A Study of Ore Genesis and Geochronology in the Sub-Volcanic Tin Belt of the Eastern Andes, Bolivia by J.N.M
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A STUDY OF ORE GENESIS AND GEOCHRONOLOGY IN THE SUB-VOLCANIC TIN BELT OF THE EASTERN ANDES, BOLIVIA by John Nigel Murray Grant A thesis submitted for the degree of Doctor of Philosophy in the University of London. Department of Mining Geology, Imperial College of Science and Technology, London, England December 1978. (I) A study of Ore Genesis and Geochronology in the Sub-Volcanic Tin Belt of the Eastern Andes, Bolivia by J.N.M. Grant ABSTRACT Most of the important mineral deposits in the southern part of the Bolivian tin belt are vein and disseminated ore systems associated with acid igneous eruptive complexes of Tertiary age. Evidence from geological, fluid inclusion and stable isotope studies suggest a genetic model involving an initially shallow magmatic supply of heat, fluid and ore components in the porphyry- and breccia-filled volcanic vents. Patterns of early pervasive rock alteration and dispersed ore mineral 'deposition were controlled by mixing of the high- temperature fluid with cool dilute water of probable meteoric origin. Later, as hydrothermal activity declined at the high level, the major vein structures were formed and tapped the deeper magmatic source. Hydrothermal circulation and ore deposition were then confined to the veins, which cross-cut the earlier patterns of pervasive alteration and mineralization. K-Ar dating of the complexes shows that their ages fall into two groups. The northern group (north of Potosi) give ages of approximately 19 Ma to 23 Ma, while the southern group give 12 Ma to 17 Ma. The ages of the northern group overlap with . previously published ages for the southernmost batholiths of the northern tin belt showing that there is not, as pre- viously believed, a major time break between them. The data show that the various metal associations of the eruptive complexes are not consistently related to their age, but appear t0 depend on the level in the crust at which the deposits were formed. This suggests that the metal zonation displayed by the tin belt as a whole, from tin-tungsten in the north to tin-silver-lead-zinc in the south, reflects the depth of formation of the deposits and hence the variation in the level of erosion, rather than changes in magma composition. CONTENTS Page Abstract List of Figures List of Tables List of Plates Acknowledgements Abbreviations and Symbols CHAPTER 1. REGIONAL GEOLOGY AND MINERALIZATION OF THE CENTRAL ANDES 1.1. The Morpho-Tectonic Units 1 1.2. Tectonics and Magmatism 6 1.2.1. Pre-Andean tectonics and magmatism 8 1.2.2. Andean tectonics and magmatism 10 1.3. Mineralization 23 1.3.1. Pre-Andean mineral deposits 24 1.3.2. Andean mineralization 25 CHAPTER 2. GEOLOGY OF THE BOLIVIAN TIN BELT AND SUB-VOLCANIC TIN DEPOSITS 2.1. General Geology and Mineralization 28 2.1.1. Geology 28 A. The northern section 30 B. The southern section 33 2.1.2. The mineral deposits 40 A. The metallogenic province 40 B. The mineral deposits of the northern section 112 C. The mineral deposits of the southern section 45 2.2. Detailed Studies of the Geology and Mineralization of Selected Deposits 48 2.2.1. Introduction 48 2.2.2. Chorolgue A. Regional setting 50 B. Rock types 51 C. Alteration types 56 D. Alteration zoning 6i (III) CHAPTER 2 (continued). Page E. Mineralization and metal zoning 63 2.2.3. Llallagua A. Regional setting 71 B. Rock types 72 C. Alteration 78 D. Mineralization 79 E. Alteration and mineral zoning 83 2.2.4. Potosi 83 A. Rock types 84 B. Alteration and mineralization 86 2.2.5. Colquechaca A. Introduction, geological setting 87 B. Alteration and mineralization 90 2.2.6. Other deposits 91 A. The San Pablo-Morococala area 91 B. Chocaya 96 C. Tatasi 98 D. Isca-Isca 100 E. Tasna 100 2.3. Discussion 101 2.3.1. Hydrothermal brecciation 101 2.3.2. Hydrothermal alteration 108 A. Pervasive alteration, types and zoning 108 • B. Vein wall-rock alteration 113 2.3.3. The chemistry of some altered and unaltered rocks A. Introduction 115 B. The whole-rock analyses 115 C. Results 117 D. Semi-quantitative analyses 130 CHAPTER 3. FLUID INCLUSION STUDIES 3.1. Introduction 135 3.2. Principles of Fluid Inclusion Study 136 3.2.1. Mechanisms of trapping of primary inclusions 137 (Iv) CHAPTER 3 (continued). Page 3.2.2. The distinction between primary, secondary and pseudo-seconcary inclusions 138 3.2.3. Leakage, and other changes since trapping 141 3.2.4. Temperature-pressure relationships and the homogenization method of microthermometry 144 3.2.5. Fluid salinity, depression