重庆秀山地区大塘坡组锰矿稀土元素地球化学研究

A Study on REE Geochemistry of the Datangpo Formation-hosted Manganese Deposits in the Xiushan Area, Chongqing, China

为探讨重庆秀山地区大塘坡组锰矿形成的古氧化还原环境,以该区小茶园和笔架山锰矿为研究对象,采用显微镜、扫描电镜-能谱仪(SEM-EDS)和ICP-MS对矿石的主、微量及稀土元素进行了地球化学分析。结果表明,小茶园锰矿的稀土元素地球化学特征和笔架山锰矿相似,其Ce和Eu为正异常;矿石矿物面扫描分析显示,小茶园锰矿的中稀土元素(MREE)主要赋存于锰碳酸盐矿物中,这与锰矿稀土元素配分模式显示的“帽”型特征相吻合;小茶园锰矿的Ce正异常存在假象,并不能指示沉积时水体的氧化环境,可能与缺氧环境有关;其稀土总量和Y/Ho比值明显区别于现代海底铁锰结核(壳),进一步暗示锰矿并非以氧化物形式沉淀;其Eu正异常与(Ca+Mn)含量呈显著正相关,且Y/Ho比值与热液流体更为接近,表明锰矿受热液(水)活动影响明显。

In order to explore the paleo-redox environment for the formation of manganese deposits occurred in the Datangpo Formation in the Xiushan area, Chongqing, the manganese ores of the Xiaochayuan and Bijiashan manganese deposits in the Xiushan area were selected as the research objects for carrying out geochemical analyse of major, trace, and rare earth elements by using microscope SEM-EDS and ICP-MS in this paper. The results show that manganese ores of the Xiaochayuan and Bijiashan deposits have similar REE characteristics with positive Ce and Eu anomalies. The surface scanning analyses of ore minerals show that the MREE mainly occurred in manganese carbonate minerals, indicating that it is consistent with the “cap”-type characteristics of REE patterns of those manganese ores.Based on diagrams of Ce/Cevs. Pr/Prand Ce/Cevs. La, it is believed that the positive Ce anomalies in REE patterns of manganese ores could be fake ones due to the manganese ore was precipitated in an anoxic environment Thus, these diagrams cannot be used to suggest that the manganese ore was formed in an oxidation environment. The ∑REY contents and Y/Ho ratios of ores are significantly different from those of the modern seafloor ferromanganese nodules(crusts), further suggesting that manganese ores of the Xiaochayuan and Bijiashan deposits were not precipitated in the form of oxides. In addition, the obviously positive correlation between positive Eu anomaly values and contents of Ca and Mn of the ores and the similarity of Y/Ho ratios of the ores to those of hydrothermal fluids indicate that manganese ores in the Datangpo Formation in the Xiaochayuan and Bijiashan deposits were obviously affected by hydrothermal activities.