Bulletin 38—Geology and mineral resources of Dwyer quadrangle, Grant, Luna, and Sierra Counties, New Mexico

By W. E. Elston, 1957, 86 pp., 10 tables, 8 figs., 8 plates.

Dwyer quadrangle is a 15-min quadrangle located in southwestern NM, southeast of the town of Santa Rita.

The major part of this report deals with the Tertiary volcanic history of the quadrangle and the petrology of the volcanic rocks. Up to 8,000 ft of volcanic rocks lie on a basement of Precambrian crystalline rocks and about 2,500 ft of sedimentary rocks ranging in age from Cambrian to Upper Cretaceous. The older volcanic rocks change with time from andesine-pyroxene andesites, via andesine-amphibole latites, to quartz-sanidine-oligoclase-biotite rhyolites. The rhyolites include a large portion of tuffs and ignimbrites. The later volcanic rocks are separated from the earlier rocks by a prominent unconformity, showing a discontinuous succession from contaminated andesites to a rocks on the rhyolite-latite-trachyte boundary, to iddingsite basalts or andesites, followed by minor rhyolites. The earlier volcanic rocks appear to have developed by some process of fractional crystallization from a grandioritic magma chemically similar to that of numerous Laramide intrusives. The later volcanic rocks apparently have been derived from basaltic magma, in part by assimilation of silicic material.

Normal faulting during, and particularly after, deposition of the volcanic rocks divided the quadrangle into several northwestward-trending fault blocks of Basin and Range type, broken by northeastward-trending transverse faults. The upthrown blocks invariably dip away from the faults, and the prevailing dip throughout the quadrangle is to the southwest. Much of the region is covered by late Tertiary or early Pleistocene fanglomerates and conglomerates, Quaternary terrace gravels, pediment gravels, and alluvium, all derived from uplifted areas.

The White Eagle fluorite mine is the only mine now being worked in the quadrangle. Mineralization in the rich Santa Rita-Central mining area immediately to the northwest is confined to prevolcanic rocks. The problem of finding areas favorable for future mineral exploration is, therefore, one of structure and volcanic stratigraphy. The thickness of the volcanic rocks varies considerably, but almost everywhere it is too great to permit potentially profitable prospecting.