National mineral assessment tract PC24 (Epithermal vein, Comstock)

Tract PC24
Geographic region Pacific Coast
Tract area 1,730sq km
Deposit type Epithermal vein, Comstock
Deposit age Tertiary

Deposit model

Model code 25c
Model type descriptive
Title Descriptive model of Comstock epithermal veins
Authors Dan L. Mosier, Donald A. Singer, and Byron R. Berger
URL https://pubs.usgs.gov/bul/b1693/html/bull5nqr.htm
Source https://pubs.er.usgs.gov/publication/b1693

Rationale

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). Three known Comstock deposits occur within Miocene felsic volcanic rocks within the tract. The abundance of Neogene basaltic to felsic volcanic and volcaniclastic rocks and the presence of known deposits of this type within the Idaho part of the Ore-Ida graben indicate the tract is permissive for undiscovered deposits of this type.
On the delineation of permissive tracts
This permissive tract for undiscovered Comstock epithermal-vein districts was drawn to include the Idaho part of the Ore-Ida graben, as interpreted from the State geologic map (Bond, 1978). This area is underlain by a voluminous pile of Miocene basaltic and felsic volcanic rocks and associated volcaniclastic rocks and encompasses Miocene and younger zone of pervasive north-south faulting. As some vein mineralization extends into Cretaceous granitic rocks occur within the tract, these rocks were also included in the tract. The tract is buried on the northeast by younger basaltic lavas of the northwest Snake River plain. The southern boundary of the tract was drawn just south of the southernmost prospects known in southwestern Idaho.
Important examples of this type of deposit
Three Comstock deposits (De Lamar, Stone Cabin, and War Eagle) are known within the permissive tract. These vein deposits variously cut Miocene rhyolites and Cretaceous granitic rocks (Asher, 1968). The contained metal contents of these three deposits overlap the mean metal content of Comstock deposits in the grade and tonnage model.
On the numerical estimates made
It is thought that all Comstock-type deposits exposed at the surface in the tract area are known, and any undiscovered deposits must be covered. Since the area has been mostly subsiding since the Miocene, the team inferred that any undiscovered Comstock vein deposit will probably still be overlain by a more disseminated hot-spring Au-Ag deposit, for which exploration is fairly incomplete. These undiscovered Comstock vein systems will probably be mined along with their overlying hot-spring deposit, and will be incorporated into the grade and tonnage of any undiscovered hot-spring deposit. For this reason, we only make an estimate for undiscovered hot-spring deposits for this area, and make no estimate for undiscovered Comstock deposits.
References
Asher, R.R., 1968, Geology and mineral resources of a portion of the Silver City region, Owyhee County, Idaho: Idaho Bureau of Mines and Geology Pamphlet 138, 106 p.
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.

Geographic coverage

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