National mineral assessment tract NR03 (Skarn Zn-Pb)

Tract NR03
Geographic region Northern Rocky Mountains
Tract area 21,300sq km
Deposit type Skarn Zn-Pb
Deposit age Mesozoic - Tertiary

Deposit model

Model code 18c
Model type descriptive
Title Descriptive model of Zn-Pb skarn deposits
Authors Dennis P. Cox


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
Skarn Zn-Pb deposits are an end-member of a spectrum of skarn deposit types that are variously copper-, zinc-lead-, or iron-rich. Skarns are metallic sulfide and oxide replacement deposits that occur in carbonate host lithologies adjacent to plutonic bodies with metal-bearing hydrothermal systems. The deposits are associated with shallow intermediate plutons, commonly those that are host to porphyry-style mineralization. The carbonate bodies that host the mineralization need not be regionally extensive, but can be small local bodies that are widely scattered essentially throughout the map area.
On the delineation of permissive tracts
The tract was delineated by excluding areas without significant carbonate rocks from the corresponding porphyry Cu tract. Permissive sedimentary packages include the Paleozoic sequence of northeastern Washington, the middle Belt carbonate units of the Belt Supergroup in northern Idaho, and the Paleozoic sequence of south-central Idaho (Bond, 1978; Stoffel and others, 1991).
Important examples of this type of deposit
No significant deposits of this type are known from the tract. A small skarn Zn-Pb deposit mined at the Horseshoe mine near Mackay, Idaho is very small compared to the grade and tonnage distributions of Mosier (1986).
On the numerical estimates made
The lack of significant known deposits of this type and the lack of known porphyry copper deposits in this tract lead the team to give a very low estimate. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more skarn Zn-Pb deposits consistent with the grade and tonnage model of Jones and Menzie (1986).
Bond, J.G., 1978, Geologic map of Idaho: Idaho Bureau of Mines and Geology, scale 1:500,000.
Mosier, D.L., 1986, Grade and tonnage model of Zn-Pb skarn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 90-93.
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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