Characterization and origin of episyenites in the southern Caballo Mountains, Sierra County, NM

A bstrAct — Brick-red, K-feldspar-rich rocks, called episyenites (altered rocks that were desilicated and metasomatized by alkali-rich solu- tions) 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. 40 Ar/ 39 Ar 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 in- dicate 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.