THE CHEMICAL AND HYDROLOGICAL EVOLUTION OF AN ANCIENT POTASH-FORMING EVAPORITE BASIN AS CONSTRAINED BY MINERAL SEQUENCE, FLUID INCLUSION COMPOSITION, AND NUMERICAL-SIMULATION

The chemical evolution of the brine in a potash evaporite basin has be en investigated by X-ray microanalysis of frozen primary inclusions tr apped in halite. A computer program based on thermodynamic equilibrium and mass balance principles has been used to simulate evaporation pat hs. The comparison between the results of calculations, the observed m ineralogy and mineral sequence, and the solute content in fluid inclus ions has placed constraints on the hydrological evolution of the basin . The upper Eocene basin of Navarra, southern Pyrenees, Spain, began a s a marine basin, evolving from a moderate to a high degree of restric tion, depositing first a basal anhydrite horizon, and then a thick seq uence of massive halite. An additional inflow of CaCl2 in the basin du ring seawater evaporation is proposed as the process responsible for t he sulfate depletion required for sylvite instead of Mg-sulfates to fo rm. Mixing of seawater with continental waters, bacterial sulfate redu ction and ''in situ'' dolomitization are discarded. The basin subseque ntly closed to the sea and evolved with decreasing volume. Alternating bands of clays-halite-sylvite and then clays-halite-carnallite were d eposited under the influence of seasonal continental recharge. Before reaching total desiccation the residual brine was diluted by continent al water. The basin then evolved under an endoreic regime, where conti nental recharge and the recycling of previously-formed halite led to d eposition of alternating beds of clays and halite.