National mineral assessment tract PC01 (Skarn Cu)

Tract PC01
Geographic region Pacific Coast
Tract area 15,900sq km
Deposit type Skarn Cu
Deposit age Mesozoic - Tertiary

Deposit model

Model code 18b
Model type descriptive
Title Descriptive model of Cu skarn deposits
Authors Dennis P. Cox and Ted G. Theodore


Confidence Number of
90% 0
50% 3
10% 5
5% 9
1% 12

Estimators: Box, Bookstrom


Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
Copper skarns 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, and consists of the Wallowa, Baker, and Huntington terranes of northeastern Oregon, west-central Idaho, and southeasternmost Washington (Brooks and Vallier, 1978; Walker and MacLeod, 1991). Plutonic rocks of Jurassic and Cretaceous(?) age are widely scattered through these terranes, and where large enough, these were excluded from the permissive tract. Small, irregular carbonate horizons occur throughout all three structurally complex terranes such that carbonate rocks could be present in the subsurface of any part of these terranes. The Izee terrane of north-central Oregon lacks carbonate rocks and was excluded from the tract.
Important examples of this type of deposit
The Peacock deposit is a copper skarn deposit located near Cuprum, east of the Snake River. Grade and tonnage information on the deposit (Terry Close, U.S. Bureau of Mines, personal commun., 1994) indicate it is larger than the median of the grade and tonnage distributions of Jones and Menzie (1986). This deposit and about a dozen other prospects occur in small carbonate bodies within a roof pendant to an Early Jurassic quartz diorite pluton in the Wallowa terrane. Numerous small prospects are also known around the Wallowa batholith in northeastern Oregon.
On the numerical estimates made
The presence of a number of copper skarn prospects, most of which have not been adequately explored, indicates reasonable potential for undiscovered deposits (including prospects that prove to be deposits with further exploration). The Payette National Forest, which makes up much of the Idaho part of the tract, has most of the known prospects. A recent U.S. Geological Survey assessment of mineral potential in the Payette National Forest (Bruce Johnson, U.S. Geological Survey, personal commun., 1994) estimated (at the 90th, 50th, 10th, 5th, and 1st percentiles) 0, 2, 3, 7, and 10 or more undiscovered copper skarn deposits consistent with the grade and tonnage model of Jones and Menzie (1986). Adding in the less prospective Oregon part of the tract, the National Assessment team estimated 0, 3, 5, 9, 12 (at the 90th, 50th, 10th, 5th, and 1st percentiles) or more undiscovered copper skarn 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.
Brooks, H.C., and Vallier, T.L., 1978, Mesozoic rocks and tectonic evolution of eastern Oregon and western Idaho, in Howell, D.G., and McDougall, K.A., eds., Mesozoic paleogeography of the western United States: Pacific section, Society of Economic Paleontologists and Mineralogists, Pacific Coast Paleogeography Symposium 2, p. 133-146
Jones, G.M., and Menzie, W.D., 1986, Grade and tonnage model of Cu skarn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 86-89.
Walker, G.W., and MacLeod, N.S., 1991, Geologic map of Oregon: U.S. Geological Survey, 2 sheets, scale 1:500,000.

Geographic coverage

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