Explained by Stephen E. Box and Arthur A. Bookstrom

On the choice of deposit models

Hot-spring Au-Ag deposits consist of precious metal-bearing siliceous sinters and silicified rocks cut by breccias and stockworks of veins and veinlets, which were deposited at or near the surface in and around felsic volcanic fields, usually associated with normal faults (Berger, 1986). Typically this disseminated mineralization focuses downward into a vein system of the Comstock type. In this tract, older mining concentrated on the vein system in underground mines. Associated hot-spring type mineralization is known to occur around the older mining districts and elsewhere in the Eocene Challis volcanic field.

On the delineation of permissive tracts

The permissive tract was delineated to include the Eocene Challis volcanic field of south-central Idaho (Bond, 1978), the host of the three known deposits.

Important examples of this type of deposit

The Eocene Challis volcanic field in south-central Idaho is host to three major deposits or districts of this type, each of which has an open-pit mine operating in 1994: the Thunder Mountain district in the Thunder Mountain caldera, the Yankee Fork district in the northeast-trending Trans-Challis fault zone, and the Champagne deposit in the Lava Creek district at the margin of the Snake River plain (a Pliocene feature). The Yankee Fork and Lava Creek districts include a number of inactive mines which exploited associated Comstock-type mineralization.

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 hot-spring deposits. 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 that exploration has not fully evaluated this area. At the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 2, 5, 7, and 8 or more deposits consistent with the grade and tonnage model of Berger and Singer (1992).

References

Berger, B.R., 1986, Descriptive model of hot-spring Au-Ag deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 143.

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.

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.