Early diagenetic pyrite morphology in a mudstone-dominated succession: theLower Jurassic Cleveland Ironstone Formation, eastern England

Diagenetic pyrite in the mudstones and ironstones of the Lower Jurassic Cle veland Ironstone Formation of eastern England exhibits two distinct morphol ogies: framboidal pyrite, commonly associated with organic matter, and euhe dral pyrite, associated with detrital clay pellets. These two morphologies are mutually exclusive in occurrence. Framboidal pyrite is present in clay- rich mudstones, ooidal ironstones, apatite-rich units and some silt-rich mu dstones. Euhedral pyrite is present in silt-rich and sand-rich mudstones. d elta(34)S isotopic analysis of six samples of pyrite suggests that both typ es of pyrite morphology precipitated during early diagenesis from porewater s with open access to overlying sea-water, although both probably acted as sites for continued pyrite precipitation during burial. It is proposed that framboidal pyrite precipitated from iron-dominated porewaters at sites of sulfide supply (i.e. in the region of organic matter as a result of bacteri al sulfate reduction) where, locally, sulfide production rates were high en ough for porewaters to reach supersaturation with respect to FeS. Euhedral pyrite also precipitated from iron-dominated porewaters, but sulfide produc tion rates from organic matter was such that FeS saturation was not reached at the sites of sulfide production. Instead, euhedral pyrite was precipita ted directly from porewater when FeS? saturation was reached. The control o ver pyrite morphology was probably the amount and reactivity of the organic matter within the deposited sediments. The sand-rich mudstones contained l ess reactive organic matter due to elastic dilution and deposition in shall ower environments with O-2-rich bottom waters. The ironstones and apatite-r ich units were deposited under very low sedimentation rates, and as a resul t organic matter contents were very low and iron reduction dominated early diagenesis, which inhibited sulfate-reduction. The presence of minor frambo idal pyrite within these units, however, suggests that sulfide reduction to ok place in micro-environments during early diagenesis. (C) 2000 Elsevier S cience B.V. All rights reserved.