Geology and Mineral Deposits of the Cornudas Mountains, Otero County, New Mexico

Abstract

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 that intrude Permian and Cretaceous sedimentary rocks, and 4) volcanic breccia dikes. 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 lower 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, bastnasite, calcio-catapleiite, vitusite, roumaite, xenotime). 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 deg. C) than the phonolite sills (760-869 deg. 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.