Explained by Warren J. Nokleberg, Dennis P. Cox, Richard J. Goldfarb, David L. Campbell, and Warren Yeend.
On the choice of deposit models
Parts of the Alaska Range and Wrangell Mountains are permissive for Late Cretaceous and early Tertiary porphyry Cu and other granitic-magmatism-related deposits because of favorable bedrock geology, two porphyry Cu occurrences and one associated Cu skarn occurrence, geochemical anomalies (Cu, Mo, Au, As, Ag, Bi, Pb, Zn, Co, Fe), and aeromagnetic anomalies. 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 for other areas of Alaska and British Columbia are provided by Hollister (1978), Hollister and others (1975), Cox (1986), and Menzie and Singer (1993).
On the delineation of permissive tracts
The favorable geologic unit defining this two-part tract is mainly the Wrangellia and Peninsular terranes where intruded by calc-alkaline granitic plutons, dikes, and sills of the Late Cretaceous and early Tertiary Kluane arc. Because concealed plutons and associated deposits may occur near the surface, the tract includes all of the Wrangellia and Peninsular terranes. About 30 areas of known and possibly concealed granitic plutons (greater than several km diameter) in the tract are delineated by large, blotchy magnetic highs on the aeromagnetic map of Alaska (Godson, 1994) and (or) by K and eU or eTh highs interpreted from sparse data on radiometric maps (J.D. Phillips, and F. Riggle, written commun., 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). Mineral resource assessments of 250,000-scale quadrangles within this tract were done by Richter and others (1975), MacKevett (1976), Singer and MacKevett (1977) Singer and others (1980), Reed and others (1979), Cox and others (1989), and Nokleberg and others (1990).
Important examples of this type of deposit
Examples of polymetallic vein deposits that may indicate porphyry copper deposits at depth in this tract are the Golden Zone mine, and the Mayflower, Nim, and Nimbus prospects in the Healy quadrangle (Hawley and Clark, 1974; Swainbank and others, 1977). An example of a porphyry Cu-Mo deposit in this tract is the Treasure Creek deposit in the Talkeetna Mountains (Richter, 1963) quadrangle. An associated gold-bearing Cu skarn deposit occurs at the Zackly deposit (Nokleberg and others, 1990). Associated placer gold deposits in the Valdez Creek, Willow Creek, and Yenta districts in the tract (Capps, 1913, 1915, 1919; Chapin, 1918; Mertie, 1919; Tuck, 1938; Jasper, 1967; Smith, 1970; Cobb, 1973, Bressler and others, 1985; Fechner and Herzog, 1990; Reger and Bundtzen, 1990) may be partly derived from porphyry Cu deposits and occurrences.
On the numerical estimates made
Tertiary and older rocks in the tract locally are covered by extensive ice fields and glaciers, and by Cenozoic sedimentary and volcanic rocks. Additional detailed exploration might result in the discovery of additional deposits or occurrences. The minimum number of undiscovered porphyry Cu deposits, consistent with the grade and tonnage model of Menzie and Singer (1993), is estimated to be:
Percentile 90 50 10 5 1
Estimated number of deposits 1 2 6 6 6
Bressler, J.R., Jones, W.C. and Cleveland, Gaylord, 1985, Geology of a buried channel system of the Denali Placer Gold Mine: Alaska Miner, January, 1985, p. 9.
Capps, S.R., 1913, The Yentna district, Alaska: U.S. Geological Survey Bulletin 534, 75 p.
Capps, S.R., 1915, The Willow Creek district, Alaska: U.S. Geological Survey Bulletin 607, 86 p.
Capps, S.R., 1919, The Kantishna region, Alaska: U.S. Geological Survey Bulletin 687, 116 p.
Chapin, Theodore, 1918, The Nelchina-Susitna region, Alaska: U.S. Geological Survey Bulletin 668, 67 p.
Cobb, E.H., 1973, Placer deposits of Alaska: U.S. Geological Survey Bulletin 1324, 213 p.
Cox, D.P., 1986, Descriptive model of porphyry Cu, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p 76.
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.
Fechner, S.A. and Herzog, D.A., 1990, Gold- and PGM-bearing conglomerate of the Valdez Creek mining district, Alaska: U.S. Bureau of Mines Open File 12-90, 52 p.
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.
Hawley, C.C., and Clark, A.L., 1974, Geology and mineral deposits of the upper Chulitna district, Alaska: U.S. Geological Survey Professional Paper 758-B, 47 p.
Hollister, V.F., 1978, Geology of the porphyry copper deposits of the Western Hemisphere: New York, Society of Mining Engineers of the American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc., 219 p.
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.
Jasper, M.W., 1967, Geochemical investigations, Willow Creek southerly to Kenai Lake region, south central Alaska: Alaska Division of Mines and Minerals Geochemical Report 14, 47 p.
MacKevett, E.M., 1976, Mineral deposits and occurrences in the McCarthy quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-773B, scale 1:250,000, 2 sheets.
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.
Mertie, J.B., 1919, Platinum-bearing placers of the Kahiltna Valley: U.S. Geological Survey Bulletin 692, p. 233-264.
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., 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.
Reger, R.D. and Bundtzen, T.K., 1990, Multiple glaciation and placer gold formation, Valdez Creek Valley, Western Clearwater Mountains, Alaska: Alaska Division of Geological and Geophysical Surveys Professional Paper 107, 29 p., 1 sheet, scale 1:63,360.
Richter, D.H., 1963, Geology of the Portage Creek-Susitna River area, Alaska: Alaska Division of Mines and Minerals Geologic Report 3, 2 sheets, scale 1:24,000.
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.
Smith, T.E., 1970, Gold resource potential of the Denali bench gravels, Valdez Creek Mining District, Alaska: U.S. Geological Survey Professional Paper 700-D, p. D146-D152.
Swainbank, R.C., Smith, T.E., and Turner, D.L., 1977, Geology and K-Ar age of mineralized intrusive rocks from the Chulitna mining district, central Alaska: Alaska Division of Geological and Geophysical Surveys Geologic Report 55, p. 23-28.
Tuck, Ralph, 1938, The Valdez Creek mining district, Alaska, in 1936: U.S. Geological Survey Bulletin 897-B, p. 109-131.