US Geological Survey


Date of this Version



Published in Chemical Geology 215 (2005)


The Summitville Au–Ag–Cu deposit is a classic volcanic dome-hosted high-sulfidation deposit. It occurs in the Quartz Latite of South Mountain, a composite volcanic dome that was emplaced along the coincident margins of the Platoro and Summitville calderas at 22.5±0.5 Ma, penecontemporaneous with alteration and mineralization. A penecontemporaneous quartz monzonite porphyry intrusion underlies the district and is cut and overlain by pyrite–quartz stockwork veins with traces of chalcopyrite and molybdenite. Alteration and mineralization proceeded through three hypogene stages and a supergene stage, punctuated by at least three periods of hydrothermal brecciation. Intense acid leaching along fractures in the quartz latite produced irregular pipes and lenticular pods of vuggy silica enclosed sequentially by alteration zones of quartz–alunite, quartz–kaolinite, and clay. The acidsulfate- altered rocks host subsequent covellite + enargite/luzonite + chalcopyrite mineralization accompanied by kaolinite, and later barite–base-metal veins, some containing high Au values and kaolinite. The presence of both liquid- and vapor-rich fluid inclusions indicates the episodic presence of a low-density fluid at all levels of the system. In the mineralized zone, liquid-rich fluid inclusions in healed fractures in quartz phenocrysts and in quartz associated with mineralization homogenize to temperatures between 160 and 390 °C (90% between 190 and 310 °C), consistent with the range (200–250 °C) estimated from the fractionation of sulfur isotopes between coexisting alunite and pyrite. A deep alunite–pyrite pair yielded a sulfur-isotope temperature of 390 °C, marking a transition from hydrostatic to lithostatic pressure at a depth of about 1.5 km. Two salinity populations dominate the liquid-rich fluid inclusions. One has salinities between 0 and 5 wt.% NaCl equivalent; the other has salinities of up to 43 wt.% NaCl equivalent. The occurrence of high-salinity fluid inclusions in vein quartz associated with mineralization, as well as in the deep stockwork veins, suggests that brines originating deep in the system transported the metals.

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