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 permissive tract is drawn to encompass the sedimentary rocks of the Quesnellia terrane in northeastern Washington (Stoffel and others, 1991). This terrane includes a belt of Triassic and Jurassic plutons that have generated a string of porphyry copper deposits in British Columbia and northern Washington, along with associated Au and Cu skarn deposits. Carbonate units are irregularly scattered through the Quesnellia terrane, such that the entire terrane is permissive.

Important examples of this type of deposit

Two large skarn Cu (+Au) deposits are known just north of the Canadian border in southern British Columbia: Phoenix and Greenwood-Motherlode. No significant deposits are known within the tract, although several small occurrences of copper-bearing skarn mineralization are known around Mesozoic plutons within the tract.

On the numerical estimates made

The lack of significant skarn Cu deposits within the tract, even around the known porphyry copper deposit at Kelsey, led the team to be conservative in their estimate. The occurrence of significant Au in both the British Columbia deposits, enough to consider them gold skarns, led the team to judge that, whereas some undiscovered skarn Au deposits may have significant copper resources, the chance of a gold-poor copper skarn is low. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 1, 3, 5, and 7 or more copper skarn deposits consistent with the grade and tonnage model of Jones and Menzie (1986).

References

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.

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.