Pyrite Characteristics and Its Environmental Significance in Marine Shale: A Case Study from the Upper Ordovician Wufeng–Lower Silurian Longmaxi Formation in the Southeast Sichuan Basin, SW China

Pyrite, as a characteristic mineral in organic-rich marine shale, is significant index for the interpretation of paleoredox conditions. In this study, based on drilling cores and focused ion beam-scanning electron microscopy (FIB-SEM), occurrence, diameter particle size distribution pyrites from 32 samples obtained Wufeng–Longmaxi Formation southeast Sichuan Basin were analyzed. The results show that pyrite displays various occurrences at macro-scale micro-scale. At (mm–cm), laminations, nodular lenticular can be found cores. micro-scale (nm–µm), common are framboids, euhedral infilled framboids. According to formation mechanism pyrites, divided into syngenetic diagenetic pyrites. framboids categorized mean framboid diameters (Mean, D) range 2.94 µm 5.33 (average 4.26 µm), with most showing 3.5 ?m 4.8 ?m. Most microcrystals d) less than 0.4 µm. Therefore, according d), three sizes: (SF, D < µm, d ? early (EDF, > µm) late (LDF, µm). Additionally, box-and-whisker charts diameter, standard deviation/skewness value ratio trace elements indicate sedimentary water body was euxinic–dysoxic environment. Euxinic conditions dominated Wufeng lower part Long11-3 section, which beneficial preservation organic matter. However, middle-upper Long13-Long12 sub-member dysoxic