Geologic description
The BOS prospect is in the Late Triassic, Eureka mafic-ultramafic complex. The host for the complex is Pennsylvanian to Permian rocks of the Slana Spur Formation which is intruded by Late Jurassic to Cretaceous granodiorite (Nokleberg and others, 1991). At the prospect, the complex is mainly a sill-like body of serpentinized peridotite, with lesser pyroxenite and gabbro (Bittenbender and others, 2007). The complex crops out along strands of the Broxson Gulch thrust fault (W.T. Ellis, oral communication, 2001).
As described by W.T. Ellis (oral communication, 2001) the mineralization consists of coarsely-crystalline clots and disseminations of pyrrhotite, chalcopyrite, and pentlandite in olivine melagabbro and feldspathic peridotite. Sulfide abundance rarely exceeds about 5 percent. A float sample contained 117 parts per billion (ppb) gold, 393 ppb platinum, 493 ppb palladium, 0.14 percent copper, and 0.4 percent nickel. Bittenbender and others (2007) could not locate mineralized rock at the Ellis's supposed location and samples they took in that area had insignificant metal values. However, after talking with industry geologists in 2004, they determined that the most likely location for what is commonly known as the BOS prospect is at the coordinates used for this record. (The Ellis location may be about a mile to the northwest but it uncertain whether there are two sites of mineralization or confusion about the location of a single location.) A sample collected by Bittenbender and others (2007) across 1 foot of peridotite with about 1 percent chalcopyrite contained 36 ppb gold, 106 ppb platinum, 86 ppb palladium, 594 parts per million (ppm) copper, and 2,030 ppm nickel. A sample of serpentinite with disseminated sulfides contained 11 ppb gold, 292 ppb platinum, 308 ppb palladium, 1,145 ppm copper, and 3,710 ppm nickel. The prospect is at the toe of a small ice field and may extend under the ice. |
