Volcano-Sedimentary Lithium Occurrences in Barstow, California, and Their Related Formation Mechanisms Determined by Stable Isotope Analyses of Carbonates and Clays

The production of lithium (Li) from underexplored volcano-sedimentary Li deposits could increase the Li stockpile for the future while diversifying the available sources of Li needed for the energy transition. Here, we investigate the occurrence, distribution, and potential enrichment mechanisms for recently discovered Li-rich mudstones within the Miocene Barstow Formation in the Mojave Desert of California. Bulk mudstone Li concentrations from the Barstow Formation range from 20 to 2,500 ppm, and the most enriched samples correspond to a greater proportion of smectite relative to illite, chlorite, and kaolinite. The Li-enriched mudstones tend to contain analcime, calcite, dolomite, and feldspar. Bulk mudstone Li concentrations covary with the oxygen isotope composition of the carbonate phases (δ18Ocarb; 17–35‰ Vienna standard mean ocean water). Smectite δ18O values mirror paired carbonate trends and contain an evaporitic slope in δ18O-δD space, suggesting both minerals formed from the same evaporatively enriched evolving water reservoir. Calculated smectite formation temperatures range from 17° to 51°C and carbonate clumped isotope formation temperatures range from 15° to 50°C, suggesting low-temperature processes drive Li enrichment. The combined geochemical and stable isotope results suggest that between 19 and 13 Ma, evaporation of ephemeral underfilled to perennial balance-filled lake waters led to the Li enrichment of pore waters and subsequent Li smectite authigenesis. We speculate the primary source of Li to ancient lake/pore waters came from the weathering of glassy volcanic ash shards to the lake and aqueous Li weathered from coeval felsic volcanic rocks in and around the Barstow basin. Evidence for Li-rich hydrothermal contributions to lake/pore waters is minor. Our study suggests that other deposits set in nearby extensional regimes may have had similar paleoenvironmental controls surrounding the formation of authigenic, Li-rich smectite.