National mineral assessment tract AK-BR13 (Placer Au)

Tract AK-BR13
Geographic region Brooks Range, AK
Tract area 108,594sq km
Deposit type Placer Au
Deposit age Quaternary

Rationale

Explained by Jeanine M. Schmidt, Karen D. Kelley, Richard I. Grauch, Byron R. Berger, John W. Cady, Gregory T. Spanski, James E. Kilburn.
On the choice of deposit models
The geologic setting and host rocks indicate that this tract is permissive for modern placer deposits as well as for preserved paleoplacer gold deposits as described by Yeend (1986). Several small- to moderate-sized placer deposits and districts (?Tertiary to Quaternary) occur, and several Au-bearing lode deposits are known.
On the delineation of permissive tracts
This tract comprises broad areas of Proterozoic(?) to Paleozoic mixed schists (predominately pelitic schists, and lesser quartzitic, mafic, calcareous, and carbonaceous schists), and sedimentary rocks of the Upper Devonian to Mississippian(?) Endicott Group (Brosge and others, 1988; Tailleur and others, 1967). The mixed schists are host to rare low-sulfide Au-quartz veins, and apparently form the bedrock upstream of most of the extensive Quaternary placer Au deposits. The Endicott group, including the nonmarine Kekituk and Kanayut Conglomerates, the marine Noatak Sandstone and the Kayak and Hunt Fork Shales, generally produces a broad distinct aeromagnetic low, and contains scattered geochemical anomalies of Zn and Pb. The sand-and conglomerate-dominant lithologies of the Endicott Group are consolidated, lithified equivalents of gravel and sand deposits. They may contain preserved paleoplacer Au deposits, because they include both beach and alluvial facies, and were shed from a continental source (Nilsen, 1981; Moore and Nilsen, 1984).
Important examples of this type of deposit
This tract includes the Wiseman, Chandalar, Kiana, Shugnak, and Koyukuk placer gold districts. The Lucky Six Creek, Little Squaw Creek, Nolan Creek, and Hammond River deposits are a few of the noteworthy placer deposits (Cobb, 1973; Dillon, 1982).
On the numerical estimates made
Because the lode sources and geomorphic setting of the existing placers have not been well established, and because the timing and genesis of the lode gold deposits is not known, no estimate of the number of undiscovered placer or paleoplacer deposits has been attempted.
References
Brosge, W.P., Nilsen, T.H., Moore, T.E., and Dutro, J.T., Jr., 1988, Geology of the Upper Devonian and Lower Mississippian (?) Kanayut Conglomerate in the central and eastern Brooks Range, in Gryc, George, ed., Geology and exploration of the National Petroleum Reserve in Alaska, 1974 to 1982, U.S. Geological Survey Professional Paper 1399, scale 1:500,000, p. 299-316.
Cobb, E.H., 1973, Placer deposits of Alaska: U.S. Geological Survey Bulletin 1374, 213 p.
Dillon, J.T., 1982, Source of lode- and placer-gold deposits of the Chandalar and upper Koyukuk districts, Alaska: Alaska Division of Geological and Geophysical Surveys Open-File Report 158, 22 p.
Moore, T.E., and Nilsen, T.H., 1984, Regional sedimentological variations in the Upper Devonian and Lower Mississippian(?) Kanayut Conglomerate, Brooks Range, Alaska: Sedimentary Geology, v. 38, p. 464-498
Nilsen, T.H., 1984, Upper Devonian and Lower Mississippian redbeds, Brooks Range, Alaska, in Miall, A.D., ed., Sedimentation and tectonics in alluvial basins: Geological Association of Canada special Paper 23, p. 187-219.
Tailleur, I.L., Brosge, W.P., and Reiser, H.N., 1967, Palinspastic analysis of Devonian rocks in northwestern Alaska, in Oswald, D.H., ed., International Symposium on the Devonian System, v.2: Alberta Society of Petroleum Geologists, Calgary, Canada, p. 1345-1361.
Yeend, Warren, 1986, Descriptive model of placer Au-PGE, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 261.

Geographic coverage

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