Sedimentary model of marine evaporites and implications for potash deposits exploration in China

Currently, no large-scale economic resources of soluble in situ potash ore have been found in China in marine strata. The marine evaporites of China mainly occur in deeply buried, older geological strata in the Sichuan, Ordos, and Tarim basins, which make it difficult to predict prospective potash deposits. Based on the analysis of facies and the unique geological background of the original small-scale plate-tectonic terrains where evaporite-bearing marine strata were deposited, we established depositional models for epeiric evaporites and the formation of K+-rich brines. These models imply that the ancient marine strata are not favorable for the formation of the large-scale economic in situ potash deposits because the pre-Late Triassic tectonic plates that later aggregated to form modern China were mainly separate isolated terrains in the paleo-ocean. On such small blocks, the episodes of epeiric seas did develop evaporite precipitation from high-salinity bottom waters below the central portions of semi-restricted basins through the classic centripetal enrichment mechanism of seawater, in which a vertical stratification leads to a downward increase in the concentration of halogen ions. However, in contrast to most massive evaporite deposits that formed on larger tectonic plates, the typical basins on these small terrains did not have a large enough radius to concentrate brine ions to the same degree. Therefore, the precipitation of potash minerals from the saline brines did not attain adequate saturation for an extended time to form commercially economic in situ potash evaporites. We conclude that the probability of discovering an in situ commercial-quality deposit of marine potash deposit is very unlikely in China. However, potassium ions of concentrated seawater could be hosted within permeable subsurface lithologies in the form of an initial K+-rich residual brine. Potentially, such an initial K+-rich brine that was retained within pores in marine evaporites and carbonates could migrate into appropriate traps under the action of the geo-hydrodynamic field to produce a secondary K+-enrichment in fluids. A related dual-step K+-enrichment mechanism by geofluids often occurs in marine petroliferous basin, which results in a coupling of the hydrocarbon and K+-rich brine accumulations. Therefore, we suggest that future exploration for economic potash concentrations within China's basins should be targeted toward potential K+-rich brines that may reside within traps of permeable carbonate reservoirs.