Sulfur isotopes and paragenesis of sulfide minerals in the Silurian Waldron Shale, southern Indiana

A diverse assemblage of sulfide morphologies and a wide range of sulfur-isotope compositions indicate that precipitation of sulfide minerals was an ongoing process during both early and later stages of diagenesis in the Silurian Waldron Shale in Indiana. Five distinct forms of sulfide minerals have been identified in samples from seven locations in southern Indiana: (1) pyritized brachiopod shells, (2) pyritized burrows, (3) pyrite cubes, (4) prismatic marcasite, and (5) cockscomb marcasite. Petrographic evidence is used to determine a probable sequence of sulfide precipitation. Isotope compositions range from about -40‰ to +40‰, and each sulfide morphology exhibits a distinctive range of δ³⁴S values. These results are consistent with a model in which sulfate reduction and subsequent precipitation of sulfide minerals occurred in a system that became progressively more closed with respect to sulfate. The sequence of sulfide mineralization reflects the chemical evolution of pore waters during diagenesis of the Waldron Shale. Redox conditions in the sediment were initially aerobic to dysaerobic, sulfate reduction occurring only in reducing microniches where brachiopod shells and then organic-walled burrows were pyritized. Sediments became more fully reducing with burial, when euhedral sulfides were formed. The transition from pyrite to marcasite precipitation suggests a decrease in pore-water pH during later diagenesis and may also indicate fluctuations in redox conditions.