Pressures of skarn mineralization at Casting Copper, Nevada, USA, based on apatite inclusions in garnet

Thermobarometry of metasomatic rocks is commonly challenging, owing to the high variance of hydrothermal mineral assemblages, thermodynamic disequilibrium, and overprinting by subsequent hydrothermal episodes. Here we estimate formation pressures of a Cu-Fe-sulfide–bearing andradite-diopside skarn deposit at Casting Copper (Yerington district, Nevada, USA) using Raman spectroscopy and elastic modeling of apatite inclusions in garnet. Andradite garnet from the Casting Copper skarn contains inclusions of hydroxylfluorapatite, calcite, hematite, magnetite, and ilmenite. Raman spectroscopy reveals that the apatite inclusions are predominantly under tension of ~–23 to −123 MPa at ambient conditions. Elastic modeling of apatite-in-garnet suggests that entrapment occurred at ~10–115 MPa, assuming a trapping temperature of ~400 °C, which is consistent with paleodepth estimates of ~2–3 km. These results provide independent constraints on the conditions of hydrothermal skarn formation at Casting Copper, and suggest that this approach may be applied to other, less-well-constrained skarn systems.