Explained by Warren J. Nokleberg, Dennis P. Cox, Richard J. Goldfarb, and Warren Yeend.
On the choice of deposit models
This tract covers part of the Yukon-Tanana terrane. It is permissive for low-sulfide Au-quartz veins (Berger, 1986) because in contains sedimentary and intermediate volcanic rocks that are metamorphosed to greenschist facies, known occurrences and prospects, and stream-sediment geochemical anomalies (Au, As, Ag).
On the delineation of permissive tracts
The favorable geologic units defining this tract are the Devonian and Mississippian metasedimentary and lesser metavolcanic rocks of southern Yukon-Tanana terrane where metamorphosed in the mid-Cretaceous to greenschist facies. The metasedimentary rocks were mainly quartz-rich shale and siltstone, quartz sandstone, minor limestone, and marl. The metavolcanic rocks were mainly andesite, but ranged in composition from rhyolite to basalt.
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), Singer and others (1977), Cox and others, 1989, and Nokleberg and others (1990).
Important examples of this type of deposit
There are no significant examples of low-sulfide Au-quartz veins in the tract. However, this deposit type is commonly associated with Au-Sb veins (Berger, 1993) that occur mainly along the southern margin of the Yukon-Tanana terrane. Minor occurrences and prospects are in the northeastern part of the tract. Local gold placer deposits in the Kantishna, Bonnifield, and Delta River districts may be derived in part from undiscovered lode deposits, or from gold-antimony deposits in this tract (Capps, 1912, 1919; Smith, 1941; Gilbert and Bundtzen, 1979; Cobb, 1973; Rose, 1965; Nokleberg and others, 1991).
On the numerical estimates made
The tract is a large area that locally is covered by thick glacial loess, and dense vegetation. The existence of favorable rock units, and of numerous occurrences of low-sulfide Au-quartz veins suggest that there are undiscovered deposits in this tract. The estimated minimum number of undiscovered deposits that are consistent with the grade and tonnage model of Bliss (1986) is:
Percentile 90 50 10 5 1
Estimated number of deposits 5 10 50 50 50
Berger, B.R., 1986, Descriptive model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239.
Berger, V.I., 1993, Descriptive, and grade-tonnage model for gold-antimony deposits: U.S. Geological Survey Open-File Report 93-194, 24 p.
Bliss, J.D., 1986, Grade and tonnage model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239-243.
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., 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.
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., 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.
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.