Geochemistry of Fluid Inclusions and Hydrothermal Alteration Vein- and Fracture-Controlled Mineralization, Stockwork '• Molybdenum Deposits
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GEOCHEMISTRY OF FLUID INCLUSIONS AND HYDROTHERMAL ALTERATION VEIN- AND FRACTURE-CONTROLLED MINERALIZATION, STOCKWORK '• MOLYBDENUM DEPOSITS by MARK STEPHEN BLOOM BSc, New Mexico Institute of Mining and Technology, 1972 MSc, New Mexico Institute of Mining and Technology, 1975 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES Department - of Geological Sciences We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA February 1983 © Mark Stephen Bloom, 1983 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Geological Sciences The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date 16 April, 1983 Abstract Molybdenum mineralization and coextensive alteration at Questa, Hudson Bay Mountain, and Endako occur in single/composite veinlets which exhibit distinctive alteration assemblages and paragenesis. Fluid inclusion populations from each ve'inlet type are compositionally distinct. Early fluorine rich, biotite-stable alteration is associated with hypersaline brines. These inclusions homogenize most frequently by halite dissolution at temperatures from 350°-600°+C. Molybdenum miner alization also coincides with quartz-sericite-pyrite alteration and inclusions having moderate to high salinity and lower (£350°C) temperatures of entrapment. Ubiquitous low-salinity inclusions in addition to inclusion types which characterize each veinlet type suggest superposition of meteoric-dominated convective circulation. These data are evidence for magmatic fluids, separate in both space and time from ingress of meteoric hydrothermal solutions. Hypersaline brines were precursors to those solutions which precipitated distinctive alteration asso• ciations, and formed by boiling/condensation of fluids released by episodic fracturing and ensuing adiabatic decompression. Highly variable and complex compositional zoning charac• terizes (K,Na)-feldspar, biotite, and muscovite solid solutions Although zoning within individual grains is common, composition al trends in the averaged compositions can be correlated with both position in the emplacement sequence and textural charac• teristics of these alteration phases. Alkali feldspars coexist ing in veinlets constitute sensitive geothermometers which pro- vide reliable depositional temperatures (£350°C). X(annite) is highest in magmatic biotite, intermediate in biotite which exhi• bits replacement textures, and significantly lower0 in veinlet assemblages. X(fluorphlogopite) systematically increases from magmatic to veinlet biotite associations; f(H20/HF) ratios com• puted from these compositions predict log f(HF) of approximately -4.0. Departures from muscovite stoichiometry persist among different veinlet assemblages and textural associations, but were not sampled with enough regularity to define trends in spa• tial disposition. Alteration geochemistry indicates that mineralization oc• curred from solutions equilibrated with veinlet assemblages. Fluid-mineral equilibria computations imply chemical character• istics of the ore-forming solutions similar to those measured in high-temperature geothermal fluids. Molybdenite solubility shows extreme dependence on temperature, pH, f(02), and f(HF). Molybdenum concentrations up to lOOppm are predicted in oxidized solutions at 350°C. In near-neutral solutions significant amounts of molybdenum are transported as Mo03F" and HMoO,", with lesser amounts as H2MoOft° and Mo02*. Chloride and sulfide com• plex concentrations are not significant. Mo03F" becomes the primary transporter of molybdenum in acid solutions and high f(HF). Table of Contents Abstract ii List of Tables vii List of Figures : viii Acknowledgements x Chapter I CHEMISTRY OF INCLUSION FLUIDS: STOCKWORK MOLYBDENUM DEPOSITS FROM QUESTA, NEW MEXICO, AND HUDSON BAY MOUNTAIN AND ENDAKO, BRITISH COLUMBIA 1 INTRODUCTION 1 GEOLOGY OF QUESTA, HUDSON BAY MOUNTAIN, AND ENDAKO 2 COGENETIC INTRUSIVE ROCKS 3 ALTERATION AND MINERALIZATION 4 1. Fracture Control 4 2. Crosscutting Relations 5 a. Pre-molybdenite Quartz 6 b. Quartz-K-feldspar-Biotite 6 c. Quartz-MoS2/Quartz-Sericite-Pyrite-MoS2 7 FLUID INCLUSION PETROLOGY 8 INTRODUCTION 8 CLASSIFICATION AND DESCRIPTION OF FLUID INCLUSIONS 10 DISTRIBUTION OF THE FLUID INCLUSIONS 12 HOMOGENIZATION DATA 14 INTRODUCTION 14 QUESTA, NEW MEXICO 15 HUDSON BAY MOUNTAIN, BRITISH COLUMBIA 17 ENDAKO, BRITISH COLUMBIA 18 INTERPRETATION OF THE FLUID INCLUSION DATA 19 