SYSTEMATIC RESEARCH ON THE ORIGIN OF THE PHOSPHORITES OF DOUSHANTUO STAGE CENTRAL GUIAHOU, CHINA

As a iind of sedimentary rocks the phosphorite is also regarded as a type of sedimentary mineral deposits so that the study on it is of petrololgical and economic significance. The origin is one of the most important subjects in phosphorite study. Taking the predeccesors ' achievements as starting points and the phosphorite deposits of Doushantuo Stage in Central Guizhou as "dissecting" objects and meanwhile using the theories and the methods of multiple sciences, this paper carried out a systematic study on the origin of the phosphorite from the aspects of the source, the deposition and the enrichment of phosphorus. The Central Guizhou phosphorite-forming region is located in the counties of Xifeng, Kaiyang, Weng' an, Fuquan, Zunyi, Meitan and Yuqing, Guizhou Province where exist two large phosphorite deposits, namely Kaiyang phbsporite deposit and Wengfu phosphorite deposit, and a series of middle to small ones. They are important phopshorite deposits of Doushantuo Stage in the Upper Yangtze region and mainly composed of sand-sized intraclast phosphorite, stromatolite (stratifera) phosphorite and algal pellet phosphorite.The main phosphate-rich environments are intertidal to subtidal ones with higher energy (shoals and algal reefs) on the shallow-water platform margin zone. Analysis of REE, especially the LREE / HREE ratio and the cateforized analysis of REE samples indicate that the phosphorus might be derived from the contemporaneous volcanic materials. The study of the textures and the compositions of phosphorites, ocean geochemistry and biochemistry shows that the assimilation and the fixation of algal is the most possible way of phosphorus deposition after which a series of enriching processes are required to form the phosphorite deposits being worth mining. The enriching processes are multistage and different enriching stages (or ways) lead to the formaion of different types of phosphorites.