CONTROLS ON ENRICHMENT OF ENERGY-CRITICAL ELEMENTS (LI, GA, REE) IN WEAKLY PERALKALINE MAGMAS OF THE MID-MIOCENE MCDERMITT VOLCANIC FIELD, OREGON AND NEVADA, BASED ON IN SITU SHRIMP-RG ANALYSES OF QUARTZ-HOSTED MELT INCLUSIONS

McDermitt Caldera is the only caldera known to be mineralized among at least four calderas on the NV-OR border that formed on eruption of weakly peralkaline rhyolite ignimbrites ~16.5-15.5 Ma, and together constitute McDermitt Volcanic Field (MVF). To test whether magmas erupted at McDermitt Caldera are unusually enriched in U, Hg, Li, Ga, and REE, we performed ~400 in situ SHRIMP-RG analyses of homogenized quartz-hosted melt inclusions from ignimbrites, lavas and dikes throughout MVF.

North of McDermitt Caldera, Whitehorse Caldera (source of the Tuff of Whitehorse Creek) is nested within two larger, previously unrecognized, overlapping calderas that new mapping suggests are sources for the Tuffs of Oregon Canyon and Trout Creek Mountains. Melt inclusions from early-erupted high-silica alkali rhyolite parts of these two ignimbrites are enriched in Li, and depleted in Ga and REE relative to those from less-evolved later-erupted portions, consistent with Ga and REE being removed by chevkinite during fractional crystallization.

The caldera-forming unit of McDermitt Caldera lacking quartz phenocrysts, we analyzed quartz-hosted melt inclusions in three lavas and dikes associated with the caldera. Concentrations of Li, Ga, and REE are similar to those in ignimbrites and lavas associated with the calderas in northern MVF. High-Zr lavas have the highest concentrations of Ga (~50 ppm), LREE (~115 ppm La), and HREE (~25 ppm Dy). Conversely, Li content is greatest (~1750 ppm) in units with the lowest Zr concentrations. The largest Li concentrations in MVF, up to 3000 ppm, were measured on inclusions from an evolved low-Zr ring-fracture lava of Whitehorse Caldera. Although Li behaves incompatibly, we suggest that the greatest enrichments occur in magmas that have incorporated a larger proportion of felsic upper crust, which leads to lower Zr concentrations and lesser peralkalinity.

Given that concentrations of energy-critical elements are similar in all magmas of MVF: (1) The known resources at McDermitt Caldera are a function of hydrothermal alteration, perhaps more intense than at other calderas in MVF because McDermitt Caldera underwent resurgence. (2) The consistently high concentrations of Li, Ga, and REE in magmas erupted across MVF suggest that there could be undiscovered resources in the northern part of the field.