National mineral assessment tract SB19 (Polymetallic replacement + skarn Zn-Pb)

Tract SB19
Geographic region Southern Basin and Range
Tract area 1,610sq km
Deposit type Polymetallic replacement + skarn Zn-Pb
Deposit age Mesozoic - Tertiary

Deposit model

Model code 19a
Model type descriptive
Title Descriptive model of polymetallic replacement deposits
Authors Hal T. Morris
URL https://pubs.usgs.gov/bul/b1693/html/bull7q0k.htm
Source https://pubs.er.usgs.gov/publication/b1693
Model code 18c
Model type descriptive
Title Descriptive model of Zn-Pb skarn deposits
Authors Dennis P. Cox
URL https://pubs.usgs.gov/bul/b1693/html/bull4u7i.htm
Source https://pubs.er.usgs.gov/publication/b1693

Estimates

Confidence Number of
deposits
90% 0
50% 1
10% 2
5% 2
1% 2

Estimators: DCox, Ludington

Rationale

Explained by Dennis P. Cox
On the choice of deposit models
The deposits in the Goodsprings or Yellow Pine district occur in Devonian and Mississippian strata and are related to poorly exposed granodiorite porphyry bodies that are noted for their very large feldspar crystals (Hewitt, 1931). Radiometric ages of these intrusions show that they are Late Triassic to Early Jurassic in age (Carr and others, 1986; Garside and others, 1993). The Goodsprings district is unique in Nevada in that a small proportion of the deposits in the district diverge markedly from typical polymetallic replacement deposits. The Boss Mine, for example, is a high-grade, oxidized copper deposit with an interior zone rich in platinum group metals. Other examples resemble Mississippi Valley deposits in their ore textures and extreme distance from a possible causative pluton, as first suggested by Hewitt (1931). Because zinc-lead skarn deposits are possible in the same environment as polymetallic replacement deposits, a model that combines the zinc-lead skarn (Mosier, 1986) and polymetallic replacement (Mosier and others, 1986) was used to represent the undiscovered districts (Mark 3 no. 92).
On the delineation of permissive tracts
Skarn Zn-Pb and polymetallic replacement deposits can be present where plutons intrude carbonate rocks. Paleozoic sedimentary rocks are hidden by a thin veneer of sediment in many parts of the area. The permissive tract was defined as an area 10 km in radius around all plutons where they intrude Proterozoic and younger carbonate rocks.
On the numerical estimates made
An undiscovered zinc-lead skarn or polymetallic replacement district might exist under cover of Cenozoic rocks and sedimentary deposits but the likelihood is small because of the limited size of the permissive area. For the 90th, 50th, and 10th percentiles, the team estimated 0, 1, and 2 or more deposits consistent with the combined grade and tonnage model (Mark 3 no. 92).
References
Carr, M.D., Evans K.V., Fleck, J.R., Frizzell, R.J., Ort, K.M., and Zartman, R.E., 1986, Early Middle Jurassic upper limit for the movement on the Keystone thrust, southern Nevada: Geological Society of America, Abstracts with Program, v. 18, no. 5, p. 345.
Garside, L.J., Bonham, H.F., Jr., Tingley, J.V., and McKee, E.H., 1993, Potassium-argon ages of igneous rocks and alteration minerals associated with mineral deposits, western and southern Nevada: Isochron West, no. 59, p. 17-23.
Hewett, D.F., 1931, Geology and ore deposits of the Goodspings quadrangle, Nevada: U.S. Geological Survey Professional Paper 162, 171 p.
Mosier, D.L.,1986, Grade-tonnage model of Zn-Pb skarns, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 90-93.
Mosier, D.L., Morris, H.T., and Singer, D.A., 1986, Grade-tonnage model of polymetallic replacement deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 101-104.

Geographic coverage

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