National mineral assessment tract AK-SC19 (Synorogenic-synvolcanic Ni-Cu)

Tract AK-SC19
Geographic region South Central, AK
Tract area 3,994sq km
Deposit type Synorogenic-synvolcanic Ni-Cu
Deposit age Triassic - Jurassic

Rationale

Explained by Warren J. Nokleberg, Richard J. Goldfarb, and David L. Campbell.
On the choice of deposit models
This tract, in the northern Chugach Mountains, is permissive for Upper Triassic and Lower Jurassic gabbroic Ni-Cu deposits because of favorable bedrock geology, one known deposit, geophysical anomalies, and geochemical anomalies (Cu, Ni). The gabbroic Ni-Cu deposit type is a modification by Nokleberg and others (1987) of the synorogenic-synvolcanic Ni-Cu deposit type of Singer and others (1986) in Cox and Singer (1986).
On the delineation of permissive tracts
The favorable geologic units for the tract are mafic and ultramafic plutonic rocks of the Border Ranges ultramafic-mafic complex of the southern part of the Peninsular terrane (Burns, 1985; Plafker and others, 1989; DeBari and Coleman, 1989; Nokleberg and others, 1994c). The tract occurs along the northern margin of Kodiak Island, on the Kenai Peninsula, and along the northern flank of the Chugach Mountains in coastal southern Alaska (Foley and Barker, 1985; Foley and others, 1985, 1987). This belt occurs sporadically along a strike distance of several hundred kilometers from Kodiak Island to the southwest to the eastern Chugach Mountains to the east.
The mainly Lower Jurassic Border Ranges ultramafic and mafic assemblage constitutes an igneous belt of ultramafic tectonites and cumulate gabbro and norite that occurs along the southern, faulted margin of the Peninsular sequence of the Wrangellia island arc superterrane immediately north of the Border Ranges fault system (MacKevett and Plafker, 1974; Burns, 1985; Plafker and others, 1989; Nokleberg and others, 1989,1994c, d, e). The ultramafic and mafic rocks are interpreted as the deep-level root of the mainly Jurassic Talkeetna island arc of the Peninsular sequence of the Wrangellia superterrane. This sequence consists of the Lower Jurassic marine andesite volcanic rocks of the Talkeetna Formation and the Middle Jurassic plutonic rocks of the Alaska-Aleutian Range batholith. If these interpretations are correct, this tract is permissive for a deep-level suite of lode deposits formed in the Talkeetna island arc that is a key component of the Peninsular sequence. The Lower and Middle Jurassic island arc rocks of the Peninsular sequence of the Wrangellia terrane are tectonically linked to a discontinuous belt of Lower Triassic blueschist and to the McHugh Complex, a pair of partly coeval subduction zone units that occur along the northern margin of the Chugach terrane (Plafker and others, 1989; Nokleberg and others, 1994c, d, e).
The distribution of the geologic units defining the tract is adapted from Nokleberg and others (1994b, c, d, e). The distribution of this belt of mineral deposits is adapted from MacKevett and others (1978), and Nokleberg and others (1987, 1993, 1994a, 1996, 1997). Mineral resource assessments of 250,000-scale quadrangles within the tract were done by Goldfarb and others (1995).
Important examples of this type of deposit
The Spirit Mountain deposit (61 18 30N, 144 12 45W: Kingston and Miller, 1945; Herreid, 1970; Nokleberg and others, 1996) is classified by Nokleberg and others (1996) as a gabbroic Ni-Cu deposit. The Spirit Mountain gabbroic Ni-Cu deposit (Nokleberg and others, 1984; Nokleberg and others, 1991) contains at least 11,000 tonnes ranging up to 6.2 % Ni and 3.4 % Cu and 0.04 % Co. (Average 0.88 % Ni and 0.9 % Cu) The deposit consists of disseminated and locally massive pyrrhotite, pyrite, chalcopyrite, pentlandite, and ullmannite (a sulphantimonide of nickel), bravoite, and minor to trace galena and sphalerite in gabbro, peridotite, and hornblendite sills and dikes that cut Mississippian Strelna Formation with an east-west trend. Ultramafic and mafic rocks may be part of the Lower Jurassic or older, informally named Border Ranges ultramafic and mafic complex of Burns (1985). The sills and dikes are believed to intrude along a major thrust fault that juxtaposes a foliated quartz diorite pluton and the upper Paleozoic rocks. The deposit(s) occur as a series of lenses 1-3 m thick that extend along strike for about 2 km. Explored with trenches, pits, and two short tunnels.
On the numerical estimates made
Because only one deposit is known, this tract is considered to be permissive. No estimate of undiscovered gabbroic Ni-Cu was attempted.
References
Burns, L.E., 1985, The Border Ranges ultramafic and mafic complex, south-central Alaska: cumulate fractionates of island-arc volcanics: Canadian Journal of Earth Sciences, v. 22, p. 1020-1038.
Debari, S.M., and Coleman, R.G., 1989, Examination of the deep levels of an island arc: Evidence from the Tonsina ultramafic-mafic assemblage, Tonsina, Alaska: Journal of Geophysical Research, v. 94, p. 4373-4391.
