REEXAMINATION OF ORE-FORMING PROCESSES IN THE EMERY MINING DISTRICT, POWELL CO., MONTANA

The present investigation was undertaken in collaboration with the Montana Bureau of Mines and Geology to re-evaluate the Emery district using modern methods of ore deposit study including fluid inclusions, stable isotopes, and mineral geochemistry. Results from this thesis will be an aid in reassessing the potential of the Boulder Batholith and surrounding geologic units to host large, undiscovered precious and/or base metal deposits. The Emery (aka Zosell) district was mined from the late 1800s until the early 1950s. Mineralized quartz veins are hosted by basaltic andesite flows of the late Cretaceous Elkhorn Mountain Volcanics (EMV). Two sets of veins are present: “bedding plane” veins and east-west trending, near-vertical fissure veins. Ore mineralogy includes pyrite, arsenopyrite, sphalerite, galena, boulangerite, Ag-rich tetrahedrite, chalcopyrite, and trace electrum. Tetrahedrite is the main Ag-bearing ore mineral. Two grains of electrum were found with XAu = 0.579 and 0.565. Electron microprobe analysis indicated the arsenopyrite locally contains gold (up to 690 ppm), and previous studies have shown high gold content in both pyrite and arsenopyrite from Emery. Gangue mineralogy was dominated by quartz during the main period of ore deposition, and by carbonate minerals (ankerite, dolomite, calcite) prior to and following the main ore-forming event. Hydrothermal alteration includes silicification and sericitization closest to the veins, grading outward to propylitic and/or carbonate alteration. The latter occurs as massive or brecciated bodies, veinlets, and as carbonate minerals replacing groundmass and phenocrysts of plagioclase and augite. Fluid inclusion homogenization temperatures in mineralized quartz veins range from 220°C to 353°C. These temperatures fit well with K/Na geothermometry based on bulk fluid inclusion leachate analyses (averaging 302 °C) and S-isotope geothermometry based on sphalerite-galena pairs (averaging 326 °C). A temperature range of 300°C to 350°C also agrees with thermodynamic calculations based on the compositions of co-existing sphalerite, pyrite, arsenopyrite, and electrum. Two broad categories of fluid inclusions were found; aqueous-rich and CO2-rich. The CO2-rich inclusions show three phases at room temperature and generally have low salinity while aqueous-rich inclusions have salinity from 0 to 10.8 wt% NaCleq, averaging 6.2 wt% NaCleq. Pressure estimates based on the phase behavior of the CO2-rich inclusions are 1.2 to 1.7 kbar while aqueous-rich inclusions indicate a pressure of 1.2 kbar. Assuming the basaltic andesite host rock is at the bottom of the EMV, lithostatic pressure would have been around 1.3 kbar during vein formation. This implies that fluid pressures may have exceeded lithostatic pressures during mineralization, especially during formation of the flat veins. Stable S and C isotopes, combined with fluid inclusion evidence, suggest that the ore-forming event at Emery involved fluids sourced from a magma that had assimilated S and possibly C from Precambrian Belt-Purcell Supergroup metasediments. Cooling, water-rock interaction, and mixing of magmatic and meteoric water are the most likely depositional mechanisms for the Ag-Au-base metal mineralization