXPRESSED AS P.P.M. Dil ACID EXTRACTABLE II2S

RESl'RONGUET CREEK

CHEEK MOUTH COMPLEX FIJOOD RAMI3 200 ppn (FOSSIL UNIT AT DEPTH) 600 ppm EBB SHIELD 300 ppm (FOSSIL UNIT AT DEPTH) 200 ppn

INTERTIDAL FLATS I.OWER COARSE-GRAINED If FIATS 900 ppn LOWER FLATS 1800 ppn FOSSIL ALGAL BANKS ) ALGAL BANKS ) 2900 ppn UPPER FLATS 3000 ppn

MARGINAL ENVIRONMENTS REDUCED SALTMARSH 5600 ppn

TRESIILIAN RIVER INTERTIDAL FIA'IS 3700 ppn

CHANNEL SLOPES 4200 ppn

IX) I NT BARS 4700 ppn 174

depth, because this measure eliminates spurious effects due to surface run-off in permeable lithologies. Fig. (57) also shows the concentration profiles for intertidal flats sediments from both Restronguet Creek and the Tresillian River and, as might be expected, there is a steady decrease in sulphate with depth. The third type of profile shows the concentration variations for cores which penetrate the fossil artificially deposited•sediments beneath the upper Restronguet marginal environments. As may be seen, modern sediments at the top of cores 21 and 22 are depleted in pore water sulphate but oxidized sediment in cores 23, 24 and 25 and all the fossil units at depth are not. Evidently, the latter contain very lou metabolizeable organic carbon concentrations.

Sulphur as authigenic monosulphide

Unlike the total sulphur concentrations the monosulphides are irregularly distributed and their concentrations are controlled by a much more complex set of factors. Thus, even a simple statistical treatment of the results is not appropriate. Fig. (58) gives the arithmetic mean monosulphide concentrations of the major environmental subdivisions in the Restronguet and Tresillian estuaries. Figs. (59), (60), (61), (62) is a series of contour plans which illustrate the broadest regional trends in monosulphide concentration for various depths below surface in Restronguet Creek.

At 5-10 cms beneath the surface of Restronguet Creek sediments (Fig. (59)), the main features are: the low value zone in the creek mouth complex, and, the very high concentrations recorded on the present day algal banks. The high concentration of 2000 - 5000 ppm, recorded near the main channel on the lower intertidal flats, occurs in some thicker near surface oozes which may mark the upper end of a partially infilled abandoned channel. Much of the sediment of the upper estuary is oxidized at this depth. At 20-35 cms depth (fig. (60)) high concentrations occur in tha FIG 59

RESTRONGUET CREEK

39 F.XTRACTABLE H.S AT IO CMS DEPTH

IOOOO PPM

i11$ 5000

2000 IOOO

SOO 38 NIL

SCALE Part of sheet SW83NW

179 081 131 fossil algal banks in the lower estuary "and on the upper intertidal flats. Essentially similar distributions are developed at 30 - 35 cms depth (fig, (61)), although much of the sediment of the upper estuary is reduced at this depth. Finally, at 40 - 50 cms, the major feature is the poorly mineralised fossil zone beneath the Tallacks Creek saltmarsh in the upper estuary. (Fig. (62)). One major feature which is not illustrated by the plans is a zone of monosulphide enrichment which coincides with the unconformable contact of the sediments of the modern creek mouth complex and the underlying fossil intertidal flats sediment. In many areas, this enrichment is due to thinly bedded sulphidic mudstreaks which occur at the unconformity, but it is also possible that it is partially due to mineralization by F^S which is diffusing out of the more sulphidic, underlying lithology. Evidence for this process is the abnormally pale colouration of the coarse grained creek mouth sediments adjacent to the contact.

Only a single plan is presented for the Tresillian environments, fig. (63), because there is remarkably little variation: the monosulphide concentrations at this 30 - 35 cms interval are very similar to those in the upper Restronguet Creek.

Detailed logs of the extractable monosulphide in the vertical core samples are presented in Figs. (69 to SO), There is little evidence of a direct relationship between total organic carbon and monosulphide concentration. Sediment grain size and the related factors of permeability and moisture content undoubtedly are as important in controlling the concentrations. Also, as mentioned above, the degree of bioturbation is of considerable significance, There are no individual vertical logs which display any of these controlling factors separately but several do give indications of the ways in which these controls operate.

Fig. (64) gives a series of extractable H2S profiles composited for several vertical sediment cores from similar environments in both the Restronguet Creek and Tresillian 182

Rivers. The exes represent increasing grain size and permeability and decreasing organic carbon content in one

direction and increasing H2S diffusion, decreasing 02 infiltration and increasing bioturbation along the other axis. .The following conclusions may be drawn from this illustration:

i) Upward diffusion of H2S is a significant factor except in the coarsest, organic deficient sediment and in the heavily bioturbated sediments. ii) Upward diffusion of H2S causes significant enrichment of sediment zones even in fairly permeable

lithologies where 02 concentrations are likely to be high. iii) Sediment grain size and thus permeability is a more significant factor than the metabolizeable organic carbon content. iv) It does not appear that a limiting maximum H^S concentration is obtained in any of the cores as a result of exhaustion of the sulphate supply.

A special case arises for the saltmarsh sediment at the head of the Restronguet Creek where a 12 cms oxidized zone is developed above the reduced lithology. This is largely as a result of the limited tidal influence and these sediments are considered separately.

Detrital Sulphur

Detrital sulphur concentrations are attributable mainly to detrital pyrite and chalcopyrite. They are derived from the recorded total sulphur concentrations in heavily oxidized sections of the sediment column or, in the absence of other evidence, from air dried sediment samples. As such, there is considerable inaccuracy in these estimates. The arithmetic mean values as derived for major environments are given in Fig. (65). 183

Fig. (65) DETRITAL SULPHUR CONCENTRATION

MAJOR ENVIRONMENTAL SUBDIVISION ARITHMETIC DIVISION MEAN DETRITAL SULPHUR CONCENTRATION ppm

RESTRONGUET CREEK CREEK MOUTH COMPLEX 760

INTERTIDAL FLATS COARSE-' GRAINED LOUER FLATS 1700 LOWER FLATS 2700 ALGAL BANKS 3900 UPPER FLATS 5000

MARGINAL ENVIRONMENTS SALTMARSH 2300 FOSSIL SEDIMENT AT OEPTH 3600

TRESILLIAN RIVER INTERTIDAL FLATS} 2000 CHANNEL SLOPES <

POINT BAR 1800

Authigenic polysulphide mineralisation

After subtraction of the sulphur present in the pore waters, in monosulphide mineral phases and in detrital sulphides from the total, a residual sulphur concentration is obtained which represents the authigenic polysulphide mineralisation in the sediment sample. This value is subject to all the contributary errors noted above for each of the component sulphur determinations but is nonetheless a parameter of considerable interest in the

187 present study. The spatial variation of authigenic polysulphide mineralisation is not known in great detail for the Tresillian River as few vertical core samples were obtained. However, the following broad estimates of the mean authigenic sulphur concentrations have been obtained:-

ENVIRONMENTAL DIVISION SUBDIVISION AUTHIGENIC S ppm

INTERTI'DAL FLATS LESS THAN 35cms 1-2000 DEPTH GREATER DEPTH 2-10,000

POINT BAR SILTY SEDIMENT 10,000 ORGANIC RICH INTERVALS 15,000

The Restronguet Creek is better known and a series of maps showing the concentrations at various depths is presented (see figs. (66-58)). As may be seen, the major mineralised environments in the 20-25 cms depth zone occur at the margin of the saltmarsh, on the saltmarsh itself and on parts of the intertidal flats. At greater depths the algal banks subdivision of the lower intertidal flats is the best mineralised with up to 1% of authigenic sulphur in places.

A comparison of these average values with the table above indicates that once again the Tresillian intertidal flats are indistinguishable from the upper Restronguet flats on the basis of sulphur concentrations alone. The highest concentration of authigenic polysulphides in the estuary occurs on the Tresillian point bars where up to 4% authigenic sulphur has been found in dark, organic rich sediment.

The spatial variation of authigenic polysulphide content is known in some detail in areas where a number of vertical core samples were obtained. The vertical plots of the sulphur analyses are shown for these cores i

FIG 69 CORE II SULPHUR FIG 70 CORE 12 SULPHUR 10 20 30 40 50 60 70 eo 90 100 110 10 20 30 40 SO 6O 70 8 0 90 DO i—¥ -I—I—f 1 4 j 1 J 1 1 If! 1 1 1 1 1 HUNDREDS PPM HUNDREDS RPM o * -JkAX- KEY TO LOGS !o

ft IP TOTAL S ! fO ft

PORE WATER S ft Q U> ft DIL ACID EXTRACTABLE S

ft PETRI TAL S O ft AUTHIG EN IC POLYSULPHIDE

! o ft I Cn ft/ /

O ft io 645

F (G 7 CORE 13 SULPHUR IO 20 30 40 5J O 60 70 8 0 90 OO •f 1 h—H i 1 HUNDREDS PPM 190 191 FiG 77 CORE 9 SULPHUR FIG 78 CORE IO SULPHUR 2 4 6 8 IO 12 14 16 18 2 4 6 8 IO 12 14 16 18 I J 1 1 F -1 1 1—I 1 1 1 1 1 1 1 +- THOUSAND PPM THOUSAND PPM 19 j / 194

in a series of figures (69 to 80). As may be seen, the total sulphur assay is represented by the outermost curve and successive deductions of the poreuater sulphate, dilute acid extractable H^S, and detrital sulphur are made in order to show the authigenic polysulphide component nearest the appropriate sedimentological unit.

Many of the cores show a reasonable degree of correlation between macroscopic sedimentological features and the various sulphur concentrations. For example unbioturbated organic rich partings in Core 13 from the Restronguet creek mouth complex are well mineralized by authigenic polysulphide. Similarly dark, fine grained sediment in Core 6 from the Tresillian point bar contains the highest sulphur concentrations. However other sedimentological features are characterized only by diffuse sulphur maxima or there is simply a general increase in authigenic sulphur content with depth, e.g. Cores 11 and 12 from the Restronguet creek mouth complex. It is thought that this behaviour results from complex interaction of the sulphate resupply, sediment permeability, bicturbation and H23 migration factors noted above. Nona appears ever to become dominant enough to enable simple relationships to be established for other than the broadest of sedimentclogicsl and environmental divisions,

4.3 TOTAL CARBON CONCENTRATIONS

A total of 42 organic carbon determinations ware performed on Restronguet and Tresillian sediment samples. An effort was made to sample all the major environments although the number of determinations for each division was commonly only three or four. Thus, no statistical treatments are appropriate although the arithmetic mean concentrations of the samples in each division are given in Fig. (81). As may be seen, for some environments there is considerable variation about this mean value which reflects the inhomogeneity of the sediments. FIG. (81)

ORGANIC CARBON CONCENTRATIONS

MAJOR ENVIRONMENTAL DIVISION SUBDIVISION ORGANIC CARBON CONCENTRATION MIN VALUE MAX VALUE MEAN % % %

RESTRONGUET CREEK

CREEK MOUTH COMPLEX 0.70 2.85 1.72

INTERTIDAL FLATS LOWER FLATS 0.52 1.44 1.26

ALGAL BANKS 1.21 3.93 2.57

UPPER FLATS 1.06 1.63 1.41

MARGINAL ENVIRONMENTS SALTMARSH

(OXIDIZED) (one £ ample) 4.21

(REDUCED) 1.95 2.99 2.39

TRESILLIAN RIVER

INTERTIDAL FLATS 0.57 3.48 1.89

CHANNEL SLOPES 1.56 1.95 1.76

POINT BARS 1.02 6.28 3.24 196 The table indicates that those environments which visually appear to contain the most organic matter produce the higher assay values. However, it is somewhat surprising that values of up to 2.85% are given in Restronguet creek mouth complex sediments. These high values ere attributed to the presence of microscopic organic debris in the mudstreaks and mudflasers. The range of concentrations recorded for the Tresillian River intertidal flats is wider than might be expected given the extensive bioturbation of these sediments and their homogeneous appearance.

