New understandings of Ni–Mo mineralization in early Cambrian black shales of South China: Constraints from variations in organic matter in metallic and non-metallic intervals

The well-known Ni–Mo ores hosted in early Cambrian black shales of South China are one of the research highlights in economic geology for the past few decades; however, their origin is complex and still debated. Here, based on a case study in the Huangjiawan ore of Zunyi City, Guizhou Province, we generate several new understandings regarding Ni–Mo mineralization through a comparative investigation of organic matter in metallic and non-metallic stratigraphic intervals, including abundance, type, maturity and relationship to mineralization. We find new direct evidence for biotic impacts on mineralization. Organic matter, and rhodophyte cystocarps (red algae) in particular, may be significantly correlated to mineralization, as it accumulates mineralized Ni and Mo. However, this organic material, as well as disseminated and amorphous organic matters, is not the sole and predominant factor controlling mineralization as implied from the nonlinear correlation between organic matter abundance/maturation and mineralization. Other fluid sources (e.g., hydrothermal and/or seawater) also contribute to mineralization, which may be influenced by hydrothermal activity. Ni and Mo may have mineralized independently, as suggested by their differential accumulation in different structures of the cystocarps, different relationships between organic matter abundance and thermal maturation and mineralized element concentration, as well as the large variation in element accumulation coefficients. The history of mineralization is complex, as Ni and Mo may be or not be deposited together during the same stage of mineralization. These results might also have broader implications for understanding the origin of sediment-hosted ores worldwide.