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Ore Genesis. in Copper Bearing Carbonatites; A

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Ore Genesis. in Copper Bearing Carbonatites; A ORE GENESIS. IN COPPER BEARING CARBONATITES; A GEOCHEMICAL, FLUID INCLUSION AND MINERALOGICAL STUDY BY RICHARD T. H. ALDOUS, B.Sc., M.I.M.M. A Thesis submitted for the Degree Philosophiae Doctor Mining Geology Division, Department of Geology, Imperial College of Science and Technology, University of London. 1980 ABSTRACT Trace amounts of copper are not uncommon in documented carbonatites. Unusually high concentrations of copper are rare, but when present occur mostly in deeply eroded complexes. Copper sulphide occurrences and parageneses in carbonatites are classified into three groups. Detailed studies of the carbonatite copper deposit at Palabora suggest mineralization occurred autometasomatically in a continuum between carbgnatite met and residual fluids at temperatures between 600'C and 200 C and low f0.7. The lack of primary aqueous inclusions in Palabora apatites is unusual for a carbonatite, they do however contain crystal- lographically orientated copper sulphide inclusions. These are thought to have grown epitaxially and indicate the presence of copper in the apatite forming media. Their dis- tribution varies with rock type. Rare earths show that the apatites are different generations in a system developing higher total rare earths and light RE/heavy RE enrichment. Melt inclusions in phoscorite olivines indicate that the earliest carbonatite magma at Palabora was unusually enriched in copper. It is concluded that copper was an integral part of the Palabora magmatism and not introduced from an extraneous source. Primary inclusions in pyroxene from a copper rich pyro- xenite diatreme at Palabora are crystalline silicate melts, with a residual portion of aqueous Cu and K rich Cl /CO brine. Secondary aqueous inclusions contain large daughters of chalcopyrite which indicate an initial fluid concentration of 3000ppm Cu. Copper bearing carbonate rich aqueous fluids, which developed from the crystallization of the pyroxenite melt, were probably precursors of copper deposition in the pyroxenite diatreme. A copper bearing sulphide, apatite, pyroch1ore, phlogo- pite, magnetite (SAPPM) assemblage from the Sokli carbonatite in Finland of hydrothermal metasomatic origin is defined and described. Geochemical and petrographic studies of the assemblage suggest a hydrothermal metasomatic origin. Fluid inclusions in apatite from the assemblage are alkali (Na:K = approx. 2:1) bicarbonate/chloride brines deduced to be the parent fluids of the assemblage. Comparison of homogenization temperatures with temperature estimates from other geother- mometers implies a formation pressure of 4 kilobars for these deposits. The nature and origin of phoscorites is reviewed in the light of melt inclusions and alteration chemistry from studies at Palabora, Sokli and Bukusu (Uganda). The theoretical aspects of copper accumulation and deposition in carbonatites are discussed. ACKNOWLEDGEMENTS I would like to express my sincere thanks to Dr. A.H. Rankin for his supervision, help and guidance