Hydrothermally induced S-34 enrichment in pyrite as an alternative explanation of the Late -Devonian sulfur isotope excursion in South China

摘要

Several Late Devonian sedimentary successions host pyrite with highly positive sulfur isotope values (delta S-34(pyrite) ). These anomalous values have been linked to marine anoxia, low sulfate concentrations in seawater, or aerobic re -oxidation of dis- solved sulfide within well -oxygenated bottom waters in a local depositional environment. Implicit to these previous models is the assumption of a biogenic genesis from microbial sulfate reduction (MSR) of these pyrites, which, in turn, can be used to understand the biogeochemical sulfur cycle and reconstruct paleoenvironmental conditions. In South China, the Late - Devonian bulk -rock sulfur isotope excursion occurs in the lower Fammenian Wuzhishan Formation of the Youjiang Basin, in both the sedimentary limestone that corresponds to the Late triangularis conodont zone at the Fuhe section and the sediment -hosted Xialei Mn deposit that lies between the Late triangularis and crepida conodont zones. To further constrain the significance of these 34 S -enriched pyrites, we carried out detailed textural and in -situ sulfur isotope examinations of pyrites in the Xialei Mn deposit. We found one type of pyrite in siliceous limestone referred to as the host rocks and three distinct pyrite types in Mn ores: Type I - pyrites with Mn-carbonate inclusion -rich cores; Type II - pyrite overgrowths surrounding an "inclusion-rich" core; and Type III - relatively inclusion -poor and mainly subhedral disseminated pyrite grains. Type I pyrites in Mn ores have negative delta S-34(pyrite) values from -7.8 parts per thousand to -2.0 parts per thousand. Types II and III pyrites, and pyrites in the host rock are characterized by highly positive delta S-34(pyrite) values (+4.2 parts per thousand to +37.6 parts per thousand), with some values being higher than that of contempo- raneous seawater sulfate. Type I pyrites with negative delta S-34 values likely formed via MSR in a closed porewater system, driven by increased sedimentation rates during a sea level lowstand. The other pyrites with highly positive delta S-34 values appear to have formed during secondary hydrothermal alteration, with their sulfur sourced through thermochemical sulfate reduction (TSR) of sulfate from dissolved barites in the underlying Liujiang Formation. This implies that the positive delta S-34 values of the Late Devonian pyrites from the Wuzhishan Formation represent a later alteration event caused by post -depositional hydrothermal fluids, and may lack a direct biogeochemical connection to the surface sulfur cycle. These results provide an important per- spective on the origin of variability in sulfur isotope records and suggest the need for careful petrographic screening and micrometer -scale analysis of sedimentary units used to reconstruct paleoenvironmental conditions. (C) 2020 Elsevier Ltd. All rights reserved.