Explained by Dennis P. Cox
On the choice of deposit models
Although copper skarn deposits are much less common in this tract than lead-zinc dominated systems, widespread igneous intrusions into carbonate rocks represent permissive conditions for copper skarn formation. Copper skarn mineral assemblages were observed in ores from the Gold Bottom-Copper Queen mine (D.P. Cox, unpub. data).
On the delineation of permissive tracts
The permissive tract is defined to be that part of the area that is underlain by Proterozoic and Paleozoic carbonate rocks, and that is near Mesozoic plutons. Small areas of basin fill more than 1 km in depth are excluded.
On the numerical estimates made
For the 90th, 50th, 10th, and 5th percentiles, the team estimated 0, 1, 1, and 2 or more copper skarn deposits consistent with the grade and tonnage model of Jones and Menzie (1986).
Jones, G.W., and Menzie, W.D., 1986, Grade-tonnage model of Cu skarns, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 86-89.