FOCUSED GROUNDWATER FLOW AND THE ACCUMULATION OF GIANT SALT DEPOSITS IN THE SALAR DE ATACAMA

The Salar de Atacama basin, the largest of the pre-Andean basins in Northern Chile, has accumulated giant deposits of halite since 5-10 Ma, while the neighboring Calama and Tamarugal basins accumulated mostly siliciclastics. The geochemistry of the water, surface and ground, which feeds the Salar de Atacama is similar to analyzed waters from the Calama and Tamarugal basins. We conclude that structural controls on drainage are responsible for NaCl accumulation in the Salar de Atacama, whereas Na and Cl remain in solution and flow through the neighboring basins.

Elemental concentration and isotope geochemical values indicate that a variety of rocks and weathering mechanisms are sources for the constituents of the Atacama salar evaporites. The composition of delta Cl-37 for inflow waters and brines (-0.4 to +0.5 per mil) are consistent with a major Cl source in Paleozoic/Mesozoic marine sediments that underlie large areas of the Altiplano. However, dissolution of buried marine evaporites or leakage of trapped brines is not consistent with our preliminary delta Li-7 data, which are much lower than seawater (< +10 versus +32 per mil). The delta Li-7 compositions suggest Li extraction from volcanic and basement rocks (from Sr isotope compositions), in part at elevated temperatures.

Assuming we have comprehensive chemistry data appropriate to inflow waters and steady state, mass balance reveals that the Salar de Atacama halite volume cannot be accounted for by all precipitation in the catchment. The measured dissolved Cl contents, the Cl concentrations of likely source rocks, and the volume of deposits can be reconciled by groundwater input focused from a much broader area of the Andean highlands, as illustrated by a simple flow model.