PhD Part 1

THE MINERALOGICAL CHARACTERISATION AND INTERPRETATION OF A PRECIOUS METAL-BEARING FOSSIL GOSSAN, LAS CRUCES, SPAIN Volume 1 Text and References A thesis submitted to the University of Cardiff in Candidature for the degree of Doctor of Philosophy Christopher Blake Department of Earth, Ocean and Planetary Sciences Cardiff University 2008 Declaration and Statements DECLARATION This work has not previously been accepted in substance for any degree and is not concurrently submitted in candidature for any degree. Signed …………………………………………………………. (candidate) Date ………………………… STATEMENT 1 This thesis is being submitted in partial fulfilment of the requirements for the degree of PhD Signed …………………………………………………………. (candidate) Date ………………………… STATEMENT 2 This thesis is the result of my own independent work/investigation, except where otherwise stated. Other sources are acknowledged by explicit references. Signed …………………………………………………………. (candidate) Date ………………………… STATEMENT 3 I hereby give consent for my thesis, if accepted, to be available for photocopying and for inter-library loan, and for the title and summary to be made available to outside organisations. Signed …………………………………………………………. (candidate) Date ………………………… ii Abstract Abstract The Las Cruces VMS deposit lies on the southern margins of the Iberian Pyrite Belt, Spain. The primary base metal massive sulphide is overlain by a supergene enriched zone and precious metal gossan that remains well preserved under approximately 150 metres of Tertiary marl. The mineralogy, mineral textures and associations of five boreholes containing precious metal gossan mineralisation were characterised using a combination of optical microscopy, SEM and XRD techniques. The mineralogy and geochemical profile of the gossan suggests that it was formed under near-surface weathering conditions, resulting in the development of the supergene zone and a mature gossan profile characterised by elevated levels of Au and Ag. The Au and Ag probably remobilised as chloride complexes under strongly acid, oxidising conditions, precipitating as high fineness Au and discrete Ag-bearing phases lower in the gossan profile. The original Fe-oxyhydroxide dominated gossan mineral assemblage has subsequently been extensively replaced by later stages of siderite, greigite, galena and high fineness Au mineralisation that reflect marked changes in the depositional environment relative to the original gossan mineral assemblage. Fluctuating oxidising and reducing conditions, coupled with biogenic processes within the Niebla Posadas aquifer, situated directly above the present day Las Cruces gossan, provide a suitable mechanism for the formation of the extensive siderite and greigite mineralisation as well as precious metal remobilisation as a thiosulphate complex under near-neutral to alkaline conditions. Strongly negative δ13C stable isotope values for the siderite are consistent with biogenic processes involving Fe3+ and/or sulphate reducing bacteria as well as a significant influence from the oxidation of methane. iii Acknowledgement Acknowledgement I would never have started my thesis if it were not for the encouragement of my mentor, Dr. Ivan Reynolds. Many thanks for your time and support during the past 17 years of my career as a mineralogist with Rio Tinto. A great deal of support has also been received from Dr. Hazel Prichard, helping me through the maze of preparing a thesis and posting me the occasional photocopy to save me the long trek into Cardiff. endless hours reading through my thesis. Finally, to my sister and parents. Thanks for providing me the love and support of a great family. Many thanks for spending About the Author The author graduated with a B.Sc. honours degree in Geology and Geography from the Cheltenham and Gloucester College of Higher Education in July 1991. Following a vacation job with Rio Tinto's Anamet Services in the summer of 1990, the author returned to Anamet Services as a technician/trainee mineralogist under the guidance of Dr. Ivan Reynolds in November 1991. Anamet Services closed in late 1997 and the mineralogy department was relocated to Clevedon where, after a few years, the authors PhD project was approved and supported by Rio Tinto. The author continued to work full time with Rio Tinto as Senior Mineralogist in the Clevedon laboratory, working on his PhD during evenings, weekends and vacation time. The Clevedon laboratory closed in December of 2008. The author now works as a consultant mineralogist. v Dedication Dedication To my mum and dad vi List of Contents List of Contents VOLUME 1: TEXT AND REFERENCES Declaration and Statements Abstract Acknowledgement About the Author Dedication List of Contents List of Figures List of Tables 1 INTRODUCTION 1.1 Introduction 1 1.2 Aims of Study 1 1.3 Thesis Outline 3 2 GEOLOGY 4 4 7 11 13 15 15 17 18 19 20 ii iii v v vi vii xv xix 1 2.1 Regional Geological Setting - The Iberian Peninsula 2.2 The Iberian Pyrite Belt 2.3 The Guadalquivir Basin 2.4 Las Cruces - Exploration History 2.5 Las Cruces - Geology and Mineralogy 2.5.1 Introduction 2.5.2 Gossan 2.5.3 Secondary Massive Sulphide 2.5.4 Primary Massive Sulphide 2.6 Las Cruces - Evolutionary History 2.7 Sample Suite 30 GOSSANS 2.8 Introduction 34 34 2.9 The Gossan Forming Process 2.10 Influences On Gossan Formation 2.10.1 2.10.2 2.10.3 2.10.4 Introduction Effect of Primary Geology on pH Effects of Climate on Gossan Formation Effects of Geomorphology on Gossan Formation 36 41 41 42 43 45 vii List of Contents 2.11 Element Mobility and Gossan Profiles 2.11.1 Introduction 2.11.2 Fe 2.11.3 Au and Ag Element Mobility 2.11.4 Au and Ag Mineralogy and Geochemical Profiles 2.11.5 Cu 2.11.6 Pb 2.11.7 As and Sb 2.11.8 Si, Sn and Ti 2.11.9 Other metals 2.12 Ancient Seafloor Weathering 2.12.1 Introduction 2.12.2 Ochres 2.12.3 Umbers 2.13 Modern Seafloor Weathering 46 46 47 50 55 59 61 64 65 69 71 71 71 73 76 2.13.1 Introduction 76 2.13.2 Modern Seafloor Fe-Oxide and Oxyhydroxide Deposits of Secondary Origin 76 2.13.3 Modern Seafloor Fe-Mn-Si Oxide and Oxyhydroxide Deposits of Primary Origin 79 2.14 Comparing Modern and Ancient Deposits 81 3 METHODS OF INVESTIGATION 3.1 Introduction 3.3 Microscopy 83 83 85 85 86 86 86 89 91 93 94 95 96 97 98 98 100 103 85 3.2 Sample Preparation 3.3.1 Transmitted Light 3.3.2 Reflected Light 3.4 Scanning Electron Microscopy 3.4.1 Qualitative Methods 3.4.2 SEM Image Collection and Enhancement 3.4.3 Image Analysis Techniques 3.4.4 Quantitative Methods 3.5 X-Ray Powder Diffraction 3.6 Fluid Inclusion Analyses 3.7 Isotope Analyses 3.8 Geochemical Whole Rock Analyses 4 BOREHOLE CR194 – SAMPLE DESCRIPTIONS 4.1 Introduction 97 4.2 Borehole CR194 - Chemistry 4.2.1 Introduction 4.2.2 Geochemical Profile 4.3 Borehole CR194 – Gossan 83 viii List of Contents 4.3.1 Introduction 4.3.2 Quartz 4.3.3 Siderite 4.3.4 Limonite 4.3.5 Fe-Clay 4.3.6 Accessory Transparent Gangue Minerals 4.3.7 Fe-Sulphides 4.3.8 Galena and Pb-Bearing Sulphides 4.3.9 Secondary Pb-bearing Phases 4.3.10 Amalgam and Hg-Bearing Phases 4.3.11 Precious Metal Mineralisation 4.3.12 Accessory Minerals 4.4 Borehole CR194 – Gossan Contact with Massive Sulphide 4.4.1 Introduction 4.4.2 163.75 to 164.60m Sample Interval - Upper Portion 4.4.3 163.75 to 164.60m Sample Interval - Middle Portion 4.4.4 163.75 to 164.60m Sample Interval - Lower Portion 4.5 Borehole CR194 – Massive Sulphide Contact with Gossan 4.5.1 Introduction 4.5.2 Clay-Rich Layer 4.5.3 Galena-Rich Layer 4.5.4 Leached Pyrite-Rich Layer 4.5.5 Lower Core 4.6 Borehole CR194 – Massive Sulphide 4.6.1 Introduction 4.6.2 Massive Sulphide 4.6.3 Massive Sulphide/Shale 4.7 Borehole CR194 – Shale 4.7.1 Introduction 4.7.2 Mineralogy 4.8 Borehole CR194 – Summary Diagram 5 BOREHOLE CR149 – SAMPLE DESCRIPTIONS 6.1 Introduction 136 5.1 Borehole CR149 - Chemistry 5.1.1 Introduction 5.1.2 Geochemical Profile 5.2 Borehole CR149 - Tertiary Sand 5.2.1 Introduction 5.2.2 General Mineralogy 5.3 Borehole CR149 - Gossan 5.3.1 5.3.2 5.3.3 5.3.4 Introduction Quartz Siderite Limonite 103 103 105 107 108 109 109 110 111 112 113 114 115 115 115 116 117 121 121 122 122 128 129 130 130 130 132 134 134 134 135 136 138 138 140 143 143 143 145 145 146 146 147 ix List of Contents 5.3.5 Accessory Transparent Gangue Minerals 5.3.6 Fe-Sulphides 5.3.7 Galena and Pb-Bearing Sulphides 5.3.8 Accessory Minerals 5.3.9 Precious Metal Mineralisation 5.4 Borehole CR149 - Gossan/Massive Sulphide Contact 5.4.1 Introduction 5.4.2 Transparent Gangue 5.4.3 Pyrite and other Fe-Sulphides 5.4.4 Galena 5.4.5 Accessory Minerals 5.4.6 Precious Metal Mineralisation 5.5 Borehole CR149 - Massive Sulphide 5.5.1 Introduction 5.5.2 General Mineralogy 5.6 Borehole CR149 – Summary Diagram 6 BOREHOLE CR038 – SAMPLE DESCRIPTIONS 7.1 Introduction 156 6.1 Borehole CR038 - Chemistry 6.1.1 Introduction 6.1.2 Geochemical Profile 6.2 Borehole CR038 - Quartz Replaced Tuffs 6.2.1 Introduction 6.2.2 Transparent Gangue Mineralogy 6.2.3 Ore Mineralogy 6.2.4 Precious Metal Mineralisation 6.3 Borehole CR038 - Quartz Replaced Tuff/Partial Massive Sulphide Contact 6.3.1 Introduction 6.3.2 Transparent Gangue Mineralogy 6.3.3 Ore Mineralogy 6.3.4 Precious Metal Mineralisation 6.4 Borehole CR038 - Partial Massive Sulphide 6.4.1 Introduction 6.4.2 Transparent Gangue Mineralogy 6.4.3 Ore Mineralogy 6.5 Borehole CR038 – Summary Diagram 7 BOREHOLE CR191 – SAMPLE DESCRIPTIONS 8.1 Introduction 169 7.1 Borehole CR191 - Chemistry 7.1.1 Introduction 7.1.2 Geochemical Profile 7.2 Borehole CR191- Tertiary Polymict Conglomerate/ Gossan Contact 7.2.1 Introduction x 148 148 149 149 150 151 151 151 152 152 152 152 154 154 154 155 156 158 158 159 161 161 161 163 164 165 165 165 165 166 167 167 167 167 168 169 171 171 172 175 175 List of Contents 7.2.2 General Mineralogy 7.3 Borehole CR191 - Upper Gossan 7.3.1 Introduction 7.3.2 Gangue Mineralogy 7.3.3 Ore Mineralogy 7.3.4 Precious Metal Mineralisation 7.4 Borehole CR191 - Middle Gossan 7.4.1 Introduction 7.4.2 Gangue Mineralogy 7.4.3 Ore Mineralogy 7.4.4 Precious Metal Mineralisation 7.5 Borehole CR191 - Lower Gossan 7.5.1 Introduction 7.5.2 Gangue Mineralogy 7.5.3 Ore Mineralogy 7.5.4 Precious Metal Mineralisation 7.6 Borehole CR191- Partial Massive Sulphide 7.6.1 Introduction 7.6.2 General Mineralogy 7.7 Borehole CR191 – Summary Diagram 8 BOREHOLE CR123 – SAMPLE DESCRIPTIONS 9.1 Introduction 189 8.1 Borehole CR123 - Chemistry 8.1.1 Geochemical Profile 8.2 Borehole CR123- Tertiary Polymict Conglomerate 8.2.1 Introduction 8.2.2 Gangue Mineralogy 8.2.3 Ore Mineralogy 8.2.4 Accessory Mineralogy 8.3 Borehole CR123 - Upper Siderite Gossan 8.3.1 Introduction 8.3.2 General Mineralogy 8.3.3 Precious Metal Mineralisation 8.4 Borehole CR123 - Middle Calcite Gossan 8.4.1 Introduction 8.4.2 Gangue Mineralogy 8.4.3 Ore Mineralogy 8.4.4 Precious Metal Mineralisation 8.5 Borehole CR123 - Lower Siderite Gossan 8.5.1 Introduction 8.5.2 Gangue Mineralogy 8.5.3 Ore Mineralogy 8.5.4 Precious Metal Mineralisation 8.6 Borehole CR123- Gossan/Shale Conglomerate Contact xi 175 177 177 177 178 179 180 180 180 181 182 183 183 183 184 185 186 186 186 188 189 190 192 193 193 193 194 194 195 195 195 196 197 197 197 198 198 199 199 199 200 201 202 List of Contents 8.6.1 Introduction 8.6.2 General Mineralogy 8.7 Borehole CR123 – Shale Conglomerate/Gossan Contact 8.7.1 Introduction 8.7.2 Transparent Gangue 8.7.3 Pyrite 8.7.4 Cinnabar and Sulphosalt Minerals 8.7.5 Precious Metal Mineralisation 8.8 Borehole CR123 – Partial Massive Sulphide/Shale 8.8.1 Introduction 8.8.2 General Mineralogy 8.9 Borehole CR123 – Summary Diagram 9 ENVIRONMENT AND FORMATIONAL MECHANISMS 9.1 Introduction 209 9.2 Siderite Formational Environment 202 202 204 204 204 205 205 205 207 207 207 208 209 210 9.2.1 Introduction 210 9.2.2 Oxic Zone (Berner, 1981) 213 9.2.3 Sulphate Reduction Zone (Curtis et al., 1986; Irwin et al., 1977) 214 9.2.4 'Methanic' or methanogenic zone (e.g. Berner 1981, Curtis et al., 1986) 215 9.2.5 Methane Oxidation 217 9.2.6 Fe3+ Reduction 219 9.2.7 Nitrate Reduction 223 9.2.8 Abiotic reactions - Thermally induced decarboxylation 223 9.3 Formation of Fe-sulphides 224 9.3.1 Introduction 9.3.2 Formation Mechanisms 9.3.3 The Role of Biological Processes 9.4 Mineral stability fields 9.4.1 Siderite 9.4.2 Fe-sulphides 9.4.3 Siderite/Fe-Sulphide Relationships 10 MINERALOGY: KEY FEATURES AND PARAGENESIS 10.1 Introduction 235 10.2 Quartz 236 10.2.1 Relative Abundance 10.2.2 Grain Size, Shape and Texture 10.2.3 Fluid Inclusion and Isotope Analysis 243 10.3.1 Relative Abundance 10.3.2 Grain Size, Shape and Textures 10.3.3 Associations 236 236 241 243 243 249 224 224 228 230 230 231 233 235 10.3 SIDERITE xii List of Contents 10.3.4 Mineral Chemistry 10.3.5 Isotope Analysis 10.3.6 Fluid Inclusion Analysis 10.4 Galena255 10.4.1 Relative Abundance 10.4.2 Grain Size and Shape 10.4.3 Associations 10.5 Fe-Sulphide Phases 10.5.1 10.5.2 10.5.3 10.5.4 10.6 Au-Bearing Phases Introduction Relative Abundance Reflected Light Characterisation Optical Properties and Occurrences of the Fe-sulphides 251 253 254 255 255 257 260 260 261 262 269 272 272 273 275 276 277 277 282 283 285 289 291 292 294 294 294 299 300 307 307 10.6.1 Relative Abundance 10.6.2 Grain Size and Shape 10.6.3 Associations 10.6.4 Chemistry 10.7 Gossan Paragenesis 10.7.1 Introduction 12 DISCUSSION AND CONCLUSIONS 10.8 Introduction 282 10.9 Seafloor Gossan Formation 10.10 Sub-Aerial Gossan Formation 10.11 Gossan reworking 10.12 Marine incursion and seawater alteration 10.13 Deep burial by Tertiary sediments 10.14 Modern day gossan and aquifer 10.14.1 10.14.2 10.14.3 10.14.4 10.15 Conclusions304 10.16 Future Investigations REFERENCES Introduction Siderite and Greigite Pb-bearing sulphides Precious metals xiii List of Contents VOLUME 2: APPENDICES Appendix 1: List of Mineral Formulae Appendix 2: Sample List Appendix 3: Assay Data Appendix 4: XRD Data Appendix 5: SEM Analyses Appendix 6: Borehole CR194 Illustrations Appendix 7: Borehole CR149 Illustrations Appendix 8: Borehole CR038 Illustrations Appendix 9: Borehole CR191 Illustrations Appendix 10: Borehole CR123 Illustrations A1 A3 A6 A15 A28 A49 A108 A133 A152 A175 xiv List of Figures List of Figures FIGURE 2.1 - A) A GEOLOGICAL MAP OF SPAIN AND PORTUGAL SHOWING THE RELATIVE POSITIONS OF THE CENTRAL IBERIAN ZONE (CIZ), THE BADAJOZCORDOBA SHEAR ZONE (BCSZ), THE OSSA-MORENA ZONE (OMZ), THE PULO DO LOBO (PL) AND THE SOUTHERN PORTUGUESE ZONE (SPZ). THE HERCYNIAN OROGENIC BELT IS SHOWN IN BLUE. PRECAMBRIAN AND PALAEOZOIC SEQUENCES IN ALPINE BELTS ARE SHOWN IN RED. FIGURE 2.1B IS A MORE DETAILED MAP OF THE AREA MARKED BY A RED RECTANGLE ON FIGURE 2.1A. THIS MAP SHOWS THE LOCATIONS OF THE MAIN VOLCANOGENIC MASSIVE SULPHIDE (VMS) DEPOSITS, INCLUDING THE LAS CRUCES DEPOSIT, POSITIONED TOWARD THE SOUTHEAST OF THE REGION UNDER THE POST PALAEOZOIC COVER. (MODIFIED FROM QUESADA, 1991) 6 FIGURE 2.2 - THE MAIN LITHOSTRATIGRAPHIC UNITS IN THE IBERIAN PYRITE BELT. 1. SHALES AND GREYWACKES 2. BLACK SHALES, SILICEOUS SHALES AND TUFFITES 3. EXHALITES (MOSTLY JASPERS) 4. SHALES, GREYWACKES, QUARTZWACKES AND QUARTZITES 5. POLYMETALLIC MASSIVE SULPHIDES AND STOCKWORKS 6. FELSIC VOLCANIC ROCKS, MOSTLY TUFFS 7. MAFIC ROCKS (SPILITES AND DOLERITES) 8. PHYLLITES AND QUARTZITES (MODIFIED FROM CARVALHO 1999). 