Barite mineralization in association with calcitic speleothem precipitates in cave structures in Silurian Aeronian carbonate rocks in Kalana quarry, Central Estonia, was studied. Barite mineralization in Kalana occurs in two generations – euhedral bladed-tabular barite zonal crystals from a few to 10 cm in size, growing on the limestone-dolomite wall-rock (generation I), and sparsely placed thin tabular crystals a few millimetres thick and up to 1 cm in size, growing on calcitic crusts (generation II). The barite crystals of generation I are frequently found embedded by paragenetically later calcitic botryoidal crusts. The Sr and S isotopic composition of barite crystals shows a trend of increasing Sr isotope ratios (from 0.7114 to 0.7120) and δ34S values (from 13‰ to 33‰) from the central parts towards the edges of zonal crystals. This suggests barite precipitation by mixing of two endmember fluids at varying ratios during barite formation: warm (up to 70 °C) reducing fluid bearing Ba, characterized by an elevated radiogenic Sr- and 34S-enriched isotopic signal, and a cooler ambient fluid bearing an isotopically lighter dissolved sulphate, characterized by lower Sr isotope ratios. The excess of radiogenic 87Sr in barite compared to Phanerozoic seawater values suggests Sr derived from a continental source, whereas sulphate was derived either from oxidized H2S or a modified seawater source. Gradual increase in δ34S values towards the outer zones could also indicate the 34S enrichment due to bacterial sulphate reduction, even though there is no paired 34S and 18O enrichment of sulphate, characteristic of bacterial reworking. This can be interpreted as indicating an open system with limited sulphate resupply where the δ18O composition of sulphate was equilibrated with warm ascending hydrothermal fluid.
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