ORIGIN OF CHLORIDE GROUNDWATERS AND BRINE S IN CRYSTALLINE MASSIFS - EVIDENCE FROM THERMODYNAMIC MODELING OF GEOCHEMICAL PROCESSES IN WATER-GRANITE SYSTEMS

Thermodynamic modeling of geochemical processes involved in interactio ns between waters of varying chemistry and granite was conducted. The results indicate that interactions between pure water and granite that hats normal abundances of chlorine can produce mineralized (containin g up to 50-100 g/l total dissolved solids [TDS]) Cl-Na(Ca) waters and brines owing to (1) an increase in solid : liquid ratios and the rewor king of large amounts of crystalline rocks by groundwaters and (2) los s of water molecules due to secondary mineral formation. This explains the occurrence of chloride waters of the first continental type (with a salinity of up to 100 g/l TDS) within most massifs of crystalline r ocks. Geochemical interactions between pure water and granite are not enough to account for the formation of high-salinity (over 200 g/l TDS ) Cl-Ca brines. In order for them to form, there must be an inflow of initial brines (e.g., of Cl-Ca, Cl-Na, and Cl-Mg composition) from the sedimentation basins around that shield. There is a fundamental diffe rence between the geochemical effect of water-rock interactions in the rmodynamically closed and open (with respect to CO2) systems. Under cl osed-system conditions, geochemical processes favor the precipitation of Ca and lower the concentrations of Na, K, and, particularly, Mg. In systems that are open with respect to CO2, geochemical processes favo r the dissolution of Na and the precipitation of Ca and Mg. These find ings provide new insights into the formation of the geochemical zoning of brines in the crust. The more closed a particular hydrogeochemical system is with respect to CO2, the higher the probability that it wil l produce Ca-type brines. Our results do not rule out the existence of other factors contributing to the formation of chloride waters and br ines in crystalline massifs. The main point of this paper is that geoc hemical interactions with crystalline rocks in favorable geochemical a nd hydrogeochemical situations can play a certain role in the formatio n of chloride waters and brines within crystalline shields so that the re may be no need to search for an external source of chloride.