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Characterisation of Hypogene Covellite Assemblages at the Chuquicamata Porpewry Copper Deposit, Crtce, Section 4500N

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Characterisation of Hypogene Covellite Assemblages at the Chuquicamata Porpewry Copper Deposit, Crtce, Section 4500N CHARACTERISATION OF HYPOGENE COVELLITE ASSEMBLAGES AT THE CHUQUICAMATA PORPEWRY COPPER DEPOSIT, CRTCE, SECTION 4500N Meghan H. Lewis Subrnitted in partial fulfilment of the requirements for the degree of Master of Science Dalhousie University Halifax, Nova Scotia December, 1996 O Copyright by Meghan H. Lewis. 1996 TABLE OF CONTENTS Page Table of contents iv List of Figures vi List of Tables vii Abstrac t xi Acknowledgements xii Chapter 1 Introduction 1.1 General S tatement 1.2 What is a Porphyry Copper Deposit? 1.3 The Nature of the Problem 1.4 Chuquicamata Porphyry Copper and Associated Deposits 1.4.1 Local Geology . 1.4.2 Previous Work on Sulphide Mineral Assemblages at Chuquicamata 1.4.3 Present Geological Models 1.5 Objectives 1.6 Methodology 1.6.1 Sulphide Petrology 1.6.2 Suiphur Isotope Thermometry 1.7 Organisation of the Thesis Chapter 2 The Cu-Fe-S and Related Systems 2.1 General S tatement 2.2 Minerdogical Associations in the Binary Cu-S System 2.3 Mineralogical Associations in the Temary Cu-Fe-S System 2.4 Coveliïte Crystal Chemistry 2.5 Expimental Studies with Covellite 2.6 Related Systems: Zn-Fe-Sand Cu-As(-Sb)-S 2.7 Highlights Relevant to this Study Chapter 3 Minerals and Mineral Assemblages at Chuquicamata 3.1 Definition of Relevant Tem 3.2 Main Ore Minerals: Texturai Occurrence, Roperties and Chemistry 3.2.1 Coveilite 3.2.2 Anilite 3.2.3 Digenite 3.2.4 Enargite 3.2.5 Bomite 3.2.6 Chalcopyrite 3.2.7 Sphalente 3.2.8 Pyrite 3.2.9 Moiybdenite 55 3.2.10 Wohmite 56 3 -2.1 i Colusite 57 3.2.12 Idaite 59 3.3 Mineral Groupingsl Assemblages 60 3.3.1 Covellite (-Digenite) Assemblages 65 3.3.2 Digenite (-Bornite) Assemblages 74 3.3.3 Chalcopyrite Assemblages 78 3.3 -4 Sphalerite Assemblages 88 3.3.5 Molybdenite Assemblages 91 3.3.6 Other Pyrite Assemblages 94 3.4 Distribution of Assemblages 95 3.4.1 Assemblages Characteristic of Quartz-sencite Alteration 95 3.4.2 Assemblages Characteristic of Potassic Alteration 97 3.4.3 Assemblages Charactersitic of Quartz-sericite Overprhting Potassic Alteration 98 3.5 Paragenetic Sequence of Assemblages 100 3.6 Hypogene vs Supergene CoveUite 107 Chapter 4 Temperature Limits on Assemblages fiom Experimentai Work on Cu-Fe-S Phases 4.1 Introduction 109 4.2 Assemblages in the Cu-Fe-SSystem 109 4.3 Other Assemblages 114 4.4 Summary 115 Chapter 5 Suiphur Isotope Study 5.1 Sulphur Isotope theory and Mechanisms of Fractionation 5.2 Method of Analysis 5.3 Purpose 5.4 Results for the Present Study 5.5 Sulphate - Sulphide Equilibria 5.6 Summary Chapter 6 Conclusions and Recommendations 6.1 Conclusions 6.2 Recommendations Appendix A Electron Microprobe Analyses Appendix B X-ray Diffraction Analyses Appendix C Hand Sample Descriptions, 4500N Section References 217 LIST OF FIGURES Chapter 1 Page Figure 1.1. Concenhic aiteration zoning in a typical porphyry copper deposit. 5 Figure 1.2. Index map locating Chuquicamata with respect to its Latitude and Longitude. 8 Figure 1.3. Simplified plan view of the Chuquicamata open pit. 9 Figure 1.4. Cross-section 45ûûN through the deposit. 