Geologic description
The rocks in the vicinity of this occurrence are a 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). The rocks are distinctly iron-stained and include white mica-bearing quartzite, white mica schist, and biotite +/- garnet +/- pyrite schist. These rocks occur in a fold hinge and form alternating 10- to 50-centimeter-thick layers of mostly quartzite and white mica schist; millimeter-thick, foliation-parallel, sulfide layers occur in the more quartz-rich layers. In one place, foliation-parallel pyrite and galena are cut by discordant pyrite veins. The quartzite and white mica schist are overlain to the north by garnet-bearing amphibolite, garnet-biotite schist, and white mica-biotite quartzite (C. Dusel-Bacon, written communication, 2002).
A sample of white mica-bearing quartzite, with pyrite and lesser galena and sphalerite contains 1,315 ppm zinc and 526 ppm lead. Petrographic analysis of this quartzite shows that the foliation is defined by fine-grained quartz with fairly uniform extinction in polygonized domains and scattered, aligned, about 0.5-millimeter-long, white mica crystals (Cynthia Dusel-Bacon, USGS, written communication, 2002). A Paleozoic age and sedimentary-exhalative origin for the sulfides are suggested by the foliation-parallel sphalerite, galena, and pyrite in quartz-rich layers and by high barium concentration (as much as 1,790 ppm) in some samples. The alternating quartz-rich and muscovite-rich layers in the rocks may represent multiple cycles of exhalite deposition alternating with ash deposition(?). Alternatively, or in addition, elevated tungsten (as much as 1,020 ppm) and possibly bismuth (as much as 4 ppm) suggest that mineralization may be related to Mesozoic metamorphism (associated with quartz sweats) or plutonism (C. Dusel-Bacon, written communication, 2002). |
| Mineral deposit model |
A sedimentary-exhalative origin for the sulfides is suggested by the foliation-parallel sphalerite, galena, and pyrite in quartz-rich layers and by high barium concentration (as high as 1,790 ppm) in some samples. The alternating quartz-rich and muscovite-rich layers in the rocks may represent multiple cycles of exhalite deposition alternating with ash deposition? Alternatively, or in addition, elevated tungsten (as much as 1,020 ppm) and possibly bismuth (as much as 4 ppm) suggests that mineralization may be related to Mesozoic metamorphism or plutonism (Cynthia Dusel-Bacon, USGS, written commun., 2002). |
| Age of mineralization |
A Paleozoic age and sedimentary-exhalative origin for the sulfides is suggested by the foliation-parallel sphalerite, galena, and pyrite in quartz-rich layers and by high barium concentration (as much as 1,790 ppm) in some samples. The alternating quartz-rich and muscovite-rich layers in the rocks may represent multiple cycles of exhalite deposition alternating with ash deposition(?). Alternatively, or in addition, elevated tungsten (as much as 1,020 ppm) and possibly bismuth (as much as 4 ppm) suggest that mineralization may be related to Mesozoic metamorphism (associated with quartz sweats), or plutonism (Cynthia Dusel-Bacon, USGS, written communication, 2002). |
