Genesis and metallogenesis of Early Cambrian manganese deposits on the northwestern margin of the Yangtze Platform, South China

• Ore-forming materials were mainly derived from submarine hydrothermal fluids. • Mineralogical and geochemical evidence indicates a stratified paleo-ocean with a “sulfidic wedge” during Mn carbonate deposition. • The Maowanli Mn deposit formed under fluctuating dysoxic–suboxic–euxinic water-column conditions. • Spherical kutnohorite suggests that microbial EPS promoted Mn carbonate precipitation. Abundant microbially textured “spherical” kutnohorite in the Early Cambrian Maowanli Mn deposit provides new constraints on the precipitation mechanism of Mn carbonates. Here we integrate petrography with EDS, EPMA, and TIMA analyses to document mineral assemblages and Mn-bearing phases, and combine major-trace element geochemistry, TOC-TS data, and C-O isotopes to constrain the sources of ore-forming materials and depositional redox conditions. The Maowanli Mn ores are dominated by kutnohorite, Mn-calcite, and alabandite, with abundant framboidal pyrite (mean diameter < 6 μm), organic matter, barite, and hyalophane. The occurrence of hyalophane, together with hydrothermal-related geochemical signatures (e.g., Al/(Al + Fe + Mn) < 0.35 and (Fe + Mn)/Ti > 20), indicates that ore-forming components were mainly supplied by submarine hydrothermal inputs. Framboidal pyrite size distributions and redox-sensitive proxies (e.g., V/(V + Ni), V/Cr, and Ni/Co) suggest that Mn mineralization occurred under a highly dynamic water-column setting with fluctuating dysoxic-suboxic-euxinic conditions. Elevated TOC (>1 wt%), coupled with Cd-Mo systematics, implies relatively high organic matter input and weakly restricted to restricted depositional conditions. Enrichments in TOC, TS, and redox-sensitive trace metals (U, Mo, and V) further support a “sulfidic wedge” was present in a stratified paleo-ocean. Slightly negative Ce anomalies (average 0.63), negative δ 13 Ccarb values (average −6.21‰), and a strong positive Mn-TOC correlation argue against a dominant transformation from Mn oxides/oxyhydroxides. We propose that “spherical” kutnohorite precipitated directly in an anoxic ferruginous environment, mediated by bacterial sulfate reduction (BSR) and extracellular polymeric substances (EPS).