of freezing point and daughter minerals 151 3.2.6. The composition of fluid inclusions 153 3.3. Experimental Methods 151E 3.3.1. Sample preparation 155' 3.3.2. Homogenization temperature determination 156 3.3.3. Freezing temperature determination 159 3.3.4. Analytical procedures 160 A. Non-destructive tests of daughter minerals 161. B. On-stage optical and chemical tests• on extracted daughter minerals 161 C. Electron-probe microanalysis 162 D. The scanning electron microscope 163 E. Crushing and leaching 167 F. Laser microprobe analysis 169 3.4. Results 175 3.4.1. Chorolque 175 A. Material studied 175 B. Classification of inclusion types 176 C. Distribution of inclusion types 179 D. Temperature and salinity data 181 E. Chemical data 190 1. Daughter mineral identification 190 2. General chemical data 193 a) The scanning electron microscope data 194 b) Data from crushing and leaching 196 c) Laser microprobe data 197 3.4.2. Llallagua A. Previous investigations 199 (v) CHAPTER 3 (continued). Page B. Material studied 201 C. Classification and distribution of inclusion types 201 D. Temperature data 205 3.4.3. Potosi 209 A. Material studied and inclusion types 209 B. Results 211 3.4.4. Other deposits 215 3.5. Discussion 218 CHAPTER 4. OXYGEN AND HYDROGEN ISOTOPE STUDY 4.1. Introduction 228 4.2. Principles 229 4.3. Analytical Methods 233 4.4. Samples Studied, and Results 235 4.5. Discussion 241 4.5.1. Surface water data 241 4.5.2. Clay data 244 4.5.3. Biotite data 245 4.5.4. Quartz-tourmaline alteration and vein data 246 4.5.5. Sericite data 248 4.6. Conclusions 251 CHAPTER 5. A GEOCHRONOLOGICAL STUDY OF THE SOUTHERN PART OF THE BOLIVIAN TIN BELT 5.1. Introduction 256 5.2. Previous Geochronological Data 256 5.3. Location and Description of Samples 258 5.4. The Principles of the Potassium-Argon Method of Age Determination 259 5.4.1. The K-Ar age equation 260 5.4.2. Extraneous argon 263 5.4.3. Argon loss 265 A. Thermal disturbance 267 B. Hydrothermal alteration 269 (vi) CIiAPTER5 (continued). Page 5.5. Experimental Methods 272 5.5.1. Sample preparation 272 5.5.2. Analytical methods 273 A. Potassium analysis 273 B. Argon isotopic analysis 274 5.6. Precision and Resolution 275 5.7. Results 279 5.7.1. San Pablo-Morococala region 279 5.7.2. Llallagua, the Salvadora stock 285 5.7.3. Colquechaca 289 5.7.4. The Potosi region 292 5.7.5. The mineralized igneous centres of the Quechisla region 298 5.8. Discussion 302 5.8.1. General considerations 302 5.8.2. Geochronology of the southern tin belt 306 CHAPTER 6. SUMMARY AND CONCLUSIONS 6.1. Origin of the Tin Belt 312 6.2. Geochronology of the Southern Part of the Tin Belt 318 6.3. A genetic Model for the Sub-Volcanic Tin Deposits 320 6.4. The Transport and Deposition of Tin in Hydrothermal Systems 328 APPENDICES I. Petrographic Descriptions of Analysed Rocks 332 II. Calculations in K-Ar Dating 340 REFERENCES 349 (vi i) LIST OF FIGURES CHAPTER 1 Page 1.1. The central Andes, morpho-tectonic units 2 1.2. The central Andes, schematic cross-section 5 1.3. Plate configuration, South American region 7 1.4. Major igneous provinces of the Eastern Cordillera of the central Andes, with locations of relevant age data 11 1.5. Tectonic segmentation of the Andes 14 1.6. Age relationships of igneous rocks in the transect across the Andes between 26o and 290 south latitude 19 1.7. Metallogenic belts of the central Andes 26 CHAPTER 2 2.1. The Bolivian tin belt, general location map 29 2.2. The northern tin belt, location map 31 2.3. The southern tin belt, location map 34 2.4. Schematic cross-sections. Mineralized eruptive complexes of the southern tin belt 36 2.5. Chorolque, general geology map 52 2.6. Chorolque, geological cross-section 53 2.7. Chorolque, plan of alteration zones 62 2.8. Chorolque, underground rock geochemistry traverse, results A. Tin 66 B. Silver 67 C. Bismuth 68 D. Copper 69 2.9. Llallagua, the Salvadora stock, geological plan and cross-section 73 2.10. Potosi, the Cerro Rico stock, geological cross-section 85 2.11. Colquechaca, surface geology 89 2.12. San Pablo stock, surface geology 93 2.13. San Pablo-Morococala region, surface geology 95 CHAPTER 2 (continued). Page 2.14. Chocaya, surface geology 97 2.15. Tatasi, surface geology 99 2.16. Alkali-silica diagrams, unaltered rocks 120 2.17. Composition variations, altered and unaltered rocks 123 2.18. K/Ca variation, altered and unaltered rocks 124 2.19. ACF diagram, altered and unaltered rocks 126 2.20. AKF diagram, altered and unaltered rocks 127 2.21. Sn/Ag variation, altered and unaltered rocks 1.33 2.22. Sn/Zn variation, altered and unaltered rocks 134 CHAPTER 3 3.1. P-T diagram for H2O 147 3.2.