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. Past mining efforts were concentrated in the Atlanta lobe of the Idaho batholith, but several Au and Hg deposits are hosted in a roof pendant of Neoproterozoic-Ordovician metasedimentary rocks. The characteristics of these sediment-hosted gold deposits (SHGDs) vary. The time-space relationships between the various types of SHGDs the batholith-hosted deposits, and igneous activity remain unclear. Currently, Midas Gold Inc. controls the property and has developed a probable reserve of 4.6 Moz Au that occurs both in the batholith and the metasediments. The objectives of our research are to document characteristics and timing of the various styles of SHGDs and to assess whether they are part of one hydrothermal system or are the result of multiple hydrothermal events that overprinted. At the West End (WE) deposit, which is the largest SHGD and located closest to the batholith, most ore is hosted by the “lower calc-silicate” unit. Two distinct vein types occur at WE, milky white veins with minor molybdenite suggestive of higher temperatures (>350^o C) that are cross-cut by banded chalcedonic silica veins that are suggestive of shallower depths and lower temperatures (<220^o C). Most of the mineralization at WE appears to be associated with veinlets and disseminations of pyrite and lesser arsenopyrite, where Au is found within As-rims on pyrite. Native Au is present in the epithermal-like veins, along with quartz and adularia. Mineralization at the nearby Stibnite pit is very similar to WE. At the Garnet pit, 2 km south of WE, mineralization is characterized by fine-grained pyrite that overprints a garnet-hornblende skarn. 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 and epithermal-like veins with high grades (≤80 ppm Au). Tentatively, there appears to be 2 stages, early disseminated mineralization hosted primarily by the lower calc-silicate unit that shares features with Carlin-type Au deposits in Nevada, and later epithermal mineralization that is likely related to the Eocene Thunder Mountain caldera.