Abstract

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.

Geochemical modeling of reaction of a magmatic fluid with Butte granite between 600° and 200°C at 1 kbar shows that the full array of alteration types—from high-temperature biotite-feldspar-andalusite to low-temperature sericite-pyrite-quartz and the main stage advanced argillic and sericitic alteration—could all have formed from a single initial fluid composition as it cooled and reacted with wall rock. The high-temperature model fluid is pH neutral and in equilibrium with biotite, feldspar, and andalusite. It is transformed upon cooling as disproportionation of aqueous SO2 makes the fluid increasingly acidic. As the acidic fluid is neutralized by rock reaction, it yields all of the observed alteration assemblages in the observed spatial order at the scale of vein envelopes and the scale of the hydrothermal system as a whole. The key implication of this finding is that one single initial magmatic fluid composition is chemically capable of producing all of the alteration and mineralization features observed in the Butte system. Thus, a single fluid composition may form porphyry copper deposits in some settings and shallow epithermal veins in others, or both ore types superimposed in single districts. The universally observed sequence of vein cutting relations in porphyry copper deposits—i.e., high-temperature quartz-chalcopyrite veins with biotite-K-feldspar envelopes cut by moderate-temperature pyrite veins with sericitic alteration, and both of these cut by low-temperature veins containing covellite and enargite with quartz-kaolinite alteration—is likely to be an inevitable consequence of cooling of a single fluid type, not a consequence of successive distinct fluid compositions expelled from an igneous source.

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