LITHIUM CLAY MINERALIZATION BY REVERSE WEATHERING: LITHIUM ISOTOPE INSIGHTS INTO A NEW GENESIS MODEL FOR THE LARGEST KNOWN LITHIUM DEPOSIT IN NORTH AMERICA

With current lithium (Li) production coming mainly from Australian pegmatite deposits (e.g., spodumene) and South American brine deposits, the development of new Li resources in North America is a growing domestic priority. The Thacker Pass Project in northern Nevada is the largest known sedimentary Li resource in the world and is positioned to alleviate some of the increasing Li demand. The deposit is hosted by clay-rich sediments in the remnants of an ancient caldera lakebed that formed following the Yellowstone hotspot-associated Middle-Miocene supereruption of the McDermitt Caldera. Li is held in a mix of smectite clay known as hectorite, with concentrations up to 4,000 ppm that gradually transitions with depth to an illite clay with concentrations up to 9,000 ppm (whole-rock).

Here we use lithium isotopes of Thacker Pass core samples to trace both hydrothermal Li fluxes to the lacustrine system and removal by reverse weathering over time. Both processes have opposite effects on lake water Li isotope signatures preserved in the Li clay. The most dramatic hydrothermal fluxes indicated by a δ⁷Li of -1.5‰ could be associated with hydrothermal events, where lake water flashes and brings extreme Li concentrations along with the high alkalinity and salinity conditions required to rapidly nucleate reactive Mg-smectites able to incorporate Li during early diagenesis. These energetic events also often bring in fresh volcanic glass shards that diagenetically alter to Al-smectite -> clinoptilolite -> analcime and K-feldspar -> albite. The initial Al-smectite could play a key role leading to the heterogeneous nucleation of Mg-smectites and thus speed up reverse weathering. We suggest K released in porewater from glass-derived K-feldspar during late diagenesis leads to illitization of the Li-rich hectorite and could explain why the more illitic clays have the most extreme Li concentrations at Thacker Pass. By contrast, the most quiescent periods indicated by a δ⁷Li of 7.1‰ could indicate fresher conditions that lead to slower reverse weathering rates, and thus more fractionated Li isotope signatures generated by more prolonged contact with the reactant pool (lake water). These fresher conditions are found to lead to the accumulation of organic matter and replacement dolomitization of the Mg-smectites during diagenesis.