9 FIGURE 2.3 - A GENERAL MAP OF THE BETIC CORDILLERA SHOWING THE POSITION OF THE GUADALQUIVIR BASIN AND LAS CRUCES (MODIFIED FROM GOMEZ ET AL., 2003). 11 FIGURE 2.4 - SUMMARY OF THE MAIN LITHOSTRATIGRAPHIC UNITS AT LAS CRUCES (KNIGHT, 2000). THE MASSIVE SULPHIDES LIE WITHIN AN APPROXIMATELY 80 METRE THICK SEQUENCE OF BLACK SHALES AND CONSIST OF GOSSAN, SECONDARY CU, PRIMARY CU/ZN AND STOCKWORK ZONES. 16 FIGURE 2.5 – AN IDEALISED, SIMPLIFIED N-S CROSS-SECTION THROUGH THE LAS CRUCES OREBODY THAT IS BASED ON THE INTERPRETATION OF DRILL CORE DATA AND BLOCK MODELLING INFORMATION PERFORMED BY RIO TINTO CONSULTANTS (R2795, 1998). CB = CU LENS BARREN, C4 = COVELLITE ZONE, CZ = PRIMARY CU/ZN, HCF = HIGH CU FOOTWALL, HC = HIGH CU. 17 FIGURE 2.6 - STAGE 1 - FORMATION OF THE LAS CRUCES PRIMARY MASSIVE SULPHIDE DEPOSIT DURING A PRIMARY HYDROTHERMAL EVENT WITH WAXING AND WANING THERMAL HISTORY (MODIFIED FROM KNIGHT, 2000). 21 FIGURE 2.7 - STAGE 2 - SUB-MARINE OXIDATION AND SECONDARY CUSULPHIDE ENRICHMENT DURING THE WANING STAGES OF HYDROTHERMAL ACTIVITY (MODIFIED FROM KNIGHT, 2000). 23 xv List of Figures FIGURE 2.8 - STAGE 3 - SUSTAINED VOLCANISM AND SEDIMENTATION LEADING TO THE BURIAL OF THE MASSIVE SULPHIDE BENEATH ~1000M PALAEOZOIC CULM SEDIMENTS (MODIFIED FROM KNIGHT, 2000). 24 FIGURE 2.9 - STAGE 4 - TILTING OF THE PRIMARY MASSIVE SULPHIDE OCCURRED DURING THE HERCYNIAN, WITH UPLIFT AND EROSION BEING FOLLOWED BY SUB-AERIAL WEATHERING AND THE DEVELOPMENT OF THE GOSSAN, SILICA CAP AND SUPERGENE CU-SULPHIDES (MODIFIED FROM KNIGHT, 2000). 25 FIGURE 2.10 - STAGE 5 - REWORKING OF THE GOSSAN AND SILICA CAP POSSIBLY PRIOR TO AND FOLLOWING THE ONSET OF THE MARINE INCURSION DURING THE MIOCENE (MODIFIED FROM KNIGHT, 2000). 26 FIGURE 2.11 - STAGE 6 - BURIAL AND PRESERVATION OF THE LAS CRUCES DEPOSIT UNDER UP TO 1000 METRES TERTIARY SEDIMENTS (MODIFIED FROM KNIGHT, 2000). 27 FIGURE 2.12 – A) A MAP OF THE LAS CRUCES DEPOSIT ILLUSTRATING THE EXTENT OF THE AU MINERALISATION (SOLID YELLOW LINE), SUPERGENE CUSULPHIDE MINERALISATION (SOLID BLUE LINE) AND THE POSITIONS OF THE BOREHOLES SELECTED FOR EXAMINATION DURING THIS INVESTIGATION. THE CONTOURS REPRESENT GRAVITY SURVEY DATA. THE RED AND PURPLE CONTOURS REPRESENT AREAS OF HIGH GRAVITY (RELATIVE TO THE SURROUNDING AREAS SHOWN IN YELLOW, GREEN AND BLUE, SCALE UNKNOWN). THE REGION OF HIGH GRAVITY IN THE CENTRAL LEFT HAND PORTION OF THE MAP REPRESENTS THE SUPERGENE ENRICHED MASSIVE SULPHIDE DEPOSIT AND THE CENTRAL UPPER REGION OF HIGH GRAVITY REPRESENTS THE PRIMARY MASSIVE SULPHIDE OREBODY. BOREHOLES CR194, CR123 AND CR038 ARE VERTICAL HOLES AND BOREHOLES CR149 AND CR191 ARE INCLINED HOLES. THE GRID SPACING IS IN UNITS OF 60 METRES. (MODIFIED DIAGRAM COURTESY OF RIO TINTO LIMITED.) 31 FIGURE 3.1 – DIAGRAM ILLUSTRATING THE ZONES OF WEATHERING IN TERMS OF EH AND PH ACCORDING TO SATO (1960). 38 FIGURE 3.2 – EH/PH DIAGRAM AT 25OC AND 1 ATMOSPHERE TOTAL PRESSURE, ILLUSTRATING THE RELATIONSHIPS BETWEEN GROUNDWATER POSITION AND MINERAL STABILITY RANGES (ANDERSON, 1990). 39 FIGURE 3.3 - IDEALISED ZONES IN THE WEATHERING PROFILE OF A VHMS ZNPB-CU DEPOSIT THAT HAS BEEN WEATHERED TO PRODUCE A MATURE GOSSAN PROFILE (SCOTT ET AL., 2001). 47 FIGURE 3.4 – EH/PH DIAGRAM ILLUSTRATING THE STABILITY RELATIONS BETWEEN IRON OXIDES AND IRON SULPHIDES IN WATER AT 25OC AND 1 ATMOSPHERE TOTAL PRESSURE AT TOTAL SULPHUR ACTIVITY OF 10-6. BOUNDARIES OF SOLIDS ARE FOR TOTAL IONIC ACTIVITY OF 10-6 (GARRELS AND CHRIST, 1965). 49 xvi List of Figures FIGURE 3.5 – EH/PH DIAGRAM ILLUSTRATING THE STABILITY RELATIONS OF SOME AU COMPOUNDS IN WATER AT 25OC AND 1 ATMOSPHERE TOTAL PRESSURE AT TOTAL DISSOLVED CHLORIDE SPECIES OF 100 AND SULPHUR ACTIVITY OF 10-1. BOUNDARIES OF SOLIDS ARE FOR TOTAL IONIC ACTIVITY OF 10-6 (GARRELS AND CHRIST, 1965). 51 FIGURE 3.6 – EH/PH DIAGRAM ILLUSTRATING THE STABILITY RELATIONS OF SOME CU MINERALS IN WATER AT 25OC AND 1 ATMOSPHERE TOTAL PRESSURE AT TOTAL SULPHUR ACTIVITY OF 10-1, CO3 ACTIVITY OF 10-3 (ANDERSON, 1990). 60 FIGURE 3.7 – EH/PH DIAGRAM ILLUSTRATING THE STABILITY RELATIONS OF PB COMPOUNDS IN WATER AT 25OC AND 1 ATMOSPHERE TOTAL PRESSURE. TOTAL DISSOLVED SULPHUR OF 10-1, PCO2 OF 10-4. BOUNDARIES OF SOLIDS SHOWN ARE FOR TOTAL IONIC ACTIVITY OF 10-6 (GARRELS AND CHRIST, 1965). 62 FIGURE 3.8 - AN ILLUSTRATION OF THE FIELD RELATIONSHIPS OF A TYPICAL SMALL UMBER HOLLOW RELATED TO SEAFLOOR FAULTING, TROODOS MASSIF, CYPRUS (AFTER ROBERTSON AND BOYLE, 1983). 75 FIGURE 4.1 - A MONOCHROME BACKSCATTERED ELECTRON IMAGE ILLUSTRATING A RATHER COMPLEX FE-OXIDE-RICH SAMPLE WITH FINE INTERGROWTHS OF GALENA. DIFFERENCES IN BRIGHTNESS REFLECT DIFFERENT MINERAL SPECIES, VARIATIONS IN MINERAL CHEMISTRY, INCLUDING OXIDATION, HYDRATION AND COMPOSITIONAL ZONING, POROSITY AND VARIATIONS IN POLISHING HARDNESS. 88 FIGURE 4.2 - THE MONOCHROME BACKSCATTERED ELECTRON IMAGE HAS BEEN FALSE COLOURED AND PERMITS THE READER TO READILY DISTINGUISH THE MINERAL SPECIES. GALENA (WHITE) OCCURS AS FINE SKELETAL AGGREGATES. LIMONITE FRAGMENTS (YELLOW-BROWN SHADES) EXHIBIT A WIDE RANGE IN BRIGHTNESS THAT REFLECTS DEGREES OF HYDRATION. DARKER BROWNS REPRESENT MORE HYDRATED FE-OXIDES (E.G. GOETHITE). THE DARKEST BROWN/BLACK PORTIONS OF THE IMAGE REPRESENT AREAS OF HIGH POROSITY. 88 FIGURE 4.3 - A TYPICAL BACKSCATTERED ELECTRON IMAGE AS CAPTURED BY THE IMAGE ANALYSIS SYSTEM. DIFFERENCES IN BRIGHTNESS OF THE MINERAL PHASES IN THE IMAGE REFLECT VARIATIONS IN MINERAL CHEMISTRY. THE WHITE AREAS CONSIST OF HIGH MEAN ATOMIC NUMBER PHASES AND MAY INCLUDE NATIVE AU OR AU-BEARING GRAINS. THE LIGHT GREY AREAS ARE FE-SULPHIDES AND THE DARKER GREY BACKGROUND IS SIDERITE. PORE SPACES ARE BLACK. 90 FIGURE 4.4 - THE SYSTEM RECOGNISES THE RANGE OF GREY SHADES OF INTEREST (RED AREAS), DEPENDING ON CRITERIA SET BY THE OPERATOR. xvii List of Figures EACH BRIGHT PHASE (OR PHASE OF INTEREST) IS AUTOMATICALLY ANALYSED BY THE ELECTRON MICROSCOPE USING THE EDX ANALYSER. 90 FIGURE 4.5 - EACH GRAIN OF INTEREST IS RECOGNISED BY THE ELECTRON MICROSCOPE AND SELECTED FOR ANALYSIS/MEASUREMENT. EACH GRAIN IS ASSIGNED A RANDOM COLOUR. 91 FIGURE 4.6 - AN EXAMPLE OF AN EDX SPECTRUM CAPTURED USING A VERY RAPID (TYPICALLY 200MSEC) EDX ANALYSIS OF EACH GRAIN. THIS IS ADEQUATE TO RECOGNISE THE PRESENCE OR ABSENCE OF AU. IN THIS EXAMPLE, THE GRAIN IS A SB-BEARING PB(SB)-SULPHIDE, RECOGNISED BY THE PRESENCE OF PB, S AND MINOR SB. 91 FIGURE 5.1 - ILLUSTRATING THE CHEMISTRY VARIATION IN BOREHOLE CR194. EACH SAMPLE INTERVAL IS DISPLAYED ON THE LEFT OF THE ILLUSTRATION, TOGETHER WITH THE LITHOCODE AS DETAILED IN APPENDIX 2. THE SAMPLE INTERVALS EXAMINED AND THE SECTIONS OF THIS THESIS IN WHICH THE MINERALOGY IS DESCRIBED ARE PROVIDED ON THE RIGHT OF THE ILLUSTRATION. THE TERTIARY CONGLOMERATE WAS NOT AVAILABLE FOR EXAMINATION. THE VARIATION IN CHEMISTRY WITH INCREASING DEPTH IS DISPLAYED ON FOUR GRAPHS IN THE CENTRE OF THE ILLUSTRATION. THE MAJOR, PRECIOUS AND DELETERIOUS ELEMENT CHEMISTRY CLEARLY EXHIBITS A SIGNIFICANT DEGREE OF VARIATION THAT REFLECTS AN EQUALLY WIDE VARIATION IN THE MINERALOGY OF EACH INTERVAL. TCP TERTIARY POLYMICT CONGLOMERATE, GHS - STRONG HEMATITIC GOSSAN, GBM - MODERATE HEMATITE MAGNETIC, MMP - MASSIVE SULPHIDE, QXM MASSIVE QUARTZ/SHALE, SXM - MASSIVE SHALE. 99 FIGURE 5.61 - DIAGRAM ILLUSTRATING THE KEY MINERALOGICAL FEATURES FOR THE 'GOSSAN', 'GOSSAN/MASSIVE SULPHIDE CONTACT', 'MASSIVE SULPHIDE/GOSSAN CONTACT', 'MASSIVE SULPHIDE', 'MASSIVE SULPHIDE/SHALE' AND 'SHALE'. 135 FIGURE 6.1 - ILLUSTRATING THE CHEMISTRY VARIATIONS IN BOREHOLE CR149. EACH SAMPLE INTERVAL IS DISPLAYED ON THE LEFT OF THE ILLUSTRATION, TOGETHER WITH THE LITHOCODE AS DETAILED IN APPENDIX 2. THE SAMPLE INTERVALS EXAMINED AND THE SECTIONS OF THIS THESIS IN WHICH THE MINERALOGY IS DESCRIBED ARE PROVIDED ON THE RIGHT OF THE ILLUSTRATION. THE SAMPLE INTERVALS EXAMINED FROM BOREHOLE CR149 CONSIST OF THE TERTIARY SAND, GOSSAN, GOSSAN/MASSIVE SULPHIDE CONTACT AND MASSIVE SULPHIDE MINERALISATION. THE VARIATION IN CHEMISTRY WITH INCREASING DEPTH IS DISPLAYED TO THE RIGHT OF THE BOREHOLE SCHEMATIC. THE MAJOR, PRECIOUS AND DELETERIOUS ELEMENT CHEMISTRY CLEARLY EXHIBITS A SIGNIFICANT DEGREE OF VARIATION THAT REFLECTS AN EQUALLY WIDE VARIATION IN THE MINERALOGY OF EACH SAMPLE INTERVAL. THE BOREHOLE DEPTHS REPRESENT DEPTH DOWN HOLE AND ARE THEREFORE NOT EQUIVALENT TO DEPTH FROM SURFACE, WITH CR149 BEING AN INCLINED HOLE. TSA - TERTIARY SAND, GHS - STRONG xviii List of Figures HEMATITIC GOSSAN, GMS - STRONG MAGNETIC GOSSAN, GEM - MODERATELY LEACHED GOSSAN, GLM - MODERATE LIMONITIC GOSSAN, GEW - WEAKLY LEACHED GOSSAN, GHM - MODERATE HEMATITIC GOSSAN, GLS - STRONG LIMONITIC GOSSAN, GLW - WEAK LIMONITIC GOSSAN, MMP - MASSIVE SULPHIDE. 139 FIGURE 6.27 - DIAGRAM ILLUSTRATING THE KEY MINERALOGICAL FEATURES FOR THE 'TERTIARY SAND', 'GOSSAN', 'GOSSAN /MASSIVE SULPHIDE CONTACT' AND 'MASSIVE SULPHIDE'. 155 FIGURE 7.1 - DIAGRAM ILLUSTRATING CHEMISTRY VARIATIONS IN BOREHOLE CR038. THE SAMPLE INTERVALS EXAMINED FROM BOREHOLE CR038 CONSIST OF QUARTZ REPLACED MASSIVE TUFFS AND PARTIAL MASSIVE SULPHIDE. THE SAMPLE INTERVALS ARE DISPLAYED ON THE LEFT OF THE ILLUSTRATION, TOGETHER WITH THE LITHOCODE AS DETAILED IN APPENDIX 2. THE VARIATION IN CHEMISTRY IS DISPLAYED TO THE RIGHT OF THE BOREHOLE SCHEMATIC. DISTINCT COMPOSITIONAL ZONES ARE CLEARLY EVIDENT, PARTICULARLY AT THE TUFF/SULPHIDE CONTACT, LARGELY REFLECTING VARIATIONS IN THE MINERALOGY OF EACH SAMPLE INTERVAL. THE SAMPLE INTERVALS EXAMINED AND THE SECTIONS OF THIS THESIS IN WHICH THE MINERALOGY IS DESCRIBED ARE PROVIDED ON THE RIGHT OF THE ILLUSTRATION. CR038 IS A VERTICAL HOLE. QTM - QUARTZ REPLACEMENT OF MASSIVE TUFF, MSPCL - PARTIAL MASSIVE SULPHIDE WITH CLAY, MPS PARTIAL MASSIVE SULPHIDE. 158 FIGURE 7.21 - DIAGRAM ILLUSTRATING THE KEY MINERALOGICAL FEATURES FOR THE 'QUARTZ REPLACED TUFFS', 'QUARTZ REPLACED TUFF/PARTIAL MASSIVE SULPHIDE CONTACT' AND 'PARTIAL MASSIVE SULPHIDE'. 168 FIGURE 8.1 - DIAGRAM ILLUSTRATING CHEMISTRY VARIATIONS IN BOREHOLE CR191. THE SAMPLE INTERVALS ARE DISPLAYED ON THE LEFT OF THE ILLUSTRATION, TOGETHER WITH THE LITHOCODE AS DETAILED IN APPENDIX 2. THE VARIATION IN CHEMISTRY IS DISPLAYED TO THE RIGHT OF THE BOREHOLE SCHEMATIC. DISTINCT COMPOSITIONAL ZONES ARE CLEARLY EVIDENT AT THE UPPER AND LOWER PORTIONS OF THE GOSSAN. THE SAMPLE INTERVALS EXAMINED AND THE SECTIONS OF THIS THESIS IN WHICH THE MINERALOGY IS DESCRIBED ARE PROVIDED ON THE RIGHT OF THE ILLUSTRATION. BOREHOLE CR191 IS AN INCLINED HOLE. TCP – TERTIARY POLYMICT CONGLOMERATE, GHW - WEAK HEMATITIC GOSSAN, GEM MODERATELY LEACHED GOSSAN, GMS - STRONG MAGNETIC GOSSAN, GES STRONGLY LEACHED GOSSAN, MMPXM - MASSIVE SULPHIDE WITH SHALE. 171 FIGURE 8.25 - DIAGRAM ILLUSTRATING THE KEY MINERALOGICAL FEATURES FOR THE 'TERTIARY CONGLOMERATE/GOSSAN CONTACT', ‘UPPER GOSSAN', 'MIDDLE GOSSAN', 'LOWER GOSSAN’ AND ‘PARTIAL MASSIVE SULPHIDE’. 188 FIGURE 9.1 - DIAGRAM ILLUSTRATING CHEMISTRY VARIATIONS IN BOREHOLE CR123. THE SAMPLE INTERVALS EXAMINED FROM BOREHOLE CR123 CONSIST xix List of Figures OF TERTIARY POLYMICT CONGLOMERATE, GOSSAN AND QUARTZ-RICH SHALES THAT HAVE BEEN PARTIALLY REPLACED BY PYRITE. THE SAMPLE INTERVALS ARE DISPLAYED ON THE LEFT OF THE ILLUSTRATION, TOGETHER WITH THE LITHOCODE AS DETAILED IN APPENDIX 2. A MARKED CHANGE IN THE CHEMISTRY OF THE BOREHOLE IS EVIDENT AT THE CONTACT BETWEEN THE SHALES AND THE GOSSAN. THE SAMPLE INTERVALS EXAMINED AND THE SECTIONS OF THIS THESIS IN WHICH THE MINERALOGY IS DESCRIBED ARE PROVIDED ON THE RIGHT OF THE ILLUSTRATION. THE BOREHOLE DEPTHS ARE EQUIVALENT TO DEPTH FROM SURFACE. TCP - TERTIARY POLYMICT CONGLOMERATE, GMS - STRONG MAGNETIC GOSSAN, QXM - QUARTZ REPLACEMENT OF MASSIVE SHALE, SXM - MASSIVE SHALE, EQU – QUARTZ VEIN. 191 FIGURE 9.35 - DIAGRAM ILLUSTRATING THE KEY MINERALOGICAL FEATURES FOR THE ‘TERTIARY POLYMICT CONGLOMERATE’, ‘UPPER SIDERITE GOSSAN', 'MIDDLE CALCITE GOSSAN', 'LOWER SIDERITE GOSSAN’, ‘GOSSAN/SHALE CONGLOMERATE CONTACT’, ‘SHALE CONGLOMERATE/GOSSAN CONTACT’ AND ‘PARTIAL MASSIVE SULPHIDE/SHALE’. 208 FIGURE 10.1 – A DIAGRAM ILLUSTRATING THE THREE DISTINCT BIOGEOCHEMICAL ENVIRONMENTS THAT MARK THE BOUNDARIES BETWEEN REGIMES OF AEROBIC AND ANAEROBIC METABOLISM. THE SCHEMATIC ILLUSTRATES THE APPROXIMATE DEPTHS THAT THE OXIC, SULPHATE REDUCING AND METHANOGENIC ZONES OCCUR, TOGETHER WITH THE TYPICAL Δ13C VALUES ASSOCIATED WITH THE CO2 GENERATED FROM THE DECOMPOSITION OF ORGANIC MATTER (MODIFIED FROM IRWIN ET AL., 1977 AND CLAYPOOL AND KAPLAN, 1974). 212 FIGURE 10.2 – EH/PH DIAGRAM ILLUSTRATING THE STABILITY OF HEMATITE, MAGNETITE AND SIDERITE AT 25OC AND 1 ATMOSPHERE TOTAL PRESSURE AND PCO2 = 10-2 ATMOSPHERE WITH TOTAL ACTIVITY OF DISSOLVED SPECIES = 10-6 (GARRELS AND CHRIST, 1965). 230 FIGURE 10.3 - PE/PH DIAGRAMS ILLUSTRATING THE STABILITY RELATIONS FOR IRON SULPHIDES IN SEAWATER AT 25OC, 1 ATMOSPHERE TOTAL PRESSURE. A) IRON ACTIVITY 10−6, SULPHUR ACTIVITY 10−2.551, C(IV) ACTIVITY 10−3.001, TROILITE AND PYRRHOTITE SUPPRESSED. B) SAME AS A WITH PYRITE ALSO SUPPRESSED. C) SAME AS B WITH MARCASITE SUPPRESSED. D) SAME AS C WITH GREIGITE AND MACKINAWITE SUPPRESSED. E) SAME AS C BUT SOLUTION CHANGED TO WORLD AVERAGE RIVER WATER WITH IRON ACTIVITY 10−6, SULPHUR ACTIVITY 10−3.902, C(IV) ACTIVITY 10−3.06. F) SAME AS C BUT IRON ACTIVITY 10−3 AND C(IV) ACTIVITY 10−2.5 (SCHOONEN, 2004). 232 FIGURE 10.4 – EH/PH DIAGRAM ILLUSTRATING THE STABILITY RELATIONS BETWEEN IRON OXIDES, SULPHIDES AND CARBONATES IN WATER AT 25OC AND 1 ATMOSPHERE TOTAL PRESSURE AT ΣCO2 OF 100 AND ΣS OF 10-6 (GARRELS AND CHRIST, 1965). 233 xx List of Figures FIGURE 11.1 - BOREHOLE CR123 - A COLOUR TRANSMITTED LIGHT PHOTOMICROGRAPH OF FIBROUS QUARTZ (WHITE AND GREY SHADES) DEVELOPED AROUND THE MARGINS OF PYRITE CRYSTALS (BLACK). THE SURROUNDING MATRIX IS FINE-GRAINED QUARTZ AND PORE SPACES. THIS IMAGE WAS TAKEN IN CROSSED POLARISED LIGHT FROM THE SHALE. THE WIDTH OF VIEW IS APPROXIMATELY 1100ΜM. 239 FIGURE 11.2 - BOREHOLE CR038 - A COLOUR TRANSMITTED LIGHT PHOTOMICROGRAPH ILLUSTRATING MORE COARSELY CRYSTALLINE QUARTZ FRAGMENTS (WHITE AND GREY SHADES, FAR LEFT AND FAR RIGHT) THAT ARE CEMENTED BY FINE-GRAINED, PARTIALLY RECRYSTALLISED CHALCEDONY (MOTTLED GREY/BLACK SHADES CENTRE OF FIELD). THIS IMAGE WAS TAKEN IN CROSSED POLARISED LIGHT FROM THE QUARTZ REPLACED TUFF. THE WIDTH OF VIEW IS APPROXIMATELY 1100ΜM. 240 FIGURE 11.3 - BOREHOLE CR149 - A COLOUR, CROSSED POLARISED TRANSMITTED LIGHT PHOTOMICROGRAPH FROM THE GOSSAN/MASSIVE SULPHIDE CONTACT ILLUSTRATING A CAVITY (CENTRE FIELD) THAT HAS BEEN FILLED BY FIBROUS CHALCEDONY (WHITE/GREY SHADES). THE SURROUNDING MATRIX IS PREDOMINANTLY CALCITE (PINKISH BROWN SHADES). THE WIDTH OF VIEW IS APPROXIMATELY 2MM. 241 FIGURE 11.4 - BOREHOLE CR194 - A COLOUR TRANSMITTED LIGHT PHOTOMICROGRAPH WITH CROSSED POLARS ILLUSTRATING THE PRESENCE OF ANGULAR SIDERITE ‘FRAGMENTS’ (PINKISH WHITE) IN A MATRIX OF QUARTZ (LIGHT AND DARK GREY SHADES) AND OXIDISED SIDERITE (BLACK). THE SIDERITE FRAGMENTS ARE MEDIUM-GRAINED, WITH DISCRETE CRYSTALLITES EXCEEDING 100ΜM IN SIZE. THE WIDTH OF VIEW IS APPROXIMATELY 2MM. 244 FIGURE 11.5 - BOREHOLE CR194 – FALSE COLOUR BACKSCATTERED ELECTRON IMAGES ILLUSTRATING A) A SIDERITE ‘FRAGMENT’ THAT ACTUALLY REPRESENTS A CAVITY FILLING. THE WIDTH OF VIEW IS APPROXIMATELY 2MM. B) COMPOSITIONALLY ZONED SIDERITE FILLING A EUHEDRAL CAVITY IN QUARTZ. THE WIDTH OF VIEW IS APPROXIMATELY 600ΜM. C) SIDERITE THAT APPEARS TO HAVE EXTENSIVELY REPLACED BARITE (LIGHT GREY). THE WIDTH OF VIEW IS APPROXIMATELY 450ΜM. D) SIDERITE FILLING CAVITIES IN BOTRYOIDAL LIMONITE. THE WIDTH OF VIEW IS APPROXIMATELY 250ΜM. VOIDS ARE BLACK. 