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, 1986). Known deposits of Comstock type occur in the Eocene Challis volcanic field, and have been mined historically (i.e., Yankee Fork district) (Anderson, 1949). Typically these deposits consist of a vein system that becomes dispersed into stockwork veining upward, sometimes capped by hot-spring sinter deposits. Older mining concentrated on the vein system, whereas, recently active mines are focused in the upper, more disseminated stockwork and hot-spring parts of the mineralized systems, typically as open pit mines.
On the delineation of permissive tracts
The permissive tract was delineated to include the Eocene Challis volcanic field of south-central Idaho (Bond, 1978). Favorable areas for epithermal districts include broad areas of known mineralization associated with a major northeast-trending, subvertical Trans-Challis fault zone (Kiilsgaard and others, 1986).
Important examples of this type of deposit
The Eocene Challis volcanic field in south-central Idaho is host to three major districts: the Thunder Mountain district in the Thunder Mountain caldera on the west, the Yankee Fork district in the northeast-trending Trans-Challis fault zone, and the Champagne deposit at the margin with the Snake River plain (a Pliocene feature). Active mines in each district exploit the stockwork, disseminated, upper parts of these hydrothermal deposits, which have the grade and tonnage characteristics of hot-spring Au-Ag deposits. However older mines, in the Yankee Fork district in particular, were concentrated on the deeper, vein portion of these hydrothermal deposits.
On the numerical estimates made
Contemporaneous faulting and volcanism in the Trans-Challis fault zone, along with two known prospect areas there, lead us to consider the area of the intersection of the fault zone and the volcanic field as favorable for undiscovered epithermal districts. Irregular cover of the favorable lower part of the volcanic field by younger lavas and ash-flows of the unmineralized upper part of the field suggests exploration has not fully evaluated this area. However, because recent mining has focused on the bulk mineable part of the mineralized systems, we expect that most undiscovered Comstock mineralization will be mined as part of an open pit operation that will result in a higher tonnage and lower grade production that better fits the hot-spring Au-Ag grade and tonnage model of Berger and Singer (1992). Therefore, since we estimate the number of undiscovered hot-spring Au-Ag deposits, we make no estimate for the number of undiscovered Comstock deposits.
Anderson, A.L., 1949, Silver-gold deposits of the Yankee Fork district, Custer County, Idaho: Idaho Bureau of Mines and Geology Pamphlet 83, 37 p.
Berger, B.R., and Singer, D.A., 1992, Grade and tonnage model for hot spring Au-Ag, in Bliss, J.D., ed., Developments in mineral deposit modeling, U.S. Geological Survey Bulletin 2004, p. 23-25.
Bond, J.G., 1978, Geologic map of Idaho: Idaho Bureau of Mines and Geology, scale 1:500,000.
Kiilsgaard, T.H., Fisher, F.S., and Bennett, E.H., 1986, The Trans-Challis fault system and associated precious metal deposits, Idaho: Economic Geology, v. 81, no. 3, p. 721-724.
Mosier, D.L., Singer, D.A., and Berger, B.R., 1986, 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.