National mineral assessment tract AK-AP01 (Besshi massive sulfide)

Tract AK-AP01
Geographic region AK Peninsula & Aleutians
Tract area 1,467sq km
Deposit type Besshi massive sulfide
Deposit age Tertiary

Rationale

Explained by Frederic H. Wilson, Stanley E. Church, Richard W. Saltus, Lawrence J. Drew, and W. David Menzie.
On the choice of deposit models
The geology of the Kodiak Islands is permissive for Besshi massive sulfide deposits as described by Cox (1986) because the tectonic setting is that of a spreading ridge underlying terrigenous sediments at a continental slope.
On the delineation of permissive tracts
This tract comprises rocks of the Ghost Rocks, Sitkalidak, Sitkinak, and other Tertiary Formations on the southeast and east side of the Kodiak Islands. These formations have a variety of lithologies, including flysch and marine sandstone and siltstone and possibly ultramafic rocks. The rocks that are most favorable for this model are the Ghost Rocks Formation and nearby associated rocks, primarily of the slightly older Kodiak Formation. In south-central Alaska, Late Cretaceous metabasalt of the Valdez Group contain Besshi massive sulfide deposits. Metabasalts are not known in the equivalent units on Kodiak Island.
Important examples of this type of deposit
No occurrences of Besshi-type deposits are known in the tract. However, the Old Harbor prospect on Sitkalidak Island (Berg and Cobb, 1967, p. 87) might be an example. For comparison, occurrences in the Orca Group (equivalent to the Ghost Rocks) include the Latouche and Beatson mines in the Seward quadrangle, the Ellamar mine in the Cordova quadrangle, and Fidalgo-Alaska mine in the Cordova quadrangle.
On the numerical estimates made
The lack of any geochemical or geophysical data, and of any known occurrences precludes making quantitative estimates of undiscovered Besshi massive sulfide deposits for this tract. The generally lower metamorphic grade of the rocks in this tract compared to those in south-central Alaska suggests a somewhat lower potential in this tract.
References
Berg, H.C., and Cobb, E.H., 1967, Metalliferous lode deposits of Alaska: U.S. Geological Survey Bulletin 1246, 254 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.
Moore, G.W., 1967, Preliminary geologic map of Kodiak Island and vicinity, Alaska: U.S. Geological Survey Open‑file Map 271, scale 1:250,000.

Geographic coverage

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