Characterization and origin of episyenites in the southern Caballo Mountains, Sierra County, NM
— V.T. McLemore, A. Smith, A.M. Riggins, Dunbar N., K.B. Frempong, and M.T. Heizler

Brick-red, K-feldspar-rich rocks, called episyenites (altered rocks that were desilicated and metasomatized by alkali-rich solutions) are found in several areas in southern and central New Mexico. These rocks contain anomalous concentrations of rare earth elements (REE, <2329 ppm), uranium (U, <9721 ppm), thorium (Th, <1378 ppm), niobium (Nb, <247 ppm) and high heavy REE (<133 ppm Yb and <179 ppm Dy). In the Caballo Mountains, the timing of metasomatism is older than late Cambrian as episyenite clasts occur in the Cambrian-Ordovician Bliss Formation that unconformably overlies episyenites and Proterozoic host rocks. ⁴⁰Ar/³⁹Ar dating of K-feldspars within the episyenites yields complex and intriguing age results that are likely related to multiple fluid-alteration events possibly during the Ancestral Rocky Mountains and Laramide orogenies. Rare U, Th, Nb, and REE minerals are found in the Caballo episyenites and could indicate potential REE mineralization at depth, including heavy REE. Synchysite is a major host of light REEs in the episyenites, while heavy REEs are concentrated predominantly in xenotime and priorite. Textural evidence and field relationships indicates that REE-bearing phases co-precipitated during metasomatism prior to deposition of the Cambrian-Ordovician Bliss Formation. The maximum age of the metasomatism forming the episyenites is between the age of the host granite (~1400 Ma) and the Bliss Formation (late Cambrian-early Ordovician). The K-feldspars in the episyenites were then re-heated during the Ancestral Rocky Mountains and Laramide orogenies.