Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
Mississippi Valley deposits are low-temperature replacement deposits of galena, sphalerite, and chalcopyrite, hosted in dolomitic carbonate rocks (Briskey, 1986). The host rocks are typically shallow-water marine carbonate rocks of the cratonic platform. Deposits within the tract near Metaline, Washington, are interpreted to be deposits of this type (Mills, 1976).
On the delineation of permissive tracts
The team used a simple approach, considering all carbonate shelf sedimentary sequences in the map area as permissive for Mississippi Valley-type (MVT) deposits. Rocks in the permissive tract are part of the Paleozoic craton-margin shelf in northeastern Washington. Permissive lithologic units include the Cambrian and Ordovician stratigraphic units in northeastern Washington (Stoffel and others, 1991).
Important examples of this type of deposit
Two districts of significant production from this deposit type are known in northeastern Washington: Metaline and Van Stone (Mills, 1977). In Canada, along strike, other districts include Robb Lake and Monarch-Kicking Horse. An alternative interpretation is that they are sediment-hosted, syndepositional exhalative deposits similar to the Irish carbonate-hosted deposits (Morton, 1992). However, the ages of the deposits in the Washington and British Columbia indicates mineralization occurred well after deposition of the host rocks, which supports the MVT deposit model. Pb isotopic data from galena of the Metaline-area deposits indicate mineral deposition well after sedimentation (Devonian mineralization in Cambrian and Ordovician sedimentary rocks; S.E. Church, U.S. Geological Survey, personal communication, 1993). Deposits along strike to the north in Canada have yielded Devonian mineralization ages.
On the numerical estimates made
The grade and tonnage models for MVT deposits are for districts, as opposed to individual deposits. The grade and tonnage of the two known districts in Washington are on the low side of the median, as are the two Canadian deposits along strike, Robb Lake and Monarch-Kicking Horse. The team decided to use a revised tonnage model for Cordilleran MVT deposits that consists only of the smaller half of the deposits in the worldwide model. There seemed to be room for one or two more districts in the favorable area west of Metaline and north of Van Stone. Two clusters of Pb-Zn prospects in that area were deemed to indicate potential for undiscovered districts. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 1, 1, 2, and 2 or more Mississippi Valley Pb-Zn deposits consistent with the modified grade and tonnage model of Mosier and Briskey (1986).
References
Briskey, J.A., 1986, Descriptive model of southeast Missouri Pb-Zn, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 220-221.
Mills, J.W., 1976, Metamorphism of the zinc-lead sulfide ores of the Yellowhead horizon, Metaline Limestone Formation, northeast Washington: Economic Geology, v. 71, no. 8, p. 1601–1609.
Mills, J.W., 1977, Zinc and lead deposits in carbonate rocks, Stevens County, Washington: Washington Division of Geology and Earth Resources Bulletin 70, 171 p.
Morton, J.A., 1992, Re-evaluation of the geology and Zn-Pb ore deposits of the Metaline mining district, northeastern Washington: Washington Geology, Washington Department of Natural Resources, Division of Geology and Earth Resources, v. 20, no. 3, p. 3-14.
Mosier, D.L., and Briskey, J.A., 1986, Grade and tonnage model of southeast Missouri Pb-Zn and Appalachian Zn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 224-226.
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.
