Characteristic of rare earth element geochemistry of the Early Carboniferous Longtou manganese ore deposit in the Yizhou district, Guangxi, China and its geological significances

The precipitation of manganese oxides in modern marine environment is usually characterized by their positive cerium anomalies (δCe > 1). Based on this, the positive Ce anomalies found in ancient sedimentary manganese deposits are used to infer that sedimentary manganese deposits were originally precipitated in the form of manganese oxides. However, the published data show that the positive Ce anomalies in this type of sedimentary manganese ore deposits are not always present. In order to find out the formation mechanism of this feature, rare earth element analyses and petrographic observations of ores from the Early Carboniferous Longtou Manganese deposit in the Yizhou area, Guangxi Zhuang Autonomous Region have been carried out in this work. The following results have been obtained. 1) There are significant differences in the REE distribution patterns of ores from the four ore layers, with the δCe values vary greatly from the first ore layer to the fourth ore layer, ranging from 0.33 to 0.53, from 0.80 to 1.61, from 0.97 to 1.64, and from 1.24 to 1.67, respectively. These mean that the positive Ce anomaly for manganese ore deposit is not common and may not be controlled by the adsorption of manganese oxides. 2) Petrographic observations show that Ce and other rare earth elements mainly occur in the form of monazite which is only found in ore samples with positive Ce anomalies and is accompanied by the presence of scheelite and tetrahedrite. 3) Ore samples with positive Ce anomalies usually have positive europium (Eu) anomalies, suggesting that the precipitation of those manganese ore deposits could be influenced by hydrothermal fluids derived from the interaction with basic-ultrabasic rocks. Based on the petrographic observations and the spatial variations of δCe and δEu anomalies of ores in four layers of the manganese ore deposit, it is believed that the positive cerium anomalies of ores in the Early Carboniferous Longtou manganese deposit in the Yizhou area, Guangxi Zhuang Autonomous Region, may be caused by the presence of rare earth minerals such as monazite with a hydrothermal origin rather than by the adsorption onto manganese oxides. Therefore, the application of cerium anomaly as a proxy for identifying the redox conditions of the formation of sedimentary manganese deposits should be combined with the corresponding petrographic observation results in order to make a comprehensive evaluation .