Hercules and Diana Hypogene Caves (Herculane Spa, Romania): Dissimilar Chemical Evolutions Documented by their Present-Day Thermal Water Discharges

Hercules Cave and Diana Cave are two small caves situated some 600 m apart, for which a hypogene origin has been previously suggested. Thermal water discharges (up to about 54°C) are hosted by each cave, and were recently made the object of an eight-month chemical monitoring operation. By plotting the concentrations of several chemical species (Na, K, Ca, Sr, Mg, SiO2, Br) against the concentration of the conservative anion Cl, it was found that both caves discharged a binary water mixture, which was derived from the same saline (and hot) parent-fluid and the same fresh (and cold) parent-fluid. This fact was substantiated mainly by the conservative behavior that Na and Br displayed both in Hercules and Diana Cave’s spring waters. Alternatively, the original water mixture appears to have systematically experienced, before reaching the sampling point in Diana Cave, significant depletions in terms of K, Ca, Sr, and SiO2, while no such depletions were recorded in Hercules Cave. Inferred depletion mechanisms include K+-Na+ cation-exchange and Ca, Sr and SiO2 precipitation in minerals. Both processes could be favored by the possibility that upstream Diana Cave, water flow occurred slowly and in a diffuse manner through a network of cracks (likely across the shaly Iuta Layers). While the absence of analogous depletions at Hercules Cave was probably due to the fact that an extended network of penetrable passages presumably extended upstream the presently known length of that cave, such an underground environment is unlikely to favor precipitation of minerals or cation-exchange processes.