Explained by Warren J. Nokleberg, Dennis P. Cox, Richard J. Goldfarb, and David L. Campbell.
On the choice of deposit models
Parts of the central and eastern Alaska Range, and the southern Yukon-Tanana Upland, are permissive for Devonian and Mississippian kuroko massive sulfide deposits (Singer, 1986; Singer and Mosier, 1986) because of favorable bedrock geologic units, known occurrences, anomalous concentrations of Cu, Pb, Zn, As, Au, and Ag in stream sediment samples, abundant sulfide minerals in heavy mineral concentrates, and favorable aeromagnetic and gravity anomalies.
On the delineation of permissive tracts
The favorable geologic unit in this tract is the marine Devonian and Mississippian felsic and intermediate metavolcanic and metasedimentary rocks in the upper structural levels of southern Yukon-Tanana terrane north of the Denali fault. These rocks have been polymetamorphosed and deformed, resulting in recrystallization and dismemberment of the massive sulfide deposits. Numerous heavy mineral concentrate samples from the tract contain chalcopyrite, sphalerite, galena, and pyrite. The tract displays broad low-amplitude aeromagnetic anomalies on the aeromagnetic map of Alaska (Godson, 1994). Gravity highs (Barnes and others, 1994) are used to extend the tract boundaries to the west and southwest in areas that are covered by Cenozoic sedimentary rocks.
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), Cox and others (1989), and Nokleberg and others (1990).
Important examples of this type of deposit
The Red Mountain, WTF, Sheep Creek, and Anderson Mountain deposits in the Healy quadrangle (Gilbert and Bundtzen, 1979), the Liberty Bell mine in the Fairbanks quadrangle (Hawley, 1976; Gilbert and Bundtzen, 1979), the Miyaoka, Hayes Glacier, and McGinnis Glacier occurrences in the western Mount Hayes quadrangle, and the Delta district deposits in the eastern Mountain Hayes quadrangle (Lange and others, 1993) are the best known examples in the area.
On the numerical estimates made
Tertiary and older rocks in the tract are locally covered by extensive ice fields, glaciers, glacial loess, and by Cenozoic basinal deposits. Because of the relatively abundant mines, deposits, and occurrences, abundant anomalies in geochemical samples, and abundant sulfide minerals in heavy mineral concentrates, several undiscovered deposits probably occur in the tract. We estimated the minimum number of undiscovered deposits, consistent with the grade and tonnage model of Singer and Mosier (1986), to be:
Percentile 90 50 10 5 1
Estimated number of deposits 3 5 25 25 25
Barnes, D.F., Mariano, John, Morin, R.L., Roberts, C.W., and Jachens, R.C., 1994, Incomplete isostatic gravity map 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 9, scale 1:2,500,000.
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.
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., 1976, Exploration and distribution of stratiform sulfide deposits in Alaska [abs.], in Miller, T.P., ed., Symposium on recent and ancient sedimentary environments in Alaska [abs.]: Alaska Geological Society Program with Abstracts, p. T1-T28.
Lange, I.M., Nokleberg, W.J., Newkirk, S.R., Aleinikoff, J.N., Church, S.E., Krouse, H.R., 1993, Devonian volcanogenic massive sulfide deposits and occurrences, southern Yukon-Tanana terrane, eastern Alaska Range, Alaska: Economic Geology, v. 88, p. 344-376.
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.
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.
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.., 1986, Descriptive model of kuroko massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 189-190.
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.
Singer, D.A.., and Mosier, D.L., 1986, Grade and tonnage model of kuroko massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 190-197.