Late Cretaceous potash evaporites in Savannakhet Basin of middle Laos: Geochemical evidences of non-marine inputs

Upper Cretaceous Nongbok Formation potash evaporites in Savannakhet Basin of middle Laos are the important materials for studying the regional evolution of ore-forming brines. The contents of trace elements in salt minerals together with their change characteristics reveal that the source of these salts was from the seawater but the supply of deep-seated water was also a significant source. The study results show that the so-called "basal" anhydrite only were the residues of primary halites which were leached by the late water-rock reaction. Slightly negative delta D and delta(18) O values of hydration water and marine nature of Br in carnallites indicate that these minerals were the products evaporated from the concentrated seawater mixed with meteoric water and that the continental water exerted influence to some extent on the evolution of ore-forming brine. These evaporites in the lower section of Nongbok Formation are divided into three different evolutionary stages: (1) Stage of marine input: the seawater entered into salt basin and voluminously hakes deposited; the transgression is characterized by the early obvious fluctuation and the late stabilization; the transition of marine to nonmarine input occurred at latest stage and then continental inputs was gradually dominant for the hydrologically closed and restricted salt basin; the components of ore-forming brine were changed by input of deep-seated fluid. (2) Stage of nonmarine input and mixed brine evaporation: the potash deposits formed during progressive evaporation; a slightly freshening event of brine happened at the early stage. (3) Stage of post-depositional reworking: the buried carnallitite was translated into the sylvitite by leaching of ground water and therefore the sylvite is the secondary mineral which substituted for precursor carnallite. Systematical analyses of trace element, oxygen and hydrogen isotope show that the nonmarine inputs played an important role during the evolution of ore-forming brine post-depositional reworking.