Fluid Inclusion and Stable Isotope Investigation of Hydrothermal Talc and Chlorite Deposits in Southwest Montan

Talc and chlorite deposits of southwest Montana formed as hydrothermal replacements of Archean and/or early Proterozoic dolomitic marble and quartzo-feldspathic gneiss. Although the hydrothermal replacement model is generally accepted, less is known about the temperature, composition, and origin of the fluids involved in talc and chlorite formation. The present study examines fluid inclusions in quartz associated with both talc and chlorite, stable hydrogen and oxygen isotopes of talc, chlorite, and quartz, and stable carbon and oxygen isotopes of carbonate minerals. The deposits being examined include the Yellowstone, Beaverhead and Willow Creek talc mines, and the Antler chlorite mine.;Fluid inclusion studies of quartz from Yellowstone show moderately high salinity of 14.3 +/-6.1 wt% NaCl eq. and a low homogenization temperature range of 110 +/-13.4 °C. All the inclusions are simple two-phase, liquid-rich fluids; the lack of CO2-clathrate or CO2(l) phases indicates a low CO2 partial pressure, which would favor talc formation at the expense of dolomite. Fluid inclusions from Antler have slightly higher homogenization temperatures of 151 +/-14.4°C but lower salinities of 11.3+/-1.5 wt% NaCl eq. These temperatures are not pressure corrected. Na/K thermometry from bulk quartz leachates give temperatures of 216 +/-43°C and 282 +/-70°C for the Yellowstone and Antler mines. Anion (Cl-Br) ratios of leachate solutions from Yellowstone and Antler have similar compositions, and plot along the seawater evaporation trend pointing to evaporated seawater as a possible fluid source. Carbonate isotopes from the Yellowstone mine show minor variation in delta13C ‰ (-2.5 to +0.9) and a wider range in delta18O ‰ (+9.2 to +27.1, VSMOW) which is consistent with other studies of dolomite associated with talc in the Ruby Range. The ranges of O-isotope compositions of talc and quartz from the Yellowstone mine are +2.7 to +3.8 ‰ and +11.1 to +15.9 ‰, respectively. The ?D values for talc range from -53.2 to -45.0. The Antler chlorite mine has a similar range of O-isotopes for chlorite and quartz of +4.0 to +7.7 ‰ and 10.7 to 10.9 ‰ respectively and ?D values in chlorite are -50.7 to -50.1 ‰. These depleted ?D values are difficult to explain without the involvement of meteoric water during the formation of hydrothermal talc and chlorite. A model is presented in which evaporated seawater, possibly sourced from the mid-Proterozoic Belt Basin, mixed with deeply circulated meteoric water to form the Montana talc and chlorite deposits.