Doña Ana County, New Mexico
If you’ve visited Las Cruces, New Mexico, you’ve been in Doña Ana County. Las Cruces is the second largest city in New Mexico, home to New Mexico State University, the gateway to White Sands, historic Mesilla, and a major agricultural center. These days it’s a quick drive along one of the interstate highways to get there.
Now try imagining - the year is 1843, and you have made the long and perilous journey by horse or mule from northern Mexico to the new land grant, El Ancon del Doña Ana. You traveled with a group of settlers because the only safety from Apache and Comanche raids was in numbers. The nearby Rio Grande had flooded the area in 1829 destroying the fledgling communities along its banks. All around you is a sea of sand with the sharp, barren peaks of the Organ Mountains (see photo above) towards the east. It’s all about the water, so your group carefully sites the village on a plateau high above the floodplain, and after digging a diversion dam and a system of acequias (irrigation ditches), construct the community according to a traditional Spanish-Mexican village plan.
According to the National Park Service, this was the beginning of Doña Ana, now the oldest permanent Hispano settlement in southern New Mexico. The village eventually became a major stop on El Camino Real. Overcrowding and other factors led to the settlement of Las Cruces.
Mining began in the Organ Mountains in the 1840s, and took off in the 1880s when the railroad reached Doña Ana County. Most mining had shut down by the 1930’s. Today there are between 43 – 100 active mining claims in the county, all in the Rincon district. Read more about mining in Doña Ana County HERE.
Overview of Doña Ana County Geological Setting
The geography and geology of Doña Ana County are similar to the other counties of this part of New Mexico. Vast expanses of sandy terrain cover most of the area, occasionally broken by up-thrusting mountain ranges. The terrain is cut by two other features – the broad flood plain of the Rio Grande, and the basalt flows of the Potrillo volcanic field.
Geologically, the old, stable continental crust of this region had been blanketed by sediments for hundreds of millions of years resulting in layers of sandstones, limestones, and shales. Tectonic activity in the late Mesozoic and early Cenozoic (the so-called Laramide orogeny) upset the flat, pancake stratigraphy with compressive, thrust-like faulting, folding, and igneous activity. Zones of crustal weakness became the foci for a series of huge magma chambers eventually erupting to form Yosemite-style volcanic complexes of ash-flow tuffs surrounding calderas. Even as the volcanic activity built-up the landscape, continuous erosion resulted in sediments blanketing the area. The tectonic setting changed about 36 million years ago as the Rio Grande rift began to take shape, producing fault-controlled basins and mountain ranges. Rhyolite volcanism continued into the Cenozoic; the Organ Mountains are part of a caldera-type volcanic complex that formed about 36 million years ago. Basalt volcanism occurred in the county about 70,000 years ago, producing the Potrillo and nearby basalt flows.
Kilbourne Hole Peridotites
Basaltic volcanism related to the extension of the Rio Grande Rift occurred in Doña Ana County about 80,000 to 70,000 years ago. The eruptions from underlying basaltic magma chambers cut through 1500 – 2000 feet of Santa Fe Group alluvial sediments. In some areas, the magma interacted with groundwater, resulting in explosive volcanic events that produced the so-called “maars” - craters – surrounded by cross-bedded layers of volcanic ash characteristic of high energy, high velocity ash flows.
The Kilbourne Hole maar and surrounding ash flow deposits contain numerous chunks of rocks presumed to have been transported intact from the upper mantle / lower crust. Called xenoliths (“foreign rocks”) they occur as nodules in the surrounding pyroclastic materials. Compositionally most consist of the minerals olivine (green) and pyroxene (black). Sometimes this rock type is called a peridotite, due to the high percentage of “peridot” (olivine).
Organ (Mountains) District
The Organ Mountains consist of granitic igneous rocks that are a batholith - the solidified remains of a large magma chamber that was underneath a large volcanic caldera complex. Batholiths are characterized by lots of quartz and sodium and/or potassium feldspars with lesser amounts of darker minerals. Over millions of years, the heat from the cooling magma body helped to drive circulation of groundwater at the edges of the intrusion and through fractures within the large structure. Late stage igneous activity in the batholith also resulted in the formation of pegmatites – veins containing large crystals of quartz, feldspar and sometimes other minerals. Economic mineral deposits of various metals such as lead, tungsten, copper, tellurium and others were formed as the circulating fluids concentrated these constituents of the magma.
All areas of the Organ District are now closed to collecting.
Feldspars - Collectible orthoclase (K(AlSi3O8)) and albite (Na(AlSi3O8)) occur in pegmatite veins within the host granitic batholith.
Wulfenite (below) - occurs in the Stephenson-Bennett lead-silver mine. The mine was first opened in 1849 then re-opened in 1887. In addition to lead and silver, minerals containing copper, tungsten, arsenic, molybdenum, vanadium and zinc have been found. The ore bodies follow a fault zone, where mineralized solutions flowed and replaced host rocks.
Altaite - occurs in the Hilltop Mine, an area discovered in the 1890s. Late stage fluids related to the last phase of igneous activity of the Organ batholith (Sugarloaf Peak Quartz Monzonite Porphyry) led to deposition of various copper-, lead- and zinc- sulfides and tellurides. The ore mineralization occurs along small folds beneath a layer of impervious shale.
Relevant Mining Districts and Minerals for Doña Ana County
Organ district: Acanthite, actinolite, adamite, albite, altaite, amethyst, anatase, andradite, anglesite, anorthite, anorthoclase, antigorite, aragonite, argentite, argentojarosite, augite, aurichalcite, azurite, barite, beudantite, biotite, bismuth, mismuthinite, mismutite, bornite, brochantite, bromargyrite, brookite, calcite, caledonite, cerussite, chalcocite, chalcopyrite, chamosite, chert, chlorargyrite, chlorite, chrysocolla, clinochlore, clinozoisite, conichalcite, copper, cordierite, cornwallite, cosalite, covellite, cuprite, descloizite, diopside, dolomite, dyscrasite, enargite, epidote, euxenite, fluorapatite, fluorite, galena, glauconite, goethite, gold, grossular, gypsum, hedenbergite, hematite, hemimorphite, hessite, hydronium-jarosite, hemimorphite, hessite, hydrozincite, ilmenite, jarosite, kaolinite, labradorite, limonite, linarite, magnetite, malachite, marcasite, microcline, mimetite, molybdenite, monazite, montanite, mottramite, muscovite, nitratine, oligoclase, orthoclase, phlogopite, phosgenite, plumbojarosite, psilomelane, pyrite, pyromorphite, pyrophyllite, pyroxene, pyrrhotite, quartz, rhodochrosite, rickardite, rosasite, rutile, scapolite, scheelite, serpentine, siderite, silver, smithsonite, sphalerite, spinel, stannite, stubelite, sylvanite, tellurium, tennantite, tetradymite, tetrahedrite, titanite, topaz, tourmaline, tremolite, turquoise, vanadinite, vesuvianite, weissite, willemite, wollastonite, wulfenite, zircon, zoisite
Kilbourne Hole - Augite, biotite, diopside, enstatite, forsterite, hercynite, olivine, pyroxene, sillimanite, spinel