GEOCHEMICAL EVALUATION AND IGNEOUS PETROGENESIS OF A HYDROTHERMALLY ALTERED EVOLVED RHYOLITE, SOUTHERN WAH WAH MOUNTAINS, UTAH

Highly evolved rhyolites represent prospective bulk-tonnage sources of many critical elements, including the rare earth elements, yet the processes that generate these rhyolites and control the deportment and concentration of these elements in these systems remain unclear. The Blawn Formation rhyolites in the Wah Wah Mountains of Southwestern Utah form a series of domes and ash beds that are part of a larger suite of high-silica, topaz-bearing rhyolites that intruded across the western US during the Cenozoic. These rhyolites are typically A-type, related to extensional tectonic regimes, and are characteristically enriched in F and other incompatible elements. The formation is divided into several separate flow units that intruded into Paleozoic to Oligocene sedimentary and volcanic units and was erupted from several vents at ~22-18 Ma, with both eruptive and intrusive units thought to be sourced from the same parental magmatic system. It is still unclear, however, whether the distribution of the critical elements (e.g. Dy, Te, Ge, In, Ga, Nb, Ta, Re, etc.) and REE within this system are controlled purely by magmatic processes or have undergone late stage to post-magmatic mobilization of critical elements. Previous experimental research has determined which complexing ligands (F, Cl, P, B, CO₂, etc.) are most stable/effective at mobilizing the rare earth and critical elements at magmatic to near-magmatic temperatures, but this work has yet to be applied to natural hydrothermal fluids or a wider variety of critical elements which are present in these high evolved lavas. Sections of the Blawn Formation have undoubtedly undergone hydrothermal alteration, providing an ideal opportunity to compare the igneous and hydrothermal controls on critical metal distribution within highly evolved rhyolitic systems.