245 FIGURE 11.6 - BOREHOLE CR194 – A DIGITISED PHOTOGRAPH SHOWING APPARENT ‘FRAGMENTS' OF SIDERITE (BROWN, OUTLINED IN RED). THESE CLASTS ARE PSEUDOMORPHS AFTER QUARTZ-RICH ROCK FRAGMENTS (WHITE/LIGHT GREY). AN EXAMPLE OF A QUARTZ-RICH ROCK FRAGMENT PARTIALLY REPLACED BY SIDERITE IS OUTLINED IN BLACK. THE WIDTH OF CORE IS APPROXIMATELY 50MM. 246 xxi List of Figures FIGURE 11.7 - BOREHOLE CR194 - FALSE COLOURED BACKSCATTERED ELECTRON IMAGE ILLUSTRATING THE PRESENCE OF GALENA (WHITE) REPLACING SIDERITE ALONG GRAIN BOUNDARIES AND HIGHLIGHTING DIFFERENT GENERATIONS OF SIDERITE MINERALISATION. LIMONITE (LIGHT BROWN, RED ARROW) IS ALSO PRESENT. THE WIDTH OF VIEW IS APPROXIMATELY 2.3MM. 247 FIGURE 11.8 - BOREHOLE CR194 - A COLOUR TRANSMITTED LIGHT PHOTOMICROGRAPH WITH CROSSED POLARS ILLUSTRATING THE PRESENCE OF LATE-STAGE, UNOXIDISED SIDERITE (LIGHT AND DARK GREY-BROWN SHADES) FILLING A CAVITY IN AN OXIDISED, OPAQUE SIDERITE MATRIX (BLACK). TINY SKELETAL GALENA CRYSTALS (WHITE ARROW, BLACK) ARE OFTEN PRESENT IN THE SIDERITE. THE SIDERITE ALSO EXHIBITS GROWTH ZONING (RED ARROWS). THE WIDTH OF VIEW IS APPROXIMATELY 4MM. 248 FIGURE 11.9 - BOREHOLE CR194 - A COLOUR TRANSMITTED LIGHT PHOTOMICROGRAPH WITH CROSSED POLARS ILLUSTRATING THE PRESENCE OF EARLY-FORMED SIDERITE CRYSTALS (DARK BROWN) THAT HAVE FORMED IN A CAVITY (DARK GREY). THE EARLY FORMED SIDERITE CRYSTALS HAVE BEEN OXIDISED AND REPLACED BY HEMATITE AND THEN OVERGROWN BY LATER STAGES OF UNOXIDISED SIDERITE (WHITE). THE WIDTH OF VIEW IS APPROXIMATELY 1100ΜM. 248 FIGURE 11.10 - BOREHOLE CR194 – FALSE COLOUR BACKSCATTERED ELECTRON IMAGE ILLUSTRATING THE PRESENCE OF SIDERITE (DARK BROWN) AND GALENA (WHITE) FILLING AND PARTIALLY FILLING CAVITIES IN HEMATITE (LIGHT BROWN SHADES). THE GALENA EXHIBITS CHARACTERISTIC SKELETAL TEXTURES. SIDERITE IS ONLY PRESENT FILLING SOME OF THE CAVITIES IN THIS SAMPLE AND APPEARS TO HAVE BEEN LEACHED FROM THE GALENAFILLED CAVITIES IN THE LOWER LEFT PORTION OF THIS IMAGE. THE WIDTH OF VIEW IS APPROXIMATELY 310ΜM. 250 FIGURE 11.11 - BOREHOLE CR191 – A COLOUR, REFLECTED LIGHT PHOTOMICROGRAPH ILLUSTRATING THE PRESENCE OF SKELETAL GALENA (PALE GREY) IN EUHEDRAL FE-SULPHIDE CRYSTALS (CREAM-WHITE). SIDERITE (DARK GREY/BLACK BACKGROUND) IS SELECTIVELY REPLACING THE FE-SULPHIDE. THE WIDTH OF VIEW IS APPROXIMATELY 85ΜM. 251 FIGURE 11.12 - BOREHOLE CR191 – FALSE COLOUR BACKSCATTERED ELECTRON IMAGE ILLUSTRATING THE SELECTIVE LEACHING OF COMPOSITIONAL ZONES WITHIN SIDERITE CRYSTALS (BROWN SHADES). THE SIDERITE IS PRESENT ALONG MARGINS OF QUARTZ FRAGMENTS (MAUVE) AND WITHIN VOIDS (BLACK). MINOR GALENA (WHITE) AND FE-SULPHIDE (LIGHT KHAKI) ARE ALSO PRESENT. THE WIDTH OF VIEW IS APPROXIMATELY 310ΜM. 252 FIGURE 11.13 - BOREHOLE CR191 – A COLOUR, REFLECTED LIGHT PHOTOMICROGRAPH ILLUSTRATING THE PRESENCE OF A FINE-GRAINED xxii List of Figures GALENA AGGREGATE (PALE CREAM) THAT IS BEING PROGRESSIVELY REPLACED FROM THE UPPER LEFT TO LOWER RIGHT BY SIDERITE (DARK GREY BACKGROUND). THE SIDERITE IS SELECTIVELY REPLACING THE FINERGRAINED GALENA, WITH ONLY THE SKELETAL GALENA SURVIVING AS A RELICT PHASE. THE WIDTH OF VIEW IS APPROXIMATELY 375ΜM. 257 FIGURE 11.14 - A MONTAGE OF FALSE COLOUR, BACKSCATTERED ELECTRON IMAGES SELECTED FROM CHAPTERS 5 TO 9, ILLUSTRATING A WIDE RANGE OF ASSOCIATIONS BETWEEN GALENA AND OTHER MINERALS (VARIOUS SCALES). A) REPLACEMENT OF PARTIALLY LEACHED, RELICT PRIMARY PYRITE BY LATE-STAGE GALENA. B) FINE-GRAINED AND POROUS GALENA AGGREGATES WITH INTERGROWN FE-SULPHIDE. C) PARTIAL REPLACEMENT OF SIDERITE BY VERMICULAR GALENA. D) AU-BEARING GRAINS IN GALENA. E) FINE GALENA RIMS ON AU-BEARING GRAINS. F) GALENA OVERGROWTH ON NATIVE AU. G) EUHEDRAL GALENA CRYSTALS IN STERNBERGITE, AU AND PYRITE. H) SKELETAL GALENA IN CERUSSITE, MIMETITE AND SIDERITE-BEARING VEIN. I) GALENA REPLACING TETRAHEDRITE. J) GALENA REPLACING QUARTZ ALONG GRAIN BOUNDARIES. K) GALENA REPLACING CALCITE ALONG MARGINS OF FRAGMENTS WITH LATER CALCITE AND FE-SULPHIDE FILLING PORE SPACES (BLACK). 259 FIGURE 11.15 - AN X-RAY DIFFRACTOGRAM CLEARLY ILLUSTRATING THE PRESENCE OF GREIGITE (PEAKS DONATED WITH BLUE VERTICAL LINES). THE UNLABELLED PEAKS REFLECT THE PRESENCE OF QUARTZ, LEPIDOCROCITE AND SULPHUR. THE LEPIDOCROCITE AND SULPHUR REPRESENT THE OXIDATION PRODUCTS OF THE FE-SULPHIDE ASSEMBLAGE. THE BROAD PEAK WIDTH FOR GREIGITE IS INDICATIVE OF A POORLY CRYSTALLINE NATURE. 261 FIGURE 11.16 – COLOUR, REFLECTED LIGHT PHOTOMICROGRAPH ILLUSTRATING TYPE 1 FE-SULPHIDE, CONSISTING OF FEATHERY, COLLOIDAL RADIATING AGGREGATES OF FE-SULPHIDE (FESAM OR MACKINAWITE/NANOPARTICULATE MACKINAWITE OF WOLTHERS ET AL. (2003)). THIS SAMPLE IS MAGNETIC. BOREHOLE CR194, 156.70M (UPPER). 100X OIL, PPL, WIDTH OF VIEW 150UM. 262 FIGURE 11.17 - COLOUR, REFLECTED LIGHT PHOTOMICROGRAPH ILLUSTRATING EUHEDRAL FE-SULPHIDE CRYSTALS (TYPE 3 FE-SULPHIDE, CIRCLED) WITH MARCASITE/PYRITE INCLUSIONS (PALER YELLOW-WHITE, WHITE ARROW) FORMING OVERGROWTHS ON A COLLOIDAL FE-SULPHIDE AGGREGATE (TYPE 1). THE COLLOIDAL FE-SULPHIDE EXHIBITS PALER COLOURED, FEATHERY INTERGROWTHS, TOWARDS THE MARGINS WHICH APPEAR STRONGLY ANISOTROPIC (TYPE 2, POSSIBLY MACKINAWITE, VERY WEAKLY DEFINED, RED ARROWS). THIS POLISHED SECTION IS MAGNETIC. BOREHOLE CR149, 151.75M. 100X OIL, 50% ZOOM PPL, WIDTH OF VIEW 105UM. 263 xxiii List of Figures FIGURE 11.18 – COLOUR, REFLECTED LIGHT PHOTOMICROGRAPH ILLUSTRATING COLLOIDAL RADIATING AGGREGATES OF FE-SULPHIDE (TYPE 1, WHITE ARROW) AND FINELY DISSEMINATED EUHEDRAL FE-SULPHIDE CRYSTALS (TYPE 3, LIGHT GREY, YELLOW ARROW) IN SIDERITE (DARK BROWN TRANSPARENT GANGUE). THE EUHEDRAL FE-SULPHIDES MAY FORM AS A REPLACEMENT OR RECRYSTALLISATION PRODUCT OF THE COLLOIDAL FESULPHIDE (WHITE CIRCLE). THE BLACK REGIONS WITHIN THE CENTRE OF THESE AGGREGATES ARE VOIDS. THIS SAMPLE IS MAGNETIC. BOREHOLE CR194, 156.70M (UPPER). 100X OIL, PPL, WIDTH OF VIEW 150UM. 264 FIGURE 11.19 - COLOUR, REFLECTED LIGHT PHOTOMICROGRAPH ILLUSTRATING EUHEDRAL FE-SULPHIDE CRYSTALS (TYPE 3, CREAM/YELLOW) IN SIDERITE (BLACK). THE POROUS CORES OF THE CRYSTALS MAY HAVE RESULTED FROM VOLUME CHANGES DURING REPLACEMENT OR BE RELICTS OF RECRYSTALLISATION/REPLACEMENT OF COLLOIDAL AGGREGATES. THIS SAMPLE IS MAGNETIC. BOREHOLE CR194, 156.70M (UPPER). 100X OIL, PPL, WIDTH OF VIEW 150UM. 265 FIGURE 11.20 - COLOUR, REFLECTED LIGHT PHOTOMICROGRAPH ILLUSTRATING EUHEDRAL FE-SULPHIDE CRYSTALS (TYPE 3, PALE PINKISH BROWN) WITH MARCASITE/PYRITE INCLUSIONS (PALER YELLOW-WHITE, WHITE ARROW) IN SIDERITE (BLACK BACKGROUND). THE CRYSTALS EXHIBIT A MARKED POROSITY (RED ARROW), POSSIBLY INDICATIVE OF VOLUME CHANGES DURING REPLACEMENT. THIS SAMPLE IS MAGNETIC. BOREHOLE CR194, 151.75M. 100X OIL, 100% ZOOM, PPL, WIDTH OF VIEW 85UM. 265 FIGURE 11.21 - COLOUR, REFLECTED LIGHT PHOTOMICROGRAPH ILLUSTRATING FEATHERY, STRONGLY ANISOTROPIC FE-SULPHIDE (TYPE 2, PINKISH CREAM SHADES, MACKINAWITE?) WITH CUBIC OVERGROWTHS OF FESULPHIDE CRYSTALS (TYPE 3, GREIGITE?, CIRCLED) IN SIDERITE (DARK BROWN/BLACK BACKGROUND). CORES OF PYRITE/MARCASITE ARE PRESENT (PALE YELLOW). THIS SAMPLE IS MAGNETIC. BOREHOLE CR149, 151.75M. 100X OIL, 100% ZOOM, PPL, WIDTH OF VIEW 85UM. 266 FIGURE 11.22 - COLOUR, REFLECTED LIGHT PHOTOMICROGRAPH ILLUSTRATING PLATELETS OF AN ANISOTROPIC FE-SULPHIDE PHASE (TYPE 4) WITH MINOR PYRITE/MARCASITE (POORLY RESOLVED, WHITE ARROW) IN A MATRIX OF SIDERITE (DARK BROWN BACKGROUND). THIS SECTION IS MAGNETIC. BOREHOLE CR123, 152.40M (LOWER). 100X OIL, 100% ZOOM, PPL, WIDTH OF VIEW 85UM. 267 FIGURE 11.23 - PLATY TEXTURES IN PYRITE (PALE YELLOW) THAT HAS PSEUDOMORPHOUSLY REPLACED TYPE 4 FE-SULPHIDE (PYRRHOTITE? DARKER PINKISH BROWN, WHITE ARROW) IN SIDERITE (DARK BROWN/BLACK BACKGROUND). THIS SECTION IS WEAKLY MAGNETIC, PROBABLY DUE TO THE PRESENCE OF DISSEMINATED GREIGITE CRYSTALS IN THE SIDERITE (NOT PRESENT IN THIS IMAGE). BOREHOLE CR149, 187.40M (MIDDLE). 100X OIL, 100% ZOOM, PPL, WIDTH OF VIEW 85UM. 268 xxiv List of Figures FIGURE 11.24 - MARCASITE (PALE YELLOW, WHITE ARROW) EXTENSIVELY REPLACES THE STRONGLY ANISOTROPIC FE-SULPHIDE PHASE (PROBABLY MACKINAWITE +/- GREIGITE, DARKER PINKISH YELLOW, RED ARROW) THAT IS PARTIALLY FILLING A EUHEDRAL CAVITY IN QUARTZ (UNDIFFERENTIATED BLACK BACKGROUND). THIS SECTION IS MAGNETIC. BOREHOLE CR191, 150.10M. 40X AIR, PPL, WIDTH OF VIEW 375UM. 268 FIGURE 11.25 - A MONTAGE OF FALSE COLOUR, BACKSCATTERED ELECTRON IMAGES SELECTED FROM CHAPTERS 5 TO 9, ILLUSTRATING A WIDE RANGE OF MORPHOLOGIES AND ASSOCIATIONS OF THE AU AND AU-BEARING GRAINS (VARIOUS SCALES). A) AN AGGREGATE OF IRREGULARLY SHAPED AUAMALGAM GRAINS IN GALENA. B) TWO NATIVE AU GRAINS RIMMED AND POSSIBLY REPLACED BY GALENA. C) AU-AMALGAM GRAINS WITH CUSPATE MARGINS THAT APPEAR TO HAVE BEEN EXTENSIVELY REPLACED BY GALENA. D) IRREGULARLY SHAPED NATIVE AU GRAINS IN CINNABAR. E) A EUHEDRAL NATIVE AU GRAIN IN LEPIDOCROCITE. F) A EUHEDRAL NATIVE AU GRAIN IN SIDERITE. G) ANHEDRAL NATIVE AU IN SIDERITE AND FE-SULPHIDE. H) NATIVE AU IN A EUHEDRAL CAVITY (BLACK) IN QUARTZ. I) EUHEDRAL NATIVE AU IN GALENA REPLACING QUARTZ. J) AU IN FE-SULPHIDE. K) EUHEDRAL AU WITH CASSITERITE. L) AU IN FE-SULPHIDE AND ANATASE. 274 FIGURE 11.26 – MONTAGE OF FALSE COLOUR BACKSCATTERED ELECTRON IMAGES ILLUSTRATING PARTIAL AND COMPLETE PARAGENETIC SEQUENCES OBSERVED DURING THIS INVESTIGATION. A) AU (YELLOW) IS FREQUENTLY LOCATED IN ISOLATION ALONG THE MARGINS OF RELICT QUARTZ GRAINS (MAUVE). B) AU WITH SIDERITE (BROWN) CEMENTING RELICT QUARTZ. C) AU WITH OVERGROWTHS OF GALENA (PALE BLUE/WHITE) CEMENTING RELICT QUARTZ. D) AU WITH OVERGROWTHS OF GALENA (PALE BLUE/WHITE) AND SIDERITE CEMENTING RELICT QUARTZ. E) AU INCLUSION IN FEMONOSULPHIDE (LIGHT GREY/BROWN) IN QUARTZ. F) AU WITH EUHEDRAL FEMONOSULPHIDE CRYSTALS AND SIDERITE CEMENT. G) A RARE EXAMPLE OF A COMPLETE PARAGENETIC SEQUENCE CONSISTING OF AU → GALENA → FEMONOSULPHIDE → SIDERITE. 278 FIGURE 12.1 - DIAGRAM ILLUSTRATING A) PRIMARY MASSIVE SULPHIDE AND SEAFLOOR GOSSAN PRESERVED UNDER CULM SEDIMENTS PRODUCED BY CONTINUED VOLCANIC ACTIVITY. B) TILTING OF THE DEPOSIT DURING THE HERCYNIAN WOULD HAVE RESULTED IN A STEEPLY DIPPING PRIMARY MASSIVE SULPHIDE AND PRESERVED SEAFLOOR GOSSAN QUITE DISTINCT FROM THE SUB-AERIALLY DERIVED, HORIZONTAL GOSSAN, SILICA CAP AND SUPERGENE MINERALISATION (MODIFIED FROM KNIGHT, 2000). 284 FIGURE 12.2 – A SCHEMATIC ILLUSTRATING THE DISTINCT BIOGEOCHEMICAL AND ABIOTIC ENVIRONMENTS THAT MARK THE BOUNDARIES BETWEEN REGIMES OF AEROBIC AND ANAEROBIC METABOLISM AND SUBSEQUENT CARBONATE AND/OR SULPHIDE MINERAL PRECIPITATION. THE SCHEMATIC ILLUSTRATES THE APPROXIMATE DEPTHS, CHANGES IN TEMPERATURE AND TYPICAL Δ13C VALUES ASSOCIATED WITH THE CO2 GENERATED FROM THE xxv List of Figures DECOMPOSITION OF ORGANIC MATTER. IN ADDITION, THE COMPETITIVE AND/OR COMPLEMENTARY PROCESSES OF NITRATE REDUCTION AND FE-/MNREDUCTION ARE ALSO INCLUDED (MODIFIED FROM IRWIN ET AL., 1977 AND CLAYPOOL AND KAPLAN, 1974). 296 FIGURE 12.3 – DIAGRAM ILLUSTRATING AN IDEALISED CROSS SECTION THROUGH THE LAS CRUCES DEPOSIT. 304 xxvi List of Tables List of Tables TABLE 11.1 - SIDERITE δ 13C AND δ 18O RATIOS 253 TABLE 12.1 - COMPARISON OF MATURE GOSSANS AND LAS CRUCES GOSSAN MINERALOGY 288 PRECISION AND ACCURACY OF AAS ANALYTICAL METHOD MAJOR ELEMENT ASSAY DATA FOR BOREHOLE CR194 MINOR/TRACE ELEMENT ASSAY DATA FOR BOREHOLE CR194 MAJOR ELEMENT ASSAY DATA FOR BOREHOLE CR149 MINOR/TRACE ELEMENT ASSAY DATA FOR BOREHOLE CR149 ASSAY DATA FOR BOREHOLE CR038 MAJOR ELEMENT ASSAY DATA FOR BOREHOLE CR191 MINOR ELEMENT ASSAY DATA FOR BOREHOLE CR191 MAJOR ELEMENT ASSAY DATA FOR BOREHOLE CR123 MINOR ELEMENT ASSAY DATA FOR BOREHOLE CR123 6 9 10 11 11 12 12 14 14 15 xxvii Chapter 1 Introduction 1 1.1 INTRODUCTION Introduction The Las Cruces Volcanogenic Massive Sulphide (VMS) deposit is situated within the Iberian Pyrite Belt (IPB), one of the world’s largest massive sulphide provinces. The IPB is approximately 250Km long and up to 70Km wide and hosts more than 80 known mines including Aznalcollar-Los Frailes and Rio Tinto in Spain and Neves Corvo in Portugal (Leistel et al., 1998). Las Cruces was discovered by Rio Tinto in 1994. The Las Cruces primary massive sulphide is essentially similar to other VMS deposits within the IPB. However, unlike other VMS deposits within the region, the gossan and supergene mineralisation at Las Cruces is undisturbed by historical mining activity or erosion being extremely well preserved under approximately 150 metres of Tertiary deposits. Early mineralogy reports on the Las Cruces gossan conducted by Rio Tinto Limited at their Anamet Services laboratory (R2643, 1996; R2644, 1996; R2696, 1997) confirmed that the mineralogy is markedly different from other VMS derived gossans described in the literature. The Las Cruces gossan consists predominantly of siderite, galena and subordinate amounts of Fe-sulphides whereas most sub-aerially derived gossans typically consist of Fe-oxyhydroxide and metal sulphates. 1.2 Aims of Study A thorough The The main focus of this investigation is the Las Cruces gossan. characterisation of the mineralogy, the mineral associations and textures through five sections of precious metal gossan provide the basis of this study. mineralogy is used to identify sequences of events in the history of the gossan to help understand the processes that have resulted in the mineralogical assemblage. Particular attention is given to the nature and mode of occurrence of siderite, greigite and galena, the dominant gossan minerals, and to Au and Ag, the only elements likely to be worthy of economic interest within the gossan. Page 1 Chapter 1 Introduction The mineralogy and geochemical profiles developed in the gossan are compared and contrasted with the mineralogy and geochemical profiles developed in gossans described in the literature, with the aim of interpreting, as far as possible, the formational history of the gossan. Reflected and transmitted light microscopy, X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) are used to identify and illustrate the different mineral species present in Las Cruces. Modern SEM-based image analysis techniques are also used to locate large numbers of precious metalbearing grains, with a large number of backscattered electron images being prepared to illustrate textural information and mineral associations. The Las Cruces deposit remains buried and as yet unexploited for its mineral wealth. Removal of the overburden has commenced by the deposits’ current owners, MK Resources Company, and it is expected that mining of the supergene Cu ore will begin in 2008. The Las Cruces VMS deposit is considered to be one of the highest grade Cu deposits in the world. With only limited information currently available on the Las Cruces gossan, this investigation provides significant detail on the mineralogy, geochemistry and styles of mineralisation that may be used as a basis for future investigations. The final data collection for this thesis took place on 28th September 2007. Page 2 Chapter 1 Introduction 1.3 Thesis Outline Chapter 2 describes the local and regional geology of Las Cruces and the Iberian Pyrite Belt and exploration history of the Las Cruces orebody. The locations of the samples selected for examination during this investigation are also discussed in Chapter 2. Chapter 