New Mexico Mineral Symposium — Abstracts

The No Agua mining district in northern Taos County, New Mexico, is composed of several mines, which have been operated for perlite and scoria since the 1950s. The mining district is located within the Taos Plateau volcanic field (TPVF), which consists of mostly Tertiary volcanics 2-5 m.y. old. The most voluminous and widespread rock type of the TPVF is the 3.6-4.5 m.y.-old Servilleta basalt (Lipman and Mehnert, 1979).

In the fall of 1998, a field trip guided by Dr. Tony Benson of the University of New Mexico at Taos was conducted to look at various rocks and structures of the TPVF. One of the stops was at the open pit of the former United Perlite Corporation mine on Brushy Mountain, which is about 12.5 mi east-northeast of Tres Piedras and 3.5 mi west of the Rio Grande. Brushy Mountain, a local lava dome of silicic rhyolite containing phenocrysts of quartz and sodic sanidine, is overlain by lava flows of olivine-augite andesite and hornblende rhyodacite. The rhyolite has yielded a K—Ar age of 22.3 m.y. (Lipman and Mehnert, 1979).

During the visit, one of the authors, Jessie M. Kline, picked up some lithophysal rhyolite from a small prospect about 100 m east of the main pit. This rock contained unknown crystals of orange and black acicular minerals in the vesicles. Following the November 1999 New Mexico Mineral Symposium, both authors traveled to Brushy Mountain to investigate the minerals of this prospect. The black acicular mineral appeared, with some certainty, to be pseudobrookite, and the quartz (amethystine) was obvious, but identification of several other minerals was in question. Further study and microprobe analysis by Paul Hiava of Sandia National Laboratory (pers. comm. 1999) has recognized the following species:

• Pseudobrookite: Black, lustrous blades up to 2 mm, some found on amethystine quartz, and also as inclusions in the quartz. The crystals are associated with tridymite, spessartine, and aegerine.
• Aegerine: Slender, prismatic crystals with steep pyramidal terminations to 1.5 mm. These crystals are orange to burnt-umber color. Aegerine is also found as short (0.2 mm), brown prismatic crystals that are opaque with a dull luster. They are often found with monazite.
• Monazite: Monazite is found as dull-yellow, subhedral crystals up to 0.4 mm in length. They are uncommon but are found with the dull-brown aegerine crystals.
• Hematite: Small brown to black tabular crystals about 0.2 mm across are sparsely found. Associated with the short, brown aegerine and monazite crystals.
• Tridymite: Aggregates of bladed, twinned, bluish-gray, transparent crystals with a vitreous
• luster are commonly found in the lithophysae. Individual crystals are about 0.5 mm across.
• Quartz: Much of the quartz at Brushy Mountain is amethystine and gemmy with a vitreous luster. Pseudobrookite is often found on and as inclusions in the quartz.
• Spessartine: Spessartine crystals are uniformly small (0.5 mm), black trapezohedrons. They are found with pseudobrookite, aegerine, tridymite, and quartz (amethyst).
• Opal: Colorless, hyaline opal commonly coats the minerals at Brushy Mountain. It fluoresces green under short wave UV.

Brushy Mountain is the first documented location for pseudobrookite in Taos County and northern New Mexico. It also appears to be the first reported occurrence of aegerine and tridymite in Taos County. Finding pseudobrookite and other minerals at Brushy Mountain suggests that investigation of other rhyolites in the TPVF might result in additional discoveries.

References:

1. Lipman, P.W., and Mehnert, H. H., 1979, The Taos Plateau volcanic field, northern Rio Grande rift, New Mexico; in Riecker, R. E. (ed.), Rio Grande rift???tectonics and magmatism: American Geophysical Union, Washington, D.C., pp. 289-311.