Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
Epithermal Au-Ag quartz-adularia vein deposits are hosted in subaerial, intermediate to felsic volcanic rocks (Mosier and others, 1986a). The Republic mining district consists of several such deposits (Tschauder, 1986). Deposits in this district occur in Eocene dacitic volcanic and volcaniclastic rocks that are widespread in northeastern Washington (Pearson and Obradovich, 1977).
On the delineation of permissive tracts
The permissive tract was delineated to include the Eocene volcanic and volcaniclastic rocks of the Republic graben and adjacent areas of northeastern Washington (Stoffel and others, 1991). Most of the permissive tract in northeastern Washington was considered to be favorable, because of the similarity of volcanic stratigraphy and structural setting, and the occurrence of three known districts. Volcanism and mineralization were broadly contemporaneous with detachment faulting above the Okanogan, Kettle, and Priest River metamorphic core complexes. The 15 or so apparently separate volcanic fields may have originally been part of a more continuous volcanic province that was extended and dismembered by detachment faulting and erosion.
Important examples of this type of deposit
The Republic and adjacent areas of northeastern Washington contains numerous volcanic-hosted deposits, prospects, and occurrences. The Golden Promise mine in Republic is an important example of this type (Tschauder, 1986). The vein system has been mined for over 300 m down dip. The prominent vein system becomes a stockwork system in the upper part, and grades into an overlying hot-spring Au-bearing sinter, which is being exploited as part of the underground mine.
On the numerical estimates made
The widely scattered favorable areas, the existence of numerous Au prospects in the region, the great productivity of the Republic district, and the poor exposure caused us to give a fairly optimistic estimate of the number of undiscovered districts. Although Comstock veins in the area grade up into more disseminated hot-spring systems, erosion has removed much of the favorable ground for hot springs, and we assessed only for Comstock deposits. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 1, 2, 4, 5, and 8 or more districts consistent with the grade and tonnage model of Mosier and others (1986b).
Mosier, D.L., Singer, D.A., and Berger, B.R., 1986a, Descriptive model of Comstock epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 150.
Mosier, D.L., Singer, D.A., and Berger, B.R., 1986b, Grade and tonnage model of Comstock epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 151-153.
Pearson, R.C., and Obradovich, J.D., 1977, Eocene rocks in northeast Washington—Radiometric ages and correlation: U.S. Geological Survey Bulletin 1433, 41 p.
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.
Tschauder, R., 1986, Gold deposits in northern Ferry County, in Joseph, N.L., ed., Geologic guidebook for Washington and adjacent areas: Washington Division of Geology and Earth Resources Information Circular 86, p. 239-254.