Despite intensive study, only partially effective correlation can be demonstrated between the chemical characteristics described above and these total organic carbon concentrations. Simple graphical methods failed to reveal any correlation at all with either dilute or concentrated acid extractable metal concentrations. Similarly, most A metal values (conc. nitric acid extractable metal - dil. hydrochloric acid extractable metal), are uncorrelatsd with organic carbon for Restronguet Creek samples. However, there appears to be a weak correlation between A Cu and organic carbon and a stronger correlation between AZn and organic carbon in the Tresillian River environments. The latter phenomenon has also been noted above in discussion of the detailed logs of the core samples^ particularly from the Tresillian Point bars, and is attributed to organic complexation of zinc in the organic rich lower Tresillian environments. The best correlation (R = 0.74) obtained is with total sulphur concentrations as illustrated in Fig. (92). This type of relationship has been described by Williams (1978) who, drawing on earlier work by Serner and Goldhaber and Kaplan, emphasized the importance of the atomic ratio of sulphur and carbon in considering sulphide mineralisation in sedimentary rocks. Williams writes that in present day deeper water anoxic sediments, an average S/C atom ratio of about 0.12 is developed but that in older and also in mineralised rocks the ratio increases to up to 1.2 for the various poorly mineralised lit'nologies and up to 42 for parts of the wall • F[G 82, . • , 5 CORRELOGRAM : ORGANIC C Vs SULPHUR

4 • BO N CA R

# NI C

RG A • O • • 2 - • • . •

• •• ••

PERCEN T ® COARSE GRAIN » SIZE 1 • •• • OXIDIZED ZONE

0 # •

IOOO 2000 PPtt i 1 1 1 • 1 1 • .iik TOTAL SULPHUR T lOO PERCENT OF FIG 83 DISTRIBUTION "OF S/C ATOM RATIO SAMPLES RESTRONGUET - T RES ILL! AN ALL AREAS

50L

O-IO 0-20 030 O-AO 0-50 060 070 ATOM RATIO S/C 199

mineralised H.Y.C. Deposit, Australia. "Fig. (83) illustrates the distribution of the S/C ratio values for samples from the Restronguet Creek and Tresillian River . As may be seen, the ratio varies from about 0.05 to 0.50 which is very similar to ranges quoted by Williams for the poorly mineralised Cambrian Alum Shale, Sweden and Carboniferous Chattanooga Shale, U.S.A. The only well mineralised lithology mentioned by Williams with a moderately similar ratio range is the Kupferschiefer, G.D.R. If authigenic polysulphide as determined in this study is compared with corresponding organic carbon values then a mean ratio more similar to that quoted in the literature for a wide range of present day sediments is obtained, although its statistical distribution cannot be determined due to the small number of sample results available. Thus Tresillian and Restronguet sediments tend to develop both the expected S/C ratio range and a good correlation between organic carbon and sulphur contents which taken together indicate that normal diagenetic fixation of H^S is proceeding. However some samples from both estuaries develop anomalous sulphur concentrations which cannot be explained by the normal processes. These concentrations cannot be caused by detrital sulphide as they are just as common in the Tresillian River. Thus they must be caused by locally anomalous reactive iron concentrations in the Tresillian River and possibly also reactive pore water copper and zinc concantrations in the Restranguet Creek. These findings are broadly in accord with those of Berner (1970) for the Black Sea and without comparing older sediments too closely with very recent materials there is also a degree of support for Williams (1978) contention that subsequent later stage overgrowth sulphide mineralization produces a platykurtic S/C atom ratio distribution. 200

CHAPTER 5

SOME CHEMICAL CHARACTERISTICS OF THE SURFACE UATERS AND SEDIMENT PORE UATERS

Surface waters on the estuarine sediments are derived from run off and rain water. Mixtures of fluids from three sources comprise the pore waters of estuarine sediments. The sources of these fluids are:- i) Estuarine water percolating down through sediment to the water table ii) Uater being expelled upwards from compacting sediment beneath the water table iii) Uater held in close association with mineral particles and other materials since deposition of the sediment

However the fluids are never physically distinct and are not directly separable by chemical means. In fact the relationship of major ionic species in the pore waters to mineral particles may be explained by the Donnan theory. This theory requires that sedimentary materials possess an electric charge on their outer surface which attracts a high concentration of oppositely charged ions from the surrounding fluid into a "layer" very near the charged surface. In order to fully compensate this inner layer, a second layer composed of a high concentration of oppositely charge ions is required. This anomalous concentration is then supposed to fall away with distance from the charged surface and eventually a stable but much lower concentration is obtained. In simple diagrammatic form: 201

+ + - +• + + 4-

+ + +• — + h + •f _ + + t

F -f- •h

+ + + - + —

LAYER II LAYER III MATERIAL LAYER I (LARGELY NGRMAL (NEGATIVE) (POSITIVE) NEGATIVE)' CONCENTRATIONS 0BTAL* CHARGE CHARGE CHARGE ZERO CHARGE

For the purposes of this study, the inner layer I is relatively unimportant in that it is not likely to be diagenetically reactive except on dissolution of the charged material itself. However, the outer layers (II and III) are likely to be progressively displaced during compaction dewatering and there is a possibility that, in the later stages of compaction within the diagenetic regime, pore fluids of steadily varying chemistry might be expelled. Unfortunately the surface water which naturally drains from an area of astuarine sediment will be from the outermost layer and will not indicate the ionic concentrations in the more closely associated layer II. In contrast to the sampling of surface water a more sophisticated extraction method is required to obtain samples of this fluid. Over the past twenty years a number of methods have been employed, most involve the artificial compaction of a sediment sample and some take into account the findings of Mangelsdorf (1969) on the effect of temperature during compaction and Manheim (1974) 202 on the effect of compaction pressure. This type of method was not adopted as firstly much more detailed compaction experiments were already planned and secondly only small quantities of sediment were routinely available. The- chemical methods for estimation of cation exchange capacity or alternative weak chemical attacks such as the acetate - acetic acid buffer or ascorbic acid methods would probably have been satis factory in determing the total associated species but would have given little in-dication of the progressive increase in ion-concentration towards the charged surface. The method finally adopted was a modification of the general system proposed by f'lurthy and Ferrell ( 1972), although a similar analysis of the results was not attempted. The principle of this modified technique is that successive treatment of the wet sediment sample with de-ionized water will progressively strip away the outer layers of adsorbed ions and give a comparative measure of the bulk adsorbtive properties of a particular sediment sample and the composition of the adsorbed layer II. It was found that these characteristics could be distinguished in all sediments by three such treatments and that for the vast majority they were distinguishable by a single treatment.

Determination of the chemistry of the pore waters

As a standard routine sediment which was stored at 4°c was subsampled to provide about 10 g of sample. This was placed in a polythene jar and 20 mis of DIU added. After thorough disaggregation and vigorous agitation for five minutes, the jars were placed in an ultrasonic bath for 55 minutes at less than 10°C. The resulting suspension was then filtered through a 0.45/*- membrane with sintered glass support to obtain a sediment residue of the same volume as the original sample before extraction. The leachate was washed from the flask and made to 25 mis with D.I.U. from which aliquots were taken for the major ion analysis. No acid or lanthanum additions were required in this case. The standard Departmental methods were 203 employed for all analyses. Chloride ion was determined by the Mohrs silver nitrate titration. Sulphate ion uas estimated by the nephelometric method described above. Calcium and magnesium were determined by atomic absorbtion using an S.P.90 spectrophotometer and !Ma+ and K+ were determined by emission spectrophotometry on the same instrument. Interference corrections were applied as required.

Replicate determinations of three consecutive leaches of two of the most common sediment types revealed that:- i) On average a first leach extracted 72% of the total ions extracted by three leaches. The second leach extracted 20.7% and the third leach about 7.3%. ii) These recoveries were independent of the sediment type and not critically dependent upon the duration of leaching nor on temperature. iii) The magnesium ion uas more quickly extracted than sulphate, sodium or potassium. The differences between the recoveries of other ions were not very significant. iv) The method is fairly consistent, given the inhomogeneous natural sample material which could neither be dried nor ground and the replication trials indicate a moderate precision. Overall, 70% of the replicate determinations were within + 18% of the established mean value with calcium giving the poorest response. The accuracy of the method could not be assessed due to the complexity and inhomogeneity of the samples.

Determination of the chemistry of the surface waters

Samples of the surface water are much more easily obtainable than representative samples of the pore waters and were simply collected in polythene bottles from shallow pits at as many sites as possible. The bottles had all been acid leached and rinsed with deionized water in the laboratory and were finally rinsed several times with water 204

from the sample site. A duplicate sample was.collected for the alkali metal and anion determinations and pH was measured as quickly as possible from the water in the sample pit. Each sample was stored at 4°C in the dark until it could be filtered. Unfortunately for some samples this period was as long as two days. After filtration through a 0.45Aimembrane the "heavy metal" samples were brought to pH2 with concentrated nitric acid and returned to the cold storage. The "alkali metal" samples were not acidified. Samples of the water for sulphide analysis were stabilised during transport by addition of alkaline zinc chloride solution and were determined immediately upon arrival at the laboratory.

Three analytical methods were available for water samples containing Fe, Mn, Cu, Pb, Zn, at Imperial College at the time of research. Of these analysis of the water sample by direct reading induction coupled plasma spectroscopy was the most suitable but facilities kindly provided by Barringer Research (Canada) extended only to checking one ' sample set determined by other means. Thus for routine work a Perkin Elmer 601 instrument was employed to determine preconcentrated water samples by A.A.S, Two preconcentration methods were compared. A SDDC (sodium diethyl dithiocarbamate) chelation followed by chloroform extraction was effective but it was slow and it would not have been suitable for very small volume samples derived from the simulated diagenesis experiments. Thus a simple evaporation prscon- centration method was'developed. Extensive trials showed" that plasma spectroscopic estimation of the dilute sample, A.A.S. determination of both the SDDC preconcentrated sample and the simple evaporation preconcentrate produced the same results to within experimental error. Thus, the evaporation method was adopted as standard. This method is simple in that a known volume of the sample is evaporated almost to dryness at 80°C in a thoroughly cleaned 50 ml beaker and diluted to volume with 1 m HCl. The formation of salt crystals (presumably NaCl) in one or two of the more 205

concentrated waters could not be shown to cause significant error.

The method for dissolved sulphide is essentially the idometric method given above for acid extractable sulphide although very much more dilute reagents were employed. The most suitable concentration was only found by trial and error as very dilute reagents allowed significant air- oxidation of the liberated iodine during titration whereas more concentrated reagents were titrated with poor accuracy. For these reasons the results obtained in this study are probably only semi-quantitative and if a sulphide specific electrode can be obtained a trial is recommended for any future studies.

2- - 2-1- 2+ + + The analyses for 50^ , CI , Ca , Mg , Ma K and pH were carried out using methods discussed above for the deionized water leaching experiments. The following diagram summarises the various elements of the analytical technique employed. 206

ANALYTICAL PRGCEDUEi

samples from Surface water artificial duplicate Surface water diagenesis for dissolved duplicate Surface waters experiments sulphide for pH

filtered through a filtered through u.45>L membrane 0,45 A- membrane in the laboratory on site acidified to unacidified stabilised unstabiiised pH2 with known with alkaline amount of HNG-, ZnCl9

stored at 4 C in dark cabinet

evaporated almost to dryness at lass than 80°C and diluted to volume uith 1 m HC1

A.A.S. determination of Fe, Fin, Cu, Pb, Zn

r l SO

determination of sulphide species by iodimetry pH FIG. (84)

COMPARISON OF SURFACE WATER SAMPLES AND

CONCENTRATIONS OBTAINED BY LEACHING SEDIMENT

ENVIRONMENTAL SUBDIVISION 7. OF SURFACE WATER CONCENTRATION RECOVERED BY LEACHING

CHLORIDE CALCIUM MAGNESIUM SODIUM POTASSIUM

RESTRQNGUET CREEK

Creek Mouth Complex 106 N/A 101 91 109

Intertidal Flats : lower 102 103 81 104 93

Intertidal Flats : algal banks 91 103 73 101 100

Intertidal Flats : upper 103 132 80 84 99

Marginal Environments 101 145 83 101 95

TRESILLIAN RIVER

Point Bars 100 83 80 91 90

ARITHMETIC MEAN 100 113 83 95 98 208

The analytical precision of the various methods was O i O i , assessed from replicate analyses. For Ca , Mg , Na , K+, the maximum relative standard deviation (expressed as a percentage of the recorded mean) uas 9% (for calcium). For 2- SO^ and 01 the relative standard deviations expressed in the same way were nearly 1Q% and Q% respectively. A better precision uas obtained for the heavy metal determinations and an overall maximum standard deviation of nearly uas obtained. Lead uas generally found to be at or b-elou the limit of detection for the methods above. Unfortunately there uere insufficient samples available for the estimation of the overall total variability of the estimates for a particular environment.

5.1 MAJOR ION CHARACTERISTICS OF SURFACE AND SEDIMENT PORE UATERS

The results of a comparison between the major ion concentrations recorded from sediment leaching experiments and from the surface water samples from different environments are given in Fig. (84). Calcium concentrations in leachates are higher, chloride and potassium concentrations are about the same, sodium concentrations are slightly lower and magnesium concentrations are much lower than those in the surface waters. These data tend to indicate that magnesium is mostly present in the inner layer of adsorbed ions around sediment materials and that sodium, chloride and potassium remain in the outer layers. Calcium is probably leached out of calcium carbonate minerals and is present in leachates in anomalous concentrations. There appears to be the expected good degree of correlation between the preferential adsorption of magnesium and the sediment grain size. For exampie least magnesium was extracted by leaching the fine silty sediments of the Restronguet algal banks.

THE SURFACE UATERS

Figs. (85-88) illustrate the distribution of some of the major ion concentrations at various sample points in

212 213

CALCIUM Na Samples

NORMAL SEAWATER

8 6

4 2 Ca2+/ CI i i Q-OI5 0-020 0025

POTASSIUM

NORMAL SEAWATER

IO|

K7ct- i i _! 1 1_ 0-018 0020 0025 0030

Fl G 89 SURFACE WATER -

HISTOGRAMS OF CONCENTRATION RATIO WITH CL NO. SODIUM Samples

IO NORMAL SEAWATER 8^

2 Naf/Cl _i L_ _i i i i i_ i i_ 0-45 OSO 0-55 0-60

MAGNESIUM © 6 NORMAL SEAWATER

2 Mg2t/CI _l 1 L _l I I I I 1 L. -I L. _l i l. 0050 0055 0060 0065 0070

FIG 90 SURFACE WATER - HISTOGRAMS OF CONCENTRATION RATIO WITH CHLORIDE ION 215 i j 217 the Tresillien River and Restronguet Creek. There are excellent concentration trends from the head to the mouth of the tributaries uith local anomalies where dilution by surface or sub-surface run off has occurred. For example the permeable creek mouth sediments give lou ionic concentrations where major brackish ebb tide run off occurs in the sub-surface. Under these conditions, the ionic ratio uith chloride gives a useful indication of the behaviour of the other ions. Figs. (89, 90) indicate that surface water in the Restronguet and Tresillian estuaries is generally enriched in potassium and deoleted in sodium and magnesium relative to "normal seauater11.

THE PORE WATERS

Restronguet Creek: The Creek Mouth Complex

The detailed leaching experiments indicate that pore water concentrations in the coarser sediments are of the order of 17-23,000 ppm chloride, 400-710 ppm calcium, 475-550 ppm potassium, 10500-13,000 ppm sodium and 950-1030 ppm magnesium. These concentrations are illustrated by the vertical profiles figs. (91), (92), and as may be seen, they generally fall with increasing depth towards the basal fossil intertidal flats sediments, e.g. Core 35. Many of the near surface sodium, calcium and chloride concentrations are greater than those in "normal seawater" and are tentative attributed to the dissolution of a localised evaporitic encrustation which commonly occurs along this shoreline. There is little significant correlation between the ion concentrations and the sedimentology other than for magnesium which appears to be abstracted from pore water in finer grained sediments.

