Genesis of acid-sulfate alteration and Au-Cu-Ag mineralization at Summitville

The Summitville Au-Cu-Ag deposit occurs within a porphyritic quartz latite, primarily in a zone of "vuggy silica " alteration characterized by leaching of all major elements except silica and iron. Vuggy silica is best developed between an elevation of 3,700 and 3,500 m where it occurs in irregular pipes and lenticular pods up to 70 m wide and is enclosed by up to 30 m of intense quartz-alunite alteration. The vuggy silica zones diminish in size with decreasing elevation and rarely exceed a width of 2 m below an elevation of 3,400 m. These deep vuggy silica zones are enclosed by quartz-kaolinite alteration with little or no alunite. Sulfide mineralization occurs primarily within the vuggy silica. The sulfide assemblage changes with decreasing elevation from covellite + luzonite _ native gold to chalcopyrite q- tennantite. Sulfides occur lining voids or in microveinlets and are accompanied by minor kaolinitc but not by alunite. Gold also occurs in a near-surface barite q- goethite q- jarosite assemblage that crosscuts the vuggy silica. Consideration of alunite, kaolinitc, and pyrite stability relations and of aluminum specia-tion at 250øC indicates that a pH of 2 or below and a logfo2 of-31 _ 1 (at 250øC) wer