Explained by D.P. Cox, Steve Ludington, B.R. Berger, M.G. Sherlock, and D.A. Singer, (USGS); and J.V. Tingley (Nevada Bureau of Mines and Geology)
On the choice of deposit models
Sedimentary exhalative zinc-lead deposits, more conveniently referred to as sedex deposits, are stratabound tabular bodies of massive zinc and lead sulfides in black shales, siltsones, or carbonate rocks (Large, 1980). There are no known examples of this type in Nevada, however Turner and others (1989) noted similarities between the Roberts Mountains assemblage and lower Paleozoic strata in the northern Canadian Cordillera that host numerous large sedex deposits. Bedded barite deposits, which occur in close association with sedex deposits in some areas, are widespread in the Roberts Mountains assemblage. Ketner (1991) described stratabound gossans containing high values of lead, zinc, and silver in lower Middle Ordovician and Lower Silurian rocks in northeastern Nevada that are strongly suggestive of sedex deposits.
On the delineation of permissive tracts
The permissive tract consists of the Roberts Mountains assemblage. The eastern part of the tract, underlain by shale and chert of the Vinini Formation, probably has a higher probability of deposits because these rocks are most similar to the host rocks typical of sedex deposits.
On the numerical estimates made
Because of the absence of known examples of sedex deposits in Nevada, our estimate of undiscovered deposits is relatively low. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 1, 3, and 5 deposits in the delineated area that are comparable in grade and tonnage to the sedimentary exhalative Zn-Pb deposits of Menzie and Mosier (1986).
Ketner, K.B., 1991, Stratigraphy and strata-bound, lead-zinc-barium mineralization of lower Paleozoic rocks in northeastern Nevada, in Raines, G.L., Lisle, R.E., Schafer, R.W., and Wilkinson, W.H., eds., Geology and ore deposits of the Great Basin—Symposium proceedings: Reno, Geological Society of Nevada, v. 2, April 1990, p. 539-551.
Large, D.E., 1980, Geologic parameters associated with sediment-hosted, submarine exhalative Pb-Zn deposits—An empirical model for mineral exploration, in Stratiform Cu-Pb-Zn deposits: Geologisches Jahrbuch, Series D, v. 40, p. 59-129.
Menzie, W.D., and Mosier, D.L., 1986, Grade-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.
Turner, R.J.W., Madrid, R.J., and Miller, E.L., 1989, Roberts Mountains allochthon—Comparison with lower Paleozoic outer continental margin strata from the northern Canadian Cordillera: Geology, v. 17, p. 341-344.