MAGMATIC ENRICHMENT OF ENERGY-CRITICAL ELEMENTS LI, GA, AND REE IN RHYOLITES OF MCDERMITT VOLCANIC FIELD, HIGH ROCK CALDERA COMPLEX, AND BUFF PEAK IDENTIFIED BY IN SITU SHRIMP-RG ANALYSIS OF MELT INCLUSIONS IN QUARTZ

Deposits containing high concentrations of energy-critical elements Li, Ga, and REE have been identified within the ~16.6-15.6 Ma McDermitt caldera on the Nevada-Oregon border (Glanzman et al., 1978; Rytuba et al., 2003). To identify potential magmatic enrichment, we analyzed homogenized melt inclusions in quartz from rhyolitic lavas and ignimbrites for 42 trace metals and volatiles using the O^2- and Cs⁺ sources on the SHRIMP-RG at Stanford University. Li concentrations are higher in an evolved intracaldera intrusion (~1500 ppm) than in ignimbrites from the volcanic field. We also analyzed the nearby Mid-Miocene Buff Peak topaz rhyolite as a potential Li source, as F-rich topaz rhyolites in the western U.S. have been shown to be sources for Li deposits due to their high magmatic concentrations (Hofstra et al., 2013). The lower Li values we find at Buff Peak (~400 ppm) suggest that the post-caldera peralkaline intrusions at McDermitt are a more likely source for the Li deposit there. Ga and REE concentrations of melt inclusions increase with increasing peralkalinity of lavas and ignimbrites from McDermitt Volcanic Field, reaching ~30 ppm Ga and ~20 ppm La in the most Zr-rich samples. To evaluate if there is a correlation between Ga and REE concentrations and the nature of the crust, we compared the Ga and REE contents of units from McDermitt, which lies on transitional crust, to samples of equivalent peralkalinity and bulk composition from High Rock Caldera Complex, Nevada, a coeval volcanic field also associated with Steens flood basalts, but located west of the 0.704 ⁸⁷Sr/⁸⁶Sr_i isopleth on accreted oceanic arc terranes. The Ga and REE contents of analyzed High Rock rhyolites are higher than McDermitt samples at any given Zr concentration, suggesting that peralkaline magmas generated in thinner, more-mafic crust are more favorable for Ga and REE enrichment. Resources of U and Hg identified at High Rock may therefore also have potential for the Ga and REE enrichment known for the U and Hg resources at McDermitt.