RARE EARTH MINERALS IN LITHIUM CLAY DEPOSITS: INSIGHTS FROM THE THACKER PASS DEPOSIT, NORTHERN NEVADA, USA

Abstract Lithium (Li) and the rare earth elements (REEs) are critical minerals for modern technologies and the energy transition due to their roles in energy storage, transport, and low-CO2 power generation and their inherently insecure supply chains. The McDermitt caldera in Nevada and Oregon hosts significant lithium resources, including the Thacker Pass deposit in Nevada—one of the largest known Li resources globally, with a mineral resource estimate of 66.1 million tonnes (Mt) of contained lithium carbonate equivalent (LCE)—as well as the McDermitt deposit in Oregon (215 Mt contained LCE). Although these deposits are known to contain some amount of the REEs as identified during exploration, these critical commodities were not the primary target during the Li clay exploration in this area. However, the criticality of the REEs and the poorly known nature of their mineralization within these lithium systems warrant the evaluation of their potential recovery as by-products of Li extraction. This study investigates the occurrence of REE-bearing minerals within the Li clays at Thacker Pass. Statistical analysis of a drill core geochemical inductively coupled plasma-mass spectrometry (ICP-MS) database shows a strong correlation between Ce and La (r = 0.9). Scanning electron microscopy observations confirmed the presence of bastnäsite, synchysite, and monazite as micrometer-sized grains within Li-rich smectite and illite mineralized zones. Semiquantitative analyses identified Ce, La, Nd, and Pr as major components of these minerals. Despite the low individual REE concentrations (Ce <300 ppm, La <150 ppm) of bulk samples from Thacker Pass, the presence of the REEs as discrete minerals suggests they could potentially be recovered as by-products of Li extraction. This is the first assessment of the REE potential of Li clay deposits, highlighting the need for further evaluation of similar deposits in Nevada and globally. These findings emphasize the potential importance of REE recovery from Li clay resources, contributing to more diverse and secure supply chains and supporting the global energy transition.