A PETROCHRONOLOGICAL APPROACH TO INVESTIGATE THE CRITICAL MINERALS: IMPLICATIONS FOR THE TIMING AND ORIGIN OF Y + HREE AND NB-TA ENRICHMENTS IN THE FELSIC ROCKS AT THE SHOULDER OF THE RIO GRANDE RIFT

This study provides detailed in-situ mineralogical and geochronological data on the one-inch rock slabs from three rhyolite laccoliths, Round Top, Little Round Top, and Little Blanca situated in Hudspeth West Texas to constrain the timing and origin of enrichment of Y + HREEs (yttrium and heavy rare earth elements), Nb and Ta. The information thus provided will facilitate designing the appropriate exploration/mining strategy to exploit these critical elements. Previous exploration activities were focused on the Round Top Mountain while little is known about the adjacent laccoliths. The Round Top is uniquely enriched in Y + HREEs with total reserves of 500,000,000 tons. However, it is not clear which mineral(s) is the primary host for these elements. This missing information is an obstacle in developing an appropriate metallurgical strategy to deplete these critical elements. Detailed automated mineralogical data for the drill holes and surface samples revealed that the average concentrations of major HREE-hosting phases in Round Top are zircon (43 %), yttrocerite (23 %), columbite (23%) and cheralite (6 %). Other phases such as cassiterite, fluocerite, thorite, and xenotime-Y constitute a minor proportion. The Y + REE dominant phases in Little Round Top are zircon (44 %), columbite (24 %), yttrocerite (14 %) and cheralite (6 %). Little Blanca is composed of zircon (42 %), columbite (30%), yttrocerite (25%), and cheralite (5 %). Generally, the grain sizes of these minerals vary from 8 to 20 microns. All zircon U-Pb dates reported here are Tera-Wasserburg lower-intercept discordia dates at 2s uncertainty. Three rhyolite samples from Little Blanca yielded zircon U-Pb LA-ICP-MS age varying from 37.4 ± 0.3 to 37.9 ±0.4 Ma. Two samples from Little Round Top yielded an age of 37.2 ± 1.0 Ma and 37.7 ± 1.0 Ma. However, zircons from Round Top yielded a slightly younger age of 36 ± 0.52. Two rhyolite samples from Round Top yielded a composite U-Pb xenotime LA-ICP-MS weighted mean ²⁰⁶Pb/²³⁸U age of 37.9 ± 1.8 Ma (2s). Zircon and xenotime U-Pb data strongly suggest that both zircon and xenotime are magmatic and record the timing of emplacement of the laccoliths, possibly crystallizing from the same source magmas. Currently, we are investigating which of these identified phases is favored to host the majority of Y + HREE, Nb, and Ta.