Controls of the Chemical Composition of Geothermal Waters

Abstract

Studies of numerous hot water geothermal systems have demonstrated that geothermal waters approach equilibrium with either primary minerals of rocks or with secondary minerals resulting from water-rock interaction. Simple calculations of the composition of water at equilibrium can be performed by using the concept of complete mineral association : the number of minerals is equal to the number of major inert elements in the solution. For each element, the ratio MeH (= a(Mez+)/[a(H+)]z) is fixed. In the temperature range 0–300°C, in the pressure range 0–1 kbar and for mineral commonly encountered in geothermal systems, MeH depends strongly on temperature, is almost independent of pressure, and depends only slightly on the nature of the minerals. Concentrations of inert elements are highly affected by the amount of mobile elements present in the solution. When this amount is high, variations in inert ions concentrations are directly related to the electric charge of the ion : elevated concentrations of Cl, for instance, increase tri- or divalent cation concentrations more than monovalent cation concentrations and decrease anion concentrations. Trace element behaviour is dominated by partition processes : dissymetry between dissolution (without trace-major fractionation) and precipitation (with fractionation) can be used to evaluate the extent of dissolution of primary minerals and precipitation of secondary minerals.