Summary:
The Carachi Pampa basin is an arid, closed basin comprised of interbedded lacustrine and alluvial sediments of gravels, sands, silts, and clays, with episodic volcanic deposits of ignimbrites, tuffs, and basalts. The basin is bounded to the east and west by north-south trending mountain ranges formed by thrust faulting exposing basement sequences in outcrops that rise to an elevation of about 5,100 m amsl. The Cerro Blanco pyroclastic complex is located on the south of the basin and is the primary source of the pyroclastic flows that deposited the ignimbrites and tuffs, while the Antofagasta de la Sierra and the Cerro Galan volcanic complex form the highlands in the north and northeast borders of the basin. The ranges to the east are composed of crystalline pre-Cambrian basement that gently slopes down to the basin floor. Red bedded sandstone and claystone sequences of the Geste and Patqia de la Cuesta Formations outcrop in the Los Colorados Range along the western edge of the basin. Extensive alluvial fan deposits were formed to the north, south, east and west of the central salar, as coarse-grained, high-energy sediments were shed from the nearby steep terrains. Altogether the basin drains a watershed area of 9,494 km2.
The center of the basin is dominated by the Quaternary basalt flows and the cider-cone of the Carachi Pampa Volcano. The volcano penetrates basin sediments to the east of the salar, with flow and air fall basalts creating a veneer over the lacustrine sediments. The volcano has a northwest-southeast striking fissure vent that is interpreted to be underlain by a northwest-southeast aligned intrusive dyke or plug of much smaller dimensions than the basalt cone has at the surface.
The salar sediments are predominantly intercalated sands and clayey silts, which constitute a leaky aquifer, with the entire sequence of sediments potentially contributing brine flow to wells. Higher brine flows are obtained from intervals with high sand content and higher permeability, with the brine grades generally comparable between geological units. The salar is surrounded on all sides by alluvial and aeolian fans of varying dimensions and significance. Most important are the Western Fan Complex and the South Fan that intercept coarse-grained sediments that contain lithium bearing brines. The North Fan is also important due to the presence of both coarse-grained sediments containing lithium bearing brines and a substantial freshwater aquifer. This freshwater overlies the lithium bearing brine.
The known sediments within the salar consist of a thin (several metre thick) salt/halite surficial layer, with interbedded clay, sand and silt horizons, accumulated in the salar from terrestrial sedimentation and evaporation of brines.
Brines within the Salt Lake are formed by evapoconcentration, interpreted to be combined with warm geothermal fluids, with brines hosted within sedimentary units.
Geology was recorded during the diamond drilling and from chip samples in rotary drill holes.