National mineral assessment tract NR40 (Sedimentary exhalative Zn-Pb)

Tract NR40
Geographic region Northern Rocky Mountains
Tract area 4,470sq km
Deposit type Sedimentary exhalative Zn-Pb
Deposit age Paleozoic

Deposit model

Model code 31a
Model type descriptive
Title Descriptive model of sedimentary exhalative Zn-Pb
Authors Joseph A. Briskey


Confidence Number of
90% 0
50% 0
10% 1
5% 2
1% 3

P(none): 0.9

Estimators: Box, DCox, Bookstrom


Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
Stratabound, massive Zn-Pb sulfide deposits form by the precipitation of sulfide and sulfate minerals from metalliferous brines that were exhaled along active submarine faults during deposition of the enclosing sedimentary sequence. The mineral deposits are dominantly iron sulfide accumulations, which enclose economic ore deposits as layers and lenses rich in Zn and Pb sulfides. These syngenetic deposits are hosted in euxinic marine sedimentary rocks in epicratonic and intracratonic basins, often associated with synsedimentary faults related to rifting and subsidence of the sedimentary basin (Briskey, 1986). The Paleozoic black shale belt in south-central Idaho (Hall, 1986; Wavra and others, 1986) is characterized by argillite, siltite, limy sandstone, shale, and siltstone, combined with the presence of synsedimentary faults, indicates that this deposit type may exist here.
On the delineation of permissive tracts
The tract was drawn to encompass the lower and upper Paleozoic, deep-marine sedimentary rocks of the black shale belt in the Wood River valley area of south-central Idaho (Hall, 1986; Bond, 1978). Abrupt facies changes and interstratified slump breccias suggest the presence of synsedimentary faults. Numerous lead and zinc occurrences within the tract, as well as one known deposit of this type, suggest the possibility of undiscovered deposits.
Important examples of this type of deposit
The Triumph deposit (Kiilsgaard, 1950) near Ketchum, Idaho is considered to be a deposit of this type. The deposit is small relative to those of the grade and tonnage distributions of Menzie and Mosier (1986). Although the grade of lead and zinc are higher than 80 percent and 60 percent, respectively, of the model deposits, the tonnage is less than 10 percent of the median value. As a result the total metal content of the Triumph deposit (production and reserves) is only about 3 percent of that of typical deposits of this type.
On the numerical estimates made
The Triumph deposit is very small relative to other deposits in the grade and tonnage distributions for sedimentary exhalative Zn-Pb deposits (Menzie and Mosier, 1986). Other prospects in the region fall well outside of the grade and tonnage fields of known deposits. The region was extensively explored for this deposit type in the 1970s and 1980s by Exxon and Noranda, with no new discoveries reported. However the occurrence of a known deposit and several prospects, along with the complex structure of the region and the possibility of structurally or stratigraphically buried deposits that could elude exploration, the team judged that there was a low but still viable chance for undiscovered deposits in this tract. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 1, 2, and 3 or more deposits consistent with the grade and tonnage model of Menzie and Mosier (1986).
Bond, J.G., 1978, Geologic map of Idaho: Idaho Bureau of Mines and Geology, scale 1:500,000.
Briskey, J.A., 1986, Descriptive model of sedimentary exhalative Zn-Pb, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 211.
Hall, W.E., 1986, Stratigraphy of and mineral deposits in Middle and Upper Paleozoic rocks of the black-shale mineral belt, central Idaho: U.S. Geological Survey Bulletin 1658-J, p. 117–131.
Kiilsgaard, T.H., 1950, Geology and ore deposits of the Triumph-Parker mine mineral belt, in Anderson, A.L., Kiilsgaard, T.H., and Frykland, V.C.,Jr., Detailed geology of certain areas in the Mineral Hill and Warm Springs mining districts, Blaine County, Idaho: Idaho Bureau of Mines and Geology Pamphlet 90, 73 p.
Menzie, W.D., and Mosier, D.L., 1986, Grade and tonnage model of sedimentary exhalative Zn-Pb, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 212-215.
Wavra, C.S., Isaacson, P.E., and Hall, W.E., 1986, Studies of the Idaho black shale belt—Stratigraphy, depositional environment, and economic geology of the Permian Dollarhide Formation: Geological Society of America Bulletin, v. 97, p. 1504–1511.

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