Oxygen and sulphur isotopes of sulphate formed by oxidation of different pyrite grain size fractions were investigated to understand oxidation mechanisms and their dependency on grain size. Both oxygen and sulphur isotope compositions indicate different oxidation mechanisms for fine and coarser pyrite grain sizes. Oxidation mechanisms with atmospheric oxygen as the oxidant seem to be more important for the finest grain size fraction < 63 µm compared to coarser grain sizes. The isotopic composition of sulphate formed by experiments with the coarser grain size fractions 63-100 µm, 100-140 µm, and 140-180 µm do not differ among each other in spite of varying surface areas. Presumably, the lack of highly reactive fine material results in similar oxidation mechanisms.
