Mosquito Fork; South Fork of the Fortymile River; Atwater Bar Mine, Fortymile District, Southeast Fairbanks Borough, Alaska, USA

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Location: Mosquito Fork is a major tributary to the South Fork of the Fortymile River. Some references to the location of placer gold workings in this area are not consistent with the river names shown on the U.S. Geological Survey 1:63,360-scale topographic map of the Eagle A-2 quadrangle (1956; revised 1971). In the past, Mosquito Fork was considered either part of the South Fork or, alternatively, the area between the junction of Dennison Fork and Walker Fork was considered part of the Mosquito Fork. In this record, all references to these two rivers will correspond to those shown on the current Eagle A-2 quadrangle topographic map (1956; revised 1971). Atwater Bar is a gravel bar at the mouth of Atwater Creek (see EA137), and references to it are grouped with the Mosquito Fork-South Fork placers. The coordinates correspond to an arbitrarily chosen point located in section 4, T. 26 N., R. 18 E., of the Copper River Meridian. This site is locality 83 of Burleigh and Lear (1994), locality 57 of Eberlein and others (1977), and locality 54 of Cobb (1972 [MF-393]).
Geology: The rocks in the vicinity of Mosquito Fork and upper South Fork predominantly are Triassic granodiorite of the Taylor Mountain batholith (Foster, 1976). Sphene from the Taylor Mountain batholith gives a uranium-lead age of 214 Ma (Aleinikoff and others, 1981); this is interpreted to represent the age of the intrusion. Minor Paleozoic, quartz mica schist and greenschist is also present. The granodiorite and schist have been cut and intruded by Tertiary basalt. In the vicinity of Chicken Creek, early Tertiary sandstone, conglomerate, and tuff are locally abundant. Quaternary alluvium, colluvium, and terrace deposits are extensive in the Mosquito Fork and upper South Fork drainages. Placer gold has been produced from both the upper South Fork and lower Mosquito Fork of the Fortymile River. The gold is very fine grained and light colored and has a fineness of approximately 900 parts of gold per thousand (Mertie, 1938). The gravel on the river floor is 2.5 inches to 10 feet thick (Yeend, 1996). Minerals in panned samples, probably all from Atwater Bar, include ilmenite, garnet, magnetite, zircon, sphene, tourmaline, pyrite, olivine, barite, and traces of scheelite, cassiterite, gold, silver, monazite, and uranium-bearing thorianite (Wedow and others, 1954; Overstreet, 1967). Dredge concentrates, probably from the Atwater dredge operated on Mosquito Fork near the mouth of Lost Chicken Creek(?) or from Atwater Bar, contained magnetite, ilmenite, marcasite, pyrite, barite, silver, and scheelite (Smith, 1968). The first record of mining for placer gold on Mosquito Fork and upper South Fork was in 1905 (Prindle, 1906 [B 284]). In 1910, four men mined at Atwater Bar near the mouth of Atwater Creek. Prospecting took place in 1914 (Brooks, 1915). An extensive drilling program was conducted in 1925, and mining took place in 1926 (Moffit, 1927; Smith, 1929). In 1936, the Atwater dredge, owned and operated by the Alaska Gold Dredging Corporation, began operating at Gibralter Bar on the South Fork about 0.9 mile below the mouth of Lost Chicken Creek (EA131). In 1936, the Atwater dredge worked upriver mining gravel 1.4 to 6.5 feet in thickness; it mined a cut 150 feet wide that extended 1,200 feet along the river (Yeend, 1996). The dredge worked for about a year and a half in this area. In 1949, prospecting occurred at Atwater Bar. Eberlein and others (1977) noted that small suction dredges were being used in small-scale placer mining operations in the 1970's. The Mosquito Fork of the Fortymile River was being actively prospected in 1992 (Swainbank and others, 1993), and placer mining along the South Fork has been done exclusively by suction dredging in recent years (Yeend, 1996). Approximately 2.5 miles above Chicken Creek on Mosquito Fork, there is an unnamed lode gold occurrence (EA129) exposed in the north canyon wall (Prindle, 1905). Smith (1968) observed flour gold in a lode deposit about 200 feet above the river and he probably was referring to this occurrence (Cobb, 1977 [OFR 77-845]). This lode occurrence and the Purdy lode (EA121), probably contributed gold to the placer deposits along Mosquito Fork. Mosquito Fork has several placer gold-bearing tributaries, including Ingle Creek (EA111) and Chicken Creek (EA128). The upper part of South Fork has several placer gold-bearing tributaries, including Mosquito Fork, Dennison Fork (EA136), and Lost Chicken Creek (EA131).
Workings: The first record of mining for placer gold on Mosquito Fork and upper South Fork was in 1905 (Prindle, 1906). In 1910, four men mined at Atwater Bar near the mouth of Atwater Creek. Prospecting took place in 1914 (Brooks, 1915). An extensive drilling program was conducted in 1925, and mining took place in 1926 (Moffit, 1927; Smith, 1929). In 1936 and 1937 a dredge operated on upper South Fork and lower Mosquito Fork. The dredge was installed in the spring of 1936 at a point about 800 feet downstream from Gibraltar Rock, a prominent bluff on the north side of South Fork about three-quarters of a mile below the mouth of Lost Chicken Creek (see EA131; Mertie, 1938). The dredge worked upstream from this point. In 1949, prospecting occurred at Atwater Bar. Eberlein and others (1977) noted that small suction dredges were being used in small-scale placer mining operations in the 1970's. The Mosquito Fork of the Fortymile River was being actively explored for placer gold in 1992 (Swainbank and others, 1993).
Age: Quaternary.

