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Gold Bench Mine, Koyukuk District, Yukon-Koyukuk Borough, Alaska, USA

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Latitude & Longitude (WGS84): 66° 58' 51'' North , 150° 38' 13'' West
Latitude & Longitude (decimal): 66.9808333333, -150.636944444


Pleistocene bones are said to have been common in the gravels of Gold Bench (Reed, 1938). See also: Ironside Bench (BT004).
Location: Gold Bench is located on a prominent bend in the South Fork Koyukuk River about 1.2 miles upstream from the mouth of John R Creek. The mine is shown on the current (1970, photorevised 1975) Bettles D-2 topographic map. The placer is about 1000 feet wide and 0.8 mile long and covers an area of about 60 to 100 acres. The location is accurate.
Geology: Gold-bearing gravels occur in a high channel along the north side of the South Fork, Koyukuk River at Gold Bench. This high channel is about 30 feet higher than the present river channel. Maddren (1913) described the deposit as surficial, fine-washed stream gravels overlying other unconsolidated deposits. The gold-bearing gravels consist predominately of schist and quartz pebbles with lesser amounts of flint and igneous rocks. The most productive deposit was an 18- to 24-inch thick layer of gravel that covers an area of about 60 acres. The gold typically rested on a 2 - to 12-inch-thick layer of reddish sand, which acted as false bedrock. Small amounts of gold occurred throughout the gravel section (Maddren, 1913). Bedrock is decomposed to blue clay that probably was derived from shale (Reed, 1938). The depth to bedrock was estimated by Reed (1938) to be about 6 feet, although Maddren reported shafts to 20 feet deep that had not reached solid rock. The gold was generally fine, well worn and very flattened. Studies of panned samples in the 1950's described a variety of accessory minerals including magnetite, hematite, garnet, pyrite, chalcopyrite, cinnabar, rutile, cassiterite, scheelite, monazite, uranothorianite, galena, sphene, and possibly bismuthinite (Wedow and others, 1952; Nelson and others, 1954). Placer concentrates contain as much as 0.18 percent equivalent uranium. The source of the gold is not known. Maddren (1913) speculated that it might have come from the Tramway Bar (WI006) area on the Middle Fork, Koyukuk or, more likely, from the hills to the south. Cobb (1973 [B1374]) thought that at least some of the gold was probably from reworked glacial deposits. The deposit was mined mostly at the surface by hand methods. A few shafts were apparently sunk, but these produced little gold. Heiner and Wolff (1968) noted mining in the 1940's using heavy equipment. Grybeck (1977) indicated mining activity through 1975.
Workings: The deposit was mined mostly at the surface by hand. A few shafts were apparently sunk, but these produced little gold. Heiner and Wolff (1968) noted mining in the 1940's using heavy equipment. Grybeck (1977) indicated mining activity through 1975.
Age: Quaternary.
Production: Maddren (1913) reported that $150,000 (approximately 7,500 oz.) in gold was produced through 1909. Figures for later production are not known.

Commodities (Major) - Au; (Minor) - Bi(?), Cu, Hg, Pb, REE, Sn, Th, Ti, U, W
Development Status: Yes
Deposit Model: Placer Au (Cox and Singer, 1986; model 39a)

Mineral List


13 valid minerals.

Major Regional Geological Units

This 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 geology at this locality but it gives a background for the region in which it is found.

Quaternary
0 - 2.588 Ma
Unconsolidated surficial deposits, undivided

Unconsolidated

Gravel, sand, silt, and peat. Floodplain deposits in Koyukuk Flats and along Yukon River composed mainly of light gray micaceous silt. Deposits occur along floodplains of major drainages and on tidal flats bordering the shores of Selawik Lake, Hotham Inlet, Eschscholtz Bay, Norton Sound, and Norton Bay. Floodplain deposits characterized physiographically by bars, oxbow lakes, meander scrolls, abandoned channels, and other evidence of recent floodplain building

