Non-thermal sulphur springs from karstic aquifers are very rare in Slovenia. One of them that I investigated from geological and hydrogeochemical standpoint is Žvepovnik spring (Zgornja Savinjska Valley, NE Slovenia). To define the dolomitic aquifer I did geological mapping, microscopical analysis of thin sections and confirmed presence of pyrite along the discordant contact with X-ray diffraction. I also measured physico-chemical properties of spring water monthly and did two samplings for detailed chemical and isotopic analysis. Water belongs to the calcium bicarbonate type and the Ca2+/Mg2+ ratio shows aquifer is mostly dolomitic with possible calcite or gypsum present. Microscopical analysis proves that aquifer of Anisian age is dolomitic with some calcite and is discordantly overlain by Oligocene deep see environment marls. Presence of pyrite and hydrothermal dolomite indicates diagenetical changes, that are presumably related to volcanic hydrothermal activity. Saturation indices (SI) for calcite, dolomite and barite show slight oversaturation, but SI for gypsum and anhydrite shows strong undersaturation. ^13CDIC value around -12,5 ‰ indicates that dissolved inorganic carbon comes from weathering of carbonate by reaction with H2CO3, which was created by decay of organic matter in soil. ^2H and ^18O values fit well with the global meteoric water line and with values from nearby Berglez spring. Tritium content indicates mixing of recent and older water. Dissolved oxygen and Eh value also show signs of a deeper aquifer. ^34S in ^18O values in sulphate don’t provide an unambiguous answer to sulphur source. Isotopic composition complies with barite being the source. There could be a similar deposit to Lepa Njiva near the spring, under the discordant contact. Or the source could be gypsum in the deeper parts of the aquifer. Other two options are oxidation of highly enriched pyrite or enrichment of the residual sulphate in the solution as a result from reduction of pyrite with SRB (sulphate reducing bacteria).
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