Explained by Warren J. Nokleberg, Dennis P. Cox, Richard J. Goldfarb, and David L. Campbell.
On the choice of deposit models
Part of the northern Prince William Sound area is permissive for early Tertiary Cyprus massive sulfide deposits as described by Singer (1986), because of favorable bedrock geologic units, several known Cyprus massive sulfide deposits, anomalous concentration of Fe, Co, Cu, Ni, and Zn in stream sediment samples, abundant sulfide minerals in heavy mineral concentrates, and favorable aeromagnetic anomalies.
On the delineation of permissive tracts
The favorable geologic units for the tract are the mafic and minor ultramafic rocks that are interpreted as dismembered parts of an oceanic ophiolite basal unit in the early Tertiary Orca Group of the Prince William terrane (Nelson and Nelson, 1993). The tract contains abundant pyrite and chalcopyrite in heavy mineral stream-sediment concentrates. The tract displays broad aeromagnetic highs that coincide with areas of basalt at the surface or areas where basalt is interpreted to occur near the surface.
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 MacKevett (1976), Tysdal and Case (1982), Nelson and others (1984), Goldfarb and others (1992), and Madden-McGuire and Winkler (1993).
Important examples of this type of deposit
The Knight Island, Twentieth Century, Pandora, Copper Bullion, Rua Cove, and Jonsy mines in the Seward quadrangle (Johnson, 1915, 1918; Moffit and Fellows, 1950), and the Threeman and Standard Copper mines in the Cordova quadrangle (Capps and Johnson, 1915) are examples of the Cyprus massive sulfides in this region (Crowe and others, 1992). About 40 small stratiform sulfide (chalcopyrite, sphalerite, and galena) occurrences are known in the tract and are interpreted as Cyprus massive sulfide deposits.
On the numerical estimates made
The tract is locally covered by extensive ice fields, glaciers, and forests. Several mines and many small occurrences are known in the tract. The mineral resource team 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 1 2 8 8 8
Capps, S.R., and Johnson, B.L., 1915, The Ellamar district, Alaska: U.S. Geological Survey Bulletin 605, 125 p.
Crowe, D.E., Nelson, S.W., Brown, P.E., Shanks, W.C., III, and Valley, J.W., 1992, Geology and geochemistry of volcanogenic massive sulfide deposits and related igneous rocks, Prince William Sound, south-central Alaska: Economic Geology, v. 87, p. 1722-1746.
Goldfarb, R.J., Case, J.E., Plafker, George, and Winkler, G.R., 1992, Maps showing areas of potential for mineral resources in the Cordova and Middleton Island 1° x 3° quadrangles, southern Alaska: U.S. Geological Survey Miscellaneous Field Investigations Map MF-2190. 1 sheet, scale 1:250,000.
Johnson, B.L., 1915, The gold and copper deposits of the Port Valdez district: U.S. Geological Survey Bulletin 622, p. 140-148.
Johnson, B.L., 1918, Mining on Prince William Sound: U.S. Geological Survey Bulletin 662, p. 183-192.
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.
Madden-McGuire, D.J., and Winkler, G.R., 1993, Mineral resource maps of the Anchorage quadrangle, southern Alaska: U.S. Geological Survey Miscellaneous Investigations Map MI-2393, scale 1:250,000, 3 sheets.
Moffit, F.H., and Fellows, R.E., 1950, Copper deposits of the Prince William Sound district, Alaska: U.S. Geological Survey Bulletin 963-B, p. 47-80.
Nelson, S.W., Miller, M.L., Barnes, D.F., Dumoulin J.A., Goldfarb, R.J., Koski, R.A., Mull, G.C., Pickthorn, W.J., Jansons, Uldis, Hoekzema, R.B., Kurtak, J.M., Fechner, S.A., 1984, Mineral resource potential of the Chugach National Forest, Alaska: US Geological Survey Miscellaneous Field Studies Map MF-1645-A, scale 1:250,000, 23 p.
Nelson, S.J., and Nelson, M.S., 1993, Geochemistry of ophiolitic rocks from Knight Island, Prince William Sound, Alaska, in Dusel-Bacon, Cynthia, and Till, A.B., eds., Geologic Studies in Alaska by the U.S. Geological Survey, 1992: U.S. Geological Survey Bulletin 2068, p. 130-142.
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., 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.
Singer, D.A.., 1986, Descriptive model of Cyprus massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 131.
Singer, D.A.., and Mosier, D.L., 1986, Grade and tonnage model of Cyprus massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 131-135.
Tysdal, R.G., and Case, J.E., 1982, Metalliferous mineral resource potential of the Seward and Blying Sound quadrangles, southern Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-880H, 2 sheets, scale l:250,000, 23 p.