National mineral assessment tract AK-SC02 (Besshi massive sulfide)

Tract AK-SC02
Geographic region South Central, AK
Tract area 1,660sq km
Deposit type Besshi massive sulfide
Deposit age Triassic - Jurassic

Rationale

Explained by Warren J. Nokleberg, Dennis P. Cox, Richard J. Goldfarb, and David L. Campbell.
On the choice of deposit models
Parts of the western Alaska Range are permissive for Triassic and Jurassic Besshi massive sulfide deposits because of geologically favorable bedrock units, a known occurrence, geochemical anomalies of Cu, Pb, Zn, As, Ag, and Co in stream sediment samples, and local aeromagnetic anomalies.
On the delineation of permissive tracts
The favorable geologic units in the tract are the marine pillow basalts that are interlayered with Triassic through Jurassic marine sedimentary rocks of the Mystic terrane. A subdued aeromagnetic birds-eye pattern might be expected over the pillow basalts, but is not seen on the aeromagnetic map of Alaska (Godson, 1994). The distribution of the geologic units defining the tract is adapted from Nokleberg and others (1994). The distribution of this belt of mineral deposits is adapted from MacKevett and others (1978) and Nokleberg and others (1987, 1993). A mineral resource assessment of the 250,000-scale Talkeetna quadrangle (Reed and others, 1979) covers part of the tract.
Important examples of this type of deposit
The best known deposit of this type is the Shellebarger Pass occurrence in the Talkeetna quadrangle (Reed and Eberlein, 1972; Bundtzen and Gilbert, 1983).
On the numerical estimates made
The tract is locally covered by extensive ice fields and glaciers. Because only one occurrence is known, no attempt was made to estimate the number of undiscovered Besshi massive sulfide deposits in this tract.
References
Bundtzen, T.K., and Gilbert, W.G., 1983, Outline of geology and mineral resources of upper Kuskokwim region, Alaska: Alaska Geological Society 1982 Symposium on Western Alaska, v. 3, p. 101-117.
Godson, Richard H., 1994, Composite magnetic anomaly map of Alaska and adjacent offshore area, in Plafker, G. and Berg, H.C., eds., The Geology of Alaska: Boulder, Colorado, Geological Society of America: The Geology of North America, v. G1, plate 10, scale 1:2,500,000.
MacKevett, E.M., Jr., Singer, D.A., and Holloway, C.D., 1978, Maps and tables describing metalliferous mineral resource potential of southern Alaska: U.S. Geological Survey Open-File Report 78-1E, 2 sheets, scale 1:1,000,000, 45 p.
Nokleberg, W.J., Bundtzen, T.K., Berg, H.C., Brew, D.A., Grybeck, Donald, Robinson, M.S., Smith, T.E., and Yeend, Warren, 1987. Significant metalliferous lode deposits and placer districts of Alaska: U.S. Geological Survey Bulletin 1786, 104 p.
Nokleberg, W.J., Bundtzen, T.K., Grybeck, Donald, Koch, R.D., Eremin, R.A., Rozenblum, I.S., Sidorov, A.A., Byalobzhesky, S.G., Sosunov, G.M., Shpikerman, V.I., and Gorodinsky, M.E., 1993, Metallogenesis of mainland Alaska and the Russian Northeast: Mineral deposit maps, models, and tables, metallogenic belt maps and interpretation, and references cited: U.S. Geological Survey Open-File Report 93-339, 222 pages, 1 map, scale 1:4,000,000; 5 maps, scale 1:10,000,000.
Nokleberg, W.J., Moll-Stalcup, E.J., Miller, T.P., Brew, D.A., Grantz, Arthur, Reed, J.C., Jr., Plafker, George, Moore, T.E., Silva, S.R., and Patton, William W., Jr., 1994, Tectonostratigraphic terrane and overlap assemblage map of Alaska: U.S. Geological Survey Open-File Report 94-194, 53 p., scale 1:2,500,000.
Reed, B.L., and Eberlein, G.D., 1972, Massive sulfide deposits near Shellebarger Pass, southern Alaska Range, Alaska: U.S. Geological Survey Bulletin 1342, 45 p.
Reed, B.L., Nelson, S.W., Curtin, G.C., and Singer, D.A., 1979, Mineral resources map of the Talkeetna quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-870-D, l sheet, scale l:250,000.

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