TEMPORAL-SPATIAL ZONING OF HYDROTHERMAL ALTERATION MINERALOGY AND GEOCHEMISTRY OF QUARTZ-ALUNITE-HOSTED EPITHERMAL AU-CU-AG ORE DEPOSITS AT SUMMITVILLE, COLORADO

The Summitville deposit is a well-studied example of a quartz-alunite-hosted Au-(Cu-Ag) epithermal ore deposit with 559 Koz Au historic production. Prior studies have documented the advanced argillic alteration mineralogic zonation around central quartz-rich ribs that contain gold and local enargite/luzonite or covellite. Our mineralogy studies expand upon previous work and document the zoning of advanced argillic altered zones >1 km laterally and >1.2 km vertically below the ore bodies. Kaolin-group clays are zoned laterally outward from central dickite-pyrophyllite ± nacrite proximal to the Au-Cu-Ag orebodies, to intermediate kaolinite-dickite ± nacrite, and to outer kaolinite in a pattern consistent with hydrothermal fluids cooling during outward flow laterally from the ore zone. The deep advanced argillic zones contain quartz-pyrophyllite-topaz to 1,400 m below the surface or >1.2 km below the ore bodies. Petrographic and SEM imaging and EMPA data demonstrate that earlier pyrophyllite was partly replaced by later muscovite along the {001} basal plane of the sheet silicate structure. Locally, zunyite (Al₁₃Si₅O₂₀(F,OH)₁₈Cl) also forms overgrowths atop and replaces earlier pyrophyllite and topaz (Al₂SiO₄(F,OH)₂). The replacement of pyrophyllite-topaz by muscovite ± zunyite indicate the later addition of K⁺ and Cl^- to stabilize the new minerals from a more neutral pH fluid compared to the magmatic-hydrothermal fluids that formed earlier advanced argillic alteration.

Whole-rock geochemical analyses indicate the addition of many trace metals (Ba, As, Cs, Sn, Tl, and W) in addition to K around the Au-Cu-Ag ore zones. Because all these metals are chloride (Cl-) or bisulfide (HS-) complexed and can be transported in late, low salinity aqueous solutions this is the likely ore fluid, whereas the early magmatic gases are incapable of such metal transport. We confirmed the pathfinder elements (i.e., As, Ba, Cs, Sn, Tl, and W) are contained in late-formed muscovite and zunyite via LA-ICP-MS spot analyses. Therefore, there is compelling evidence that late chloride-bearing fluids also transported common pathfinder elements, and formed muscovite and zunyite in the deposit, and by inference also likely transported and deposited Au-(Cu-Ag) in ores as native gold with local enargite-luzonite and covellite.