GEOCHEMICAL CONDITIONS OF DEPOSITION IN THE UPPER DEVONIAN PRYPIAC' AND DNIPRO-DONETS EVAPORITE BASINS (BELARUS AND UKRAINE)

Abstract

Earlier studies of individual fluid inclusions in salt minerals of some Devonian evaporite basins showed that the brines in those basins were of chloride type with a high concentration of Ca2+. The genetic significance of this occurrence was subject to different interpretations. We have studied in detail samples taken from the East European Dnipro-Donets and Prypiac' evaporite basins aiming to establish the chemical composition of fluid inclusions in studied evaporites and then, using other geochemical information, to establish a geochemical model of the Devonian evaporite basins. Our study showed that salt deposition in the Devonian Dnipro-Donets and Prypiac' evaporite basins was because of intensive evaporation of mainly marine brines of chloride type with a relatively high Ca2+ content. In turn, the ratios between Na, K, and Mg corresponded to ratios in recent marine brines. Salt minerals precipitated in bottom water conditions. The occurrence of one-phase fluid inclusions indicates the temperatures of halite-precipitating brines <43°C. Gas content varied from 10 to 300 g/L and was controlled by the pressures existing at the basin bottom during halite growth. The recorded high Ca2+ contents, characteristic of brines in the Dnipro-Donets rift basin, is due to intensive discharge of connate highly mineralized Ca2+-rich solutions; however, the parent marine water was also Ca2+ rich.