Petrological and Ni-Mo isotopic evidence for the genesis of the Ni- and Mo-sulfide extremely enriched early Cambrian black shale from Southwest China

The black shale in the Lower Cambrian Niutitang Formation in Southwest China is anomalously enriched in sedimentary exhalative Ni- and Mo-sulfides. The occurrence of large fluctuation of ocean redox conditions and thriving of benthic animals highlight the uniqueness of the stratum and its link to the Cambrian Explosion. Here, we report our observation of the high-resolution structures of the Ni- and Mo-extremely enriched sulfidic spheroids and measurements of their Ni- and Mo-isotopic compositions. Mo-sulfides appear as clusters of oval spheroids disseminated with pyrite framboids inside; differently, Ni-sulfides fill fractures or appear as coronas of the Mo-sulfidic spheroids. These observations indicate that the Mo-sulfide and the Ni-sulfide are mineralized by different processes. The Ni isotopic compositions of the spheroids and the groundmass fall on the mixing line between seawater (low [Ni]/[Mo] and heavy Ni isotopes) and hydrothermal fluids (high [Ni]/[Mo] and light Ni isotopes), confirming the hydrothermal-fluid-addition of Ni to the deposit. In contrast, the Mo isotopic compositions of the spheroids and the groundmass consistently exhibit their seawater-source. Along with our analyses of biomarkers and X-ray absorption fine structure of Mo, we deduce that Mo in the seawater was reduced and deposited as sulfidic spheroids, while Ni had an additional source from hydrothermal fluids that enhanced the biogeochemical cycles during their depositions.