Gold Bench Mine, Koyukuk District, Yukon-Koyukuk Borough, Alaska, USA
|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.
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)
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.
0 - 2.588 Ma
|Unconsolidated surficial deposits, undivided|
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|
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.