throughout the course of this work. Other members of staff from the Royal School of Mines also provided valuable advice, instruction or assistance, including:- Prof. R. Davis, Dr. S. Parry, Dr. C. Halls, Dr. R. Parker, P. Suddaby, N. Wilkinson, R. Curtis, B. Foster, E. Morris and P. Watkins. I am grateful to Palabora Mining Company and Rautaruukki Oy for making samples available and for their generous hospitality. I should particularly like to thank H. Vartiainen for his help. Thanks are also due to the following who provided samples for the study:- Dr. J. Baldock, S. Eriksson, Dr. A. Woolley, the Department of Geology, Leeds University, The Ugandan Geological Survey and Goldfields of South Africa. I thank the Natural Environment Research Council - who financed this work, Blue Circle Industries who printed the black and white plates and Mrs. D. Babbage for typing the final copy. My deepest thanks go to my wife, who not only typed the draft, but also translated numerous Russian papers. My Wife, Children and Parents provided an enormous amount of material and non-material support without which this work would not have been possible. ii Table of Contents Page INTRODUCTION 1 CHAPTER 1. A REVIEW OF THE OCCURRENCE AND BEHAVIOUR OF COPPER IN THE FORMATION OF CARBONATITES 7 1.1. THE OCCURRENCE OF COPPER AND SULPHUR IN CARBONATITES 7 1.1.1. The Sulphide Mineralogy of Carbonatites 9 A. Iron Sulphides, Pyrrhotite and Pyrite 10 B. Copper Sulphides 11 C. Pentlandite and Millerite 12 D. Molybdenite 15 E. Lead and Zinc Sulphides 15 F. Polymetallic Sulphides, Sulphosalts and Native Elements 16 1.1.2. Carbonatite Complexes with Pronounced Copper Enrichment 17 A. Copper Mineralization at Palabora 19 B. Bukusu, Uganda 19 C. Sokli, Finland 20 D. Glenover, South Africa 21 E. Kovdor, Kola Peninsula 21 F. Vuori Yarvi 22 G. Beloziminsk 22 1.1.3. A Classification of Copper Bearing Sulphide Parageneses 23 1.2. THE ORIGIN OF COPPER AND SULPHUR IN CARBONATITES 26 1.2.1. The Origin of the Copper 26 1.2.2. The Origin of the Sulphur 30 1.2.3. The Behaviour of Copper and Sulphur in Silicate Melts and Carbonatites 30 iii Page 1.3. CONTROLS AND P.T.V.X. OF ORE FORMING CARBONATITE FLUIDS 35 1.3.1. Fluid Inclusions 36 1.3.2. Experimental Work 39 1.3.3. Geochemistry 40 1.3.4. Sulphur Isotopes, T and f02 41 1.3.5. Structural Control 45 1.4. SUMMARY AND CONCLUSIONS 46 CHAPTER 2. THE CARBONATITE COPPER DEPOSIT AT PALABORA, SOUTH AFRICA 48 2.1. INTRODUCTION AND BACKGROUND GEOLOGY 48 2.1.1. Introduction 48 2.1.2. The General Geology 48 2.1.3. The Loolekop Pipe 51 2.1.4. The Mineralization at Loolekop 54 2.1.5. The Aim of the Present Work 57 2.2. ORE MICROSCOPY 58 2.2.1. Previous Work 58 2.2.2. The Present Work 60 2.2.3. The Relationship between Sulphides and Palabora Carbonatites 61 2.2.4. Sulphide Behaviour in Transgressive Carbonatite Dykes 72 2.2.5. Implications of Sulphide Carbonate Relationships 73 2.2.6. The Sulphide Paragenesis 75 2.2.7. The Oxide Minerals 82 2.2.8. The Ore Paragenetic Sequence 85 2.3. GEOCHEMISTRY OF THE SULPHIDE ASSEMBLAGE 87 2.3.1. Present Work 87 2.3.2. Deductions from Published Figures 89 2.4. THE ENVIRONMENT OF FORMATION OF PALABORA SULPHIDES 91 2.4.1. Temperature