Cox, D.P., and Singer, D.A., 1986, eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, 329 p.
Foley, J.Y., Mardock, C.L., and Dahlin, D.C., 1987, Platinum-group elements in the Tonsina ultramafic complex, southern Alaska: Metallurgical Society, Process Mineralogy VII, p. 165-195
Foley, J.Y., Barker, J.C., and Brown, L.L., 1985, Critical and strategic mineral investigation in Alaska: Chromium: U.S. Bureau of Mines Open File Report 97-85, 54 p.
Foley, J.Y., and Barker, J.C., 1985, Chromite deposits along the Border Ranges fault, southern Alaska: U.S. Bureau of Mines Information Circular IC-8990, 58 p.
Goldfarb, R.J., Carter Borden, J., and Winkler, G.R., 1995, Geochemical survey of the Valdez 1°x3° quadrangle, south-central Alaska: U.S. Geological Survey Bulletin 2084, 77 p., 1 sheet, scale 1:250,000.
Herreid, Gordon, 1970, Geology of the Spirit Mountain nickel-copper prospect and surrounding area: Alaska Division of Mines and Geology Geologic Report 40, 19 p., 1 sheet, scale 1:20,000.
Kingston, Jack, and Miller, D.J., 1945, Nickel-copper prospect near Spirit Mountain, Copper River region, U.S. Geological Survey Bulletin 943-C, p. 49-57.
MacKevett, E.M., Jr., and Plafker, George, 1974, The Border Ranges fault in south-central Alaska: U.S. Geological Survey Journal of Research, v. 2, no. 3, p. 323-329.
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., Berg, H.C., Brew, D.A., Grybeck, Donald, Robinson, M.S., Smith, T.E., Yeend, Warren, 1994a, Metallogeny and major mineral deposits of Alaska, 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, p. 855-904.
Nokleberg, W.J., Bundtzen, T.K., Berg, H.C., Brew, D.A., Grybeck, Donald, Robinson, M.S., Smith, T.E., Yeend, Warren, and 54 contributors, 1994b, Metallogenic map of significant metalliferous lode deposits and placer districts of Alaska, 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 11, scale 1:2,500,000.
Nokleberg, W.J., Bundtzen, T.K., Dawson, K.M., Eremin, R.A., Goryachev, N.A., Koch, R.D. Ratkin, V.V., Rozenblum, I.S., Shpikerman, V.I., Frolov, Y.F., Gorodinsky, M.E., Melnikov, V.D., Ognyanov, N.V., Petrachenko, E.D., Petrachenko, R.I., Pozdeev, A.I., Ross, K.V., Wood, D.H., Grybeck, Donald, Khanchuk, A.I., Kovbas, L.I., Nekrasov, I.Ya., and Sidorov, A.A., 1996, Significant metalliferous lode deposits and placer districts for the Russian Far East, Alaska, and the Canadian Cordillera U.S. Geological Survey Open-File Report 96-513-A, 385 p.
Nokleberg, W.J., Bundtzen, T.K., Dawson, K.M., Eremin, R.A., Ratkin, V.V., Shpikerman, V.I., Goryachev, N.A., Khanchuk, A.I., Koch, R.D., Rozenblum, I.S., Gorodinsky, M.E., Frolov, Y.F., Pozdeev, A.I., Parfenov, L.M., and Sidorov, A.A., 1997, Mineral deposit and metallogenic belt maps of the Russian Far East, Alaska, and the Canadian Cordillera: U.S. Geological Survey Open-File Report 97-161, 2 sheets, 5 million scale; 5 sheets, 10 million scale).
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., Foster, H.L., and Aleinikoff, J.N., 1989, Geology of the northern Copper River Basin, eastern Alaska Range, and southern Yukon-Tanana Basin, southern and east-central Alaska, in, Nokleberg, W.J., and Fisher, M.A., eds., Alaska Geological and Geophysical Transect: Field Trip Guidebook T104, 28th International Geological Congress, p. 34-63
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., 1994d, Tectonostratigraphic terrane and overlap assemblage map of Alaska: U.S. Geological Survey Open-File Report 94-194, 53 p., scale 1:2,500,000.
Nokleberg, W.J., Parfenov, L.M., and Monger, J.W.H., and Baranov, B.V., Byalobzhesky, S.G., Bundtzen, T.K., Feeney, T.D., Fujita, Kazuya, Gordey, S.P., Grantz, Arthur, Khanchuk, A.I., Natal'in, B.A., Natapov, L.M., Norton, I.O., Patton, W.W., Jr., Plafker, George, Scholl, D.W., Sokolov, S.D., Sosunov, G.M., Stone, D.B., Tabor, R.W., Tsukanov, N.V., Vallier, T.L. and Wakita, Koji, 1994c, Circum-North Pacific tectono-stratigraphic terrane map: U.S. Geological Survey Open-File Report 94-714, 221 pages, 2 sheets, scale 1:5,000,000; 2 sheets, scale 1:10,000,000.
Nokleberg, W.J., Plafker, George, and Wilson, F.H., 1994e, Geology of south-central Alaska, in Plafker, George, and Berg, H.C., eds., The Geology of Alaska: Boulder, Colorado, Geological Society of America, The Geology of North America, v. G-1, p. 311-366.
Plafker, George, Nokleberg, W.J., and Lull, J.S., 1989, Bedrock geology and tectonic evolution of the Wrangellia, Peninsular, and Chugach terranes along the Trans-Alaska crustal transect in the Chugach Mountains and southern Copper River Basin, Alaska: Journal of Geophysical Research, v. 94, p. 4255-4295.
Singer, D.A., Page, NJ., and Menzie, W.D., 1986, Descriptive model of synorogenic-synvolcanic Ni-Cu, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 28-31.

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