The Gucheng Mn ore deposit is a typical Datangpo type manganese ore deposit in the northern margin of Yangtze platform. Ore bodies occur mainly in Mn bearing black shale series of the Datangpo Formation of Nanhua System, with rhodochrosite as the main ore bearing mineral. The Gucheng Mn ore deposit is characterized geochemically by high ΣREE contents, PAAS normalized REE patterns similar to that of ferromanganese nodules in modern ocean bottom, and positive Ce anomaly, indicating that manganese was precipitated firstly as Mn oxides or hydroxide and then changed into Mn carbonate during digenesis. The redox sensitive trace elements couples, like Th/U, V/Cr, V/(V+Ni) rations, indicate a sub oxic to oxic depositional environment, and this is in consistent with the conclusion that manganese was precipitated as Mn oxides or hydroxide. Our study indicates that metallogenic mechanism of the Gucheng Mo deposit contains two processes (deposition and transformation): Mo was buried under the hypoxia zone after precipitation as Mn oxides or hydroxides in the upper oxic water, and transformed to Mn carbonate resulting from reaction with organic materials due to Mn2+ released from oxides and CO2-3 from oxidation of organic materials during the diagenesis, which can be simply described by the follow formula: 2MnO2 + CH2O + HCO3-=2MnCO3 + H2O + OH-.