Abstract

A complex of lithological-geochemical studies was carried out in rocks of the Upper Jurassic–Lower Cretaceous Bazhenov Formation and their transition zones to host rocks: composition of rocks, ratio of organic carbon and sulfide sulfur (C/S values), and distribution of several redox indicators (degree of pyritization, content of the authigenic uranium, and values of Mn/Al and Mo/Mn ratios). Morphology of pyrite was examined with a scanning electron microscope. Two main types of pyrite were revealed: (a) framboidal pyrite formed during early diagenesis with the participation of bacterial activity; (b) cryptocrystalline pyrite formed during diagenesis (early stage included) and, presumably, mesocatagenesis, i.e., medium substage of catagenesis. It was established that some part of the cryptocrystalline pyrite predated the framboidal variety. Pyritization of rocks with the cryptocrystalline pyrite formation took place at the lower and upper boundaries of the Bazhenov Formation in the over- and underlying rocks, where the low-carbon pyritic rocks are formed. Intense pyritization is also observed in the high-carbon rocks near the top of the Bazhenov Formation, where the pyrite–kerogen rocks are formed. Both low-carbon pyritic and pyrite–kerogen rocks are characterized by C/S < 1.5; rocks of the Bazhenov Formation, by C/S > 2. The low-carbon pyritic and pyrite–kerogen rocks occur near boundaries of the lithologically different members (mainly biogenic rocks in the Bazhenov Formation and terrigenous varieties in the host rocks) that were deposited under different redox conditions existing in the water column and near the marine paleobasin floor. Boundaries of such members could serve as geochemical redox barriers at late stages of lithogenesis. The pyritic and pyrite–kerogen rocks were likely pyritized during diagenesis because of the deposition of pyrite on the geochemical redox barriers from fluids that contained iron sulfides and migrated from the high-carbon Bazhenov sequence. Presumably, pyritization continued during the catagenesis owing to thermogeochemical processes of the organic matter transformation.

中文翻译：

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黑色页岩/主岩过渡带岩石的黄铁矿化：西西伯利亚巴热诺夫组的证据

摘要

在上侏罗统-下白垩统巴热诺夫组及其过渡带至宿主岩的岩石中进行了复杂的岩石地球化学研究：岩石的组成，有机碳与硫化物硫的比率（C / S值）以及几个氧化还原指标（黄铁矿化程度，自生铀的含量以及Mn / Al和Mo / Mn比值）。用扫描电子显微镜检查黄铁矿的形态。揭示了两种主要的黄铁矿类型：（a）在成岩早期就形成了细菌活性参与的黄菊黄铁矿；（b）在成岩作用（包括早期）和大概中速成岩作用（即中速成岩作用）期间形成的隐晶黄铁矿。已经确定，隐晶黄铁矿的某些部分早于黄菊变种。在上层和下层岩石的Bazhenov组的上下边界发生了具有隐晶黄铁矿形成的岩石的黄化作用，形成了低碳黄铁矿岩石。在Bazhenov组顶部附近的高碳岩石中也观察到强烈的黄铁矿化，形成了黄铁矿-干酪根岩石。低碳黄铁矿和黄铁矿-干酪根岩的特征都是C / S <1.5。低碳黄铁矿和黄铁矿-干酪根岩发生在沉积的岩性不同的成员（主要是巴真诺夫组中的生物成因岩和宿主岩中的陆源变种）的边界附近在水柱中和海洋古生物的底部附近存在不同的氧化还原条件。此类成员的边界可以在成岩的晚期充当地球化学的氧化还原屏障。黄铁矿和黄铁矿-干酪根岩很可能在成岩过程中发生了黄铁矿化，因为黄铁矿从含硫化铁并从高碳巴热诺夫序列中迁移出来的流体沉积在地球化学氧化还原层上。据推测，由于有机物转化的热地球化学过程，在催化作用中黄铁矿化持续进行。