Bulletin-13—Geology and Economic Features of the Pegmatites of Taos and Rio Arriba Counties,
New Mexico

By E. Just, 1937, 73 pp., 3 figs., 3 plates.

This bulletin consists of three parts; Part I: General description of the Picuris and Petaca areas, Part II: The Picuris area, and Part III: The Petaca area. In two areas in Taos and Rio Arriba Counties, Precambrian rocks have yielded commercial quantities of non-metallic minerals. The Picuris area, a few miles southwest of Taos on the east side of the Rio Grande, includes what has been variously called the Embudo, Rinconada, Picuris, or Copper Hill district. In this district the rare lithium minerals, lepidolite and spodumene, have been produced at the Harding mine. The Petaca area, several miles west of the Rio Grande, includes the Bromide-Hopewell, La Madera and Petaca districts. Scrap and plate mica have been mined for many years from a number of workings in the La Madera and Petaca districts.

Although both areas have been productive of metallic minerals, particularly the Bromide-Hopewell district, this report, except for furnishing some details of the regional geology, is concerned primarily with non-metallic minerals rather than with ore deposits. The areas and their geology are described, and some pertinent economic considerations are discussed, with a view to aiding operators already established as well as fostering new exploitation. Possibilities of new development that are considered include minerals as yet unexploited as well as minerals that have been mined.

Very little systematic study has been made of the Precambrian rocks of this part of NM, and practically no separation or naming of lithologic units has been attempted. In making a start in this direction and in presenting some details of Precambrian structure and geologic history, it is hoped that a systematic beginning has been made in unraveling the complicated Precambrian geology of this part of the state.

The Rio Grande in southern Taos County and eastern Rio Arriba County has a southerly course and has cut deeply into the broad, rather flat surface of a thick series of Quaternary basalt flows. The continuity of the basalt surface is broken only by the gorges of the Rio Grande and Taos Creek and a few minor groups of hills, including the Precambrian inlier known as Blue Hill. To the east of the basalt plain are the high, north-south ranges of the Sangre de Cristo Mountains. The Picuris area includes a conspicuous, triangular group of mountains extending westward from the Sangre de Cristo ranges to the Rio Grande.

Between this great basaltic mass and a dissected plateau that lies about 25 mi west of the Rio Grande, the hills and mountains of the Petaca area rise above the plain of basalt and Tertiary sediments which surrounds them and buries their edges. The rocks of the Petaca area, consisting of resistant Precambrian formations, are exposed from a point northwest of Hopewell southward to Ojo Caliente. Although this area includes mountains of magnitude similar to those of the Picuris area, they do not stand out so conspicuously when viewed from a distance, as they are more surrounded by rugged topography, particularly on the west, where the general plateau surface is as high as the general level of the Precambrian rocks. The Picuris area drains westward into the Rio Grande, and the Petaca area drains southward into the Rio Chama.

The region is sparsely inhabited but is accessible by good roads. It is traversed by the narrow-gauge line of the Denver & Rio Grande Western railroad, which extends from Santa Fe to Alamosa, CO, where it connects with a standard-gauge line of the same company. The climate is very agreeable in summer, and the winters are not severe enough to prevent mining operations.