2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 83-13
Presentation Time: 11:25 AM


KORZEB, Stanley, Montana Bureau of Mines and Geology, Montana Tech University, 1300 W. Park Street, Butte, MT 59701, SCARBERRY, Kaleb, Montana Bureau of Mines & Geology, 1300 West Park Street, Butte, MT 59701, ZIMMERMAN, Jarred L., Geological Engineering, Montana Tech, Butte, MT 59701 and GAMMONS, Christopher, Geological Engineering, Montana Tech of The University of Montana, 1300 West Park Street, Butte, MT 59701, skorzeb@mtech.edu

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 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.