Polyhalite

Abstract: Topographically closed basins may be hydrologically open as a result of seepage losses to underlying or surrounding ground-water systems. In such cases, these losses can have a substantial control over the suite and the thicknesses of evaporite minerals formed in the basin. The ratio of ground-water outflow to inflow (flux ratio) in hydrologically open basins is as important in determining the mineralogy and thicknesses of evaporite deposits as the solute composition of the inflow water. Attainment of steady state flux ratios permits large thicknesses of two or three minerals to form rather than thin veneers of many minerals. The presence or absence of glauberite, mirabilite, halite, bloedite, polyhalite, and hexahydrite, caused by subtle changes in the ground-water seepage is illustrated using an example from the Southern High Plains of Texas and New Mexico. However, the model is general and is applicable with any solute composition including that of seawater and the use of surface rather than ground water. An analytical, lumped parameter, solute mass balance model is developed to define the concept of a ground-water flux ratio as it applies to topographically closed basins in which evaporation exceeds precipitation. Diffusion, advection, and density-driven flow are proposed as mechanisms by which solutes can escape to the ground water from these closed basins. The geochemical reaction computer program PHRQPITZ is used to document the effects of various flux ratios on the mineralogy and thickness of deposits. Solute analyses used in conjunction with the model can be used to screen prospective basins as well as to provide insights for exploratory drilling program.

Abstract: Waters in modern evaporite systems are marine, non‐marine, or hybrid but mineralogies in most ancient systems are not so simple that marine and non‐marine brines can be easily interpreted from the chemistry of their precipitates. Complications arise related to subsurface brine mixing and back‐reactions both at the surface and in the subsurface. The precipitation order of ancient bittern salts from seawater may have been dependent on flux rates of river inflow relative to flux rates through mid‐ocean ridges. In ancient continental systems the chemistry of the inflow waters was a fundamental control on the subsequent mineral paragenesis. Our lack of hydrogeochemical understanding of ancient evaporite systems has led to the ‘potash problem’. Potash evaporites, traditionally interpreted as marine salts, fall into two categories: (i) potash deposits characterised by MgSO4 salts, such as polyhalite, kieserite and kainite; and (ii) potash deposits characterised by assemblages containing halite, sylvite and carna...