Controls on U enrichment in organic-rich shales from the Triassic Yanchang Formation, Ordos Basin, Northern China

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The Chang 7 lacustrine organic-rich shale in the Upper Triassic Yanchang Formation has high U contents, with a mean of 10.2 ppm. Previous studies have proposed that the redox conditions and high total organic carbon (TOC) content provide advantageous physical-chemical conditions for syngenetic U enrichment; however, the processes, mechanisms, and factors controlling U enrichment are still unclear. In this study, we examined the relationships between TOC and major, and trace element concentrations to better understand the factors that control U enrichment. Results indicate that U enrichment in the Chang 7 organic-rich shale is mainly controlled by redox conditions, sources of U, and sedimentation rate. Under oxic conditions, U is predominantly enriched in authigenic phosphate nodules; however, under anoxic conditions, it is mainly enriched in organic matter. Therefore, organic matter plays a more important role in controlling U enrichment than once thought due to its high content in the Chang 7 organic-rich shale, although the authigenic phosphate nodules have a higher enrichment efficiency. In addition to favorable redox conditions, deep hydrothermal fluids and volcanic ash can provide sources for U, and low sedimentation rate during shale deposition may create an environment conducive to U enrichment. The results of this study provide critical information about U enrichment in organic-rich shales and provide guidance for further research on the coexistence of U enrichment and high organic matter contents in source rocks.