2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 22
Presentation Time: 9:00 AM-6:00 PM


RUSSIN, Teresa Z., Quaternary Resource Investigations, L.L.C, 13588 Florida Blvd, Baton Rouge, LA 70819 and FIFAREK, Richard H., Department of Geology, Southern Illinois University, Carbondale, IL 62901-4324, terrirussin@yahoo.com

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. % NaCl­eq). Barite from late barite+galena +sphalerite±gold veins analyzed for this study has a range of δ34S values (19.3 - 31.8 ‰) that reflects SO42- -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. % NaCl­eq.

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.