Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
Au-Ag-Te vein deposits, characterized by gold telluride minerals and fluorite, occur as veins and breccia bodies related to hypabyssal or extrusive alkalic rocks (Cox and Bagby, 1986). This description fits a class of gold occurrences in northeastern Washington associated with Mesozoic alkalic igneous complexes. In British Columbia, some deposits are also associated with Eocene alkalic intrusive bodies.
On the delineation of permissive tracts
The permissive tract was defined primarily by the location of Paleozoic and Mesozoic rock units of the Quesnellia island arc terrane (Stoffel and others, 1991), which hosts the Late Triassic to Early Jurassic alkalic intrusive bodies in a late-stage shoshonitic volcanic-intrusive setting (Mortimer, 1986). Eocene alkalic rocks occur sporadically within the Quesnellia terrane near the Canadian border, and have some associated Au prospects. The tract includes several known alkalic intrusive centers (Fox, 1973), with several known telluride-bearing gold deposits.
Important examples of this type of deposit
The Rossland camp near Rossland, British Columbia (40 km north of the Canada-U.S. border) is considered by some to be a deposit of this type (Fyles, 1984). This deposit is associated with Early Jurassic alkalic volcanic rocks and shallow intrusive bodies. Its tonnage is greater than over 90 percent of deposits of this type. The Comstock and Gold Dike deposits in the Shasket Creek district (a few km south of the Canada-U.S. border north of Republic, Washington) occur in syenitic dikes cutting older Quesnellia terrane rocks (Herdrick and Bunning, 1984; Tschauder, 1989). The grade and tonnage of these deposits are slightly less than the median for this deposit type.
On the numerical estimates made
As defined by Bliss and others (1992), alkaline Au-Te deposits are small, exhibiting a median of 1.78 million metric tons of ore with a median Au grade of nearly 10 g/t. The Rossland deposit is much larger than the median size and Shasket Creek is slightly smaller than the median. Favorable areas for undiscovered deposits are located south of Rossland in the U.S., near the Shasket Creek intrusive bodies, and near the Similkameen alkalic pluton to the west. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 1, 2, 4 or more deposits consistent with the grade and tonnage model for Au-Ag-Te deposits associated with alkaline rocks (Bliss and others, 1992).
Bliss, J.D., Sutphin, D.M., Mosier, D.L., and Allen, M.S., 1992, Grade-tonnage and target-area models of Au-Ag-Te veins associated with alkaline rocks: U.S. Geological Survey Open-File Report 92-208, 15 p.
Cox, D.P., and Bagby, W.C., 1986, Descriptive model of Au-Ag-Te veins in Cox, D. P., and Singer, D. A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 124.
Fox, K.F., Jr., 1973, The geology of alkalic complexes in north-central Washington: U.S. Geological Survey Open-File Report 73-73, 192 p.
Fyles, J.T., 1984, Geological setting of the Rossland mining camp: British Columbia Ministry of Energy, Mines and Petroleum Resources, Bulletin 74, 61 p.
Herdrick, Melvin, and Bunning, B.B., 1984, Geology of the Gold Dike mine, Ferry County, Washington: Washington Geologic Newsletter, v. 12, no. 1, p. 20-22.
Mortimer, N., 1986, Late Triassic, arc-related potassic igneous rocks in the North American Cordillera: Geology, v. 14, p. 1035-1038.
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.
Tschauder, R., 1986, Gold deposits in northern Ferry County, in Joseph, N.L., ed., Geologic guidebook for Washington and adjacent areas: Washington Division of Geology and Earth Resources Information Circular 86, p. 239-254.