The Bayan Obo Group of Middle-Upper Proterozoic is composed of a series of continental margin rift sedimentary rocks distributed in central Inner Mongolia, China. Based on structural characteristics six sedimentary cycles, represented by the Dulahala Formation, the Jianshan Formation, the Halahuogete Formation, the Bilute Formation, the Baiyinbaolage Formation and the Hujiertu Formation, can be recognized, in which the Jianshan and Bilute formations are important parts. They contain abundant organic materials and show significant enrichment of a variety of oreforming elements (Au, P, Fe, and REEs）. Samples of carbonaceous shale collected from the black rock series were analyzed for trace elements and rare earth elements that were used to demonstrate the paleoenvironmental settings. Remarkable enrichment can be recognized of a variety of trace elements, including Hg, B, As, Mn, Sb, Au, Ag, Pb, P, Ba, U and Mo. The high content of B in the black rock series represents a highsalinity sedimentary condition; high V/(V+Ni） and Ce/La indicate an anoxic sedimentary environment for these shales. The Zn-Co-Ni diagram and the high contents of Ba, As, Sb and B are important indicators of hydrothermal sedimentation. The differences between the Jianshan Formation and Bilute Formation in REE patterns, Ce and Eu anomalies, La/YbΣREE and La/YbCe/La diagrams suggest that the Jianshan Formation consists of dominating terrigenous matters with few hydrothermal sediments, while the Bilute Formation formed in a deeper depth contains a higher proportion of hydrothermal sediments than the Jianshan Formation does.
