Geology, Geochemistry, and Potential Origins of the Basin Volcano-Sedimentary Lithium Deposit, Kaiser Spring Volcanic Field, Northwestern Arizona, USA

The Basin volcano-sedimentary lithium deposit in the Kaiser Spring volcanic field, northwestern Arizona, hosts a combined indicated, inferred, and measured mineral resource of 641 million metric tonnes (Mt) of mineralized rock with grades of 823 ppm lithium (Li). Basin contains 2,809 kt of Li carbonate equivalent (LCE). Geologic mapping, logging of drill core, and geochemistry analyses shed new light on the geologic setting, stratigraphy, clay sedimentology, Li mineralization, and potential origins of this small, deep depocenter within the eastern half of the bimodal Kaiser Spring volcanic field, which contains 12 high-silica rhyolite domes. Basin stratigraphy consists of two Li-rich clay units, the Upper Clay and Lower Clay, both in sharp contact and interbedded with tuffs, basalt lava flows, coarse volcanic and nonvolcanic alluvial conglomerates, and volcanic sinter. These rocks were deposited and subsided into a semicircular, synclinal basin—potentially a maar crater—lacking basin-bounding faults that would have controlled the formation of accommodation space. Stratigraphic correlations from mapped surface geology and drill core in the southeastern side of the basin suggest the depocenter is at least 300 m deep. Lithium ore principally occurs as continuous, stratified zones of magnesian smectite-group clays in both the Upper and Lower Clay. The Upper Clay contains averages of 778 to 983 ppm Li with a high-grade zone of >1,200 ppm Li, whereas the Lower Clay averages 690 ppm Li. The high-grade zone in the Upper Clay is coincident with anomalous molybdenum (Mo), with weighted average concentrations between 69 and 206 ppm, though Mo concentration is highly variable throughout the interval. Potential lithium sources include hydration of Li-rich high-silica rhyolite dome vitrophyre, dissolution of volcanic ash, and hydrothermal fluid circulation. The origin of the Li-rich clay remains unresolved but could include the neoformation of magnesian smectite at an alkaline lake bottom, hydrothermal fluid alteration of volcanic ash or smectite, and/or diagenesis of Li-rich volcanic ash deposits. Although Basin is the first volcano-sedimentary deposit described in a bimodal volcanic field and in a maar crater, it is analogous to many other volcano-sedimentary deposits—including those located at McDermitt caldera/Thacker Pass, Nevada; Rhyolite Ridge, Nevada; Sonora, Mexico; and Clayton Valley, Nevada; and the lesser-known Big Sandy, Arizona, and Lyles clay/Thompson Valley, Arizona—in that Li was mobilized from proximal or interbedded Li-rich rhyolitic tuffs and lavas, Li was concentrated in a closed-hydrologic basin, and Li is dominantly trapped in magnesian smectite-group clays.