Geologic description
Dowry Creek, a west tributary to upper Salmon River, flows eastward from headwaters on the northeast flank of the Red Mountain ultramafic pluton. It has been placer mined over about 0.6 miles of its one-mile length, from its confluence with the Salmon River upstream to an elevation of 450 feet. Platinum on Dowry Creek was probably discovered in 1928, the year it was discovered on nearby Clara Creek (HG010). At least some mining occurred in 1935 and a dragline, first installed on Clara Creek in 1935, was probably moved to Dowry Creek and worked the paysteak out by 1941. Bedrock in Dowry Creek is mostly dunite of the Red Mountain ultramafic pluton. Dowry Creek contained about 5 feet of gravel and about 90,000 cubic yards were placer mined. The mean of 5 analyses of Pt-rich samples from Dowry Creek, recomputed free of impurities, is 87.56% Pt, 7.34% Ir, 1.47% Os, 0.12% Ru, 1.25% Rh, 0.46% Pd, and 1.80% Au (Mertie, 1976). Serpentinized dunite at the head of the creek is overlain by a 120-foot-wide and 2-foot-thick unmined deposit of cobbles, boulders, and clay that contains 0.0215 ounce of platinum-group metals and 0.0006 ounce of Au per cubic yard (Fechner, 1988, p. 73). Gold particle fineness measurements were 738 and 828. A sample of 3 feet of bank material at an elevation of 600 feet contained 0.0035 ounce of platinum-group metals per cubic yard, and two samples of placer mine tailings contained 0.0007 and 0.0015 ounce of PGM per cubic yard (Fechner, 1988, p. 73). Microprobe analyses of platinum-rich grains showed 0.8 to 1.5 percent Rh, 0.4 to 0.9 percent Ru, 63.4 to 80.2 percent Pt, 7.2 to 18 percent Ir, 1.2 to 6.8 percent Os, and 6.6 to 7.5 percent Fe. The grains consisted of iron-platinum alloy containing 8 to 30 percent Fe; iron-platinum alloy with minor osmiridium inclusions; and osmiridium, hollingsworthite, osmium, xingzhongite, iridarsenite, irarsite, sperrylite, and tulameenite. |
