THERMOBAROMETRY OF THE CASTING COPPER SKARN, NV, BASED ON APATITE INCLUSIONS IN GARNET

This study presents the first application of mineral-inclusion thermobarometry to a hydrothermal system. 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. We use Raman spectroscopy to measure the pressure within mineral inclusions, because the magnitude of the peak shift is proportional to the pressure within the inclusion. In this study, we analyzed samples from the Casting Copper skarn in the Yerington district, NV, which contain abundant apatite within andraditic garnet. Raman spectroscopy was used to determine the internal pressures of these inclusions and these pressures were interpreted using an elastic model to account for the compressibility/expansivity of apatite and garnet. The data show that apatite inclusions from Casting Copper are mostly under tension when analyzed at ambient conditions, and the elastic model restores the inclusions to pressures of <100MPa at an inferred trapping temperature of ~500°C, consistent with previous thermobarometric estimates for the Casting Copper skarn. The good performance of this method suggests that mineral-inclusion thermobarometry represents a viable new method for thermobarometry of hydrothermal rocks.