National mineral assessment tract AK-EC14 (Plutonic porphyry Au)

Tract AK-EC14
Geographic region East Central, AK
Tract area 84,106sq km
Deposit type Plutonic porphyry Au
Deposit age Cretaceous - Tertiary

Estimates

Confidence Number of
deposits
90% 1
50% 3
10% 5
5% 9
1% 13

Estimators: Grybeck, Cox, Light, Lee, Phillips

Rationale

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
The Fort Knox deposit near Fairbanks is the potentially the largest lode gold deposit ever found in Alaska, and is the type example of this model.
On the delineation of permissive tracts
The plutonic porphyry Au deposit is a relatively new model (Hollister, 1992), and the composition, age, and other variables that constrain the permissive plutons are not well defined. The Fort Knox deposit is in a granitic to granodioritic stock having an age of about 93 Ma (Newberry and others, 1995). It is not clear, however, what the age range or composition of other possible host stocks may be. Therefore, this tract is defined by the distribution of known or inferred felsic to intermediate plutons in east-central Alaska. If undiscovered deposits of this type are near the surface, placer gold is probably the best indicator.
Important examples of this type of deposit
The Fort Knox deposit in the Fairbanks occurs in a small zoned stock of granite to granodiorite composition. Native gold is disseminated along fractured zones associated with quartz veins. Sulfides are rare; trace amounts of bismuthinite and bismuth tellurides are common (Bauman, 1990). The deposit is being developed as a bulk-tonnage, low-grade mine with published reserves of 158 million tonnes containing 0.83 grams per tonne (Bundtzen and others, 1994). The True North deposit is in the Livengood quadrangle.
On the numerical estimates made
The Fort Knox deposit has reported reserves of more than 4 million ounces of gold (Bundtzen and others, 1994). The estimated minimum number of undiscovered plutonic porphyry Au deposits, consistent with the deposit data reported by Hollister (1992), is:
Percentile 90 50 10 5 1
Estimated Number of deposits 1 3 5 9 13
References
Bauman, F.W., 1990, Geology of the Fort Knox gold deposit, Fairbanks, Alaska; Alaska Miner, January, p. 14-15.
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.
Hollister, V.F., 1992, On a proposed plutonic porphyry gold deposit model: Nonrenewable resources, v. 1, p. 293-302.
Newberry, R.J., McCoy, D.T., and Brew, D.A., 1995, Plutonic-hosted gold ores in Alaska; Igneous vs. metamorphic origins: Resource Geology Special Issue No. 18, p. 57-100.

Geographic coverage

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