Commodities (Major) - Au; (Minor) - Ag, Sn, U?, W
Development Status: Yes; small
Deposit Model: Placer Au (Cox and Singer, 1986; model 39a).

References

Aleinikoff, J.N., Dusel-Bacon, Cynthia, and Foster, H.L., 1981, Geochronologic studies in the Yukon-Tanana Upland, east-central Alaska, in Albert, N.R., and Hudson, T., eds., The United States Geological Survey in Alaska--Accomplishments during 1979, U.S. Geological Survey Circular C-823-B, p. 34-37. Brooks, A.H., 1915, The Alaskan mining industry in 1914: U.S. Geological Survey Bulletin 622-A, p. 15-68. 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 Parker, G.L., 1911, Placer mining in the Yukon-Tanana region: U.S. Geological Survey Bulletin 480-F, p. 153-172. 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. Mertie, J.B., Jr., 1938, Gold placers of the Fortymile, Eagle, and Circle districts, Alaska: U.S. Geological Survey Bulletin 897-C, p. 133-261. Moffit, F.H., 1927, Mineral industry of Alaska in 1925: U.S. Geological Survey Bulletin 792, p. 1-39. Overstreet, W.C., 1967, The geologic occurrence of monazite: U.S. Geological Survey Professional Paper 530, 327 p. Prindle, L.M., 1905, The gold placers of the Fortymile, Birch Creek, and Fairbanks regions, Alaska: U.S. Geological Survey Bulletin 251, 89 p. Prindle, L.M., 1906, Yukon placer fields: U.S. Geological Survey Bulletin 284, p. 109-127. Prindle, L.M., 1908, The Fortymile gold-placer district, in Brooks, A.H., ed., Mineral Resources of Alaska, Report on progress of investigations in 1907: U.S. Geological Survey Bulletin 345, p. 187-197. Smith, P.S., 1929, Mineral industry of Alaska in 1926: U.S. Geological Survey Bulletin 797, p. 1-50. Smith, P.S., 1937, Mineral industry of Alaska in 1935: U.S. Geological Survey Bulletin 880-A, p. 1-95. Smith, P.S., 1938, Mineral industry of Alaska in 1936: U.S. Geological Survey Bulletin 897-A, p. 1-107. Smith, P.S., 1939, Mineral industry of Alaska in 1937: U.S. Geological Survey Bulletin 910-A, p. 1-113. Smith, P.S., 1939, Mineral industry of Alaska in 1938: U.S. Geological Survey Bulletin 917-A, p. 1-113. Smith, P.S., 1941, Mineral industry of Alaska in 1939: U.S. Geological Survey Bulletin 926-A, p. 1-106. Smith, P.S., 1942, Mineral industry of Alaska in 1940: U.S. Geological Survey Bulletin 933-A, p. 1-102. Smith, W.H., 1968, Geochemical investigation of a portion of the Fortymile district, Alaska: Alaska Division of Mines and Minerals Geochemical Report 16, 17 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 52-165, 124 p. Wedow, Helmuth, Jr., White, M.G., and Tolbert, G.E., 1954, Reconnaissance for radioactive deposits in east-central Alaska, 1949: U.S. Geological Survey Circular 335, 22 p. 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.

Mineral List

Cassiterite

Gold

Scheelite

3 entries listed. 3 valid minerals.