Paleo-ocean chemistry characteristics of the Cryogenian Datangpo Formation in South China and implications for manganese metallogenesis

Paleo-ocean chemical variations play a critical role in understanding the genesis of manganese (Mn) ore deposits within the Cryogenian Datangpo Formation of South China. Based on lithological observations and multiple geochemical proxies—including iron speciation, molybdenum (Mo), uranium (U), vanadium (V), manganese (Mn) concentrations, Sr/Ba ratios, the Chemical Index of Alteration (CIA), and pyrite sulfur isotopes (δ³⁴S_py)—from drill core ZK0602 in Chongqing, South China, the lower black shales of the Datangpo Formation were deposited under euxinic fresh-brackish water conditions and can be subdivided into four intervals (Intervals I, II, III, and IV). Notably, relatively high Mn concentrations and metallogenesis are genetically linked to ferruginous marine conditions in interval II, which is sandwiched between two euxinic fresh-brackish intervals (I and III). An updated conceptual framework is proposed to decipher the co-evolutionary dynamics among redox condition, salinity fluctuations, and manganese metallogenesis. During intervals I and III, Sturtian deglaciation-driven warming enhanced chemical weathering, increasing terrigenous input to the basin and reducing water-column salinity. Concurrently, elevated nutrient inputs from both open-ocean and terrigenous sources heightened surface-water primary productivity and subsequent anaerobic organic matter mineralization in the water column, inducing bottom-water sulfidization and promoting pyrite formation. In contrast, during interval II, as glacial stadials emerged within the interglacial period, chemical weathering weakened, terrigenous input decreased, and the salinity of the basin water body rose to marine levels. Diminished nutrient supplies from both oceanic and terrestrial sources decreased primary productivity, reducing the downward flux of organic matter and sulfate. This shift favored manganese reduction as the dominant organic matter oxidation pathway. Under ferruginous conditions, Mn ore precipitated near the water–sediment interface.