GEOLOGY OF THE RARE EARTH ELEMENTS DEPOSITS IN THE CORNUDAS MOUNTAINS, OTERO COUNTY, NEW MEXICO

Re-examination of the geology in the Cornudas Mountains as part of the USGS Earth Mapping Resources Initiative (Earth MRI) program is warranted in light of today’s increased demand for rare earth elements (REE), niobium (Nb), and zirconium (Zr) that are essential in most of our electronic devices and other uses. New detailed mapping, petrography, ⁴⁰Ar/³⁹Ar geochronology, and geochemical analyses have provided a better understanding of the emplacement of these intrusions and associated REE deposits. The Cornudas Mountains form the northern Trans–Pecos alkaline magmatic province in the southern part of the North American Cordilleran alkaline-igneous belt.⁴⁰Ar/³⁹Ar geochronology results indicate the intrusions were emplaced in two pulses: 37.14-34.5 and 32.48-26.95 Ma. REE, Zr, and Nb have the highest potential in the Wind Mountain and Chess Draw areas, specifically the lower unit of the Wind Mountain nepheline syenite laccolith, phonolite and breccia dikes, and hydrothermal skarns. The spatial distribution of REE, Zr, and Nb is discontinuous and localized. Downhole profiles of chemical analyses of drill core suggest certain zones are more enriched in these elements than others. Although the REE concentrations of the Wind Mountain and Chess Draw intrusions and altered areas are below normal economic grades (i.e. <5% total REE), some of these minerals could be separated magnetically, to produce a potential REE concentrate for leaching. REE are found in eudialyte, zircon, monazite, bastnäsite, calcio-catapleiite, vitusite, roumaite and xenotime. Fluorine is not a major constituent in these mineral deposits, unlike REE deposits at Round Top (Texas), Gallinas or Capitan Mountains (New Mexico). Pyrochlore is the predominant Nb mineral, and zircon and eudialyte are the predominant Zr minerals. Chess Draw samples have higher Nb than Wind Mountain samples, whereas Wind Mountain samples have higher Zr. New mapping and chemical analyses in Chess Draw indicates the aeromagnetic anomalies from the geophysical surveys conducted by the USGS could be the top of syenite and phonolite intrusions, possibly as sills, laccoliths, or plugs, which could host additional REE, Zr, and Nb deposits, but drilling is required. None of the other intrusions, outside of Chess Draw and Wind Mountain, have elevated REE (>800 ppm), Zr or Nb.