Pegmatite Resources in The Southwest

Abstract

Although quantitative appraisal of pegmatite resources is at present precluded by numerous and serious gaps in basic data, it is possible to trace the occurrence of pegmatite deposits in the Southwest, to classify them geologically, and to indicate areas and types of deposits that offer maximum or minimum economic promise. The pegmatites of the Southwestern States occur chiefly in (1) the Rocky Mountain belt of northern New Mexico; (2) the Basin and Range belt of southeastern California, southern Nevada, west-central and southern Arizona, southwestern New Mexico, western Texas, and adjacent parts of Mexico; (3) the Peninsular Range belt of southern California and Baja California; and (4) the Sierra Nevada belt of south-central California. The pegmatites in the two first-named belts are pre-Cambrian, and occur mainly in schists and gneisses, whereas those in the California belts are late Mesozoic in age and occur mainly in igneous rocks. Nearly all forms of pegmatite bodies are represented, and those bodies of commercial promise show well defined internal zoning. Many also contain fracture fillings and replacement bodies of one or more minerals. Nearly all the pegmatites are truly granitic in composition, with abundant potash feldspar. Trends in distribution of commercially desirable minerals can be summarized with respect to beryl, feldspar, gem minerals, lithium minerals, mica, and tantalum-columbium minerals. These trends are only partly reflected by commercial exploitation to date, as many of the deposits are undeveloped. Most Southwestern pegmatite deposits face the handicap of relatively high transportation costs to centers of demand, but there are fair possibilities for future development. These probably could be improved by increased use of geologic data in exploration, development, and mining, and especially by improvements in mining and milling techniques. It seems likely that numerous exposed pegmatite deposits remain to be discovered in the Southwestern region.