Abstract Complex pyrite textures associated with large changes in isotopic and trace element compositions are routinely assumed to be indicative of multi-faceted processes involving multiple fluid and sulfur sources. We propose that the features of ore-stage pyrite from roll front deposits across the world, revealed in exquisite detail via high-resolution trace element mapping by LA-ICP-MS, reflect the dynamic internal evolution of the biogeochemical processes responsible for sulfate reduction, rather than externally driven changes in fluid or sulfur sources through time. Upon percolation of oxidizing fluids into the reduced host-sandstones, roll front systems become self-organized, with a systematic reset of their activity cycle after each translation stage of the redox interface down dip of the aquifer. Dominantly reducing conditions at the redox interface favor the formation of biogenic framboidal pyrite (δ34S from −30.5 to −12.5‰) by bacterial sulfate reduction and the genesis of the U mineralization. As the oxidation front advances, oxidation of reduced sulfur minerals induces an increased supply of sulfate and metals in solution to the bacterial sulfate reduction zone that has similarly advanced down the flow gradient. Hence, this stage is marked by increased rates of the bacterial sulfate reduction associated with the crystallization of variably As-Co-Ni-Mo-enriched concentric pyrite (up to 10,000′s of ppm total trace contents) with moderately negative δ34S values (from −13.7 to −7.5‰). A final stage of pyrite cement with low trace element contents and heavier δ34S signature (from −6.9 to +18.8‰) marks the end of the roll front activity cycle and the transition from an open to a predominantly closed system behavior (negligible advection of fresh sulfate). Blocky pyrite cement is formed using the remaining sulfate, which now becomes quickly heavy according to a Rayleigh isotope fractionation process. This ends the cycle by depleting the nutrient supplies for the sulfate-reducing bacteria and cementing pore spaces within the host sandstone, effectively restricting fluid infiltration. This internally-driven roll front activity cycle results in systematic, large S isotope and trace element fractionation. Ultimately, the long-time evolution of the basin and fluid sources control the metal endowment and evolution of the system; these events, however, are unlikely to be preserved by the roll front, as a direct result of its hydrodynamic nature.

中文翻译：

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内部驱动的生物地球化学循环导致铀滚动前沿系统黄铁矿中的大S同位素和微量元素分馏

摘要 与同位素和微量元素组成的巨大变化相关的复杂黄铁矿结构通常被认为是涉及多个流体和硫源的多方面过程的指示。我们建议，通过 LA-ICP-MS 的高分辨率痕量元素映射，世界各地辊锋沉积物的矿石阶段黄铁矿的特征反映了负责硫酸盐还原的生物地球化学过程的动态内部演化，而不是外部驱动的流体或硫源随时间变化。当氧化流体渗入还原的主砂岩中时，滚动前沿系统变得自组织，在含水层的氧化还原界面向下倾角的每个平移阶段之后系统地重新设置它们的活动周期。氧化还原界面处的主要还原条件有利于通过细菌硫酸盐还原和 U 矿化的成因形成生物黄铁矿（δ34S 从 -30.5 到 -12.5‰）。随着氧化前沿的推进，还原硫矿物的氧化导致细菌硫酸盐还原区的溶液中硫酸盐和金属的供应增加，细菌硫酸盐还原区也同样沿流动梯度向下推进。因此，这个阶段的特点是细菌硫酸盐还原速率增加，与富含 As-Co-Ni-Mo 的同心黄铁矿结晶有关（高达 10,000 ppm 的总痕量含量），δ34S 值为中等负值（从-13.7 至 -7.5‰）。具有低微量元素含量和较重 δ34S 特征（从 -6.9 到 +18. 8‰) 标志着滚锋活动周期的结束，以及从开放到主要封闭系统行为的转变（新鲜硫酸盐的平流可忽略不计）。使用剩余的硫酸盐形成块状黄铁矿水泥，根据瑞利同位素分馏过程，硫酸盐现在很快变重。这通过耗尽硫酸盐还原细菌的养分供应和在宿主砂岩内胶结孔隙空间来结束循环，有效地限制流体渗透。这种内部驱动的滚动前沿活动循环导致系统的、大的 S 同位素和微量元素分馏。最终，盆地和流体源的长期演化控制了系统的金属禀赋和演化；然而，这些事件不太可能被滚动锋保留，