Our data show significant negative Ce anomalies (Ce/Ce* as low as 0.4) occurring in kerogen, which indicate an oxygenated surface environment of primary productivity in consistent with the concept that the organic matter is mainly derived from organisms in the euphotic zone. Mass balance calculation suggests that the kerogen-associated REE can dominate the measured black shale REE budget, while similarity between our measured REE patterns and those of similarly aged phosphorites indicates that the REE content of ancient phosphorites may have also derived initially from organic matter.
The redox-sensitive trace elements, such as U, V, Mo, and their ratios of U/Al, V/Al and Mo/Al in black shales show different correlation patterns with TOC contents. The upper black shales show a good metal/TOC correlation, but such a correlation is absent in the lower part. The lower black shales exhibit much higher metal enrichments compared to Black Sea sulphidic (euxinic) sediment. This is taken to indicate the presence of sulphidic bottom waters during the deposition of the lower black shales, including the Ni-Mo ore layer. In contrast, anoxic, non-sulphidic conditions occurred during the deposition of the upper black shales.
Taking all these geochemical data together, we suggest that the early Cambrian South China seaway was strongly stratified and stagnant, and that euxinic bottom water conditions may have led to enrichment of the redox-sensitive metals such as U, V and Mo in the lower black shales, and in one case the occurrence of a polymetallic Ni-Mo sulphide ore bed bearing an extraordinarily extreme metal enrichment, which, according to the Mo/TOC and Ni/TOC ratios and much other geochemical evidence, may have been additionally influenced by hydrothermal input of metals within the rift basin as suggested by a number of previous studies.