Abstract Several Late Devonian sedimentary successions host pyrite with highly positive sulfur isotope values (δ34Spyrite). These anomalous values have been linked to marine anoxia, low sulfate concentrations in seawater, or aerobic re-oxidation of dissolved 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 34S-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 δ34Spyrite values from −7.8‰ to −2.0‰. Types II and III pyrites, and pyrites in the host rock are characterized by highly positive δ34Spyrite values (+4.2‰ to +37.6‰), with some values being higher than that of contemporaneous seawater sulfate. Type I pyrites with negative δ34S 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 δ34S 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 δ34S 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 perspective 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.

中文翻译：

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热液诱导黄铁矿中 34S 富集作为华南晚泥盆世硫同位素偏移的另一种解释

摘要 几个晚泥盆世沉积层序包含具有高正硫同位素值的黄铁矿（δ34Spyrite）。这些异常值与海洋缺氧、海水中的低硫酸盐浓度或局部沉积环境中充氧良好的底层水中溶解的硫化物的有氧再氧化有关。这些先前模型的隐含假设是这些黄铁矿的微生物硫酸盐还原 (MSR) 的生物成因假设，反过来，可用于了解生物地球化学硫循环和重建古环境条件。在华南地区，右江盆地下法门统五指山组发生晚泥盆世大块岩硫同位素偏移，在对应于阜河剖面的晚三角牙形牙形石带的沉积灰岩和位于晚三角牙形牙形石牙形牙形石带之间的沉积物夏雷锰矿床中。为了进一步限制这些富含 34S 的黄铁矿的重要性，我们对夏雷锰矿床中的黄铁矿进行了详细的结构和原位硫同位素检测。我们在硅质石灰岩中发现了一种称为主岩的黄铁矿，在锰矿石中发现了三种不同的黄铁矿类型：I 型——具有富含锰碳酸盐包裹体的黄铁矿；II 型 – 围绕“富含包裹体”的核心的黄铁矿过度生长；和类型 III – 相对较少的包裹体，主要是半自形浸染的黄铁矿颗粒。Mn 矿石中的 I 型黄铁矿具有负的 δ34Spyrite 值，从 -7.8‰ 到 -2.0‰。II型和III型黄铁矿，主岩中的黄铁矿和黄铁矿的特征是δ34Spyrite值很高（+4.2‰至+37.6‰），其中一些值高于同期海水硫酸盐的值。δ34S 值为负的 I 型黄铁矿可能通过 MSR 在封闭孔隙水系统中形成，这是由于海平面低位期间沉积速率增加所致。其他具有高 δ34S 值的黄铁矿似乎是在二次热液蚀变过程中形成的，其硫来源于下伏柳江组溶解重晶石中硫酸盐的热化学硫酸盐还原 (TSR)。这意味着五指山组晚泥盆世黄铁矿的正δ34S值代表了由沉积后热液引起的后期蚀变事件，并且可能缺乏与地表硫循环的直接生物地球化学联系。这些结果为硫同位素记录变异的起源提供了重要的视角，并表明需要对用于重建古环境条件的沉积单元进行仔细的岩相筛选和微米级分析。