Explained by Dennis P. Cox, Steve Ludington, and Michael F. Diggles
On the choice of deposit models
Volcanogenic massive sulfide deposits are located in the foothills of the Sierra Nevada, in belts of marine felsic to intermediate volcanic rocks. Volcanogenic massive sulfide deposits have been an historically important source of Cu, Zn, Ag, and Au in this region. Relatively high grades of polymetallic ores, simple metallurgy, and potential for large deposits make these deposits attractive exploration targets, and exploration for, and development of, these deposits continues. Although both kuroko and Cyprus types of massive sulfide deposits are possible in this tract, the team only assessed the kuroko type. 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, many of which are included in the Sierran kuroko model, are in Jurassic rocks.
On the delineation of permissive tracts
All map units in the foothills of the Sierra Nevada that contain sequences of submarine volcanic rocks have been included and define the permissive tract for volcanogenic massive sulfide deposits. The tract extends westward under the Great Valley where the depth to Jurassic basement is no more than one kilometer, based on drillhole data (Wentworth and others, in press).
Important examples of this type of deposit
The Penn mine is one of the larger examples of volcanogenic massive sulfide deposits from the Sierra Foothills metavolcanic terranes. It produced 38,000 metric tons of copper, 10,000 metric tons of zinc, 66 metric tons of silver, and 2 metric tons of gold. The Blue Moon and Western World deposits have been actively explored in recent years.
On the numerical estimates made
This region has been explored extensively in the past, focusing on easily-observed surface gossans. There are seven known deposits and 19 smaller occurrences in the area, and we judged that about a quarter of those occurrences could be developed into deposits with further exploration and development. This, coupled with consideration of substantial amounts of concealed potential host rocks, guided our estimate for the 50th percentile. The substantial concealed area also guided our estimate for the 10th percentile. For the 90th, 50th, and 10th percentiles, the team estimated 2, 13, and 25 or more deposits consistent with the Sierran kuroko grade and tonnage model of Singer (1992).
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.
Wentworth, C.M., Fisher, G.R., and Jachens, R.C., in press, Contour map of the Great Valley basement, California: U.S. Geological Survey Miscellaneous Field Studies Map, scale 1:750,000.