National mineral assessment tract AK-AP11 (Placer Au)

Tract AK-AP11
Geographic region AK Peninsula & Aleutian Is.
Tract area 51,342sq km
Deposit type Placer Au
Deposit age Quaternary

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
Placer gold can be expected to occur in unconsolidated deposits downstream from any of the gold‑bearing lode deposit types. Therefore, the placer gold permissive area is quite broad. In addition, extensive glaciation over much of the tract area has mobilized unconsolidated deposits, possibly concentrating some into gold placers and eroding others.
This tract is permissive for porphyry copper deposits, and for polymetallic or epithermal gold veins. Erosion of any of these deposit types could result in a placer gold deposit.
On the delineation of permissive tracts
This tract, on the Alaska Peninsula, includes Permian to Quaternary age rocks of the Chignik subterrane and overlap sequences. Pre-Tertiary rocks are dominantly clastic and minor carbonate sedimentary rocks and very minor volcanic rocks. Tertiary and Quaternary rocks are largely volcanic and volcaniclastic, and small plutons and stocks are widely distributed in the tract.
Important examples of this type of deposit
The Mary Lou mine on Popof Island beach at Sand Point (Wilson and others, 1988) is an example of the small placer deposits that can be expected in this tract. Additionally, shoreline placer occurrences on the Bering Sea coast, derived presumably from glacial deposits, are known but may be ephemeral.
On the numerical estimates made
Because of the relative ease of exploring for placer gold deposits, much of the state has been thoroughly explored. However, this part of the state is probably not as well explored as better known areas, and small deposits may yet be discovered. There is a low probability that any major new placer districts will be discovered, but it is likely that a few relatively small new deposits will be found in the known districts. A thorough understanding of Quaternary geologic history is necessary to a rational search for additional deposits in this tract. No estimate of the number of deposits was attempted.
References
Wilson, F.H., White, W.H., and DuBois, G.D., 1988, Brief descriptions of mines, prospects, and mineral occurrences in the Port Moller and Stepovak Bay quadrangles, Alaska Peninsula: U.S. Geological Survey Open‑File Report 88‑666, 128 p., scale 1:250,000, 1 map sheet.

Geographic coverage

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