Explained by Marti L. Miller, Thomas K. Bundtzen, Dennis P. Cox, John E. Gray, and Jeffrey D. Phillips
On the choice of deposit models
In southwest Alaska, mixed oceanic lithologies that show greenschist facies metamorphism are considered permissive for low-sulfide Au-quartz vein deposits. The local presence of placer gold in association with these rocks is consistent with the model designation (Berger, 1986).
On the delineation of permissive tracts
This tract consists of two main areas and a third smaller area: (1) The central area lies south of Kako Creek in the Russian Mission and Holy Cross quadrangles. This area is underlain by a mixed lithology that includes part of the Angayucham-Tozitna terrane (mixed PzMz basalt, ultramafic, and sedimentary rocks) overlain by volcanic and volcaniclastic rocks of the Yukon-Koyukuk terrane. The tract was extended west over a covered region on the basis of gamma ray geophysics. The regional metamorphic facies map (Dusel-Bacon, 1991) shows prehnite-pumpellyite facies in this area, but 1992 field work indicates it is actually greenschist facies (T.K. Bundtzen, oral commun., 1993). Five placer gold mines have operated within this area. (2) The southern area forms a belt through the Bethel, Goodnews, and Hagemeister Island quadrangles and contains the Kilbuck terrane and part of the Goodnews terrane. The Goodnews terrane lithologies include mudstone, basalt, serpentine, ophiolitic rocks, and schists. The Kilbuck terrane contains mafic and felsic schists. Metamorphic grade across much of the belt is low greenschist, although locally higher grades exist (Dusel-Bacon, 1991). Gold placers in the area south of the Kilbuck terrane may indicate the presence of either low-sulfide Au-quartz veins or an unexposed pluton. (3)The northern area lies in the north-central Iditarod and south-central Ophir quadrangles. It is underlain by greenschist-facies metavolcanic rocks and lesser quartz-mica schist of the Ruby terrane (Miller and Bundtzen, 1994). The area is outlined on the basis of permissive greenschist facies rocks alone; no gold occurrences are known in the northern area.
Important examples of this type of deposit
The Arnold prospect in the Russian Mission quadrangle, area (1) described above, is probably a low-sulfide Au-quartz vein deposit.
On the numerical estimates made
Given the presence of permissive rock types of the appropriate metamorphic grade and the occurrence of placer gold deposits in the two main areas of the tract, the assessment team concluded that the likelihood of a low-sulfide Au-quartz vein deposit was high. The third small area discussed above (underlain by Ruby terrane), was excluded from the estimate. The estimated minimum number of undiscovered low-sulfide Au-quartz vein deposits, consistent with the grade and tonnage model of Bliss (1986) is:
Percentile 90 50 10 5 1
Estimated number of deposits 1 2 3 4 6
Berger, B.R., 1986, Descriptive model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239.
Bliss, J.D., 1986, Grade and tonnage model of low-sulfide gold-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239-243.
Dusel-Bacon, Cynthia, 1991, Metamorphic history of Alaska: U.S. Geological Survey Open-file Report 91-556, 48 p., 2 sheets, scale 1:2,500,000.
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 archeological sample localities: U.S. Geological Survey Miscellaneous Field Studies Map MF-2219-A, 48 p., scale 1:250,000.