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

The Summitville Au-Cu-Ag deposit occurs within a porphyritic quartz latite, primarily in a zone of 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 + or - native gold to chalcopyrite + 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 + goethite + jarosite assemblage that crosscuts the vuggy silica.Consideration of alunite, kaolinitc, and pyrite stability relations and of aluminum speciation at 250 degrees C indicates that a pH of 2 or below and a log f (sub O 2 ) of -31 + or - 1 (at 250 degrees C) were necessary to produce vuggy silica alteration. The chalcopyrite + tennantite mineralization occurring at depth in the vuggy silica zone is indicative of less oxidizing and less acidic conditions. The covellite + luzonite + or - gold assemblage requires conditions intermediate between those necessary for vuggy silica alteration and for deep sulfide mineralization. Barite + gold mineralization is interpreted as a near-surface assemblage produced under highly acidic, oxidizing conditions.These textural and geochemical relationships indicate that sulfide and gold mineralization postdate the formation of vuggy silica alteration in the deposit and are not directly related to the highly acidic solutions necessary for this early acid-leaching event. The change from extreme acid-leaching to copper sulfide and precious metal deposition may represent a transition from a magmatic-vapor-dominated to a liquid-dominated system, leading to more reducing and less acidic conditions during sulfide deposition.