THE OCCURRENCE AND δ34S OF AUTHIGENIC PYRITE IN MIDDLE JURASSIC BRENT GROUP SEDIMENTS

Abstract

Minor amounts of authigenic pyrite are common throughout the Middle Jurassic Brent Group, often being well developed within the delta‐plain fades of the Ness Formation. Petrographic analyses of pyrite samples from the Lyell and Murchison fields reveal two stages of pyrite authigenesis, both of which occurred during an early stage of burial.The first phase of pyrite authigenesis is represented by the development of nodules and finely‐disseminated cement within shale, mudstone, and siltstone fades, and can include the development of bladed marcasite nodules. These cements are interpreted as having formed within organic‐rich sediment soon after deposition, and pre‐date all other authigenic precipitates (typically calcite, quartz and kaolinite). “Early” pyrite cements display a wide range of δ34S(‐14.9 to +42.5% CDT), and some nodules analysed display isotopically‐heavy cores and considerably lighter margins. These isotopically‐zoned nodules are problematical. They could be interpreted as forming via progressive “closed system” bacteriogenic reduction and isotopic fractionation of sea‐water sulphate, with incorporation of heavy H2S into the earliest‐formed pyrite occurring as a result of its upward diffusion through the sediment column, following isotopic fractionation at depth. However, there are a number of problems with this type of interpretation. These include the fact that diffusion will favour the 32S isotope, so that the first‐formed cements should still be isotopically “light” not “heavy”; also, there is a mass‐balance problem, in that no low δ34S sulphides were detected.Coarsely‐crystalline pyrite euhedraform nodular cements mainly within sandstones (δ34S ‐2.6 to +12.8%CDT), and are interpreted as a “later” diagenetic phase. This second phase of sulphide cementation also pre‐dates the main phases of quartz and kaolinite authigenesis within sandstones, but post‐dates an early phase of kaolinite, and may have been partly coeval with some early calcite authigenesis.High organic contents and the early establishment of reducing conditions led to early formation of pyrite within fine‐grained brackish and marine sediments. The lowest measured end‐member δ34S of sandstone‐hosted pyrite cements is less isotopically depleted than that within fine‐grained argillaceous fades, perhaps indicating that sandstone‐hosted pyrite cements began to form subsequent to the onset of burial and bacteriogenic reduction within mudstones. Compaction of marine/brackish‐water shales and mudstones (or sulphate diffusion from these fades) may have supplied an already isotopically‐fractionated source of sulphate to porous sandstone lithologies, where “later” pyrite cements precipitated.