Genesis and geological significance of pyrite in the Cretaceous shale of Songliao Basin, NE China

        Pyrite is an important component in reconstructing the biogeochemical conditions and sedimentary environments of the earth's surface in the past. The study of Cretaceous pyrite in the Songliao Basin not only helps to clarify the formation mechanism of high-quality source rocks in the area, but is also important for reconstructing the Cretaceous paleoenvironment. Pyrite in the Qingshankou Formation includes euhedral pyrite, anhedral pyrite, fine-grained pyrite aggregates, pyrite framboids and polyframboids. According to this genetic division, the euhedral pyrite can be divided into "authigenic type" and "secondary type". The "authigenic type" euhedral pyrite is directly precipitated from solution, while "secondary type" euhedral pyrite is formed by recrystallization of pyrite framboids. The "secondary type" can be further divided into "compaction type" and "cementation type" type, indicating that the transformation of pyrite framboids into secondary euhedral pyrite is controlled by compaction and cementation, respectively. Anhedral pyrite is usually precipitated on the surface of iron-rich clay minerals (e.g., chlorite), or by metasomatism of other minerals, biological skeletons, and microorganisms. Pyrite framboids are transformed from greigite during the syndiagenetic stage. Under the same redox conditions, higher water flow energy conditions enhance the abundance of pyrite framboids, increase the number of microcrystalline layers, and lead to larger diameter pyrite framboids. The sediments in the K₂qn¹ Formation were deposited in a semi-arid to semi-humid climate, in an anoxic and reducing environment. The sedimentary lacustrine basin was a highly restricted environment with brackish to saline water. In this environment, circulation was weak, resulting in fewer pyrite framboids with fewer microcrystalline layers and smaller diameters. The restricted environment resulted in abnormally high δ³⁴S_py values. The enhanced development of euhedral pyrite with heavier sulfur isotope values and the low occurrence of pyrite framboids with lighter sulfur isotope values is also an important reason for the abnormally high δ³⁴S_py values. This study provides a new understanding of the genetic mechanism of different types of pyrite.