12 Chapter 2 Figure 2.1. Phase relations of condensed phases in the central portion of the copper-sdphur system. 18 Figure 2.2. Minerals reponed within the Cu-Fe4 system. 22 Figure 2.3 a. Schematic phase relations in the centrai portion of the Cu-Fe-S system at 400°C. and b. at 300°C (after Craig and Scott, 1979). 26 Figure 2.4. Structure of coveilite, CuS. 27 Chapter 3 Figure 3.1. Cu vs Fe plot of covellites fkom Chuquicamata. 41 Figure 3.2. Cu-Fe-S diagram showing fields of covellite and blue-remaining coveibte £Yom Chuquicamata- 42 Figure 3.3. Cu vs Fe plot showing Fe content of digeaites from Chuquicamata. 45 Figure 3.4. Cu-Fe-S plot showing digenites from Chuquicamata 46 Figure 3.5. Zn vs Cu plot showing variation in sphalente compositions from Chuquicamata. Figure 3.6 Sample cu205. DDH 2967,5 14.90m. Reflected light photograph of a euhedral pyrite grain surromded by massive digenitecovellite. 66 Figure 3.7 a. Sample cu192. DDH 2967,325.93m. Refiected light photograph of €me-grained sub-rounded thin-lamellar grains of coveiiite disseminated in vein sulphate. 70 Figure 3.7 b. Sample cu214. DDH 2967.21 1-60m. Reflected light photograph of typical sample of supergene ore. 71 Fiewe 33.. Sample cu204. DDH 2967.5 l3.OSm. Reflected iight photograph of pyrite grains with spaces hfiied by curved tabular wolframite. 72 Figure 3.9 a. Sample cu441. DDH 2242, 145.32m. Reflected iight photograph of a sample of fault gouge. 75 Figure 3.9 b. Sample cu467. DDH 2242,261.27m. Reflected light photograph of massive vein digenite with rims of colusite. 76 Figure 3.10 a. Sample cu476. DDH 2242,288.10m. Refiected light photograph of bornite and digenite in an exsolution relationship. 79 Figure 3.10 b. Sample cu49 1. DDH 2234,306.79m. Reflected light photograph of bornite and - digenite in a replacement relationship. 80 a Figure 3.1 1. Sample cu450. DDH 2242, 16 1.98rn. Reflected light photograph of semi-massive coveUite + idaite. 85 Figure 3.12 a. Sample cu455. DDH 2242, 193.86m. Reflected light photograph of coveliite replacing chalcopyrite. 86 Figure 3.12 b. Sample cu494. DDH 2234,219.84m. Reflected light photograph of metastable assemblage covellite + chalcopyxite. 87 Figure 3.13. Sample cu507. DDH 2234, 113.6%~ Reflected light photograph of possible supergene sphalerite. 92 Figure 3.14. Sample cu433. DDH 2242,56.45m. Plane iight photograph of a cut section of drill core through a %lue vein'. 93 Figure 3.15. Chuquicamata cross-section 450N,with alteration zones. 102 Figure 3.16. Distribution of covellite-digenite assemblages in section 4500N. 103 Figure 3.17. Distribution of sphalente assemblages in section 4500N. 104 Figure 3.1 8. Distribution of chalcopyrite assemblages in section 4500N. IO5 Figure 3.19. Inferred (ore) minerd paragenesis for section 4500N. Chapter 4 Figure 4.1. Cu-Fe-S system at 300'~isotherm. 111 Figure 4.2. Estimated temperatures of rnineralisation for assemblages in section 4500N. 116 Chapter 5 Figure 5.1. Schematic diagram of a basic mass spectrometer. 122 Figure 5.2. Plot of sulphate and sulphide 6% perd values vs delta (A) values of coexisting suiphate-sulphide pairs from Chuquicamata nd El Salvador. 128 - Figure 5.3. Estimate of 6% value for total sulphur for the Chuquicamata system 130 Figure 5.4 a Stability fields of sulphur phases in a pH-log f, diagram at 250°c, and b. 6% values of sulphides and suiphates at 25OoC. 132 Figure 5.5. Mole bctions of aqueous sulphur species relative to total suiphur content plotted on a ph-log fO2diagram. 