Paleo-ocean environmental fluctuations controlling manganese mineralization and organic matter enrichment in the Cryogenian Datangpo Formation, Western Hunan, China

Against the backdrop of profound Neoproterozoic climatic and oceanic changes, the Cryogenian Datangpo Formation in western Hunan hosts distinct vertical successions of manganese (Mn) deposits and organic-rich shales. However, the paleoceanographic processes driving this transition remain unclear. This study presents a systematic analysis of mineralogy, elemental geochemistry, carbonate carbon isotopes, and whole-rock strontium isotopes of the member Ⅰ of the Datangpo Formation in the Minle Mn deposits, western Hunan. Results show that the basal Datangpo Formation contains two Mn ore layers composed mainly of subhedral, granular microcrystalline rhodochrosite, characterized by light carbonate carbon isotope values (–9.74 ‰ to –9.23 ‰), high initial 87 Sr/ 86 Sr ratios (0.714259–0.715730), and geochemical signatures indicative of a hydrothermal–hydrogenetic mixed origin. These features indicate that the ore-forming materials of the Minle Mn deposit were likely derived from hydrothermal activity in deep-water basins, with an additional contribution from terrigenous weathering owing to its depositional setting proximal to the palaeoland. These materials initially precipitated as Mn oxides under fluctuating oxic–dysoxic conditions during the interglacial onset, and ultimately transformed into rhodochrosite during diagenesis through interaction with organic matter. Subsequently, climatic warming intensified continental silicate weathering, supplying substantial nutrients and stimulating an increase in primary productivity. Concurrently, a progressively stable anoxic environment favored organic matter preservation, ultimately forming the black shale with excellent hydrocarbon potential. We propose that the post-glacial transition from fluctuating oxic-dysoxic to more sustained anoxic water column was the critical control driving the shift from Mn mineralization to organic matter enrichment, providing a framework for predicting Mn ore and petroleum-bearing horizons in the Datangpo Formation.