Dual pathways controlling Mn-carbonate mineralization in redox-stratified basins: Geochemical constraints from the Ortokarnash deposit (Western Kunlun, China)

The formation of sedimentary manganese deposits—vital sources of a critical metal for green technologies—is intimately linked to redox-stratified basins, yet whether Mn-carbonates precipitate directly in anoxic water columns or form diagenetically in sediments has remained unresolved for decades. To resolve this dichotomy, we present integrated geochemical constraints (stable isotopes, rare earth elements, pyrite framboid morphometry, and mineral chemistry) from two laterally adjacent profiles in the Upper Carboniferous Ortokarnash deposit (Western Kunlun, China), a giant sedimentary manganese deposit. In the deeper OP-1 profile, laminated textures, syndepositional pyrite framboids (2–5 μm), pronounced positive Ce anomalies, and moderately negative δ13C values (−8.72‰ to −6.77‰) collectively constrain an authigenic pathway: Mn-carbonate precipitated directly in anoxic, H2S-rich bottom waters via reductive dissolution of Mn oxides and mixing with dissolved inorganic carbon. In contrast, the shallower OP-2 profile is characterized by massive textures, highly negative δ13C values (−22.83‰ to −21.67‰), elevated total organic carbon (7.02–8.29 wt%), and conserved Ce anomalies, indicating a diagenetic pathway: Mn-carbonate formed in organic-rich porewaters through microbial reduction of preexisting Mn oxides coupled with organic matter oxidation. These results demonstrate that two distinct pathways concurrently control Mn-carbonate mineralization in a single redox-stratified basin. We propose a unified dual-pathway model that reconciles competing genetic hypotheses and provides a general framework for interpreting Mn-carbonate formation in both Phanerozoic and Precambrian redox-stratified systems. This study highlights the power of multiproxy geochemical constraints in deciphering complex mineralization processes in redox-sensitive basins, with broad implications for reconstructing ancient redox stratification, understanding ore formation linked to Earth’s oxygenation, and guiding exploration for manganese resources in analogous settings.