HYPOGENIC CAVES IN SW ROMANIA: A MINERALOGICAL AND STABLE ISOTOPE APPROACH

Over 150 caves are known to develop in the Jurassic and Lower Cretaceous limestone that outcrops downstream Cerna Valley and its tributaries in southwestern Romania. Except for Salitrari Cave (~1.5 km in length), most of these cavities are rather short and less complex in terms of morphology and mineral variety. The Cerna Valley (especially in, and around the city of Băile Herculane) is well known for its thermal springs that were used for cure ever since Roman times. Thermal water pooling or flowing through some of the caves or hot steam rising along fractures from deeper thermal water pools causes the cave air temperatures to be as high as 45ºC. In addition, the thermal waters in a number of these caves are H2S-enriched and therefore, occurrences of large gypsum and other sulfate deposits (mainly in the form of crusts) are somehow normal. Bat colonies and associated guano deposits are common in most caves of this region. The most unique one is Adam Shaft, which hosts over 10,000 bats and the temperature inside is ~45ºC with a relative humidity of 100%. This presentation highlights the results of our X-ray diffraction, scanning electron microscope, and electron microprobe analyses investigations. In addition, stable isotope measurements (d34S) on sulfate speleothems were performed. The minerals identified in the investigated caves, are: calcite, aragonite, gypsum, anhydrite, pickeringite, halotrichite, apjohnite, kalinite, tamarugite, chalchantite, apatite-(CaOH), brushite, darapskite, and nitratine. The phosphates were precipitated in a typical vadose environment, whereas nitrates and most sulfates are of a more complex origin. The Barzoni Cave (upper part of the valley) shows the lowest d34S values (-23 – -28‰), whereas gypsum from Sălitrari and Aburi caves (middle and upper lower section, high elevation) display a wide range of d34S values (from -19.8 to +6.5‰). The situation changes completely in caves that open close to the Cerna Valley (Despicătura and Diana) where enriched sulfur isotopic values were obtained (11 to 19‰). The range in values reflects the increase in completeness of the reduction of sulfate. However, these values also suggest that the resulting cave sulfate isotopes depend not only on the source of the sulfur but also on the isotopic value of the reduced sulfide.