Circular 44—Geology and petrography of the Stendel perlite deposit, Socorro County, New Mexico

By R. H. Weber, 1957, 24 pp., 2 tables, 1 fig., 2 plates.

A local sequence of the late Tertiary-early Quaternary (?) volcanics in the western foothills of the Magdalena Mountains comprises several flow units of lithoidal rhyolite and rhyolite glass, with thin interbeds of tuff and breccia. Lithoidal rhyolite, at the base of the mapped section, is overlain by a tabular body of porphyritic perlite. The congenesis of the perlite and the rhyolite is demonstrated by their nearly identical chemical composition (water-free basis), phenocryst assemblage, and intimate to gradational contact relationships. The high viscosity of the perlite resulted in widespread brecciation of the mass during the flow stage.

The perlite was attacked by hydrothermal solutions, resulting in the development of zeolites (?), silica minerals, and montmorill-onite as the principal products of the selective alteration of the glassy matrix. The phenocrysts show very little attack. Chemical analyses of unaltered and altered perlite, converted to a water-free basis, show that the SiO₂ and Al₂O₃contents were essentially unchanged during zeolitic (?) alteration, whereas H₂O, CaO, and MgO were added, and Na₂O and K₂O were partially leached. FeO was oxidized largely to Fe₂O₃. 

Upward in the section two rhyolitic flow units with the vitrophyric basal phases, and a separate lens of vitrophyre, are interbedded with pumiceous lapilli tuffs and vitric breccias. The tuffs also are locally altered. Laboratory furnace tests of the perlite show that it will expand at moderate temperatures to a cellular product that may prove acceptable as a lightweight plaster aggregate. Utilization of the perlite will be impeded by the prevalence of zones of nonexpansible alteration minerals and spherulites, which eliminate 76% of the mapped segment of the deposit from consideration as a source of commercial-grade perlite.

Perlite, the volcanic glass that may be expanded by appropriate heat treatment to a cellular material especially valuable for lightweight plaster aggregate, has become the basis for an industry of significant economic importance in NM. The value of production in 1956 exceeded one million dollars. This report summarizes the preliminary results of one segment of a continuing field and laboratory study of perlite deposits of the state by the NM Bureau of Mines and Mineral Resources.

The Stendel deposit was being prospected in 1950 when the mapping was undertaken, but to date has not been mined. It was selected for detailed study in part because of the variety of contained impurities which would affect its commercial utilization, should this be attempted. Field relationships show the perlite to be a tabular layer or lens within a sequence of rhyolite flows and tuffs. Chemical analyses of the perlite and associated rhyolite, when converted to a water-free basis, are essentially the same, although the two rock types have a markedly different appearance in hand specimen.