Explained by Dennis P. Cox and Steve Ludington
On the choice of deposit models
Volcanogenic massive sulfide deposits in western United States and have been an historically important source of Cu, Zn, Ag, and Au. Relatively high grades of polymetallic ores, simple metallurgy, and potential for large deposits make these deposits an attractive exploration target. Besshi massive sulfide deposits form in marine sedimentary rocks associated with subordinate marine basalt flows and breccias (Fox, 1984).
On the delineation of permissive tracts
One massive sulfide deposit, Island Mountain, occurs in Cretaceous rocks of the Franciscan Complex in California. Island Mountain is difficult to classify as to massive sulfide subtype but, as it is in a sedimentary rather than volcanic environment, it most resembles a Besshi-type deposit. We have, therefore, included all of the Franciscan Complex in a permissive tract for Besshi deposits even though only one example is known and only a small amount of this map unit is probably favorable for the occurrence of volcanogenic massive sulfide deposits. The Franciscan Complex is composed chiefly of graywacke, siltstone, chert, and greenstone (Dickinson and others, 1982).
In western Oregon and Washington, the submarine basalt and minor intercalated sedimentary rocks of the Tertiary Crescent Formation and lithologically equivalent units are considered permissive for Besshi deposits although no occurrences are known.
Important examples of this type of deposit
The Island Mountain massive-sulfide deposit, which has produced 120,000 metric tons of pyrrhotite-rich ore averaging 3.3 percent Cu, 33 g/t Ag, and 2 g/t Au, was classified as a Besshi-type deposit by Koski and others (1993). Host rocks are a mélange of sandstone, siltstone, and minor chert and greenstone.
On the numerical estimates made
The team made a low estimate because only one Besshi type deposit is known. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more undiscovered deposits consistent with the Besshi grade and tonnage model of Singer (1986).
Dickinson, W.R,. Ingersoll, R.W., Cowan, D.S., Helmhold, K.P., and Cuczek, C.A., 1982, Provenance of Franciscan sandstones in coastal California: Geological Society of America Bulletin, v. 93, p. 95–107.
Fox, J.S., 1984, Besshi-type volcanogenic sulfide deposits—A review: Canadian Institute Mining and Metallurgy Bulletin, v. 77, no. 864, p. 57-68.
Koski, R.A., Lamons, R.C., Dumoulin, J.A., and Bouse, R.M., 1993, Massive sulfide metallogenesis at a late Mesozoic sediment-covered spreading axis—Evidence from the Franciscan complex and contemporary analogues: Geology, v. 21, p. 137-140.
Singer, D.A., 1986, Grade and tonnage model of Besshi massive sulfide deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 136-138.