Greigite from carbonate concretions of the Ediacaran Doushantuo Formation in South China and its environmental implications

Greigite (Fe3S4) is a ferrimagnetic iron sulfide that commonly forms as a precursor of pyrite in anoxic environments where the supply of reactive Fe outpaces that of sulfide (H2S and HS−). Because of its metastability and sensitivity to redox changes during burial, greigite has been rarely documented in rocks older than the Cretaceous. Here we report well-preserved greigite in carbonate concretions of the upper Doushantuo Formation (ca. 551 Ma) in the Yangtze Gorge area, South China. Greigite in the carbonate concretions coexists with anhedral and framboidal pyrite, and is distributed in clay-rich carbonates with card-house microtextures and dolomitic spherical structures indicative of early diagenetic formation during shallow burial. Preservation of greigite in carbonate concretions of the upper Doushantuo Formation implies that these concretions maintained a closed micro-system since their formation and that they provide information about the ancient depositional and early diagenetic environments.

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► Well-preserved greigites are identified from carbonate concretions in the Ediacaran Doushantuo Formation.
► Close association of greigites and microfabrics of clay minerals suggests their early diagenetic origin.
► Preservation of greigite and associated microstructures in ca. 551 Ma carbonate concretions requires a closed system during subsequent burial and diagenesis.
► Presence of greigite in old carbonate concretions provides information about the ancient depositional and early diagenetic environments.