GEOLOGY OF THE THACKER PASS DEPOSIT IN THE MCDERMITT CALDERA, NEVADA: THE LARGEST AND HIGHEST-GRADE KNOWN SEDIMENTARY LITHIUM RESOURCE IN THE UNITED STATES (Invited Presentation)

As the electrification of the automobile industry intensifies, demand for lithium (Li) for use in Li-ion batteries is projected to surpass expected supply by mid-decade. This is resulting in a “lithium rush” for new Li resources, including those hosted in sedimentary rocks. Most published sedimentary resources contain whole-rock Li concentrations considerably less than 2,000 ppm Li. The one notable exception is the Thacker Pass deposit in the mid-Miocene McDermitt Caldera in northern Nevada. This resource, hosted in the clay-rich caldera lake sediments, has an average resource grade of ~3,000 ppm Li, with whole-rock concentrations up to 9,000 ppm Li. These sediments are present throughout much of the ~40x30 km caldera, leading Castor and Henry (2020) to estimate a total Li mass of ~120 Mt Li (640 Mt LCE) within the McDermitt Caldera. To better understand the geology and metallogeny of this uniquely high-grade and large deposit, Lithium Americas Corp. launched a new multi-institutional collaborative research project with over 20 researchers from universities and federal labs across North America in Summer 2021. New data from this collaboration indicate that the bulk of the Li-bearing clays likely precipitated as neoformed Mg-rich, Al-poor (Stillings et al., 2018) trioctahedral smectites (hectorite) within a high-pH, high-alkalinity closed caldera lake system. At the base of the sedimentary section, the hectorite likely underwent thermal diagenesis to an Li-rich illite due to degassing of the underlying intracaldera Tuff of Long Ridge (Ingraffia et al., 2020), which provided sustained high temperatures and added fluids rich in F, Cl, and additional Li to the system. This illitic zone is extremely enriched in Li, with whole-rock concentrations consistently over 4,000 ppm Li and up to 9,000 ppm Li. We hypothesize that the McDermitt Caldera lake sediments have a distinctively higher grade than other sedimentary resources because (1) the magma chamber was initially enriched in Li (Benson et al., 2017); (2) the sediments formed in a closed lake system favorable for the precipitation of Mg-smectites; and (3) prolonged degassing and devitrification of intracaldera tuff added excess Li mass to the base of the system. The ongoing research with external collaborators aims to expand and improve upon this model using advanced mineral structure characterization techniques, Li and stable isotopic analyses, K-Ar, ⁴⁰Ar/³⁹Ar, and U-Pb geochronology, trace element geochemistry, hydrogeochemistry, and 3D geological mapping.