Mesozoic - Paleozoic
66 - 541 Ma
Igneous: extrusive; Extrusive: mafic

Basalt, olivine basalt, tholeiite, alkali basalt, basanite, pillow basalt, flood basalt or metamorphosed equivalent

Interior western Alaska, Yukon-Koyukuk Basin

Sedimentary basin; Wilson & Hults, unpublished compilation, 2007-08

Cretaceous
66 - 145 Ma
Cretaceous sedimentary rocks
Jurassic - Devonian
145 - 419.2 Ma
Devonian-Jurassic volcanic: mafic rocks

References for regional geology:

Data provided by Macrostrat.org

J.C. Harrison, M.R. St-Onge, O.V. Petrov, S.I. Strelnikov, B.G. Lopatin, F.H. Wilson, S. Tella, D. Paul, T. Lynds, S.P. Shokalsky, C.K. Hults, S. Bergman, H.F. Jepsen, and A. Solli. Geological map of the Arctic. doi:10.4095/287868. Geological Survey of Canada Map 2159A.

Garrity, C.P., and Soller, D.R.,. Database of the Geologic Map of North America: adapted from the map by J.C. Reed, Jr. and others (2005). U.S. Geological Survey Data Series 424 .

Geological Survey of Canada. Generalized geological map of the world and linked databases. doi:10.4095/195142. Open File 2915d.

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.



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References

Bottge, R.G., 1986, Maps summarizing land availability for mineral exploration and development in northcentral Alaska, 1985: U.S. Bureau of Mines Open-File Report 70-86, 14 sheets. Cobb, E.H., 1972, Metallic mineral resources map of the Bettles quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-387, 1 sheet, scale 1:250,000. Cobb, E.H., 1973, Placer deposits of Alaska: U.S. Geological Survey Bulletin 1374, 213 p. Grybeck, Donald, 1977, Map showing known mineral deposits of the Brooks Range, Alaska: U.S. Geological Survey Open-File Report 77-166-C, 45 p., 1 sheet, scale 1:1,000,000. Heiner, L.E., and Wolff, E.N., eds., 1968, Mineral resources of northern Alaska, Final report, submitted to the NORTH Commission: University of Alaska, Mineral Industry Research Laboratory Report No. 16, 306 p. Maddren, A.G., 1910, The Koyukuk-Chandalar gold region: U.S. Geological Survey Bulletin 442-G, p. 284-315. Maddren, A.G., 1913, The Koyukuk-Chandalar region, Alaska: U.S. Geological Survey Bulletin 532, 119 p. Mulligan, J.J., 1974, Mineral resources of the Trans-Alaska Pipeline corridor: U.S. Bureau of Mines Information Circular 8626, 24 p. Nelson, A.E., West, W.S., and Matsko, J.J., (1952) 1954, Reconnaissance for radioactive deposits in eastern Alaska: U.S. Geological Survey Circular 348, 21 p. Overstreet, W.C., 1967, The geologic occurrence of monazite: U.S. Geological Survey Professional Paper 530, 327 p. Reed, I.M., 1938, Upper Koyukuk region, Alaska (Wiseman, Chandalar, and Bettles): Alaska Territorial Department of Mines Miscellaneous Report 194-7, 201 p. Schrader, F.C., 1904, A reconnaissance in northern Alaska across the Rocky Mountains, along Koyukuk, John, Anaktuvuk, and Colville rivers and the Arctic coast to Cape Lisburne, in 1901, with notes by W.J. Peters: U.S. Geological Survey Professional Paper 20, 139 p. Wedow, Helmuth, Jr., 1953, Preliminary summary of reconnaissance for uranium and thorium in Alaska, 1952: U.S. Geological Survey Circular 248, 15 p. Wedow, Helmuth, Jr., White, M.G. and Moxham, R.M., 1952, Interim report on an appraisal of the uranium possibilities of Alaska: U.S. Geological Survey Open-File Report 51, 123 p. White, M.G., 1952, Radioactivity of selected rocks and placer concentrates from northeastern Alaska: U.S. Geological Survey Circular 195, 12 p.

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