Explained by Donald Grybeck, Thomas D. Light, William J. Keith, Dennis P. Cox, Gregory K. Lee, and Jeffrey D. Phillips.
On the choice of deposit models
Placer gold can be expected to occur in unconsolidated deposits downstream from any of the gold‑bearing lode deposits, and therefore the permissive area for placer gold is extensive.
On the delineation of permissive tracts
This tract, in east-central Alaska, is defined by the distribution of its known deposits (Cobb, 1973).
Important examples of this type of deposit
The majority of placer gold production in east-central Alaska has been from the Fairbanks, Circle, Livengood (Tolovana), Fortymile, Rampart, and Hot Springs (Tofty) districts. Placer deposits in the Fairbanks district have produced 8 million ounces of gold, the Circle district produced more than 1 million ounces of gold, and the other districts each produced 1/2 million or less ounces of gold ( Bundtzen and others, 1994).
On the numerical estimates made
Exploring for placer gold deposits is relatively easy, and because of the numerous gold rushes from the 1890’s to the 1920’s much of the state has been thoroughly explored. There is a low probability that major new placer districts will be discovered, but a few relatively small new deposits probably will be found. Such small placer gold deposits may be outside the known districts, and might be mined during periods of high gold prices. No attempt was made to estimate the number of undiscovered deposits.
Bundtzen, T.K., Swainbank, R.C., Clough, A.H., Henning, M.W., and Hansen, E.W., 1994, Alaska’s Mineral Industry: Alaska Division of Geological and Geophysical Surveys Special Report 48, 84 p.
Cobb, E.H., 1973, Placer deposits of Alaska: U.S. Geological Survey Bulletin 1374, 213 p.