Geochemistry of brines in Vendian deposits of the Siberian platform

Results of the geochemical studies on natural brines from regional Vendian reservoirs (Lower-Danilov, Tira, Nepa and Vilyuchan) of the Siberian platform are presented. TDS values of the studied groundwaters and brines range from 17.3 to 583.1 g/dm(3), where brines with salinity 280 - 400 g/dm(3) are predominant. Brines identified within the Ayan, Bratsk, Balaganka, Upper Chona, Shamanka and other areas, have the highest TDS values.The most widely spread brines belong to five chemical types of Cl Na to Cl Ca composition. TDS concentrations vary widely from 197 to 580 g/dm(3) in the Lower Danilov and Tira horizons, and from 170 to 470 g/dm(3) in the lower-sitting Nepa and Vilyuchan horizons. Na Cl brines whose TDS concentration reaches the halite precipitation phase should be assigned to infiltrationformed brines on account of salt rock leaching, while strongly metamorphosed brines (S > 300) mostly of Cl and Ca Cl Ca-Mg composition - to sedimentogenic type accounting for those descended from Cambrian salt-bearing deposits, thereby largely explaining the phenomenon of inverse hydrogeochemical zonality in subsalt horizons. The third genetic type, of Cl Ca-Na composition, is rooted in the processes of mixing sodium and calcium types of brines.