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Volcanic Stratigraphy, Alteration Zoning, and Vein Paragenesis of the Sascha-Pelligrini Low- Sulphidation Epithermal System, Santa Cruz, Argentina

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Volcanic Stratigraphy, Alteration Zoning, and Vein Paragenesis of the Sascha-Pelligrini Low- Sulphidation Epithermal System, Santa Cruz, Argentina Queensland University of Technology School of Natural Resource Sciences Volcanic Stratigraphy, Alteration Zoning, and Vein Paragenesis of the Sascha-Pelligrini Low- Sulphidation Epithermal System, Santa Cruz, Argentina By Quinn Eric Smith B. App. Sc. (QUT) 2009 Supervisor: Assoc. Prof David Gust A Thesis submitted for the degree of Master of Applied Science (Queensland University of Technology) KEYWORDS Epithermal, Low-sulphidation, Gold-Silver, Vein Paragenesis, Deseado Massif, Chon Aike, Santa Cruz, Argentina I Abstract The Sascha-Pelligrini low-sulphidation epithermal system is located on the western edge of the Deseado Massif, Santa Cruz Province, Argentina. Outcrop sampling has returned values of up to 160g/t gold and 796g/t silver, with Mirasol Resources and Coeur D‟Alene Mines currently exploring the property. Detailed mapping of the volcanic stratigraphy has defined three units that comprise the middle Jurassic Chon Aike Formation and two units that comprise the upper Jurassic La Matilde Formation. The Chon Aike Formation consists of rhyodacite ignimbrites and tuffs, with the La Matilde Formation including rhyolite ash and lithic tuffs. The volcanic sequence is intruded by a large flow-banded rhyolite dome, with small, spatially restricted granodiorite dykes and sills cropping out across the study area. ASTER multispectral mineral mapping, combined with PIMA (Portable Infra- red Mineral Analyser) and XRD (X-ray diffraction) analysis defines an alteration pattern that zones from laumontite-montmorillonite, to illite-pyrite- chlorite, followed by a quartz-illite-smectite-pyrite-adularia vein selvage. Supergene kaolinite and steam-heated acid-sulphate kaolinite-alunite-opal alteration horizons crop out along the Sascha Vein trend and Pelligrini respectively. Paragenetically, epithermal veining varies from chalcedonic to saccharoidal with minor bladed textures, colloform/crustiform-banded with visible electrum and acanthite, crustiform-banded grey chalcedonic to jasperoidal with fine pyrite, and crystalline comb quartz. Geothermometry of mineralised veins constrains formation temperatures from 174.8 to 205.1°C and correlates with the stability field for the interstratified illite-smectite vein selvage. Vein morphology, mineralogy and associated alteration are controlled by host rock rheology, permeability, and depth of the palaeo-water table. Mineralisation within ginguro banded veins resulted from fluctuating fluid pH II associated with selenide-rich magmatic pulses, pressure release boiling and wall-rock silicate buffering. The study of the Sascha-Pelligrini epithermal system will form the basis for a deposit-specific model helping to clarify the current understanding of epithermal deposits, and may serve as a template for exploration of similar epithermal deposits throughout Santa Cruz. III CONTENTS INTRODUCTION ........................................................................................... 1 REGIONAL GEOLOGICAL SETTING ............................................................................................ 3 SASCHA-PELLIGRINI EPITHERMAL SYSTEM ............................................................................... 7 METHODS ..................................................................................................... 9 FIELD INVESTIGATIONS ........................................................................................................... 9 SAMPLE PREPARATION AND ANALYTICAL TECHNIQUES ............................................................ 10 Short Wave Infrared (SWIR) spectrometry ................................................................... 10 X-ray Diffraction ............................................................................................................. 11 Microscopy .................................................................................................................... 12 Geochemistry ................................................................................................................ 13 Remote sensing ............................................................................................................. 14 RESULTS .................................................................................................... 16 STRATIGRAPHY .................................................................................................................... 16 PETROGRAPHY .................................................................................................................... 21 Chon Aike Formation ..................................................................................................... 21 La Matilde Formation ..................................................................................................... 22 Other units ..................................................................................................................... 23 GEOCHEMISTRY ................................................................................................................... 26 STRUCTURAL SETTING .......................................................................................................... 33 EPITHERMAL VEINING ........................................................................................................... 36 Quartz textures .............................................................................................................. 36 VEIN GEOCHEMISTRY AND MINERALOGY ................................................................................ 39 GEOTHERMOMETRY ............................................................................................................. 42 ALTERATION ........................................................................................................................ 43 Regional Alteration – Multispectral Mineral Mapping .................................................... 43 Prospect Alteration – PIMA and XRD............................................................................ 46 ALTERATION GEOCHEMISTRY ................................................................................................ 55 DISCUSSION ............................................................................................... 61 VOLCANOLOGY .................................................................................................................... 61 Depositional setting ....................................................................................................... 61 Eruption styles ............................................................................................................... 63 Magma Petrogenesis..................................................................................................... 68 HOST ROCK CONTROL AND STRUCTURAL MODEL .................................................................. 70 ALTERATION ZONING ............................................................................................................ 74 VEIN PARAGENESIS .............................................................................................................. 82 SUPERGENE OVERPRINT ...................................................................................................... 88 SUMMARY ............................................................................................................................ 89 CONCLUSION ............................................................................................. 93 REFERENCES ............................................................................................ 96 APPENDIX 1.............................................................................................. 109 APPENDIX 2.............................................................................................. 115 APPENDIX 3.............................................................................................. 119 APPENDIX 4 (MAP) .................................................................................. 121 APPENDIX 5 (MAP) .................................................................................. 122 IV List of Figures Figure 1. Conceptual models and genetic classifications of epithermal deposits .................... 3 Figure 2. Location of the Sascha-Pelligrini study area. ............................................................ 6 Figure 3. Sample and prospect locations ................................................................................. 7 Figure 4. Comparison of accepted results for rock standard SEQG566 with returned analysis. ...................................................................................................................14 Figure 4. Stratigraphy for the Sascha-Pelligrini study area. ...................................................19 Figure 5. Interpretive geology, structure and mapped veining for the Sascha-Pelligrini study area. ...............................................................................................................20 Figure 7. XRF major element results for Sascha-Pelligrini volcanic samples ........................29 Figure 8. ICP-MS trace element results for Sascha-Pelligrini volcanic samples. ..................30 Figure 9. REE spider diagrams for Sascha-Pelligrini volcanic samples. ...............................31 Figure 10. REE geochemistry normalised to upper crust, Bajo Pobre and lower crust xenoliths ................................................................................................................32
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