Characteristics of the morphology, distribution, sulfur isotopes, and genesis of pyrite in source rocks of the Upper Triassic Chang 7 Member, Ordos Basin, China

ABSTRACT The Ordos Basin, one of the major petroliferous basins in northern China, hosts the Chang 7 Member (also called Chang 7 Member in some literature) of the Upper Triassic Yanchang Formation as its most important source rock. This member is rich in pyrite and has drawn considerable interest owing to its significantly positive pyrite sulfur isotopes (δ 34 S py ), which has been attributed to bacteria sulfate reduction (BSR) in a closed system or BSR triggered by the inflow of exogenous sulfate. Numerous studies have indicated that the Chang 7 Member is atypical hydrothermal deposits, where pyrite formation may be closely linked to hydrothermal fluids. Core samples from this member in the southwestern Ordos Basin were selected to investigate the sedimentary paleoenvironment and pyrite genesis through scanning electron microscopy (SEM), X-ray diffraction (XRD), trace element, and δ 34 S py analyses. Although geochemical indicators suggest that the Chang 7 Member was deposited in a freshwater-to-brackish environment, the total organic carbon (TOC)-total sulfur (TS) relationships and elemental geochemical signatures [V/(V+Ni)] reveal that some samples plot within the fields of brackish to even sulfurized anoxic depositional conditions. This anomaly is interpreted as being related to the hydrothermal activity occurring simultaneously within the basin. Hydrothermal activity intensity indicators reveal that a strong thermal fluid event occurred during the middle depositional period of the Chang 7 3 sub-member in the study area. The δ 34 S py values in the source rocks range from 3.23‰ to 11.40‰, showing remarkable similarity to δ 34 S values of pyrite derived from deep hydrothermal fluids, particularly those associated with the Late Triassic magmatic-hydrothermal activities in the Qinling orogenic belt. It is inferred that deep-sourced volcanic-hydrothermal fluids from the orogenic belt along the southwestern margin of the basin likely served as the principal source of sulfur and iron for authigenic pyrite in the source rocks, with a minor contribution from BSR-derived pyrite. Hydrothermal activity profoundly influences the lake-floor sedimentary environment and delivers abundant nutrients that trigger massive algal blooms in the lake. These factors together dominate the deposition of the high-quality source rocks of the Chang 7 Member.