The Evolution of Closed-Basin Bines - 1970

Mining & Minerals - Evaluations / Critical Mineral Resources

Hardie and Eugster reported in 1970 that the evolution of closed-basin brines has been investigated with the aid of a model based on evaporative concentration in equilibrium with the atmosphere. Water compositions are restricted to Si02, Ca, Mg, Na, K, HCO" CO" SO. and Cl. Evaporation is simulated by a computer program and saturation tests are performed with respect to calcite, sepiolite and gypsum. When saturation occurs, the solids are removed from interaction with the brine. Calculations are terminated at an ionic strength near five.

Sixty-seven subsurface and many closed basin inflow waters have been subjected to the calculations. The calculated brines exhibit the same compositional diversity as the natural brines. However, very few generalizations can be made with respect to brine compositions derived from different rock types. Examination of the detailed mechanisms which determine the eventual fate of a particular water showsthat the critical decisions are made very early in the evaporative sequence, at a stage when the calculations are still accurate. Calcite is always the first phase to appear, and it separates carbonate-rich from carbonate-poor brines. The appearance of sepiolite complicates matters and may cause some paths to change direction. Gypsum precipitation also represents a crucial divide.

A flow sheet for brine evolution emerges from the examination of the mechanisms. It relates the four major brine groups to the evolutionary steps. Group a brines (Na-CO,-SO₄-Cl brines) can be obtained by precipitation of calcite and sepiolite. Group b (Na₂SO₄-NaCI) and Group c brines (Na-Mg-Ca-CI) form by calcite, sepiolite and gypsum precipitation, the choice depending upon the calcium-to-sulfate ratio at the time of gypsum saturation. Na-Mg-SO₄-Cl brines (Group d) can only be obtained by retarding sepiolite precipitation, for instance, through using low silica values. Because of the multiple equilibria involved, simple inspection of a water analysis does not reveal its eventual fate upon evaporation.

Calculations are not yet feasible with respect to concentrated brines. Therefore, the fate of concentrated brines has been analyzed by standard graphical procedures based on experimental determinations of brine equilibria. Three janecke projections are constructed and they define the last stages of evaporation for any specific brine. The validity of the model has been tested on several closed basins: Deep Springs, Lake Magadi, Saline Valley, Great
Salt Lake and Abert Lake. The correspondence between calculated and natural brine compositions is generally very good. In fact, the model has provided new insights into the hydrology of some of these basins. The chief limitations of the model are related to the silica removal mechanism, to the artificially fixed Na/K ratio and to the compositional restrictions imposed by the model.