Explained by Marti L. Miller, Thomas K. Bundtzen, Dennis P. Cox, John E. Gray, and Jeffrey D. Phillips.
On the choice of deposit models
The presence of siltstone, shale, chert, and carbonate rocks that were deposited in a deep-water environment suggests the possibility of sedex deposits as described by Briskey (1986). Selection of this model type is supported by one bedded barite deposit, a model type considered to form distal to sedex occurrences.
On the delineation of permissive tracts
This tract encompasses the deep-water facies rocks of the Farewell terrane (as defined by Decker and others, 1994), which are found in the McGrath, Lime Hills, and Sleetmute quadrangles. The Farewell terrane comprises two sequences: the Middle Cambrian through Middle Devonian White Mountain sequence and the overlying Upper Devonian through Lower Cretaceous Mystic sequence. Deep-water facies consisting of siltstone, shale, chert, and carbonate rocks are contained in parts of both sequences. The region is poorly known; reconnaissance geochemical data are not available.
Important examples of this type of deposit
No examples of sedex deposits are known in the tract, but the Gagaryah barite deposit in the Lime Hills quadrangle (Bundtzen and Gilbert, 1991) may be related to sedex mineralization.
On the numerical estimates made
For estimating the number of undiscovered deposits, the median deposit is assumed to be 15 mt containing 5.6% Zn, 2.8% Pb, and 30 g/t Ag (Menzie and Mosier, 1986). The estimated number of undiscovered deposits is:
Percentile 90 50 10 5 1
Estimated number of deposits 0 1 2 3 5
Briskey, J.A., 1986, Descriptive model of sedimentary exhalative Zn-Pb, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 211-212.
Bundtzen, T.K., and Gilbert, W.G., 1991, Geology and geochemistry of the Gagaryah barite deposit, western Alaska Range, Alaska, in, Reger, R.D., ed., Short Notes on Alaskan Geology 1991: Alaska Division of Geological and Geophysical Surveys Professional Report 111, p. 9-20.
Decker, John, Bergman, S.C., Blodgett, R.B., Box S.E., Bundtzen, T.K., Clough, J.G., Coonrad, W.L., Gilbert. W.G., Miller, M.L., Murphy, J.M., Robinson, M.S., and Wallace, W.K., 1994, Geology of southwestern Alaska, in Plafker, George, and Berg, H.C., eds., The geology of Alaska: Boulder, Colorado, Geological Society of America, The Geology of North America, v. G1, p. 285-310.
Menzie, W.D., and Mosier, D.L., 1986, Grade and tonnage model of sedimentary exhalative Zn-Pb, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 212-215.