Carnallite

Abstract: Modern potash salt deposits and associated brines of the Qaidam Basin, western China, demonstrate that some anomalous marine evaporites may have formed from nonmarine brines instead of seawater. Qaidam Basin brines are derived from meteoric river inflow mixed with small amounts of CaCl spring inflow similar in composition to many saline formation waters and hydrothermal brines. Evaporation of springenriched inflow yields a predicted mineral sequence including carnallite, bischofite, and tachyhydrite that is identical to several anomalous marine evaporites. Other mixtures of river and spring inflow produce the salt assemblage expected from evaporation of seawater.

Abstract: This Quaternary evaporite succession is little affected by time and burial, and provides unique evidence bearing on problems of the genesis of evaporites in general. At least 3200 feet of halitic evaporites were deposited in a basin formed by subsidence in a rift zone. A shallow barrier of volcano-tectonic origin prevented direct circulation of marine waters, limiting influx to the amount required to balance evaporation. The basin succession, from the base, is marine reef limestones, redbeds, gypsum-anhydrite and halite, and potash-rich strata. The upper potash interval consists of a basal zone rich in kainite, an intermediate carnallite zone, and an uppermost sylvite zone. The first two are considered to be primary, and the sylvite zone to be formed before burial by selective surface leaching of MgCl 2 from the carnallite by meteoric waters. The theoretically predictable kainite zone, absent in most older deposits, may be preserved here because of youth and shallow burial. Absence of the magnesium sulfate layer that should theoretically underlie the kainite zone in this and older deposits is, therefore, probably due to nondeposition rather than later removal, but the reasons are not understood. Mineral assemblages suggest temperatures increasing from 25 degrees C to 55 degrees C.