Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
Skarn Au deposits are a subset of a spectrum of skarn 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 throughout the map area. Since a significant porphyry copper deposit and a significant skarn Au deposit both occur within the tract, the gold skarn model is clearly appropriate.
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
Only one gold-bearing skarn, Buckhorn Mountain (Hickey, 1992), is known in northeastern Washington. This skarn is localized within rocks of the Quesnellia terrane around a pluton of uncertain but probable Mesozoic age, and lies to the south of a cluster of gold skarn deposits in southern British Columbia (Theodore and others, 1991), which also occur around Mesozoic plutons in the Quesnellia terrane. The known deposit has very large tonnage greater than 90 percent of the deposits in the tonnage distribution of Theodore and others (1991), with a contained Au content more than 10 times larger than the median Au content from the grade and tonnage distributions.
On the numerical estimates made
Seven deposits of this type are known in the Quesnellia terrane, all but one in British Columbia. The Buckhorn deposit in Washington, a prospect for nearly one hundred years, was only recently proved to be a significant deposit, and at least five more similar prospects are known in Okanagan County, Washington (Derkey and others, 1990). The association of this deposit type with magnetite mineralization makes buried targets easy to delineate. Since the discovery of the Buckhorn deposit, exploration for this deposit type has been brisk. For these reasons the team expressed cautious optimism about the potential for undiscovered deposits in this tract. Because the grade and tonnage model of Theodore and others (1991) includes some very small deposits, we felt we could estimate more accurately using a model that is truncated to include only those deposits with more than 15,000 metric tons of mineralized rock. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 1, 3, 5, and 7 or more deposits that are comparable in grade and tonnage to the gold skarn grade and tonnage model (truncated) of Theodore and others (1991).
Derkey, R.E., Joseph, N.L., and Lasmanis, Raymond, 1990, Metal mines of Washington—Preliminary report: Washington Division of Geology and Earth Resources Open-File Report 90-18, 577 p.
Hickey, R.J., 1992, The Buckhorn Mountain (Crown Jewel) gold skarn deposit, Okanogan County, Washington: Economic Geology, v. 87, no. 1, p. 125-141.
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.
Theodore, T.G., Orris, G.J., Hammarstrom, J.M., and Bliss, J.D., 1991, Gold-bearing skarns: U.S. Geological Survey Bulletin 1930, 61 p.