Sulphur in Pennsylvania coals of Atlantic Canada: geologic and isotopic evidence for a bedrock evaporite source

Abstract

Higher sulphur contents in coals are commonly explained by the proximity of the original peat to marine waters during deposition, as the sulphate ions in seawater provide an abundant source of sulphur. However, Pennsylvanian coals of the Maritimes Basin, which commonly contain 5–8% sulphur, are associated with a predominantly freshwater biota and alluvial sedimentary features, and evidence for marine beds is scare. Several workers have suggested that the sulphur was derived from dissolution of thick sulphate-evaporites in the underlying Mississippian Windsor Group, rather than from Pennsylvanian seawater. We set out to test this hypothesis using a biogeochemical model derived from studies of sulphur isotopes in modern peat swamps. During the bacterial decomposition of plant material, the H 2S generated from groundwater sulphate is fixed as metal sulphides which are enriched in the lighter 32S isotope by 15% or more. Sulphides formed chemically during burial will be much less fractionated. Isotope ratios of sulphur-bearing products can thus be used to trace the original source of the sulphur by comparison with a source of known isotopic composition. Eight evaporite samples from the upper Windsor Group in the Sydney Basin have a mean δ 34S value of 15%. Sixteen pyrite samples from five coal seams in the overlying Morien Group range from 14.8% to −5% δ 34s. The pyrite values follow the pattern predicted for derivation from Windsor sulphates. Disseminated pyrite generally contains more highly fractionated sulphur than nodular pyrite, suggesting that much of the former was generated by bacterial action whereas the latter was produced during later burial. Isotopic ratios for soluble sulphate in the coal correspond closely to those for the associated pyrite. Because Mississippian and Pennsylvanian seawaters showed a similar range of δ 34S values, the test does not preclude a Pennsylvanian marine source of sulphate. Both geological and isotopic data, however, are consistent with a Windsor evaporite source.