Rhyolitic calderas - the explosive “supervolcanoes” like modern-day Yellowstone that spread ash continent-wide - have long been recognized as hosts for traditional metal resources such as Ag, Au, Pb, Zn, and Cu, which form by circulation of hydrothermal fluids driven by the heat of the magmatic intrusions below. Recently, calderas have been foci of exploration for non-traditional resources that are essential to clean-energy technologies. The shallow levels of calderas have features that make them particularly suited for creation of resources of energy-critical elements such as lithium, gallium, and rare earth elements: abundant faults and fractures that provide pathways for mineralizing fluids, a source of metals in the vast quantities of volcanic ash formed in the explosive eruptions that lead to caldera collapse, and caldera lakes in which easily altered glassy ash accumulates and the elements are concentrated. In Mid-Miocene calderas associated with the inception of the Yellowstone hotspot, zones of hydrothermal alteration along caldera-collapse ring faults contain Ga and REE enrichments to near-economic levels. Most significantly, Li-rich clays form on relatively low-temperature alteration of the fine-grained ashy sediments. These Li-rich clays in McDermitt Caldera on the Nevada/Oregon border constitute the largest Li resource in the United States. In order to understand what controls the concentrations of energy-critical elements in magmas, we analyzed melt inclusions trapped inside quartz crystals from units erupted in a variety of tectonic settings. Ga and REE concentrations are highest in alkalic rhyolites that form within thin or extending continental crust. Li enrichment is associated with more aluminous magmas, and the highest concentrations are found in magmas that form within thick continental crust. The magma that erupted at McDermitt Caldera was not extraordinarily enriched in Li (or Ga and REE), but the volume of ash formed in this caldera-forming “supervolcano” was sufficiently large that leaching of the ash provided enough Li for it to be concentrated into the largest resource in the United States. In the Great Basin there are dozens of calderas with preserved caldera lake sediments that constitute targets for further exploration for energy-critical elements.
Speaker: Gail Mahood, Stanford
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Stanford, CA 94305
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