Restronguet Creek: The Intertidal Flats

The lower and upper intertidal flats sediments exhibit similar pore water concentrations which are generally lower than those from the creek mouth complex. The chloride

221 concentrations vary from 15,000 to 21,000 ppm (mean 18,400 ppm), the calcium concentrations vary from 260 to 550 ppm, the potassium concentrations vary from 400 to 600 ppm, the sodium concentrations vary from 9000 to 12,000 ppm, and the magnesium concentrations' vary from 650 to 920 ppm.- The detailed vertical logs illustrate the variation of these concentrations uith depth, see figs. (93, 94, 95), but show only slight correlation betueen pore uater chemistry and sedimentology. Chloride and calcium concentrations in particular correlate well uith the appropriate surface uater samples and approach the concentrations in normal seauater quite closely. .'There is only a slight correlation betueen sedimentology and higher sodium concentrations and this appears to indicate that in more sulphidic environments adsorbed sodium is being released to pore waters as iron oxides in the sediments are dissolved. The tendency for magnesium to be abstracted by finer sediment is also evident although the phenomenon is not as obvious in more organic rich beds. This may be partially attributed to the reduction of the surface chemical effects around fine grained sediment t particles by a coating of organically derived carbon compounds. As discussed above, potassium is enriched in these pore waters although (the vertical profiles do' not indicate any likely source lithologies.

Fig. (96) illustrates the vertical profile obtained for Core 30 uhich contains several cycles of algal sediments. As may be seen, most ions correlate slightly uith the lithology and most of the effects discussed above are displayed.

Restronguet Creek: Marginal Environments Tuo core samples of mudcracked algal flats sediments marginal to the saltmarsh are illustrated in figs. (97), (98). The chloride concentrations in modern sediments vary from about 14000 to 18000 ppm. Calcium concentrations in these modern sediments vary from 200 ppm to 600 ppm and similarly the potassium concentrations vary from 310 ppm to 450 ppm, the sodium concentrations vary from 6000 ppm

CORE 26 PORE WATER CHEMISTRY Fig. 97

Ca,K,Mg TENS PPM Ca,K,Mg/Cl ION RATIO 10 20 30 40 5p 60 70 60 90 IOO , .Ol .02-03 -04 -05 .06 «07 .06 .09 -IOO I 1 1 1 1 4- 1 1 1 1 1 I F 1 1 1 1 4—1 1 1 1 Y No HUNDREDS PPM Na/CL RATIO xlO"'

rMo GJ to to 225

to 11,500 ppm and the magnesium concentrations vary from 500 to 1000 ppm. Beneath these sediments the fossil artificially deposited units are of quite different pore uater chemistry uhich varies considerably even from one core to the next. There is some evidence to indicate that run off dilution along the unconformable contact is a significant concentration control. Potassium behaves anomalously in this environmental subdivision as there is no significa'nt • concentration discontinuity at the unconformity and contrary to the trends discussed above, potassium does not appear to be anomalously concentrated in these pore waters. Most of the other ions behave as in the other Restronguet environments although magnesium cannot be shown to be absorbed from the porewaters by fine grained sediment.

The core samples of saltmarsh sediment exhibit varying pore water concentrations which are much influenced by dilute surface-run off. Magnesium seems to be somewhat enriched in pore waters from highly sulphurous sediments and this effect may be due to the high organic carbon content or to dissolution of iron oxide coatings around clay mineral grains. Potassium is somewhat enriched in these upper pore waters although minor quantities appear to be removed by the sediments at greater depth.

Tresillian River: Intertidal Flats

The chloride concentrations in these sediments are not available but probably fall to only about 9000 ppm in the upper reaches of the estuary. However, the calcium concentrations average about 300 ppm, the potassium concentrations reach about 550 ppm, the sodium concentrations vary from 7000 to S000 ppm and the magnesium concentrations vary from 500 to 1000 ppm. Fig. (99) illustrates the concentration profiles developed in one core sample and as may be seen, calcium is abstracted in the deeper sediment zones. In contrast, potassium is supplied to the pore waters at all levels. As discussed above, magnesium is CORE 42 PORE WATER CHEMISTRY Fig. 99

Cq ,K, Mg TENS PPM Ca,K,Mg/Cl ION RATIO 10 20 30 40 50 6O 70 80 90 DO , -OI -02-03 -04 -05 -06 -07 -OS .09 400 | 1 1 1 1 4- 1 1 1 1 1 I + 1—I 1 1 1 1 1—I 1—H Na HUNDREDS PPM Na /CL RATIO * IO'1 O O 3 sss

O O

O O fO to i

O q uj U)

>o O

o o in CJI

ss Ca K Mg Na K Ca Na Mg O O in in 227

generally abstracted from the pore waters and is more

closely associated with fine grained sediment than any of

the other ions.

Tresillian River: Point Bars

The recorded concentrations of the ions are rather

similar to those noted above for the Restronguet intertidal

flats. Chloride varies from 9000 ppm to 21,000 ppm at the

confluence of the tributary although the mean is about

15,000 ppm-. Calcium varies from 140 ppm to 340 ppm,

potassium varies from about 300 ppm to 550 ppm, sodium varies

from a low value of 7000 ppm to about 9000 ppm and magnesium

varies from 500 ppm to 1000 ppm. The general findings

discussed above are all exhibited in these sediments and

there is a significant enrichment of potassium in most pore waters when compared to the normal seawater proportion.

See figs. (100), (101), (102).

BEHAVIOUR OF MA00R I0N3

These detailed studies indicate that each of the major

pore water components has a characteristic mode of behaviour which is exhibited to varying degrees in different sedimentary environments .

Chloride

The chloride concentrations are characteristically virtually independent of the sediment type and decline

from the mouth of the Restronguet estuary, where the concentrations appear to be evaporitically enhanced, to the head of the Tresillian River, There are a few examples of low chloride concentrations which are generally produced by surface run off dilution.

Calcium

The calcium concentrations vary significantly from one CORE AO PORE WATER CHEMI ST RY Fig. 100

Ca%K , Mg TENS PPM Ca ,K , Mg / CI ION RATIO IO 2-0T 30 40 5fQ- 0O 70 BO 90 IOO , -0! 02 03 -0^—i4 -05— -06 -07 -08 -09 4-i0 1-

O Na HUNDREDS PPM O Na / CL RATIO x IO"' 3 55 3 55 Qfi O o / / 555 / h / i d/ cf i T O 55 o l\J / ! / to 555 1 /' <' /' 55 O O Li 1 • • 1 Li X 1 1 A i i I m f Q O •f" > •i VV •i* V. f - 1 \ t c V 55 \ \ - 55 O i \ O Cr» » \ ui il l A T O \ ' A O Ch \ \ \ 1 SSS 'V Ca K Mg,Na K Ca Na Mg \i

* CO RE 46 PORE WATER CHEMISTRY Fig. 102

Ca,K,Mg TENS PPM Ca,K,Mg/CL ION RATIO IO 20 30 40 50 6O 70 80 90 DO , -pi -Q2 -CG .04 .05 .06 .07 .OS .09-IOO I 1 1 t 1 4- 1 1 1 1 1 I- 1—H 1 1 1 1 1 1 1 -1 Nd HUNDREDS PPM Na/CL RATIO xlO O O 3 T 5 O O 5 I / / O po 1 s o 5 Q (jj a> C a K Mg K Ca Na Mg o O 4 •U o O Cn ui

O o o* 231 environment to another. There is a general tendency for abstraction of pore uater calcium by fine grained sediment although this effect is masked by the varying degree of bioturbation on the lower Restronguet intertidal flats.

Some anomalously high calcium concentrations are produced by partial dissolution of calcium carbonate minerals.

Potassium

The pore uater potassium concentrations are much higher than uould be expected from a simple sea uater source.

Thus, the sediments appear to be supplying potassium to the pore waters in both the Restronguet Creek and the Tresillian

River although the effect is reduced at the head of the

Restronguet Creek. Individual sedimentary types are not distinguishable by their pore uater potassium concentrations uhich indicates that in these environments the K ion is particularly mobile.

Sodium

The sodium concentrations vary uith the sedimentary environment but there are clearly many factors affecting them. The most significant trend identified in this study is for sodium to be supplied to pore uaters in those upper intertidal flats environments uhich contain the highest acid extractable 1^5 concentrations. Elseuhere in rather similar sediments there is no discernable trend. Finer grained sediments at the mouth of the Restronguet Creek tend to adsorb sodium in areas where bioturbation is most intense.

Magnesium

There is a clearly recognisable tendency for magnesium to be removed from the pore uaters by fine grained sediment in all environments. Houever, one or tuo samples of the highly organic, sulphurous sediment at depth on the algal banks and beneath the oxidised saltmarsh appear to be 232

releasing magnesium to the pore uaters. This effect

possibly results from reduction of iron oxides and development

of thin coatings of organic materials around finer grained

sediment particles thereby reducing the cation exchange

capaci ty •

5.2 TRACE METAL AiMD pH CHARACTERISTICS OF SURFACE CATERS

RESTROiMGUET CREEK

The surface uater samples from Restronguet Creek are

colourless and generally odourless. Only one sample from

the highly reduced zone of Tallacks saltmarsh contained

traces of hydrogen sulphide. A uide range of pH values uas

found although the variation does not appear to be entirely

consistent. It appears that saltmarsh uaters are the most

alkaline uith values from pH 7.15 to 7.55 although fresh

uaters flouing into the marsh give pH values from 6.3 to

6.6. The intertidal flats in the upper • estuary average

about pH 7.2 but become more alkaline dounstream. The tidal

flats associated uith the creek mouth complex have a mean

pH of 7.3. The creek mouth complex itself exhibits a uide

range of pH values (from 7.0 to 7.45) without obvious

consistency. Five sample sites shoued some precipitation of

orange-broun iron and manganese oxides as the sample uas

being collected. Three sites were on the Tallacks saltmarsh

and one on either side of the crest of the creek mouth

complex.

I ron

Dissolved iron concentrations in Restronguet Creek

surface uaters range from 70 ppb to 4000 ppb but the majority of samples fall into the range 200 ppb to 800 ppb uith a median value of about 450 ppb. As considerable

care uas taken during the filtration of these samples, the

high values do not result from contamination by colloidal

mineral particles. These concentration levels far exceed

the known solubilities of iron sulphide ana thus it seems

235

most likely that they are given by dissolved organo-metallic

complexes. . ( Duinker et al, (1974)). Fig. (103)

illustrates the distribution of dissolved iron concentrations

in Restronguet Creek and as may be seen the highest values

occur in a zone of tidal mixing beneath -the surface of the

Tallacks saltmarsh. This mixing zone was sampled in the

field in a line of pits which were excavated at right angles

to the main channel. As the rising tide mixed with saltmarsh

groundwaters, flocculation of reddish metal oxides occurred

and a wedge of metal enriched groundwater was pushed laterally

and updip within the permeable saltmarsh sediments themselves.

After a short interval the metal oxides were deposited on the

floor of the pit and in the pore spaces and the high dissolved metal concentrations described above were gradually diluted

by the flowing tide, see fig. (104). A similar situation

probably exists on the flood ramp where a wedge of iron

enriched groundwater appears to be pushed updip into the lower intertidal flats environment to give a concentr-ation maximum at the contact with the permeable flood ramp and ebb shield sands. Evidence of diagenetic modification of these enriched waters is given by the lower Restronguet sediments, where an excellent inverse correlation between dissolved iron concentration and dilute acid extractable

H^S is obtained for deep samples. It seems likely that during diagenesis of these waters the organic complexes are metabolised in such a way that iron is firstly liberated into solution and is then abstracted to form iron sulphide.

Manganese

The manganese concentrations in Restronguet Creek vary from nil to several thousand ppb. However, more

commonly concentrations range from nil to about 700 ppb.

The concentration distribution is bimodal and consists of one population with a median of about 90 ppb and another with a median of about 420 ppb. The highest values occur

at the margin of the creek mouth complex and in the Tallacks

Creek area. As may be seen from fig. (105) there is

excellent correlation between the manganese concentrations

PPB. FIG. 106

Mn SURFACE WATERS - ALL AREAS 600 MANGANESE Vs ACID EXTRACTABLE H,S

500

400

300

• •

200

100 *

THOUSAND PPM H 2 S 238 and the sedimentary environments dounstream from Tallacks

Creek,, It is clear that in these areas dissolved manganese decreases as the sediment grain size decreases, and organic carbon and hydrogen sulphide contents increase. Fig. (106) illustrates the degree of correlation between dissolved manganese ana dilute acid extractable sulphide concentration of the enclosing sediment.

Thus there is a general similarity of behaviour between iron and manganese although their concentrations correlate rather poorly in individual samples. The variation in concentration levels within a particular environment are much more consistent than for iron and unlike iron there are no extreme concentration maxima given for very organic rich intervals. Cn balance, the available evidence indicates that the manganese concentrations are probably controlled by inorganic chemical reactions. The inverse correlation between H^S and dissolved manganese tends to indicate that in environments with strongly negative Eh manganese is speedily mineralised as manganiferous calcite, rhodo- chrosite or possibly even the manganese sulphide albandine.

Independent evidence for the formation of these species could not however be obtained.

Copper

The copper concentrations in Restronguet Creek sediments range from about 40 opb to 2750 ppb with modal values at

75 ppb, 260 ppb and 630 ppb. Fig. (107) illustrates the concentration distribution obtained from individual samples.

