ORIGIN OF THE LATE AURIFEROUS BARITE-BASE METAL SULFIDE AND GOETHITE-HEMATITE STAGES AT THE SUMMITVILLE HIGH-SULFIDATION CU-AU-AG DEPOSIT, COLORADO

The Summitville high-sulfidation Cu-Au-Ag deposit is hosted by a quartz latite volcanic dome near the margin of the Platoro-Summitville caldera complex in SW Colorado. Previous studies suggest that early acid-sulfate alteration formed at 200 - 250 ºC from magmatic vapor condensate and was followed by main stage enargite+covellite+gold mineralization deposited at 200 - 275 ºC from saline magmatic fluids (~10 - 25 wt. % NaCl­_eq). Barite from late barite+galena +sphalerite±gold veins analyzed for this study has a range of δ³⁴S values (19.3 - 31.8 ‰) that reflects SO₄^2- -H₂S isotopic equilibration of magmatic sulfur (~0 ‰) at 130 to 175 °C. Barite δ¹⁸O values (8.3 - 13.9 ‰) indicate the late stage fluids were still dominantly magmatic in origin but contained a component of meteoric water. Growth zones in barite defined by emplectite (CuBiS₂) wires have primary L-V fluid inclusions that yield unreliable homogenization temperatures but consistent freezing point depressions corresponding to an average salinity of 5.1 wt. % NaCl­_eq.

Weathering generated a goethite+hematite±jarosite±scorodite±gold overprint in the upper part of the deposit. ⁴⁰Ar/³⁹Ar dates on jarosite suggest that exposure to surficial waters had occurred by ~ 9 to 7 Ma. Barite and bladed alunite inclusions in goethite+hematite represent residual mineral fragments; as confirmed by alunite ⁴⁰Ar/³⁹Ar dates of 23.38 to 22.68 Ma that match published dates for alteration and dome emplacement. Alunite also is present in goethite+hematite as small (<50 μm), zoned, rhombohedral crystals that have an isotopic composition (δ³⁴S = 0.8 ‰; δ¹⁸O = 4.7 ‰) consistent with a weathering origin.

The late barite-base metal sulfide±gold stage was triggered by cooling and oxidation of high-sulfidation magmatic fluids due to mixing with meteoric water. The diluted fluids (~5 wt. % solutes) contained Ba, Fe, Cu, Pb, Zn, Bi, Ag, Au and sulfate and sulfide sulfur. After preservation by younger volcanics for ~15 M years, the deposit was modified by acidic, oxidizing supergene fluids that remobilized and broadly enriched the top of the deposit in Cu and Ag and locally in native gold (5 wt. % Ag). Overall, the amount of gold deposited decreased during the evolution of the deposit but dramatically increased in coarseness and grade.