Salmon River Bench Mine, Salmon River - Red Mountain District, Goodnews Bay, Bethel Borough, Alaska, USAi
|Regional Level Types|
|Salmon River Bench Mine||Mine|
|Salmon River - Red Mountain District||Mining District|
|Latitude & Longitude (WGS84):||58° 56' 23'' North , 161° 41' 60'' West|
|Latitude & Longitude (decimal):||58.93972,-161.70000|
|Köppen climate type:||Dfc : Subarctic climate|
Location: This record describes the Salmon River Bench mine, and the subsurface extension of the deposit as defined by drilling. The Salmon River Bench mine is along the east side of Salmon River valley. Dredge mining has taken place from opposite the mouth of Dowry Creek at a surface elevation of about 275 feet, south 2 miles to opposite the mouth of Platinum Creek at a surface elevation about 175 feet. The area of dredge tailings is commonly 600 to 700 feet wide. The map site is at the approximate midpoint of the placer tailings. This mine is included in locality 12 of Cobb (1972 [MF 362]; 1980 [OF 80-909]). The subsurface extension of this deposit has been traced southward by drilling for a distance of about 10 miles, to within 3/4 mile of the coast of Kuskokwim Bay.
Geology: Dredge mining of the Salmon River Bench has taken place from opposite the mouth of Dowry Creek on the north (surface elevation of about 275 feet), south 2 miles to opposite the mouth of Platinum Creek (surface elevation about 175 feet). The area of dredge tailings is commonly 600 to 700 feet wide. The paystreak is on weathered bedrock, in the overlying 10 feet of clay and gravel, and in fractured bedrock. It has been traced by drilling for a distance of about 10 miles, to within 3/4 mile of the coast; it is 600 feet to as much as 1,000 feet wide in places. The bedrock bench does not contain local incisions and has a general dip to the south. As a result, overburden thickness changes from as little as 15 feet at the north end of the paystreak to more than 200 feet within a mile of the mouth of Salmon River. The unconsolidated deposits on this bench are clay and about 20 percent gravel, which forms lenses and streaks as much as 70 feet thick. The gravels contain commonly faceted, subangular to rounded pebbles and cobbles coated with clay; the clay makes recovery of PGM grains difficult (Mertie, 1969, p. 82). Non-siliceous pebbles and cobbles are decomposed. Unlike other unconsolidated deposits in placer mines of the Salmon River area, some layers in the bench overburden are frozen. South of Happy Creek, the upper part of the overburden locally is thick, clean, and little-weathered gravel (Mertie, 1976, p. 32). Five samples of the bench tailings contained PGM contents from below detection to 0.0024 ounce per cubic yard (Fechner, 1988, p. 191-192). PGM grains from Salmon River valley and bench tailings contain 0.6 to 1.1 percent Rh, 0.4 to 0.7 percent Ru, 60.3 to 85.5 percent Pt, 3.8 to 25.6 percent Ir, 1.2 to 6.3 percent Os, and 5.9 to 8.9 percent Fe (Fechner, 1988, p. 81). The PGM-bearing phases identified in these samples included iron-platinum alloy containing 8 to 30 percent Fe; iron-platinum alloy with minor osmiridium inclusions; and osmiridium, sperrylite, and tetraferroplatinum (Fechner, 1988, p. 81). Small amounts of cinnabar and traces of native mercury have been identified in dredge concentrates (Mertie, 1976, p. 38). Two small diamonds were identified in the nondissolved residue from 8 PGM granules (Mertie, 1976, p. 17). This placer is spatially removed from the Red Mountain ultramafic pluton on the west side of Salmon River valley, from which many younger alluvial placers have been derived (eg. HG010, HG011, HG013, HG014, and HG027). Some higher and older bench remnants exist along the east side of Salmon River valley and reworking of these deposits has contributed to the Salmon River bench placer. In general, it appears that the Salmon River bench placer has a long and complicated history. Its proximity to the coastline and low elevation (in places actually below present sealevel), suggests that Quaternary sea level changes could have influenced its development.
Workings: Dredge mining, started on Salmon River in 1937, has taken place from opposite the mouth of Dowry Creek to the north (surface elevation of about 275 feet) south 2 miles to opposite the mouth of Platinum Creek (surface elevation about 175 feet). The area of dredge tailings is commonly 600 to 700 feet wide. Drilling has delineated the extensions of the bench placer deposit southward to within 3/4 mile of the mouth of Salmon River on Kuskokwim Bay. It becomes deeper and apparently uneconomic to the south.
Production: A significant part of the estimated 650,000 ounces of PGM and 15,600 ounces of gold produced from the Salmon River area placers (Cobb, 1980 [OF 80-909]) was recovered from the Salmon River bench placer.
Reserves: Fechner (1988, p. 81) lists measured and indicated reserve estimates from private company files that total over 63 million cubic yards with grades ranging from 0.0017 to 0.0084 ounce of PGM per cubic yard.
Commodities (Major) - Au, PGM; (Minor) - Cr, diamond, Hg
Development Status: Yes; medium
Deposit Model: Placer PGE-Au (Cox and Singer, 1986; model 39b)
Regions containing this locality
|North America Plate||Tectonic Plate|
Select Mineral List TypeStandard Detailed Strunz Dana Chemical Elements
Commodity ListThis is a list of exploitable or exploited mineral commodities recorded at this locality.
9 valid minerals.
Detailed Mineral List:
|ⓘ Iridium var: Osmiridium|
List of minerals arranged by Strunz 10th Edition classification
|Group 1 - Elements|
|Group 2 - Sulphides and Sulfosalts|
|Group 4 - Oxides and Hydroxides|
List of minerals arranged by Dana 8th Edition classification
|Group 1 - NATIVE ELEMENTS AND ALLOYS|
|Metals, other than the Platinum Group|
|Platinum Group Metals and Alloys|
|Semi-metals and non-metals|
|Group 2 - SULFIDES|
|AmXp, with m:p = 1:1|
|AmBnXp, with (m+n):p = 1:2|
|Group 7 - MULTIPLE OXIDES|
|Unclassified Minerals, Mixtures, etc.|
List of minerals for each chemical element
|Ru||ⓘ Iridium (var: Osmiridium)||(Ir,Os,Ru)|
|Os||ⓘ Iridium (var: Osmiridium)||(Ir,Os,Ru)|
|Ir||ⓘ Iridium (var: Osmiridium)||(Ir,Os,Ru)|
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org
0 - 2.588 Ma
|Unconsolidated surficial deposits, undivided|
Age: Pleistocene (0 - 2.588 Ma)
Reference: Wilson, F.H., Hults, C.P., Mull, C.G, and Karl, S.M. (compilers). Geologic map of Alaska. doi: 10.3133/sim3340. U.S. Geological Survey Scientific Investigations Map 3340, pamphlet 196. 
|Mesozoic - Paleozoic|
66 - 541 Ma
|Paleozoic-Mesozoic volcanic rocks|
Age: Phanerozoic (66 - 541 Ma)
Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529.