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.