Willow bench Mine, Iditarod District, Yukon-Koyukuk Borough, Alaska, USA
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Geology: The Willow Bench is probably an an ancestral fluvial channel of Willow that is now perched on a bench 65 to 100 feet above modern Willow Creek. The Willow Bench is a reflection of the asymmetry of the Willow Creek basin, which formed due to differing freeze and thaw environments on the northwest versus the southeast slopes (Bundtzen and others, 1992). Auriferous gravels on the Willow Bench are overlain by 25 to 32 feet of organic-rich, frozen muck. Based on churn drilling, the auriferous gravel deposit is 10 to 12 feet thick, 150 to 650 feet wide, and can be traced for nearly 2.5 miles. Air photo analysis summarized in Bundtzen and others (1992) suggest that the pay channel may extend further to the southwest and southeast in other ancestral channels of Willow Creek before spilling into Bonanza Creek. In addition to gold, the principal heavy minerals in concentrates include zircon, cinnabar, chromite, magnetite, and ilmenite, (Cobb, 1976 [OFR 76-576]; Bundtzen, Cox, and Veach, 1987; Bundtzen and others, 1992). The gold averages 874 fine, about 10 points richer than Happy Creek (ID104). The source of the placer gold and heavy minerals on the Willow bench is probably auriferous stockwork veins in intrusive rocks on Chicken Mountain (Bundtzen and others, 1992). There are few monzonite cobbles in the Willow bench paystreak, possibly reflecting its considerable distance from Chicken Mountain (Mertie, 1936). Mertie and Harrington (1924) believed that placer gold in the Willow Bench was derived from a different lode source than Chicken Mountain. Alternatively, the dike swarm exposed at the head of the Willow Creek Mine might be the source of the placer gold in the Willow bench deposit. Based on examination of published and unpublished records, Bundtzen and others (1992) estimated that Willow Creek and the Willow bench (ID105) produced at least 41,948 ounces of gold and 5,033 ounces of silver, mainly from 1910 to 1986.
Workings: The Willow bench mine was discovered in 1910 and is possibly the deposit described as the Haggerty Bench (Maddren, 1911). The bench deposit was originally mined by ground sluicing about 12 feet of overburden. The ground was reported to have about $1.25 of gold per bedrock foot (with gold at $20.67 per ounce). Due to the thick, frozen nature of the overburden, the Willow Bench was mined for many years by hydraulic mining. In the early 1980s, Flat Creek Placers, Inc. (John and Richard Fullerton), planned to remove the overburden in preparation for a large scale mine. However, their company was unable to obtain permits from the U.S. Environmental Protection Agency (EPA) for hydraulic removal of overburden and the project was discontinued in 1986. No mining has occurred since.
Age: Mammoth and horse remains from overburden on the Willow bench yielded radiocarbon ages of about 28,000 years BP (J.T. Kline, unpublished data, 1986). The ancestral Willow bench auriferous fluvial deposits are believed to be Late Tertiary in age, analogous to other similar placers in Interior Alaska (Hopkins and others, 1971).
Production: Based on examination of published and unpublished records, Bundtzen and others (1992) estimated that Willow Creek and the Willow bench (ID105) produced at least 41,948 ounces of gold and 5,033 ounces of silver, mainly from 1910 to 1986.
Reserves: There may be additional placer gold resources in extensions of the Willow bench to the southwest and southeast.
Commodities (Major) - Au; (Minor) - Ag, Cr, Hg, Sn, U, W, Zr
Development Status: Yes
Deposit Model: Au placer deposit (Cox and Singer, 1986; model 39a).
6 entries listed. 6 valid minerals.
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Bundtzen, T.K., and Miller, M.L., 1997, Precious metals associated with Late Cretaceous-early Tertiary igneous rocks of southwestern Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 242-286. Bundtzen, T.K., Cox, B.C., and Veach, N.C., 1987, Heavy mineral provenance studies in the Iditarod and Innoko districts, western Alaska: Process Mineralogy VII, The Metallurgical Society, p. 221-246. Bundtzen, T.K., Miller, M.L., Laird, G.M., and Bull, K.F., 1992, Geology and mineral resources of Iditarod mining district, Iditarod B-4 and eastern B-5 quadrangles, southwestern Alaska: Alaska Division of Geological and Geophysical Surveys Professional Report 97, 46 p., 2 sheets, scale 1:63,360. Cobb, E.H., 1972, Metallic mineral resources map of the Iditarod quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-363, 1 sheet, scale 1:250,000. Cobb, E.H., 1976, Summary of references to mineral occurrences (other than mineral fuels and construction material) in the Iditarod and Ophir quadrangles, Alaska: U.S. Geological Survey Open-File Report 76-576, 101 p. Hopkins, D.M., Matthews, J.V., Wolfe, J.A., and Silberman, M.L., 1971, A Pliocene flora and insect fauna from the Bering Sea region: Paleoecology, vol. 9, p. 211-231. Maddren, A.G., 1911, Gold placer mining developments in the Innoko-Iditarod region: U.S. Geological Survey Bulletin 480-I, p. 236-270. Mertie, J.B., Jr., 1936, Mineral deposits of the Ruby-Kuskokwim region, Alaska: U.S. Geological Survey Bulletin 864-C, p. 115-245. Mertie, J.B., Jr., and Harrington, G.L., 1916, Mineral resources of the Ruby-Kuskokwim region, in Brooks, A.H., and others, Mineral Resources of Alaska, Report on Progress of Investigations in 1915: U.S. Geological Survey Bulletin 642-H, p. 223-266. Miller, M.L., and Bundtzen, T.K., 1994, Generalized geologic map of the Iditarod quadrangle, Alaska showing potassium-argon, major oxide, trace element, fossil, paleocurrent, and archeological sample localities: U.S. Geological Survey Miscellaneous Field Studies Map MF-2219-A, 48 pages; 1 sheet, scale 1:250,000. Miller, M.L., Bundtzen, T.K., and Gray, J.E., 2005, Mineral resource assessment of the Iditarod quadrangle, west-central Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2219-B, scale 1:250,000, pamphlet.