Abstract
The McDermitt caldera (Oregon–Nevada, USA) hosts the largest volcano-sedimentary (VS) Li resource globally. This study presents an integrated geological, mineralogical, and isotopic investigation of the McDermitt deposit in Oregon, with the aim of constraining the processes responsible for Li enrichment in clay-type VS Li mineralization. Our study reveals that Li mineralization occurs in stratiform lacustrine units composed of hectorite-bearing claystones interbedded with siliceous–carbonate beds and altered ash fall units. Lithium enrichment is mainly controlled by the formation of Mg–Li smectites (hectorite) associated with hydrous silica, and locally by zeolitization of tuffaceous precursors – a pathway that can be explained by devitrification of volcanic glass under near-neutral to slightly alkaline conditions. Geochemical and isotopic data indicate that mineralization formed through fluids at average temperatures of 133 °C and δD values of − 107‰, consistent with heated meteoric waters in a geothermal setting. Unlike the nearby Thacker Pass deposit, illitization of smectite did not occur, reflecting differences in the total amount of Li available and in the pH chemistry of deeper fluids circulating on a caldera-scale.