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University of Alberta Geochronology, Petrography, Geochemical Constraints, and Fluid Characterization of the Buriticá Gold Deposit, Antioquia Department, Colombia. by Guillaume Lesage A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Master of Science Earth and Atmospheric Sciences ©Guillaume Lesage Fall, 2011 Edmonton, Alberta Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author’s prior written permission. Abstract Buriticá is a low- to intermediate-sulfidation epithermal gold deposit located 75 km north of Medellín, in Colombia. It is hosted by the Buriticá andesite porphyry, a shallow intrusive rock dated at 7.41 ± 0.40 Ma (40Ar/39Ar on hornblende), which intrudes the Cretaceous Cañasgordas Group and Buriticá stock. Gold mineralization is associated with a proximal sericite-adularia and a distal epidote-rich propylitic alteration, and is hosted in two different sets of veins respectively striking and dipping around 072°/87°S and 105°/87°S. Hydrothermal sericite yields an age of 7.74 ± 0.08 Ma. Sulfur isotope and fluid inclusion data indicate that a hot and saline fluid (>300°C, average 5.5 wt.% NaCl equiv.) of magmatic origin mixed with a cooler and less saline meteoric fluid. Boiling conditions were observed in quartz crystals coeval with the timing of gold mineralization, therefore boiling is believed to be the main control on gold precipitation. Acknowledgements I would like to thank Jeremy Richards, whose supervision and advice during this project have been very important and much appreciated. Jeremy was very patient with me as I was perfecting my English writing skills, and his comprehensive reviews of my manuscript greatly improved it. Field and research expenses were financed by Continental Gold of Colombia Inc. and in part by the Society of Economic Geologists. 40Ar/39Ar geochronological analyses were conducted by Terry Spell, from at the Nevada Isotope Geochronology Laboratory. Sulfur isotope analyses were conducted by Stephen Taylor, from the Isotope Science Laboratory of the University of Calgary. X-ray diffraction analyses were conducted by Diane Caird, from the University of Alberta. Lithogeochemical analyses were conducted at Activation Laboratories Ltd. in Ancaster, Ontario. Finally, fluid inclusion analyses were conducted by myself at the University of Alberta. Thanks Jeremy for letting me use your lab. Thanks to all the Continental Gold geologists who helped me while I was mapping in the field: Stuart Moller, Alejandro Murillo, Jimmy Torres, Mauricio Cruz. Elkin—thank you for having carried my heavy samples around in the mountains. My fellow graduate students and close friends for their scientific input, their positive energy, and for the poker nights: Jean-François Gagnon, Neil Fernandes, Alberto Reyes, Stefan Lalonde, Tyler Hauck, Kurt Borth, Catherine Johnson, Shawna White, Hilary Corlett, Jamie Kraft, and all the others. I would also like to mention all those people who contributed to this project by making my life in Edmonton more fun and interesting, namely Marie- Maude Pontbriand, Francis O’Brien, Jonathan Lavoie-Lapointe, Gabriel Trahan, Alexandra Prescott, Audrey Roy, Cybelle Morin. Thanks for being there. Table of Contents 1. Introduction 1 2. Tectonic History of Colombia 4 3. Ore Deposits of Colombia 14 4. Buriticá Historical Background 16 5. Regional Geologic Setting 17 6. Buriticá Deposit Geology 21 6.1. Volcanic and sedimentary rocks 21 6.2. Plutonic rocks 24 6.3. Breccia 30 6.4. Alteration 32 6.5. Structural setting 36 6.6. Mineralized veins 39 6.7. Mineralization Paragenesis 40 7. Analytical Methods 46 7.1. X-ray diffraction methods 46 7.2. Lithogeochemical methods 46 7.3. Geochronology methods 47 7.4. Sulfur Isotope Methods 48 7.5. Fluid Inclusion methods 49 8. Results 50 8.1. Lithogeochemistry 50 8.2. Geochronology 57 8.3. Sulfur Isotope Analyses 60 8.4. Fluid Inclusions 61 9. Discussion 70 9.1. Timing and tectonic setting of the Buriticá porphyry and gold deposit 70 9.2. Characteristics of ore-forming fluids in the Buriticá gold deposit 72 9.2.1. Importance of boiling for gold mineral precipitation 73 9.3. Classification of the Buriticá gold deposit 74 9.4. Comparison with other late Miocene gold deposits in Colombia 76 9.4.1. La Colosa porphyry gold deposit characteristics 76 9.4.2. Marmato epithermal gold deposit characteristics 78 9.4.3. Similarities with the Buriticá epithermal gold deposit 80 9.5. Models for formation of late Miocene gold deposits in Colombia 81 9.6. Mineral exploration implications 82 10. Conclusion 83 References 85 Appendix A: Sample Descriptions and Locations 99 Appendix B: Petrographic Descriptions 111 Appendix C: X-Ray Diffraction Data 126 Appendix D: Structural Data 130 Appendix E: Lithogeochemical Data 136 Appendix F: Ar-Ar Data 148 Appendix G: Fluid Inclusion Data 152 List of Tables Table 7-1. Measured standard data. 49 Table 8-1. Sulfide crystallization temperatures calculated from sulfur isotope fractionation between coeval galena and sphalerite. 