古城锰矿是扬子地台北缘典型的“大塘坡”式锰矿,赋存于南华系大塘坡组含锰黑色岩系中,含锰矿物主要为菱锰矿.古城锰矿稀土总量高、类似现代海底铁锰结核的“帽式”稀土元素配分模式和Ce的正异常等地球化学指标指示古城锰矿Mn是以锰氧化物或氢氧化物形式沉淀,锰碳酸盐是后期转化而成.Fe/Mn比值低,较低的V、U含量和中等程度的Mo富集以及氧化还原敏感元素对Th/U、V/Cr、V/(V+Ni)比值等地球化学指标指示古城锰矿沉积环境为一种氧化一次氧化的状态,与锰是以锰氧化物或氢氧化物形式沉淀的结论一致.综合认为,古城锰矿成矿机制包括沉淀和转化两个过程,在氧化还原分层的海洋系统中,Mn在上部氧化性的水体中以氧化物或氢氧化物形式沉淀,之后下沉被掩埋在缺氧带之下,在成岩过程中和有机物质相互作用,锰氧化物或氢氧化物被还原释放出的Mn2+和有机碳被氧化释放出的CO32-结合形成锰碳酸盐,其转化可以方程2MnO2+CH2O+HCO3-=2MnCO3+H2O+OH-简单表示.%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 2REE 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 Mn2+ released from oxides and CO2-3 from oxidation of organic materials during the diagenesis, which can be simply described by the follow formula; 2MnO2 + CH2O + HCO3-=2MnCO3+ H2O + OH-.
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