40AR/39AR GEOCHRONOLOGY AND GEOCHEMISTRY OF REE-RICH EPISYENITES IN THE CABALLO MOUNTAINS, SOUTHERN NEW MEXICO: POTENTIAL RELATIONSHIP TO HYDROTHERMAL FLUIDS DERIVED FROM CAMBRIAN-ORDOVICIAN ALKALINE MAGMATISM

The Caballo Mountains contain many REE-enriched bodies that are metasomatic in origin (episyenite). They have similarities to fenites surrounding late Cambrian carbonatites in Colorado and New Mexico, and are unconformably overlain by the late Cambrian to early Ordovician Bliss Formation (~500 Ma). However, alkaline intrusions are not exposed, and previous ⁴⁰Ar/³⁹Ar geochronology on K-feldspar from these episyenites yielded post-500 Ma dates that are in conflict with field constraints. New ⁴⁰Ar/³⁹Ar K-feldspar analyses from New Mexico and Colorado seek to resolve this conflict. K-feldspar separates were characterized by BSE analysis, and then analyzed by Ar/Ar. This is paired with Sr and Pb-isotopes of feldspar and whole rock geochemistry to determine the chemistry and sources of fluids involved in alteration. The episyenites are also compared to K-fenites related to known Cambrian intrusions (Amethyst carbonatite, CO and Lobo Hill, NM). Sr and Pb-isotope measurements and geochemical analyses are currently ongoing.

Secondary feldspar in the Caballo Mountains is anhedral and non-perthitic, and shows relatively flat age spectra ranging from ~370-480 Ma. The primary feldspar is perthitic and rimmed by secondary feldspar, and shows an age gradient ranging from ~575 to 975 Ma with no evidence of Cambrian Ar-loss. Secondary feldspar from fenites at the Amethyst mine is similar texturally, and displays flat spectra ranging from 375-475 Ma, relative to primary feldspar ages of ~950-1300 Ma. Overall, the metasomatic K-feldspar age spectra are younger than would be expected if they are in fact related to Cambrian intrusions. Whole rock geochemistry indicates that alteration adds K, Rb, Ba, REE, U, Th, F, and Y, and removes SiO₂, Na, and Ca, which is similar to many potassic fenites. However, loss of Sr and dramatic increases in U and Th are suggestive of additional fluid processes. If fenitization is demonstrated, the episyenites would be potential targets for REE exploration.