Dennison Fork Prospect, Fortymile District, Southeast Fairbanks Borough, Alaska, USA
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Geology: The rocks in the lower part of the Dennison Fork valley consist predominantly of the Triassic granodiorite of the Taylor Mountain batholith (Foster, 1976). A rubidium/strontium date on biotite from granodiorite of the batholith gives an age of approximately 190 Ma (Wasserberg and others, 1963). The granodiorite has been intruded by minor Tertiary gabbro. Quaternary alluvium, colluvium, and terrace deposits are extensive in the Dennison Fork drainage. Considerable prospecting and drilling has been carried out near the mouth of Dennison Fork, but there is no record of actual placer mining. A ditch was dug in 1911 to work the bench gravels (Porter, 1912); despite completion of the ditch, no mining occurred (Ellsworth and Davenport, 1913). Gold near the mouth of Dennison Fork is very fine grained. In 1918, there was extensive prospecting of high benches near the mouth of Dennison Fork,and plans were made to build a 21-mile-long ditch (Martin, 1920). Additional systematic prospecting of a large body of gravel occurred in 1920 (Brooks, 1922). A sample from the foot of the bluff at the junction of Dennison and Mosquito Forks contained trace scheelite (White and others, 1963).
Workings: Considerable prospecting and drilling has been carried out near the mouth of Dennison Fork, but there is no record of actual placer mining. A ditch was dug in 1911 to work the bench gravels (Porter, 1912); despite completion of the ditch, no mining occurred (Ellsworth and Davenport, 1913). Gold near the mouth of Dennison Fork is very fine grained. In 1918, there was extensive prospecting of high benches near the mouth of Dennison Fork,and plans were made to build a 21-mile-long ditch (Martin, 1920). Additional systematic prospecting of a large body of gravel occurred in 1920 (Brooks, 1922). A sample from the foot of the bluff at the junction of Dennison and Mosquito Forks contained trace scheelite (White and others, 1963).
Age: Quaternary.
Commodities (Major) - Au; (Minor) - W
Development Status: None
Deposit Model: Placer Au (Cox and Singer, 1986; model 39a).
Mineral List
2 entries listed. 2 valid minerals.
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References
Brooks, A.H., 1922, The Alaskan mining industry in 1920: U.S. Geological Survey Bulletin 722-A, p. 7-67. Brooks, A.H., and Martin, G.C., 1921, The Alaskan mining industry in 1919: U.S. Geological Survey Bulletin 714, p. 59-95. Burleigh, R.E., and Lear, K.G., 1994, Compilation of data for phase 1 of the mineral resource evaluation of the Bureau of Land Management Black River and Fortymile River subunits: U.S. Bureau of Mines Open-File Report 48-94, 116 p. Cobb, E.H., 1972, Metallic mineral resources map of the Eagle quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-393, 1:250,000 scale, 1 sheet. Cobb, E.H., 1973, Placer deposits of Alaska: U.S. Geological Survey Bulletin 1374, 213 p. Cobb, E.H., 1977, Summary of references to mineral occurrences in the Eagle quadrangle, Alaska: U.S. Geological Survey Open-File Report 77-845, 122 p. Eberlein, G.D., Chapman, R.M., Foster, H.L., and Gassaway, J.S., 1977, Map and table describing known metalliferous and selected nonmetalliferous mineral deposits in central Alaska: U.S. Geological Survey Open-File Report 77-168-D, 132 p., 1 map sheet, scale 1:1,000,000. Ellsworth, C.E., and Davenport, R.W., 1913, Placer mining in the Yukon-Tanana region, in Brooks, A.H., ed., Mineral resources of Alaska, report on investigations in 1912: U.S. Geological Survey Bulletin 542-F, p. 203-222. Foster, H.L., 1976, Geologic map of the Eagle quadrangle, Alaska: U.S. Geological Survey Miscellaneous Investigations Series, Map 922, 1 sheet, scale 1:250,000. Martin, G.C., 1920, The Alaskan mining industry in 1918: U.S. Geological Survey Bulletin 712, p. 11-52. Porter, E.A., 1912, Placer mining in the Fortymile, Eagle, and Seventymile River districts: U.S. Geological Survey Bulletin 520-G, p. 211-218. Powers, J.B., 1935, Brief history of the Fortymile and Eagle Mining Districts to 1935: Alaska Territorial Department of Mines Mineral Report MR 60-2, 19 p. Swainbank, R.C., Bundtzen, T.K., Clough, A.H., Hansen, E.W., and Nelson, M.G., 1993, Alaska's Mineral Industry 1992: Alaska Division of Geological and Geophysical Surveys Special Report 47, 80 p. Wasserberg, G.J., Eberlein, G.D., and Lanphere, M.A., 1963, Age of Birch Creek Schist and some batholithic intrusion in Alaska [abs.]: Geological Society of America Special Paper 73, p. 258-259. White, M.G., Nelson, A.E., and Matzko, J.J., 1963, Radiometric investigation along the Taylor Highway and part of the Tanana River, Alaska: U.S. Geological Survey Bulletin 1155, p. 77-82.
