Isotopic composition of vein calcite and its fluid inclusions: Implication to paleohydrological systems, tectonic events and vein formation processes

The isotopic composition of calcite and the deuterium content of its fluid inclusions were determined in a Tertiary vein system that developed in a tectonically active region. The vein system is composed of three sets (I, II, III) with different trends and their relative age has been established by cross-cutting relations. Each set has its characteristic calcite δ¹⁸O- and δ¹³C-values: − 7.9 and + 1.59‰ for set 1; − 15.3 and + 0.54‰ for set II; and −15.9 and + 1.73‰ for set III, respectively, while fluid-inclusion δD of the oldest set (I, −23.99‰) resembles that of Israeli coastal plain rainwater and groundwater δD-values of the youngest set (III, −49.69‰) resemble rain- and groundwaters of Mount Hermon. We attribute the δD differences to the isotopic “altitude effect” and conclude that the major change in the hydrologic system was related to the mountain formation and the elevation of the Hermon region. Formation temperatures for set I (35°C) and set III (64°C) are calculated using the relationship between δ¹⁸O and δD of meteoric water and suggest that vein formation occurred at shallow depths (1.7 km) in this region. The calcite δ¹³C indicates that the country rock was the source of vein calcite and that δ¹³C was modified by ∼ 1‰ due to pressure release. A more general model for vein formation, based on the pressure dependence of calcite solubility and cycles of increasing pore pressure that cause hydrofracturing and pressure drop, is discussed.