Explained by Gregory T. Spanski
On the delineation of permissive tracts
The Trans-Pecos volcanic province is a dominantly alkaline system becoming increasingly alkalic from west to east. Barker (1977) and Price and others (1987) divide the province along a north-northwest trending boundary into an alkalic eastern belt and a western alkali-calcic belt. Magmatic activity, lasting from about 48 Ma until 32 Ma, occurred under compressive tectonic conditions after the Laramide orogeny. The broad compositional range demonstrated in eruptive units from this period suggest that the magma chambers at depth had achieved a high degree of differentiation. Epithermal quartz veins are primarily spatially associated with late stage intrusive events that resulted in resurgent doming at many of the known volcanic centers in the western alkali-calcic belt. The permissive tract therefore includes all of the area in the Trans-Pecos volcanic province lying west of the alkalic/alkali-calcic boundary and extending to a depth of one kilometer.
Important examples of this type of deposit
No examples of significant size are known in the permissive tract. However, small, low-grade prospects and occurrences that may be epithermal in character are known to exist in association with late-stage intrusive phases in the Infiernito, Chinati Mountains, and Quitman Mountains calderas (Price and others, 1983).
Rationale for Numerical Estimates
The Trans-Pecos region of Texas has been prospected since the middle of the 19th century without the discovery of a major deposit. The known calderas within the permissive tract have been well exposed by erosion, lessening the probability for the existence of concealed late-stage intrusions. The apparent absence of often-associated placer gold deposits is not meaningful because the known prospects are very silver-rich. Finally, the volcanic rocks in the permissive tract are compositionally enriched in potassium, compared with the calc-alkaline rocks with which the deposit type is most commonly associated. However, because of the favorable shows in the Chinati and Infiernito calderas and the potential for discovery of an early volcanic center concealed beneath the volcanic cover produced by later events, we made a small estimate. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more districts consistent with the combined Comstock-Sado grade and tonnage model.
Barker, D.S., 1979, Magmatic Evolution in the Trans-Pecos Province, in Walton, A.W., and Henry, C.D., eds., Cenozoic geology of the Trans-Pecos Volcanic Field of Texas: Texas Bureau of Economic Geology Guidebook 19, p. 4-9.
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.
Price, J.G., Henry, C.D., Barker, D.S., and Parker, D.F., 1987, Alkalic Rocks of Contrasting Tectonic Settings, in Morris, E.M. and Pasteris, J.D., eds., Trans-Pecos Texas—Mantle metasomatism and alkaline magmatism: Geological Society of America Special Paper 215, p. 335-346.