Open-file Report - 619
Geology and Mineral Deposits of the Cornudas Mountains, Otero County, New Mexico

Re-examination of the rare earth elements (REE) deposits in the Cornudas Mountains is warranted in light of today’s economic importance of critical minerals, including REE that are essential in most of our electronic devices. New mapping, petrography, ⁴⁰Ar/³⁹Ar geochronology, and geochemical analyses have provided a better understanding of the emplacement of these intrusions and associated mineral deposits. The Cornudas Mountains form the northern Trans–Pecos alkaline magmatic province in the southern part of the North American Cordilleran alkaline-igneous belt. The igneous rocks in the Cornudas Mountains were emplaced in two pulses at 37.14-34.5 and 32.48-26.95 Ma, just prior to or during the early phases of Rio Grande rift extension, and consist of 1) larger nepheline syenite–syenite laccoliths and plugs, 2) phonolite plugs, sills, and dikes, 3) smaller syenite plugs and dikes, and 4) volcanic breccia dikes that intrude Permian and Cretaceous sedimentary rocks. New USGS geophysical data indicate that some of these intrusions extend deep into the subsurface, with additional buried intrusions potentially at depth. The focus of REE exploration is along the basal unit (PEnsp2) of the Wind Mountain nepheline syenite laccolith, as well as within syenite–phonolite and volcanic breccia dikes, plugs and skarns and carbonate-replacement deposits in Chess Draw. Some samples contain as much as 3110 ppm total REE. REE could be leached from a mineral concentrate of the REE-bearing minerals (eudialyte, zircon, monazite, bastnäsite, calcio-catapleiite, vitusite, roumaite, xenotime). Niobium and zirconium are also found with the REE and have potential for economic recovery. Another potential mineral resource is nepheline syenite for use as ceramics, glass or other industrial use. We incorporate whole rock and clinopyroxene chemistry of each intrusion into the clinopyroxene-liquid geothermobarometer (Masotta, 2013) to determine the temperatures and pressures of emplacement. This thermometer provides higher crystallization temperature estimates for the syenite intrusions (857-1027°C) than the phonolite sills (760-869°C). We then use the barometric estimates (0.3-3.3 kbar) to calculate emplacement depths (1.2-12.3- km). Pairing these depths with the new geochronology, minimum exhumation rates for intrusions in the Cornudas Mountains are estimated that range from 0.04-0.34 mm/yr crystallization temperatures and exhumation rates provide additional information to aid in developing a model for the formation of REE deposits.

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