National mineral assessment tract NR12 (Polymetallic replacement)

Tract NR12
Geographic region Northern Rocky Mountains
Tract area 21,300sq km
Deposit type Polymetallic replacement
Deposit age Mesozoic - Tertiary

Deposit model

Model code 19a
Model type descriptive
Title Descriptive model of polymetallic replacement deposits
Authors Hal T. Morris


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

P(none): 0.99

Estimators: Box, Bookstrom


Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
These deposits are hydrothermal epigenetic deposits that consist of silver-, lead-, zinc-, and copper-bearing minerals in massive lenses, pipes, and veins in carbonate sedimentary rocks near igneous intrusions (Morris, 1986). Associated igneous rocks are commonly calc-alkaline and porphyritic. Types of alteration include dolomitization and silicification. On a district scale, the deposits are commonly zoned from a copper-rich central area, through a wide lead-silver zone, outward to a zinc- and manganese-rich fringe. A permissive tract for polymetallic replacement deposits in Idaho and northeastern Washington is delineated based on the widespread occurrence of Jurassic, Cretaceous, and Tertiary calc-alkaline plutonic rocks and carbonate-bearing sedimentary sequences.
On the delineation of permissive tracts
The tract was delineated by excluding areas without significant carbonate rocks from the corresponding porphyry Cu tract. Host sedimentary packages include Paleozoic rocks in northeastern Washington, the Wallace Formation of the Middle Proterozoic Belt Supergroup in northern Idaho, and Paleozoic rocks in south-central Idaho (Stoffel and others, 1991; Bond, 1978).
Important examples of this type of deposit
The Clayton Silver deposit in south-central Idaho is considered to be a polymetallic replacement deposit (Ross, 1937). The deposit occurs as an elongate cigar-shaped stratabound replacement of a carbonate member of the Cambrian Kinnikinnik Quartzite. The mine produced significant silver, lead, and zinc.
On the numerical estimates made
The low estimate for undiscovered porphyry copper deposits for this tract, the rarity of significant prospects of this deposit type, and the long exploration history of the region led the team to make a very low estimate for undiscovered deposits of this type. For the 90th, 50th, 10th, and 5th percentiles, the team estimated 0, 0, 0,0, and 1 or more deposits consistent with the grade and tonnage model of Mosier and others (1986).
Bond, J.G., 1978, Geologic map of Idaho: Idaho Bureau of Mines and Geology, scale 1:500,000.
Morris. H.T., 1986, Descriptive model of polymetallic replacement deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 99-100.
Mosier, Dan L., Morris, H.T., and Singer, D.A., 1986, Grade and 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.
Ross, C.P., 1937, Geology and ore deposits of the Bayhorse region, Custer county, Idaho: U.S. Geological Survey Bulletin 877, 161 p.
Stoffel, K.L., Joseph, N.L., Waggoner, S.Z., Gulick, S.W., Korosec, M.A., and Bunning, B.B., 1991, Geologic map of Washington-Northeast quadrant: Washington Division of Geology and Earth Resources, Geologic Map GM-39, scale 1:250,000.

Geographic coverage

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