PETROGENESIS AND HYDROTHERMAL ALTERATION OF AN EVOLVED RHYOLITE, THE BLAWN FORMATION, SOUTHERN WAH WAH MOUNTAINS, UTAH

The Red Beryl (RBR) and Tetons units of the Blawn Fm. are two A-type rhyolite domes within the Wah Wah Mountains of Utah with different igneous evolution and alteration histories. The RBR is subdivided into two units, RBR Early and RBR Late, based on eruption timing and notable differences in concentrations of select trace elements. These rhyolites were derived from mantle- or lower crustal-derived basalts that underwent high degree fractional crystallization with little to no crustal contamination. The RBR Late unit is strongly depleted in several incompatible elements compared to the other undepleted rhyolites in the Blawn Fm. These incompatible elements were removed by the crystallization of fluorite, cerianite, monazite, and xenotime from the evolving melt. This indicates that extensive differentiation prior to eruption can generate rhyolites that are depleted in incompatible elements, some of which are classified as critical for current technology and the energy transition. This indicates the crucial nature of knowledge of A-type rhyolite petrogenesis for mineral prospectivity analysis.

Some critical element-bearing mineral phases within the Blawn Fm. are anhedral, porous, and present in the groundmass or in pores within the rhyolite, whereas others are euhedral and typically present as inclusions within phenocrysts. These variations in crystal texture and mineral associations indicate different primary igneous and secondary hydrothermal mineral assemblages. Alteration indices as a proxy for hydrothermal alteration indicate that alteration and secondary mineralization did not affect trace element concentrations (REE, Nb, Ta, Y, etc.) of the rocks, neither upgrading nor degrading them beyond the concentrations established during igneous processes. However, the change in mineralogy and porosity during alteration may have concentrated these elements into smaller volumes of secondary minerals that are more easily processable during potential mining and extraction. This study indicates that although concentrations of elements of interest, such as the REE, Nb, Ta, Y, and others, may not increase during the alteration of A-type rhyolites, this alteration may significantly increase the economic potential of these units by increasing leachability and processability of the altered units.