60 Table 8-2. Comparative table of fluid inclusion homogenization temperatures (Stage 1 average per sample) vs. sulfur isotopes crystallization temperatures for the same samples. 69 Table 9-1. Middle Cauca metallogenic belt deposit summary table. 77 List of Figures Figure 1-1. Geological provinces of South America, and geographic locations, main deposit types, and sizes of major gold belts. 2 Figure 1-2. Geological provinces of Colombia and geographic locations of major Miocene gold deposits. 3 Figure 2-1. Litho-tectonic map of the Northern Andes. 5 Figure 2-2. Paleogeographic reconstruction of the Archean cratons between the Neoproterozoic and the Late Carboniferous. 7 Figure 2-3. Schematic cross-sections depicting the closure of a back- arc basin. 9 Figure 2-4. Major Mesozoic and Cenozoic intrusive rocks of Colombia. 13 Figure 3-1. Metallogenic belts of Colombia. 15 Figure 4-1. Underground geology map of the Yaragua mine level 1. 18 Figure 5-1. Regional geology map of the Buriticá area and its surroundings. 20 Figure 6-1. Detailed geology map of the Buriticá area. 22 Figure 6-2. Barroso Formation outcrop photos. 24 Figure 6-3. Buriticá stock outcrop photos. 26 Figure 6-4. Buriticá stock hand sample photos and thin section photomicrographs. 28 Figure 6-5. Buriticá andesite porphyry hand sample photos and thin section photomicrographs. 29 Figure 6-6. Yaragua breccia hand sample photos. 31 Figure 6-7. Photos of alteration facies in the Buriticá stock and andesite porphyry in hand specimen. 33 Figure 6-8. Transmitted light photomicrographs of alteration facies in the Buriticá stock and andesite porphyry. 34 Figure 6-9. Photos of structural features observed in the Buriticá area. 37 Figure 6-10. Structural data measured in the Buriticá area, plotted on the lower hemisphere of an equal-area stereographic net. 38 Figure 6-11. Mineralized vein paragenesis. 40 Figure 6-12. Transmitted light photomicrographs of Stage 1 sphalerite crystals showing color zonation from dark brown and yellow to late light yellow sphalerite. 41 Figure 6-13. Photos of the San Antonio and Centena veins in hand specimen. 42 Figure 6-14. Reflected light photomicrographs of Stage 1 and Stage 2 vein minerals. 43 Figure 6-15. Reflected light photomicrographs of Stage 3 vein minerals. 45 Figure 8-1. Total alkali (Na2O + K2O) versus silica diagram. 51 Figure 8-2. Harker diagrams. 52 Figure 8-3. Chondrite-normalized REE diagram. 54 Figure 8-4. Primitive mantle-normalized extended trace element diagram. 56 Figure 8-5. Apparent 40Ar/39Ar age spectra for two samples of igneous hornblende. 58 Figure 8-6. Apparent 40Ar/39Ar age spectra for two samples of hydrothermal sericite. 59 Figure 8-7. Transmitted light photomicrographs of primary fluid inclusions. 63 Figure 8-8. Histogram showing the first melting temperature distribution for fluid inclusions from different host minerals. 65 Figure 8-9. Histograms showing homogenization temperatures and salinity distribution for fluid inclusions from different host minerals. 66 Figure 8-10. Scatter plot of fluid inclusion assemblage average homogenization temperatures vs. average salinity for Stage 1 to Stage 3. 67 Figure 8-11. Scatter plot of homogenization temperatures vs. salinity for fluid inclusions pertaining to four fluid inclusion assemblages from Stage 1 and Stage 2. 68 Figure 9-1. La/Yb versus Sc/Ni diagram showing fields for different tectonic settings. 71 List of Symbols and Abbreviations Symbol Description °C Degree Celsius 1s.d. One standard deviation 40Ar* Radiogenic argon a Fractionation factor Adu Adularia Ag Native silver amu Atomic mass unit Ap Apatite Ar Argon Au Native gold Aug Augite Bi Biotite Cal Calcite CF-EA-IRMS Chl ChloriteContinuous flow-isotope ratio mass spectrometry cm Centimeter Cpy Chalcopyrite CRFS Cauca-Romeral Fault System deg Degree detect. Detection Ep Epidote equiv. Equivalent FUS-ICP Fusion-inductively