Genesis and model of nodular pyrite in Jurassic coal-bearing strata, Ordos Basin: Evidence from petrography, mineralogy, and geochemistry

In warm, humid environments, pyrite accumulates during sedimentary periods. Mud in shale and organic matter within algal pellets promotes framboidal pyrite formation via microbial sulfate reduction (MSR) during early diagenesis. With continuous material supply and diagenesis, framboidal pyrite evolves into euhedral to massive pyrite during intermediate diagenesis. Subsequent hydrothermal activity precipitates anhedral/colloidal pyrite, which ultimately results in the development of macroscopic nodular and disseminated pyrite. • Pyrite’s multi-stage evolution links micro-morphology to macro-distribution. • Pyrite enrichment is controlled by sandstone-shale-coal differentiation. • Diagenesis & hydrothermal intrusion drive pyrite differentiation. Abundant nodular pyrite has been documented within the Jurassic Yan’an Formation coal-bearing strata of the Ordos Basin, yet its genetic mechanisms remain to be fully elucidated. This study focuses on the No. 2 coal seam and its overlying shale and sandstone in the Lucun No. 2 Mine. Through thin-section identification, scanning electron microscopy, X-ray diffraction, electron microprobe analysis, in-situ sulfur isotope analysis, specular reflectance spectroscopy, and whole-rock geochemical tests (for major, trace, and rare earth elements), the sedimentary paleoenvironmental differences between various lithologies are explored. This approach helps reveal the material sources, genesis mechanisms, and formation models of nodular pyrite in coal-bearing sequences. The results demonstrate that: 1) The pyrite in the interlayer of the shale roof and floor primarily comprises nodular pyrite and disseminated pyrite. Microscopically, it occurs as euhedral–subhedral pyrite and anhedral pyrite, which results from the filling and development of framboidal pyrite into euhedral/massive pyrite during the middle diagenesis period, along with the alteration of hydrothermal intrusion into anhedral/colloidal pyrite during later stage. 2) Sandstone, coal, and shale are mainly composed of quartz minerals, with pyrite exhibiting the highest content in coal. The in-situ sulfur isotopes, burial history, and vitrinite reflectance all corroborate the findings associated with the middle diagenesis period. 3) Pyrite undergoes initial material accumulation during sedimentation, with superimposed hydrothermal overprinting during subsequent diagenesis. In this process, sulfur supply primarily derives from argillaceous components deposited in shale and organic matter-rich algal pellets, while iron is mainly metabolically extracted from water by microbial activity. Subsequent hydrothermal fluid infiltration provided additional iron and sulfur sources for later-stage pyrite formation. 4) Under conditions of sufficient material accumulation during sedimentation, framboidal pyrite continue to accumulate, develop, and evolve into euhedral/massive pyrite during the middle stage of diagenesis. Moreover, hydrothermal intrusion during the later diagenetic process leads to the formation of anhedral/colloidal pyrite, which ultimately accumulates and develops into macroscopic nodular and disseminated pyrite, respectively. This study investigates the genesis and evolutionary patterns of pyrite in coal-bearing strata, offering theoretical support for understanding both the sedimentary evolution of these systems and their relationship with pyrite formation.