CHEMISTRY AND MINERALOGY OF THE LREE-ENRICHED CORNUDAS MOUNTAINS OF TEXAS

The Cornudas Mountains are a series of alkaline intrusions located approximately 95 kilometers northeast of El Paso, Texas, clustered near the border between New Mexico and Texas. These intrusions were emplaced 37-27 Ma (McLemore, 2018) and form the northern part of Trans-Pecos magmatic province in Texas. Geochemical and mineralogical studies were conducted on two laccoliths, Chattfield Mountain and San Antonio Mountain, as well as on a low-lying sill, Washburn Mountain, situated on the Texas side. These intrusions are intermediate in composition and rich in nepheline, analcime, anorthoclase, aegirine, and amphibole. They are alkaline, ferroan, peralkaline to metaluminous, and display within-plate geochemical affinity, suggesting that the complex formed during the transition from Laramide compression to Rio Grande rift extension. Harker variation and trace element diagrams record magma differentiation trend from the oldest Chattfield Mountain (~36.8 Ma) to younger Washburn Mountain (~31.5 Ma) and San Antonio Mountain (~27.9 Ma). The initial magma shows Na decrease over time, resulting in a relative decrease of alkalinity and incompatible elements, such as Rb, Nb, Zr, U, Th, and REEs.

Average REE+Y concentrations from the whole-rock geochemical data are 770 ppm (n=14), 543 ppm (n=22), and 589 (n=21) for Chattfield Mountain, Washburn Mountain, and San Antonio Mountain, respectively. Energy-dispersive spectras obtained from the scanning electron microscope revealed the presence of monazite in these intrusions. Washburn Mountain samples are relatively more enriched in fluorine (average of 864 ppm; n=15) and fluorocarbonate minerals such as bastnäsite and parisite are identified in the intrusion. The Trans-Pecos Magmatic Province is known to host REE-enriched deposits and the elevated REE+Y concentrations recorded for Cornudas Mountains of Texas suggest potential for additional critical mineral resources in this region.