Summary:
The deposit type is a brine aquifer within a salar basin. Lithium is enriched in the aquifer brine as a result of evapo-concentration of small concentrations of lithium that enter the salar in fresh or brackish water – commonly associated with geothermal activity. Because lithium does not precipitate into a solid form while hydrated (unlike halite and other evaporite minerals), the brine becomes enriched in lithium over time.
Conceptual Model of Salar de Rio Grande
Based on the available information, Salar de Rio Grande appears to be a relatively mature salar. Basin margins are interpreted to be fault controlled. The margin of the basin is dominated by Volcanic units. Volcanic units are not known to occur in the basin. Depth to bedrock was interpreted to be more than 300 meters below land surface (mbls) based on vertical electrical sounding (VES) geophysical surveys (AMINCO, 2022a) but additional studies are required to define basement depth.
Evaporitic sediments saturated with brine were interpreted with variable thickness in the central and western areas of the salar. Fine-grained sediments also saturated with brine were recognized based on data obtained from the VES survey. In northern area of the salar evaporitic sediments saturated with brine are covered by coarse and fine grain sediments. In the northeastern area, coarse and fine grain sediments were recognized as likely alluvial deposits with a freshwater/saltwater interface.
The principal sources of water entering the Project area are from surface water coming into the basin from the basin margins. To date, surface water flow has not been formally measured.
Some groundwater inflow from natural recharge along the mountain fronts via alluvial fans is also believed to exist. In both cases, there appears to be limited mixing of the fresh water and brine in the basin due to density differences and is likely to be in the upper part of the aquifer. As a result, the fresh water entering the Project tends to stay in the upper part of the aquifer system on the edges of the basin, without moving to the center part of the salar. These freshwater discharge areas tend to support altiplanic vegetation. Evaporation of fresh water in the basin over time results in concentration of the dissolved minerals and ultimately results in brine generation.
In terms of fresh groundwater inflow, a freshwater thickness of about 13 meters was identified in the alluvial at north of the salar boundary (Sulfa X property). As conceptually expected, the fresh water was found in the upper part of the aquifer, on top a deep brine column.
Local Geology
One of the most important characteristics that define the Geological Province of Puna is the presence of evaporitic basins, where important deposits of borates, sodium sulfate, and brines with high lithium concentrations. The Rio Grande Salar occupies one of these endorheic (internally drained) basins. The oldest rocks that outcrops in the area are of Upper Permian age and correspond to La Tabla Formation/Llullaillaco Plutonic Complex, which is located in the northeast section of the area. The Tabla Formation/Llullaillaco Plutonic Complex is composed of porphyries, breccias, ignimbrites, and lavas of dacitic to rhyolitic composition with fine-grained dikes intrusions of dacitic to andesitic composition.
The Tabla Formation/Llullaillaco Plutonic Complex is covered by the Vizcachera Formation which is divided into a lower member composed of sandstone, pellites and red onglomerates, and upper member composed of medium to coarse conglomerates with intercalations of fine silt beds and sandy layers. The Vizcachera Formation outcrops in the east side of the basin and lies unconformably above the Geste Formation (which is not exposed in the immediate area) and unconformably with overlying formations.
The Permian and Miocene units are intruded by Lower and Upper Miocene stratovolcanoes that include dacitic-andesitic lava flows and intrusions of dacitic and rhyodacitic domes integrated by the Quebrada del Agua, Cori and Cave complexes located in the north of the geologic map.
In some areas Lower Miocene volcanic rocks are covered by Sijes Formation of Upper Miocene age, which is composed of fine to medium-grained sandstone and sandstone conglomerates. Outcrops of the Sijes Formation are distributed along the edges of the Rio Grande Salar and interrupted by pyroclastic flows and ignimbrites in the south border of the salar.
Lower and Upper Pliocene age volcanic rocks, including Archibarca ignimbrite are distributed in the southeast, south, and southwestern area of Rio Grande. The volcanics are represented by a set of eroded stratovolcanoes, porphyry lavas, dykes, domes, and pyroclastic deposits of andesitic and rhyodacitic composition.
Pleistocene Los Caletones ignimbrite is present in the form of an extended subhorizontal mantle, in the surroundings of the La Casualidad mine and it lays with unconformity on Lower Pliocene lavas. Pleistocene age basalts are present on the upper margins of the basin on the west side of Rio Grande. These are often associated with ignimbrites of similar age and overlay uncomformably on almost all the units that make up the regional stratigraphic column.
Quaternary sediments are abundant in the vicinity of the salar evaporitic crust. These sediments are forms alluvial-colluvial deposits represented by alluvial fans, wind-blown sands, and stream and valley sediments.
Mineralization
The mineralization for the project consists of a lithium-enriched brine that is contained within the pore spaces of the sedimentary strata formed by evaporitic processes within the salar basin. The boundaries of the mineralization are suspected to be the basin boundary, although some lithium-enriched brine may be contained in the fractures and/or pores of the older rocks that form the basin boundary. Distribution and chemical composition of the brine in the salar sediments is not currently known. The actual length, width, depth, and continuity of the brine-bearing aquifer is currently not accurately known; however, with exception of freshwater areas on the edges of the salar, NOA Lithium Brines anticipates lithium-rich brine to occur in most of the aquifer in the basin to effectively land surface.