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). Within this tract, widespread occurrences of hydrothermal alteration, silica sinter, and anomalous concentrations of gold, silver, arsenic, antimony, and mercury in rocks, soil, and stream sediments indicates the presence of numerous extinct geothermal systems with precious metal contents (Peters and others, 1994). Commonly Comstock-type vein mineralization occurs in the deeper parts (below 100 m) of the geothermal systems.
On the delineation of permissive tracts
The permissive tract was delineated to include all the area of Neogene rocks within the Ore-Ida graben, a north-south structural feature controlling the distribution of Miocene volcanic and sedimentary rocks (Peters and others, 1994). Since much of the Neogene section is buried under younger Neogene rocks, essentially all of the prospective Comstock vein horizons are buried.
Important examples of this type of deposit
The Grassy Mountain deposit is a large hot-spring Au-Ag deposit within the tract (Wheeler, 1988). Some parts of the deposit contain deeper vein-style epithermal mineralization that could be classified as Comstock-type. However all of the grade and tonnage of the Grassy Mountain deposit are included in the hot-spring model, since the deposit would be bulk-mined as one deposit.
On the numerical estimates made
An important factor considered in the development of estimates of the number of these deposits is that grade and tonnage values used in constructing the model include data from mining districts, as opposed to individual mines or deposits. District data is used where individual mines or deposits are spaced less than one mile apart, in which case the production and reserve data for the mines or deposits are aggregated. In this case several widely spaced hot-spring deposits at or near the surface may at depth be related to a system of epithermal veins, whose spacing is such that it would be treated as a district. This accounts for the lower estimated number of these deposits in comparison to the estimate for the number of hot-spring deposits for the same tract. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 2,3,4,6, and 8 or more districts (Spanski, 1994) consistent with the Au-Ag 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.
Peters, S.G., Albino, George, Brooks, H.C., Evans, J.G., Carlson, R.R., Lee, G.K., Griscom, Andrew, and Halvorson, P.F., 1994, Deposit models and tracts for undiscovered metallic resources in the Malheur-Jordan-Andrews Resource Area, southeastern Oregon, in Smith, C.L., ed., Mineral and energy resources of the BLM Malheur-Jordan Resource Area, southeastern Oregon: U.S. Geological Survey Administrative Report, p. B1-B21.
Spanski, G.T., 1994, Quantitative resource assessment for locatable minerals, in Smith, C.L., ed., Mineral and energy resources of the BLM Malheur-Jordan Resource Area, southeastern Oregon, U.S. Geological Survey Administrative Report, p. C1-C12.
Wheeler, G.R., 1988, Epithermal gold deposits in eastern Oregon: Mining Engineering, v. 40, no. 11, p. 1033-1035.