Bulletin-42—Geology of Costilla and Latir Peak Quadrangles, Taos County, New Mexico

By P. F. McKinlay, 1956, 32 pp., 1 fig., 1 plate.

This report covers the geology and ore deposits of the northern half of the Taos Range of the Sangre de Cristo Mountains and the Costilla Plain to the west. The purpose of the survey was to delineate any areas of ore mineralization and to gather detailed information on the geology. The study was made by the NM Bureau of Mines and Mineral Resources in cooperation with the USGS. The area covered, Costilla and Latir Peak quadrangles, is in the northern part of Taos County, NM. Field work began in the summer of 1947 and continued during the summers through 1950.

Principal topographic features of the two quadrangles are the rugged Sangre de Cristo Mountains and the Costilla Plain, the latter including the Rio Grande gorge. Elevations range from about 7,500 ft on the Costilla Plain to over 12,700 ft in the mountains. The Costilla Plain and Sangre de Cristo Mountains have quite different geologic settings. In the Mountains, Precambrian rocks crop out over two-thirds of the area. Tertiary volcanics, intrusives, and interbedded-sediments crop out over much of the remaining area. Scattered outcrops of Permo-Carboniferous sediments also occur, and Quaternary deposits of stream gravel, fan gravels, glacial debris, etc., locally cover the older rocks.

The Precambrian rocks include amphibolites and hornblende schists, interlayered quartzites and quartz-mica schists, metagabbro, and massive quartzite intruded by granite. The granite is the predominant rock type. The Tertiary volcanics are andesite, latite, and rhyolite flows, tuffs, and breccias, with interbedded siltstones, sandstones, conglomerates, and basalt flows. The percentage of sediments increases northward, where the sequence is known as the Amalia Formation. The Tertiary volcanic activity was accompanied by the intrusion of latite sills and dikes, rhyolite plugs and dikes, monzonite porphyry dikes, and granite stocks. The exact age and correlation of most of the Tertiary rocks are questionable. The Costilla Plain, in contrast, is covered in a thick sequence of late Tertiary to Quaternary gravels and interbedded basalts.

The region has been subjected to at least six periods of deformation. Precambrian rocks were first tightly folded into northeast trending folds, then intruded by granite, and later folded along axes that trend from north to 20° west. Trust faults and extensive granulation occurred during the late phase of the Precambrian deformation. Uplift of the region in carboniferous-Permian time is indicated by the conglomeratic nature of the Carboniferous-Permian sediments and by the absence of recognizable Pennsylvanian marine rocks. During the Laramide revolution, the metamorphic rocks and Precambrian granite were thrust over the sedimentary rocks to the east. Late Tertiary normal faulting elevated areas of Precambrian rocks and lowered the areas that now are covered mainly by volcanic rocks. The Sangre de Cristo Mountains were raised over 7,000 ft and tilted to the east in the Pliocene and Pleistocene.

The Precambrian metamorphic rocks in general show low-grade metamorphism, although near Precambrian granite bodies, thermal recrystallization has produced coarse sillimanite and muscovite gneisses. Migmatite rocks are common in the amphibolite and hornblende schist near the Precambrian granite. Mylonite schists are locally abundant in the Precambrian rocks. The Tertiary intrusives had little metamorphic effect on the intruded rocks.

In the southern part of Latir Peak quadrangle, the Tertiary volcanic rocks were altered by hydrothermal solutions. Chlorite, pyrite, and quartz are the principal alteration minerals. A small amount of gold mineralization accompanied or followed the alteration solutions.

The survey indicates that the favorable areas for future economic mineral exploration are the alteration area south of Ortiz Peak, and the small pegmatites southeast of Costilla. Approximately 10,000 acre-ft per year of undeveloped ground water could be made available on the Costilla Plain by shallow 250-ft wells.