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 paleo-ground surface in and around felsic volcanic fields, usually associated with normal faults (Berger, 1986). The tract is underlain by Miocene arkosic sedimentary rocks derived from the Idaho batholith and capped by lavas of the Columbia River Basalt Group. Hot-spring mercury mineralization, with subeconomic gold grade, at the Idaho Almaden mine near Weiser (Andserson, 1941) indicates the existence of extinct geothermal systems. Although rhyolitic magmatism typically associated with the hot-spring deposits (Berger, 1986) is not known, the occurrence of extinct hot spring systems indicates the tract is permissive for this deposit type.
On the delineation of permissive tracts
The tract was drawn around the Miocene Payette Formation, host rock of the Idaho Almaden mine, northwest of the younger basalts of the western Snake River plain (Bond, 1978). The surrounding area, where the Payette Formation may occur within a km of the surface beneath overlying rocks of the Columbia River Basalt Group, is also included.
Important examples of this type of deposit
No examples of this deposit type are known in the tract. However, hot-spring mercury system at the Idaho Almaden mine near Weiser has similar structural characteristics and abundant silica deposits characteristic of this deposit type. Gold occurs in subeconomic grades in the Idaho Almaden deposit.
On the numerical estimates made
The lack of known deposits within the tract and the dearth of information available on any exploration activity precluded estimation of the number of undiscovered deposits of this type.
Anderson, Alfred L., 1941, Geology of the Idaho Almaden quicksilver mine: Idaho Bureau of Mines and Geology Pamphlet 55, 9 p.
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.
Bond, J.G., 1978, Geologic map of Idaho: Idaho Bureau of Mines and Geology, scale 1:500,000.