As may be seen, the distribution is very erratic although low values occur in lower Restronguet Creek mouth environments ana high values occur at Tallacks Creek. It is thought that these very variable concentrations are given by the differential effects of four main processes:-

F 1 G 1,08

•

4 -

CD OL QJ •

CO Q Z • 3 • C

• z • o 2 » 1— • < cr i— z LXJ o z o • • o • • • SURFACE WATERS <* • • (ALL AREAS) o 1 • z N • • •• • Zn vs. Cu

i • i i i * 2i 4 6 8 IO 12

COPPE R CONCENTRATION HUNOREOS PR 8, 241

i) complexation of copper as organometallic molecules

in organic rich zones with subsequent partial mobiliza-

tion into sediments updip and marginal to the tidal

wedge.

ii) liberation and subsequent mineralization of copper

as copper sulphides following biological metabolization

of organometallic complexes. iii) dilution of complexed copper concentrations by rising

flood tides . iv) sedimentation of copper adsorbed onto flocculated

metallic oxides ahead of the rising tidal wedge.

In general, high copper concentrations at Tallacks Creek result from the mixing of fresh and salt waters to form an enrichment front which is progressively diluted towards the main channel. Intermediate concentrations are common in intertidal flats environments where the various processes appear to be in equilibrium. Low concentrations occur in the creek mouth complex where dilution by estuarine waters has occurred.

Zinc

The behaviour of zinc is similar to that of copper and a range of concentrations from 113 ppb to 4400 ppb is generated. fig. (108) indicates that the zinc concentrations, although greater, correlate well with the copper concentrations, and the same processes appear to control the concentrations ofboth metals. fig. (109) illustrates the distribution of the dissolved zinc concentrations.

Lead

Studies of the lead concentration distribution in

Restronguet Creek proved to be inconclusive as many of the initial analyses could not be subsequently repeated. It was found that the same sample might give a range of concentrations from • 110 ppb to 450 ppb. This indicates that the sample volumes available are insufficient.

243

Therefore, it is only possible to conclude that dissolved

lead concentrations of up to 600 ppb are present at Tallacks

Creek whereas there is negligible dissolved lead in the creek

mouth surface waters. Very feu of the tidal flats samples

contained detectable dissolved lead,

TRESILLIAN RIVER

Host of the surface water samples from the Tresillian

River were colourless and odourless although three samples

taken on the louer point bar uere yellouish-broun ana the

colouration remained after filtration through a 0.45/11-

membrane. These samples also smelled of hydrogen sulphide.

One sample from the higher area of the point bar precipitated

yellouish-broun iron and manganese oxides before it could

be acidified. The recorded pH varied from 7 to 7.3 uith

an average of 7.05. There is little consistent variation

from the head to the mouth of the tributary.

I ron

The recorded dissolved iron concentrations vary from

200 ppb to 1050 ppb uith an excellent lognormal distribution.

The geometric mean value is about 340 ppb. There is little systematic variation of iron concentrations in the Tresillian

River, although values do appear to increase slightly

touards its mouth. The values do not correlate directly uith the sedimentary environments. The highest value

(1050 ppb) occurs just belou the crest of the louer point bar and may be given by mixed salt and fresh waters flouing doundip on the ebb tide. Elseuhere, diagenetic processes in organic rich sediments have destroyed any enriched pore uater metal concentrations uhich may have formed on the

flood tide. These sediments have the same pore uater iron concentrations as finer grained Restronguet Creek flats and may be regarded as their lateral equivalents. 244

Manganese

The manganese concentrations in Tresillian River sediments range from nil to more than 500 ppb and are b-imodally distributed. The distribution is complex and with the current sample density it is only possible to indicate an approximate louer median value of 70 ppb and an upper median value of 220 ppb. The lower value occurs throughout the tidal flats environments and the upper

Tresillian point bar environment. This evidence indicates quite clearly that mixing processes are not dominant in the

Tresillian River environments. This is probably partly due to the decreased permeability of the sediments and a short high tide interval. It is also possible that the sediments are so organic rich and thus diagenetically active, that any pore water enrichments are destroyed more swiftly than in

Restronguet Creek. The lower point bar analyses tend to support this hypothesis for in more permeable sediments a high median value of 200 ppb was recorded. However, in a zone of very high organic carbon and hydrogen sulphide content which is just as permeable as the surrounding sediment, the highest manganese concentrations is 75 ppb and the mean is 56 ppb.

As expected, the manganese pore water concentrations are very similar to those in the finest grained Restronguet intertidal flats sediments. There is an excellent pattern of decreasing manganese values from the coarse grained

Restronguet Creek mouth complex to the head of the Tresillian

River with local anomalies where more permeable sediments occur,

Copper

All the copper concentrations found in the Tresillian

River pore waters are less than 30 ppb. The median value of the slightly skewed distribution is about 45 ppb. There is little or no systematic variation in any environment nor from the head to the mouth. As the concentrations are 245 unaffected by the presence of hydrogen sulphide or by pH, it appears that the copper species are in equilibrium uith the enclosing sediment. Overall, the values are much less than those for similar sediment in Restronguet Creek and it is concluded that copper pollution of the Carnon

River at least doubles the copper concentration of Restronguet

Creek environments. It is clear that the effect of this pollution does not extend to the Tresillian River.

Zinc

The zinc concentrations are very much higher and more variable than the copper values. A range of zinc values from 50 to 450 ppb was obtained uith a median value of about 150 ppb. The sample density is insufficient to attribute a mean concentration to any particular environment and the variation of zinc values is difficult to interpret.

Houever, it appears that there is no overall decline in zinc towards the head of the estuary. Thus, the range of observed zinc concentrations, as for copper, is probably due to statistical variation under equilibrium conditions.

There is no strong evidence that Tresillian River pore waters are polluted uith heavy metal from the Carnon River although the concentrations in equivalent lithologies from the tuo estuaries are only' broadly similar.

Lead

No reliable analyses indicating significant lead concentrations in the Tresillian River pore uaters ware obtained. 246

CHAPTER 6

PART I - CONCLUSIONS

Part I of this study reports the results of a geological

and geochemical investigation of the sediments in two of the major tributaries of the Falmouth Estuary, Cornwall,

Restronguet Creek is visibly polluted with mining effluent via the Carnon River, and the Tresillian River, which is situated some distance to the north-west, may also be contaminated to a small degree. A continuous sequence of sedimentary environments is developed in these tributaries and for the purposes of this study eight major divisions are recognised:- the creek mouth complex, intertidal flats, channel slopes, point bars, tributary creeks and streams, marginal environments and artificial environments. The latter division encompasses those sediments which were deposited as a result of man's activities and are distinct from naturally deposited buried sediments which are encountered in the sub-surface of the

Restronguet Creek. The Restronguet Creek mouth complex is predominantly composed of sand sized sediment with sporadic mudstreaks and passes laterally into bioturbated silty intertidal flats sediment. The finer grained upper sub-division of the latter environment also occurs widely in the Tresillian

River and small marginal saltmarshes are developed in both estuaries.

Sediments rich in organic material occur in the algal banks subdivision of the lower Restronguet intertidal flats and comprise a large part of the point bar environment in the

Tresillian River.

The sediments in both tributaries are predominantly composed of marine derived quartz and detrital muscovite, chlorite and kaolin of fluvial provenance. In general, the proportions of chlorite and kaolin increase towards the head of the Tresillian River although this estuary is undoubtedly polluted with some kaolin-bearing china clay waste. Feldspar, tourmaline, cassiterite, stannite (kasterite), pyrite and 247

chalcopyrite were all identified in the sediments by X.R.D.

Authigenic iron rich chlorite is being emplaced in the algal

banks sediments of the lower Restronguet intertidal flats

and quasi oolitic iron oxide pellicles enclose some detrital

silicates in Restronguet creek mouth sediment. Thus very low

grade zoned iron mineralisation is developed in the lower

part of the Restronguet Creek and varies from iron bearing

silicate in nearshore algal banks, to pyrite mineralisation

in the surrounding intertidal flats sediments and quasi

oolitic iron oxide in distal channel margin sediments.

Framboidal pyrite, detrital sulphides and rare chalco-

pyrite flakes up to 200^/iw in diameter also occur in the

Restronguet Creek mouth sediments. Framboidal pyrite, authigenic

cubic pyrite overgrowths, dusty authigenic chalccpyrite and

stellate aggregates of authigenic chalcopyrite are more

common in intertidal flats environments. In addition die-

genetic stannite occurs in lower Restronguet intertidal flat

sediments. Authigenic pinkish bornite is more common towards

the head of the Restronguet Creek and forms overgrowths to

authigenic bladed chalcopyrite, datrital chalcopyrite and

detrital purple bornite. The marginal environments at the

head of the estuary are characterised by a diversity of

sulphide minerals many of which occur in discrete chemical micro-environments. Supergene enrichment phenomena are

common and chalcocite is developed in very limited areas.

In these environments, however, the major diagenetic effect

appears to be widespread development of pinkish bornite which

does not replace either diagenetic or detrital pyrite. The

Tresillian River sediments are well mineralised with pyrite

framboids and pyrite overgrowths. One flake of diagenetic

chalcopyrite was identified in coarse grained marginal sediment.

The highest recorded nitric acid extractable metal

concentration in the tributary estuaries is an iron assay of

69,500 ppm which was obtained from a sample of algal bank

sediment at the margin of the lower Restronguet intertidal

flats. Authigenic iron rich chlorite is forming in this

environment. The highest mean nitric acid extractable 248 manganese, copper, lead and zinc concentrations all occur

in upper Restronguet intertidal flats sediment and are

849 ppm, 4080 ppm, 459 ppm and 3174 ppm respectively. In

general, there is good correlation between iron and copper,

and copper and zinc concentrations and these relationshios

have enabled anomalously high copper and zinc concentrations to be recognised in upper intertidal flats and buried algal banks sediments. Anomalously high iron concentrations occur in creek mouth complex sediments. Elsewhere metal concentrations appear to be inversely related to sediment grain size and do not correlate with the detrital silicate mineralogy. Manganese is somewhat different in that local

E^ variations as indicated by the acid extractable F^S concentrations also appear to exert a significant influence.

Vertical metal concentration profiles indicate that lenticular bodies of metal rich sediment occur in the sub-surface of the marginal saltmarsh environment and in the creek mouth complex.

These are probably formed by tidal flux through- permeable marginal sediments.

Metal concentrations in the Tresillian River are between one half and one tenth of those in the equivalent Restronguet

Creek sediments and indicate the provenance of these high metal values by pollution from the tributary Carnon River.

Some anomalous zinc and iron concentrations occur in the lower

Tresillian point bar sediments and these may mark the limit of metal dispersion upstream from Restronguet Creek.

The total sulphur concentrations recorded in both

Restronguet Creek and the Tresillian River are much more regular than the metal concentrations and may be directly related to variation in major sedimentologicai characteristics from the mouth of the Restronguat Creek to the head of the

Tresillian River. The sulphur occurs in a variety of ionic species and, rarely, as native sulphur. A series of detailed vertical profiles illustrating the variation of pore water sulphate, detrital sulphide, authigenic monosulphide, and authigenic polysulphide concentrations demonstrate that up to 5500 ppm of authigenic monosulphide and more than 249

15,000 ppm of authigenic polysuiphide can occur in these

sediments. Highly mineralised mudstreaks and organic rich

beds occur in the creek mouth and point bar sediments.

Up to 6,3^ of organic carbon is known to occur in lower

Tresillian point bar sediments but the concentrations are

highly variable in both tributary estuaries. There is a

satisfactory correlation between authigenic polysuiphide and

organic carbon concentrations although the sulphur/carbon

atomic ratio is somewhat higher than that reported for other

present day sedimentary environments. This degree of sulphide mineralisation is partially attributed to very high available

iron concentrations. It is not due to detrital sulphide

contamination as the ratios from relatively uncontaminatad lower Tresillian River sediments are also high.

2+ 2+ + + — The concentrations of Ca , fig , K , Na and CI recorded during leaching experiments on small sediment sampies correlate well with the concentrations obtained from bulk samples of the surface waters collected at the same sampling points. However these detailed leaching studies demonstrate the tendency of fine sediments to abstract calcium and magnesium from the porewaters. Potassium is more concentrated than in the parent seawater and is mobile in estuarine porewaters.

In some limited areas magnesium is released into the porewater on dissolution of the iron oxide pellicle enclosing detrital silicate grains. The maximum dissolved trace metal concentrations in these sampies are all obtained from the Restronguet

Creek and for iron, manganese, copper and zinc are 4000 ppb,

700 ppb, 2750 ppb and 4400 ppb respectively. The dissolved lead concentrations are about 450 ppb which is at the limit of detection. The concentrations in the Tresillian River are generally similar although copper values are anomalously low. (Metal enrichments are induced in the sediment porewaters by intrusion of saline estuarine waters at high tide but these are short lived due to metal oxide flocculation and dilution effects. In general, the abnormally high metal concentrations are attributable to organic complexation processes and are thus liable to diagenetic modification as 250

some of the organic compounds are probably biologically degradable. An exception is the concentration of dissolved manganese which appears to be controlled by purely inorganic chemical reactions probably involving the precipitation of majiganiferous carbonates or sulphides.

Thus, this preliminary study of the major features of the sediments occurring in the field areas has identified a number of interesting diagenetic processes which themselves warrant further study in more detail. In particular, the iron mineralisation in the lower Restronguet Creek and the relation' ship between supergene and diagenetic copper mineralisation at the head of the Restronguet Creek could profitably be further investigated using existing techniques. In addition, however, the study indicates the types of material most suitable for experimental high temperature compressive testing and provides the essential background data against which the experimental results must be assessed. 251

PART II THE EXPERIMENTAL 5TUDIE5

CHAPTER 7

EXPERIMENTAL APPARATUS, PROCEDURES AND RESULTS

The experiments described below ere essentially

feasibility studies firstly to establish whether it is

possible to simulate diagenetic environments and sediment

burial processes in the laboratory, secondly to investigate

some geologically interesting, high temperature environments

and thirdly to assess the future potential of these experimental investigations.

7.1 APPARATUS, EQUIPMENT AND EXPERIMENTAL PROCEDURES

The study was carried out using two main pieces of

equipment. The "Big Cell" is a so phisticated apparatus

developed to simulate sediment burial to a depth of over

10,000 feet ( n/ 3 km) with control over the sediment

temperature and pore fluid pressure. Development of this

apparatus began in 1957 and was continued up to and including

the period of this tastwork, (February - November 1977).

