Explained by Marti L. Miller, Thomas K. Bundtzen, Dennis P. Cox, John E. Gray, and Jeffrey D. Phillips
On the choice of deposit models
Placer gold (Yeend, 1986) can be expected to occur in unconsolidated deposits downstream from any of the gold‑bearing lode deposit types. Therefore the placer gold permissive area for southwest Alaska is quite broad. In addition, glaciation over parts of the tract has redistributed unconsolidated deposits, possibly concentrating some into gold placers and eroding others.
On the delineation of permissive tracts
The placer gold tract encompasses most of southwestern Alaska. It represents an extension of the peraluminous granite porphyry Au, epithermal vein, low-sulfide Au-quartz vein, and plutonic porphyry Au tracts, and in part the polymetallic vein tract. A portion of the polymetallic vein tract was excluded because it contains no known placer occurrences despite relatively extensive placer exploration. The placer gold tract was extended into a covered region of the Dillingham quadrangle because it contains one small granodiorite body and an indication in the geophysical data of unexposed plutons. The eastern boundary of the tract is defined by the glacial front on the west flank of the Alaska-Aleutian Range.
Important examples of this type of deposit
All of the known placer districts in southwestern Alaska are contained in the tract (Cobb, 1973). They include the Goodnews Bay, Marshall, Aniak, and Iditarod districts, all of which are wholly contained within the region, and also the Innoko and McGrath districts, which straddle the boundary with the west-central region.
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 it is likely that a few relatively small new deposits will be found in the known districts. No estimate of undiscovered resources was made.
Cobb, E.H., 1973, Placer deposits of Alaska: U.S. Geological Survey Bulletin 1374, 213 p.
Yeend, Warren, 1986, Descriptive model of placer Au-PGE, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 261.