New Mexico Mineral Symposium — Abstracts

The Jarilla or Orogrande mining district is in the Jarilla Mountains in south-central New Mexico. The district is near the town of Orogrande, 46 mi northeast of El Paso, Texas and 35 mi south of Alamogordo, New Mexico on US-54. The district has been the site of modest metal production but has always held a lure for mineral collectors, exploration geologists, and promoters. The name Orogrande (Spanish for large gold) alludes to a tale about the origin of serious exploration in the district after a large nugget was found by placer mining. It is still not clear if the 6-7-oz nugget was actually found in place or if the place was salted with the nugget to stimulate mining activity in the district. The nugget is reported to still exist in a private collection.

The district is in a horst block in the center of the Tularosa Basin. The district consists of an elongate structural dome centered about a granite stock of Tertiary age. Flanking the dome are sedimentary rocks of Paleozoic age, mainly shales and carbonates. Igneous dikes are most common in the northern and southern parts of the range and intrude both the stock and sedimentary rocks. Contact metamorphism is common at the contacts of the granite stock and limestones. This metamor-phism hosts most mineralization thus far discovered in the district. The district has been the target for porphyry copper mineralization in the main intrusive mass. Basin and Range-type faulting has produced minor complications in the geologic structure of the district and exposed the horst block that forms the Jarilla Mountains. The mountains were later dissected by streams that produced a number of arroyos and small canyons and led to the formation of the placer deposits.

More than 41 minerals have been identified in the district. Each is associated with a particular episode of ore mineralization. In addition to the primary minerals commonly found in limestones, shales, and granites, many contact metamorphic, skarn, and ore sulfide minerals are found. Weather¬ing of the ore deposits produced secondary minerals, mainly copper types that display striking color contrasts with their matrix material.

Primary minerals, formed directly from crystallization of igneous rocks, include fine orthoclase crystals. The crystals can be found as single crystals or as complex twins from the unit mapped as orthoclase adamellite by Schmidt and Craddock (1964). These crystals can be found in pod-shaped bodies near the Little Joe workings. In addition, a large dike of the granite porphyry is found through the center of the range in a north-south orientation. The best collecting is along this dike, which displays a distinct chill zone on the margin and large megacrysts of orthoclase in the center.

Interactions of magmatic fluids with carbonate host rocks produced large-scale metamorphism of the limestones. Minerals from the contact metamorphic suites include euhedral specimens of both garnet and magnetite that can be found mainly in the southern part of the district. Euhedral magnetite crystals (2.5 cm on edge) replaced by hematite (as "martite" pseudomorphs) can be collected on the hill above the Lucky mine workings. Euhedral, but discolored garnets with numerous inclusions of calcite and pyroxene can also be found at this location. South of the Lucky mine, immediately across the road at the Lincoln workings, are prospects where small hematite roses (0.3 cm diameter) can be found on the dumps or underground. Reddish-brown grossular to andradite garnets can be separated from calcite at the Cinco de Mayo, Iron Duke, Iron Queen, and Iron King mines. The garnets occur as individual grains or as complex intergrowths.

Secondary minerals, formed by the weathering of earlier-formed species, are common in the district. The district is renowned for turquoise, which is still produced from the "turquoise workings," part of the old Iron Mask workings. Chrysocolla and malachite form lovely specimens on black manganese oxides and tenorite from the large stope in the Lucky mine. Often, clear scalenohedrons of calcite formed over the chrysocolla or malachite.

The easy access to the mines makes the Jarilla district a favorite of weekend collectors and geology students. Many collectors visit the area annually and still manage to find good material. Recently reported finds of quartz crystals with chlorite inclusions continue to add lure to the district. Some workings in the district are rarely visited by the "weekenders" and represent a good prospecting target for the more adventurous. Prospectors should have in hand the publication of Schmidt and Craddock (1964), the NMBMMR Open-file Report 370 by Robert North, USGS Orogrande North 71/2-min quadrangle map, and plenty of water.
 

References:

1. North, R. M., 1982, Geology and ore deposits of the Orogrande mining district, Otero County, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Open-file Report 370, 18 pp.
2. Schmidt, P. G., and Craddock, C., 1964, The geology of the Jarilla Mountains, Otero County, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Bulletin 82, 55 pp.