The "Small Cell" is a -simple apparatus constructed from

eauipment in the Engineering Geology Laboratory at Imperial

College during 1977.

The Big Cell

This apparatus consists of a consolidation cell unit,

an axial pressure system, handling and control equipment

and data output systems. The consolidation cell unit is shown in figure (110) which indicates the location of the

specimen. The most important features are the heating and

temperature recording apparatus (h, 9, 10), the pore fluid connections to drained and undrained ends, (1 and 3) and the

pressure balanced measurement piston, (11). The cell

construction is designed to be as insensitive as possible

to pressure and temperature effects up to 10Q°C and 252

lower plug

1. Pore fluid connection to undrained 8. Pore fluid connection to drained , . end n en<3 2. Thermal insulation 9. Temperature probe (lower) 3. Wiper blade relaxed shape 10. Temperature probe (upper) 4. ptfe liner (not employed 1977) 11. Pressure-balanced piston 12. Connections to ah transducer, 5. Wiper blade in posi.tion upper heater, upper temperature 6. Porous steel drainage plate probe 7. Connections to lower heater h = heater

nqure 110 Cell general assembly 253

10,000 psi (^v 70 FiPa) . Unf ortunetely, in this environment

steel must be employed in the construction and there is

contact of both the sediment sample and the pore fluid with

the steel components especially at the porous drainage plates

and in the pore fluid.connections. Sample contact with the

drainage plate is minimised by a filter paper (Whatman 42)

lining.

The axial pressure system consists of a small priming pump which is employed mainly to expel air from the sample and from the sampie holder and to actuate the pressure balanced pencil piston, and the main ram pump. This pump may be automatically controlled by addition or subtraction of weights from the pencil piston which increases/decreases the axial pressure by 5.1 psi (0.0345 MPa) per unit weight. The handling and control equipment consists of a mechanical hoist and supporting legs to allow easy operation of the consolidation cell and electrical equipment which controls the rate of loading of the sample and its temperature. Thus, a predetermined rate of loading can be employed by setting a timer to release a weight (steel ball), into a receptacle at the pencil piston which actuates the'ram pump. The temperature control is a simple thermostat which is increased manually as.required.

The data output equipment gives five pieces of information printed on paper tape and may be preset to record at chosen intervals. These intervals are short at the beginning of the test when compaction is swift, but are gradually lengthened as the test proceeds. The time elapsed since the test began, the axial pressure (P1), the pore fluid pressure

(ll ) , the temperature, and a measure of the sample compaction are printed out. Measurement of the sample compaction is somewhat complicated as a linear displacement transducer with a travel of only 0.6 inches is used. Thus, at the beginning of the test the screw jack (see figure 110) can be adjusted to give a reading of +0.3 inches which will become -0,3 inches as the sample is consolidated. At this point the jack is screwed down by about 0.6 inches to return the reading to 254 about 0.3 inches. All these operations are recorded on the paper tape and it is a simple matter to obtain the A h (chance in sample thickness) values by subtraction or the incremental sample thickness provided the final thickness is known after the complete loading and unloading cycle. The pore fluid pressure is measured by a pressure transducer connected to a capillary tube which is attached to the upper end of the sample.

In addition, there are alarms installed to give warning of the need to operate the screwjack, (as above), or of the approach of the limit of travel of the apparatus towards the end of the test when the drainage anvil assembly can be endangered.

The "Small Cell"

This apparatus is illustrated in figure (111). It consists of the loading frame which enables a constant rate of strain to be applied to the samples via a piston and coaxial seal assembly in which temperature or pore fluid pressure measurement probes may be inserted. The consolidation cell is of stainless steel and drainage through a sintered stainless steel plate and L'hatman 42 filter papers is obtained at the base. The heating apparatus is a simple band heater controlled by a thermostat mounted on the outside of the consolidation cell. Measurement and control equipment are essentially limited to the proving ring which indicates the applied axial pressure and the pore fluid pressure gauge.

In practice, the rate of strain is adjusted to obtain consistent low pore fluid pressures which indicate satisfactory drainage from the sample. The test cannot be continued overnight and thus a continuous increase in axial pressure is not feasible. The relationship between the heating band thermostat temperature and the sample temperature is simple and can be allowed for during the experiments.

Experimental difficulties

A number of difficulties were both anticipated and actually encountered during the testwork. Most were common 255

FIG 111 "SMALL CELL" APPARATUS EMPLOYED 256 to both pieces of apparatus and exerted a continuing influence on the design of the experiments and the expectation of

results. The major problems encountered were:-

(i) Lack of pore fluid pressure control for both pieces

of apparatus.

(ii) Contamination of solid and pore fluid samples in

contact uith stainless steel.

(iii) Contamination of pore fluid samples by hydrau-lic oil

during some testuork using the Big Cell.

(iv) Difficulty in obtaining uncontaminated samples

at temperatures greater than 100°C.

(v) Inaccuracy of volume measurements and axial pressure

values due to thermal expansion of the Small Cell

apparatus.

Experimental difficulties: Pore fluid pressure control

There is no control system available to regulate the pore fluid pressure on either the Big or Small Cells. The physical compaction of uet sediment has been described by a number of workers and an excellent summary given in Niagara

(1978) Chapter 3, has been drawn upon to provide the basis for the following discussion. As may be readily appreciated, the sample in the consolidation cell will behave initially as solid silicate grains entirely enveloped in pore fluid. At low axial pressure, there is little contact between silicate grains and much of the applied pressure is supported by the pore fluid. Drainage of the sample occurs at a constant maximum rate which in the early stages is less than compaction rats and thus the pore fluid pressure attains a maximum value.

Later whan the pore fluid is partially drained the silicate grains begin to take load. Eventually, an equilibrium condition is obtained whereby the drainage is sufficiently rapid to induce only a minor pore fluid pressure ana the increase in axial pressure is accommodated by increased silicate grain - grain contact. This situation differs from 257

that occurring in nature in tuc major respects:-

(i) The experimental drainage rate is constant and

relatively rapid for the types of sediment considered.

In nature, the rate is dependent on the host rock

permeability through which the fluid eventually

escapes. These experiments are essentially modelling

the situation in which a sample rock is in contact

with a good aquifer.

(ii) Ps explained by Magara (1978) p49, the pore fluid pressure

at depth is hydrostatic plus a transmitted component

from the overburden pressure. The efficiency with which

this component is transmitted depends upon the drainage

rate which is in turn controlled by pore configuration,

diagenetic precipitation in the pore space, enclosing

host rock permeability and geothermal gradient. In these

experiments the efficiency is very low and most of the

pore fluid pressures are insignificant. This is

important in that virtually all the applied axial

pressure is supported by grain - grain contacts.

Thus, in order to derive geologically meaningful analogues of the experimental conditions as developed, certain assumptions must be made. In these studies, ail the samples are well drained thus:-

Overburden pressure = bearing pressure + hydrostatic pressure

(grain to grain) (pore fluid pressure)

In general, it may be assumed that the overburden pressure gradient is 1 psi/vertical foot (of burial) and the hydrostatic pressure gradient is 0,44 psi/vertical foot. Thus, the bearing pressure ( ) is given by

1 = ( CT ) + 0.44 ) psi/vertical foot

= 0,56 psi/vertical foot

To derive the burial depth simulated by the present experimental testwork with uncontrolled pore fluid pressure, the following relationship must be applied:- 258

Simulated burial depth = (Axial pressure recorded - pore fluid pressure)

0.55

For the investigation of grain - grain effects, this approach is quite satisfactory. However, given current experimental

conditions, it is not possible to develop a pore fluid

pressure which is a significant proportion of the expected hydrostatic pressure for simulated depths in excess of

1,000 feet. Provided that temperature effects dominate the chemistry of the pore fluids then this drawback is not

particularly serious. For trace metal components this is most probably the case.

During each test, a rough estimate of the simulated burial depth was obtained in order that the geologically appropriate temperature could be selected. At the end of the test, the calculation was repeated over all the recorded axial and pore fluid pressure range and accurate simulated burial depths estimated.

Experimental Difficulties: Contamination

There is extensive contact of the sample sediment and its pore fluids with stainless steel components. These components are manufactured from low impurity austenitic stainless steel En58J supplied by British Steel Corporation,

The steel has excellent corrosion resistance characteristics but cannot be expected to be completely inert when in contact with saline water at high temperatures. Thus saline water containing known concentrations of metal ions was subjected to a test cycle during which the applied axial

pressure was adjusted to prevent boiling within the cell

except for the final sample at 18C°C. Fig. (112), Iron and manganese increase during the test and are probably derived from the steel cell and drainage plate. Lead remained approximately constant but zinc increased twofold. 259

Fig. -112 Test blank contamina tion survey

Expelled Temperature F e Pin Cu Pb Zn Solution (Concentr ation - ppm)

Initial 15°C (Nominal) 19.0 1.6 0.75 0.5 2.30

rn, 25°C 19.0 1 .7 0.73 0.5 2.55

B 64°C 24.0 1 .8 0.45 0.5 3.2

C B4°C 23.0 1 .9 0.42 0.8 3.1

D 105°C 25.0 1 .8 0.33 0.3 3.5

E 1 20°C 21 .0 1.8 0.34 0.7 3.8

F 1 50°C 34.0 2.0 0.22 0.5 5.4

G 1 80°C (Boiled) 1.7 1.0 0.05 N .D 0.06 260

It is likely that zinc is derived from the fluorinated hydro-

carbon pressure seals rather than the stainless steel vessel.

In contrast, copper fell by two thirds during the test and

might possibly be involved in a slow electrochemical reaction

at the drainage plate. The final sample indicated that should

boiling occur within the cell, the expelled water vapour will

have precipitated most of the dissolved ions in the delivery

tube and is essentially distilled water.

If significant chemical trends are to be obtained for

pore waters expelled from this apparatus, these results must

be taken into account. In general for temperatures up to

120°C, any supposed increase or decrease in metal concentration must be at least by a factor of two to be considered significant.

Smaller changes must be attributed to experimental error. Any

samples which have boiled in the consolidation cell are

indicative only of the composition of the mixture of water

vapour and fluid which would have been generated in nature

by a sudden drastic reduction in pore fluid pressure. Metal

contamination of the solid sample is unlikely to be significant

during a test cycle and the major pore fluid anions and

cations are unaffected by the composition of the apparatus.

Experimental Difficulties: Hydraulic Oil Contamination

A further difficulty encountered during testwork is the

escape of hydraulic oil due to seal failure into the pore

fluid sample during parts of some tests using the 5ig Cell,

The oil is not miscible with the pore fluid but could abstract

the dissolved organic material from it. Thus, if contamination

is evident, the concentration and composition of dissolved

organic chemicals and associated organo-metallic complexes must be regarded as suspect. 261

Experimental Difficulties: Sample Collection

Various methods of obtaining fluid samples from high

temperature - high pressure tests with the small cell were

tried. Whilst the pore fluid is not boiling within the cell,

the delivery tube is also at high temperature and boiling of

the expelled sample is likely as the pore fluid pressure

falls. Best results were obtained with a water cooled combined

glass and steel delivery tube operating on the Liebig principle.

Experimental Difficulties: Thermal Expansion Effects

At higher temperatures the coefficient of thermal ^

expansion of the metal components of the Small Cell becomes important and unavoidable inaccuracy is introduced into the

estimates of axial pressure sample thickness and theoretical pore volume reduction. 'where possible a calculated correction has been applied. The problem is much reduced at the lower operating temperatures of the Big Cell where the components are of better design and the accuracy of measurement of these parameters can be guaranteed to be less than + 1%.

Experimental Procedures

The basic procedures discussed in this section were employed for both Big Cell and Small Ceil experiments. The sediment samples were collected by manually coring the required stratum as exposed in a pit with large diameter (35 cms) plastic barrel so as to completely fill the tube which was then sealed and made air-tight. A small diameter (10 cms) core sample was then taken adjacent to the sampled site in order to provide sedimentological reference samples and to locate any strata of particular interest. The samples were swiftly transported to be stored at a low temperature. In the laboratory the sample canister was opened and the 262 appropriate sediment interval selected, trimmed to size and placed within the consolidation cell in contact with filter papers (Uhatman No. 42), at the top and base. The consolidation cell assembly consisting of steel chamber, steel drainage plates, PTFE wiper blades and the specimen was placed upon the drainage anvil and all electrical and pore fluid delivery connections made. A calibrated disposable syringe was attached to the pore fluid outlet to collect expelled pore waters.

Initial compaction of the specimen using the priming pump expelled any entrapped air from the system and produced large volumes of pore fluid. Thereafter, the apparatus functioned automatically to give preset increases in axial pressure.

Attention was only required for operation of the screw jack, collection of the sample syringes and control of the sample temperature. Separate pore fluid samples were collected for heavy metal and major ion analyses. It was found that nitric acid was efficient in preventing the formation of iron oxide precipitates during collection of the heavy metal cation sample and sufficient was added into the collection syringe to eventually obtain a 1-2M sample solution. The actual concentration depended upon the accurate prediction of the expelled pore volume which was not always possible. In general, the metal cation samples were of about 10 mis plus nitric acid and the anion samples were of about 5 mis, although in some cases up to 40 mis were collected for particular samples.

At the end of the test, the specimen was automatically unloaded and cooled before extraction from the cell. Its final thickness was measured by micrometer ana up to ten solid samples were taken by sawing the compacted sediment into representative block samples.

Analyses of the solid and fluid sampies were performed by the methods discussed above for estuarine materials.

7.2 THE TE5TW0RK

A total of eight field sampies plus three spikes and blanks were tested in the period February to October 1977. 263

Restronguet Creek end Tresillien River sediment was tested

in Big and Small Cells in about equal proportions. Two of the

eight sampies were contaminated uith hydraulic oil towards

the end of the tests and a number of the expelled pore fluid

samples were discarded. • A further two samples boile-d at the

end of the test and these results are also discounted. Some

of the more organic rich samples gave brownish or blackish

pore fluid samples which are discussed separately below.

