INITIAL EMPLACEMENT AND SUBSEQUENT REDISTRIBUTION OF CRITICAL METALS IN A HYDROTHERMALLY ALTERED RHYOLITE SYSTEM

Highly evolved rhyolites represent prospective economically viable, domestic sources of rare earth elements (REE) and other critical metals. At this time, the processes that control the igneous distribution and subsequent hydrothermal redistribution of the critical metals (including REE) in these systems remain unclear. The Cenozoic Blawn Fm. rhyolite (18-23 Ma) is a high-Si, topaz-bearing rhyolite located in the Wah Wah Mountains of southwestern Utah, one of a series of highly evolved rhyolites emplaced contemporaneously across the western US. These evolved rhyolites crop out in a region between the Trans-Pecos Range of western Texas through New Mexico and western Utah and into eastern Nevada, with the majority located in Nevada and Utah. These rhyolites are A-type, formed in extensional tectonic regimes, and are characteristically enriched in F and other incompatible elements. A number of experimental petrographic studies have determined which complexing ligands (e.g., F, Cl, P, B, CO₂, etc.) are most stable over a variety of hydrothermal conditions. These studies found that F and Cl are particularly effective at mobilizing the critical elements, but this work has yet to be extended to natural hydrothermal fluids at near-magmatic temperatures and low pressure (Burt et al., 1982; Lewis et al., 1998; Migdisov et al., 2009). Previous studies indicate that melt inclusions within quartz from the Blawn Fm. contain anomalously high quantities of F (~3340 ppm), Cl (~580 ppm), and H₂O (5.98-7.99 wt. %) with highly variable amounts of CO₂ (3-972 ppm; Lowenstern et al., 1994). The hydrous nature of the Blawn Fm., the abundance of critical elements and complexing ligands such as F and Cl, as well as the high F:Cl ratios of the hydrothermal fluids that interacted with the Blawn Fm. provide an ideal setting to examine the relative REE- and critical metal-complexing capabilities of F- vs. Cl-rich hydrothermal fluids at near-magmatic temperatures.