Explained by Alan R. Wallace, Steve Ludington, and Virginia McLemore
On the choice of deposit models
Southwestern New Mexico contains several Tertiary porphyry copper prospects, as well as calc-alkaline plutons. In addition, some of these intrusive rocks have a halo of polymetallic veins that might reflect a concealed porphyry copper deposit associated with the intrusive bodies. Immediately east of the permissive area, porphyry copper-gold deposits are associated with a sequence of alkaline plutons that, although slightly older, are part of the same magmatic sequence as the rocks in southwestern New Mexico. Thus, assuming the two deposit types are related, southwestern New Mexico may contain undiscovered porphyry copper deposits related to calc-alkaline plutonism like those described by Cox (1986).
On the delineation of permissive tracts
The general rule for delineating permissive areas for porphyry copper deposits of middle Tertiary and younger age was to exclude all areas that show no evidence for plutonism of this age. In addition, areas of plutonism that are overwhelmingly gabbroic or granitic, or strongly alkaline, were excluded. Manifestation of plutonism may include exposed plutonic terranes, known polymetallic vein, skarn, and(or) replacement deposits, unexposed plutons, inferred to exist by geophysical or other means, and any other inferred magmatic trends.
Important examples of this type of deposit
No significant deposits are known in southwestern New Mexico, but at least 5 prospects—White Signal, Alum Mountain, Bitter Creek, Cooperas Creek, and Steins Creek—are known that are probably middle Tertiary in age.
On the numerical estimates made
Much of the permissive area for deposits of this age in southwestern New Mexico is not deeply eroded, and deposits that might be present could easily be covered by younger materials. The area concealed is at least half the total. The known prospects have not been explored very thoroughly, and the assessment team was hampered by a distinct lack of information. Nevertheless, exploration in the area for several decades has failed to discover a viable Tertiary deposit. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 1, and 2 or more deposits consistent with the worldwide porphyry copper grade and tonnage model (Singer and others, 1986).
Cox, D.P., 1986, Descriptive model of porphyry Cu, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 76.
Singer, D.A., Mosier, D.L., and Cox, D.P., 1986, Grade and tonnage model of porphyry Cu, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 77-81.