Summary:
Lithium is found in a number of different geological deposit types. The most common are pegmatite bodies, associated with granitic intrusive rocks, and continental brines in salt lakes (salars).
Continental lithium brines in salars settings (salt lakes) are found principally in Argentina, Chile, Bolivia and China, with lithium carbonate or lithium chloride produced from these projects. Lithium is rarely found in continental oilfields, where the accompanying produced water is enriched in lithium, probably deriving lithium from evaporite sequences in the stratigraphy.
Lithium brine projects can also be subdivided into two broad ‘deposit types’ with different characteristics (Houston et. al., 2011), which consist of:
• Mature salars (those containing extensive thicknesses – up to hundreds of meters - of halite (salt), such as the Salar de Atacama (Chile), and the Livent Hombre Muerto operation (northern Catamarca, Argentina); and
• Immature salars, which are dominated by clastic sediments, with limited thicknesses of halite, such as the Olaroz salar in Jujuy Argentina and the Silver Peak deposit in Nevada, USA, where brine is extracted from porous volcanic ash units.
Immature salars
Immature salars conversely have brine hosted in pore spaces controlled by the porosity and permeability associated with individual layers within the salar sequence. A degree of compaction occurs with increasing depth below surface, but unlike in mature samples significant porosity and permeability characteristics may continue to depths of hundreds of meters in these salars (such as the producing Olaroz salar and the adjacent Cauchari salar in Northern Argentina and at the Silver Peak lithium brine mine in Nevada).
The porosity and permeability characteristics may be variable between units, and units with low productivity for brine extraction can alternate with more productive units, due to differences between sediments such as sand and gravel and finer grained silts and clays. The presence of different stratigraphic units in clastic salars typically results in differences in the distribution of the contained brine and influences the recovery of brine as reserves from the defined brine resource, with lower resource to reserve conversion ratios than are typical in hard rock mining situations. It is very important to consider the characteristics of the host aquifers in each salar, together with the aquifer geometry and physical properties, particularly specific yield and specific storage hydrogeological characteristics.
Buried salars
Salars contain sequences of sedimentary deposits with clastic sediments (clay, silt, sand, gravel) and evaporites (principally salt). These sediments progressively accumulate and the surface of the salar consists of salt or fine sediments such as clays. In some cases, due to changes in climate or tectonic events salars are buried by alluvial fan sediments prograding from the margins of basins. In extreme cases salars may be entirely covered by alluvial fan sediments, such that there is no salar surface in the middle of a closed drainage basin. However, brine can remain in place in the sequence of salar or clastic sediments beneath the alluvial fans which will often contain fresh to brackish water.
The Olaroz Project contains buried targets beneath the Archibarca alluvial fan in the southwest of the basin and in the north of the basin, where AMT electrical geophysics suggests the presence of brine beneath the Rosario Delta. These areas off the surface of the salar have not yet been explored at Olaroz, but are likely to contain significant volumes of brine in addition to that defined directly below the surface of the salar.
Geology & Mineralisation
The Olaroz salar is located in the elevated Altiplano-Puna plateau of the Central Andes. The Puna plateau of north-western Argentina comprises a series of dominantly NNW to NNE trending reverse fault- bounded ranges up to 5,000-6,000 m high, with intervening internally drained basins at an average elevation of 3,700 m. High evaporation rates, together with reduced precipitation, have led to the deposition of evaporites in many of the Puna basins since 15 Ma, with borate deposition occurring for the past 8 Myr. Precipitation of salts and evaporites has occurred in the centre of basins where evaporation is the only means of water escaping from the hydrological system.
Mineralization in the Olaroz salar consists of lithium dissolved in a hyper-saline brine, which is about eight times more concentrated than seawater. The lithium concentration is the product of the solar evaporation of brackish water which flows into the salar as groundwater and occasional surface water flows. The concentrated brine with lithium is distributed throughout the salar in pore spaces between grains of sediment. The brine also extends a considerable distance away from the salar, beneath alluvial gravel fans around the edges of the salar. These areas are largely unexplored by the company to date. In addition to lithium, there are other elements, such as sodium, magnesium, and boron, which constitute impurities that are removed in the ponds and processing plant.
Brine projects differ from hard rock base, precious and industrial mineral projects due to the fluid nature of the mineralisation. Therefore, the term ‘mineralisation’ should be considered to include the physical and chemical properties of the fluid (brine), as well as the flow regime controlling fluid flow.
The brines from Olaroz are solutions nearly saturated in sodium chloride with an average concentration of total dissolved solids (TDS) of 290 g/L and average fluid density of 1.20 g/cm3. In addition to extremely high concentrations of sodium and chloride typical in these salar settings the Olaroz brine also contains significant concentrations of Li, K, Mg, Ca, Cl, SO4 and B.
The Olaroz salar is large and the brine is rather homogeneous, although there are some trends in the concentrations of lithium and other elements through the salar sediments. Brine concentrations are lower close to the margins of the salar and in areas where there is significant recharge by runoff. The Mg/Li ratio averages 2.3, with the SO4/Li ratio averaging 23.
Brine quality is evaluated through the relationship of the elements of commercial interest lithium and potassium and the consideration of other elements that must be removed to provide a high-quality lithium product. Other components of the brine constitute impurities, including Mg, Ca, B and SO4.