Explained by Steve Ludington
On the choice of deposit models
The petrochemical characteristics of the Trans-Pecos Texas volcanic field are similar to those of the contiguous belt of alkaline intrusions in New Mexico that contain porphyry copper-gold deposits and prospects.
On the delineation of permissive tracts
The permissive tract is delineated on the basis of the extent of the alkaline Trans-Pecos Texas volcanic field, using the criteria used for porphyry copper-gold deposits in New Mexico.
Important examples of this type of deposit
Two prospects that resemble porphyry copper deposits are associated with the Oligocene West Chinati caldera (Price and others, 1983), and the group assigned them to the porphyry copper-gold model. Gold assays from the prospects are not available, but the setting is very similar to the SOL porphyry prospect in New Mexico which is associated with the Organ caldera (Seager, 1981), where the alkaline volcanic and intrusive rocks are also of Oligocene age.
On the numerical estimates made
The numerical estimate is based upon the presence of alkaline igneous rocks and two known prospects. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 1, and 2 or more deposits consistent with the porphyry copper-gold grade and tonnage model (Singer and Cox, 1986).
Seager, W.R., 1981, Geology of Organ Mountains and southern San Andres Mountains, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Memoir 36, 97 p.
Singer, D.A., and Cox, D.P., 1986, Grade and tonnage model of porphyry Cu-Au, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 110-114.
Price, J.G., Henry, C.D., and Standen, A.R., 1983, Annotated bibiliography of mineral deposits in Trans-Pecos Texas: Bureau of Economic Geology, Mineral Resource Circular, no. 73, 108 p.