Geochemistry and Petrography of Beartrack Mine, Lemhi County, Idaho

Abstract

The Beartrack Mine is located in east-central Idaho and is currently being explored by Meridian Gold Company, a subsidiary of the Yamana Gold Company. Production between 1994 and 2000; yielded a total of 608,741 ounces of gold. Mining focused on a shear zone associated with the Panther Creek Fault (PCF), which locally separates metasediments of the Proterozoic Yellowjacket Formation and a Proterozoic quartz monzonite. Dikes of unknown ages cross cut the two formations. Sericitic alteration is dominant and follows the PCF extending laterally up to 200 meters, grading into fresh country rock. The sericite and microcline in the altered lithologies were used for 40Ar/39Ar dating and the mineralization age is interpreted as being 68 Ma. The deposit was classified as a greenstone-hosted lode gold deposit (Hawksworth and Reynolds, 1997), due to the nature of quartz-stockwork veining and fluid inclusions containing liquid and vapor CO2. This paper disputes that fact, as there is evidence for it being an epithermal deposit that was reworked by the PCF. In 2012, a drill program encountered two substantial gold intercepts, much deeper than previous exploration and mining targets. Disseminated, free gold occurs in partially to fully re-healed PCF. On average, 1100 feet below surface, several drill holes intercepted silver-rich base metal-bull quartz-adularia veins of unknown age. Silver values range from 10 to 307 ppm and are hosted in tennantite-tetrahedrite. Other important metals in the veins are galena and free gold. Primary fluid inclusion data from the base metal quartz veins yield two homogenization temperature ranges between 204-2160C and 241-2470C. Optically determined paragenetic relationships in these veins, showed two stages of quartz veining, which corresponds with fluid inclusion data. Those temperatures also fall in the range of an epithermal deposit. Fluid inclusion homogenization temperatures and style of veining suggests two different hydrothermal systems at Beartrack. Correlation matrices show that silver is associated with copper, mercury, lead, and antimony likely due to its elevated levels in the base metal veins. Gold only correlates with arsenic in the South Pit. Silver to gold ratios versus elevation show no zonation in the overall system. However, silver values are higher in the PCF, bull quartz-base metal veins, and the silicified breccia zones relative to sericite altered and fresh Yellowjacket and quartz monzonite formations.