MAGMATIC EVOLUTION AND HYDROTHERMAL ALTERATION OF THE REE-ENRICHED ROUND TOP RHYOLITE LACCOLITH, TRANS-PECOS REGION, TEXAS

The magmatic rocks at the Round Top hypabyssal rhyolite are enriched in Li, Be, F, Rb, Y, Zr, Nb, Sn, REEs, and Th. REE-bearing minerals are mainly accessory phases, including HREE-rich xenotime, zircon, tantalite, columbite, cassiterite, yttrocerite and yttrofluorite. The rhyolite consists of K-spar ~50%, quartz ~29%, plagioclase ~11%, biotite ~4%, and opaque oxides ~2%. The rhyolites are slightly peraluminous , high-silica, alkaline, with elevated HREE concentrations and a large negative Eu-anomaly. The rhyolite has high concentrations of Zr, but little zircon is present in thin section, SEM, and mineral separates, consistent with a soluble, alkali, F-rich, magmatic system.

REEs are found throughout the rhyolite in sub-micron groundmass. This ubiquitous dissemination coupled with HREE mineral phases present as secondary, interstitial phases is indicative of late- to post-magmatic hydrothermal alteration. Geochemical modeling offers a better understanding of the geochemical evolution and REE complexing behavior in these rocks, and determines whether crystal fractionation or precipitation from magmatic hydrothermal fluids was the main mechanism for HREE-enrichment and mineralization. K/Rb ratios are lower than chondritic values, typical of highly evolved magmatic systems. Zr/Hf ratios are low (average=13.1), shifting toward lower values with increasing evolution of the silicate melt and suggest high-temperature hydrothermal interaction. Sr/Eu and Eu/Eu* ratios (average=848 and 0.048, respectively) are typical of a system having undergone extensive magmatic differentiation (average=848), and may be further depleted in Eu due to preferential fractionation into a coexisting F- and Cl-rich aqueous fluid phase. Y/Ho ratios are similar to the chondritic average (average=25.9), and would typically suggest that complexing with fluorine had little influence on mineralization. The rhyolites at Round Top, however, have significantly elevated concentrations of both Y and Ho, characteristic of a high-silica magmatic system rich in H₂O, Li, B, F, P and/or Cl transitional between pure silicate melt and hydrothermal fluid. The pervasive hydrothermal conditions, high fluorine activity, and thorough differentiation contributed to the late-stage precipitation of REE-bearing minerals at Round Top.