Neoproterozoic microbial processes in chemical sediment diagenesis:evidence from the Aberfeldy barite deposits

Microbial- and diffusion-controlled diagenetic processes were significant in the formation of sulfide-, sulfate- and carbonate-rich stratiform mineralization hosted by Neoproterozoic graphitic metasediments near Aberfeldy in Perthshire, Scotland. In 1-10m thick beds of barite rock, barite δ34S of +36 ±1.5 ‰ represents the isotopic composition of contemporaneous seawater sulfate. Pronounced vertical variations in δ34S (+30 to +41 ‰) and δ18O (+8 to +21 ‰) occur on a decimetre-scale at bed margins. These excursions are attributed to early diagenetic alteration, while the barite sediment was fine-grained and porous, due to pulsed infiltration of isotopically diverse porefluids into the marginal barite. Fluid-mediated transfer of barium and sulfate into adjacent sediments contributed to barium enrichment and growth of millimetric sulfate crystals cross-cutting sedimentary lamination. Witherite, barytocalcite and norsethite occur only as small inclusions within crystals of pyrite. Barium carbonate formation is ascribed to early diagenetic processes that generated very low sulfate activity in pore waters. Subsequently, sulfidation reactions produced secondary barite + non-barium carbonates in the rock matrix. Comparatively low δ34S (+16 to +22 ‰) in secondary barite indicate sulfur derived from reduced sulfide. Similar sulfide δ34S ratios in the host sediments are consistent with microbial reduction of seawater/ porewater sulfate