NEW INTERPRETATIONS OF VEIN GENESIS FOR THE EMERY MINING DISTRICT, POWELL COUNTY, MONTANA

The Emery (Zosell) mining district is located 10 km east of Deer lodge, Powell County, Montana. Polymetallic veins were mined here from the late 1880’s to the mid-1950’s.Two types of veins were mined; a north-south striking vein extending through the district and a group of eight, east-west veins. The north-south vein occupies a bedding-plane fissure in the volcanic host rock. The east-west veins occupy steep to vertical normal faults crosscutting the host rock. These veins are hosted by Cretaceous amygdaloidal-basaltic andesite that correlates with the Elkhorn Volcanics. Most of the veins are narrow (< 1 m) and have similar mineralogy and alteration. Propylitic, sericitic/argillic alteration is district wide forming a lithocap, carbonate and silicic alteration occurs in the vicinity of the veins.

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 251^oC to 348^oC, (average 298^oC). 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.