Endoskarn and Cu-Zn mineralization at the Empire mine, Idaho, USA

Abstract

The Empire mine is a Cu-Zn skam associated with the granite porphyry phase of the Mackay Stock, which consists of quartz monzodiorite, granophyre, granite porphyry, Mackay Granite, and numerous dikes. Both granite porphyry and Mackay Granite have high F and also have unusual, extremely vermicular quartz phenocrysts. Both endoskarn and exoskam are developed at the Empire mine, with more endoskam than exoskarn. The alteration of the intrusive rocks began with weak disseminated diopsidic pyroxene, actinolite, and titanite. Further endoskam formed by veins or as massive replacements of intrusive rocks. The earliest formed endoskarn veinlets contain scapolite, with or without wollastonite halo. This was followed by wollastonite-dominant (± Carich plagioclas and hedenbergitic pyroxene) veins as fronts or envelopes on garnet-dominant veins. Early pyroxene is diopsidic whereas pyroxene in distal/late veinlets is hedenbergitic. Similarly, garnet becomes more Fe-rich with time. In exoskam, all the pyroxene is diopsidic and garnet andraditic. Magnetite precipitated after garnet-pyroxene in both endoskarn and exoskam. Zn sulfide precipitated together with Cu in proximal locations, associated with retrograde quartz+calcite+chlorite. Massive endoskarn and exoskarn replacement formed at 500–550°C, whereas slightly higher temperatures were recorded by late minerals at the metasomatic front, >600°C. The highest temperatures, >700°C, occur in gamet-dominant veins that probably represent conduits insulated by earlier skarn. During retrograde alteration, quartz, calcite, chlorite, fluorite, and chalcopyrite precipitated in both endoskarn and exoskarn at 250–300°C. The extremely vermicular texture of quartz phenocrysts, abundant endoskarn, and proximal deposition of Zn, are all caused by the high F contents of the magma and magmatic fluid.