National mineral assessment tract AK-WC03 (Besshi massive sulfide)

Tract AK-WC03
Geographic region West Central, AK
Tract area 13,309sq km
Deposit type Besshi massive sulfide
Deposit age Paleozoic - Mesozoic


Explained by Bruce M. Gamble, Robert G. Eppinger, John W. Cady, Stanley E. Church, Byron R. Berger, and Gregory T. Spanski
On the choice of deposit models
No massive sulfide deposits are known in this tract. The presence of altered basalts containing pyrite, and locally high copper values may suggest submarine hydrothermal activity, and suggest that the area is permissive for Besshi massive sulfide deposits as described by Cox (1986).
On the delineation of permissive tracts
This tract encompasses mafic rocks of Proterozoic(?), Paleozoic, or Mesozoic age (Murchey and Harris, 1985; Pallister and others, 1989). The tract consists of mafic and ultramafic oceanic assemblages of the Kanuti, Tozitna, and Innoko terranes (Patton and others, 1989; Nokleberg and others, 1994). Also included are pillow basalts, chert, gabbro, and diabase of the Narvak complex. Stream sediment geochemical anomalies are sparse and consist of scattered Cu, Zn, and rare As.
In west-central Alaska, the Ruby and Nixon Fork terranes, which comprise Precambrian(?) to lower Paleozoic pelitic schist, metabasites, quartzite, orthogneiss, and marble, are permissive for Besshi massive sulfide deposits. Arsenic is the most common stream sediment anomaly; Zn, W, Cu, and rare Au, Ag and Bi are present locally.
Important examples of this type of deposit
No Besshi massive sulfide deposits are known in this tract.
On the numerical estimates made
No deposits of this type are known in the area, and no estimates of the number of undiscovered deposits have been attempted.
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.
Murchey, B.L., and Harris, A.G., 1985, Devonian to Jurassic sedimentary rocks in the Angayucham Mountains of Alaska: possible sea mount or oceanic plateau deposits [abs]: EOS, Transactions of the American Geophysical Union, v. 66, no. 46, p. 1102.
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, 45 manuscript p., scale 1:2,500,000.
Pallister, J.S., Budahn, J.R., and Murchey, B.L., 1989, Pillow basalts of the Angayucham terrane: oceanic plateau and island crust accreted to the Brooks Range: Journal of Geophysical Research, v. 94, no. B11, p. 15,901-15,923.
Patton, W.W., Jr., Box, S.E., Moll-Stalcup, E.J., and Miller, T.P., 1989, Geology of west-central Alaska: U.S. Geological Survey Open-File Report 89‑554, 41 p.

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