Special aspects of siderite mineralization in the Jurassic-Lower Cretaceous clay-terrigenous deposits of Western Siberia

Research subject. Siderite of Jurassic and Lower Cretaceous sediments of the West Siberian sedimentary basin. Aim. To determine the patterns of the structure and composition of siderite mineralization and its dependence on sedimentation conditions and stages of changes of surrounding rocks. Materials and methods. Siderite mineralization in different Jurassic and Lower Cretaceous facies of Western Siberia has been studied. The morphology of siderite was determined by optical and electron microscopy using an energy dispersive spectrometric microprobe. The contents of Fe, Mg, Ca, Mn, Al, Sr were determined in hydrochloric acid extracts using the ICP-MS method. The isotopic composition of C and O in siderites was determined using the FinniganTM MAT 253 isotope ratio mass spectrometer. Results. A variety of siderite forms has been established. Pelitomorphic and globular siderite predominate, to a lesser extent micro-, fine-crystalline and spherulite. The siderites of continental sediments are cleaner in chemical composition, and increased Ca and Mg contents are recorded in marine sediments. Minor variations of δ 13 C (–5.5...+5.5‰) were noted in marine siderites, and a wide range of δ 13 C values (–8.7...+13.5‰) is typical for continental ones. According to the isotopic composition of oxygen, siderite of continental sediments has a distinctly (depleted isotope) composition (δ 18 O – +13.5...+17.6‰) compared with coastal and shallowwater marine (δ 18 O – +22.3...+24.4‰). When studying the structural and morphological varieties of siderite, it was found that the early diagenetic generations are characterized by a cleaner composition, while the slightly manifested catagenetic ones are characterized by an increased content of Ca, Mn, and Mg. This pattern can be used when conducting a stadium analysis of enclosing rocks. Conclusions. Siderite of sandy-siltstone and siltstone-clay rocks of the Jurassic and Lower Cretaceous of Western Siberia were formed under the influence of a number of factors under different conditions and at different stages of diagenesis. They are characterized by a wide range of structural and morphological varieties, a significant range of isomorphic impurities and variability of isotopic composition. According to a number of parameters (content of Ca, Mg, Sr, δ 18 O), siderite of continental sediments is distinctly different from marine sediments. Versatile precision studies will facilitate the use of siderite as an indicator of sedimentary environments and post-sedimentation changes.