National mineral assessment tract AK-BR16 (Polymetallic vein)

Tract AK-BR16
Geographic region Brooks Range, AK
Tract area 9,782sq km
Deposit type Polymetallic vein
Deposit age Cretaceous


Explained by Jeanine M. Schmidt, Karen D. Kelley, Richard I. Grauch, Byron R. Berger, John W. Cady, Gregory T. Spanski, James E. Kilburn.
On the choice of deposit models
Polymetallic vein deposits (Cox, 1986) can be associated with intrusive rocks of any age and a wide range of compositions. Therefore, it is difficult to limit the permissive area of deposits without significantly more geologic control than exists in most of Alaska. This model also serves as somewhat of a catch‑all for deposits of unknown type that, given minimal information, do not appear to fit into other categories. Areas that are not considered permissive are predominantly where there is sufficiently detailed geologic information to indicate that deposits do not exist.
Numerous small occurrences of contact metasomatic skarns or disseminated and veinlet base metal mineralization are known in the area (Grybeck and others, 1977; Grybeck and Nelson, 1981). Petrologic and geochemical information on the plutons is limited or absent but geologic factors are permissive for numerous mineral deposit types, including polymetallic vein deposits.
On the delineation of permissive tracts
This tract comprises all the Proterozoic, Devonian or Cretaceous granitic plutons in the Brooks Range, and includes all potential host rocks within 5 km of the plutons. The timing and grade of metamorphism of the host rocks, and their lithologies are not known in enough detail to differentiate areas of higher or lower favorability. The geochemical data from stream sediment samples show anomalous levels of Sn, W, and/or Mo in addition to the base metals Cu-Pb-Zn. There is very little, if any, gold associated with the intrusions.
Important examples of this type of deposit
No polymetallic vein deposits are known in this tract.
On the numerical estimates made
There is insufficient geologic information to estimate the number of undiscovered polymetallic vein deposits in this tract.
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.
Grybeck, Donald, Beikman, H.M., Brosge, W.P., Tailleur, IL., Mull, C.G., 1977, Geologic map of the Brooks Range, Alaska: U.S. Geological Survey Open-File Report 77-166B, 2 sheets, scale 1:1,000,000.
Grybeck, Donald, and Nelson, S.W., 1981, Mineral deposit map of the Survey Pass quadrangle, Brooks Range, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-1176-F, 1 sheet, scale, 250,000.

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