Cold seep activity in the early Cambrian: Evidence from the world-class shale-hosted Tianzhu barite deposit, South China

Abstract

Barite (BaSO4) is widely formed in modern and ancient sedimentary environments by the interaction of barium- and sulfate-bearing fluids from a variety of sources. A word-class barite deposit, hosted by lower Cambrian black shale in the Tianzhu area of the Yangtze block, is a well-known stratiform barite ore deposits. Its origin remains highly debated. A variety of genetic models have been proposed, including biological enrichment, cold seep and submarine exhalative hydrothermal activity, which have differing implications for paleo-oceanic environmental interpretation and resource exploration. We conducted a detailed sedimentological, mineralogical and geochemical study of the Tianzhu barite deposit. It dominated by anhedral barite with variable carbonate-pyrite-hyalophane associations and siliciclastic admixtures. These typically display porous and pore-filling structures, as well as dendritic and rosette-like growth forms, indicating in situ precipitation near the sediment-water interface. Based on vertical variation in sedimentary and mineralogical features, four vertically successive Units (I–IV) can be recognized. Barite samples in Units I and III display 87Sr/86Sr ratios within a very narrow range (0.7084 to 0.7085), nearly identical with coeval seawater values; while others display elevated ratios (0.7086 to 0.7279) that likely reflect contributions from modified porewater and/or siliciclastic admixture. Barites also display seawater-like and higher δ34Sbrt values (32.3 ‰ to 48.6 ‰) with respect to coeval seawater sulfate, and δ34Sbrt-δ18Obrt co-variation trends extending from seawater sulfate to higher δ-values. Co-occurring authigenic dolomites display moderately negative δ13Ccarb values (−18.3 ‰ to −7.2 ‰). These features suggest precipitation at the sulfate-methane transition zone (SMTZ) near the sediment-water interface. Overall, these characteristics of the Tianzhu barite deposit most closely resemble features associated with barite enrichment in modern and ancient cold seeps. This genetic model differs from previous interpretations, and does not require a restricted basin with seawater sulfate-depletion. This study has implications for understanding of oceanic barium‑sulfur cycling and environmental change in the early Cambrian, and formation of shale-hosted stratiform barite deposit in the geological record.