Explained by Marti L. Miller, Thomas K. Bundtzen, Dennis P. Cox, John E. Gray, and Jeffrey D. Phillips.
On the choice of deposit models
Gold-bearing deposits in this tract are associated with Upper Cretaceous and lower Tertiary peraluminous granite porphyry and rhyolite dikes that intrude Cretaceous turbidite and volcanic rocks. Little is known about these occurrences, but they appear to represent a distinct model type, herein called peraluminous granite porphyry gold deposits. The peraluminous granite porphyry gold deposits have some features in common with polymetallic veins (Cox , 1986) and felsic intrusion-associated veins (Sangster, 1984), but their association with peraluminous dikes across much of southwest Alaska (Bundtzen and Miller, 1997) makes designation of a distinct model type more useful in understanding and predicting their occurrence. The felsic dikes are probably structurally controlled, but their origin is unknown. The model is more fully described by Bundtzen and Miller (1997).
On the delineation of permissive tracts
The extent of the permissive tract is defined by the known or inferred distribution of peraluminous granite porphyry dikes intruding the turbidites of the Cretaceous Kuskokwim Group or the volcanic and sedimentary rocks of the Yukon-Koyukuk terrane. The dikes are granitic in composition, contain about equal amounts of plagioclase and potassium feldspar, and range from porphyritic to aphanitic. They are depleted in heavy REE, and have high Rb/Sr ratios, suggesting a possible crustal melt source (Miller and Bundtzen, 1994). As further information becomes known, the permissive area for deposit occurrences of this type may be revised.
Important examples of this type of deposit
Examples of this deposit type include the Stuyahok occurrence in the Holy Cross quadrangle (Miller and others, 1996) and the Donlin Creek deposit in the Iditarod quadrangle. Exploration and drilling at Donlin Creek yielded a preliminary resource of 3.6 million ounces of gold (Dodd, 1996).
On the numerical estimates made
Insufficient geologic information about the distribution of deposits, or about the controls for ore genesis preclude the possibility of estimating quantity or size of undiscovered deposits, but additional deposits are expected.
Bundtzen, T.K., and Miller, M.L., 1997, Precious metals associated with Late Cretaceous-early Tertiary igneous rocks of southwestern Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral deposits of Alaska: Economic Geology Monograph 9, p. 242-286.
Cox, D.P., 1986, Descriptive Model of polymetallic veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 125.
Dodd, Stan, 1996, Donlin Creek project, southwest Alaska [abs], in Alaska mining—no longer just a dream: Alaska Miners Association, Abstracts, 1996 Annual Convention, November 6-8, 1996, Anchorage, Alaska, p. 27-28.
Miller, M.L., and Bundtzen, T.K., 1994, Generalized geologic map of the Iditarod quadrangle, Alaska, showing potassium-argon, major-oxide, trace-element, fossil, paleocurrent, and archaeological sample localities: U.S. Geological Survey Miscellaneous Field Studies Map MF-2219-A, 48 p., scale 1:250,000.
Miller, M.L., Bundtzen, T.K., Keith, W.J., Bailey, E.A., and Bickerstaff, Damon, 1996, Geology and mineral resources of the Stuyahok area, part of Holy Cross A-4 and A-5 quadrangles, Alaska: U.S. Geological survey Open-File Report 96-505-A, 47 p., scale 1:63,360.
Sangster, D.F., 1984, Felsic intrusion-associated silver-lead-zinc veins, in Eckstrand, O.R., ed., Canadian mineral deposit types: A geologic synopsis: Geological Survey of Canada Economic Geology Report 36, p. 66.