Salar de Uyuni in Bolivia holds a large percentage of the world's lithium reserves. Since the lithium is dissolved as LiCl in the brine, it makes extraction much easier than the alternative: Li bearing minerals in granitic pegmatites.
What is the source of lithium in the brine? Where does it come from?
Due to its increasing economic importance, Lithium deposits are coming under more intense scrutiny, by organisations such as the United States Geological Survey (1). The Salar de Uyuni deposit in Bolivia is one of the deposits of lithium brine that is undergoing modelling and research.
The USGS noted that all lithium brine deposits have some very similar characteristics, climatically, they are in arid environments, be near one or more aquifers and have had sufficient time for the accumulation of the lithium from weathering processes, other commonalities include the tectonic settings, associated volcanic/hydrothermal activity and local petrology (lithium bearing rocks) (1).
The geological setting of the Salar de Uyuni is described by the USGS (1) as being situated in the Altiplano of Bolivia and Chile - a high, internally drained plateau along the crest of the Andean convergent boundary orogen, that includes closed basins. Active faulting is present, causing in subsidence, allowing sediments to accumulate.
Observations made by Hofstra et al. (2013) (2) in similar lithium brine deposits in the United States came to the conclusion that melt inclusions in quartz phenocrysts in rhyolite tuffs in association with hydrothermal deposits of tin, molybdenum and beryllium have been found to be heavily enriched in lithium (to the order of 1000's ppm). Hofstra et al. (2013) surmise from these observations that the source of the lithium has been leached out over time from the nearby rhyolitic tuffs of the Bolivian tin belts to the east of the Salar de Uyuni (2),(3).
The lithium is then leached out of the rhyolite by meteoric water, draining into the basins and concentrated by evaporation in the local arid environments.
References
(1) Bradley et al. 2013, A Preliminary Deposit Model for Lithium Brines, USGS Open-File Report 2013–1006
(2) Hofstra et al. 2013, Silicate Melt Inclusion Evidence for Extreme Pre-eruptive Enrichment and Post-eruptive Depletion of Lithium in Silicic Volcanic Rocks of the Western United States: Implications for the Origin of Lithium-Rich Brines*, Economic Geology
(3) Arce-Burgoa and Goldfarb, 2009, Metallogeny of Bolivia, Metalliferous Ore Deposits of Bolivia
Adding to Sabre Tooth's summary:
In addition to weathering of felsic volcanics as ST describes, another aspect that has received attention over the years has been direct input to the salar (or playa) from hot spring sources. The idea being that the lithium is coming in as magmatic, not just meteoric water. There is no question that hot springs are typically associated with lithium brines - but since the causal relationship is not yet conclusive, there could be an underlying confounding variable.
A paper published recently by Daisuke Araoka (2013), notably covering the same part of Nevada as Hofstra (2013), finds isotopic evidence for the hot spring source over the weathering of felsic volcanics. So the debate is not quite over. There are some unusually high Li levels in silver and gold vein mineralization adjacent to Clayton Valley. I'm not sure how that fits in - the Ag-Au mineralization is quite a bit older but it could still relate to the bigger story. I don't know the Uyuni area as well as Argentine salares, but I don't think the hot spring model has been pushed there too much although I do hair it mentioned in Argentina occasionally.
A good geochemical discussion on sources in the Andes is provided in Risacher (2003). His work also has application in developing exploration techniques. The amount of research (aka funding) that has gone into porphyry deposit genesis and exploration methods dwarfs the amount that has gone into lithium brines. And much of the work that has been done has been by a few companies that have had no motivation to publish their work in the way that the academic groups researching porphyries have. Consequently there is still work to be done in this area. Risacher's paper, and others by him on the same subject are readable and interesting examples of applied geochemistry in genetic models and potential exploration methods.
