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

On the choice of deposit models

Volcanogenic massive sulfide deposits in western Washington 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 attractive exploration targets. Kuroko massive sulfide deposits form in marine volcanic rocks of intermediate to felsic composition that include rhyolite, dacite, and subordinate basalt and associated sedimentary rocks. These rock types are present in tectonostratigraphic terranes derived from island-arc volcanism. The Sierran kuroko model, which is defined to be restricted to deposits of Triassic and Jurassic age, is used for this tract because the only known deposit is hosted in a belt of gneiss and hornblende schist of Late Triassic age.

On the delineation of permissive tracts

All map units containing sequences of submarine volcanic rocks in the Northern Cascades have been included and define the permissive area for kuroko massive sulfide deposits. Although some of these areas may be permissive for Cyprus and Besshi deposits, the team assessed only for kuroko, because of lack of detailed stratigraphic information in the area.

Important examples of this type of deposit

The Holden deposit, in western Washington, produced 9 million metric tons of ore averaging 1.06 percent Cu, 0.2 percent Zn, 6.8 grams per metric ton Ag, and 2 grams per metric ton Au between 1938 and 1957 (McWilliams, 1958). This deposit is larger than all other deposits in the Sierran kuroko grade and tonnage model, but this difference is not deemed sufficient to rule out the use of that model.

On the numerical estimates made

The Holden deposit is in a part of the permissive tract that may continue under rocks of the Columbia River Basalt Group. The area that is covered may be the site of undiscovered deposits. For the 90th, 50th, 10th, and 5th percentiles, the team estimated 0, 0, 1, and 2 or more Sierran kuroko deposits consistent with the grade and tonnage model of Singer (1992).

References

Dragovitch, J.D., and Derkey, R.E., 1994, A Late Triassic island-arc setting for the Holden volcanogenic massive sulfide deposit, North Cascades, Washington: Washington Geology, v. 22, no. 1, p. 22-39.

McWilliams, J.R., 1958, Mining methods and costs at the Holden mine, Chelan Division, Howe Sound Company, Chelan County, Washington: U.S. Bureau of Mines Information Circular 7870, 44 p.

Singer, D.A., 1986, Descriptive model of kuroko massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 189.

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.