Geologic description
Fox Gulch is where platinum was first discovered in the Salmon River area (1926). It has been placer mined for about 0.4 mile upstream from its junction with Platinum Creek. The headwaters of Fox Gulch are in dunite of the Red Mountain ultramafic pluton, and it also flows across border zone clinopyroxenite and hornblende-bearing rocks. In the area of mining, however, the bedrock is part of an assemblage that includes sheared argillite, graywacke, and mafic to intermediate, fine-grained igneous rocks that are difficult to identify because of their decomposed character where exposed in mining cuts (Mertie, 1940). These strata are included in a regional sedimentary and volcanic assemblage that ranges in age from Paleozoic to Mesozoic (Hoare and Coonrad, 1978).
Gravels in lower Fox Gulch are up to 12 feet thick, but they thin upstream. The gravels are coarse, subangular, and locally derived and contain boulders averaging 8 to 12 inches in diameter. The average width of the pay streak was 75 feet and it extended at least 2,600 feet upstream from the mouth of Fox Gulch. Platinum was concentrated in the lower few feet of gravel, on bedrock, and in fractures in bedrock. Chromite is common in the heavy-mineral concentrates and some chromite grains are intergrown with PGM. The U. S. Bureau of Mines collected 4 samples from the present drainage. These samples contained 0.0009 to 0.0378 ounce of PGM per cubic yard (Fechner, 1988, p. 80) The highest grade sample was from unmined material in the upper part of the gulch. One sample of tailings in Fox Gulch contained 0.0121 ounce of PGM per cubic yard (Fechner, 1988). Microprobe analyses of PGM grains from these samples showed 1.3 to 1.9 percent Rh, 0.7 to 1.0 percent Ru, 37.7 to 48.6 percent Pt, 26.7 to 41.3 percent Ir, 9.8 to 13.4 percent Os, and 4.1 to 8.4 percent Fe. PGM-bearing phases that were identified in these samples included iron-platinum alloy containing 8 to 30 percent Fe; iron-platinum alloy with minor osmiridium inclusions; and hollingsworthite, irarsite, iridarsenite, iridium, sperrylite, and platarsite (Fechner, 1988, p. 80). The mean of 26 analyses of PGMs (recalculated to exclude impurities) recovered during mining from Fox Gulch and Platinum Creek above Squirrel Creek was 63.71% Pt, 28.01% Ir, 5.39% Os, 0.47% Ru, 1.82% Rh, 0.23% Pd, and 0.37% Au. Six samples from Fox Gulch, recalculated free of impurities, averaged 50.56 percent Pt, 39.14 percent Ir, 7.74 percent Os, 0.71 percent Ru, 1.62 percent Rh, 0.14 percent Pd, and 0.09 percent Au (Mertie, 1976). Fechner (1988) estimates that there are 160,000 cubic yards of tailings on Fox Gulch that contain 0.012 ounce of PGM per cubic yard and 20,000 cubic yards of unmined material that contain 0.02 ounce of PGM per cubic yard. |