133 LIST OF TABLES Chapter 2 Page Table 1. Minerals and phases of the Cu-S system. 20 Table 2. Minerals and phases of the Cu-Fe-S system. 21 Chapter 3 Table 3. Chuquicamata, Section 4500N. Table 4. Chemical composition of hypogene coveliite from Chuquicamata. Table 5. Chernical composition of blaubleibender coveilite from Chuquicamata. Table 6. Chemical composition of digenite from Chuquicamata. Table 7. Chemicai composition of enargite frorn Chuquicamata. Table 8. Chemical composition of bornite from Chuquicamata, Table 9. Analyses of possible anomalous bomites fiom Chuquicamata Table 10. Chemical composition of chalcopynte £rom Chuquicamata. Table 1 1. Chemicai composition of sphalerite fkom Chuquicamata. Table 12. Chemical composition of pyrite fiom Chuquicamata. Table 13. Chemicai composition of colusite fkom Chuquicamata. Table 14. Chemicai composition of idaite from Chuquicamata. Table 15. Distribution of ore minerais in section 4500N. Chapter 4 Table 16. Summary of invariant points for the condensed Cu-Fe-ssystem. Chapter 5 Table 17. Sulphur isotope results as 6% values for sulphides and sulphates from Chuquicamata. 125 Table 18. Regression analysis for suiphide-sulphate data points from Chuquicamata. ABSTRACT Covellite (CuS) has traditionally been described as a mineral formed by supergene oxidation processes, although its occurrence as a hypogene mineral is weU documented. However. it may be difficult to distinguish hypogene covellite fIom that of supergene origin without consideration of associated der& and geologic setthg. Also, the mineralogical relationships sunounding covellite in the Cu-Fe-S system are poorly constrained with respect to temperature, particularly above 250°C. This thesis was plaaned as a fkt step towards the characterisation of coveliite. attempting to develop practical criteria for the differentiation of hypogene from supergene covellite in the Chuquicamata porphyry copper deposit, Chile. Approximately 120 samples of ore were collected hm3 drillholes that intersect the major alteration zones (e.g. potassic, quartz-sencitic, chloritic), withm tbe 4500N coordmate section, recornmended by mine staff to be representative of the deposit. The petrology and mineralogy of the samples were studied using reflected light microscopy, x-ray diffraction, and the electron microprobe. Both supergene and hypogene coveliite are prominent ore minerals in the studied section of Chuquicamata This study indicates that hypogene and supergene covellite differ both texturally (in - habit and mineral assemblages), and compositiondy with respect to minor constituents (specifically iron content). Hypogene coveUites from Chuquicamata contain signincant Fe, which varies systematicaily fiom O to < 5 weight %, and appears to indicate crystallisation dong the coveiiite-idaite tie line. The Fe content of hypogene coveaite has not been adequately documented before. In contrast, supergene covellite in the studied samples rarely deviates fiom the stoichiornetric composition CuS. The hypogene sulphide assemblages were studied in the context of experimentailydetermined sulphide phase equilibria to pose constraints on temperatures of ore deposition.
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