Date of Award
Master of Science
The Summitville high-sulfidation gold deposit is hosted by a volcanic dome consisting of the South Mountain Quartz Latite that was erupted 23 Ma ago during formation of the Platoro Caldera complex in the San Juan volcanic field of south central Colorado. Alteration and mineralization developed during or shortly after dome emplacement as a result of metal-rich magmatic fluids and vapor emanating from a crystallizing intrusion at depth. Copper, arsenic, silver and gold are enriched in the deposit with two of the last paragenetic stages, the barite-base metal sulfide and goethite stages, containing the highest gold grades. Barite contains magmatic sulfur with a range of δ34S values (19.3 – 31.8 ‰) that reflects SO42– – H2S isotopic equilibration over a temperature range of 115 to 180 °C, which is consistent a calculated temperature of 147 °C based on the sulfur isotopic fractionation between intergrown barite and galena. Barite δ18O values (19.3 – 31.8 ‰) indicate the barite fluids contained a minor component of meteoric water. Barite growth zones with acicular crystals (wires) of emplectite (CuBiS2), a mineral not previously reported from Summitville, contained primary fluid inclusions that yielded a wide range of vapor to liquid homogenization temperatures as a result of stretching but consistent freezing point depressions that infer an average salinity of 5.1 wt. % NaCl equivalent. These data indicate that the barite fluids were modestly enriched in magmatic sulfur and metals (Cu, As, Bi, Pb, Zn, Au, Ag) and that barite precipitation was likely triggered by cooling during mixing with meteoric waters. The goethite stage developed during weathering of the deposit that likely occurred during uplift and exposure around 9 to 7.7 Ma ago based on 40Ar/39Ar dates on jarosite. The assemblage goethite + hematite ± jarosite ± scorodite ± gold filled open spaces and coated barite of the barite-base metal sulfide stage. Inclusions of barite and bladed alunite in the iron oxide/hydroxide (FeOx) matrix have the texture, composition and isotopic values of their earlier formed counterparts in the deposit and therefore represent residual material that survived the weathering event. Three 40Ar/39Ar dates on the bladed alunite ranged from 23.15 to 22.88 Ma, confirming their origin as magmatic hydrothermal alteration alunite. Layers of compositionally zoned, small rhombohedral crystals of alunite were also discovered in the FeOx matrix. Although these could not be successfully dated, they had a distinct texture, chemistry and isotopic composition (δ34S = 0.8 ‰ δ18O = 4.7 ‰) that indicated they formed during the weathering event. The acidic, oxidizing supergene fluids remobilized and broadly enriched the top of the deposit in Cu and Ag and locally in native gold (5 wt. % Ag).
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