Controls on Mn carbonate precipitation in the formation of Cryogenian Datangpo manganese deposits, southeastern Chongqing, South China

Abstract

The Cryogenian Datangpo Formation (ca. 663–654 Ma) in South China hosts globally significant sedimentary Mn deposits whose genetic mechanisms remain debated. To clarify their formation processes, the present study conducted integrated petrographic, mineralogical, and geochemical analyses of a new drill core from the Xiaochayuan subbasin (Nanhua Basin), southeastern Chongqing. Geochemical evidence show that Mn was predominantly derived from hydrothermal activity associated with extensional tectonics in the Nanhua Rift Basin during the breakup of the Rodinia Supercontinent. Variations in the Chemical Index of Alteration (CIA), together with regional correlations, indicate that Mn accumulation occurred under fluctuating cold and warm climatic conditions following the Sturtian glaciation. Enrichment of redox-sensitive elements and consistently low Mo/TOC ratios demonstrate Mn carbonate-bearing strata were deposited in a highly restricted, anoxic-euxinic environment. Positive Ce anomalies coupled with distinctly negative δ¹³C_carb values in the Mn carbonate rocks, provide compelling evidence for Mn oxide reduction mediated by organic matter degradation. The negative correlation between Mn content and δ¹³C_carb further supports that anaerobic oxidation processes generated HCO-3 supersaturation in porewaters. Microtextural analyses reveal that authigenic minerals with abundant nucleation sites, such as dolomite, pyrite and quartz, acted as catalytic substrates that facilitated Mn carbonate precipitation through heterogeneous nucleation. Collectively, these results establish a genetic framework in which organic matter accumulation, post-glacial climatic oscillations, hydrothermal influx, oceanic anoxia, and intensified continental weathering synergistically controlled the formation of these black shale-hosted Mn deposits.