Abstract Chert nodules are widely developed in Ediacaran deposits of South China, yet the mechanism responsible for their formation remains disputed. Petrological and geochemical studies of chert nodules and host shale of the Ediacaran Doushantuo Formation of the Yangtze Block, South China, were carried out to more fully assess silicification of these deposits. Doushantuo chert nodules display a concentrically layered internal structure dominated by quartz and lesser concentrations of calcite, carbonate fluorapatite, organic matter, pyrite, and sphalerite. Individual layers become thinner from nodule centers to edges. Our results suggest that (1) Doushantuo host black shale accumulated under persistent anoxic bottom water conditions and (2) chert nodules formed as a consequence of the anaerobic oxidation of methane focused along sulfate-methane transition zones at shallow burial depth. Nodules likely originated from porous proto-nodules that consisted of 13C-depleted authigenic calcite produced by the anaerobic oxidation of methane. Decaying of organic matter within nodules helped to create porosity that was filled by authigenic calcite and also released phosphate and zinc ions to pore water, thus favoring precipitation of carbonate fluorapatite and sphalerite. Silica supplied principally from the diagenesis of clay minerals of the host shale cemented the nodules and replaced early formed calcite. The progressive infilling of residual porosity by 18O-depleted pore fluids associated with increasing burial depth is reflected in the strongly negative δ18Ocarb values documented from the nodules. Thus, the formation of Doushantuo chert nodules reflects a protracted diagenetic history initiated by bacterial sulfate reduction and anaerobic oxidation of methane close to and within the sulfate-methane transition zone and continued in association with the diagenetic release of silica from host shale as well hydrocarbon generation with increasing burial depth.

中文翻译：

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华南扬子地块埃迪卡拉纪陡山沱组黑色页岩燧石结核成因

摘要 华南埃迪卡拉纪矿床中广泛发育燧石结核，但其形成机制仍有争议。对华南扬子地块埃迪卡拉纪陡山沱组燧石结核和寄主页岩进行了岩石学和地球化学研究，以更全面地评估这些矿床的硅化作用。陡山沱燧石结核显示出以石英为主的同心层状内部结构，方解石、碳酸盐氟磷灰石、有机质、黄铁矿和闪锌矿的浓度较低。从结节中心到边缘，各个层变得更薄。我们的研究结果表明，(1) 豆山沱寄主黑色页岩在持续缺氧的底水条件下堆积；(2) 燧石结核是由于甲烷厌氧氧化而形成的，这些燧石结核集中在浅埋深的硫酸盐-甲烷过渡带上。结核可能起源于多孔原结核，由甲烷厌氧氧化产生的 13C 耗尽的自生方解石组成。结核内有机质的腐烂有助于产生孔隙，孔隙被自生方解石填充，并向孔隙水中释放磷酸盐和锌离子，从而有利于碳酸盐氟磷灰石和闪锌矿的沉淀。二氧化硅主要来自寄主页岩的粘土矿物的成岩作用，胶结了结核并取代了早期形成的方解石。与埋藏深度增加相关的 18O 耗尽的孔隙流体对残余孔隙度的逐渐充填反映在从结核中记录的强烈负的 δ18Ocarb 值中。因此，豆山沱燧石结核的形成反映了在硫酸盐-甲烷过渡带附近和内部由细菌硫酸盐还原和甲烷厌氧氧化引发的长期成岩历史，并与宿主页岩中二氧化硅的成岩释放以及油气生成有关。随着埋藏深度的增加。