Explained by Warren J. Nokleberg, Dennis P. Cox, Richard J. Goldfarb, and David L. Campbell.
On the choice of deposit models
A large part of the Chugach Range in south-central Alaska is permissive for early Tertiary Chugach-type low-sulfide Au-quartz vein (Bliss, 1992) deposits because of favorable bedrock geology, favorable regional metamorphism, known mines and occurrences, and geochemical anomalies (Au, W) (Goldfarb and others, 1995).
On the delineation of permissive tracts
The favorable geologic unit defining this tract is mainly the Upper Cretaceous flysch of the Valdez Group where metamorphosed to greenschist facies. Also favorable are: (1) the northern margin of the lower Tertiary Orca Group of the Prince William terrane where metamorphosed to lower greenschist facies; (2) the metasedimentary rocks of the Orca Group within a few kilometers of granitic plutons; and (3) the southern margin of the Upper Triassic through mid-Cretaceous McHugh Complex in the southern Seldovia quadrangle. Chugach-type low sulfide Au-quartz vein deposits occur both in (meta)sedimentary rocks and in Eocene and Oligocene granitic plutons intruding the Valdez and Orca Groups. Hydrothermal muscovite from gold-bearing veins has been dated at 53 Ma in the Port Valdez district (Winkler and others, 1981), at 52 Ma in the Hope-Sunrise district (Mitchell and others, 1981), and at 57 Ma in the Nuka Bay district (Borden and others, 1992).
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 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 Cliff mine in the Valdez quadrangle, the Granite, Crown Point, Alaska Oracle, Gilpatrick, Lucky Strike, Palmer Creek, and Kenai-Alaska mines in the Seward quadrangle, the Monarch and Jewel mines in the Anchorage quadrangle, and the mines of Nuka Bay district (Alaska Hills, Beauty Bay, Goyne, Lost Creek, Sonny Fox, and Nualaska) in the Seldovia quadrangle are all examples of low-sulfide Au-quartz veins in this tract (Johnson, 1914; Martin and others, 1915; Park, 1933; Tuck, 1933; Richter, 1970; McGee, 1972; Mitchell, 1979; Mitchell and others, 1981; Stuwe, 1984).
On the numerical estimates made
Chugach-type low-sulfide Au-quartz vein deposits (model 36a.1) are relatively small, averaging only about 3000 metric tons with an average grade of 6.2 grams per ton (Bliss, 1992), as compared to the generic low sulfide Au-quartz vein model (model 36a), which average 30,000 metric tons with 16 grams per ton Au (Bliss, 1986). Because the Chugach-type deposits are relatively small, a potential exists for a correspondingly larger number of deposits than would be expected for the generic low-sulfide Au-quartz veins. However, because this deposit model is only expected to have a total contained gold content of about 18,600 grams (about 600 oz.) the potential for development is significantly reduced.
There is a high probability of numerous undiscovered deposits in the tract. A reasonable estimate for the number of undiscovered deposits is about 0.02 deposits per km2 (Bliss, 1986; J.D. Bliss, written commun., 1995) or about 20 deposits per 1,000 km2.. For the tract size, this ratio indicates that as many as 600 undiscovered deposits may exist at the 50% percentile. Accordingly, the mineral resource team estimated the number of undiscovered low-sulfide Au-quartz vein deposits that are consistent with the grade and tonnage model of Bliss (1992) to be:
Percentile 90 50 10 5 1
Estimated number of deposits 80 600 800 800 800
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.
Bliss, J.D., 1992, Grade and tonnage model of Chugach-type low-sulfide Au-quartz veins, in, Bliss, J.D., editor, Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, p. 44-46.
Borden, J.C., Goldfarb, R.J., Gent, C.A., Buruss, R.C., and Roushey, B.H., 1992, Geochemistry of lode gold deposits, Nuka Bay district, southern Kenai Peninsula: US Geological Survey Bulletin 2041, p. 13-22.
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.
Goldfarb, R.J., Leach, D.L., Miller, M.L., and Pickthorn, W.J., 1986, Geology , metamorphic setting, and genetic constraints of epigenetic lode-gold mineralization within the Cretaceous Valdez Group, south-central Alaska, in Keppie, J.D., Boyle, R.W., and Haynes, S.J., eds., Turbidite-hosted gold deposits: Geological Association of Canada Special Paper 32, p. 87-105
Goldfarb, R.J., Snee, L.W., and Pickthorn, W.J., 1993, Orogenesis, high-T thermal events, and gold vein formation within metamorphic rocks of the Alaskan Cordillera: Mineralogical Magazine, v. 57, p. 375-394.
Johnson, B.L., 1914, The Port Wells gold-lode district: U.S. Geological Survey Bulletin 592-G, p. 195-236.
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., 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.
Martin, G.C., Johnson, B. L., and Grant, U.S., 1915, Geology and mineral resources of Kenai Peninsula, Alaska: U.S. Geological Survey Bulletin 587, 243 p
McGee, D.L., 1972, Kodiak Island and vicinity, Alaska—Geology and mineral resources: Alaska Division of Geological and Geophysical Surveys Open-file Report 31, 7 p.
Mitchell, P.A., 1979, Geology of the Hope-Sunrise (gold) mining district, north-central Kenai Peninsula, Alaska: Unpublished M.Sc. thesis, Stanford University, 123 p.
Mitchell, P.A., Silberman, M.L., and O’Neil, J.R., 1981, Genesis of gold mineralization in an Upper Cretaceous turbidite sequence, Hope-Sunrise district: U.S. Geological Survey Open-File Report 81-103, 18 p.
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.
Park, C.F., Jr., 1933, The Girdwood district, Alaska: U.S. Geological Survey Bulletin 849-G, p. 381-424.
Richter, D.H., 1970, Geology and lode-gold deposits of the Nuka Bay area, Kenai Peninsula, Alaska: U.S. Geological Survey Professional Paper 625-G, 16 p.
Stuwe, K., 1984, Granitoid intrusions and gold mineralization in the western Port Wells mining district, Prince William Sound, Alaska: Unpublished M.Sc. thesis, University of Leoben, Austria.
Tuck, R., 1933, The Moose Pass-Hope district, Kenai Peninsula, Alaska: U.S. Geological Survey Bulletin 849-I, p. 469-530.
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.
Winkler, G.R., Silberman, M.L., Grantz, Arthur, Miller, R.J., and MacKevett, E.M., Jr., 1981, Geologic map and summary geochronology of the Valdez quadrangle, southern Alaska: US Geological Survey Open-File Report 80-892-A, 2 sheets, scale 1:250,000.