GEOCHEMISTRY OF BOLIVIAN SALARS, LIPEZ, SOUTHERN ALTIPLANO - ORIGIN OF SOLUTES AND BRINE EVOLUTION

The southern Bolivian Altiplano is a volcanic area which contains numerous undrained basins occupied by playas and saline lakes, locally named salars. Solutes carried by springs and rivers into the salars originate mostly from the alteration of the volcanic rocks and the re-solution of ancient buried evaporites. Both weathering and hydrothermal alteration lead to Na-HCO3 inflow waters of similar composition. High contents of lithium and boron are not specifically related to hydrothermal activity. Evaporite leaching leads to Na-Cl inflow waters of higher salinity. Atmospheric precipitation contributes only a small amount of Ca and SO4, and very little Na or Cl. The calculated evaporative evolution of the inflow waters shows that about half of them should become highly alkaline brines and the other half should end as near-neutral brines. However, alkaline soda lakes are rare in this region. Most lake brines are of the Na-Cl and Na-Cl-(SO4) types. To understand this anomaly one characteristic sequence of evaporating waters was studied in detail. Precipitation of calcite and Mg-smectites accounts only partially for the reduction of alkalinity. A further loss of alkalinity is probably related to the oxidation of wind-blown native sulfur in peripheral ponds. Wind-blown sulfur is also deposited around the lakes, on the drainage basins. In the rainy season, ephemeral streams carry along sulfur particles in permanent pools of the preconcentration areas. A hydrologic control of brine chemistry is suspected. Inflow waters, that should become alkaline, appear to evolve to neutral brines if they remain in contact with atmospheric dust and peripheral colluvium for a long time. On the contrary, inflow waters to soda lakes are generally close to the shore and have only minimal contact with atmospheric dust and peripheral sediments. Presently, all lakes are at relatively low levels, and inflow waters often drain an important area of the drainage basin before reaching the topographic low. In the past, lakes were at higher levels and less interaction occurred between inflow waters, atmospheric dust, and peripheral sediments. As a result, the chemistry of some lakes may have changed during the Quaternary.