Discovery of phosphate concretions in the upper Sinian Fengtai Formation at the southeast margin of the North China block and their implications

Phosphate concretions have been found in the basal and middle parts of the Fengtai Formation at the southeast margin of the North China block, which indicates that the Fengtai Formation (at least a part of it) probably formed in the warm marine environment. The geochemical characteristics of the phosphate concretions indicate that the depositional environments of the two horizons of phosphate concretions are different: the base of the Fengtai Formation formed in a marine environment with relatively strong mantle activity, while the middle part formed in a closed environment with relatively weak mantle activity.