Geologic description
The rocks in the vicinity of this occurrence are Paleozoic amphibolite-facies quartzite of the Fortymile River assemblage that has been intruded by granitic and ultramafic rocks of Jurassic age (Foster, 1976; Dusel-Bacon and others, 2002). Sulfides occur as layers within biotite + quartz +/- staurolite +/- garnet schist and biotite quartzite. The sulfides are present in two modes: (A) a finely laminated, rusty-weathering, approximately 1-meter-thick oxidized layer, including a 1- to 2-centimeter-thick layer of massive pyrite or limonite after pyrite; and (B) a massive, foliation-parallel, quartz layer (quartz sweat?), in which layers of galena are apparently concordant with metamorphic layering. Petrographic analysis shows that the quartz-rich layer contains minor white mica and rare patches of sericitized and (or) kaolinitized feldspar; quartz grains are strained and have sutured contacts; tiny, thin, scattered white mica crystals occur between some quartz grains. A sample from mode A contains 260 ppm copper, 130 ppm lead, 2.8 ppm silver, and 20 ppb gold. A sample from mode B contains 6,350 ppm lead, 137 ppm copper, 13 ppm silver, and 295 ppb gold. Slightly elevated bismuth (as much as 10 ppm) and tungsten (as much as 680 ppm) in one quartz-rich rock suggests mineralization may be pluton related (Cynthia Dusel-Bacon, USGS, written communication, 2002). |
