New Mexico Mineral Symposium — Abstracts

Tourmaline is a distinctive mineral of the metamorphic rocks exposed in the rugged Pilar cliffs at the west edge of the Picuris Range, southwest Taos County. The Cliffs, along the east bank of the Rio Grande south of the town of Pilar, extend from approximately 6,000 ft elevation at river level to a plateau at approximately 7,500 ft. From north to south, these Precambrian metamorphic rocks include quartz-muscovite-feldspar "quartz-eye" schist of the Glenwoody Formation that is structurally overlain by massive quartzite of the Ortega Formation (Hondo Group) and staurolite-, garnet-, and biotite-bearing schist and quartzite of the Rinconada Formation (Hondo Group). The Glenwoody and Ortega Formations are separated by the south-dipping Pilar shear zone, and it has been debated which is actually the younger sequence of rocks. The Glenwoody Formation, believed to be metamorphosed rhyolite, perhaps an ash-flow tuff, has a reported age of about 1,700 Ma (Early Proterozoic) and has been tentatively correlated with the Vadito Group exposed farther southeast in the Picuris Range (Bauer and Helper, 1994; see also Bauer, 1984, 1987, 1993; Gresens and Stensrud, 1974; Manley, 1984; Montgomery, 1953). Manganese-rich layers near the top of the Glenwoody Formation are distinctive in containing the red manganese-bearing epidote mineral, piemontite, and Grambling (1984) has described the bright green, ferric iron and manganese-bearing andalusite (viridine) found at the Glenwoody—Ortega contact.

Tourmaline occurs throughout the Glenwoody Formation and in about the lowest 100 m of the Ortega Formation. The tourmaline ranges from brown to reddish brown to black, and it varies from fine needles to lustrous prisms up to several centimeters in length and 5 mm or more in diameter. The tourmaline ranges from dravite (magnesium-rich) to schorl (iron-rich). Rocks hosting tourmaline include mica schist, fine-grained quartz-feldspar¬mica schist, quartz pods and augen, and simple pegmatites. One —50-m-thick dark layer in the Ortega is rich in tourmaline and almandine garnet largely pseudomorphed to the hydrous iron silicate mineral, hisingerite. Other minerals associated with the tourmaline include quartz; white, pinkish-purple to coppery-red, and green (fuchsite) varieties of muscovite; piemontite, epidote, clinozoisite, zoisite, and pink zoisite (thulite); tremolite; green andalusite (viridine), sillimanite, and kyanite; and rare accessory minerals including allanite and stibiotantalite (several of these mineral identifications have been made by Paul F. Hlava, Sandia National Laboratories, and Virgil E. Lueth, New Mexico Bureau of Mines and Mineral Resources). The red micas are not known to be lithium-bearing and are colored mainly by ferric iron, rahter than manganese. The piemontite, purple muscovite and fuchsite, thulite, plus blue idocrase (cyprine) and gahnite from the Pilar area have been described by DeMark and Hlava (1987) and DeMark and Kline (1997).

References:

1. Bauer, P. W., 1984, Stratigraphic summary and structural problems of Precamrian rocks, Picuris Range, New Mexico; in Baldridge, W. S., Dickerson, P. W., Riecker, R. E., and Zidek, J. (eds.), Rio Grande rift: northern New Mexico: New Mexico Geological Society, Guidebook 35, pp. 199-204.
2. Bauer, P. W., 1987, Precambrian geology of the Picuris Range, north-central New Mexico: Unpublished PhD dissertation, New Mexico Institute of Mining and Technology, Socorro, New Mexico; New Mexico Bureau of Mines and Mineral Resources, Open-file Rept. 325, 280 pp.
3. Bauer, P. W., 1993, Proterozoic tectonic evolution of the Picuris Mountains, northern New Mexico: Journal of Geology, v. 101, pp. 483-500.
4. Bauer, P. W., and Helper, M. A., 1994, Geology of Trampas quadrangle, Picuris Mountains, Taos and Rio Arriba Counties, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Geologic Map 71, scale: 1:24,000.
5. DeMark, R. S., and Hlava, P. F., 1987, A new occurrence of cyprine (blue idocrase) in New Mexico: New Mexico Geology, v. 9, pp. 21-22.
6. DeMark, R. S., and Kline, J. M., 1997, Gahnite, margarite, and other new mineral occurrences from Taos County, New Mexico: New Mexico Geology, v. 19, pp. 28.
7. Grambling, J. A., 1984, The Precambrian rocks near Pilar; in Baldridge, W. S., Dickerson, P. W., Riecker, R. E., and Zidek, J. (eds.), Rio Grande rift: northern New Mexico: New Mexico Geological Society, Guidebook 35, pp. 305-306.
8. Gresens, R. L., and Stensrud, H. L., 1974, Recognition of more metarhyolite occurrences in northern New Mexico and a possible Precambrian stratigraphy: The Mountain Geologist, v. 11, pp. 109-124.
9. Manley, Kim, 1984, First-day road log, from Taos to Embudo, Dixon, Penasco, Vadito, Tres Ritos, Holman Hill, and back to Taos via U.S. Hill and Talpa; in Baldridge, W. S., Dickerson, P. W., Riecker, R. E., and Zidek, J. (eds.), Rio Grande rift: northern New Mexico: New Mexico Geological Society, Guidebook 35, pp. 303-319.
10. Montgomery, Arthur, 1953, Pre-Cambrian geology of the Picuris Range, north-central New Mexico: New Mexico Bureau of Mines and Mineral Resources, Bulletin 30, 89 pp.