Explained by Warren J. Nokleberg, Dennis P. Cox, Richard J. Goldfarb, and David L. Campbell.
On the choice of deposit models
The northern Prince William Sound area is permissive for Late Cretaceous and early Tertiary Besshi massive sulfide deposits (Cox, 1986; Singer, 1986) because of favorable bedrock geologic units, known mines and abundant occurrences, anomalous concentrations of Fe, Co, Cu, Ni, and Zn in stream sediment samples (Goldfarb and others, 1992), abundant sulfide minerals in heavy mineral concentrates, and favorable aeromagnetic anomalies.
On the delineation of permissive tracts
Two geologic units are favorable in this tract: (1) an extensive unit of Late Cretaceous marine metabasalt and associated metamorphosed flysch along the southern margin of the Valdez Group in the southern part of the Chugach terrane; and (2) the abundant early Tertiary marine basalts and associated marine sedimentary rocks of the Orca Group in the Prince William terrane. On the aeromagnetic map of Alaska (Godson, 1994), 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 Midas mine in the Valdez quadrangle (Johnson, 1915; Moffit and Fellows, 1950) is an example in the Valdez Group. Examples in the Orca Group are the Blackbird, Beatson, and LaTouche mines on LaTouche Island in the Seward quadrangle (Johnson, 1915), and the Ellamar, Fidalgo-Alaska, and Schlosser mines in the Cordova quadrangle (Capps and Johnson, 1915). An estimated 200 small stratiform sulfide (chalcopyrite, sphalerite, and galena) occurrences are known in the tract and are interpreted as Besshi massive sulfide deposits (Crowe and others, 1992).
On the numerical estimates made
The tract is partly covered by extensive ice fields, glaciers, and forests, and is truncated by the southern Alaska coastline. The tract contains several large mines (listed above) and many small occurrences, some of which are Cyprus-type deposits (AK-SC06). Assuming a general ratio of Besshi to Cyprus massive sulfide deposits of 10:1 (Cox and Singer, 1986), about five undiscovered Besshi massive sulfide deposits are estimated to occur in this tract. The mineral resource team estimated the minimum number of undiscovered Besshi massive sulfide deposits, consistent with the grade and tonnage model of Singer (1986), to be:
Percentile 90 50 10 5 1
Estimated number of deposits 5 10 30 30 30
Capps, S.R., and Johnson, B.L., 1915, The Ellamar district, Alaska: U.S. Geological Survey Bulletin 605, 125 p.
Cox, D.P., 1986, Descriptive model of Besshi massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 136.
Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 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.
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.
Goldfarb, R.J., Carter Borden, J., and Winkler, G.R., 1995, Geochemical survey of the Valdez 1°x3° quadrangle, south-central Alaska: U.S. Geological Survey Bulletin 2084, 77 p., 1 sheet, scale 1:250,000.
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.
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.
Johnson, B.L., 1915, The gold and copper deposits of the Port Valdez district: U.S. Geological Survey Bulletin 622, p. 140-148.
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.
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, Grade and tonnage model of Besshi massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 136-138.
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.