SALINITIES OF INCLUSION FLUIDS 19 ESTIMATES OF PRESSURE AND DEPTH 22 TEMPERATURE AND SALINITY DISPERSION 30 PRESSURE CORRECTION 32 SOURCE AND EVOLUTION OF THE INCLUSION FLUIDS 33 EVOLUTION OF INCLUSION POPULATIONS 36 CONCLUSIONS 42 Chapter II GEOCHEMISTRY OF HYDROTHERMAL ALTERATION 44 INTRODUCTION 44 ALTERATION PETROGRAPHY 45 RELATIVE AGE RELATIONS 45 TEXTURAL VARIATIONS 46 V METHOD OF INVESTIGATION AND ANALYTICAL TECHNIQUES 49 DATA PRESENTATION FOR MULTICOMPONENT SOLID SOLUTIONS ..52 INTRODUCTION 52 COMPONENTS AND COMPOSITIONAL SPACE .....53 1. Dioctahedral Layer Silicates 55 2. Trioctahedral Layer Silicates 58 ACTIVITY-COMPOSITION RELATIONS 61 ALKALI FELDSPARS 61 SCHEELITE-POWELLITE 65 WHITE MICA 67 TRIOCTAHEDRAL LAYER SILICATES 68 DISCUSSION 69 BINARY SOLID SOLUTIONS 69 MUSCOVITE/SERICITE SOLID SOLUTIONS 72 BIOTITE SOLID SOLUTION 76 CONCLUSIONS 86 Chapter III THEORETICAL PREDICTION OF FLUID-MINERAL EQUILIBRIA 89 INTRODUCTION 89 THEORETICAL CONSIDERATION OF THE THERMODYNAMIC MODEL ..90 THERMOCHEMICAL DATA AND CONVENTIONS 90 COMPUTATION OF FLUID CHEMISTRY 93 LIMITATIONS OF THE MODEL 95 ASSERTIONS AND PHYSICAL DESCRIPTION OF THE MODEL 95 TEMPERATURE-PRESSURE 96 SALINITY, IONIC STRENGTH, AND A(H20) 97 THERMODYNAMIC COMPONENTS AS CONSTRAINTS FOR SOLUTE SPECIES 98 DISCUSSION OF FLUID-MINERAL EQUILIBRIA 105 ACTIVITY RATIOS OF SOLUTE SPECIES 105 1. Volatile Species 105 2. [a(K+)/a(H+)] and [a(Fe2 +)/a(H+)2] 107 3. [a(Mg2+)/a(H+)2] and [a(Ca2 +)/a(H+)2] 109 4. [a(Al3 + )/a(H+)3] and a(H,SiO,) 114 AQUEOUS MOLYBDENUM SPECIATION 117 ABSOLUTE SOLUTE CONCENTRATIONS 127 1. Hydrogen Ion Concentration 129 2. Error Propogation , 129 3. Molybdenum Concentration and Precipitation Mechanisms .130 CONCLUSIONS 135 LITERATURE CITED 137 vi APPENDIX A - MEAN ELECTRON MICROPROBE ANALYSES OF HYDROTHERMAL ALKALI FELDSPARS 147 APPENDIX B - MEAN ELECTRON MICROPROBE ANALYSES OF HYDROTHERMAL TRIOCTAHEDRAL MICAS 152 APPENDIX C - MEAN ELECTRON MICROPROBE ANALYSES OF HYDROTHERMAL DIOCTAHEDRAL MICAS 159 APPENDIX D - MEAN ELECTRON MICROPROBE ANALYSES OF HYDROTHERMAL AMPHIBOLES FROM HUDSON BAY MOUNTAIN 162 APPENDIX E - MEAN ELECTRON MICROPROBE ANALYSES FOR SCHEELITE-POWELLITE FROM HUDSON BAY MOUNTAIN 163 vii List of Tables I. Average WDS electron microprobe analyses of solid so• lution phase standards 51 II. Solid solution descriptors 56 III. Mean WDS electron microprobe analyses of hydrothermal white mica solid solutions 57 IV. Mean WDS electron microprobe analyses of hydrothermal biotite solid solutions 59 V. Activity-composition relations of natural solid solu• tions 62 VI. Mean WDS electron microprobe analyses of hydrothermal alkali feldspar solid solutions 63 VII. Mean WDS electron microprobe analyses of scheelite solid solutions from Hudson Bay Mountain 66 VIII. Thermochemical data for molybdenum and fluorine- bearing minerals 92 IX. Dissociational equilibria for thermodynamic components and expressions for solute activities 100 X. Thermochemical data for gases and aqueous molybdenum species 119 XI. Computed fluid-mineral equilibria at 350°C 128 XII. Error propogation in the distribution of aqueous spe• cies using the Monte Carlo method ; 131 vi i i List of Figures 1. Fluid Inclusion Types Observed in Fracture-Controlled, Molybdenum-Mineralized Veinlets and Stockworks 11 2. Fluid Inclusion Homogenization (filling) Temperatures 16 3. Temperature-Salinity Determinations for Types A, C, and D Inclusion Fluids 20 4. Compositions of Hypersaline Fluids and the "Halite Trend" for Type D Inclusion Fluids 23 5. Vapor Pressure Estimates for Specific Type C and D Inclusions or Inclusion Pairs 24 6. Fluid Inclusion Populations from Questa (B), Hudson Bay Mountain (A), and Endako(C) 26 7. Solubility Curve Approximating the Questa and Hudson Bay Mountain Halite Trends in the NaCl-KCl-H20 System .37 8. Temperature-Pressure Diagram Showing the Effect of Isenthalpic Cooling 41 9. Distribution of End Members in Binary Solid Solutions 70 10. Distribution of End Members in Hydrothermal White Mica Solid Solution 73 11. Distribution of End Members' in Hydrothermal Biotite Solid Solution 77 12. Compositional Spaces for Hydrothermal Mica Solid Solutions 82 13. Phase relations in the system MoS2-FeS~FeS2-H20 in the presence of steam-saturated aqueous solution at 350°C 106 14. Phase relations in the system K20-A1203-Si02-H20-HF in the presence of quartz and steam-saturated aqueous solu• tion at 350°C 108 15. Phase Relations in the System K20-Al203-Si02-FeO-MgO- H20-H2S-HC1-HF in the Presence of Quartz and Steam- Saturated Aqueous Solution at 350°C 110 16. Phase