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Geological Society, London, Special Publications; 2007; v. 285; p. 265-273;
DOI: 10.1144/SP285.15
© 2007 Geological Society of London

Ancient basins

Sulphur isotopic composition of K–Mg sulphates of the Miocene evaporites of the Carpathian Foredeep, Ukraine

S. Hryniv1, J. Parafiniuk2 & T. M. Peryt3

1 Institute of Geology and Geochemistry of Combustible Minerals, National Academy of Sciences of Ukraine, Naukova 3a, 79053 Lviv, Ukraine (e-mail: Sophia_Hryniv{at}ukr.net)
2 Institute of Geochemistry, Mineralogy and Petrology, University of Warsaw, Zwirki i Wigury 93, 02-089 Warszawa, Poland
3 Panstwowy Instytut Geologiczny, Rakowiecka 4, 00-975 Warszawa, Poland

Miocene evaporites of the Carpathian Foredeep host an interesting sulphates group of potash deposits, including about 20 sulphates minerals. The study of sulphur isotopic composition of 10 of the sulphates minerals from the Kalush–Holyh and Stebnyk potash deposits shows that only the basal Ca-sulphates (anhydrite) from the Kalush–Holyn potash deposits has {delta}34S values typical of Neogene marine evaporites (+21.0{per thousand}). Potash minerals related to the deposits (polyhalite, anhydrite, kainite, langbeinite and kieserite) show {delta}34S values from +15.28{per thousand} to +17.54{per thousand}, and the weathering zone minerals (picromerite, leonite, bloedite, syngenite and gypsum) show values ranging from +14.73{per thousand} to +18.22{per thousand}. The recorded depletion of sulphur isotopic composition of the salt minerals of potash deposits (and their weathering zone) was probably caused by one or more of the following isotope fractionation factors: bacterial reduction of sulphate, effect of crystallization and inflow of surface waters containing sulphates enriched in light sulphur isotopes due to pyrite oxidation. Accordingly, the observed sulphur isotopic composition of minerals from these potash deposits demonstrates the depletion of the original brines and continual inflow of new (concentrated) seawater. Similar sulphur isotopic composition of minerals from the potash deposits and their weathering zone points out that there was no significant sulphur isotope fractionation during weathering and in this case inflow of surface water has an insignificant influence on sulphur isotopic composition.