The Butte Magmatic-Hydrothermal System: One Fluid Yields All Alteration and Veins

摘要

Mine exposures and deep drilling reveal that the Butte porphyry Cu-Mo ores occur in two, internally zoned domes with alteration assemblages containing biotite, K-feldspar, sericite, and chlorite. A third body of pervasive quartz-sericite-pyrite alteration lies between and overlaps the Cu-Mo-mineralized domes, and has roots that extend to a depth of more than 2 km. The porphyry system is cut by large, throughgoing fissure veins containing copper, zinc, lead, and manganese. Each of the two Cu-Mo-mineralized domes has concentric zones of widely overlapping shells of centimeter-scale stockwork veinlets. The deepest and innermost zone consists of barren quartz veinlets that grade upward to a thick zone of quartz-molybdenite veinlets that cut veins of the next outward zone, consisting of quartz-sulfide veinlets with biotite-K-feldspar-sericite-andalusite alteration envelopes. The biotite-dominant alteration grades upward to a shell of abundant quartz-magnetite-chalcopyrite-pyrite veins with pale green inner envelopes of sericite-K-feldspar-chlorite bordered by biotite-bearing outer envelopes. Farther upward, the outer envelopes change from biotitic to chloritic, and veins of quartz-pyrite-chalcopyrite with chlorite-sericite-K-feldspar envelopes become abundant, intermixed with pyritic veins with gray sericite and an outer envelope with remnant primary biotite dispersed in sericite-pyrite that has replaced primary feldspars. In the outermost of the concentric shells, biotite crackle veins contain pyrite and epidote, and millimeter-scale veinlets containing sphalerite and galena are bordered by K-feldspar-bearing propylitic alteration. Between the two domes, a zone of intense pervasive sericitic alteration formed around a stockwork of pyrite-quartz veinlets bordered by inner envelopes of gray sericite-pyrite-quartz and an outer selvage of sericitized feldspars and remnant primary biotite.