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Genetic aspects of commercial perlite deposits in New Mexico


New Mexico Bureau of Geology and Mineral Resources, Bulletin 154, p.171-185, 1996

ABSTRACT- New Mexico supplies perlite from three commercial deposits; a world-class deposit at No Agua Peaks in Taos County, a smaller but high-yield deposit near Socorro and a moderate-yield deposit near Grants. We interpret commercial perlite deposits in New Mexico as nothing more than chemically weathered (hydrated), glass-mantled, high-silica rhyolite volcanoes of late Cenozoic age (3.3-7.8 Ma) that are accessible to open-pit mining. The extreme viscosity of high-silica rhyolite lava (75.0-77.5 wt.%SiO2) favors formation of very thick glassy zones across the chilled surfaces of the slowly extruded flows. Perlite is not a primary igneous rock; it is formed by the slow diffusion of meteoric water at low temperature into nearly anhydrous primary obsidian.

The maximum economic potential of a perlite deposit (tonnage of accessible glass) is largely predetermined by the eruptive history of its parent volcano. Glassy, subhorizontal tops of microvesicular (i.e. permeable), high-silica lava flows are the most favorable precursors of commercial perlite deposits. Large, steep-sided, high-silica lava domes are much more common than high silica flows, but the large domes yeild relatively little commercial perlite because their complex cooling histories form an intimately interleaved mixture of of glass and crystalline rhyolite. Numerous investigations demonstrate that high-silica rhyolite magma chambers contain strong, vertically oriented, chemical gradients prior to eruption. For example, rubidium is strongly concentrated at the top of the chamber, thus the initial eruption is highest in Rb, and the last magma to reach the surface is lowest in Rb.

Most of New Mexico's perlite production is from 3.9 Ma high-silica rhyolite lava flows at the No Agua Peaks volcanic center. The primary vent at No Agua is defined by a 270 m high Rb-rich composite lava dome, known as west peak, which contains minor perlite reserves. Slightly less Rb-rich and hotter lava then squeezed out from under the west flank of the dome to form relatively thin (75 m) but highly vesicular lobate flows that now consist of about 50% recoverable perlite mined by Dicaperl. Finally, the Rb-poor lavas were extruded from a large radial dike on the northeast flank of the dome. These thicker (12m) flow lobes (mined by Harborlite) contain about 20% recoverable perlite.

The relatively small Socorro deposit (mined by Dicaperl) consists of 95% microvesicular perlite and lacks the crystalline core common to most larger lava domes. The Socorro deposit is a zone of weathering (hydration) superimposed on a 7.85 Ma, glassy high-silica rhyolite lava dome at least 90 m thick and about 840 in diameter. Sedimentation completely buried the high-silica lava dome by about 4 Ma, it was then uplifted along a Quaternary fault zone and exhumed.

The Grants perlite deposit (mined by U.S. Gypsum) is a large high-silica rhyolite lava dome that consists of over 95% crystalline rhyolite. The glassy eastern toe of the dome yields moderate amounts of perlite. This 3.3 Ma lava dome, 210 m high and 2.4 km wide, was buried by ash and lava flows from the Mount Taylor stratovolcano and was later exhumed.