NEW INTERPRETATIONS OF VEIN GENESIS FOR THE EMERY MINING DISTRICT, POWELL COUNTY, MONTANA
Based on samples from mine dumps, vein mineralogy consists of quartz-carbonate gangue with arsenopyrite, pyrite, sphalerite, galena, boulangerite, chalcopyrite and freibergite (in decreasing order of abundance). Mineral paragenesis indicates five stages of mineralization with most sulfide mineralization taking place during stages two and three. Homogenization temperatures of fluid inclusions in quartz from mineralizing stages two and three ranged from 251oC to 348oC, (average 298oC). Temperatures based on sulfur isotopic composition of co-existing sulfide-mineral pairs overlap with the temperatures suggested by the fluid inclusion data. Salinities determined from ice melting temperatures for stage 2 and 3 mineralization ranged from 5.3 to 9.6 (average 7.2) equivalent wt% NaCl. The presence of three phase inclusions with a CO2 vapor bubble and a rim of liquid CO2 suggest a moderate formation depth. Sulfur from pyrite, arsenopyrite, sphalerite and galena have δ34S values ranging from 1.0‰ to 8.7‰. Pyrite from stage 2 mineralization gave the highest δ34S values averaging 8.0‰ and galena from stage 3 mineralization yielded the lowest δ34S values averaging 2.6‰. Zircons from a granodiorite intrusion that cuts the Cretaceous age volcanics yielded a Proterozoic age of 1.77 Ga suggesting that Proterozoic rocks were assimilated into the granodiorite. Isotope and fluid inclusion data suggest the veins originated from a magmatic source which could be an exploration target for a porphyry system.