DIVERSITY OF SEDIMENTARY ROCK-HOSTED GOLD DEPOSITS IN THE STIBNITE MINING DISTRICT, IDAHO

The Stibnite mining district in central Idaho historically produced Au (900,000 ounces), Ag, Sb, W and Hg, mostly from deposits in the Atlanta lobe of the Idaho batholith. However, several Au and Hg deposits are hosted in a roof pendant of Neoproterozoic-Ordovician metasedimentary rocks. Currently, Midas Gold Inc. controls the property and has developed a probable reserve of 4.6 Moz of Au. Our research is aims to document various styles of SHGDs and their time-space relationships with intrusions and uplift and to assess whether they are part of one hydrothermal system or are the result of multiple overprinting hydrothermal events. At the West End (WE) and Stibnite pits, which are the largest and closest SHGDs to the batholith, most ore is hosted by a “lower calc-silicate” unit. Three vein types occur at WE: early milky white veins with minor molybdenite (Re/Os 86.3±0.4 Ma), suggestive of higher temperatures (>350C), which are cross-cut by intermediate, medium grained, white-gray quartz veins with minor pyrite. Both of these vein sets are cut by epithermal banded chalcedonic silica veins with local K-feldspar and bladed calcite, which are suggestive of lower temperatures (<220C), shallower depths, and boiling conditions. Most of the mineralization at WE appears to be associated with the intermediate quartz veinlets and disseminations of pyrite and lesser arsenopyrite, where native Au and arsenopyrite inclusions are found within zoned arsenian pyrite rims on pyrite. Native Au is present in the epithermal-like veins, along with quartz and K-feldspar. At the Garnet pit, 2 km south of WE, mineralization is characterized by fine-grained zoned arsenian pyrite that overprints a garnet-hornblende skarn with calcite veins. In the Saddle and Fern area, 2.5 km SE of WE, SHGDs are primarily hosted in marble and are mostly characterized by jasperoid bodies with high grades (≤80 ppm Au), epithermal-like veins and irregular and poorly developed arsenian pyrite rims on pyrite. There appears to be at least 2 stages of gold mineralization. Early disseminated and veined mineralization formed between ~86 and 50 Ma. Sometime during this period, there was significant uplift, after which later epithermal mineralization, likely related to the Eocene Thunder Mountain caldera, overprinted the SHGDs.