By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Otero County, in southern New Mexico along the Texas border, is relatively poor in mineral deposits, yet a few small mines in the Jarilla district produced a total of about 16,500 ounces of gold through 1959.
The Jarilla (Orogrande) district is in the Jarilla Mountains about 50 miles north-northeast of El Paso in the southwest corner of Otero County. The first prospecting was done in 1879, but little interest was generated until turquoise was discovered about 20 years later (Jones, 1904, p. 194). Gold and copper lodes were mined on a small scale, and a little gold was recovered from dry placer operations. The most active period was 1905-18; the district was dormant from 1948 through 1959.
The Jarilla Mountains are underlain by Carboniferous limestone intruded and domed by an irregular mass of fine-grained monzonite porphyry. Near the contact, the limestone is metamorphosed to a skarn of garnet, diopside, epidote, quartz, and tremolite. The ore deposits are in fracture zones and along bedding planes in the metamorphosed limestone.
Specularite and gold- and silver-bearing pyrite and chalcopyrite are the chief primary ore minerals; oxidized ore contains much limonite, malachite, and chrysocolla (Graton, in Lindgren and others, 1910, p. 185-186). The placer ground that has been worked is on the southeastern slope of the Jarilla Mountains. Most of the placer gold has been recovered with some form of dry washer.
Black sand constitutes approximately 1 percent of the gravel and is reported to run about $40 per ton in gold, which is equivalent to about 40 cents in gold per ton of gravel. The black sand also carries magnetite, ilmenite, hematite, and zircon (Wells and Wootton, 1940, p. 14).
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