National mineral assessment tract PC32 (Hot-spring Au-Ag)

Tract PC32
Geographic region Pacific Coast
Tract area 1,730sq km
Deposit type Hot-spring Au-Ag
Deposit age Tertiary

Deposit model

Model code 25a
Model type descriptive
Title Descriptive model of hot-spring Au-Ag
Authors Byron R Berger
URL https://pubs.usgs.gov/bul/b1693/html/bull0g4n.htm
Source https://pubs.er.usgs.gov/publication/b1693

Estimates

Confidence Number of
deposits
90% 0
50% 1
10% 3
5% 5
1% 5

Estimators: Box, DCox, Bookstrom

Rationale

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). Three epithermal Au-Ag vein deposits, each with associated hot-spring-type mineralization, occur within or beneath 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 hot-spring Au-Ag deposits was drawn to include the Idaho part of the Ore-Ida graben, as interpreted from the State geologic map (Bond, 1978). This tract is underlain by a voluminous pile of Miocene basaltic and felsic volcanic rocks and associated volcaniclastic rocks and encompasses a Miocene and younger zone of pervasive north-south faulting. Minor amounts of Cretaceous granitic rocks are exposed beneath the Miocene rocks and locally contain epithermal mineralization; they are also included in the tract. The tract is buried on the northeast by younger basaltic lavas of the northwest Snake River plain.
Important examples of this type of deposit
Three epithermal Au-Ag deposits (De Lamar, Stone Cabin, and War Eagle) are known within the permissive tract. Two of the deposits (Delamar, Stone Cabin) grade from disseminated hot-spring type mineralization to more focused vein-type mineralization in different parts of the deposits (Asher, 1968). These two deposits have gold contents at and just below the mean Au content of hot-spring Au-Ag deposits. Silver contents are well above the mean for this deposit type.
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, we concluded that any undiscovered Comstock vein deposit will probably still be overlain by a more disseminated Au-Ag hot-spring 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 deposit for the area of this tract, and make no estimate for undiscovered Comstock-type deposits. since a lot of the tract is covered by rock younger than the favorable Miocene rhyolite horizon, much of the ground is open for the occurrence of undiscovered deposits. Exploration for this deposit type has been incomplete, having begun only in the 1980s. For the 90th, 50th, 10th, 5th and 1st percentiles, the team estimated 0, 1, 3, 5, and 5 or more undiscovered hot-spring Au-Ag deposits, consistent with the grade and tonnage model of Berger and Singer (1992).
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.
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 of 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.

Geographic coverage

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