Large accumulations of 34S-enriched pyrite in a low-sulfate marine basin: the Sturtian Nanhua Basin, South China

•First investigation of sulfur isotopes in pyrite and CAS from Cryogenian strata of the Nanhua Basin, South China.•Heavy δ34S compositions in pyrite (+29 to +72‰, mean +55‰) and CAS (+39 to +70‰, mean +56‰); △34S is −12‰ to +14‰.•Both pyrite and CAS δ34S show water-depth gradients within the Nanhua Basin due to low seawater sulfate concentrations.•High total sulfur content (mean 2.2 ± 1.1%) despite evidence for low contemporaneous seawater sulfate concentrations.•These features can be explained by massive hydrothermal emissions of 34S-enriched H2S with partial oxidation to sulfate.