New Mexico Geological Society Annual Spring Meeting — Abstracts

Episyenite bodies produced by metasomatism of Precambrian basement rocks occur in the Caballo Mountains and Lobo Hill, NM and at the Amethyst prospect in Fremont County, CO. Many of these episyenites are REE-enriched and are being explored for their economic potential. Previous studies have shown similarities to metasomatism via alkaline intrusive complex derived fluids (fenitization). This hypothesis is reasonable in NM as the late Cambrian-early Ordovician Bliss Formation unconformably overlies these rocks and in CO due to their spatial relationship to known Cambrian magmatism. However, previous ⁴⁰Ar/³⁹Ar geochronology yielded variable age results; perhaps due to post-formation thermal episodes or fluid alteration. The main goal of this study is to determine the timing of alteration that produced these episyenites. Primary and secondary feldspars were characterized using BSE analysis and these individual crystal fragments were analyzed for Ar/Ar geochronology. Rb and Sr isotope analysis and whole rock geochemistry is used to help determine the origin and chemistry of the altering fluids. Secondary feldspar is comprised of fine-grained aggregates of anhedral and unexsolved feldspar that are near Or₁₀₀, while primary K-feldspar has exsolution textures typical of basement K-feldspar. Secondary K-feldspars from the episyenites yield relatively flat age spectra with integrated ages from 338±3 to 429±10 Ma, and plateau ages from 316-418Ma, with younger ages that coincide with Ancestral Rocky Mountain uplift. Age spectra from episyenites in the Amethyst prospect are also nearly flat with integrated ages from 392 to 416 Ma to gently rising with ages ~450-500 Ma in higher temperature steps. Primary K-feldspar age spectra from both CO and NM have gradients from ~560 to1090 Ma and indicate cooling below ~150°C by late Precambrian. Thus, the young ages for secondary K-feldspar are not consistent with late Cambrian plutonic emplacement followed by cooling through argon closure near 400 Ma. That is, the young secondary K-feldspars ages are not the result of elevated post-Cambrian regional temperatures. It is interesting that these young ages are recorded in both NM and CO episyenites, and that the episyenite ages in the Caballo Mountains coincide with Ancestral Rocky Mountain activity. Whole rock geochemistry from the Caballo Mountains shows an increase in K₂O as well as Fe₂O_3t and MgO, and losses of SiO₂ and Na₂O. Most of the trace elements are enriched in episyenites relative to background rocks. Pb, Th, and U show the most enrichment, but increases in Y, Zr, and F are notable. REE also increase with the degree of alteration, with HREE increasing more readily than LREE. These elements can be mobile in peralkaline settings, but lack of strong enrichment for Ba, V, Sc and Sr do not support this idea. The data collected so far suggests that if alteration due to Cambrian alkaline magmatism formed the episyenites, later fluid processes are required to explain younger ages and inconsistencies in trace elements. Overall young ages suggest post-Cambrian low temperature fluid alteration of the episyenites, but it remains puzzling that this additional alteration event did not significantly affect basement Precambrian K-feldspar or overlying sedimentary rocks.