3 includes a literature review on gossans, the processes involved in gossan formation and predominant influences on gossan formation. The geochemical profiles, mineralogy and element mobility are discussed. Submarine gossan formation in modern and ancient deposits is also discussed. The methodologies employed during this investigation, including sample preparation techniques, reflected and transmitted light microscopy, scanning electron microscopy and X-ray powder diffraction are given in Chapter 4. Chapters 5 through to 9 include the major and minor element geochemistry and geochemical profiles of the Las Cruces gossan, together with detailed descriptions of each of the boreholes selected for examination. The illustrations are provided in Appendices 6 to 10. Chapter 10 describes the environment and formational mechanisms for siderite and greigite. Chapter 11 summarises the key mineralogical features of the Las Cruces gossan. Chapter 12 discusses the evidence presented in the previous chapters and compares how the mineralogy and geochemistry of the Las Cruces gossan fit with the model of formation described by Knight (2000), the only other significant academic work on this deposit to date. Conclusions are drawn from the evidence presented in the previous chapters. This chapter concludes with a brief discussion on how future investigations may be focussed. Page 3 Chapter 2 Geology 2 2.1 GEOLOGY Regional Geological Setting - The Iberian Peninsula The The Iberian Peninsula is largely underlain by a Hercynian belt of approximately 750 km in length, extending in a NW-SE direction (Figure 2.1a, blue). progressively accreted during the Pan-African/Cadomian Hercynian belt consists of a number of discrete zones or terranes that were and Hercynian Orogenies. These zones are the Cantabrian Zone (CZ), West Asturian-Leonese Zone (WALZ), Galicia Tras-os-Montes Zone (GTZ), Central Iberian Zone (CIZ), the Badajoz-Cordoba Shear Zone (BCSZ), the Ossa-Morena Zone (OMZ), the Pulo do Lobo (PL), the Southern Portuguese Zone (SPZ) and the Porto Tomar Shear Zone (Figure 2.1a). Precambrian and Palaeozoic sequences in Alpine belts are shown in red (Leistel et al., 1998). The Central Iberian Zone belongs to the Iberian Autochthon onto which the other zones were accreted (Ribeiro et al., 1990; Quesada, 1991). The BadajozCordoba Shear Zone is a major suture formed during the Pan-African and Hercynian Orogenies (Quesada, 1991). The CIZ and accreted OMZ underwent a passive margin type evolution in the northern margin of Gondwana until the onset of the Hercynian orogeny in early to mid-Devonian (Leistel et al., 1998). The Pulo do Lobo Zone is a complex ophiolite sequence formed as a result of the subduction of oceanic lithosphere at the outer margin and underneath the OMZ (Leistel et al., 1998). The collision of the Southern Portuguese Zone with the Ossa-Morena Zone resulted in the lateral escape of units that coincided with bimodal magmatism, hydrothermal circulation and ore deposition (Leistel et al., 1998; Oliveira, 1990 and Quesada et al., 1991). These marginal units represent what is known today as the Iberian Pyrite Belt. The tectonic setting was extensional and epicontinental and developed during the Hercynian plate convergence, culminating in thinskinned deformation and accretion of the South Portuguese terrane to the Iberian Palaeozoic continental block (Leistel et al., 1998) (Figure 2.1a). Page 4 Chapter 2 Geology Additional divisions of the Southern Portuguese Zone include the Baixo Alentejo Flysch Domian and the SW Portugal Domain. These sub divisions are related to the breakdown of a Devonian platform resulting from the continent-continent collision that occurred throughout the early Carboniferous (Saez et al., 1996). The northern sector of the Southern Portuguese Zone consists of siltstones of Precambrian to upper Palaeozoic age that have been metamorphosed to greenschist facies. These rocks are thought to be the source of the clastic materials of the Iberian Pyrite Belt sediments (Strauss and Madel, 1974). The central sector of the Southern Portuguese Zone makes up the Iberian Pyrite Belt. The southern sector comprises marine sandstones, shales and limestones of Devonian to Carboniferous age (Oliveira, 1983). Page 5 Chapter 2 Geology Figure 2.1 - a) A geological map of Spain and Portugal showing the relative positions of the Central Iberian Zone (CIZ), the Badajoz-Cordoba Shear Zone (BCSZ), the Ossa-Morena Zone (OMZ), the Pulo do Lobo (PL) and the Southern Portuguese Zone (SPZ). The Hercynian orogenic belt is shown in blue. Precambrian and Palaeozoic sequences in Alpine belts are shown in red. Figure 2.1b is a more detailed map of the area marked by a red rectangle on Figure 2.1a. This map shows the locations of the main Volcanogenic Massive Sulphide (VMS) deposits, including the Las Cruces deposit, positioned toward the southeast of the region under the post Palaeozoic cover. (Modified from Quesada, 1991) Page 6 Chapter 2 Geology 2.2 The Iberian Pyrite Belt Situated within late Devonian to The Las Cruces deposit lies within the Iberian Pyrite Belt (IPB), in the south west part of the Iberian Peninsula (Figure 2.1b). middle Carboniferous rocks, covered in places by Tertiary-Quaternary terrace and alluvial deposits, the IPB is 250Km long and 25-70Km wide. The IPB hosts a huge quantity of volcanic-hosted massive sulphide (VMS) mineralisation with more than 80 known mines totalling 1700Mt of sulphides and containing 14.6Mt Cu, 13.0Mt Pb, 34.9Mt Zn, 46100t Ag and 880t Au (Leistel et al., 1998). Mining of the outcropping deposits in the IPB dates back to the Chalcolithic era (5000–3000BC) with Tartassians, Phoenicians and Romans extracting Cu, Au and Ag from oxide and supergene zones overlying the massive sulphide orebodies (Strauss et al., 1990). With recent cessation of the use of pyrite for sulphuric acid production, large scale mining in the IPB belt is now limited. Only five mines remain active in the belt today, namely Soteil-Migollas, Aznalcollar-Los Frailes, Rio Tinto and Tharsis in Spain and Neves Corvo in Portugal (Leistel et al., 1998). The locations of these deposits are shown in Figure 2.1b. With the discovery of the Neves-Corvo Cu-Sn deposit in 1977, renewed interest in exploration for deep, 'blind' VMS deposits resulted in a number of new discoveries, Las Cruces being one of the more recent and significant additions. Throughout the Phanerozoic, Europe has been subjected to three continuous compressional and extensional periods of geotectonic activity, namely the Caledonian Orogeny (circa 600–350Ma), Variscan Orogeny (circa 550-250Ma) and Alpine Orogeny (circa 250-0Ma) (Rickard, 1999). The Iberian Pyrite Belt was formed during the Variscan Orogeny, during the development of pull-apart basins alongside continental margins. The Variscan Orogeny resulted from the closure of the pre-Mediterranean Tethys Ocean, climaxing at around 300Ma and is partly synonymous with the Hercynian Orogeny of Northern Europe (Rickard, 1999). All the sequences of the Pyrite Belt were deformed during the Hercynian orogeny, which was accompanied by low-grade regional metamorphism, ranging from zeolite to lower greenschist facies (Munha, 1983). Page 7 Chapter 2 Geology The massive sulphide deposits of the IPB exhibit specific features that aid in their identification and characterisation, including mineralogy and geochemistry, Pb isotope data, hydrothermal alteration and structure. The geochemistry of a large part of the basic lavas associated with the IPB are comparable to those of mantle-derived basalts emplaced in extensional tectonic settings and the associated acidic rocks were produced by melting of a basic crustal protolith at low to medium pressures and a steep geothermal gradient (Leistel et al., 1998). The IPB consists of an extremely complex succession of Late Devonian to Middle Carboniferous rocks resulting from several facies variations and intense tectonic deformation overlain by Tertiary to Quaternary sediments (Oliveira, 1990). The stratigraphy of the IPB has classically been sub-divided into three principle units, the Phyllitic Quartzite (PQ) formation, the Volcano-Siliceous (VS, Devonian-Carboniferous) complex and the Culm (or Flysch, Upper Carboniferous) group (Schermerhorn, 1971). The main lithostratigraphic units in the Iberian Pyrite Belt are illustrated in Figure 2.2. The PQ formation, estimated to be greater than 1000m in thickness (Strauss, 1970), consists of Late Devonian shale, quartz sandstone and rare conglomerate that essentially form the base of the IPB. The depositional environment is thought to be a shallow epicontinental sea (Leistel et al., 1998). Dating of the upper 30m thick sequence of carbonates and bioclastic lenses indicate late Famennian age (late Devonian circa. 367–362Ma) (Van den Boogard et al., 1980). Towards the top of the unit, the uniform nature of the PQ formation changes and is marked by high energy sedimentary deposits registering the tectonic evolution of the IPB basins (Moreno et al., 1996). Page 8 Chapter 2 Geology Figure 2.2 - The main lithostratigraphic units in the Iberian Pyrite Belt. 1. Shales and greywackes 2. Black shales, siliceous shales and tuffites 3. Exhalites (mostly jaspers) 4. Shales, greywackes, quartzwackes and quartzites 5. Polymetallic massive sulphides and stockworks 6. Felsic volcanic rocks, mostly tuffs 7. Mafic rocks (spilites and dolerites) 8. Phyllites and quartzites (modified from Carvalho 1999). The VS complex dates from late Famennian to middle Visean (circa 342–339Ma) (Oliveira, 1990) and varies in thickness between 100 and 600m (Leistel et al., 1998). Exposure of the VS complex is restricted to the IPB. Although somewhat variable between zones in the IPB, the VS complex essentially consists of alternating felsic and mafic, sub-aerial to sub-marine volcanics within detrital and Page 9 Chapter 2 Geology chemically derived sediments (Saez et al., 1996). al., 1998):- The most complete VS sequence, evident in some units of the southern branch of the belt are (Leistel et 1. A lowermost rhyolitic sequence (VA1), with fine to coarse-grained pyroclastics and lavas 2. A second rhyolitic sequence (VA2), with pyroclastics and lavas 3. A third rhyolitic sequence (VA3), mainly reworked tuffs and siliceous shale. 4. Basic lava, locally pillowed, intercalated between VA1 and VA3; basic dykes and sills injected into the lower part of the complex (possibly feeder zones). 5. A purple shale situated directly below VA3. 6. A pelite-black shale and sandstone containing beds of jasper and rare limestone, interstratified with VA1 to VA2 volcanics. The Culm facies or Baixo Alentejo flysch group is a thick turbidite formation forming a south-westward prograding detrital cover that is diachronous over the underlying VS complex. The thickness of this facies is estimated to be up to 3000m (Strauss and Madel, 1974). Moreno (1993) describes three stratigraphic units for the Culm facies:1. The Basal Shaly Formation (BSF), consisting of volcanic and non-volcanic sediments, marking the end of volcanic activity in the region and the beginning of autochthonous sedimentation of pelagic clay. 2. The Culm Facies Turbidite Formation (CFTF), consisting of turbidite sequences of sandstones, shales and minor conglomerates. 3. The Shallow-Platform Sandy Unit (SPSU) consisting of shales and sandstones reworked and redeposited following the erosion of volcanic uplands. Page 10 Chapter 2 Geology 2.3 The Guadalquivir Basin The Guadalquivir Basin is situated along the eastern end of the IPB. Las Cruces lies along the western margin of the Guadalquivir Basin and is buried under approximately 150m of Tertiary sediments. This relatively flat lying area ranges from 15 to 50 metres above sea level. The Guadalquivir Basin lies between the Iberian Foreland to the north and the Betic Cordillera to the south (Figure 2.3). The Betic Cordillera is the northern segment of an arcuate orogen that extends over 600Km westward across the Gibraltar Arc into the Rif Chain. The inner part of this orogen is occupied by the extensional basin of the Alboran Sea. The cordillera contains numerous Neogene basins, including the Sado Basin to the NW and the Guadalquivir Basin in the SE (Sanz de Galdeano and Vera, 1992). Figure 2.3 - A general map of the Betic Cordillera showing the position of the Guadalquivir Basin and Las Cruces (modified from Gomez et al., 2003). The Guadalquivir Basin was formed during the Alpine orogeny (Miocene to Recent) as the African Plate continued to collide with the Eurasian Plate. Dewey et al. (1989) determined that this area experienced in the order of 200Km of N-S convergence between the mid-Oligocene (circa 30Ma) and late Miocene (circa 6Ma), followed by approximately 50Km of WNW-directed oblique convergence in the late Miocene to recent times. The evolution of the Guadalquivir Basin ended in the Messinian (circa 6Ma) when the basin was partially filled by Miocene sediments, consisting predominantly of marine marls (Fernandez et al., 1998). Page 11 Chapter 2 Geology The basement of the Guadalquivir Basin consists predominantly of PQ, VS and Culm Palaeozoic sediments that dip gently in a SSE direction. The Miocene sediments increase in thickness towards the south, reaching a maximum thickness of ~15Km. Fernandez et al. (1998) suggest that the Iberian Massif to the north of the Guadalquivir Basin provided clastic infill to the basin. These clastics were subsequently redistributed along the ENE-WSW axis of the basement by turbiditic currents. The external zones of the Betic Cordillera provide a gravitational infill to the south, producing sedimentary deposits known as 'olistostromes', consisting predominantly of chaotic mixtures of Triassic evaporites, clays, limestones and Upper Cretaceous to Palaeozoic limestones (Fernandez et al., 1998). There has been significant erosion of the Miocene sediments in recent times and the Las Cruces deposit may at one time have been buried by as much as 1000m of sediment (Knight, 2000). Page 12 Chapter 2 Geology 2.4 Las Cruces - Exploration History The Las Cruces volcanogenic massive sulphide deposit is situated on the western margin of the Guadalquivir basin, in the southern region of the Iberian Pyrite Belt in the Seville Province of Andalucia approximately 15km northwest of the city of Seville (Figures 2.1 and 2.3). After an absence from the Pyrite Belt of several years, in 1990, Rio Tinto began the exploration programme that led to the Las Cruces discovery. The block of ground that contains Las Cruces (Faralaes II) is covered almost entirely by recent Tertiary sediments and was applied for in 1991 on the supposition that Palaeozoic rocks prospective for base metals were concealed below the Tertiary sediments. The presence of Boliden's Aznalcóllar orebody in exposed Palaeozoic rocks 12.5km to the west gave weight to this supposition. The initial exploration method used by Riomin Exploraciones (Rio Tinto Mining and Exploration) was gravity surveying. This is a costly technique and in 1992 Rio Tinto departed from traditional practice by reducing the density of the survey points from 50 - 100/km² to 11 - 15/km² thus allowing more extensive and faster survey coverage of all available prospective ground. the Las Cruces deposit were:• Early 1994, a gravity anomaly of exceptional size