Unnamed Occurrence (ARDF - EA161; at head of Turk Creek), Fortymile District, Southeast Fairbanks Borough, Alaska, USA
Location: This magnetite prospect covers about 1 square mile about 2.8 miles south-southeast of Boundary, near the top of the ridge in the headwaters of Turk Creek. This occurrence is informally called the Turk Creek Magnetite prospect or the Minnesota Mountain prospect. The coordinates are the approximate center of the claims, in section 15, T. 26 N, R. 22 E., of the Copper River Meridian; the location is accurate within one-half mile.
Geology: Magnetite-bearing serpentinized ultramafic rocks crop out over about 1 square mile on the east side of the top of the ridge near VABM Minnesota in the headwaters of the south fork of Turk Creek (Foster, 1969 [B 1271-G]; R.L. Flynn, unpub. data, 2000). Field relations suggest that this large ultramafic body is thrust over greenschist-facies metamorphic rocks to the east, and many of the smaller, lens-like serpentinized ultramafic bodies in the Boundary area appear to be imbricated along thrust faults (Foster and others, 1985). The magnetite-bearing serpentinized ultramafic rocks define a strong magnetic high in the Alaska Division of Geological and Geophysical Surveys airborne geophysical data (Burns and Liss, 1999; Alaska Division of Geological and Geophysical Surveys and others, 1999). Magnetite crystals range from one- to three-eighths of an inch in diameter. Assays of five rock samples range from 7.66 to 8.43 percent iron (Saunders, 1960). The ultramafic rocks are probably metamorphosed, serpentinized dunite that consists of large grains of magnetite surrounded by alteration rims of brucite and chlorite in a matrix of antigorite and relict olivine (Foster, 1969 [B 1271-G]). The rocks are cut by late veins of magnetite and magnesite. Locally, minor silica-carbonate alteration is present (R.L. Flynn, unpub. data, 2000). Nearby, Turk Creek (EA160) has produced placer gold. Five claims were staked on magnetite-bearing rock in the vicinity in 1959 (Saunders, 1960).
Workings: Magnetite-bearing rock south of Boundary was discovered by placer miners in 1958; they staked five claims in 1959, and the property was visited by R.H. Saunders, Territorial mining engineer in 1959 (Saunders, 1960). Saunders reported assays fortfive samples that contain from 7.66 to 8.43 percent iron.
Alteration: The serpentinized ultramafic rocks in the headwaters of the south fork of Turk Creek are probably metamorphosed, serpentinized dunite that consists of large grains of magnetite surrounded by alteration rims of brucite and chlorite in a matrix of antigorite with relicts of olivine. The rocks are cut by late veins of magnetite and magnesite (Foster, 1969 [B-1271-G]). Minor silica-carbonate alteration is present locally (R.L. Flynn, unpub. data, 2000).
Commodities (Major) - Fe
Development Status: None
ReferencesAlaska Division of Geological and Geophysical Surveys, Geoterrex-Dighem, and Stevens Exploration Management Corporation, 1999, CD-ROM containing profile and gridded data and section lines of 1997 geophysical survey data for part of the Fortymile mining district, Alaska, southern Eagle and northern Tanacross quadrangles: Alaska Division of Geological and Geophysical Surveys Public-Data File 99-9, 2 CD-ROMs. Asher, R.R., 1970, Geochemistry and geology, Boundary area, Fortymile district, Eagle A-1 quadrangle, Alaska: Alaska Division of Mines and Geology Geochemical Report 23, 32 p., 2 sheets, scale 1:63,360. Burns, L.E., and Liss, S.A., 1999, Portfolio of aeromagnetic and resistivity maps of part of the Fortymile mining district, Alaska, southern Eagle and northern Tanacross quadrangles: Alaska Division of Geological and Geophysical Surveys Public-Data File 99-6, 20 p. Foster, H.L., 1969, Reconnaissance geology of the Eagle A-1 and A-2 quadrangles, Alaska: U.S. Geological Survey Bulletin 1271-G, p. G1-G30 Foster, H.L., and Keith, T.E.C., 1974, Ultramafic rocks of the Eagle quadrangle, east-central Alaska: U.S. Geological Survey Journal of Research, v. 2, no. 6, p. 657-669. Foster, H. L., Cushing, G. W., Keith, T. E. C., and Laird, J., 1985, Early Mesozoic tectonic history of the Boundary area, east-central Alaska: Geophysical Research Letters, v. 12, p. 553-556. Keith, T.E.C., and Foster, H.L., 1973, Basic data on ultramafic rocks of the Eagle quadrangle, east-central Alaska: U.S. Geological Survey Open-File Report 73-140, 4 sheets. Mortensen, J.K. (compiler), 1999, Yukonage--An isotopic age database for the Yukon Territory, in Gordey, S.P., and Makepeace, A.J., compilers, Yukon Digital Geology: Indian and Northern Affairs Canada, Exploration and Geological Services Division, Yukon Region. Saunders, R.H., 1960, Report on a magnetite occurrence near Boundary (Turk Creek): Alaska Division of Mines and Minerals Prospect Evaluation Report PE 60-8, 3 p.
1 entry listed. 1 valid mineral.
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