Explained by Dennis P. Cox, Steve Ludington, and Michael F. Diggles

On the choice of deposit models

Volcanogenic massive sulfide deposits in the Klamath Mountains have been a moderately important source of Cu, Zn, Ag, and Au. Relatively high grades of polymetallic ores, simple metallurgy, and potential for large deposits make these deposits a attractive exploration targets. Kuroko massive sulfide deposits form in marine volcanic rocks of intermediate to felsic composition that include marine rhyolite, dacite, and subordinate basalt derived from island-arc volcanism. The Sierran kuroko model, which is defined to be restricted to deposits of Triassic and Jurassic age (Singer, 1992), was selected because the known deposits are in Jurassic rocks.

On the delineation of permissive tracts

All map units containing sequences of submarine volcanic rocks have been included and define the permissive tract for volcanogenic massive sulfide deposits. Paleozoic, Triassic, and Jurassic rocks are present in the tract. Although some of this tract may also be permissive for Besshi deposits, the team assessed only for Cyprus and kuroko types, because of lack of detailed stratigraphic information in the area.

Important examples of this type of deposit

The Gray Eagle mine in the Happy Camp district, California, produced over 50,000 metric tons of copper. Two smaller deposits are known in Oregon.

On the numerical estimates made

In addition to the Gray Eagle deposit, there are two kuroko-type occurrences. The estimators decided that there was an even chance of an equal number of undiscovered deposits, and that belief guided our estimate. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 3, 7, 15, and 20 or more Sierran kuroko deposits consistent with the grade and tonnage model of Singer (1992).

References

Koski, R.A., and Derkey, R.E., 1981, Massive sulfide deposits in ocean-crust and island-arc terranes in southwestern Oregon: Oregon Geology, v. 43, no. 9, p. 119-125.

Singer, D.A., 1992, Grade and tonnage model of Sierran kuroko deposits, in Bliss, J.D., ed., Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, p. 29-32.