PRESSURES OF SKARN MINERALIZATION AT CASTING COPPER, NEVADA, BASED ON APATITE INCLUSIONS IN GARNET

This study presents the first application of mineral-inclusion thermobarometry to a hydrothermal system. Thermobarometry of metasomatic rocks is commonly challenging, owing to the high variance of hydrothermal mineral assemblages, thermodynamic disequilibrium and overprinting by subsequent hydrothermal episodes. As mineral inclusions and their corresponding host minerals are exhumed to the surface and cool to ambient temperatures, volume perturbations may be different owing to differences between their physical properties. The difference in compressibility and thermal expansivity between the inclusion and its host results in the development of pressurization of the inclusion. Raman spectroscopy was used to quantify the internal pressures of each inclusion because Raman spectra are pressure-dependent. The internal pressures of these inclusions were applied to an elastic model which accounts for the different rates of expansion of the minerals involved as a function of changes in pressure and temperature to provide us with the original pressure conditions under which the mineral inclusion was enclosed. In this study, we estimate formation pressures of a Cu-Fe-sulfide-bearing andradite-diopside skarn deposit at Casting Copper (Yerington district, NV) using Raman spectroscopy and elastic modeling of apatite inclusions in garnet. Andradite garnet from the Casting Copper skarn contains inclusions of hydroxyl-fluorapatite, 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 suggest entrapment occurred at ~10 to 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-constrained skarn systems.