| The Empire mine is a Cu-Zn skarn associated with the granite porphyry phase of the Mackay Stock, which consists of quartz monzodiorite (50.4 +/- 1.8 Ma), granophyre (48.8 +/- 0.8 Ma), granite porphyry (48.2 +/- 0.7 Ma), Mackay Granite (47.3 +/- 0.9 Ma), and numerous dikes. Both granite porphyry and Mackay Granite have high F content and 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. The consequent endoskarn formed by veins replacing the intrusive rocks or as massive replacements. The earliest formed veinlets are filled with Na and Cl-rich 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. The temperature at the metasomatic front was >600°C and in the garnet-dominant veins >700°C. The massive endoskarn replacement formed at lower temperature, 500--550°C, at similar temperature as massive exoskarn. 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 precipitates after garnet-pyroxene in both endoskarn and exoskarn. During retrograde alteration, quartz, calcite, chlorite, fluorite, and chalcopyrite precipitate in both endoskarn and exoskarn at 250--300°C. Zn sulfide precipitates together with Cu in proximal locations.; The unusual features of the deposit, namely the extremely vermicular texture of quartz phenocrysts, abundant endoskarn, and proximal deposition of Zn, are all associated with the high F content of the magma and resulting processes during the magmatic-hydrothermal transition. This is explained by a model involving equilibrium among three coexisting phases, melt, quartz phenocrysts, and magmatic hydrothermal fluid, resulting in corrosion of the quartz phenocrysts which is facilitated by the high F in the fluid. High F in the magma also lowers 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. |
