Modern potash evaporites (carnallite-MgKCl 3 . 6H 2 O) occur at the Qarhan salt plain, Qaidam Basin, China. Field and petrographic evidence shows that carnallite formed at the surface on the margins of Dabusun Lake is an extensively modified ephemeral deposit. Carnallite preserved at depth (0-13 m) consists of coarse crystals formed as cement in primary halite layers and as displacive crystals in muds. Carnallite from the subsurface of Qarhan salt plain displays textures identical to ancient potash evaporites from the Permian Salado Formation, New Mexico and the Devonian Prairie Formation, Saskatchewan, Canada. The early diagenetic formation of carnallite on the northern shoreline of Dabusun Lake is probably a modern phenomenon, because 1) groundwater brines at or above carnallite saturation occur to depths of up to 17 m; and 2) brines trapped in fluid inclusions in primary halites (from 0 to 45 m depth) are well below carnallite saturation but become progressively more concentrated toward the modern surface. Fluid inclusion brines in the youngest halites (less than 1 m depth), reflecting modern surface brines, are the most concentrated lake waters observed. Sinking and cooling of these surface brines to temperatures consistent with measured temperature profiles produces carnallite supersaturated conditions because of the strong temperature control on carnallite solubility.