MICROTHERMOMETRY OF FLUID INCLUSIONS IN THE EMPIRE MOUNTAINS SKARNS, SIERRA NEVADA, CA: IMPLICATIONS FOR FLUID δ18O VALUES AND ORE FORMATION

Skarn deposits at Empire Mountain in the south-central Sierra Nevada batholith have variable, low δ¹⁸O values, that suggest extensive infiltration of meteoric water throughout all stages of the hydrothermal system (D’Errico et al., 2012, Geology). As one of the shallowest recognized hydrothermal systems from the Mesozoic Cordillera, the Empire Mountain skarns permit a detailed study of shallow fluid flow in North American batholiths and provide critical insight into the volatile budget of convergent arcs. The Empire Mountain Skarns show a clear petrogenetic sequence: 1) early growth of distinct granoblastic red (locally andraditic) garnet; 2) secondary overgrowth of beige garnet filling veins and fractures in the red garnetite; 3) late stage, cross-cutting veins containing euhedral quartz + epidote ± calcite. The secondary veins indicate alteration of primary skarn garnet and magmatic plagioclase as H₂O and CO₂infiltrated during decreasing temperatures. Late stage quartz is associated with Au-Ag-Zn mineralization.

Primary fluid inclusions in the Empire Mountain skarns and associated alteration zones are used to examine the thermal history of the hydrothermal system and to evaluate the isotopic composition of skarn-forming fluids. Microthermometry of primary fluid inclusion assemblages within late stage, grossular garnets yield homogenization temperatures of 310–325 °C; early andradite garnets lack discernible primary fluid inclusion assemblages. Primary inclusions in euhdral quartz, including samples from the Empire Mine homogenize at 215–255°C. Using δ¹⁸O values of garnet and quartz, and homogenization temperatures as minimums, calculated δ¹⁸O values of water in equilibrium with minerals are as follows: red garnet ~ 0.5–1.0‰, beige garnet ~4.3‰, and quartz = –1.7 to –6.5‰. Initial waters had low δ¹⁸O values that increased during growth of beige garnet, which suggests greater proportion of magmatic or metamorphic water input. Early meteoric water infiltration into the hydrothermal system was facilitated by the extensive brecciation caused by a shallow, hot intruding magma. Late stage alteration and ore formation was accompanied by the resumption of an increased proportion of meteoric water infiltration.