An SEM study of microfossils in the black shale of the Lower Cambrian Niutitang Formation, Southwest China: Implications for the polymetallic sulfide mineralization

•We investigated the fine structures of the Ni–Mo sulfide ore layer by SEM.•Abundant well-preserved microfossils that resemble green algae are described.•Algal biomass accumulation may explain the formation of the sulfide ore layer.

The black shale in the Lower Cambrian Niutitang Formation on the southeastern margin of the Yangtze Platform hosts a layer of polymetallic sulfide ores containing extremely high concentrations of Ni (up to 3.8 wt.%) and Mo (up to 7.5 wt.%). Abundant micrometer-sized microfossils were observed in the Ni–Mo sulfide enriched layer under scanning electron microscopy. The microfossils include vesicles with clear organic wall structures and permineralized internal contents. These structures resemble unicellular green algae. Both biomass-adsorbed transition metals and biological structural protein metals could be accumulated and mineralized from the blooming algae, followed by anoxic microbial reduction and immobilization. The biotic impact may be a substantial mechanism for the massive Ni- and Mo-sulfide deposition in a typical restricted ocean basin environment.