Explained by Alan R. Wallace and Virginia McLemore
On the choice of deposit models
The characteristics of some known prospects meet many of the published criteria for porphyry copper deposits. Deposits often related to porphyry copper deposits, such as polymetallic replacement and vein deposits, are quite common in New Mexico. Middle Tertiary and younger deposits are assessed because Laramide intrusions, which generated many porphyry copper deposits in nearby Arizona and southwesternmost New Mexico, are not present in this area.
On the delineation of permissive tracts
The general rule for delineating permissive areas for porphyry copper deposits is to exclude all areas that show no evidence for intrusive activity during middle Tertiary time. In addition, areas where intrusions are overwhelmingly gabbroic or granitic, or strongly alkaline, are excluded. Manifestation of intrusive activity may include exposed intrusive terranes, known polymetallic vein, skarn, and(or) replacement deposits, unexposed intrusions, inferred to exist by geophysical or other means, and any other inferred magmatic trends. The permissive tract for deposits of this age is continuous from northernmost Colorado to the border with Mexico (exclusive of southwestern New Mexico), but the probability of occurrence is much lower throughout much of northern New Mexico.
Important examples of this type of deposit
No deposits are known in the permissive tract, and no specific prospects were identified, other than those classified as porphyry copper-gold, which were assessed separately.
On the numerical estimates made
Much of the permissive area for deposits of this age in New Mexico has not been deeply eroded since the Miocene, and many deposits that might be present could be covered by surficial deposits. The area concealed by younger material is somewhat less than half the total. The known prospects have not been thoroughly explored, and the assessment team was hampered by a lack of reliable information. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more deposits consistent with the worldwide porphyry copper grade and tonnage model (Singer and others, 1986).
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.