Explained by Dennis P. Cox, Steve Ludington, and Michael F. Diggles
On the choice of deposit models
Volcanogenic massive sulfide deposits form in belts of marine felsic to intermediate volcanic rocks, and have been an historically important source of Cu, Zn, Ag, and Au, due to their relatively high grades and simple metallurgy. The Sierran kuroko model, which is defined to be restricted to deposits of Triassic and Jurassic age (Singer, 1992), was selected because the host rocks in this tract are primarily Jurassic in age.
On the delineation of permissive tracts
On the west side of the Southern California batholith, there are Jurassic metavolcanic rocks that host gold and base-metal deposits. Rocks of this type also underlie a small area in the south. These rocks are permissive for both low-sulfide Au-quartz vein deposits and for kuroko deposits (Weber, 1963).
Important examples of this type of deposit
There are no known kuroko deposits in the tract. Known mineralized areas in the Julian district are low-sulfide Au-quartz vein deposits.
On the numerical estimates made
The fact that there are no known deposits of this type in the tract convinced us that a small estimate was appropriate. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more Sierran kuroko deposits consistent with the grade and tonnage model of Singer (1992).
Singer, D.A., 1992, Grade and tonnage model of Sierran kuroko deposits, in Bliss, J.D., ed., Developments in deposit modeling: U.S. Geological Survey Bulletin 2004, p. 29-32.
Weber, F.H., Jr., 1963, Geology and mineral resources of San Diego County, California: California Division of Mines and Geology County Report 3, 309 p.