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 been investigated by X-ray microanalysis of frozen primary inclusions trapped in halite. A Computer program based on thermodynamic equilibrium and mass balance principles has been used to simulate evaporation paths. The comparison between the results of calculations, the observed mineralogy and mineral sequence, and the solute content in fluid inclusions has placed constraints on the hydrological evolution of the basin. The upper Eocene basin of Navarra, southern Pyrenees, Spain, began as a marine basin, evolving from a moderate to a high degree of restriction, depositing first a basai anhydrite horizon, and then a thick sequence of massive halite. An additional inflow of CaCl 2 in the basin during seawater evaporation is proposed as the process responsible for the sulfate depletion required for sylvite instead of Mg-sulfates to form. Mixing of seawater with continental waters, bacterial sulfate reduction and "in situ" dolomitization are discarded. The basin subsequently closed to the sea and evolved with decreasing volume. Alternating bands of clays-halite-sylvite and then clays-halite-carnallite were deposited under the influence of seasonal continental recharge. Before reaching total desiccation the residual brine was diluted by continental water. The basin then evolved under an endoreic regime, where continental recharge and the recycling of previously-formed halite led to deposition of alternating beds of clays and halite.