Geochemistry of Cryogenian Datangpo manganese deposits in the southeastern Yangtze Platform of South China: implications for the origin of metallogenesis and depositional environment

Sedimentary manganese (Mn) deposits in the southeastern Yangtze Platform of China have been newly discovered with considerable potential for resource exploration. Such deposits formed due to dynamic changes in the global environment, including the break-up of the supercontinent Rodinia, Snowball Earth glaciation, and the Neoproterozoic Oxygenation Event. The Mn-bearing sequences are hosted in the basal Datangpo Formation and are interbedded with black carbonaceous shale in a series of graben sub-basins in the Nanhua Rift Basin. We investigated the origin of these Mn deposits by analysing the major and trace elements, total organic carbon, and stable carbon and oxygen isotopes. Positive Eu anomalies were observed, indicating that the Mn was derived from a hydrothermal system. Redox-related elements, including Ce, Mo, U, V, S, and P, indicated that the depositional environment of the Mn layers was oxidative and that Mn2+ was initially oxidized to Mn oxides. However, the host rock (i.e. black carbonaceous shale) was deposited under dysoxic - anoxic and even sulfidic water conditions. The alternating distribution of Mn ore and host rock suggested alternating redox conditions in the sub-basins. A negative correlation between the Mn content and & delta;C-13(carb) values (-10 & PTSTHOUSND;--7 & PTSTHOUSND;) and a deficiency of sulphide minerals indicate that the diagenetic degradation of organic matter (& delta;C-13(org) from - 34 & PTSTHOUSND; to - 31.9 & PTSTHOUSND;) took place through Mn oxide reduction rather than microbial sulphate reduction and can contribute to a considerable source of bicarbonates. The approximately homogeneous & delta;C-13(carb) depletion and their apparent discrepancy with & delta;C-13(org) indicate that the post-depositional genesis of the Mn carbonate ore occurred in a relatively open early diagenetic pore water system with a considerable contribution from seawater-derived carbon sources.

Automated summary

Sedimentary manganese deposits with considerable resource exploration potential have been newly discovered in the southeastern Yangtze Platform of China. The origin of these deposits was investigated by analyzing major and trace elements, total organic carbon, and stable isotopes. The Mn deposits were derived from a hydrothermal system, while the host rock was deposited under dysoxic - anoxic and even sulfidic water conditions. The diagenetic degradation of organic matter through Mn oxide reduction contributed to a considerable source of bicarbonates, and the post-depositional genesis of the Mn carbonate ore occurred in a relatively open early diagenetic pore water system with a significant contribution from seawater-derived carbon sources.