Explained by Dennis P. Cox
On the choice of deposit models
Plutons, mainly of Jurassic age, are closely associated with copper- and lead-zinc skarns and polymetallic replacement and vein deposits in the Great Basin of California. It is possible that one or more of these plutons could have given rise to a porphyry copper system, although few examples are known. Outcrops of felsic porphyry with an aplitic groundmass and pervasive sericitic alteration was observed at the Copper Queen mine east of Trona (D.P. Cox, unpub. data) suggesting that favorable conditions for porphyry mineralization might have occurred, at least, locally. In addition, records in the Bureau of Land Management office in Ridgecrest, Calif., indicate that there is a deposit believed to be a porphyry copper deposit being explored in the El Paso Mountains.
On the delineation of permissive tracts
The permissive tract is a nearly continuous area that encompasses many small plutons that intrude Precambrian metamorphic rocks and Precambrian and Paleozoic sedimentary rocks in a highly faulted part of the Mesozoic continental margin. Small areas of basin fill more than 1 km in depth are excluded.
On the numerical estimates made
Porphyry systems that are well exposed at the surface are marked by highly visible alteration patterns. Since no such alteration has been observed, it is likely that any deposits awaiting discovery would be largely concealed by alluvium or Tertiary sedimentary or volcanic rocks.
For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 1, and 2 or more porphyry copper deposits consistent with the grade and tonnage model of Singer and others (1986).
Singer, D.A., Mosier, D.L., and Cox, D.P., 1986, Grade-tonnage model of porphyry copper, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 77-81.