Platinum Creek

Mine, Active?

Commodities and mineralogy

Geographic location

Quadrangle map, 1:250,000-scale HG
Quadrangle map, 1:63,360-scale D-6
Latitude 58.92
Longitude -161.73
Nearby scientific data Find additional scientific data near this location
Location and accuracy Platinum Creek is a west tributary to Salmon River. The confluence of Platinum Creek and Salmon River is about 4.5 miles upstream of the mouth of Salmon River on Kuskokwim Bay. The map site is at the mouth of Squirrel Creek, at the approximate midpoint of placer mine workings along Platinum Creek. This mine is included in locality 12 of Cobb (1972 [MF 362]; 1980 [OF 80-909]).

Geologic setting

Geologic description

Platinum Creek flows northeast along the southeast border of the Red Mountain ultramafic pluton; three of its west tributaries, Fox Gulch (HG028), Dry Gulch (HG027), and Squirrel Creek (HG013), have headwaters within the Red Mountain pluton. Bedrock in Platinum Creek is sheared argillite, graywacke, and mafic to intermediate, fine-grained igneous rocks that are difficult to identify because of their extensively decomposed character where exposed in mining cuts (Mertie, 1940). These country rocks to the Red Mountain pluton are included in a regional sedimentary and volcanic assemblage that ranges in age from Paleozoic to Mesozoic (Hoare and Coonrad, 1978).
Platinum was discovered on Fox Gulch in 1927 and placer mining began on Platinum Creek in 1928. It has been placer mined, mostly by dragline operations, over about 1 mile of its length below Fox Gulch. Gravels in Platinum Creek were about 12 feet thick below 2 to 3 feet of vegetation-rich material. The locally derived gravel is mostly rounded to subangular but just downstream from Fox Gulch it is coarser: 8 to 12 inch, subangular, boulders are common, and some are as much as 3 feet across (Mertie, 1940). The pay streak was commonly 100 to 125 feet wide, but it was wider near the mouths of Fox Gulch and Squirrel Creek and it widened to about 400 feet near the mouth of Platinum Creek. At the mouth of Squirrel Creek there were two pay streaks, one continuing downstream from Squirrel Creek and one continuing downstream from upper Platinum Creek (Mertie, 1976, p. 25). Platinum was concentrated in the lower few feet of gravel, on bedrock, and in fractures in bedrock. One cleanup on Platinum Creek just below the mouth of Fox Gulch yielded 0.02 ounce of PGM per cubic yard. Most of the platinum is fine, the largest nugget recovered by 1937 weighed 1/4 ounce. Some of the platinum is intergrown with chromite. In addition to chromite, the heavy mineral concentrates contain much ilmenite and magnetite. The U. S. Bureau of Mines collected six samples from the present drainage upstream from Dry Gulch that contained a trace to 0.0093 ounce of PGM per cubic yard. A sample from the unmined area above the confluence with Fox Gulch contained 0.0028 ounce of PGM and 0.0006 ounce of Au per cubic yard (Fechner, 1988, p. 80). An estimated 50,000 cubic yards of tailings are present above the mouth of Squirrel Creek; two samples from them contained 0.0028 and 0.0006 ounce of PGM per cubic yard. The mean of 26 analyses of PGMs (recalculated to exclude impurities) 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. The mean of 38 analyses from lower Platinum Creek valley was 77.03% Pt, 16.76% Ir, 3.62% Os, 0.27% Ru, 1.44% Rh, 0.34% Pd, and 0.84% Au (Mertie, 1976). Microprobe analyses of the PGM-bearing grains in U. S. Bureau of Mines samples showed them to contain 0.8 to 1.6 percent Rh, 0.5 to 1.0 percent Ru, 45.6 to 77.8 percent Pt, 8.1 to 37.8 percent Ir, 3.2 to 13.4 percent Os, and 4.1 to 8.4 percent Fe (Fechner, 1988, p. 80). 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; osmiridium; osmiridium with subordinate iron-platinum alloy; and hollingsworthite, prassoite, kashinite, sulrhodite, ehrlichmanite, iridosmine, osmium, iridarsenite, iridium, sperrylite, and tulameenite. The gold fineness measured in one sample was 714 (Fechner, 1988, p. 80).
Geologic map unit (-161.732226006044, 58.9192159205445)
Mineral deposit model Placer PGE-Au (Cox and Singer, 1986; model 39b)
Mineral deposit model number 39b
Age of mineralization Quaternary.

Production and reserves

Workings or exploration Platinum was discovered on Fox Gulch in 1927 and placer mining began on Platinum Creek in 1928. It has been placer mined, mostly by dragline operations, over at least 1.1 miles of its length below Fox Gulch.
Indication of production Yes; small
Reserve estimates There are about 50,000 cubic yards of tailings with an estimated average grade of about 0.0026 ounce of PGM per cubic yard on Platinum Creek above the mouth of Squirrel Creek (Fechner, 1988, p. 81).
Production notes If 0.02 ounce of PGM per cubic yard was recovered from 50,000 yards of pay, Platinum Creek above the mouth of Squirrel Creek produced 1,000 ounces of PGM.

References