Explained by Bruce M. Gamble and Thomas D. Light
On the choice of deposit models
Numerous areas in west-central Alaska are permissive for placer gold deposits. The Seward Peninsula contains large placer gold deposits and numerous low-sulfide gold-quartz vein occurrences. The Koyukuk magmatic arc, Ruby and Nixon Fork terranes each contains small gold occurrences associated with plutons.
On the delineation of permissive tracts
On the Seward Peninsula, this tract includes all of the Nome Group rocks exclusive of high-grade (amphibolite and above) portions, which are confined to the mountain ranges. The Nome Group consists chiefly of pelitic schist, calc-schist, pure and impure marble, graphitic quartzite and quartz-mica schist, and mafic schist. Metamorphic grade within the tract is blueschist having a pronounced greenschist overprint (Till and others, 1986). Known placer Au deposits and low-sulfide gold-quartz vein deposits are confined to these rocks. Stream sediment and panned concentrate samples commonly contain anomalous As and Sb, and, locally, Au, W, Bi, and other elements.
Low-level gold geochemical anomalies are scattered across the Koyukuk magmatic arc, and gold lodes are associated with plutons throughout the area.
Stream-sediment geochemical samples in the Ruby terrane, which comprises Precambrian(?) to lower Paleozoic pelitic schist, metabasites, quartzite, orthogneiss, and marble, locally show anomalous values of such metals as As, Zn, W, Cu, Au, Ag and Bi. Stream-sediment samples in the Nixon Fork terrane, which includes greenschist facies pelitic- and calc-schist, greenstone, and minor felsic metaplutonic and metavolcanic rocks, show scattered Au, Zn, and As anomalies.
Mafic and ultramafic oceanic assemblages of the Kanuti, Tozitna, and Innoko subterranes are included in the tract because they contain phyllite and argillite of the Slate Creek thrust panel; pillow basalts, chert, gabbro, and diabase of the Narvak complex; and mafic-ultramafic rocks of the Kanuti complex (Patton and others, 1989b). The Narvak complex is the most common assemblage in west-central Alaska (Patton and others, 1989a).
Important examples of this type of deposit
The famous placer gold deposits of the Nome and Council districts (Collier and others, 1908; Yeend, 1988)) are included in this tract, as well as the smaller Kougarok, McGrath, Hughes, Fairhaven, Ruby, Innoko, and Koyuk districts (Nokleberg and others, 1987).
On the numerical estimates made
Because of the relative ease of exploring for placer gold deposits, much of the state has been thoroughly explored. There is a low probability that major new placer districts will be discovered, but a few relatively small new deposits probably will be found.
No estimate of the number of undiscovered deposits was attempted.
Collier, A.J., Hess, F.L., Smith, P.S., and Brooks, A.H., 1908, The gold placers of parts of Seward Peninsula, Alaska: U.S. Geological Survey Bulletin 328, 343 p.
Nokleberg, W.J., Bundtzen, T.K., Berg, H.C., Brew, D.A., Grybeck, Donald, Robinson, MS., Smith, T.E., and Yeend, Warren, 1987, Significant metalliferous lode deposits and placer districts of Alaska: U.S. Geological Survey Bulletin 1786, p. 104.
Patton, W.W., Jr., Box, S.E., Moll-Stalcup, E.J., and Miller, T.P., 1989a, Geology of west-central Alaska: U.S. Geological Survey Open-File Report 89‑554, 41 p.
Patton, W.W., Jr., Box, S.E., and Grybeck, Donald, 1989b, Ophiolites and other mafic-ultramafic complexes in Alaska: U.S. Geological Survey Open-File Report OF 89‑648, 27 p., scale 1:5,000,000.
Till, A.B., Dumoulin, J.A., Gamble, B.M., Kaufman, D.S., and Carroll, P.I., 1986, Preliminary geologic maps and fossil data, Solomon, Bendeleben, and southern Kotzebue quadrangles, Seward Peninsula, Alaska: U.S. Geological Survey Open-File Report 86‑276, 62 p., scale 1:250,000.
Yeend, Warren, Kaufman, D.S., and Till, A.B., 1988, Map showing placer gold in the Solomon, Bendeleben, and southern part of the Kotzebue quadrangles, western Alaska: US Geological Survey Miscellaneous Field Studies Map MF‑1838-C, scale 1:250,000.