PHOSPHORITES OF DENGYING FORMATION DURING SINIAN PERIOD AND THEIR GENESIS
震旦纪灯影组磷块岩及其生成特点

Of the phosphorites in Dengying Formation there are four types, viz., micritic, intraclastic, algal and finecrystal phosphorites. The major minerals making up the micritic phosphorite are micritic apatite and small amounts of illite and quartz. The micritic apatite was directly precipitated from marine water. The intraclastic phosphorite consists of phosphatic intraclasts and various cements or matrix. The intraclaste occurred in sedimentary basin due to the break of phosphatic deposit. The mineral constituents of the cement are sparry apatite, dolomite and quartz, which were precipitated from pore water between the intraclasts. The constituents of matrix are dolomite and illite. They deposited together with intraclasts at the same time. The algal phosphorite consists of phosphatic oncolite and micritic dolomite matrix. The finecrystal phosphorite was mainly produced by recrystallization of the micritic apatite and formed under the influence of siliceous solution. The phosphorites tended to deposit in the pericontinental sea, rather than in the epicontinental sea. In the pericontinental sea, phosphorite usually accumulated in the shallow water region associated with deep water. The background of deep and shallow water environments was tectonic depression and carbonate rock platform respectively. The former provided mineragenetic material, and the latter provided advantageous sedimentary environment for the mineralization of phosphate.