National mineral assessment tract AK-BR17 (Zn-Pb-Ag veins)

Tract AK-BR17
Geographic region Brooks Range, AK
Tract area 59,440sq km
Deposit type Zn-Pb-Ag veins
Deposit age Devonian - Mississippian

Rationale

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
Several dozen vein-breccia occurrences of this type help define this Brooks Range tract (Schmidt and Werdon, 1993). The deposits consist of color banded sphalerite growing in apparent open-space filling veins and surrounding wall rock clasts in simple breccias up to several meters in width. Quartz and argentiferous galena infill open space in both veins and breccias; in rare cases more than one generation of brecciation or veining are indicated. Alteration associated with the mineralization is limited to minor de-carbonation of wall rocks (only in marine sandstones with calcareous cement), minor silicification rare muscovite formation within microns to millimeters of the vein walls. This deposit model is apparently unique to the Brooks Range, but has similarities to the Shawangunk deposits of New York and vein breccia deposits at Bad Grund, Germany. There is NO spatial or genetic association of these veins with either volcanic or plutonic igneous rocks, which distinguishes them from the igneous-related polymetallic vein model. The Pb-Zn-Ag veins are probably derived by dewatering of a large clastic sedimentary basin (such as the Late Devonian to Mississippian Endicott Group).
On the delineation of permissive tracts
This tract includes all known exposures of sedimentary rocks of the Upper Devonian to Mississippian(?) Endicott Group. These rocks include the nonmarine Kekituk and Kanayut Conglomerates, the marine Noatak Sandstone and the Kayak and Hunt Fork Shales. These units generally produce a broad distinct aeromagnetic low, and contain only scattered geochemical anomalies of Zn and Pb.
Important examples of this type of deposit
Mineral occurrences such as Husky, Story Creek, Whoopee Creek, and Kady are examples of Brooks Range type Pb-Zn-Ag veins in quartzites and siltstones (Schmidt, 1997). No vein-breccia deposits cut rocks older than Late Devonian (Frasnian), or younger than Early Mississippian (Kinderhook Kayak Shale and Isikut Formation), suggesting that their age of formation is Early to Middle Mississippian.
On the numerical estimates made
Because no grade and tonnage data are available for any of the vein breccia occurrences, no local grade-tonnage distribution curve could be constructed. No attempt was made to estimate the number of undiscovered deposits.
References
Schmidt, J.M., 1997, Shale-hosted Zn-Pb-Ag and barite deposits of Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 35-65.
Schmidt, J.M., and Werdon, M.B., 1993, Clastic-hosted stratiform, vein-breccia, and disseminated Zn-Pb-Ag deposits of the northwestern Brooks Range, Alaska: Are they different expressions of dewatering of the same source basin? [abs], Geological Society of America Abstracts with Program, volume 25, no. 5, p. 143.

Geographic coverage

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