SKARN AND MINERALIZATION AT THE EMPIRE MINE, IDAHO

The Empire mine is a Cu-Zn skarn 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 exoskarn are developed at the Empire mine, with more endoskarn than exoskarn. The alteration of the intrusive rocks began with weak disseminated diopsidic pyroxene, actinolite, and titanite. Further endoskarn formed by veins or as massive replacements of intrusive rocks. The earliest formed exoskarn veinlets contain scapolite, with or without wollastonite halo. This was followed by wollastonite-dominant (± Ca-rich plagioclase 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 exoskarn, all the pyroxene is diopsidic and garnet andraditic. Magnetite precipitated after garnet-pyroxene in both endoskarn and exoskarn.

Massive endoskarn and exoskarn replacement formed at 500-550 ºC, whereas slightly higher temperatures were recorded by late minerals at the metasomatic front, >600 ºC. The highest temperatures, >700 ºC, occur in garnet-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 ºC. Zn sulfide precipitated together with Cu in proximal locations.

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. This is modeled as follows: during rapid ascent of the magma, F-rich water bubbles exsolve and adhere to quartz phenocrysts, thereby accelerating their dissolution at bubble contacts and producing a vermicular texture. High F in the magma also lowered the solidus temperature of the melt, producing low-temperature magmatic fluid from which Zn precipitated in proximal locations. High F in hydrothermal fluids also promotes the dissolution of silicates, which allows abundant endoskarn formation.