Explained by Warren J. Nokleberg, Dennis P. Cox, David L. Campbell, and Richard J. Goldfarb.
On the choice of deposit models
Parts of the Talkeetna Mountains and western Alaska Range are permissive for kuroko massive sulfide deposits of Jurassic age (Singer, 1992) because of favorable bedrock geologic units. Because of the Jurassic age of the host rocks, the Sierran kuroko model of Singer (1992) represents the best estimates of tonnage and grade for undiscovered deposits.
On the delineation of permissive tracts
The favorable geologic unit in the tract is the Lower Jurassic andesite and associated basalt and volcaniclastic rocks of the Talkeetna Formation in the Peninsular terrane. Generally, aeromagnetic and gravity data are insufficient to help delineate the tract. The aeromagnetic map of Alaska (Godson, 1994) locally displays broken magnetic highs that may reflect covered volcanic rocks within the Talkeetna Formation.
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 Singer and others (1980), Reed and others (1979), Cox and others (1989), and Nokleberg and others (1990).
Important examples of this type of deposit
No kuroko deposits are known within this tract, but the Johnson River prospect occurs in correlative rocks in the Kenai quadrangle (Steefel, 1987), and is a kuroko massive sulfide deposit. At least 11 sporadic lenses of stratiform or strata-bound sulfides, mainly pyrite, exist in the Talkeetna Formation in the northern Chugach Mountains and may be related to marine volcanism (Newberry, 1986).
On the numerical estimates made
Prospective host rocks in the tract are partly covered by extensive ice fields, glaciers, and by Cenozoic basinal deposits. The mineral resource team estimated that the minimum number of undiscovered deposits consistent with the grade and tonnage model of Singer (1992) is:
Percentile 90 50 10 5 1
Estimated number of deposits 0 1 2 4 10
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.
Heran, W.D., 1992, Geophysical model of massive sulfide deposits, in Heran, W.D., ed., Codocil to The geophysical expression of selected mineral deposit models: U.S. Geological Survey Open-file Report 94-174, p.4-15.
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.
Newberry, R.J., 1986, Mineral resources of the north-central Chugach Mountains, Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigation 86-23, 44 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.
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.
Singer, D.A.., 1992, Grade and tonnage model of Sierran kuroko massive sulfide, in Bliss, J.D., ed., Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, p. 29-32.
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.
Steefel, C.I., 1987, The Johnson River prospect, Alaska: gold-rich sea-floor mineralization from the Jurassic: Economic Geology, v. 82, p. 894-914