BRAISED IGNIMBRITE: INTRA-CALDERA IGNIMBRITE COOLING AS A MECHANISM FOR Li ENRICHMENT

The transition away from fossil fuels requires new energy sources and storage which has increased demand for critical minerals (CM) especially lithium (Li). Despite its societal importance and recent efforts, we do not understand the mechanisms by which Li is leached from low-concentration volcanic rocks located within closed basin systems (e.g., calderas). One leading hypothesis for Li and other CM enrichment is hydrothermal alteration, as elemental diffusion in glass has an exponential relationship to temperature. We use numerical simulations of hydrothermal systems to understand how the erupted ignimbrites within caldera cool and thus affect the distribution of Li and similarly deposited critical minerals (e.g., REE, As, Sb, Rb, Mg). Using the finite difference code HYDROTHERM, we calculate time-temperature paths for hydrothermal vents at an idealized, sediment filled caldera. We find that ignimbrite cooling has two significant effects on hydrothermal circulation: 1. increased temperature (up to 600 ºC) and 2. prolonged hydrothermal cooling even after the ignimbrite becomes fluid saturated. This second effect is due to decreased geotherm and fluid circulation at the magma chamber, and has not been a focus in previous studies. For example, at 1 ka post-eruption even with a modest 600 m ignimbrite produces vents 250 ºC warmer than a magma-only system. At 2 ka post-eruption, the hydrothermal vents remain 220 ºC warmer. To understand the effect of cooling on Li concentrations, we calculate the total Li budget available for leaching from ignimbrites at 34 calderas, where intracaldera ignimbrite thickness and size are reported using both a conservative [Li] (100 ppm) and a high estimate from melt inclusions (1000 ppm). In either case, hydrothermal circulation driven by ignimbrite cooling leads to orders of magnitude higher degrees of [Li] concentration within intracaldera rocks and sediments. Future work is required to understand how these reaction kinetics vary within global caldera systems and if the Li-rich brines implied these simulations are retained to form economic deposits or are lost to subsequent fluid flow. That said, we propose a novel and testable mechanism for Li enrichment in volcanic settings.