Abstract:

Lower Cambrian black shales deposited widely in South China，which are mainly composed of the black shales, black dolomitic shales, black arenaceous shales and black siliceous shales. There are thick mineral deposits of phosphorus, vanadium deposit, molybdenum and nickel in the black shales. Petrologic characteristics and elemental geochemical characteristics of black shales were studied systematically, and the causes of enrichment of trace elements were analyzed in detail. The results show that the black shales were formed in reductive environment of gentle slope in shallow to bathyal milieu. The layered element enrichment belts were of sedimentary origin, and abundant substances were mainly from oceanic hydrothermal fluid. In general, the contents of Co, Be, Mn and Ga are relatively low. Cu, V, Ni and Mo have high content, and most of them are higher than North American Shale Composite (NASC). Element layers of Ni and Mo are usually located above the phosphorites and the sulfide and sulfate minerals in black shales and black dolomitic shales are the main carriers. The content of TOC is as high as 12.2%, which is much higher than any other element layers. With the increase of siliceous component in shales, the contents of Ni, Mo, Fe and Co decrease significantly. Many elements, such as V, mainly occur in hydromicas in the black siliceous shales. The content of V has a positive relationship with the content of siliceous components in shale. REE in black shales varies from 76.22×10-6 to 290.67×10-6 and LREE/HREE changes in the range of 1.21 to 4.22, which indicate that light rare earth element has high enrichment. The value of Sr/Ba ranges from 0.04 to 0.34, δEu is positively anomalous and the distribution patterns of REE normalized by North American shales are gentle leaning to LREE. These evidences indicate the existence of oceanic hydrothermal fluid during the deposition process.

Key words: black shales, trace element, REE, enrichment causes, Lower Cambrian, middle and lower Yangtze area, South China