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Salmon River Bench

Producer in Alaska, United States with commodities Gold, Chromium, Diamond, Mercury
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Geologic information

Identification information

Deposit ID 10307622
Record type Site
Current site name Salmon River Bench

Geographic coordinates

Geographic coordinates: -161.70268, 58.93925 (WGS84)
Relative position 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.
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Geographic areas

Country State
United States Alaska

Commodities

Commodity Importance
Gold Primary
Chromium Secondary
Diamond Secondary
Mercury Secondary

Materials information

Materials Type of material
Chromite Ore
Cinnabar Ore
Diamond Ore
Gold Ore
Mercury Ore
Sperrylite Ore
Iron Ore
Platinum Ore
Osmium Ore
Iridium Ore
Tetraferroplatinum Ore

Mineral occurrence model information

Model code 120
USGS model code 39b
BC deposit profile C01. C02
Deposit model name Placer PGE-Au

Nearby scientific data

(1) -161.70268, 58.93925
ListMap

Comments on the geologic information

  • Geologic Description = 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.
  • Age = Quaternary.

Economic information

Economic information about the deposit and operations

Development status Producer

Comments on exploration

  • Status = Active

Mining district

District name Goodnews Bay

Comments on the production information

  • Production Notes = 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.

Comments on the reserve resource information

  • 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.

Comments on the workings information

  • Workings / Exploration = 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.

Reference information

Links to other databases

Agency Database name Acronym Record ID Notes
USGS Alaska Resource Data File ARDF HG032

Bibliographic references

  • Deposit

    Mertie, J.B., Jr., 1969, Economic geology of platinum minerals: U.S. Geological Survey Professional Paper 630, 120 p.

  • Deposit

    Cobb, E.H., 1980, Summaries of data and lists of references to metallic and selected nonmetallic mineral deposits in fifteen quadrangles in southwestern and west-central Alaska: U.S. Geological Survey Open-File Report 80-909, 103 p.

  • Deposit

    Fechner, S.A., 1988, Bureau of Mines mineral investigation of the Goodnews Bay mining district, Alaska: U.S. Bureau of Mines Open-File Report 1-88, 230 p.

  • Deposit

    Cobb, E.H., 1972, Metallic mineral resources map of the Hagemeister Island quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-362, 1 sheet, scale 1:250,000.

Comments on the references

  • Primary Reference = Mertie, 1969

General comments

Subject category Comment text
Deposit Model Name = Placer PGE-Au (Cox and Singer, 1986; model 39b)

Reporter information

Type Date Name Affiliation Comment
Reporter 18-MAR-01 Travis L. Hudson Applied Geology