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Bulletin 82—The geology of Jarilla Mountains, Otero County, New Mexico

By P. G. Schmidt and C. Craddock, 1964, 55 pp, 13 figs., 2 plates.

The Jarilla Mountains are a small range of hills in southern NM. The oldest exposed rocks are of Desmoinesian age; they consist of cherty gray limestone with minor sandstone and shale and are correlated with the Bug Scuffle member of the Gobbler Formation. This unit is incompletely exposed but is 822 ft thick. Rocks of early to middle Wolfcampian and probably latest Pennsylvanian age crop out in the northern and eastern parts of the range. This section is also incompletely exposed but consists of 1,200 ft of interbedded thin limestone, siltstone, shale, and conglomerate correlated with the Hueco Formation. The Laborcita Formation and possibly the Bug Scuffle Member and the Hueco Formation were deposited in the Orogrande Basin, a local subsiding element on the marine shelf of late Pennsylvanian and early Permian time.

Igneous stocks, dikes, and sills of probable Tertiary age were intruded into the Paleozoic rocks. Three main stages of injection are recognized: a massive syenodiorite, followed by monzonite-adamellite of somewhat variable composition, then by adamellite with large orthoclase phenocrysts. Themonzonite-adamellite has in places undergone severe hydrothermal alteration. Metasomatism and thermal metamorphism, locally intense, accompanied the intrusions. Beds of shaly siltstone were recrystallized to a scapolite-diopside-garnet rock; the limestones were mainly changed to marble. Adjacent to igneous contacts, however, the limestones were locally converted to andradite-speclarite skarns. Gold, silver, copper, lead, and iron mineralization was spottily developed in some of the skarns.

The Jarilla Mountains consist structurally of a dome formed by the forceful injection of the magmas. The sedimentary rocks dip uniformly away from the igneous core and are cut by many small radial faults. Numerous xenoliths, sills, and dikes occur in the southern part of the range, which is interpreted as a roof area of the intrusives. Mining of the copper, gold, lead, and silver lode ores and placer gold was begun about 1900, and iron mining activity about 1913. Except for brief periods of placer operations, mining activity ceased about 1930. There are no known ground-water reservoirs of potable water near the Jarilla Mountains. Water is piped 36 mi from the Sacramento Mountains.

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