The Mineralogy and Sulfur Isotopic Composition of Sulfates In Caves of Southwestern Romania

Over 30 caves are known to develop in the Jurassic and Cretaceous limestone that outcrops along the Cerna Valley in southwestern Romania. There are three aspects that strike when entering most caves on Cerna: the amount of guano deposits, the temperature, and the impressive sulfate deposits. The temperature anomalies are rather uncommon in ordinary cave environment. However, along Cerna Valley one can measure cave air temperatures (in some cavities) as high as 40 ºC. This situation is due to the presence of thermal water pooling or flowing through the caves or to the hot steam that rises along fractures from deeper thermal water pools. Some of the thermal waters are enriched in H₂S and therefore, occurrences of large gypsum and other sulfate deposits are somehow normal.

This preliminary study presents the results of ~57 mineral samples that were investigated by means of X-ray diffraction, scanning electron microscope, and electron microprobe analyses. In addition, oxygen and sulfur measurements (δ ¹⁸O and δ ³⁴S) on sulfate speleothems were performed with the scope of ruling out the origin of cave sulfates (i.e., vadose, hypogene, bacteriogenic, etc.). The minerals identified so far in Salitrari, Ion Barzoni, Diana, Adam, Despicatura, and Grota cu Aburi caves, are: calcite [CaCO₃], aragonite [CaCO₃], gypsum [CaSO₄•2H₂O], anhydrite [CaSO₄], pickeringite [MgAl₂ (SO₄)₄•22H₂O], halotrichite [Fe²⁺Al₂(SO₄)₄•22H₂O], kalinite [KAl(SO₄)₂•11H₂O], melanterite [FeSO₄•7H₂O], hydroxylapatite [Ca₅ (PO₄)₃(OH)], carbonate-hydroxylapatite [Ca₅ (PO₄,CO₃)₃(OH)], brushite [CaHPO₄•2H₂O], darapskite [Na₃ (SO₄)(NO₃)•H₂O], and nitratine [NaNO₃]. The phosphates were precipitated in a typical vadose environment, whereas nitrates and most sulfates are of bacteriogenic/ hypogene origin.

Based on our preliminary isotopic data, the gypsum from the Barzoni Cave shows to be dominantly hypogene in origin with a depletion in the heavy isotope of sulfur and a slight enrichment in oxygen isotopic values (δ ³⁴S (SO4) = -23 – -28‰ and δ ¹⁸O (SO4) = 0.2 – 4.3‰).