THE TEXTURE TYPES, GENESES AND ORE-FORMING STAGES OF SEDIMENTARY PHOSPHORITE

The classification of sedimentary rocks according to texturegenesis can organically associate the texture tpes with their depositional environments. Therefore, the author classified and described the Chinese phosphorites with this method. In view of the field investigations and the surveys of more than 1400 slices of the phosphorite taken from varied parts of China, the author discussed further the genses of the different kinds of phosphorites and the varied stages of oreforming of phosphorites. According to the differences of the texture types, the Chinese phosphorites can be classified into four groups; 1. The chemical sedimentary texture group Phospholutite is its representative type. It is formed by chemical or colloid-chemical sedimentary process in bottom water enriched with phosphates. 2. The bio-sedimentary texture group It includes pellet, oncolite, stromatolite, guano and boi-clastic phosphorites. They originated from the organism itself or bio-mechanical and bio-chemical processes. 3. The granular texture group. It includes intragranular (i.e.,intraclast, oolite,pisolite) phosphorites and extra-granular (i.e., extraclast) phosphorites .They are formed by the process of physical enrichment, i.e. by mechanical breaking and winnowing process of sea water, being controlled by the energy condition of water basin. 4. The diagenesis texture group It includes nodule, recrystal and replaced phosphorites. They are controlled by diagenetic environment and physicochemical condition in diagenetic process. There are three kinds of cements in phosphorite, i. e, phosphate (with argillaceous at times) cements,carbonate cements and silicate cements. From deep water to shallow water, the changing regularity of the cement kinds is from silicate, phosphate to carbonate. The ore-forming process of phosphorites is very complex, it can be summed up into the following six changing stages: Stage l.The phospates are enriched together in sea basin. It is suggested that phosphorus came from sea-resource, terrestrial resource, and volcanic resource.The phosphates are enriched in sea basin and constituted some local phosphate-rich areas. Stage 2.The chemical sedimentary stage in bottom water.Under the influences of chemical, colloid-chemical and bio-chemical processes, the phosphates in phosphorus-rich sea-water precipited on the interface of sediment/water,forming phospholutites. Stage 3. The phosphate deposited down because of organism or bio-mechanic and bio-chemical processes. With the phosphates constituted biotic cells and tissues and formed biogenetic series phosphorites, organism imbibing phosphorus from sea water. Stage 4.The physical enrichment stage. The nodules and the broken materials of the phosphorites formed early on sea-floor were enriched gradually by scouring, winnowing of sea water, forming intragranular and extraclastic phosphorites. This stage is the most important one among all the stages for industrial phosphorites. Stage 5. The cements precipited from phosphate-rich interstitial water. When the intergranular pore spaces and other pore spaces were filled with phosphate-rich pore water, the phosphate cements would gradually deposit. Stage 6. The changing in diagenetic process. Recrystallization or replacement took place in the diagenesis process of all kinds of phosphorites formed early, forming replaced phosphorite and micrite-medial phosphorites. In the phosphates- rich ooze, the phosphorus removed toward some centres, forming nodule phosphorites. By above-mentioned stages, plentiful types of phosphorite were formed. Surely, the six stages above couldn't develop completely in all deposits, some stages might be absent.