Abstract: The Hongshuihe manganese deposit is located in the transitional zone between the northern margin of the East Kunlun Orogen and the southern margin of the Qaidam Basin. The orebodies are hosted in a weakly metamorphosed siliciclastic-carbonate succession of the Langyashan Formation, yet the depositional environment, sources of manganese, and ore-forming processes remain poorly constrained. This study integrates measured stratigraphic sections, mineralogical observations, and whole-rock geochemical data to investigate the depositional evolution of the Mn-bearing succession, the sources of ore-forming materials, the redox conditions of the water column, and the processes of Mn mineralization. The ores consist mainly of rhodochrosite, calcian rhodochrosite, manganoan calcite, rhodonite, and Mn oxides, recording three principal stages: formation of Mn carbonates during sedimentation and diagenesis, tectono-hydrothermal recrystallization of Mn silicates, and near-surface oxidation. The Mn-bearing succession was deposited in a shallow-marine platform-restricted basin system and is composed mainly of carbonate rocks and mixed siliciclastic–carbonate assemblages. Mn mineralization is concentrated at key stratigraphic intervals marking the transition from siliciclastic-dominated to carbonate-dominated sedimentation, indicating that facies migration and reorganization of the depositional system exerted an important control on the localization of the Mn-rich horizons. V/Cr, Ni/Co, V/(V+Ni), Ce anomalies, and Mo-U enrichment patterns collectively indicate that the water column during Mn deposition was generally oxic to suboxic rather than persistently strongly reducing or euxinic. The weak correlations of MnO with Al₂O₃ and TiO₂, together with the (Fe+Mn)/Ti and Al/(Al+Fe+Mn) discrimination diagrams, suggest a significant volcanic-hydrothermal contribution of Mn- and Si-bearing components to the basin, with additional terrigenous detrital input and subsequent redistribution within seawater. Overall, the Hongshuihe manganese deposit records a multistage evolutionary history comprising sedimentary-early diagenetic mineralization, subsequent tectono-hydrothermal modification, and supergene oxidation. The deposit is therefore interpreted as a sedimentary manganese deposit influenced by volcanic–hydrothermal input and subsequently modified by tectono-hydrothermal and supergene processes..