The inter-relationship between lithium brines and clays in the playa lake environment C. Tucker Barrie, CTBA Geoconsultants, Ottawa, Canada barriect@sympatico.ca The majority of Earth’s economic lithium resources reside in brines associated with Pliocene – Pleistocene - Holocene playa lakes or salars, and many of these settings also have lithium-bearing clay deposits. The ultimate source for lithium for brines and clays is generally considered to be contemporaneous lithium-enriched felsic tuffs and flows, which after deposition, hydrate and weather to clay, and lose cations to groundwater. The groundwater collects within interior basins under semi- arid to arid conditions and becomes a brine through evaporation over time. The balance between groundwater evaporation and tectonic activity dictates the maturity of a playa lake environment. “Mature” play lakes or salars have evaporate minerals in abundance, as is the case for most salars of South America, whereas “immature” , clastic-dominant interior basins are more common in tectonically active settings in the SW USA. Geothermal systems can play an important role in the lithium budget between brine and clay deposits in both “mature” and “immature” playa lake settings. Laboratory experiments indicate that high temperature geothermal fluids promote hectorite formation. Hectorite locks lithium into the octahedral sites of the smectite crystal structure. This process can rob the brine of lithium in preference to magnesium and other cations. At lower temperatures, other clays (illite, kaolinite) have capacity to adsorb lithium on platy crystal surfaces and edges. Additionally, algal mats and other organic material, and soluble lithium-bearing salts within sediments in the playa lake environment may strongly influence the lithium budget. This presentation examines lithium brines and clays of Clayton Valley, Nevada, Salar de Pocitos, Salta Province, Argentina (Quiron II property – Alba Minerals), and other South American salars, where geothermal systems are active. Initial studies of Clayton Valley lithium clay deposits indicate that a significant portion of lithium is water soluble, which may be a critical factor for an economically recoverable lithium resource.