CHARACTERIZING HIGH REE OCCURRENCES ASSOCIATED WITH EPISYENITES IN THE CABALLO AND BURRO MOUNTAINS, NEW MEXICO

Extraordinarily potassium feldspar-rich replacement rocks, termed “episyenites”, in the Caballo and Burro Mountains, New Mexico, have anomalously high concentrations of U, Th and REE. Investigations of these episyenites aim to increase understanding of their overall genesis, the timing of metasomatism and REE resource potential. Field and electron microprobe investigations suggest episyenites are formed by interaction of K-rich metasomatic fluids with Precambrian granitic host rocks, resulting in altered rocks with bulk compositions that contain up to 15 wt.% K₂O. In the Caballo Mountains, timing of metasomatism is constrained to be older than late Cambrian as episyenite clasts occur in the Є-O Bliss Sandstone that unconformably overlies metasomatised basement. This stratigraphic timing control is not available in the Burro Mountains.

A Cambrian-Ordovician magmatic event is well documented in southern Colorado and New Mexico, characterized by intrusion of carbonatite and alkaline complexes with associated K-metasomatism. Direct dating of the metasomatism using the 40Ar/39Ar method on sub-milligram fragments of metasomatic K-feldspar yield complex and intriguing age results. In the Caballo Mountains, age spectra range from nearly flat to highly disturbed with total gas ages (TGA) between ~40 and 460 Ma. Individual fragments with flat spectra from single samples vary in TGA by ~140 Ma (~320 to 460 Ma). Flat spectra with highly variable ages indicate multiple recrystallization events within single samples. Preliminary microprobe imaging does not reveal obvious multiple generations of metasomatic K-feldspar in single samples, but further work is required to relate age and microtexture. The overall youthfulness of the results is not compatible with the hypothesis that a single metasomatic event related to regional Є-O alkaline magmatism was ubiquitously responsible for all metasomatism. Perhaps, fluids circulating along the unconformity interface also contributed to heterogeneous growth of metasomatic K-feldspar. In the Burro Mountains, samples near Tertiary intrusions are too outgassed to record accurate metasomatism ages. However, one sample yields a plateau age at ~540 Ma that may record late Cambrian metasomatism caused by an unexposed pluton in the subsurface.