Summary:
Pastos Grandes and Pozuelos are both classified as salars, which are brine formations containing elevated levels of metals in solution, typically as salts. Salars can be found at elevations from 1,000 [m] to more than 4,000 [m] above sea level. They typically represent the end product of a basin infill process that starts with the erosion of the surrounding relief, beginning with deposition of colluvial talus and fan gravels and grading upwards to sheet sands and playa silts and clays as the basin fills. There are numerous variations on the model and the literature provides ample discussion of the relevant tectonic and sedimentary processes involved in both general and specific terms (Hardie et al, 1978; Reading, 1996; Warren, 1999; Einsele, 2000); and specifically with regard to the Altiplano-Puna (Ericksen and Salas, 1989; Alonso et al, 1991; Chong et al., 1999, Bobst et al, 2001; Lowenstein et al., 2003; Risacher et al., 2003; Vinante and Alonso, 2006).
Lithium brine projects differ significantly from hard rock mining projects due to their fluid nature. The important considerations of a brine deposit are the contained elements and chemistry of the brine and the characteristics of the host aquifer, such as aquifer extent, thickness, internal variations/heterogeneity and the physical aquifer properties, particularly porosity.
Lithium brine projects can be subdivided into two broad ‘deposit types’, depending on the salar characteristics (Houston et. al., 2011):
- Mature salars (those containing extensive thicknesses – often hundreds of meters - of halite, such as the salar de Atacama, the salar Pozuelos and the salar Hombre Muerto operation), and;
- Immature salars, which are dominated by clastic sediments, with (usually) limited thicknesses of halite. Examples are salar de Pastos Grandes, salar Pocitos and salar Olaroz/Cauchari.
Mineralization at salar de Pozuelos and Pastos Grandes consists of a lithium and potassium enriched brine of the SO4-Na-Cl variety. The brine is present throughout the salar as one or more aquifers separated by semi-permeable layers composed of sand/clay/halite mixtures. Borates (as ulexite) are primarily present within the upper 1 [m] – 2 [m] of the playa zone in the northern sector of the salar de Pozuelos basin. Borates at Pastos Grandes may be present in thin zones at almost any depth in the salar.
Sampling of brines at Pozuelos shows that the Mg:Li ratio is typically in the range of approximately 5:1 to 7:1, with lithium grades varying across the salar and with depth, but typically in the range of approximately 300 [mg/l] to approximately 600 [mg/l]. Brines at Pastos Grandes show relatively low surface values for lithum (100 [mg/l] – 300 [mg/l]) but can be locally higher. Samples recovered at depth show increased lithium values typically in the range of 300 [mg/l] to approximately 600 [mg/l]. The average Mg:Li ratio at Pastos Grandes is approximately 6.3:1.
Calcium and sulphate values at Pozuelos and Pastos Grandes are complementary, permitting blending of the brines to achieve efficient precipitation of undesirable salts.