As stated in the introduction at the beginning of the

chapter, the aim of this testwork was to establish the

feasibility of laboratory simulation of sediment burial, to

investigate some of the more interesting higher temperature

environments and to assess the future potential of the method.

Therefore, the experimental pressure/temperature regimes were planned with respect to a specific geological objective.

Due to widespread familiarity with Imperial units these are

retained in this part of the discussion.

REGIME A - Simulation of burial under a "normal pressure

temperature regime" with an axial pressure increase

of 1 psi (0.0068 MPa) vertical foot, a hydrostatic

pore pressure gradient of 0.44 psi (0.C03piPa)

vertical foot, and a geothermal gradient of 1°C/

100 vertical feet (30.7 m).

(Samples BR6, Restronguet intertidal flats sediment,

BR3, Restronguet creek mouth complex sediment, 8R4, Tresillian

lower point bar sediment, and BR1, Tresillian intertidal flats

sediment were subjected to this regime.)

REGIME 3 - Simulation of slow burial under abnormally high

geothermal gradients.

Sample BR9, Restronguet saltmarsh sediment was subjected

to a geothermal gradient five times normal.

REGIME C - Simulation of the imposition of extreme temperature

regimes upon either normal or accelerated burial 264

rates. This regime is thought to be geologically

analogous to the initial stages of contact

metamo rphism.

Samples BR7, Tresillian lower point bar, BR5, Tresillian upper point bar and BR2, Restronguet upper intertidal flats were subjected to this regime.

Unfortunately, in the time available, it was not possible to duplicate any of the samples in any of the three experimental regimes.

7.3 CHEMICAL CHARACTERISTICS OF THE EXPELLED PORE FLUIDS

TRACE METAL CONCENTRATIONS

The heavy metal concentrations in pore waters expelled under the experimental conditions discussed above are illustrated in a series of figures Nos. 113 to 120. For sample BR5 a wide range of elements were analysed through the good offices of Messrs. Barringer Research (Canada) and these results are also included in fig. (117). For the other samples only manganese, copper and zinc concentrations are judged to be sufficiently accurate for inclusion in this report. For sample

BR1 manganese is excluded due to contamination and zinc determinations were not performed. The lead concentrations are given for sampie BR 1 only and as may be seen they are at about the limit of detection. All the figures are annotated to show the temperature/pressure regime which was developed.

The appropriate natural pore water concentration medians

(as discussed in Chapter 5.2), are also shown for comparison purposes. As may be seen, there is generally close agreement between these ranges and initial expelled pore fluid concentrations for most samples. However those from the upper point bar in the Tresiliian River and from the creek mouth complex in the Restronguet estuary are anomalously enriched in manganese and a sample from the lower point bar in the

Tresillian River is depleted in zinc. The former anomalies may well be due to contamination and these assays are FIG in EXPELLED PORE WATERS SAMPLE BR1 (NORMAL P/T REGIME)

iooo-

800

66°

59 \22( 600- / o- Pb 29' ° 3 5° ppb 25

400-

200

200 - ppb l 80°C IOO 22 25 29 35^ .59° Cu JJL jd MEDIAN

t 2345 6789 IO SIMULATED BURIAL DEPTH (THOUSAND FEET =307METRES)

SIMULATED BURIAL DEPTH THOUSAND FEET(- 307 METRES) FIG 116-

EXPELLED PORE WATER - SAMPLE BR 4 P/T REGIME NORMAL

ppb IOOO oil\ \ 32° o57°C \ M n 600 - 19° / Q —o o' l I \ 0*" UL O 3 7° 200 -Oo MEDIAN

300 -

200 - 19 EDIAN IT - Zn IOO 5 7°C 200

IOO Cu 37 I]* 57° C MEDJAN

2 3 4 5 6 SIMULATED BURIAL DEPTH THOUSAND FE E T (= 307 MET R E S ) 269

FIG 119 EXPELLED PORE WATER SAMPLE BR 7 (EXTREME THERMAL* REGIME)

300 ppb 25 Mn 0 1 90° - 25° IOO h I 25' O \ a c\- wi

300 Zn __ MEDIAN- IOO 90° 25 110° 200 Cu IOO 25 25c 25 90 25 MEDIAN

SIMULATED BURIAL DEPTH THOUSAND FEET (» 307 ME T RE s) FIG120 EXPELLED PORE WATER "SAMPLE BR 9 (GEOTHERMAL GRADIENT X5 NORMAL)

600 _ __ 50° —

ppb

V \ Mn . i MEDIAN > 600 ppb ^ ^ 200 \ . - - \J20°

20° . J MEDIAN 2 300 ppb 600

" \ 20° "^L-l 20

200 "

#20° T MEDIAN 630 ppb

400

" \ 75° 200 - V^ 20° 50° 50°

I 2 SIMULATED BURIAL DEPTH - TROUSAND FEET 307 METRE s) 273

treated with great caution.

Manganese

In general there is.a general increase in expelled pore

fluid manganese concentrations with temperature and applied

axial pressure for samples from the lower and upper Restronguet

tidal flats, the Restronguet saltmarsh and one area of the lower Tresil-lisn point bar. There is a general decrease for samples from another area of the lower Tresillian point bar, the upper Tresillian point bar and the Restronguet creek mouth complex. Figs. (113 to 120) also indicate that manganese concentrations do not vary erratically even with instantaneous temperature increases of up to 50°C, at least up to a limiting temperate range of about 100°. In the higher temperature range of 100° - 130°C there is commonly an instantaneous increase or decrease in manganese concentrations which then rise with subsequent temperature increases.

Copper

The behaviour of copper is somewhat different in that virtually all the tests give a similar result at least up to temperatures of about 100°C. In comparison with manganese there appears to be much less dependence upon the nature of the sample material. Thus, the initial pore fluid concentrations agree very closely with the pore water concentrations obtained in field samples but may be very slightly less in a feu cases. There is then a decline in concentration to a limiting base level, (not the limit of detection), which is maintained up to the temperature range 110 - 13Q°C.

The response of tested materials in the range 110° -

130°C is comp-lex. For example, in test 3R9 a surge 'of pore water is generated from saltmarsh sediment at about 'i20°C and is initially enriched in copper but becomes progressively more dilute as the expelled volume decreases, A final very lou concentration (omitted in fig. (120)) seems to be caused by boiling within the delivery apparatus during the initial 274 stages of a machine failure and may be discounted.

Unfortunately, this temperature range uas examined in only tuo other tests namely BR5 at 130°C and BR7 at 1'1Q°C uhen a slight concentration decline uas obtained. Houever, both tests are on samples of poorly mineralised Tresillian point bar sediment and the experimental temperatures are at the upper and louer ends of the suggested temperature range.

Above about 130°C the results from tuo out of the three samples tested shou an initial increase of several hundred ppb in pore fluid copper concentration although in one case these high concentrations seem to fall auay at the highest experimental temperatures employed. The results from the third sample tested BR7 shou a slight but continuous decline in pore uater cooper concentrations up to the maximum temperature of i75°c.

Zinc

The initial zinc concentrations also agree closely uith the recorded range found in the field samples grounduaters although there is a significant anomaly in BR7 uhere the expelled fluid is very depleted. In the temperature range up to 110°C pore fluid zinc concentrations are someuhat variable but in five of six relevant tests an initial decline is folloued by a slight increase in concentration at about

50°C. In the range 110 - 130°C tuo out of the three tests shou slight or significant instantaneous increases in the zinc concentrations folloued by a fall in one test, (6R5).

At higher temperatures, spectacularly high zinc concentrations of several ppm are recorded in tuo out of the three relevant tes ts.

Other Elements

The expelled pore uaters from a single test (BR5) uere subjected to multielement analysis by an induction coupled plasma spectrophotometer method at the Canadian laboratories of Barringer Research (Canada) Inc. The author did not carry 275 out these analyses in person but duplicate analyses lie within the field of experimental error of his own determinations of

Cu, Fe, Mn, Zn and Pb and therefore the results for Ti, Ag,

V, Co, Ba, P, Al, 3, Mo, Cr and Ni are also judged to be acceptable. As may be seen from fig. (117) these elements fall into four distinct concentration classes which exhibit similar behaviour on compressive testing. Silver and titanium occur in the expelled porewaters in very low concentrations

( /V 100 ppb) at temperatures of up to 57°C and fail to trace values above this temperature. Vanadium and cobalt normally occur at concentrations of between 100 ppb and 500 ppb.

Vanadium at least shows a marked increase in concentration at the highest temperatures. Barium, phosphorous 3nd aluminium occur at concentrations in the range 200 ppb to 6 ppm at temperatures of up to 84°C. Above this temperature the concentrations increase markedly. Finally boron, molybdenum, chromium and nickel occur at normal concentrations in the range 300 ppb to 10 ppm but these increase dramatically at the higher temperatures. The increase is most marked in the case of boron where the concentration begins to rise from 7 ppm at temp-eratures of about 60°c and reaches 65 ppm at 170°C.

The behaviour of molybdenum, chromium and nickel is rather similar but the increased concentrations are observed at

B4°C for Mo and 170°C for Ni and Cr. The reasons for the complex behaviour of these elements cannot be deduced from a single test especially as blank and spike determinations could not be performed for most of the elements. The results are therefore included mainly as a stimulus to further work.

MAJOR ION CONCENTRATIONS

As discussed above, special unacidified samples were collected for the determination of the major anions and cations. Unfortunately, the metal cation determination required most of the expelled pore fluid and thus the major ion studies are not as complete. The results are presented 2 - in figs. (121 - 123) where the variation of CI , SD^ , Na+,

Ca^+, Mg^+ and pH with time, increasing axial pressure

I I FIG 124 EXPELLED PORE WATERS - SAM PLE BR5 (EXTREME THERMAL REGIME) ppm \

No x 10" 21 CI x 10" WW- 7-4 \ + • PH 2000 21 21' 7-0 i \ 21 \ \ 21°

IOOO cl' so/" - K*

® NcF Mg2+ Ca2+

SIMULATED BURIAL DEPTH —• THOUSAND FEET (= 307 METRES) 280

ppm (CIX10"') FIG 128 ( Na x 1Cf') - EXPELLED PORE WATERS

- SAMPLE BR 9 20° SO/- __ — 2000 • — (GEOTHERMAL GRADIENT X5 NORMAL) 50

• - B-O

PH _ 7-8 20° > _ > - 7-6

O i I 1 i i I I I 1 2 SIMULATED BURIAL DEPTH THOUSAND FEE T (* 307 METRES) ^ 284

end temperature is given. On each graph the approximate

range of concentrations recorded in the relevant field samples

of surface uater is also quoted. As may be seen, there is

close agreement for most ions at low temperature.

Sodium and calcium in middle and upper Restronguet

sediments shou little activity up to about 80°C although

there is a rapid decline in concentrations above this

temperature. In the creek mouth sample houever, sodium and

calcium concentrations are initially much lower than the pore

water concentrations but increase considerably with temperature.

This discrepancy is attributed to the evaporative near surface + 2+ Na , Ca pore water enrichment effect discussed in Chapter

5 for certain of the upper creek mouth sedimentary units,

fit the sampled depth, this enrichment is likely to be negligible

and thus equivalent samples are not available for comparison.

The concentration increase with temperature may be correlated with the general tendency of creek mouth sediment to release

sodium and calcium to the pore waters as described in Chapter 5.

Similarly, the potassium and magnesium concentrations derived

from the same Restronguet Creek sediment samples seldom

correlate closely with the equivalent groundwater concentrations.

Potassium is generally much more concentrated than in the surface water samples which are themselves more concentrated

than average sea water. Magnesium is not anomalous although concentrations from tidal flats sediments are lower than those obtained from surface waters which themselves tend to be lower

than normal seawater. During testing the potassium concentra-

tions increased continually to 160°C which is the maximum test

temperature for which potassium analyses are available. The

increase in concentration does not vary greatly with sediment

type. The lack of variation with sediment type tends tc

confirm the general mobility of potassium in the Restronguet

Creek which was also indicated by the leaching tests. The

behaviour of magnesium is less regular and the concentration increases slightly with ternperatures up to 30°C in the upper

tidal flats sediments, falls slightly in the middle tidal flats

and saltmarsh samples and falls considerably in the creek mouth sample. There is good correlation with the results of 285 the sediment leaching tests, as it was demonstrated in

Chapter 5 that in general the more reduced, sulphurous sediments released magnesium to the pore waters whereas the lower tidal flats and the creek mouth sediments tended to absorb magnesium. Increased temperature and pressure appear to accelerate these processes.

The expelled pore waters of the Tresillian River samples for which major cation analyses are available show marked concentration trends with increased temperature and pressure for most ions. Sodium concentrations from lower point bar samples generally correlate quite closely with analyses of the relevant surface waters although analyses from the upper point bar sediment samples are much lower than those from the corresponding water samples. As noted in Chapter 5, the sodium content of the surface waters is in any case considerably lower than the normal seawater concentration. Thus, the slight fall in sodium concentrations with increased temperature up to

100°C and the greater fall above this temperature demonstrates that the expelled pore waters are indeed depleted in sodium.

This effect is greatest for upper point bar sediments. The calcium concentrations approximate very closely to the pore water concentrations which correlate well with analyses of normal seawater. There is a marked fall in calcium concentrations with increased temperature. This agrees closely with the findings of the leaching tests given above where it was found that deeper,-more mature sediment is abstracting calcium from the pore water. Again, elevated temperature and pressure appear to accelerate these processes. Thus, expelled pore waters are likely to be depleted in calcium. Potassium concentrations also approximate closely to groundwater analyses which are themselves slightly higher than normal seawater.

Uith increasing temperature and pressure there is little change in concentration until about 80 - 90°C when tne values increase quite dramatically.

