Petrologic characteristics and genetic analysis of kaolinite claystone in coal-bearingstrata of Shanxi Formation, Longdong Area

The kaolinite claystone in the coal-bearing strata is a characteristic coal associated mineral resource in China, with various types of genesis, which can provide geological significance such as stratigraphy, chronology or tectonic geology. A set of thick-bedded grayish-white kaolinite claystone is developed in the coal-bearing strata of Shanxi Formation, Longdong area, Ordos Basin. The mineral composition and rock type were identified by core observation, thin section identification, scanning electron microscope analysis, X-ray diffraction (XRD) analysis and major element analysis. And the genetic type was preliminarily identified. The results show that the gray-white kaolinite claystone is mainly composed of clay minerals (74.96% on average), followed by quartz (14.28% on average) and siderite (10.46% on average). The clay minerals contained are dominated by kaolinite (76.2% on average) which is mostly cryptocrystalline aggregates. Quartz is mostly microcrystalline. And siderite is mostly lumped, oolitic, and disseminated. It can be further divided into three categories: gray-white siderite-bearing kaolinite claystone, gray-white sideritic kaolinite claystone and gray-white kaolinite claystone. The Hinckley index (Hi) of the grayish-white kaolinite claystone is low (0.25 on average), reflecting the genesis of redeposition after continental transport. The peak half height width (FWHM) of peak (001) is 0.269~0.567, and the calculated crystallinity ranges from 1.68 to 3.23, which is low overall, reflecting the the genesis of redeposition after continental transport or the genesis of redeposition after continental transport genesis of volcanic material alteration type. The ratio of major elements SiO₂/Al₂O₃ of is 2.02, which is characterized by high SiO₂, high Al₂O₃, high TFe₂O₃, and very low TiO₂, K₂O, MgO, CaO, and Na₂O, reflecting that it is not of volcanic ash alteration. The distribution of major elements contents is different from that of weathering residual sedimentary rocks. It is believed that the grayish white kaolinite claystone is formed by transport and redeposition after weathering, and belongs to the sedimentary genesis type. The high chemical alteration index (CIA) reflects the high intensity of chemical weathering and the tropical climate conditions in the sediment source areas of the rocks at the time of formation. The development of grayish white kaolinite claystone reflects the presence of tectonic activities in the late depositional stage of the Shanxi Formation and the transformation of the humid and hot climate in the source area of Longdong region, which can be used as a marker layer for the contrasting division of small layers and has geological significance of event stratigraphy. The development of siderite also provides evidence for the development of weakly reducing environment of delta front-shallow lake deposition in the study area.