Genesis and Geological Significance of Siderite in the First Member of the Nantun Formation of Dongming Sag, Hailar Basin

Multiple siderite beds developed in the first member of the Lower Cretaceous Nantun Formation (K1n1) in the basin. The results show that the siderites in K1n1 of the study area are mostly stratiform or massive, with three micromorphological features (dense micronized crystals, bands, and paragenesis with quartz and calcite). The siderite beds are mainly composed of siderite, clay, quartz, calcite, and feldspar. Under the microscope, charcoal, algal fossils, granular pyrite crystals, vein-like siliceous bands, etc., were observed. The oxides in the siderite beds include Fe2O3, SiO2, Al2O3, etc. The trace elements are typically characterized by high Mn and Be contents; low Sr/Ba, Th/U, and Al/Ti ratios; and high V/Cr ratios. These indicate weakly reducing, freshwater depositional paleoenvironments. The δ13Cv-PDB and δ18Ov-PDB values of siderite are −0.20–1.11‰ (mean: 0.62‰) and −18.22‰ to −10.14‰ (mean: −14.23‰), respectively, which shows that the carbon in siderite came mainly from carbonate dissolution. The Fe-bearing rocks in the source area migrated to the basin after undergoing physical and chemical weathering, and when the resultant Fe2+ concentration reached saturation, Fe2+ combined with CO32− in the water bodies to form authigenic siderite.