Sandoval County, New Mexico

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Located in north-central New Mexico, Sandoval county has a varied, diverse geology and lots of minerals that go with it. Some mineral deposits were probably known and worked in more ancient times, modern mining became possible with the arrival of the Atchison Topeka and Santa Fe Railway in the 1870’s. Sandoval county had numerous mines that operated intermittently until about 1960. Currently there are about 190 active mines in Sandoval County.

Overview of Sandoval County Geologic Setting

If you’re traveling in New Mexico, you’ve been in Sandoval County if you headed up towards Farmington passing Santa Ana Pueblo, Zia Pueblo, Jemez Pueblo and Cuba. Bandelier National Monument, Jemez Springs, and the Valles caldera (like the Valle Grande where parts of Longmire were filmed) are all in Sandoval County. The south- and north-western parts of the state are largely desert, now part of the Navajo Nation’s reservation lands.

This diverse landscape reflects the geology of this area. If someone had tried to identify the most geologically diverse and complex part of New Mexico and turn it into a county, they might have come up with Sandoval County. Bounded on the east by the Rio Grande River (marking the center line of the Rio Grande rift zone), on the north by more rift-related mountains and moving into the Brazos uplift and Uncompaghre highlands, and on the west by the broad flat surfaces of the San Juan Basin, there’s a lot going on. Much of the county was shaped into the present-day topography by Rio Grande rift-related volcanism and faulting, accompanied by infilling of rift basins by sediment shed from the fault block mountains. Another major geological feature is the Jemez volcanic complex, similar to Yosemite or Long Valley in size and eruptive style and products.

This complex geology results in diverse mineralogy in parts of the county. Mining geologists will often designate a particular area as a mining district where a particular group of minerals can be found. The original mining was for economically valuable resources such as copper, gold and silver. Few active mines remain, but many of the areas have collectible minerals of little economic value.

Mining Districts and Minerals of Sandoval County

The mining districts and other areas where minerals of mining interest have been reported in Sandoval County include the following; this list is from the compilation Minerals of New Mexico (by Stuart A. Northrup, 3^rd edition revised by Florence A LaBruzza, UNM Press, 376pp). Please note that these minerals are not necessarily clusters of large, sparkly crystals; many are found as microscopic crystals in host rock.

Cochiti District
Argentite, arsenopyrite, augite, biotite, bromargyrite, carnotite, chalcedony, chalcopyrite, chlorite, covellite, cuprite, dickite, dufrenite, euclase, galena, gold, hematite, limonite, magnetite, malachite, oligoclase, opal, orthoclase, proustite/pyargyrite, pyrite, quartz, sanidine, siderite, sphalerite, stevensite, stibnite, titanite, tourmaline, zircon

Jemez Springs District

Albite, apatite, aragonite, azurite, barite, bytownite, calcite, chabacite, chalcocite, chevkinite, copper, covellite, cristobalite, gold, selenite, hematite, hypersthene, labradorite, limonite, malachite, muscovite, natrolite, oligoclase, pyrite, quartz, sanidine, serpentine, sulfur, tridymite

Jemez Sulfur District

Dufrenite, opal, sulfur

Nacimiento Mountains District

Antlerite, autunite, azurite, bornite, brochantite, cerussite, chalcocite, chrysocolla, copper, covellite, cuprite, selenite, hematite, jarosite, limonite, malachite, pyrite, quartz, silver, spangolite, sphalerite, vivianite

Placitas District

albite, anglesite, anthophyllite, autunite, azurite, barite, biotite, calcite, carbon, chalcoalumite, chalcopyrite, chlorargyrite, chlorite, chrysocolla, coffinite, enstatite, epidote, fluorite, galena, garnet, gold, selenite, hematite, ilmenite, kaolinite, limonite, magnetite, microcline, orthoclase, pyrite, quartz, schorl, sillimanite, silver, tenorite, uraninite, uranophane, zircon

Bandelier Tuff

Allanite, fayalite, magnetite

Cabezon Peak area

Antigorite, augite, calcite, diopside, dolomite, enstatite, labradorite, olivine, spinel

San Ysidro Area

Barite, calcite, celestite, gypsum

Nacimiento Mine, Sandoval County, New Mexico, 2016. Photo by Erin Delventhal

Sierra Nacimiento Petrified Wood

Most of Taos Rockers specimens from Sandoval County are from the Nacimiento Mountains district, known for copper mineralization. The Sierra Nacimiento are not particularly high. They resulted from uplift of much older rock strata during multiple phases of crustal upheaval affecting the ancestral Rocky Mountains (the most recent is termed the Laramide orogeny). The uplift exposed these older rock strata and the economically valuable copper, gold and silver deposits that drew the mining industry to the area.

The host rocks of the mineralized area consist of Triassic sedimentary rocks; they were being deposited when the dinosaurs first appeared. Vast layers of sand and clay were deposited in intermontane basins. Climate in that region had higher rainfall, so there were also areas of lush vegetation, including some now extinct trees, gingkos, pines, and cycads (not extinct, they resemble modern palm trees but do not flower).

When trees collapsed they were buried by the rapid sedimentation of the regional stream system. After burial, the soft, sandy sediments became cemented, turning into sandstones. The tree remains were replaced by minerals percolating through the sedimentary rocks.

In the case of Nacimiento petrified wood, the cycad material was replaced by copper minerals. Over time, the copper itself was altered by interacting with groundwater to form new copper minerals, especially chalcocite, malachite and azurite, giving the beautiful black, bright green and blue fossilized wood we find today.

Where did the copper come from? One of several working hypotheses (each supported by field studies) suggest that the copper began percolating through the Triassic age sandstones shortly after they were deposited. The copper came from copper-rich sediments that washed into the stream system from neighboring highlands.

Azurite Balls

Striking specimens of bright blue azurite balls in a matrix of pale sandstone also come from the Sierra Nacimiento. This is an example of the copper mineralization that occurred in an area that had no vegetation, so there is no petrified wood. Copper-rich fluids circulated through the sandstones, randomly precipitating azurite when the concentrations and conditions permitted. Sometimes the azurite forms a layer coating a surface; in other instances, the azurite crystal precipitation formed discrete spheres.