and strength was detected at Las Cruces. • This was largely responsible for the discovery of Las Cruces. The key steps in the discovery of The first scout hole to investigate the anomaly was drilled in May 1994, showing that the cause was a large concealed sulphide deposit (the first hole returned insignificant amounts of base metal, the sulphide consisting almost entirely of pyrite). • September 1994, the first promising secondary Cu mineralisation was intersected grading 3.77 per cent Cu over 42 metres. Page 13 Chapter 2 Geology • June 1995, very high-grade secondary Cu mineralisation was intersected close to the central part of the main orebody, grading 19.49 per cent Cu over 17 metres. • June 1995 to October 1996, progressive delineation of secondary Cu-rich mineralisation at shallow depth continued, together with the confirmation of thick zones of primary Zn-Cu mineralisation. • October 1996, Las Cruces moved to project status, with a first phase feasibility study being completed in September 1998. • The Las Cruces deposit was sold to MK Gold Company (now MK Resources Company), a subsidiary of US-based Leucadia National Corporation, in 1999. Inmet Mining Corporation is now the majority owner of the project after it acquired 70 percent from MK Resources in August 2005. Page 14 Chapter 2 Geology 2.5 2.5.1 Las Cruces - Geology and Mineralogy Introduction The mineralisation occurs in volcano-sedimentary rocks of Devonian to Carboniferous age. Similar associated deposits include Neves Corvo in Portugal and the Rio Tinto, Soteil and Aznalcollar mines in Spain. The Las Cruces deposit is covered by a thick layer of Tertiary sediments of Miocene age, dating from circa 6Ma. These sediments have prevented the erosion of the Las Cruces orebody and have resulted in a high degree of preservation of the supergene massive sulphide mineralisation and associated gossan. Although Las Cruces lies outside of the confines of the IPB, the basement rocks are the same as those described for the IPB and consist of the Phyllitic Quartz formation, the Volcano-Sedimentary sequence and the Culm facies. The host rock lithology for Las Cruces is typical for the IPB. The footwall is dominated by highly deformed acid volcaniclastics, with interbedded shales becoming increasingly important to the west (Knight, 2000). The main lithostratigraphic units at Las Cruces are illustrated in Figure 2.4. Page 15 Chapter 2 Geology Figure 2.4 - Summary of the main lithostratigraphic units at Las Cruces (Knight, 2000). The massive sulphides lie within an approximately 80 metre thick sequence of black shales and consist of gossan, secondary Cu, primary Cu/Zn and stockwork zones. The volcaniclastics include high level intrusive lavas and tuffs which, in part, have been altered due to seawater interaction (Knight, 2000). Hydrothermal alteration is prevalent in the footwall sequence, with some zoning around the stockwork and chloritic and sericitic alteration throughout. Kaolinitic alteration is evident in the centre of the footwall, with some late-stage carbonate replacement and silicification also being evident (Knight, 2000). The massive sulphide deposit is hosted within an approximately 80 metre thick sequence of black shales with some volcanic material. The black shale host is no more pyritic than other shale units in this sequence (Knight, 2000). The hangingwall consists predominantly of shales with subordinate interbedded volcaniclastics which, in places, are almost indistinguishable from the footwall volcaniclastics, although some extensive zones of brecciation are present (Knight, 2000). Page 16 Chapter 2 Geology The Las Cruces orebody consists of a gossan cap that overlies secondary and primary massive sulphide mineralisation. The massive sulphide orebody consists of a number of discrete primary and secondary sub-lenses comprising HC (HCH and HCL), CZ (primary Zn and primary Cu), C4 and CB (Figures 2.4 and 2.5). These lenses are described in greater detail in the following section. E l e v a( m t i )o n 3 0 - 0 A i r T e r t i a r y M a r l a n d S a n d A - 1 0 0 u G o s s a n H C F H C H a n g i n g w C a l l Z H C F - 2 0 0 C C - 3 0 0 B 4 F o o t w a l l a n d S t o c k w o r k Figure 2.5 – An idealised, simplified N-S cross-section through the Las Cruces orebody that is based on the interpretation of drill core data and block modelling information performed by Rio Tinto consultants (R2795, 1998). CB = Cu lens Barren, C4 = covellite zone, CZ = primary Cu/Zn, HCF = High Cu Footwall, HC = High Cu. 2.5.2 Gossan The gossan is situated within the Carboniferous hangingwall, directly above the HC secondary massive sulphide orebody and is developed in the pre-Tertiary oxidising zone. The gossan ranges from between 0 and 20 metres in thickness Page 17 Chapter 2 Geology (R2795, 1998). In 1997, the gold bearing gossan resource was estimated to be 133,000 tonnes averaging 6.7ppm Au (R2703, 1998). The mineralogy of the gossan is dominated by the presence of siderite and quartz together with the more typical assemblage of Fe-oxides and Fehydroxides. Sulphide minerals are also abundant throughout the gossan and include galena and Fe-sulphides. Textural evidence suggests that the gossan has been subjected to some degree of reworking and mechanical transportation. The gossan is markedly enriched in Au and Ag relative to the underlying massive sulphide mineralisation. The gossan is the main focus of this thesis and is described in greater detail in Chapters 5 to 9. 2.5.3 Secondary Massive Sulphide The HC (High Cu) sub-lens represents the secondary supergene enriched zone and contains the bulk of the economic mineralisation. It is a flat-lying tabular unit with a strong undulating footwall and flatter hangingwall. The bulk of the secondary mineralisation occurs in the central and eastern part of the deposit. Two thicker areas of secondary massive sulphide in the SW and NE are linked by a thinner region of secondary massive sulphide forming a dumbbell shaped lens (Knight, 2000). The HC lens is divided into HCH (High Cu, High density) and HCL (High Cu, Low density). The HCH lens is interpreted as the supergene replacement of massive sulphides and the HCL is interpreted as the supergene replacement of partial massive sulphides and associated wallrocks. The HCF (High Copper Footwall) is a low tonnage, discontinuous, disseminated sulphide lens occurring just below the HC footwall (R2795, 1998). Localised E-W trending faults have produced permeable zones within the orebody, increasing the depth of penetration of the supergene fluids, resulting in an increase in thickness of the supergene enrichment (R2795, 1998). The mineralogy and textures observed in the secondary massive sulphide reflect the nature of the primary mineralisation and the degree of supergene alteration. The HC zone consists of pyrite and digenite together with subordinate amounts of Page 18 Chapter 2 Geology chalcocite, covellite, chalcopyrite, bornite, tetrahedrite-tennantite, enargite and galena (Knight, 2000). Gangue minerals include quartz, barite, calcite and alunite. The C4 sub-lens is a small secondary massive sulphide lens at the base of the primary sulphide orebody dominated by pyrite and covellite. By 2005, the present owners of the deposit, Inmet Mining estimated that at a 1.0 percent Cu cutoff, the measured plus indicated resource for the combined HCH, HCL and C4 lenses is 15.6 million tonnes averaging 6.89 percent copper, with an additional inferred resource of 0.360 million tonnes averaging 8.66 percent copper. 2.5.4 Primary Massive Sulphide The CZ (Cu-Zn) sub-lens represents the main primary massive sulphide orebody and consists of a tabular structure dipping to the north at an angle of approximately 35o, flattening towards the west of the deposit. The upper portion of this zone is typically Zn-rich relative to the lower Cu-rich portion (R2795, 1998). The relative proportions of the dominant ore, gangue and accessory minerals vary significantly, largely reflecting the primary depositional processes. The ore is typically fine-grained and exhibits a range of textural features reflecting variations in the degree of recrystallisation (Knight, 2000). Pb and 2.5 per cent Zn (R2703, 1998). Pyrite is the dominant mineral together with subordinate amounts of chalcopyrite, sphalerite and galena. Accessory minerals include tennantite-tetrahedrite, arsenopyrite, enargite, cassiterite and Bi-bearing sulphosalts. Gangue minerals include quartz, barite, clays, dolomite and calcite (Knight, 2000). The CB (Cu lens, Barren) is a barren sub-lens within the primary massive sulphide orebody. The footwall rocks are volcano-sedimentary silicate-rich rocks containing some mineralised stockwork structures. The hangingwall rocks consist predominantly of unmineralised Carboniferous volcano-sedimentary silicate-rich rocks that are overlain by a thick sequence of Tertiary sand and marl sediments (R2795, 1998). In 1997, the CZ resource was estimated to be 13.9 million tonnes at 2.2 per cent Cu, 0.9 per cent Page 19 Chapter 2 Geology 2.6 Las Cruces - Evolutionary History The emplacement of the Las Cruces orebody has many similarities to other massive sulphide deposits of the IPB. This similarity ends with the events that post date the emplacement of the primary massive sulphide and resulted in the preserved supergene mineralisation and gossan that is observed today. The only significant works to date on the formation of the massive sulphide orebody is by Knight (2000), who produced a model for the development of the primary and secondary mineralisation based on the mineralogy, stable isotopes, fluid inclusions and noble gas data. Knight (2000) concludes that the paragenetic sequence that resulted in the formation of the present day deposit at Las Cruces included seven distinct events (Figures 2.6 to 2.10):• Stage 1 - A primary hydrothermal event with waxing and waning thermal history resulting in temporal and spatial mineralogical zoning (Figure 2.6). Knight (2000) proposes that a suite of primary ore facies developed under characteristic hydrothermal conditions whereby cycles of volcanic activity and episodes of diffuse flow lead to focussed fluid discharge and the formation of massive sulphide deposits over time. At Las Cruces, this initially resulted in the primary precipitation within, and replacement of, the host black shales. This was followed by diffuse flow of mixed hydrothermal and seawater fluids, leading to the replacement and overgrowth of different generations of pyrite. Saez et al. (1999) also suggest that interaction between the black shales and hydrothermal fluids highlights one of the main differences between southern IPB massive sulphides and other VMS deposits. Page 20 Chapter 2 Geology Figure 2.6 - Stage 1 - formation of the Las Cruces primary massive sulphide deposit during a primary hydrothermal event with waxing and waning thermal history (modified from Knight, 2000). Saez et al. (1999) note that Pb isotope data suggest a single (or homogenised) metal source derived from both the volcanic piles and the underlying Devonian rocks. The authors also consider that the IPB deposits had magmatic activity as the heat source, but the environment was not strictly volcanogenic, with many of the evolutionary stages possibly occurring in conditions similar to those of sediment hosted massive sulphides. Saez et al. (1999) suggest that dispersion of hydrothermal fluids may have been restricted and therefore focussed by the black shales, with massive sulphides subsequently forming by deposition and replacement processes (citing Almodovar et al, 1998). The mineralogy and chemistry of the massive sulphide mound was modified over a period of hundreds or thousands of years by cycles of hydrothermal diagenesis, with each hydrothermal cycle involving a waxing stage, in which prograde Page 21 Chapter 2 Geology diagenesis occurs, a period of peak hydrothermal conditions and a waning stage, in which retrograde diagenesis occurs (Knight 2000, citing Knott, 1994). At Las Cruces, early diagenetic conditions are represented by a prograde assemblage, which developed as a result of hydrothermal insulation within the mound. Increased massive sulphide thickness provided both thermal and chemical insulation of the hydrothermal fluids from the surrounding seawater. Increased intensity of the hydrothermal system resulted in the development of the Zn-Fe-Pb-(Cu) sulphides (Knight, 2000). The peak hydrothermal stage is associated with pervasive, focussed, high temperature mineralisation, with hydrothermal fluid temperatures >300 oC, resulting in the development of a high temperature, chalcopyrite-rich core with a cooler, outer margin rich in sphalerite (Figure 2.7). (Knight, 2000). • These conditions are analogous to the conditions in the central conduit of a black smoker chimney Stage 2 - Oxidation during the waning stages of the hydrothermal system resulting in the formation of secondary Fe oxides/hydroxides and secondary Cu sulphides (Figure 2.7). During the waning stages of hydrothermal activity, long term, low temperature (~100oC–300oC) fluid circulation and diffuse venting of white smoker chimneys replaced those of the focussed high temperature activity. covellite (Knight, 2000). These changes resulted in the late overgrowth of silica, minor sphalerite, galena, barite and Page 22 Chapter 2 Geology Figure 2.7 - Stage 2 - Sub-marine oxidation and secondary Cu-sulphide enrichment during the waning stages of hydrothermal activity (modified from Knight, 2000). The retrograde hydrothermal conditions also lead to increasing conductive cooling and seawater mixing, generating a low pH and oxidation. It is likely that some oxidation of the massive sulphide orebody occurred during the waning stages of sub-marine hydrothermal activity, similar to that described for modern seafloor sulphide deposits. Knight (2000) provides evidence of Fe-oxide dustings in silica samples suggesting the oxidation may have taken place at a similar time to the late-stage silicification event that is also strongly correlated to a phase of secondary Cu-sulphide mineralisation. signatures. The secondary Cu-sulphides exhibit a slightly enriched 34S isotope signature most likely caused by the addition of reduced seawater sulphate. Fluid inclusion and oxygen isotope data for the associated quartz also confirm modified seawater type solutions (Knight, 2000). This evidence supports the theory of oxidation and supergene enrichment during the waning stages of hydrothermal activity. Due to This event, which took place at temperatures of 5ppm) over several metres of core, thereby improving the chances of locating and identifying any Au-bearing phases. Some consideration was given to the spatial distribution of the boreholes. Boreholes CR149, CR194 and CR123 are situated due south of the main supergene enriched massive sulphide mineralisation. The gossan is mechanically and chemically reworked and may also occur some distance from the original source. Boreholes CR038 and CR191 provided information on the nature of the precious metal mineralisation away from the main supergene orebody and it was predicted that these might differ somewhat from those in direct contact with the underlying massive sulphide orebody. The Las Cruces site is situated approximately 30 to 35 metres above