Thus, if these results can be extrapolated to burial diagenetic environments then the initially expelled pore waters would appear to be slightly enriched in potassium 286 relative to normal seawater and would be even more enriched at the higher temperatures. Higher temperatures and pressures appear to have accelerated a process operating at normal temperatures and pressures. The magnesium concentrations are as expected on the upper point bar but are very much higher than the relevant analyses of the field samples of pore water from the lower point bar. The concentrations appear to be unchanged by temperatures of up to about 30°c although thereafter there is firstly a steady decline and then a dramatic fall above about 80 or 90°C. At normal temperatures and pressures, leaching experiments have shown that magnesium is progressively abstracted from pore waters by the sediment and this trend also appears to be accelerated by elevated temperature and pressure conditions.

Figs. (121-128) indicate that variation in chloride and sulphate concentrations with temperature and pressure is somewhat erratic. However, in certain tests fixed temperatures were maintained for short periods whilst the pressure was raised and in these examples, (Tresillian River upper and lower point bar samples BR5 and 8R7) , a number of check and duplicate major ion samples were taken. Sulphate and chloride concentrations are reproduceable within experimental error.

Thus, the greater part of the concentration variation must be attributable to natural processes. As may be seen the

Restronguet Creek samples BR2, 3 and 6 give chloride concen-' trations of the same magnitude as the surface water analyses.

However, the saltmarsh sample BR9 appears to be greatly enriched.

The creek mouth sampie shows little concentration variation up to about 60°C but falls markedly at 80°C. The middle tidal flats sample shows considerable variation in concentration in the range 25 - 40°C but only a slight net increase to

60°C. The concentration in the upper tidal flats sample increases sharply at 80°C and the saltmarsh sample shows little variation up to about 50°C. With the available data, it is impossible to attempt to explain such variation in behaviour. The sulphate concentrations are almost as erratic but there are more evident general trends. The creek mouth and lower tidal flats samples show a general decrease in 287

expelled sulphate concentrations uith increasing temperature.

The concentration in the upper tidal flats sample increases uith temperatures above 8Q°C but the saltmarsh concentrations remained constant up to 50°C. As the possible reactions involving sulphate ions in these kinds of sediment are so complex, it is not possible to establish whether a significant proportion of pore water sulphate was metabolised by sulphate reducing bacteria during the test. However, the concentration curves do give some indication that for samples from the creek mouth and lower intertidal flats at least there was abstraction of sulphate from the expelled waters in the temperature range

25 - 50°C and this could be due to bacterial activity. In other sampies and at higher temperatures other processes appear to become more important.

OTHER OBSERVATIONS

In four of the eight tests a brown discolouration appeared in the pore water samples at about 80 C and became more intense at subsequent higher temperatures. The four r samples were of the organic rich upper and lower Tresillian point bar sediments and Restronguet saltmarsh sediments. A direct correlation between the total organic carbon content of the solid sample, the temperature attained and the intensity of brown colouration was visually quite obvious. Filtration at 0.42/1^ did not affect the colouration. On treatment with concentrated nitric acid, the clear brown pore fluid samples became milky white and slightly gelatinous. A detailed trace element analysis failed to reveal inorganic species, such as silica, which even remotely could have contributed to this phenomenon. It was concluded that the colouration is produced by dissolved organic species which probably undergo complex hydrolysis, oxia ation and polymerization reactions with nitric acid. The detailed evaluation of these species is beyond the scope of the present work.

Some of the expelled pore water samples with this

dissolved organic matter present also contain very high

trace metal concentrations. Thus some at least of these 288

anomalous metal concentrations could be in the form of metal

chelate compounds. However not all high metal concentrations occur in discoloured samples with dissolved organic matter

present in visible concentretions. Clearly this is a field worthy of further investigation.

7.4 CHEMICAL CHARACTERISTICS GF THE SGLID RESIDUES

Each of the tested sediment samples was obtained from a

field site which is surrounded by other grab and core sample

points and which lies within an environmental division whose general sedimentological and geochemical characteristics are known and have been discussed above. The test sample and 2 surrounding sample sites all lie within a 250 m rectangle in each case.

TRACE METAL CONCENTRATIONS

As discussed in Chapter 4, one piece of evidence for

the mineralogical mode of occurrence of a metal is the difference between dilute acid and concentrated nitric acid extractable metal. This different (in ppm) is called here the

A metal value and is a distinctive parameter for sediments in most of the environments in the studied estuaries. In general a high A metal value indicates that a relatively high proportion of the metal occurs in polysulphide and organically complexed forms. However this measure is very sensitive to normal analytical imprecision ana a better indicator is: -

A metal value x 100 % total HNO^ extractable metal

In this study this measure is called the metal parameter.

In order to compare the results obtained for samples tested

at high temperatures and pressures and their surrounding field

control samples the ratio of their metal parameters (called

the metal factor) is employed. Thus in Fig. (129) results

from tested samples may be compared directly with average

results from all relevant surrounding field samples. General Fig. 129

A METAL VALUES FOR SELECTED SAMPLES

(A Metal) Metal parameter Metal Factor (UNO^Me - HC1 Me) AMe/UN03Me %

Fe Mil Cu PL) Zii Fe Mil Cu Pb Zn Fe Ma Cu Zn

BR1 CEIL SAMITE 10420 74 181 18 186 30.1 26.0 24.1 6.0 16.3 1.4 1.34 1.55 2.01 APPROPRIATE AREA 6240 46 96 12 65 21.4 19.4 15.5 7.0 8.1

B112 CELL SAMPLE " 9760 87 1284 541 14.2 11.6 23.5 17.6 0.63 0.90 0.90 0.89 APPROPRIATE AREA 16120 96 1373 4 632 22.5 12.8 26.0 1.1 19.8

BR3 CELL SAMPLE 7416 48 473 19 187 16.7 10.7 44.2 9.5 17.8 1.28 0.65 1.08 1.02 APPROPRIATE AREA 5650 75 443 3 183 13.0 16.5 40.9 1.4 17.3 BIM CELL SAMPLE 18173 218 107 19 25 59.3 67.9 28.3 11.9 3.6 1.5 1.54 1.15 _ APPROPRIATE AREA 11281 95 84 37 Nil 39.6 44.0 24.6 23.6 Nil

BR5 CEIL SAMPLE 5347 21 250 17 94 18.5 9.5 45.5 9.3 12.4 0.75 0.5 2.54 1.48 APPROPRIATE AREA 6832 35 74 5 53 24.7 19.0 17.9 3.3 8.4

BR6 CEIL SAMPLE 16220 95 1235 27 408 25.0 14.4 29.0 8.3 16.7 1.37 1.10 0.91 0.87 APPROPRIATE AREA 11798 96 1194 13 505 18.3 13.0 32.0 3.6 19.2

BR7 CEIL SAMPLE 13400 68 393 13 121 42.9 30.2 71.5 6.6 15.9 1.08 0.69 2.9 APPROPRIATE AREA 11281 95 84 37 Nil 39.6 44.0 24.6 23.6 Nil

BR9 CELL SAMPLE 8360 76 2459 25 122 12.8 11.9 48.6 7.4 4.6 0.72 0.60 5.1 0.70 APPROPRIATE AREA 11112 130 300 13 146 17.8 20.0 9.5 3.3 6.6 290

trends in these values are directly correlsble uith the experimental test conditions and are not masked by the influence of sample variability and initial metal, sulphur or carbon contents. For example anomalously high A Cu metal values, indicating conversion of copper monosulphide to polysulphide (cnaicopyrite) during the test, are as common for copper rich samples as copper poor ones.

A ranked series of metal factors illustrates the important trends induced by different experimental regimes:-

METAL RANKED METAL FACTORS

Fe (1.5) (1.4) (1.37) (1 .28) 1 .03 0.75 0.72 0.63 BR4 BR 1 BR 6 BR 3 BR7 BR5 BR 9 BR 2 Fin (1.54) (1,34) (1.10) 0.90 0.69 (0.65) 0.60 0.50 BR4 3R1 BR 6 BR 2 ER7 BR3 BR9 BR5 Cu 5.12 2.90 2.54 (i .55) (1.15) (1 .08) 0.9 (0.09) BR9 BR7 BR 5 BR 1 BR4 BR3 BR 2 BR6 Zn (2.01) 1 .43 (1 .02) 0.89 (0.87) 0.70 BR 1 BR5 BR3 BR 2 BR6 BR9

Those fa c to rs indi ca ted ( ) are for tests which aim to simulate burial under a normal pres sure - temper ature re gims (Regime A es above). The oth er tes t s aim to simulate va rious environments uith abnormally high heat flow. Clearly a normal regime tends to produce high iron and manganese factors and very low copper factors. Thus it is concluded that under normal burial conditions early formed iron and manganese rich oxide, sulphide, silicate and carbonate phases are progressively broken down and replaced by pyrite, albandine (f'lnS) and possibly manganiferous organic complexes. Under high heat flow conditions this conversion of iron oxide to pyri te does not apoear to be favoured. Perhaps the most significant result is the production of very high copper metal factors by high heat flow regimes in a wide variety of materials. This almost certainly indicates that widespread recrystallization of early formed copper monosulphide to chalcopyrite has occurred in the temperature range 80° - 200°C. 291

SULPHUR CONCENTRATIONS

As discussed above in Chapter 4.2, sulphur in the sediments is mainly present as easily decomposed monosulphide or as bisulphide. These two major components are estimated by direct analysis of the hydrogen sulphide liberated by dilute acid decomposition and by difference with the known total sulphur concentration. Fig. (130) gives the results of these analyses for the tested samples and the corresponding field samples.

Fig.(130)

TOTAL SULPHUR AND ACID EXTRACTASLE SULPHIDE CONCENTRATIONS

Test & field Total sulphur dilute acid HCl Ext, S SULPHIDE sample (ppm) extractaole total 5 FACTOR H2S (ppm) (%)

BR 1 6660 2160 3 2.5 0.59 AREA 6620 3670 55.3

BR2 10000 1660 16.6 0.78 AREA 8510 1820 21.4

BR3 2200 510 23.0 1.31 AREA 1930 340 17.6

BR4 28200 3130 11.1 0.42 AREA 21050 5490 26.0

BR5 8840 3180 36.0 0.67 AREA 8910 4790 53.7

BR6 11880 4420 37.2 1 .1 5 AREA 10050 3240 32.3

BR7 16130 2410 14.9 0.57 AREA 21050 5490 26.0

BR9 11 420 N . A , N . A . N . A . AREA 13170 6570 49 .9

The percentage (dilute acid extractable H25/total sulphur) is given for both tested and field samples and these are related by a simple proportion (sulphide.factor = tested percentage/ field percentage). A high sulphide factor indicates that 292 proportionally more sxtractable hydrogen sulphide occurs in the tested sam pie than in the field sample. A ranked series may be obtained:-

RANKED FACTORS

(1.31} (1 . 15) 0 .78 0.67 (0.59) 0.57 (0.42) BR3 • BR6 BR2 BR5 BR 1 SR7 BR4

From this series it is possible to conclude that at experimental temperatures and pressures there is a tendency for lower Restronguet Creek sediments to form monosulphide minerals and for Tresillian River sediments to form more stable bisulphide minerals. The samples which were subjected to normal pressure/ temperature gradients are indicated by the brackets ( ) and as may be seen the other tests which were carried out at abnormally high temperatures do not seem to have produced a significantly different response. Thus, these effects are definitely induced by temperatures of less than 80°C and effective pressures of less than 5,000 psi which are conditions common to all the tests.

The tested sediment samples were also analysed for dilute acid ex tractable hydrogen sulphide after being air dried. The percentage of the extractable hydrogen sulphide which was recovered after drying was found in Chapter 4 to be inversely related to the moisture content of the sample. In general, very high values were obtained from lower Restronguet samples which are rather coarse-grained and very low values were obtained from fine grained Tresillian point bar sediments. However, it was not possible to conclude by study of the field samples that Restronguet Creek sediments were preferentially mineralised by more stable copper, lead and zinc monosulphides. Fig. (131) gives the percentages (dilute acid extractable H^S (dry)/dilute acid extractable

H2S (wet)) / for tested samples and corresponding field area. As may be seen, these values vary considerably within an area and the mean value must be treated with caution as in some cases less than five samples were analysed. However, 293

Fig. 131

COMPARISON OF DILUTE ACID EXTRACTABLE H S IN WET AND AIR DRIED z0i SEDIMENT SAMPLES

| Test & field HC1. H0S (air dried) (%) FACTOR i sample HC1. I^S (wet samples)

BR1 77% 2.53

AREA 19 - 53% mean 30.4% BR2 106% 2.0

AREA 36 - 69% mean 53% BR3 68 - 100% mean 84% 0.45

AREA 83 - 350% mean 187% BR4 42% 2.33

AREA 14 - 21% mean 18% BR5 32% 1.06

AREA 21 - 38% mean 30% BR6 31% • 0.40

AREA 33 - 191% mean 78% BR7 104% 5.78

AREA 14 - 21% mean 18% BR9 N.D N.D

AREA 14 - 42% mean 31%

BR3 = Restronguet creek mouth complex

BR6 = Restronguet lower intertidal flats

BR2 = Restronguet upper intertidal flats

BR5 = Tresillian upper point bar

BR1 = Tresillian intertidal flats

BR7 = Tresillian lower point bar

BR4 = Tresillian lower point bar

BR9 = Restronguet salxmarsh 294

the factor (% result for tested sample/mean % result for field area) gives a useful measure of the behaviour of a sample after compressive testing and a ranked series is obtained:-

8ANKED FACTGRS

5.78 (2.53) (2.33) 2.0 1 .06 (0.45) (0.40) BR7 ' BR 1 BR4 BR2 BR5 BR3 BR6

A high factor indicates that a great deal more hydrogen sulphide was recovered from the air dried sample after temperature and pressure testing than from corresponding air dried field samples. In general, this means that the simulated diagenetic conditions have induced recrystallisation of copper, zinc and iron monosu1 phides to poorly crystallized polysulphides which are partially resistant to air oxidation but are decomposed by dilute acid liberating hydrogen sulphide. As may be seen, this series agrees very well with that given above relating dilute acid extractable hydrogen sulphide to total sulphur content. Thus it is concluded that the monosulphide minerals in sam pies from the lower point bar and tidal flats in the Tresillian River and the upper tidal fiats in the Restronguet Creek begin to recrystallise into more stable forms at temperatures of less than 80°C and effective pressures of less than 5000 psi. A sample from the upper Tresillian point bar appears to have been relatively unaffected and samples from the lower Restronguet Creek have developed less stable sulphide minerals.

