Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
These stratabound, massive Zn-Pb sulfide deposits form by the precipitation of sulfide and sulfate minerals from metalliferous brines that were exhaled along active submarine faults during deposition of the enclosing sedimentary sequence. The mineral deposits are dominantly Fe sulfide accumulations, which enclose economic ore deposits as layers and lenses rich in Zn and Pb sulfides. These syngenetic deposits are hosted in euxinic marine sedimentary rocks in epicratonic and intracratonic basins, often associated with synsedimentary faults (Briskey, 1986). Paleozoic off-shelf black shale units in northeastern Washington were deposited in a rifted continental slope-and-rise setting, and are appropriate hosts for this deposit type.
On the delineation of permissive tracts
All sedimentary units with significant black shales in northeastern Washington (exclusive of the Belt Supergroup and stratigraphic equivalents) are regarded as permissive for this deposit type. The permissive tract is defined by lithologic units that include the Late Proterozoic Windermere Group, the lower Paleozoic Metaline Formation and the Paleozoic Covada Group (Stoffel and others, 1991).
Important examples of this type of deposit
Three small prospects occur in Paleozoic deep-water strata in northeastern Washington, although they are too small to be considered significant deposits. Two of these occur in the Covada Group, an upper Paleozoic continental slope-and-rise sequence. One prospect is known east of the Covada Group in lower Paleozoic shales of the Metaline Formation (continental shelf-slope environment). Numerous barite occurrences and Pb and Zn prospects in the area could also be examples of this deposit type, although other Pb- and Zn-bearing deposit types are also present in the area.
On the numerical estimates made
This area is considered to be part of the rifted early Paleozoic continental margin. None of the prospects is large enough to be comparable to deposits in the grade and tonnage models. Cominco has had an exploration effort in northeastern Washington for many years, so the assessment team considered the area to have been thoroughly explored. However exposure in the area is poor, structure is complex, and forest cover is extensive, so that the team judged that some undiscovered deposits might remain. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 1, 2, and 4 or more for deposits consistent with the grade and tonnage model of Menzie and Mosier (1986).
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.
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.
Stoffel, K.L., Joseph, N.L., Waggoner, S.Z., Gulick, S.W., Korosec, M.A., and Bunning, B.B., 1991, Geologic map of Washington-Northeast quadrant: Washington Division of Geology and Earth Resources, Geologic Map GM-39, scale 1:250,000.