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
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.