Lithium Isotope Constraints on Quantitative Lithium Uptake by Reverse Weathering: A New Genesis Model for the Largest Known Lithium Resource in the World

Abstract

This study presents a new reverse weathering model for lithium ore genesis at the Thacker Pass Project, the largest known lithium resource in the world. The formation of Li-rich claystones in the closed McDermitt Caldera paleolake system required hydrogeochemical conditions (a_Mg²⁺, a_SiO₂(aq), and pH) that drove the nucleation of reactive, Li-trapping Mg-silicates during brine evapoconcentration. Early-stage mineralization was marked by highly saline, alkaline brines that enabled the crystallization of extremely Li-rich illite (~1% Li) during diagenesis, while transitions to mixed-layer illite-smectite and hectorite reflect progressive freshening as solute inputs to the paleolake waned. The model integrates Li brine ore-genesis with analogs from modern East African Rift Valley alkaline lakes and ancient carbonate-rich basins associated with Mg-silicates, supported by low δ⁷Li values ranging from -1.5 to 7.1‰ that lack vertical trends - indicating bulk-scale, quantitative Li uptake during mineralization. Episodic hydrothermal pulses are recorded in isotopic shifts, with depleted δ⁷Li linked to more active hydrothermal inputs. These findings support a genesis model where Li enrichment was primarily governed by lacustrine salinity and alkalinity driven by evapoconcentration, coupled with the neoformation of Li-bearing clays. The maintenance of alkaline conditions during diagenesis has also been found to be essential in preserving the resource by limiting dissolution, silicification, or dolomitization of the Mg-silicates.