and Depth 91 2.4.2. Mineral Equilibria 91 iv Page 2.4.3. Magnetite-Ilmenite Pairs 93 2.4.4. Conditions of fS2 and f02 97 2.5. SUMMARY AND CONCLUSIONS 98 2.5.1. Summary 98 CHAPTER 3. INCLUSIONS AND RARE EARTH ELEMENTS IN PALABORA APATITES 100 3.1. INTRODUCTION ,100 3.2. APATITE DISTRIBUTION 100 3.3. TYPES OF INCLUSION IN THE APATITE 102 3.3.1. Aqueous Inclusions 102 3.3.2. a) Irregular Shaped Non Aligned Solid and b) Multisolid Inclusions 103 3.3.3. Elongate Inclusions Aligned Parallel to the C-Axis of the Apatite 103 3.3.4. Explanation and Deductions from Crystallographically Controlled Types 112 3.4. RARE EARTHS IN PALABORA APATITES 114 3.4.1. Previous Work 114 3.4.2. The Present Work 116 3.4.3. Results 116 3.5. SUMMARY AND CONCLUSIONS 123 CHAPTER 4. MELT INCLUSIONS IN OLIVINE AT PALABORA 126 4.1. INTRODUCTION 126 4.2. THE INCLUSIONS 126 4.3. PREVIOUS REPORTS OF MELT INCLUSIONS IN CARBONATITES 128 4.4. COMPOSITION OF THE INCLUSIONS 131 4.5. HEATING STUDIES • 133 4.6. SIMILAR INCLUSIONS FROM OTHER COMPLEXES (THIS WORK) 136 Page 4.7. CONCLUSIONS 136 CHAPTER 5. COPPER RICH AQUEOUS ALKALI CARBONATE INCLUSIONS IN PYROXENES FROM THE GUIDE COPPER MINE, PALABORA 140 5.1. INTRODUCTION 140 5.1.1. The Sulphide Assemblage 142 5.2. INCLUSIONS TYPES 146 5.2.1. Type 1: Solid Inclusions 146 5.2.2. Type 2: Secondary Aqueous Inclusions • 146 5.2.3. Type 3: Aqueous Multisolid Inclusions 146 5.2.4. Inclusions in Orthoclase 150 5.2.5. Similar Inclusions from the main Pyroxenite at Palabora 150 5.3. IDENTIFICATION OF DAUGHTER AND TRAPPED PHASES 151 5.4. HEATING STUDIES 153 5.5. INCLUSION COMPOSITION AND IMPLICATIONS 157 5.6. CONCLUSIONS 161 CHAPTER 6. HYDROTHERMAL ORE DEPOSITION AT THE SOKLI CARBONATITES, FINLAND 164 6.1. INTRODUCTION 164 6.2. THE GEOLOGY OF SOKLI 166 6.2.1. General Geology and Previous Work 166 . 6.2.2. The Present Work 167 6.3. SULPHIDES AT SOKLI 183 6.4. APATITE AS AN ACCESSORY MINERAL AT SOKLI 188 6.5. THE SULPHIDE APATITE PHLOGOPITE (YROC ORE MAGNETITE ASSEMBLAGE(S) 191 6.5.1. Mode of Occurrence 191 6.5.2. Petrography of the SAPPM Assemblage 192 • vi Page 6.5.3. Similar Assemblages in Other Complexes 200 6.6. PYROCHLORE 201 6.6.1. The Occurrence of Pyrochlore 201 6.6.2. Pyrochlore Chemistry 203 6.6.3. Discussion of Results and Comparison with other Complexes 210 6.7. TEMPERATURE OF FORMATION OF THE SAPPM ASSEMBLAGE '212 6.7.1. The Sulphides 212 6.7.2. Ilmenite/Magnetite Pairs from the SAPPM Assemblage 213 6.7.3. Geobarometry using the Sphalerite- Pyrrhotite-Pyrite Equilibrium 215 6.7.4. Proposed Temperature of Formation 215 6.8. A GEOCHEMICAL EVALUATION OF THE SAPPM ASSEMBLAGE 218 6.8.1. Objectives 218 6.8.2. Methods of Sample Selection and Analysis 219 6.8.3. The Results 220 6.8.4. Correlation Matrices for Boreholes 274 and 275 227 6.8.5. Conclusions reached from the Analytical Study 228 6.9. SUMMARY AND CONCLUSIONS 229 CHAPTER 7. INCLUSIONS IN APATITE FROM THE SAPPM ASSEMBLAGE, SOKLI 231 7.1. INTRODUCTION 231 7.2. THE INCLUSION-TYPES IN SOKLI APATITES 232 7.2.1. Aqueous/Vapour Inclusions - Type 1 232 • 7.2.2. Monophase Aqueous Inclusions - Type 2 237 7.2.3. Monophase Gas Inclusions - Type 3 237 7.2.4.
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