SR ISOTOPE STUDY OF VEIN AND CAVE CALCITES FROM SOUTHERN ISRAEL

The strontium isotope compositions of secondary calcites from the Negev, southern Israel, were compared to those of the marine carbonate host rocks, which range in age from Triassic to Eocene, in order to understand fluid source and migration through fracture systems in dominantly carbonate strata. The Sr isotopes of these calcite clusters are divided into two groups: (1) calcites with 87Sr 86Sr values greater than those of the host carbonates; and (2) calcites with 87Sr 86Sr values close to the values of the host carbonate. 1. (1) These secondary calcites were found on the main tectonic lines of the Negev (faults and fold axes) and are enriched in 87Sr ( 87Sr 86Sr = 0.707709-0.709151) relative to the marine carbonate country rocks ( 87Sr 86Sr = 0.707251-0.70755, with one exception). These calcites are associated or crossed by thin veins filled by Fe- and Mn-oxides. 2. (2) Secondary calcites with 87Sr 86Sr values close to those of the marine carbonate country rocks (0.7073-0.7077) are found in karstic caves and veins, and are located in sites which are not on the major faults and fold axes. These calcites are not crossed by Fe- and Mn-oxides. The isotopic results indicate that the solutions from which the secondary minerals of the first group precipitated were not in isotopic equilibrium with the marine host rocks. The possible source of the precipitating solutions can be either surface rain descending through the fault system or ascending groundwaters from the deep Nubian aquifer (Paleozoic to Early Cretaceous in age). In both cases there would be a limited interaction with the host sandstone rock which usually is depleted in Sr. The similarity of the 87Sr 86Sr values to those of the host rocks in the second group suggests that the main source of Sr in these calcite crystals was from the dissolution of the marine carbonate country rocks by rain- and flood waters. © 1990.