Explained by Warren J. Nokleberg, Dennis P. Cox, and Richard J. Goldfarb.
On the choice of deposit models
Parts of the Maclaren terrane in the Alaska Range are permissive for low-sulfide Au-quartz vein deposits (Berger, 1986) because of favorable bedrock geology, favorable regional metamorphism, known occurrences, and stream-sediment geochemical anomalies (Au, W).
On the delineation of permissive tracts
The favorable geologic unit defining the tract is the Upper Jurassic(?) flysch (argillite and metagraywacke) of the Maclaren Glacier metamorphic belt of the Maclaren terrane (Smith, 1981; Nokleberg and others, 1985), where metamorphosed to lower greenschist facies in the early Tertiary. One of Alaska’s largest gold placer mines at Valdez Creek occurs downstream from the area of known deposits. The Maclaren Glacier metamorphic belt displays classic Barrovian-type metamorphism in which the lower, greenschist facies portions are judged to be highly favorable for low-sulfide Au-quartz vein deposits. The Maclaren terrane is interpreted as the offset equivalent of metamorphic rocks that contain major low-sulfide Au quartz vein deposits in the Juneau region of southeastern Alaska (Nokleberg and others, 1985, 1993).
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), Cox and others (1989), and Nokleberg and others (1990).
Important examples of this type of deposit
The Lucky Hill, Timberline Creek, Gold Hill, and Surprise occurrences and prospects in the Healy quadrangle (Smith, 1981; Adams and others, 1992) are examples of low-sulfide Au-quartz vein deposits in this tract. The associated gold placer mines in the Valdez Creek district (Chapin, 1918; Capps, 1919; Tuck, 1938; Smith, 1970; Cobb, 1973; Bressler and others, 1985; Fechner and Herzog, 1990; Reger and Bundtzen, 1990) may be derived in part from undiscovered low-sulfide gold Au-quartz vein deposits.
On the numerical estimates made
This tract locally is covered by large ice fields and glaciers, therefore much of the area has not been available for exploration. Because of the presence of numerous low-sulfide Au-quartz veins, a major placer mine, and the limited outcrop area, the assessment team concluded that this tract probably contains at least one undiscovered low-sulfide Au-quartz vein deposit. The 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 1 3 4 4 4
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.
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.
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., 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., 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.
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., Jones, D.L., Silberling, N.J., 1985, Origin, migration, and accretion of the Maclaren and Wrangellia terranes, eastern Alaska Range, Alaska: Geological Society of America Bulletin, v. 96, p. 1251-1270
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.
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.
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.
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.
Smith, T.E., 1981, Geology of the Clearwater Mountains, south-central Alaska: Alaska Division of Geological and Geophysical Surveys Report 60, 72 p.
Tuck, Ralph, 1938, The Valdez Creek mining district, Alaska, in 1936: U.S. Geological Survey Bulletin 897-B, p. 109-131.