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.
On the delineation of permissive tracts
On western Kodiak Island, west of the Border Ranges fault, the bedrock is metamorphic, ultramafic, and plutonic rocks of Triassic and older ages (Moore, 1967). The central portion of Kodiak Island, southeast of the Border Ranges fault, consists of Upper Cretaceous graywacke and mudstone of the Kodiak Formation and lower Tertiary plutons on the Kodiak Islands. The southeast portion of Kodiak Island comprises rocks of the Ghost Rocks, Sitkalidak, Sitkinak, and other Tertiary Formations, which consist of a variety of lithologies, including flysch and marine sandstone and siltstone, and possibly ultramafic rocks. This latter area is tentatively correlated with the Orca Group of Prince William Sound.
Several occurrences of bedrock gold mineralization and of shoreline placer gold have been reported in this tract, which is overlain by fluvial and glacial deposits that are largely of local derivation. Because of the glaciation and fluvial erosion and the known occurrence of shoreline placer gold, the area is considered permissive for placer gold deposits.
Important examples of this type of deposit
Most of the placer occurrences northwest of the Border Ranges fault were in beach deposits, reworked from glacial deposits. As such, they tend to be ephemeral and typically were mined during periods of low tide (Martin, 1913). Gold has been mined in small-scale operations from several localities, including Cape Alitak, along 50 km of beaches between Cape Alitak and Old Red River, and in the vicinity of Olga Bay (see Capps, 1937). In the Orca Group-equivalent rocks, there are two reported placer occurrences: one each on Chirikof Island and Tugidak Island (see Smith, 1933); there also is an unsubstantiated report of PGE being dredged offshore of the tract at Cape Chiniak.
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 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.
There is inadequate information to estimate the number of undiscovered placer deposits.
Capps, S.R., 1937, Kodiak and adjacent islands: U.S. Geological Survey Bulletin 880‑C, p. 111‑184.
Martin, G.C., 1913, Mineral deposits of Kodiak and the neighboring islands: U.S. Geological Survey Bulletin 542, p. 125‑136.
Moore, G.W., 1967, Preliminary geologic map of Kodiak Island and vicinity, Alaska: U.S. Geological Survey Open‑File Map 67-161, scale 1:250,000.
Smith, P.S., 1933, Mineral Industry of Alaska in 1930: U.S. Geological Survey Bulletin 836-A, p. 1-83.