MINERALOGY, GEOCHEMISTRY, AND CHRONOLOGY OF THE CABALLO AND BURRO MOUNTAINS REE-BEARING EPISYENITES

Extraordinarily potassium feldspar-rich red rocks, termed “episyenites”, in the Caballo and Burro Mountains, New Mexico, have anomalously high concentrations of U, Th and REE. Field and electron microprobe investigations of outcrop distribution and mineralogical textures suggest episyenites formed by interaction of K-rich metasomatic fluids with Precambrian granitic host rocks, resulting in K-feldspar-rich rocks with bulk compositions with up to 16 wt.% K₂O. The secondary feldspars are significantly less fractured than primary igneous feldspar, display no perthititic textures, and contain micron size hematite inclusions. The most reddened episyenites are composed largely of interlocked K-feldspar crystals which display no igneous texture. Investigation of rocks with the highest concentrations of U, Th, and REE indicate complex mineralogy associated with fluid alteration, particularly in areas that were mafic silicates in the original granite, now replaced by a combination of secondary chlorite, carbonate, apatite, a Ti bearing phase (possibly rutile), LREE-rich phase, possibly parisite, and a co-existing HREE-rich phase that contains Dy, Er, and Yb. The LREE and HREE phases appear to be co-precipitating in places, suggesting that the REE are partitioning between two distinct phases from a single metasomatic fluid. The HREE-bearing phase is also found as an overgrowth on igneous zircon.

In the Caballo Mountains, timing of metasomatism is constrained to be older than late Cambrian as episyenite clasts occur in the Є-O Bliss Formation that unconformably overlies metasomatised basement. 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 Mts. 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). 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 responsible for all metasomatism. However, one sample from the Burro Mts. yields a plateau age at ~540 Ma that may record late Cambrian metasomatism caused by an intrusion in the subsurface.