Explained by Warren J. Nokleberg, Dennis P. Cox, Richard J. Goldfarb, David L. Campbell, and Warren Yeend.

On the choice of deposit models

This tract, in the southern Yukon-Tanana Upland, is permissive for Late Cretaceous and early Tertiary porphyry Cu and other granitic-magmatism-related deposits because of favorable bedrock geology, porphyry Cu and Cu skarn occurrences in the extension of the tract to the north in east-central Alaska, and geochemical anomalies (Cu, Mo, Au, As, Ag, Bi, Pb, Zn). This porphyry Cu model (17) includes porphyry Cu (BC-Ak type) (17.1), porphyry Cu skarn (18a), porphyry Cu-Au (20c), and porphyry Cu-Mo (21). Details of the deposit model for this tract and other areas are provided by Hollister and others (1975), Cox (1986), Menzie and Singer (1993) .

On the delineation of permissive tracts

The favorable geologic units in the tract are the Late Cretaceous and early Tertiary granitic plutons of the Kluane arc that intrude Mesozoic and older rocks of the Yukon-Tanana and adjacent terranes, and carbonate rocks altered to skarn in areas where intruded by Late Cretaceous and early Tertiary igneous rocks. The extension of this tract to the north, in east-central Alaska, includes various small fragments of Paleozoic continental-margin sedimentary rocks, turbidite basin deposits, and oceanic crustal rocks in the Livengood, Manley, and Wickersham, White Mountain terranes that are intruded by Late Cretaceous and early Tertiary granitic plutonic rocks (Nokleberg and others, 1994). To the southwest in southwestern Alaska, the extension of this tract includes the porphyry Cu and other granitic-magmatism-related deposits related to intrusion of the Late Cretaceous and early Tertiary Kuskokwim Mountains volcanic-plutonic belt (Nokleberg and others, 1994).

The principal mineral deposit models are porphyry Cu and Cu skarn, but related coeval granitic-magmatism deposits could also occur. Local zonation of geochemical anomalies may exist. Local porphyry Cu occurrences in the extension of the tract to the north in east-central Alaska exhibit multiple phases of intrusion and propylitic and sericitic alteration. Areas of known and possibly large concealed granitic plutons (greater than several km diameter) in the tract are delineated by large, blotchy magnetic highs and (or) by eU or eTh cells interpreted from sparse data on radiometric maps.

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). This tract is the same area as tract AK-SC24 for plutonic porphyry Au deposits and tract AK-SC07 for gold-antimony vein deposits. This tract corresponds to part of the East-Central Alaska belt of granitic magmatism deposits described by Nokleberg and others (1993). Mineral resource assessments of 250,000-scale quadrangles within the tract were done by Richter and others (1975), Singer and others (1977), Reed and others (1979), Cox and others (1989), and Nokleberg and others (1990, 1991).

Important examples of this type of deposit

The Taurus deposit, which has inferred reserves of 126 million tonnes (140 million tons) containing 0.30% Cu and 0.34grams/tonne (0.01 oz/ton) Au, and 0.03% Mo (Bundtzen and others, 1996) occurs in the Tanacross quadrangle. In the extension of this tract to the north are a wide variety of polymetallic vein, Sb-Au vein, porphyry Cu, and Sn greisen mines, deposits, and occurrences (Nokleberg and others, 1987, 1993). Associated placer gold deposits in the Bonnifield, Delta River, and Kantishna districts in the tract (Capps, 1912, 1919; Smith, 1941; Rose, 1965; Cobb, 1973; Bundtzen ,1981; Gilbert and Bundtzen, 1979; Nokleberg and others, 1991) may be partly derived from porphyry Cu or other granitic-magmatism-related occurrences.

On the numerical estimates made

The tract is partly covered by extensive glacial loess, deep Tertiary weathering, and dense vegetation that could obscure porphyry Cu deposits. Because two major porphyry occurrences are in the extension of the tract to the north, undiscovered deposits probably occur in the tract. The minimum number of undiscovered porphyry Cu deposits, consistent with the grade and tonnage model of Menzie and Singer (1993), are estimated to be:

Percentile 90 50 10 5 1

Estimated number of deposits 1 5 7 7 7

References

Bundtzen, T.K., 1981, Geology and mineral deposits of the Kantishna Hills, Mount McKinley quadrangle, Alaska: Fairbanks, Alaska, University of Alaska, M.S. thesis, 237 p, 4 sheets, scale 1:63,360.

Capps, S.R., 1912, The Bonnifield region, Alaska: U.S. Geological Survey Bulletin 501, 64 p.

Capps, S.R., 1919, The Kantishna region, Alaska: U.S. Geological Survey Bulletin 687, 116 p

Cobb, E.H., 1973, Placer deposits of Alaska: U.S. Geological Survey Bulletin 1324, 213 p.

Cox, D.P., and Singer, D.A., 1986, eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, 329 p.

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.

Gilbert, W.G., and Bundtzen, T.K., 1979, Mid-Paleozoic tectonics, volcanism, and mineralization in north-central Alaska Range, in Sisson, A., ed., The relationship of plate tectonics to Alaskan geology and resources: Alaska Geological Society, Symposium, 1977, p. F1-F21.

Hollister, V.F., Anzalone, S.A., and Richter, D.H., 1975, Porphyry copper deposits of southern Alaska and contiguous Yukon Territory: Canadian Mining and Metallurgy Bulletin, v. 68, p. 104-112.

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.

Menzie, W.D., and Singer, D.A., 1993, Grade and tonnage model of porphyry Cu deposits in British Columbia, Canada, and Alaska, U.S.A.: U.S. Geological Survey Open-File Report 93-275, 9 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., Lange, I.M., Roback, R.C., Yeend, Warren, and Silva S.R., 1991, Map showing locations of metalliferous lode and placer mineral occurrences, deposits, prospects, and mines, Mount Hayes quadrangle, eastern Alaska Range, Alaska: U.S. Geological Survey Map MF-1996-C, 42 p., 1 sheet, scale 1:250,000.

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.

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.

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.

Rose, A.W., 1965, Geology and mineral deposits of the Rainy Creek area, Mt. Hayes quadrangle, Alaska: Alaska Division of Mines and Minerals Geologic Report 14, 51 p.

Singer, D.A., Curtin, G.C., and Foster, H.L., 1977, Mineral resources map of the Tanacross quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-767-E, 1 sheet, scale l:250,000.

Smith, P.S., 1941, Mineral industry of Alaska in 1939: U.S. Geological Survey Bulletin 926-A, p. 1‑106.