Source and distribution of pyrite and inorganic sulfur isotopes in the Saray and Pınarhisar Coalfields, North Thrace Basin, Turkey

This study is mainly concerned with sulfur isotope (δ³⁴S) values of inorganic sulfur species associated with the North Thrace Basin's coals (Saray and Pınarhisar coalfields). Sulfur isotope analyses were performed on pyritic sulfur, native sulfur, sulfate sulfur (rozenite) and the precipitated sulfate (P-BaSO₄) (leached sulfate from the coal). The δ³⁴S values of pyritic sulfur, rozenite, native sulfur and P-BaSO₄ samples in Saray coals ranging from −6.9‰ to −37.3‰ are comparable to the δ³⁴S values of Pınarhisar coals which change from −1.1‰ to −59.3‰. A wide range of strong negative δ³⁴S values in both coalfields, particularly in Pınarhisar site, indicate that bacterial sulfate reduction (BSR) and subsequent pyrite formation were the key processes in the peat depositional environments. As in the δ³⁴S values of both coalfield sites, comparable Sr/Ba and Th/U ratios and pyrite forms (cleat filling and partly massive) of the coals imply that besides marine sulfate hydrothermal solutions, originated from volcanism being active during peat accumulation in the basin, also contributed to the sulfur pools in the coal depositional environments. Overall, the δ³⁴S values of inorganic sulfur species, and the largest sulfur isotope fractionation between δ³⁴S _P-BaSO4 and the δ³⁴S_pyrite, obtained for the first time for the Thrace Coal Basin, suggest that bacterial reduction of sulfate, possibly marine and hydrothermally derived, and subsequent pyrite formation and its fast burial may have been contributed to sulfur budget in the basin.