7.5 SULPHIDE MINERALOGY OF THE SOLID RESIDUES

Six samples of the solid residue from the high temperature/ pressure experiments were impregnated with either araldite or carbowax and polished. A reflected light microscopical study of these 3ampies indicates that a number of remarkable mineralogical changes have been induced by the experimental conditions. 295

Restronguet Creek: Creek Mouth Complex

Sample 3 R3 was of fine to medium grained sandy sediment from the flood ramp area of the Restronguet Creek mouth complex and was subjected to Regime A testwork, i.e. simulation of "normal" geothermal temperature/hydrostatic pressure gradients to a maximum temperature of 30°c, The microscopical studies indicate that considerable recrystallisation of the sulphide minerals-has occurred. For example, in the fresh field sample this sandy sediment was well mineralised by ordered and disordered hematite and .iron oxide, detrital pyrite, chalcopyrite, sphalerite and arsenopyrite and by rare diagenetic pyrite framboids and cubes which were commonly rimmed by earthy iron oxide. In the tested sample however, the most obvious sulphide phase is well crystallised pyrite which occurs as an overgrowth to detrital pyrite grains and also as lath like and encrusting grains marginal to the 'surrounding detrital silicates, (Plate 31). In addition, very fine grained-but clearly visible dusty pyrite encloses these silicate grains and forms an outer overgrowth to detrital pyrite. This phase also occurs in considerable concentrations near organic debris and forms overgrowths to altered diagenetic framboids which occur in the sediment. In one or two examples atoll textures are developed. The evidence indicates that this fine grained phase replaces earthy iron oxide (Plate 32). In the finer grained sands there is little evidence of any well crystallised pyrite overgrowths and the major change appears to be the widespread development of typical pyrite framboids uithou.t atoll textures. Other sulphides are relatively rare in these sediments although chalcopyrite grains are rimmed by both bornite and chalcopyrite and overgrown by a late bornite phase which is attributed to recrystallisation during the tastwork. This bornite phase also encloses detrital silicate grains and poorly crystallised diagenetic pyrite in some of the organic rich silty beds, (Plate 33). There are rare examples of botryoidal\chalcopyrite and probably bornite with an internal radiating crystal structure encrusting woody organic debris, Sphalerite is moderately common and occurs as irregular blebs within 296

PLATE 31 Pyrite overgrowths on detritai grains Field 60^ Gil

PLATE 32 Atoll textured pyri te Field 60/ju Oil

PLATE 33 Encrusting borni te Field 60/U, Gil 297 polycrystalline pyrita aggregates. The pyrite itself appears to be diagenetic and by inference the sphalerite is also assumed to be diagenetic although it does not exhibit any characteristic textures. It is not, however, en exsolution phase although it does now occur at pyrite grain boundaries. Detrital arsenopyrite does not appear to be altered in any way and stannite was not identified.

Restronguet Creek: Intertidal Flats

One sample of silty sediment from the upper intertidal flats environment was satisfactorily impregnated and this had been tested under Regime B in which a geothermal gradient of about ten times normal had been applied. The maximum temperature attained was 2Q0°C. The field samples equivalent to BR2 all exhibited diagenetic pyrite overgrowths on altered detrital pyrite, diagenetic pinkish bcrnite overgrowths on altered detrital chalcopyrite and discrate diagenetic bornite grains. In these field samples there appeared to be a natural sequence of recrystallisation from detrital chalcopyri (commonly with purple flame bornite) and purple bornite, (with bluish flame chalcocite) , to pinkish diagenetic bornite and earthy hematite. However, there was no evidence at all of chaicopyrite forming discrete mineral grains in the field samples. The tested sample indicates that quite remarkable mineralogical changes have occurred during the testwork. Pyrite is disseminated throughout the sample as virtually submicroscopic aggregates and occurs rarely as large framboids Chaicopyrite is uoiouitous and forms discrete lath-like grains or botryoidal encrustations which may or may not be overgrown by crustiform pinkish bornite. (See Plates 34, 35). Plate 36 appears to illustrate how a sulphidation front has developed and produced chaicopyrite grains with late pinkish bornite overgrowths marginally. Bornite is widespread as an alteration phase of bornite-chalcooyrits and chalcocite- bornite detritai grains found in the field samples but this process seems to have been further promoted by the experiments conditions as in Plate 37. In this example the relationship oetueen the two phases of bornite is vary clear. Although PLATE 34 Crusti form chalco pyri te and pinkish bornite Field 60ytw Oil

PLATE 35 C rusti fo rm chalco pyri te and pinkish bornite Field Oil

PLATE 36 Sulphi dation front forming pinkish bornit overgrouth Field 60Oil 299

PLATE 37 Two phases of bornite overgrowth Field 60^ Oil

PLATE 38 Gas bubbles developed in compacting sediment? Field 85yto Oil 300 discrete bornite grains do occur in field samples they appear to become more coarsely crystalline and more commonly composed of lath-like crystal aggregates after the experimental treatment. Thus, it is clear that the testuork has accelerated a normal t-rend towards the replacement of other copper sulphides by bornite although there is clear evidence that local anisotropy of copper or sulphide concentrations can produce other copper minerals. For example, Plate 38 illustrates an environment in which, probably, r^S has formed gas bubbles in the compacting sediment and permitted the development of chalcopyrite grains without enclosing bornite. In addition, the experimental conditions have created an environment favourable to the widespread crystallisation of pyrite, (and possibly chalcopyrite and bornite) microcrystals.

Tresillian River: Lower Point Bar

Apart from a few detrital pyrite grains the only sulphides identified in field samples of point bar sediments were pyrite framboids. Very fine grained sphalerite was suspected to be present but could not be positively identified. In the tested sample, however, recrystallisation of pyrite framboids has occurred with a considerable increase in mean grain size of the mineral aggregates. The photographs (Plate 39,40) indicate that most pre-existing framboids have been internally disordered by the experimental conditions and are ove-rgrown by a freshly formed phase of slightly pinkish pyrite which probably explains some of_the acid extractable h^S results discussed above. A continuous sequence of forms from disordered framboids to atoll textured grains and polyhedral pyrite aggregates can be recognised in the photographs. As may be seen from these photomicrographs the pyrite occurs within or at the margins of the most organically rich zones.' Sphalerite appears to be present at the margins of some better crystallised polyhedral pyrite grains but its optical characteristics are poor and it does not exhibit any distinguishing texture. Chalcopyrite was not identified in the sample. PLATE 39 Pyrite overgrowths on framboids Field 60/f^ Oil

PLATE 40 Pyrite overgrowths on framboids Field 300 Oil 302

Tresillian River: Intertidal Flats

The field samples discussed above were mineralised by rare detrital copper minerals, detrital pyrite and framboidal pyrite. One altered grain of chalcopyrit'e with marginal bornite was identified in the tested sample but there was little evidence to indicate that any of the bornite was recently crystallised. Most of the pyrite framboids were partially disordered with some overgrowths of pinkish pyrite but the most important change is illustrated in photograph Plate 41 which shows fine grained pyrite encrusting a woody detritai fragment. This type of pyrite'was not identified in the field sampies and has almost certainly crystallised during the experimental testwork. Chalcopyrite was obtained from pan concentrates of the field sediment samples but could not be positively identified in the tested sample.

Tresillian River: Upper Point Bar

The field samples discussed sbove^ were rich in various detrital iron minerals including pyrite', magnetite, ilmenite and hsmatite and these appear, on heating and compaction, to have been generally overgrown by diagen,etic pyrits. The major pre-existing diagenetic form was framboidal pyrite and this was found to have largely recrystallised during the experimental testwork to give an initially disordered framboid and then a more regular compact form surrounded by freshly crystallised microframboids and pyrite crystallites. See Plate 41. Perhaps the most important change is the general recrystallisation of sulphides in the sediment matrix with a significant increase in grain size. As shown in Plates 40 and 41, pre- existing macroscopic pyrite crystallites and microframboids which were clustered near the margins of organic debris or in slightly less permeable zones have recrystallised to form much coarser more compact pseudo-framboidai forms. Microscopic pyrite appears to have developed at the margins of detrital silicates and as a general dissemination within the organic rich sediment matrix. Other sulphides are rare and only one example of chalcopyrite was positively identified. 303

PLATE 41 Pyrite encrusting a uoody fragment Field 60/2-Oil

•

- V ' ^ T*: 1

% • v

PLATE 42 Chalccpyri t encrusting rthy hematite Field 60 A/Cm Gil 304

As may be seen in Plate 42, chalcopyrita occurs as an 8 - 1O^l thick encrustation of detrital hematite and may well have formed in a local copper rich environment provided by the decomposition of a copper sulphide grain and the subsequent adsorption of copper by the iron oxide. Elsewhere it was virtually impossible to assess whether the finest graiped sulphides were pyrite or chalcopyrite given the very poor polishing characteristics of the samples and the tendency of these sulphides to tarnish, rapidly. 305

CHAPTER 8

PART II - CONCLUSIONS

Part II of this study presents the results of experimental high temperature and high pressure simulation of burial compaction and exhumation processes on samples of the estuarine materials described in Part I. It is concluded that the experimental work was generally successful and the feasibility of the experimental method has been proven. However the levels of metal and organic chemical contamination induced by the construction materials required for the experimental apparatus did eliminate many of the expelled pore water metal concentration data and poorly controlled pore fluid pressures severely inhibited the simulation of very early, shallow, low temperature burial regimes. Notwithstanding these difficulties eight sampies of various estuarine materials were subjected to a total of three different experimental' pressure-temperature regimes. The methods are certainly suitable for study of the major ions and trace manganese, copper and zinc in expelled pore waters. Unfortunately iron concentrations are highly contaminated and lead is below the detection limit.In addition improved pore fluid pressure control would greatly increase the range of geological environments which can be simulated with this equipment.

Sufficiently close agreement is obtained between the appropriate field samples of near surface water and ths initially expelled pore waters to demonstrate that geologically significant concent ration changes are induced by elevated temperatures and pressures. However these changes can only be detected with the present apparatus in sediment which has been "buried" to depths greater than 3GQ0 m and thereby heated to more than 1Q0°C. Under these simulated conditions manganese is strongly depleted and zinc slightly depleted in expelled pore water. Under more extreme conditions, between 110°C and 13Q°C a postulated pore water viscosity change seems to facilitate an anomalous expulsion of water which is initially enriched in most trace metal components. 306

Above 15Q°C very significant heavy metal enrichment of pore water is given for most sample types and this may be accompanied by discolouration of the porewater by dissolved organic matter. These more extreme simulation regimes indicate that swift exhumation of-this type of sediment under very high heat flow conditions, perhaps near an igneous intrusion, can produce pore fluids with heavy metal concentrations of many parts per million.

Under normal geological conditions the burial of sediment from the Restronguet Creek to 10,000 feet (to 3,000 metres) would produce 'very little change in the major cation concentrations in the pore water. The concentration of potassium is abnormally high in ail the samples and the only concentration change recorded is the depletion of magnesium and enrichment of calcium in samples from the creek mouth sediments. The behaviour of sediment from the Tresillisn River is entirely different although all the pore water samples are enriched in potassium when compared with normal seawater. For these samples magnesium, sodium and calcium concentrations, however, all fall with increasing temperature and pressure. One possible reason for this difference in behaviour between the sediments from the two estuaries is that the Tresillian sediment contains much more kaolin and far less quartz than the Restronguet sediment and thus adsorption/desorption reactions are more important. The ion filtration effect is unlikely to be significant over most of the experimental tamperatura and pressure range although it is possible that the effect became significant towards the end of some of the tests. A much more likely explanation of these observations is that normal adsorption/desorption mechanisms known to operate at ambient temperatures and pressures, are accelerated by increased temperatures and pressures and probably account for a much greater proportion of any concentration change than does the ion filtration effect.

The major anion concentrations and oh show a general tendency to fall with increasing depth of burial. The sulphate and chloride concentrations correlate well with the 307 major cation concentrations although the sulphate is possibly being somewhat depleted by continuing bacterial metabolisation. The two samples from the lower Restronguet Creek and creek mouth complex which gave rising pH values also contain the most detrital carbonate.

The metal concentrations in tested samples do not vary dramatically under normal pressure-temperature experimental regimes. A steady conversion of poorly ordered iron monosulphide to pyrite and recrystallization of early formed framboidal pyrits is indicated by metal factor ratios and extractable sulphide ratios. These findings are confirmed by polished section mineralogical studies and similar results are obtained from higher temperature tests under geologically more extreme conditions. These more extreme conditions favour the recrystallization of microcrystalline copper sulphides and microscopic pinkish authigenic bornite (Cu^FeS^) to chalcopyrita (CuFeS2) although local chemical microenvironments appear to exert continuing control over the precise copper mineralogy. There is no convincing evidence of any replacement of pyrite by copper minerals under the environmental conditions tested. APPENDIX

DETAILED SEDIF1ENT0L0GICAL LOGS

VERTICAL CORE SAMPLES KEY CORE No

O SURFACE 3 SILT

SAND JJU- CROSS STRATIFIED SAND SANDY FLASER BEDDING FLASER BEDDING o ro MUDSTREAK MUDSTONE O SCOURED SURFACE GJ BEDDING PLANE MUDFLAKES O A s WEAKLY BIOTURBATED ss MODERATELY O STRONGLY (n sss PYGOSPIO BURROW o ORGANIC MATTER c> s* ROOT BIOTURBATION o SLUMP

MUDCRACKS o IRON OXI DE CEMENT 03 X f- CONGLOMERATE Q_ rOcPc UJ 0 vO Q

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Q 1 •o RESTRONGUET : SALTMARSH CONSTRUCTION (TALLACKS CK.) CORE No o 27 3

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48 AO sss sss

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% a

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ro 3 318 319

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