Isotopic evidence of the origin of mineralized waters from the Central Carpathian Synclinorium, SE Poland

Abstract

Stable isotopes of hydrogen and oxygen were determined in 45 samples of water (27 samples of oil-associated waters, 17 samples of mineral waters used by spas, 1 sample of surface river water) from the Central Carpathian Synclinorium, covering a stratigraphic range of flysch sediments from Upper Cretaceous to Oligocene. Moreover, oxygen isotope compositions of authigenic calcite (vein and cement) from core samples of four boreholes were made to evaluate isotopic equilibrium between waters and diagenetic carbonates as a function of temperature. The saline and brackish waters (TDS from≅1 g/l to 48.9 g/l) considered here, generally belong to four hydrogeochemical classes: Na-Cl, Cl-HCO3-Na, HCO3-Cl-Na and HCO3-Na. Their isotopic composition causes them to fall to the right of Global Meteoric Water Line (GMWL) showing enrichment in 18O and 2H. On the other hand, relative to Standard Mean Ocean Water (SMOW) they are depleted in 2H and both depleted and enriched in 18O. The observed isotopic composition can be explained by the three-component mixing of surface water, diagenetically modified sea water (kind of connate water) and metamorphic water. The mixing is accompanied by an exchange of oxygen isotopes between water and carbonate cements causes 18O enrichment of interstitial waters. The contribution of isotopic exchange between water and clay minerals in shales was evaluated only theoretically basing of the literature.