National mineral assessment tract PC21 (Epithermal vein, Comstock)

Tract PC21
Geographic region Pacific Coast
Tract area 31,300sq 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

Estimates

Confidence Number of
deposits
90% 2
50% 4
10% 7
5% 11
1% 16

Estimators: Rytuba, Peters, Raines, Evans, Spanski

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, 1986a). In southeastern Oregon, widespread Neogene felsic, intermediate, and mafic volcanic rocks, along with considerable Neogene normal faulting, provide a compositionally and structurally appropriate environment for epithermal vein occurrences. Known Au-Ag epithermal occurrences are of the quartz-adularia type.
On the delineation of permissive tracts
The tract encircles essentially all Neogene volcanic and associated sedimentary rocks in southeastern Oregon (Peters and others, 1994), outside of the Ore-Ida graben. The eastern and southern tract boundaries are arbitrarily drawn at the State boundaries. The western boundary is drawn to separate much less favorable ground in central Oregon. Some basaltic volcanic rocks are included which may not be appropriate for the occurrence of Comstock veins. However insufficient data are available to outline only the compositionally appropriate volcanic rocks.
Important examples of this type of deposit
No significant Comstock-type Au-Ag deposits are known from this tract. However a number of hot-spring geothermal systems are known, and deeper, unexposed parts of these systems are likely to include tabular vein deposits of Comstock type.
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, 4, 7, 11, and 16 or more districts (Spanski, 1994) consistent with the grade and tonnage model of Mosier and others (1986b).
References
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 Areas, 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.

Geographic coverage

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