MICROSTRUCTURES OF PISOLITIC AND OOLITIC PHOSPHORITES AND THEIR GENETIC CHARACTERISTICS
鲕粒,豆粒磷块岩的显微结构和生成特点

There are six types of pisolitic and oolitic phosphorites in the Doushantuo, Yuhucun and Xinji Formation, viz., ruduceous, sandy, dolomitic, siliceous, micritic pisolitic phosphorites and siliceous oolitic phosphorite. These phosphorites consist of phosphatic pisolites, oolites and various cements or matrices. The mineral composition forming lamella of pisolite and oolite is mainly micritic apatite that was directly precipitated from marine water. It has concentric laminated structure. The nucleus are mainly quartz sand, dolomite clast and various phosphatic intraclast. The intraclasts occurred in a sedimentary basin and were produced from broken phosphatic deposits. The mineral constituents of the cements are quartz, dolomite, limonite and a small amount of kaolinite, which were precipitated from pore water among the pisolites or oolites. The constituents of matrices are quartz sand, metamorphic rock gravel, dolomite and phosphatic intraclasts. They deposited together with pisolites and oolites at the same time. The pisolitic phosphorites were mainly deposited in high-energy environments of tidal flat and tidal shoal, and the oolitic phosphorite deposited in low-energy environments of deep water zone.