Explained by Stephen E. Box and Arthur A. Bookstrom
On the choice of deposit models
Besshi massive sulfide deposits are thin, sheetlike bodies of massive to well-laminated pyrite, pyrrhotite, and chalcopyrite within marine clastic sedimentary deposits and mafic tuffs associated with submarine mafic volcanic rocks (Cox, 1986). Besshi-type volcanic-hosted massive sulfide ore deposits produce copper and zinc, with gold and silver as common byproducts. Paleozoic rocks of the Covada Group or Kootenay terrane in northern Washington and southern British Columbia are host to deposits and prospects that have characteristics similar to Besshi-type massive sulfide deposits.
On the delineation of permissive tracts
The genesis of this deposit type is uncertain due to the amount of regional deformation and metamorphism of the deposits in the type locality in Japan. However, the general exploration criteria for these deposits include: (1) clastic terrigenous sedimentary rocks and interbedded mafic volcanic tuffs and breccia host rocks; (2), local association with black shale, oxide-facies iron formation and red chert; (3) formation from submarine hot springs related to basaltic magmatism in rifted basins in island-arc or back-arc tectonic settings.
The Kootenay terrane or Covada Group consists of a Paleozoic continental slope and rise sequence west of the North American Paleozoic continental shelf sequence (Smith and Gehrels, 1992). Sporadic occurrences of ferruginous chert associated with volcanic rocks indicate synvolcanic hot-spring activity, which is believed to be conducive to the formation of Besshi-type massive sulfide deposits. The permissive tract was drawn to include all of the Covada Group in the U.S. (Stoffel and others, 1991).
Important examples of this type of deposit
In British Columbia, two important deposits of this type are known from this belt of rocks. The Goldstream deposit has reserves of 3.2 million metric tons containing 4.5 percent copper, 3.1 percent zinc, and 20 grams per metric ton of silver (Hoy, 1991). The True Blue deposit further south is another deposit of this type in British Columbia. In Washington, prospects and small deposits of this type are known but none have been major producers.
On the numerical estimates made
The lack of known major deposits in Washington or immediately across the border in British Columbia, along with the long history of exploration in northern Washington, cause us to consider this tract to have low potential for undiscovered deposits of this type. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, 1 or more deposits consistent with the grade and tonnage model described by Singer (1986).
References
Cox, D.P., 1986, Descriptive model of Besshi massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 136.
Hoy, Trygve, 1991, Volcanogenic massive sulphide deposits in British Columbia, in McMillan, W.J., and others, eds., Ore deposits, tectonics and metallogeny in the Canadian Cordillera: Ministry of Energy, Mines, and Petroleum Resources, Paper 1991-4, p. 89-123.
Smith, Moira, and Gehrels, G.E., 1992, Stratigraphic comparison of the Lardeau and Covada Groups: implications for revision of stratigraphic relations in the Kootenay Arc: Canadian Journal of Earth Sciences, v. 29, p. 1320-1329.
Singer, D.A., 1986, Grade and tonnage model of Besshi massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 136-138.
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.
