The geochemistry of Li-rich brines in the northern Puna salars: possible sources and distribution patterns

GARCÍA M.G

San Salvador de Jujuy

Workshop; 3rd International Workshop on Lithium, Industrial Minerals and Energy; 2016

UNJU

The increased globaldemand for lithium (Li) in batteries for hybrid electric vehicles and otherelectronics has triggered a great interest on the main reservoirs of thiselement worldwide. Lithium is mostly produced from pegmatite deposits and evaporativebrines [1] and therefore these Li ores are the focus of the more recentprospects. The world´s largest lithium-bearinglacustrine evaporite basins are located in the Puna Plateau, where several hydroclimaticand geological settings converge to develop lithium-rich brine deposits. Aridclimate, tectonically driven subsidence, igneous and geothermal activity, closedbasins containing a salar in which one or more aquifers are hosted and thepresence of large outcrops of Li-bearing rocks are some important commonfeatures in all producing lithium brine deposits [2] including those located inthe northern Puna region of Argentina. The understanding of the sources andmechanisms that control the formation of Li-rich brines requires therefore an integralapproach from several fields of the Earth Sciences. This has prompt amultidisciplinary study that is currently being carried out by researchers fromINECOA (Instituto de Ecorregiones Andinas) and CICTERRA (Centro de Investigacionesen Ciencias de la Tierra).The study is focusedon salars located in the Northern Puna region, and particularly in theOlaroz-Cauchari and Guayatayoc basins. The Salar de Olaroz-Cauchari occupies aN-S orographic depression sectioned by the huge prograding alluvial fan of theArchibarca River, which acts as a barrier between the Olaroz (in the North) andCauchari (in South) salt pans. Guayatayoc is a playa lake system locatedtowards the east of the Olaroz-Cauchari salars just at the eastern border ofNorth Puna. Typical features for Li-rich brines are found in these salars. Climateis arid; the annual precipitation averages 60 mm and concentrates in one season(austral summer), while evaporation rates are higher than rainfall resulting ina net water deficit in the region. Ponds of geothermal hot water have beenobserved in the northern border of the Olaroz salar (see Franco et al.´sabstract in this workshop), while hot springs are found near the Coranzulí andTuzgle volcanos (see López Steinmetz et al.´s abstract in this workshop),located towards the North and South respectively of the studied salars. Inaddition, large volumes of ignimbrites and volcanic rocks outcrop at thecatchments of rivers that discharge into the salars. Eventhough the content ofLi in these rocks remains still unknown the presence of the Li-rich micastaenolite and eucriptite in the Guayatayoc playa lake sediments [3] suggeststhat Li-bearing minerals could be important constituents in such rocks. At the moment, thereis no agreement on the source of Li in the brines hosted at the salarsdepocenter. However, two types of sources are the most widely accepted: 1)Weathering of felsic volcanic rocks and 2) geothermal activity associated withnearby volcanic systems or underlying magmas [4], [5]. Contributions of Li fromrock weathering have been little explored but more recent efforts wereconducted to use the 7Li/6Li ratio as a tracer of mineral/waterinteraction because secondary minerals (the solid products of primary mineral´salteration) preferentially incorporate 6Li, driving the isotopecomposition of residual waters to heavier values [6], [7], [8]. Geothermalwaters have also been considered as one important source of Li+ tothe brines, as they contain elevated concentrations of lithium and some otherrelated trace elements (i.e., B, As, Cs) [10], [11], [12]. Finally,contributions from volcanic ash deposits accumulated in the nearby regions ofthe volcanoes have not yet been considered as valuable sources of Li. However,leaching experiments carried out with northern Puna [13] and Patagonian [14]ashes revealed that this element is rapidly released during the ash-waterinteraction, contributing high concentrations of Li+ to aqueousreservoirs. Once in solution, Li ion is susceptible of being incorporated intothe smectites interlayers [15], [16] and thus, it is expected that a significantproportion of Li could be associated with the clayed fraction of the sediments.Other elements in solution, such as boron and potassium, may be recovered asbyproducts or coproducts; in addition, brines can also contain undesirable elementsthat create problems in processing (magnesium) or toxic elements (arsenic) thatrequire care in waste disposal [17].The integral conceptualmodel exposed above summarizes the deep Earth and surface pathways of Li in thestudied salt pans and are the subject of the ongoing research activities of theINECOA and CICTERRA´s geology and low-temperature geochemistry groups.