sea level. Page 30 Chapter 2 Geology Figure 2.12 – a) A map of the Las Cruces deposit illustrating the extent of the Au mineralisation (solid yellow line), supergene Cu-sulphide mineralisation (solid blue line) and the positions of the boreholes selected for examination during this investigation. The contours represent gravity survey data. The red and purple contours represent areas of high gravity (relative to the surrounding areas shown in yellow, green and blue, scale unknown). The region of high gravity in the central left hand portion of the map represents the supergene enriched massive sulphide deposit and the central upper region of high gravity represents the primary massive sulphide orebody. Boreholes CR194, CR123 and CR038 are vertical holes and boreholes CR149 and CR191 are inclined holes. The grid spacing is in units of 60 metres. (Modified diagram courtesy of Rio Tinto Limited.) Borehole CR194 exhibits extensive Au mineralisation with grades in excess of 14ppm Au in the gossan and in excess of 13ppm Au within the supergene enriched massive sulphide. The Ag mineralisation is extensive, particularly towards the base of the gossan, with grades exceeding 1100ppm. The gossan in borehole CR194 lies directly above the supergene enriched massive sulphide mineralisation. The supergene massive sulphide contains elevated Cu values in addition to deleterious elements (from a mining perspective), including As, Bi, Hg and Sb. The supergene massive sulphide mineralisation lies above a Cu- Page 31 Chapter 2 Geology enriched shale. This borehole was selected for examination due to the extensive precious metal mineralisation and the central position relative to the underlying massive sulphide and supergene Cu sulphide mineralisation. The gossan in borehole CR149 also lies directly above the supergene enriched massive sulphide mineralisation with Au contents ranging between 0.67 and 48.54ppm Au between 170.20 and 190.00 metres down hole. This borehole is an inclined hole, the angle of dip being approximately 60 degrees. Therefore, the depths are not representative of the vertical extent of the mineralisation. The Au mineralisation is confined to the gossan with elevated Cu values occurring in the underlying massive sulphides. The Ag content of this borehole is relatively low with a significant increase in the Ag content (~730ppm) occurring at the contact between the gossan and massive sulphide. Relatively high levels of As, Bi, Hg, Sb and Sn occur throughout the gossan and massive sulphide. This borehole was selected for examination because of the extensive precious metal mineralisation and the central position relative to the underlying massive sulphide and supergene Cu sulphide mineralisation. The Au lens/gossan zone in borehole CR038 occurs between 150.80 and 157.25 metres and exhibits extensive precious metal mineralisation (1.33–11.31ppm Au, 3.8–1240ppm Ag). This borehole lies towards the margins of the precious metal mineralisation for the Las Cruces orebody and away from the main massive sulphide zone. The underlying geology is that of partial massive sulphide that largely represents pyrite-rich shales and wall rocks that exhibit some degree of supergene enrichment. This borehole was selected for examination because of the extensive precious metal mineralisation and the marginal location relative to the massive sulphide mineralisation. Borehole CR191 is also extensively mineralised with respect to Au (0.61– 12.04ppm) with the Ag content (5.3–58.6ppm) being less significant than previous boreholes. The Au zone occurs between 137.95 and 153.85 metres. However, this borehole is an inclined hole, the angle of dip being approximately 70 degrees. Therefore, the depths are not representative of the vertical extent of the mineralisation. Borehole CR191 was selected for examination because of the Page 32 Chapter 2 Geology extensive Au mineralisation and the marginal position relative to the main supergene massive sulphide mineralisation. The gossan zone of boreholes CR123 occurs between 152.40 and 172.85 metres. The Au content of the core is relatively high (1.47–56.55ppm), with moderate amounts of Ag (13.6–175.3ppm) also being present. This borehole lies on the margin of the Au and secondary Cu mineralisation, towards the southern most region of the Las Cruces orebody. The gossan lies above partially supergene enriched pyritised shales and wall rocks. Borehole CR123 was selected for examination because of the extensive Au mineralisation and the marginal position relative to the main supergene massive sulphide mineralisation. Of the five boreholes selected for the current study, only borehole CR038 had been examined previously (R2644, 1996). This earlier investigation revealed that the bulk of the precious metal mineralisation occurred in the form of relatively coarse native Au grains, with discrete grains commonly exceeding 25µm in maximum dimension. Reports R2643 (1996), R2644 (1996) and R2696 (1997) also provided some initial mineralogical information on the nature and mode of occurrence of the precious metal mineralisation in other boreholes from the Las Cruces gossan. However, only limited information was available on the textures and association of the precious metal mineralisation, with the bulk of the investigation being based on crushed reject materials from assay sampling. The five boreholes consist of several hundred metres of core, with the upper 100 metres typically consisting of marl and unmineralised overburden. The field geologists often discarded this material, as it had no commercial value with only mineralised intersections (with respect to Au and/or Cu), and material immediately above or below the mineralisation being retained for examination. It was therefore not possible to examine material from all sample intervals within each borehole. Subsequently, the samples selected for investigation consist predominantly of Au-bearing gossan and material directly above and directly below the Au lens. However, borehole CR194 contains significant Au values within the massive sulphide zone and the sample suite therefore also included this Au-bearing material. Page 33 Chapter 3 Gossans GOSSANS 2.8 Introduction Gossans have been a source of great interest since ancient times, with the earliest prospectors recognising gossans as the surface expressions of base and precious metal-bearing orebodies. Recently, gossan evaluation has been focussed on characterising the mineralogy and geochemistry of these oxidised outcrops, with the aim of differentiating between barren and fertile gossans and ironstones. This has become particularly important in the field of exploration geology, because in many mineralised terrains, gossans provide the only visible indication of potentially economic ore hidden at depth. Some of the more notable work has been by Blain and Andrew (1977) and Andrew (1978, 1984), with reviews on gossan typology, mineralogy and geochemistry for both base and precious metal-bearing orebodies. Recent literature on gossans is somewhat limited relative to those produced on the underlying orebodies. This may be related to the degree of economic interest in gossans. Although many gossans contain economic quantities of metals, their value is often less than that in the underlying orebody. The bulk of detailed papers on gossans have focussed on pathfinder geochemistry, identifying economic sub-surface mineralisation. Many gossans, particularly those in the Iberian Pyrite Belt, in which this study is focussed, have been mined since before Roman times and much of the gossan has long been removed. English language publications of Iberian Pyrite Belt massive sulphide deposits are common, but information on the nature of the respective gossans is scarce, being confined largely to Spanish and Portuguese research papers held in university and research departments. Limited information is available on gossans and massive sulphides in the Rio Tinto mine, Spain (Kosakevitch et al., 1993 and Williams, 1933-34 and 1950). Page 34 Chapter 3 Gossans Nickel (1984), Taylor and Sylvester (1982), Taylor and Appleyard (1983) and Scott et al. (2001) have produced detailed accounts of gossan profiles associated with both barren (base/precious metal-poor) and fertile (base/precious metal-rich) orebodies. Boyle (1995) examined the gossan of the Murray Brook Deposit, New Brunswick. Hannington et al. (1986, 1988) and Herzig et al. (1991) provide examples of the weathering and formation of gossans in present day seafloor sulphides. The geochemistry of gossan forming processes is reviewed by Blain and Andrew (1977) and Andrew (1978, 1984). Thornber (1975, 1976) and Thornber and Wildman (1984) provide experimental data on the chemical and electrochemical processes of gossan formation, and associated formation of carbonates, sulphates and oxide minerals. Mann (1984) and Webster and Mann (1984) studied the mechanisms of precious metal mobilisation effects of climate and geomorphology on gossan formation. The following section consists of a literature review of gossan forming processes, gossan geochemistry and geochemical profiles, element mobility, gossan typology and mineralogy, especially those developed above polymetallic, pyrite hosted Cu-Pb-Zn massive sulphide deposits and/or Au-bearing, sulphide-rich orebodies, similar to the Las Cruces massive sulphide. Page 35 Chapter 3 Gossans 2.9 The Gossan Forming Process Most hypogene sulphide minerals are unstable under near-surface weathering conditions, particularly in the presence of weathering agents such as waterdissolved oxygen, carbon dioxide and ionic species. These cause the sulphide body to re-equilibrate electrochemically (Blain and Andrew, 1977) and the sulphide minerals oxidise to form sulphates and the metal-sulphur bonds are broken, releasing metal cations that are either dissolved in the co-existing groundwaters or precipitated as insoluble oxidate minerals. This gives give rise to more stable secondary sulphide and oxide mineral assemblages. The residues of Fe-bearing minerals and varying amounts of introduced silica, are commonly the most abundant constituents of a gossan above massive sulphides (Blain and Andrew, 1977). As sulphide minerals corrode to stable oxide, carbonate and sulphate phases near the water table, they become disconnected from the main sulphide ore zone, are poor conductors and no longer contribute to the major electrochemical corrosion processes acting on the orebody (Blain and Andrew, 1977). At the water table, a dramatic increase in Eh results in decomposition of Fe-sulphides, producing goethite and a low pH environment (Taylor and Sylvester, 1982):4FeS2 + 10H2O + 15O2 → 4FeOOH + 8SO42– + 16H+ Thornber and Wildman (1984) compare the results of reacting different ore types under varying conditions over a wide pH range. They highlight high and low pH processes of Fe hydrolysis, where Fe is a major metal being released from a sulphide (e.g. pyrite and/or Fe-S-hosted orebodies). 1. The high pH process (pH>7). Base metals, including ferrous Fe will be hydrolysed and mixed Fe-Cu hydroxycarbonates and hydroxysulphates form for Cu, and mixed Fe-Pb hydrocarbonates form for Pb. The Fe is located in these initial compounds as a green rust where it is effectively Page 36 Chapter 3 Gossans bound as ferric hydroxide. produces no further acid:- Subsequent oxidation of this hydroxide 4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)3 2. The low pH process (pH10-3.2 (Cl- activity) (Webster and Mann, 1984). Figure 3.5 illustrates that in the presence of high chloride, Au is soluble in certain acid, oxidising solutions. Page 50 Chapter 3 Gossans Figure 3.5 – Eh/pH diagram illustrating the stability relations of some Au compounds in water at 25oC and 1 atmosphere total pressure at total dissolved chloride species of 100 and sulphur activity of 10-1. Boundaries of solids are for total ionic activity of 10-6 (Garrels and Christ, 1965). Experimental evidence suggests that very acid chloride solutions generated by ferrolysis are responsible for the dissolution of Au and Ag (Mann, 1984). Natural waters in near-surface conditions will be oxidised by the atmosphere and can contain abundant chlorine from the dissolution of salts (Ross, 1997). The dissolution of Au to form a Au-chloride complex is expressed in the chemical reaction below. Page 51 Chapter 3 Gossans 4Au0 + 16Cl- + 3O2 + 12H+ → 4AuCl4- + 6H2O The reaction requires the presence of oxygen, acid (H +), and a notably large concentration of chloride ions (Mann, 1984). Such chlorine-rich groundwaters have been sampled at Kalgoorlie, Western Australia, where Cl concentrations ranged between 21,000 to 107,000mg/L (Grey et. al., 1992). Further examples of Au and Ag believed to have been remobilised as chloride complexes under oxidising conditions include Westonia and Yilgarn Block, Western Australia (Webster and Mann, 1984, Mann, 1984) where dissolution of Ag and/or Au is a relatively frequent occurrence along the rim of nuggets, especially when there are adjoining Fe-oxides. Secondary Au precipitated by reduction at the site of Fe oxidation is often of higher fineness (lower Ag content) than the primary Au (Webster and Mann, 1984; Mann, 1984 and Saunders, 1991). This feature can be readily explained by the nature of Au- and Ag-chloride complexes and their behaviour under near surface weathering conditions. Following the release of Au and Ag from primary Au and electrum grains as chloride complexes, the supergene solutions migrate downward through the weathering profile and reducing conditions are encountered. The Au-chloride is subsequently re-precipitated by reduction of the AuCl4- ion with Fe2+ (Mann, 1984). 