Origin of early Cambrian black-shale-hosted barite deposits in South China: Mineralogical and geochemical studies

The barite deposits hosted in early Cambrian black shales in South China are a classic example of sediment-hosted stratiform barite deposits, and their origin is controversial. In this paper, we address the origin of these deposits by a comprehensive integrated study of mineralogy and geochemistry, based on a case study of the Dahebian deposit in Tianzhu County, Guizhou Province, which contains the largest global reserve of barite. Particular attention was paid to a comparison between ore and non-ore horizons, as this has not been fully addressed previously. The results indicate that the ore horizon is dominated by barite with intergrown hyalophane, quartz and pyrite. The hyalophane is zoned, implying hydrothermal activity. In contrast, the non-ore horizons contain minor amounts of granular barite and are free of hyalophane, suggesting that these horizons experienced weaker hydrothermal activity. The contrasting trace and rare earth element compositions of the ore and non-ore samples indicate dilution of complex geochemistry with multiple ore-forming processes and materials during barite mineralization. The barite mineralization was rapid, as demonstrated by the relatively low terrigenous input in the ore samples compared with the non-ore samples. The ore samples have positive Eu anomalies (>3.0), indicative of hydrothermal activity during mineralization. Organic geochemical analyses provide evidence of the development of abundant organic matter and hydrothermal activity. The sulphur isotopic values (δ34S) of the ore samples show an enrichment relative to contemporaneous early Cambrian seawater and substantial variation (+36.7‰ to +43.8‰), indicative of sulphur derived from seawater that was affected by sulphate-reducing bacteria and hydrothermal activity in a restricted marine environment. To summarize, the barite deposit in the Dahebian site was most likely formed in a generally restricted and reducing marine basin environment with rapid sedimentation during a period of hydrothermal activity. These results have general implications for the future study of other black-shale-hosted deposits worldwide.