In situ characterization of pyrite from multiple microenvironments in middle Jurassic strata, northeastern Ordos Basin, China: Evaluation of synsedimentary/diagenetic fluid evolution

The sulfur isotopic composition and trace element content recorded in sedimentary pyrite have been widely used as a proxy to establish the geochemistry of pyrite-forming fluids. However, a continuous and complete evolution pro-cess of these fluids from the sedimentary period to diagenetic period remains not well constrained. The middle Jurassic strata (the Yan'an Formation containing coal seams and its overlying Zhiluo Formation hosting U deposits in the sandstone) in the northeastern Ordos Basin contain four kinds of microenvironments favorable for pyrite for-mation, namely coal, carbonaceous debris in barren gray sandstone, barren gray sandstone without carbonaceous debris and U mineralized gray sandstone from bottom to top, providing a good opportunity to decode pyrite forma-tion in response to synsedimentary/diagenetic fluid evolution. In these microenvironments, pyrite named as PC, PD, PB and PM respectively, displays a variety of micro morphologies (framboidal, euhedral, infilling, irregular and cement) formed during sedimentary and diagenetic stages. PC has & delta;34S values ranging from -36.1 to +26.5 %o and low contents of total trace elements (from 6.7 to 58 ppm of Co + Ni + As + Se + Mo). PD and PB present & delta;34S signatures from -33.6 to +14.4 %o and from +9.7 to +22.2 %o, respectively, with moderate trace element con-tents (from 21 to 111 ppm and from 23 to 80 ppm, respectively). PM resulting from the oxidation of PD and PB shows negative & delta;34S values (from -16.9 to -11.1 %o for framboidal pyrite and from -56.7 to -34.8 %o for euhedral pyrite + pyrite cement) and high trace element contents varying from 350 to 931 ppm. Detailed sulfur isotope and trace element microanalyses indicate that S involved in the formation of pyrite is derived from the fractionation pro-cess of organic sulfur in the coal, while Fe and trace elements originate from the surface-derived meteoric water. A paragenetic sequence from early PC, to PD and PB, and finally PM implies an evolution trend of & delta;34S values increasing first and then decreasing (bacterial sulfate reduction for framboids and Ostwald ripening for euhedral pyrite and py-rite cement of PM) and gradual enrichment of trace elements from synsedimentary to diagenetic fluids. The quan-titative results can contribute to a better understanding of synsedimentary/diagenetic fluid evolution both locally and regionally for sedimentary strata in the low-temperature system.& COPY; 2023 Published by Elsevier B.V.