4AuCl4- + 3Fe2+ + 6H2O → Au0 + 3FeOOH + 4Cl- + 9H+ This reaction is thought to occur near the water table, where Fe2+ would be present in the weathering profile (Mann, 1984). Ag-chloride complexes are not initially affected by the encounter with reducing conditions, because of the relative redox potentials of Fe 2+/Fe3+ and Ag/AgCl0 (Mann 1984) and will remain in solution, migrating downward (Saunders, 1991). The solubility of Ag is comparatively high as a chloride complex with respect to Page 52 Chapter 3 Gossans Au (Webster, 1986) and therefore the refinement of Au in supergene process is the product of the different stabilities of Au- and Ag-chloride complexes (Saunders, 1991). The proposed mechanism of Au and Ag transportation as chloride complexes has also been suggested for gossans associated with modern day seafloor sulphides. Hannington et al. (1988) have documented precious metal-bearing grains from supergene zones from the Mid Atlantic Ridge and note the relatively high purity of the native Au grains. Au and Ag mobilisation as chloride complexes is not the only viable mechanism and a number of other possibilities are discussed in the literature. Garrels and Christ (1965) suggest that under neutral to alkaline reducing conditions, Au is soluble as a AuS- complex (Figure 3.5). More recent experimental works by Vlassopoulos and Wood (1990) show that in groundwaters circulating through oxidising orebodies, Au(S2O3)23- (thiosulphate), AuHS0 and Au(HS)2- are the stable solution species. Webser and Mann (1984) also suggest that thiosulphate complexes are the stable species under alkaline oxidising conditions, citing examples including the Upper Ridges Mine, Papua New Guinea, where, under neutral to basic, moderately oxidising conditions, found in the vicinity of the weathering carbonate veins, Au and Ag may be complexed by thiosulphate to form Au(S2O3)23- and Ag(S2O3)23- or a mixed complex. Au of low fineness (high Ag content) is re-precipitated by reduction at the water table, as, unlike the chloride complexes, both the Au and Ag thiosulphate complexes destabilise under similar pH and Eh conditions (Webser and Mann, 1984). Thornber (1992) comments, however, that although sulphate complexes are more stable than chloride complexes, because most natural waters have higher activities of chloride than sulphate, chloride complexes are more important for geochemical mobility. Boyle (1995), on the Murray Brook precious metal-bearing gossans, notes that during progressive oxidation and physico-chemical erosion of the gossan zone, Au was transported downward in the groundwaters, probably as an Au 0 colloid Page 53 Chapter 3 Gossans complex, to be concentrated in the lower horizons of the gossan profile. Boyle's hypothesis was based on leaching experiments and microprobe analyses, which indicate that Au is present in the gossan as sub micron composite sols of Au-Agsilica. A number of other mechanisms for Au and Ag mobility are mentioned in the literature. These include organic ligands, such as humic acid, cyanide complexes CN- or SCN-, which can form locally from biogenic processes (Webster and Mann, 1984). New thermodynamic data and theoretical calculations for gold hydrolysis demonstrate that in conditions prevailing for most supergene waters the complex that should control the solubility is AuOH(H2O)0 rather than AuCl4(Vlassopoulos and Wood, 1990). Although it is generally accepted that only one complexing agent is active in a deposit (Mann, 1984), Angelica et al. (1996) suggest that more than one complexing agent may have been active at different stages of gossans development. Angelica et al. (1996) describe a lateritised gossan in Brasil and suggest the most accepted model for the dissolution of Au during the gossan formation in this case is through thiosulphate complexes, in oxidising, neutral to alkaline environments. In a second stage, however, the authors suggest that during the laterisation of pre-existing gossan, other physiochemical conditions may have prevailed in a more oxygenated environment, resulting in a new remobilisation of Au through the combination of humates, thiocyanates and also H2O-OH complexes. Recent studies (Lengke and Southam, 2005; Reith and McPhail, 2006) have shown that bacteria in the natural environment may play an important role in both the mobilisation and reprecipitation of Au and other metals. Experimental studies by Reith and McPhail (2006) have shown that aerobic and anaerobic microbiota in auriferous soils from the Tomakin Park Gold Mine, New South Wales, Australia are capable of dissolving finely disseminated Au bound within the soil fractions. In the anoxic experiment, the maximum concentrations of solubilised Au were lower than that of the oxic experiment. The authors show that Au can be solubilised in in vitro studies with heterotrophic bacteria and found that Au amino Page 54 Chapter 3 Gossans acid complexes dominated. When the amino acids are utilized more rapidly than they are produced, the Au-ions, if present in solution, are left without complexing ligands and become unstable in solution, precipitating and/or re-adsorbing to the solid soil phases. Southam and Beveridge (1996) have shown that octahedral gold was formed through indirect bacteria involvement when organic acids were released from dead bacteria, which then formed complexes with gold in solution and finally transformed to crystalline octahedral gold. In carbon limited system such as quartz/Au veins, the resident microbiota also released Au, but the Au release appears to be linked to a different microbially mediated Au solubilisation process, probably Fe or sulphide oxidation. Fe- and sulphur-oxidising bacteria such as strains of Acidithiobacillus sp. and Leptospirillum sp. have been observed to mediate the release Au by breaking down the sulphides in sulphidic Au ore (Reith and McPhail, 2006). The organisms use Fe2+ and sulphide as electron donors in their metabolisms and oxidise them to Fe3+, thiosulphate, and sulphate respectively. Reith and McPhail (2006) note that despite the studies undertaken to date little is known about the mobility of Au and its interactions with microorganisms in a complex natural environment. The species or groups of bacteria and other microorganisms that are important in affecting Au mobility need to be identified more specifically and the speciation of Au needs to be identified. 2.11.4 Au and Ag Mineralogy and Geochemical Profiles The review of selected gossans in the literature reveals that similarities occur in both the precious metal mineralogy and the resultant profiles developed within the gossan. The mineralogy and profiles are therefore discussed in greater detail in this section. Williams (1933-34) describes the Rio Tinto gossan in detail, and, although limited analytical techniques were available at the time, significant detail on the nature of the precious metal mineralogy and geochemical profile was obtained. One of the Page 55 Chapter 3 Gossans key features noted by Williams was the development of a precious metal layer at the base of the gossan. Williams describes this enrichment as being due to a concentration of the traces of Au and Ag that were originally present in the sulphide deposits. Williams comments that jarosite is the dominant mineral in this earthy, precious metal bearing layer and as well as Au and Ag, this layer is also marked by enrichment in Pb, Sb, Bi and Se. Ag has been identified as cerargyrite and is also probably present as acanthite (Williams, 1933-34). Much of William's work has been verified by Vinals et al. (1995) who confirm that Ag is present in a number of forms, including members of the beudantite-jarosite group of minerals, cerargyrite (plus or minus some bromide and iodide), acanthite and Hg/Ag sulpho-halides. Vinals et al. (1995) also confirm the presence of micrometre-sized native Au grains and note that the majority of the Au contained in the ore is probably submicroscopic. In the Salomon-Cerro Colorado area of Rio Tinto, Spain, the base of the oxide zone is 10 to 40m deep and the contact is generally sharp. An earthy precious metal layer (1 to 2m vertical interval) below the oxide zone, overlies a thin horizon of leached pyrite. A well developed zone of secondary sulphide enrichment (30 to 40m vertical interval) grades into the hypogene ore (Blain and Andrew, 1977). At Lagoa Salgada an increase in precious metals (Au and Ag) is evident in its supergene enrichment zone, with Au contents reaching a maximum value of 2.38ppm. Ag occurs, at least in part, as relatively coarse grains of amalgam (AgHg alloy) that may be visible in hand specimen (Oliveira et al., 1998). Lopez Garcia et al. (1988) on Sierra de Cartagena, southeastern Spain note that in horizon 1, derived from the oxidation of a magnetite and siderite primary assemblage, Ag occurs mainly as cerargyrite and native metal. In horizon 2, derived from the oxidation of a pyrite and marcasite primary assemblage, it occurs principally in jarosite and as native Ag. These differences in Ag mineralogy are largely controlled by acid generation during oxidation of different Page 56 Chapter 3 Gossans primary geologies. A contributing factor to the formation of cerargyrite was the proximity of this area to the sea, which resulted in an important source of windborne chlorine (Lopez Garcia et al., 1988). In the Eastern Lachlan Fold Belt, NSW, Australia, Scott et al. (2001) describe the Au and Ag distribution and associations for a number of deposits within the region. The authors note that for Woodlawn and Currawang, Ag may be severely depleted in the gossanous outcrop relative to the original ore but substantial enrichment may occur in the supergene sulphide, sulphate and carbonate zones. Au is also significantly enriched in the carbonate zone of the gossan, relative to the primary sulphide. Scott et al. (2001) also comment that Ag is retained in high concentrations in the gossans of Kangiara, Lewis Ponds, Peelwood and Mt Costigan and note correlations between Ag-Pb and Ag-Sb contents. This may relate to gossan maturity and different weathering susceptibilities of the primary Ag-bearing phases, as Ag may be present in more than one phase within the primary orebody. Ag initially concentrates in Cu-rich secondary sulphides, although with continued weathering, they are concentrated in the Pb-bearing alunite-jarosite minerals and to a lesser extent the Fe-oxides (Scott et al., 2001). Au contents typically increase with Ag content in the gossans of the Lachlan Fold Belt, except in four Ag-rich deposits. Au contents also increase with As except in the five high As gossans. The Au grades of the gossans commonly represent a significant enrichment relative to the primary ores, although the immature gossans are less likely to show the extreme enrichment of some mature gossans (Scott et al. 2001). At the Murray Brook deposit, New Brunswick, Boyle (1995) notes a strong correlation between Au, Sn and Si. However, much of Boyle's microscopic interpretations appear to be based on a single occurrence of Au in the gossan as a small grain of Au-Ag-silica gel-like material. Because halide minerals are not present in the Murray Brook gossan or other gossans in the Bathurst Camp area, it is unlikely that Au was transported as a halide complex (Boyle, 1995). Page 57 Chapter 3 Gossans Boyle suggests that the precipitation of silica may have had a controlling effect on Au concentration and cites Fujii and Haramura (1976) and Fujii et al. (1977) who have shown that colloidal silica is a good precipitating agent of Au sols, and that acid silica solutions act as a reducing media for Au3+. As Si solubility decreases with decreasing pH, groundwaters moving down into the oxidising pyrite-rich zones would precipitate silica during the oxidation of pyrite (Boyle, 1995). Boyle's hypothesis for the close association between Au, Sn and Si are to some degree corroborated by microscopic interpretation of the primary ore. Boyle remarks that in the primary ore, Sn is concentrated mainly in the pyrite-rich zones and, because cassiterite has been shown to be very resistant to weathering processes, the Au-rich zones in the gossan represent the former positions of primary pyrite-rich zones. Boyle (1995) observed that native Ag in these ores have a physical appearance similar to physically re-worked grains (e.g. from a placer), however, much of the Ag occurs in jarosite group minerals with some Ag in the pyrite-quartz sand occurring as acanthite. Costa et al. (1999) and Angelica et al. (1996) have studied a number of lateritised gossans from South America and conclude that Au mineralisation is closely associated with the Fe oxyhydroxides in the gossans, with a great range of Au compositions in the different parts of the profile. The higher Au values coincide with the respectively greater goethite and hematite contents of the profile (Angelica et al 1996). Ag was detected only in the upper part of the profile with Cu, Mo, Sn and As also present in high values and exhibiting a good correlation with Au (Angelica et al. 1996). Costa et al. (1999) reveal that the gossan elements (Au, As, B, Cu, Mn, Mo, Ni, Pb, Sn, W, Y and Zn) display good correlations and these persist in laterite, latosols and colluvium. The authors suggest that this behaviour reinforces the primary nature of these materials, controlled mainly by minerals that are still preserved as resistates in the supergene materials. The most important are dravite (B), wolframite (W), cassiterite (Sn), and Au (Costa et al. 1999). Page 58 Chapter 3 Gossans 2.11.5 Cu Cu, Pb, As and Sb often exhibit a close correlation in weathering profiles and their presence and/or absence may also be indicative of the degree of profile maturity and subsequently reflect conditions under which the gossan has formed. This is evident in many of the gossan profiles studied in the literature. In mature gossan profiles, Cu is typically depleted in the uppermost portions of the gossan, but may be concentrated in the lower gossan within the supergeneenriched zone in the form of secondary Cu sulphides. These secondary Cu sulphide minerals may also host a significant proportion of As and Sb as well as Ag and include chalcocite, enargite/luzonite and chalcanthite (Scott et al., 2001). Cu is largely absent from the upper part of most mature gossan profiles, including those of Rio Tinto, Lagoa Salgada and Murray Brook. Angelica et al. (1996) revealed the presence of bornite, cuprite, malachite, chalcocite, native Cu, azurite and chrysocolla in the secondary sulphide enrichment zone of a lateritised gossan in the Amazon region. The absence of Cu in mature gossan profiles is largely a result of the relatively high solubility of Cu under the acid, oxidising conditions that often prevail during the weathering of massive sulphide orebodies (Figure 3.6). Under oxidising, near-neutral pH conditions, Cu-sulphates may form (e.g. chalcanthite), but their high solubility often results in rapid redissolution and reprecipitation with Fe oxides and hydroxides (Anderson, 1990). At high Eh and pH, Cu-oxides are the stable species. The Cu-sulphides dominate under strongly reducing conditions (Figure 3.6) Page 59 Chapter 3 Gossans Figure 3.6 – Eh/pH diagram illustrating the stability relations of some Cu minerals in water at 25oC and 1 atmosphere total pressure at total sulphur activity of 10-1, CO3 activity of 10-3 (Anderson, 1990). Less mature gossans may contain elevated levels of Cu. Cu is particularly abundant in the surficial gossan of the immature profile at Currawang where it is present, partly, as malachite, although substantial amounts are also retained in the Fe-oxides and plumbojarosite (Scott et al., 2001). At Mugga Mugga, Cu is depleted at the base of the weathered zone, with levels increasing up the profile. The Cu is incorporated into the hematite structure as well as being adsorbed by Page 60 Chapter 3 Gossans goethite (Taylor and Sylvester, 1982). The authors also note a similar trend for Ag in this deposit. Similarly, the Teutonic Bore contains between 500 and 1000ppm Cu in the upper part of the gossan, the bulk of which has co-precipitated with Fe, presumably in the form of Fe-oxyhydroxides. Nickel (1984) refers to Thornber and Wildman (1984) noting that the coprecipitation of cations is favoured by a high pH, in the case of the Teutonic Bore, probably resulting from the high level of carbonates in the groundwater and partly dissolved carbonate species from the ore (Nickel, 1984). At the Dulgald River Lode, Taylor and Appleyard (1983) note that Cu appears in relatively high concentrations within the bulk of the gossan profile, indicative of an immature gossan profile developed during near neutral to alkaline conditions. 