Explained by Steve Ludington and Gregory T. Spanski
On the choice of deposit models
Polymetallic replacement deposits consist of hydrothermal, epigenetic, Ag, Pb, Zn, and Cu sulfide minerals in massive lenses, pipes, and veins in limestone, dolomite, or other reactive rocks near contacts with intrusions.
On the delineation of permissive tracts
These deposits form where intermediate to felsic intrusive rocks, which are the principal sources of the metals, are emplaced into or near carbonate rocks. Therefore, the permissive tract for polymetallic replacement deposits in Texas is that area where carbonate rocks are present at or within a kilometer of the surface in the area where middle Tertiary and younger igneous rocks are also found. Undiscovered districts postulated in this tract are only those associated with calc-alkaline intrusions. Areas characterized by alkaline rocks were assessed for alkaline Au-Te deposits, which may include some replacement ore.
Important examples of this type of deposit
The Shafter district was an important polymetallic replacement deposit in the past, and other areas (Altuda, Loma Plata) seem to conform to the descriptive model (Price and others, 1983). Very important polymetallic replacement deposits are found just to the west of trans-Pecos Texas in the Sierra Madre Oriental of Mexico, at Santa Eulalia (Megaw, 1990) and Naica (Querol and Vallejo, 1990).
On the numerical estimates made
The team judged that, even though the known occurrences are not impressive, the permissive area is large and the level of exploration probably has not been very intense. The Mexican deposits appear to be related to calc-alkaline intrusions, whereas trans-Pecos Texas east of this tract features primarily alkaline magmatic systems. For the 90th, 50th, and 10th percentiles, the team estimated 0, 1, and 2 or more deposits consistent with the grade and tonnage model for polymetallic replacement deposits of Mosier, Morris, and Singer (1986).
Megaw, P.K.M., 1990, Mineralization in the Santa Eulalia mining district, Chihuahua, Mexico, in Clark, K.F., ed., Mexican silver deposits: Society of Economic Geologists Guidebook Series, v. 6, p. 27-39.
Mosier, D.L., Morris, H.T., and Singer, D.A., 1986, Grade and tonnage model of polymetallic replacement deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 101-104.
Querol, F., and Vallejo, A., 1990, Geology and genesis of Naica ore deposits, Chihuahua, in Clark, K.F., ed., Mexican silver deposits: Society of Economic Geologists, Guidebook series, v. 6, p. 47-58.
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.