National mineral assessment tract AK-SC01 (Basaltic Cu/Kennecott Cu)

Tract AK-SC01
Geographic region South Central, AK
Tract area 25,441sq km
Deposit type Basaltic Cu/Kennecott Cu
Deposit age Cretaceous


Explained by Warren J. Nokleberg, Dennis P. Cox, David L. Campbell, and Warren Yeend.
On the choice of deposit models
This tract covers parts of the Alaska Range and Wrangell Mountains, and is permissive for Kennecott Cu deposits because of favorable bedrock geologic units, known major mines, geochemical anomalies (Cu, As, Au, Ag, Hg), and locally abundant chalcopyrite and bornite in stream-sediment samples.
On the delineation of permissive tracts
The geologic units that define this tract are the Upper Triassic Chitistone Limestone and correlative units where underlain by the Upper Triassic Nikolai Greenstone. The mafic volcanic rocks of the Nikolai Greenstone exhibit elevated copper concentrations. The major Kennecott Cu deposits are interpreted as having been derived by a combination of hydrothermal and (or) metamorphic processes in the mid-Cretaceous (Nokleberg and others, 1993) from the upper part of the Nikolai to form the large deposits in the Kennecott Cu district (Bateman and McLaughlin, 1920) that occur in the overlying Upper Triassic Chitistone Limestone. Because the Nikolai is interpreted as the source of copper, the distribution of the Nikolai defines the permissive extent of both basaltic copper and Kennecott Cu deposit types. On the aeromagnetic map of Alaska (Godson, 1994), the tract displays local birds-eye magnetic anomalies that are characteristic of (meta) basalt. The Kennecott Cu deposits in this tract may be genetically associated with formation of Cu-Ag quartz vein deposits (tract AK-SC05) in that both are interpreted to have formed during mid-Cretaceous low-grade regional metamorphism (Nokleberg and others, 1993).
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). Mineral resource assessments of 250,000-scale quadrangles within the tract were done by Richter and others (1975), MacKevett (1976), Singer and others (1977, 1980), Cox and others (1989), and Nokleberg and others (1990).
Important examples of this type of deposit
The famous Cu mines of the Kennecott district (Bonzana, Erie, Green Butte, Jumbo, Mother Lode) (Bateman and McLaughlin, 1920; MacKevett, 1976) occur in this tract. These mines were included in the basaltic copper model described by Cox (1986). Considerable debate exists over the inclusion of the Kennecott Cu deposits in the basaltic copper model, because these deposits, although interpreted as derived from the basalts, are formed in carbonate rocks as much as 100 meters above the basalt (Bateman and McLaughlin, 1920). The associated Cu-Ag quartz vein deposits in the Kennecott district and elsewhere in the tract are classified into a metamorphic-related Cu-Ag quartz vein deposit model by Nokleberg and others (1993).
On the numerical estimates made
No tonnage and grade model exists for Kennecott Cu deposits. Therefore, no estimate of the number of undiscovered deposits was attempted.
Bateman, A.M., and McLaughlin, D.H., 1920, Geology of the ore deposits of Kennecott, Alaska: Economic Geology, v. 15, p. 1-80.
Cox, D.P., 1986, Descriptive Model of Basaltic Cu, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 130.
Cox, D.P., Light, T.D., Csejtey, Béla, and Campbell, D.L., 1989, Mineral resource assessment map of the Healy quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2058-A, scale 1:250,000.
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., 1976, Mineral deposits and occurrences in the McCarthy quadrangle, Alaska: U.S.G.S. Miscellaneous Field Studies Map MF-773-B, 2 sheets, scale 1:250,000.
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, MS., Smith, T.E., and Yeend, Warren, 1987, Significant metalliferous lode deposits and placer districts of Alaska: U.S. Geological Survey Bulletin 1786, p. 104.
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., Lange, I.M., Singer, D.A., Curtin, G.C., Tripp, R.B., Campbell, D.L., and Yeend, Warren, 1990, Mineral resource assessment maps of the Mount Hayes quadrangle, eastern Alaska Range, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-1996-A, 4 sheets, scale 1:250,000, 22 p.
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.
Richter, D.H., Singer, D.A., and Cox, D.P., 1975, Mineral resources map of the Nabesna quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-655-K, 1 sheet, scale l:250,000.
Singer, D. A., Csejtey, Bèla, Jr., and Miller, R.J., 1980, Map and discussion of the metalliferous and selected non-metalliferous mineral resources of Talkeetna Mountains quadrangle, Alaska: U.S. Geological Survey Open-File Report 78-558-Q, 34 p., 1 sheet, scale 1:250,000.
Singer, D.A., and MacKevett, E.M., Jr., 1977, Mineral resources map of the McCarthy quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-773-C, l sheet, scale l:250,000.

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