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Bulletin 47—Kyanite occurences in the Petaca district, Rio Arriba County, New Mexico

By A. F. Corey, 1960, 70 pp., 9 tables, 11 figs, 9 plates, 1 index.

The kyanite deposits of the Petaca district, in north-central NM, are approximately 55 airline mi north of Santa Fe. They occur on and near La Jarita Mesa, a part of the long, southeast-trending prong of the San Juan Mountains. The mesa surface, a dissected remnant of the Pliocene Santa Fe peneplain, ranges in altitude from 6,500 to 9,500 ft and is bordered on the east and west by the canyons of the Rio Tusas and the Rio Vallecitos, respectively.

The Proterozoic Petaca schist series, chiefly quartzites and quartz-mica schists with intercalated metavolcanic rocks, underlies the mesa and is flanked by sedimentary and volcanic rocks of Tertiary age. Numerous pegmatite bodies and quartz veins in the metamorphic terrane are related genetically to a nearby late Precambrian granite batholith. The Petaca schist series and associated younger Precambrian rocks include 10 lithologic types: quartzite, quartz schist, sericite schist, kyanite schist, metaconglomerate, garnet schist, biotite schist, amphibolite, pegmatite, and quartz.

The dominant structural feature in the district is a succession of large isoclinal anticlines and synclines with axes that strike and plunge northwestward. These folds are overturned to the northeast. Superimposed on their flanks are numerous smaller folds of varying sizes but similar attitude. Foliation, well developed in most of the metamorphic rocks, is essentially parallel to original bedding; on the limbs of folds it strikes north-northwest and dips south-southwest. Conspicuous minor crenellations in the micaceous rocks attest to reapplication of stress along the original directions related to folding. Joint sets in the competent rocks controlled the emplacement of quartz veins, and many pegmatite bodies also cut across the foliation of the host rocks.

Kyanite occurs in a major group of deposits with commercial potentialities lying within an area of 3 mi2 in the north-central portion of la Jarita Mesa, and in a minor group of deposits containing very low-grade ore. The six deposits of the major group comprise 25 lenses of kyanite schist that are conformable with the country rock. Each lens of kyanite-bearing rock is enclosed in a shell of sericite schist that in turn grades outward into quartz schist and thence into quartzite.

The masses of kyanite schist, containing essential quartz, kyanite, and mica, were produced mainly by the progressive metamorphism of pelitic silt lenses. Lenticular or irregular masses of coarse-grained quartz-interlacing kyanite rocks and quartz-rosette kyanite rocks within the kyanite schist were formed in the absence of stress toward the end of the metamorphic period. Either metamorphic differentiation or deposition from hydrothermal solutions contaminated by dissolved kyanitic material is believed to have been responsible for the development of these bodies. Still later in Precambrian time, hydrothermal solutions with assimilated kyanitic material formed quartz-kyanite veins along joints in the lenses of kyanite schist.

The most favorable economic features of the major kyanite deposits are: (1) the presence of kyanite schist of apparent commercial grade, (2) a moderate tonnage of exposed ore and potentially large ore reserves, and (3) the occurrence of scrap mica as a possible byproduct from the sericite schist. The most unfavorable factor is the isolated location of the deposits with respect to railroads and markets.

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