François Risacher

The origin of brines and salts in Chilean salars: a hydrochemical review

Abstract Northern Chile is characterized by a succession of north–south-trending ranges and basins occupied by numerous saline lakes and salt crusts, collectively called salars. Fossil salt crusts are found to the west in the extremely arid Central Valley, while active salars receiving permanent inflows fill many intravolcanic basins to the east in the semiarid Cordillera. Sea salts and desert dust are blown eastward over the Cordillera, where they constitute an appreciable fraction of the solute load of very dilute waters (salt content Evaporative concentration of inflow waters leads to sulfate-rich or calcium-rich, near-neutral brines. Alkaline brines are almost completely lacking. The alkalinity/calcium ratio of inflow waters is lowered by the oxidation of native sulfur (reducing alkalinity) and the deposition of eolian gypsum (increasing Ca concentration). Theoretically, SO4-rich inflow waters and their derived SO4-rich brines should be found in the intravolcanic basins of the Cordillera because of the ubiquity of native sulfur, while Ca-rich brines should prevail in sedimentary basins where Ca-rich minerals are abundant. This relation is perfectly observed in the salar de Atacama, the largest in Chile. However, several salars located within the volcanic Cordillera belong to the Ca-rich group. Inflows and brines may have acquired their Ca-rich composition in Pleistocene time when their drainage basins were mainly sedimentary. Later on, recent lava flows and ignimbrites covered the sedimentary formations. Underground waters may have kept their early sedimentary signature by continuous recycling. However, the weathering of volcanic rocks tend to slowly shift the water compositions from the Ca-rich to the SO4-rich type.

Authigenic smectite on diatom frustules in Bolivian saline lakes

Abstract Observation under the electron microscope of diatom frustules from Bolivian Altiplano saline lakes shows that many of these are coated with particles occurring as tiny sheets. The frustules can be found to be almost completely replaced by these sheets. Isolated sheet aggregates seem to have resulted from completely transformed frustules. Section observations of altered frustules bear out that the sheets have grown from biogenic silica through replacement. Selected area diffraction, dark field observation, microdiffraction, and elemental microanalysis show that the particles on the diatom frustules consist of a poorly crystallized MG-smectite. The unambiguous localisation of this authigenesis allows us to reconstruct its hydrochemical and sedimentological environment. Observation of the most recent lake sediments has pointed out that at least two main conditions are required for this authigenesis at 5°C: saturation with respect to amorphous silica, and a pH above 8.2. Variations in the Mg concentration have no significant effect.

Geochemistry of Bolivian salars, Lipez, southern Altiplano: Origin of solutes and brine evolution

Abstract 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-HCO 3 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 SO 4 , 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-(SO 4 ) 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.

Geochemical evolution of brines in the Salar of Uyuni, Bolivia

Recent analyses of brines from the Salars of Uyuni and Coipasa have been compared with published chemical data for Lakes Titicaca and Poopo in order to evaluate solute compositional trends in these remnants of two large Pleistocene lakes once connected by overflow from the northern to the southern part of the Bolivian Altiplano. From Titicaca to Poopo the water chemistry shows an increase in concentration of Cl and Na somewhat greater than the total solutes, suggesting preferential inputs of these elements or significant losses of other constitutents downstream from Lake Titicaca proper. In contrast, Ca and SO4 increase to a lesser extent than do total dissolved solids, and carbonate species are relatively constant, suggesting solute losses most readily related to the precipitation of calcite and gypsum. Between Poopo and Coipasa the proportions of Ca, SO4 and CO3 continue to decrease. At Coipasa and Uyuni, the great salars frequently evaporate to halite saturation and widespread halite crystallization is accompanied by an increased proportion of K, Mg and SO4 in residual brines. Notably high concentrations of Li and B in brines from the south end of Uyuni, near the mouth of the Rio Grande de Lipez, are clearly in excess of that expected for the solute concentration trends for inflow from Coipasa and the northern Bolivia Altiplano, as shown by comparison with Mg and K. This supports other indications that the major source of Li and B are the recent rhyolitic volcanic rocks prevalent in the drainage of the Rio Grande.

Hydrochemistry of two adjacent acid saline lakes in the Andes of northern Chile

Numerous closed-basins in central Andes contain saline lakes and salt crusts (salars). Almost all of them are neutral or alkaline lakes. Only two lakes out of 84 studied in Bolivia and Chile are acid lakes. They are located in two adjacent intravolcanic basins in northern Chile. The origin of acidity is due to the juxtaposition of two factors: the very strong hydrothermal alteration of volcanic rocks in the drainage basins and the high sulfur content in the whole area. Primary minerals are replaced by amorphous silica, limonite, chlorite, white mica and elemental sulfur. The buffer capacity of the volcanic rocks has been considerably lowered and is no longer sufficient to neutralize the sulfuric acid generated by oxidation of native sulfur. Dissolution modeling of chlorite, white mica and sulfur in a neutral inflow water leads to a composition very much like that of the acid inflows. The modeled acid solution is oversaturated with respect to alunite by several orders of magnitude, exactly as observed in natural acid inflows. Some of the alunite reported in the altered zones of the drainage basins could be supergene in origin. The simulation of evaporation of acid inflows shows a good agreement between salar brines and computed brines, indicating that the acidic brines do not originate in the past hydrothermal activity, but result from recent supergene processes: the leaching of the severely altered volcanic rocks and the direct evaporation of the resulting acidic waters. Aluminium shows a conservative behavior in both salars reflecting the drastic lowering of the saturation index of Al-sulfate minerals in evaporating acid brines, which is confirmed by the lack of detection of alunite within both salars.

A computer program for the simulation of evaporation of natural waters to high concentration

Abstract EQL/EVP is a FORTRAN 90 program that simulates the evaporation of dilute waters as well as concentrated brines. The code calculates stepwise the composition of the evaporated solution and the amounts of precipitated minerals. Activity coefficients are based on the Pitzers interaction model which allows calculation to high ionic strength. The Newton–Raphson method is used to solve a set of linear mass-balance and non-linear mass-action equations. The simulation may be carried out in equilibrium mode where minerals are allowed to redissolve into the solution or in fractional crystallization mode where minerals are removed as they precipitate. Temperature dependence of various parameters and mineral solubility products are tentatively included between 0 and 50°C. The code processes all invariant points: those where the activity of water is constrained by several minerals and the invariant end-points. The code may be a useful tool for understanding hydrological and geochemical processes in arid regions.