Bulletin-51—Geology of the Questa Molybdenum (Moly) Mine area, Taos County, New Mexico

By J. H. Schilling, 1956, 87 pp, 1 table, 29 figs., 5 plates, 1 index.

The Questa molybdenum mine (Moly mine), 6 mi east of Questa, in the Taos Range of the Sangre de Cristo Mountains, Taos County, NM, is unique in containing high-grade molybdenite ore in fissure veins, whereas in most other mines, production is from low-grade, disseminated deposits. The true nature of the veins was first recognized in 1916. In 1920 the Molybdenum Corporation of America acquired the property, and the mine has been in almost continuous production since then. Total production to Jan. 1, 1956, was 18,095,000 pounds of molybdenite. Mining is by overhand, horizontal slicing in still-supported stopes; concentration is by flotation.

The Taos Range is made up chiefly of Precambrian metamorphic rocks overlain by tertiary volcanic rocks. In the vicinity of the mine, Precambrian amphibolites and quartz-biotite schist of the amphibolite complex are overlain by Cabresto metaquartzite, and all these units are intruded by Precambrian granite. Late Paleozoic and Mesozoic sediments crop out along the eastern edge of the range and in small scattered areas elsewhere in the range. At the mine, conglomerates, arkosic graywackes, siltstones, and limestones overlie the Precambrian and are correlated tentatively with the Pennsylvanian-Permian Sangre de Cristo Formation. A thick sequence of Miocene volcanics overlies the older rocks. The lower series of this complex includes andesite and quartz latite flows, breccias, and tuffs; these in turn are overlain by an upper series of rhyolite flows, breccias, and tuffs. Numerous dikes, sills, and small plugs of rhyolite and andesite may be the intrusive equivalents of this volcanic complex. Late Tertiary soda granite stocks and dikes intrude the older rocks. Quaternary alluvial gravels cover the valley bottoms and terraces, and mud-flows form fans in red River Canyon at the mouths of some of the side gulches.

Folding is not an important structural feature of the Taos Range. Faults are common, the majority being divisible into three systems: (1) north-trending thrust faults along the east edge of the range, active during late Cretaceous and early Tertiary time, with thrusting toward the east; (2) north to northwest-trending high-angle frontal faults along the wet edge of the range and associated parallel faults throughout the range, active during late Tertiary and early Quaternary time; and (3) east to northeast-trending high-angle faults throughout the range, active during the Precambrian or Paleozoic, with later Tertiary and Quaternary movement along some faults. Fractures parallel the east to northeast-trending fault system. The Precambrian rocks show foliation, which most commonly strikes northeast and dips vertically.

Important local structures are superimposed on the regional structures of the Taos Range, forming the structural pattern in which the ore deposits occur. The mine is located in a downfaulted zone several miles wide, trending east, and extending across the range. In this zone, the Miocene volcanics and older rocks have been downfaulted in an irregular pattern. This downfaulting occurred during and shortly following the extrusion of the volcanics. Some areas within the downfaulted zone were brecciated intensely during the faulting.