2.11.6 Pb Pb is one of the least mobile metals and is commonly observed throughout a large number of the gossans in a variety of forms. Figure 3.7 illustrates the stability fields for Pb compounds under conditions that resemble near-surface weathering conditions. This illustration serves to confirm that Pb is soluble only under the most extreme acid or alkaline conditions. Galena is the stable phase under most reducing conditions, with anglesite and cerussite dominating under acid and alkaline oxidising conditions respectively (Garrels and Christ, 1965). Scott et al. (2001) comment that Pb is strongly retained in both mature and immature gossans of Woodlawn and Currawang respectively. Despite its immobility, the mineral hosts for Pb change significantly during weathering. The great bulk of the Pb in the primary ores reviewed here occurs in the form of galena. However, the respective gossans typically exhibit a wide variety of Pbrich species. Nickel (1984) notes that for the Teutonic Bore, Pb has been found as a major component in twelve secondary minerals, the chief ones being cerussite and plumbogummite. Page 61 Chapter 3 Gossans Figure 3.7 – Eh/pH diagram illustrating the stability relations of Pb compounds in water at 25oC and 1 atmosphere total pressure. Total dissolved sulphur of 10-1, pCO2 of 10-4. Boundaries of solids shown are for total ionic activity of 10-6 (Garrels and Christ, 1965). Scott et al. (2001) note that high Pb gossans (Pb >4%) are typically immature, probably due to the lesser abundance of pyrite and hence less acid conditions during weathering. The authors identify a close association between Pb-As and Pb-Sb in some gossans but not in the primary ore suggesting that As and Sb are distributed between several phases in the ore but become associated with Pb in alunite-jarosite during prolonged weathering. However, in many immature Page 62 Chapter 3 Gossans gossans, the Pb-As and Pb-Sb associations have not had time to develop and the Pb is mainly present in oxidate phases like cerussite that typically contain very low contents of As and Sb (Scott et al., 2001). At Woodlawn, the carbonate zone contains acicular crystals of cerussite and the sulphate zone anglesite. The supergene sulphide zone contains anglesite and relict galena. At Currawang gossans retain boxwork textures, indicating that the profile is immature, and contains cerussite. The sulphate zone material consists of dark Fe-oxides with a basic Pb sulphate. Alunite-jarosite minerals are also present, intergrown with the Fe-oxides (Scott et al., 2001). At the Dulgald River Lode gossan, Pb minerals include plumbian jarosite, plumbogummite and anglesite (Taylor and Appleyard, 1983). At the immature Mugga Mugga gossan, Pb is retained and even concentrated in the lower part of the profile, where it occurs as secondary sulphate, arsenate and phosphate minerals of the alunite-jarosite series. Pb is, however, depleted in the upper part of the profile (Taylor and Sylvester 1982). Pb has co-precipitated with Fe-oxides in the immature gossans of the Teutonic Bore (Nickel, 1984). Oliveira et al. (1998) note that the Lagoa Salgada gossan contains high Pb and As values in the form of mimetite crystals. Williams (1933-34) notes a marked enrichment in Pb at the base of the Rio Tinto gossan associated with the precious metal layer and Vinals et al. (1995), revealed that Pb occurs as solid solutions of beudantite-plumbojarosite-potassium jarosite. Pb was also detected, but only occasionally, as anglesite associated with gangue species (Vinals et al., 1995). Cerussite may also be present in minor amounts (Williams, 1933-34). Lopez Garcia et al. (1988) on Sierra de Cartagena, south-eastern Spain, comment that Pb was leached from the primary ores and precipitated as Pb- and Ag-bearing jarosites, anglesite, cerussite, Pb-bearing coronadite and goethite. The Pb bearing minerals of the Sierra de Cartagena gossan differ depending on the composition of the primary ore. Anglesite, cerussite, Mn-oxides and goethite occur in horizon 1, a gossan formed under weakly acid conditions resulting from a low Fe-sulphide content in the primary ore and acid buffering from associated Page 63 Chapter 3 Gossans carbonates. Pb occurs in Mn-oxides and jarosite in horizon 2, a primary geology rich in pyrite, forming strongly acidic conditions during weathering. In addition, the ore textures also differ from one horizon to the other with pseudomorphic textures frequently observed in horizon 1, but in horizon 2 primary textural features have largely been obliterated (Lopez Garcia et al. 1988). Supergene galena occurs throughout the transition zone of Broken Hill, Northern Zimbabwe. Taylor (1958) relates this considerable migration of Pb in the zone of weathering to a former period of aridity and increased salinity of the groundwater. Cerussite is abundant in the oxide ore. Taylor highlights the abundance of pyromorphite as an indication that the chloride ion is present. Blain and Andrew (1977) also conclude that solutions enriched in chloride and bicarbonate ions favour the dissolution of galena, thus enhancing the Pb content of the solutions from which secondary sulphides may subsequently precipitate. 2.11.7 As and Sb Arsenic and Sb are often closely associated in ores and gossans. In the Currawang and Woodlawn deposits, the As and Sb occurs predominantly in tetrahedrite-tennantite and enargite-luzonite solid solution series in the primary ore and in alunite-jarosite minerals in the gossan, whereas in more As-rich primary ores of the Lachlan fold belt, As is largely present as arsenopyrite, occurring as scorodite in the profiles of immature gossans (Scott et al., 2001). The gossan of the Dulgald River Lode contains elevated levels of As and Sb possibly introduced as a result of leaching of the surface gossan (Taylor and Appleyard, 1983). Similarly, in the Mugga Mugga massive sulphide deposit of the Yilgarn Block, Taylor and Sylvester (1982) note that the anomalous concentrations of As and Sb in the surface gossan result from the precipitation of secondary Pb-bearing minerals of the alunite-jarosite series. The authors note that the concentration of As immediately above the water table in secondary Pb minerals is followed by a trend of slightly increasing As content up the profile. This distribution reflects the low mobility of As in weakly acidic solutions and its ready co-precipitation with Fe-oxides (citing Boyle and Jonasson, 1973). Page 64 Chapter 3 Gossans Similarly, at Murray Brook, Boyle (1995) notes that Pb, As, Sb and Bi correlate strongly and occur within specific horizons. As with Dulgald River, Currawang and Woodlawn, these elements are typically associated with the precipitation of jarosite-group minerals within the gossan. Boyle also notes that borehole sections rich in these metals contain lower than average Au contents, indicating that precipitation of these hydroxyl-sulphate-oxide minerals has had little control on the localisation of Au. Williams (1933-34) notes enrichment in Sb associated with the precious metal layer at the gossan/sulphide contact of the Rio Tinto deposit. Vinals et al., (1995) comment that As was detected in members of the beudantite-plumbojarositepotassium jarosite solid solutions, appearing as powdery aggregates of zoned and skeletal crystals, which could suggest a formation through successive crystallisation re-dissolution processes. Sb was observed as fine-grained oxides of the stibiconite-bindheimite group (Vinals et al., 1995). Vink (1996) predicts that under both acid and alkaline oxidising conditions, Sb is highly mobile as SbO3-(aq) and as Sb2O42-(aq) under strongly reducing alkaline conditions. In the absence of sulphur, As is highly mobile under almost all conditions, with native As only occurring under very strongly reducing conditions. The high mobility of As means that arsenate and arsenite ionic species are widely available for forming precipitates with many types and combinations of cations, hence the wide variety of As-bearing species often observed in gossans (Vink, 1996). 2.11.8 Si, Sn and Ti The breakdown of silicate minerals during the gossan forming process may result in the supersaturation of SiO2 in the mineralising solutions. Below pH 9, silica is in solution as the uncharged molecule Si(OH)4 and above pH 9 as Si(OH)4(Thornber, 1985). Si is a common constituent of the gossans reviewed during this investigation, occurring predominantly as quartz. Quartz is essentially a resistate phase and exhibits a close correlation with other resistate phases, including cassiterite (Sn) Page 65 Chapter 3 Gossans and TiO2 in a number of the gossans. A significant proportion of the Si content of these gossans is, however, present in the form of remobilised Si that appears to have formed as a result of the dissolution of wall rocks and associated Si-rich gangue minerals. This is typically followed by the subsequent reprecipitation of the Si, largely as chert/jaspers in specific zones in the gossan. Blain and Andrew (1977) note that it is quite likely that the acid buffering, hydrolysis reactions of silicate wall rocks account for the release of silica. The dissolution and mobilisation of Si is evident in the sub-rounded nature of quartz grains in the gossan of the Flambeau mine, Wisconsin, U.S.A. Ross (1997) suggests that rounding the quartz grains occurs during dissolution of quartz grain edges by acidic supergene alteration fluids. Citing Morris and Fletcher (1987), Ross (1997) also proposes a hypothesis that a reaction between ferrous Fe in solution and quartz may have formed a thin layer of ferrous silicate that would subsequently oxidise to form a hydrous Fe oxide (goethite), while rapidly releasing silica into solution. Thus, the presence of ferrous Fe would greatly increase the solubility of quartz, as opposed to the solubility of quartz in a solution devoid of ferrous Fe (Ross, 1997). May (1977) describes a 5 metre thick gossan and siliceous cap that is in sharp contact with the massive sulphide of Flambeau. Boyle (1995) describes the dissolution, reprecipitation and subsequent accumulation of silica in the Murray Brook gossan. The bulk of the silica is present as euhedral quartz and in lesser amounts, as amorphous silica. The author notes that most of the quartz exhibits a chalky white appearance due to attack by acidic solutions. During oxidation, the silicate minerals, and to a much lesser extent, primary quartz, are dissolved by acidic solutions to form cation complexes, silicic acid, and colloidal silica. During changes in pH and electrolyte composition with depth, the colloidal silica becomes unstable in solution and precipitates lower in the oxidising profile as amorphous silica (Boyle, 1995). In Mugga Mugga, Taylor and Sylvester (1982) note that there has been an absolute accumulation of silica immediately above the sulphide zone, occurring in the form of a massive and slightly ferruginous chert derived from rock weathering Page 66 Chapter 3 Gossans and deposition from groundwaters. The authors also note that generally SiO2 and Fe2O3 contents vary antipathetically with an overall decrease of SiO2 and increase in Fe2O3 up the profile. Solomon (1967) describes cherts associated with the fossil gossan of Mt. Lyell, Tasmania and compares them to the goethite-veined cherts overlying pyritic ore on the Cerro Colorado at Rio Tinto, Spain (citing Williams, 1933-34). Oliveira et al. (1998) note the gossan associated with Lagoa Salgada is more siliceous near to the base. Taylor and Appleyard (1983) indicate that Si and Sn are essentially immobile in the weathered zone of the Dulgald River Lode, illustrating the resistate nature of the primary minerals within which these elements occur. The presence of remobilised and reprecipitated cherts is also recognised at Skouriotissa, Cyprus, where Constantinou and Govett (1972) note that the cherts are common but are restricted to the ochre (oxide) sulphide contact. The authors also confirm that their deposition was probably controlled by pH conditions, both directly in their effect on the precipitation of colloidal silica (optimum pH 4.5), (citing Okamoto et al., 1957) and indirectly as they affected the precipitation of Fe and aluminium hydroxides (Constantinou and Govett, 1972). An alternative mechanism for the formation of siliceous materials within gossans is discussed by Hannington et al. (1986, 1988, 1991a, b, c and d) where hydrothermal activity has continued intermittently during weathering of modern seafloor sulphide mounds, resulting in localised silicification. Sn2+ and Sn4+ are important in the aqueous gossan environment. As a cation, Sn2+ is quite soluble below pH 5, occurring as the anion Sn(OH)3- above pH 9, with some solubility of Sn(OH)20 at intermediate pH. Sn4+ is essentially insoluble. In the primary ore, Sn4+, usually in the form of cassiterite, is highly resistate in nature and will usually remain in the gossan. Sn 2+, often occurring in the sulphide minerals, may coprecipitate with the Fe-oxides, but will eventually oxidise to Sn 4+ and form cassiterite (Thornber, 1985). Page 67 Chapter 3 Gossans Sn is a common accessory component of several of the massive sulphide associated gossans described in the literature. Scott et al. (2001), referring to Woodlawn and Currawang, note that in the primary ore, Sn is present as cassiterite and stannite (ideally Cu2FeSnS4), with the latter breaking down to additional cassiterite during weathering. Sn is residually concentrated in mature gossans (Scott et al., 2001). Oliveira et al. (1998) on Lagoa Salgada note that the values of Sn are relatively high, especially in the gossans with the bulk of the Sn occurring as cassiterite (Oliveira et al., 1998). At Murray Brook, Sn, as cassiterite, is conservative and correlates strongly with Au, and Si (Boyle, 1995). At Mugga Mugga there is some concentration of Sn above the water table. There is some suggestion of slight concentration of Sn in the kaolinite zones indicating possible derivation from the amphibolite rather than the sulphide mineralisation (Taylor and Sylvester, 1982). At Rio Tinto, Sn was detected as anhedral grains (5-100um) of cassiterite commonly associated with Sb oxides (Vinals et al., 1995). Titanium has very low mobility under almost all environmental conditions, mainly due to the high stability of TiO2 under all but the most acidic of conditions (Brookins, 1988). Rutile, brookite and anatase are the naturally occurring polymorphs of TiO2, with rutile being the most common, particularly in the primary massive sulphide ores. These forms of TiO2 are highly resistate phases that are largely retained and often concentrated during the gossans forming process. TiO 2 therefore often exhibits a close correlation with other resistate phases, notably quartz (Si) and cassiterite (Sn). Taylor and Sylvester (1982) comment that at Mugga Mugga, Ti appears to be concentrated low in the weathering profile, partially as a result of residual concentration. Ti also occurs as a minor constituent of other less resistate phases, notably amphibole and biotite, phases that may be predominantly leached from surrounding wall rocks. Dimanche and Bartholome (1976) suggest that Ti is not entirely immobile during weathering. Skrabal (1995) notes that Ti may exist in a fully hydrated form, TiO(OH)2, in water above pH 2, being transported in a Page 68 Chapter 3 Gossans colloidal state rather than as a dissolved ion. Hutton et al. (1972) suggest that Ti4+ is mobilised more readily in the presence of organic acids at pH