FLUID INCLUSION AND PETROGRAPHIC ANALYSIS OF THE EMERY MINING DISTRICT, SOUTHWEST MONTANA

Polymetallic veins of the Emery (Zosell) mining district, southwest Montana, were mined from the late 1880s to the mid-1950s. The deposits are shallow dipping, bedding-plane fissure veins. Country rock is amygdaloidal andesitic basalt of presumed late Cretaceous age, possibly correlative with the Elkhorn Volcanics. Most of the veins are narrow (< 1 m) and consist of one set of shallow-dipping veins concordant to layering in volcanic host rocks, and a second set of vertical veins striking E-W. Both sets of veins have similar mineralogy and alteration, and the vertical veins show little change in tenor along the mined length (F. Robertson, 1953, MBMG Memoir 34).

Samples collected from dumps at the Hidden Hand, Emma Darling, Emery Lode and Bonanza mine were examined under reflected light and with scanning electron microscopy (SEM-EDS). Mineral assemblages are relatively simple, consisting of (in decreasing order of abundance) arsenopyrite, pyrite, sphalerite, galena, boulangerite, chalcopyrite, and Ag-bearing tetrahedrite, in a quartz-carbonate gangue. Two small grains of electrum were found with compositions of XAu = 0.609 and 0.616. Silver was found to be mostly in the tetrahedite, which contains 9.70 to 15.91 wt% Ag. XFeS in sphalerite is 0.027-0.087, suggesting that the ore fluids had a moderately low S2 fugacity, consistent with the presence of electrum. Fluid inclusion homogenization temperatures range from 137°C to 298°C, averaging 249°C. Temperatures based on the sulfur isotopic composition of co-existing sulfide-mineral pairs collected by one of us (SK) broadly overlap with the fluid inclusion data. Salinities based on ice melting temperatures range from 4.6 to 8.4 (average of 7.3) equivalent wt% NaCl. Some fluid inclusions are 3-phase, with a central CO2 vapor bubble and a rim of liquid CO2. The presence of 3-phase fluid inclusions suggests a moderate depth of formation, and that the veins in question are not of shallow epithermal origin. Given the moderate salinities, it is hypothesized that the ore-forming fluids have a magmatic origin, perhaps related to emplacement of the late Cretaceous Boulder Batholith, which crops out 10 km to the east.