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.
The geology of Dwyer quadrangle, previously undescribed, is the subject of this report. In the handling of material, special emphasis has been given to the Tertiary volcanic history of the quadrangle and the petrology of its volcanic rocks. No topographic map of Dwyer quadrangle was available when the geologic map was made. All geologic features were marked on aerial photographs taken in 1937 by the Soil Conservation Service, and were later projected on a planimetric sheet. A scale of 1:31,680 was used throughout.
Altitudes of points on the structure profile were determined with two aneroid barometers, one of them stationary. Shorter traverses were measured by tape and Brunton compass. As rock exposures are excellent throughout the quadrangle, it was possible to trace almost all contacts. The summers of 19501952 were spent in the field. Considerable time during 1952 and early 1953 was devoted to office and laboratory studies at Columbia University.
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