Sedimentary Character And Metallogenic Mechanism Of Stromatolitic Phosphorite Of Dengying Formation, Weng'an County Of Guizhou Province, China

Phosphorite deposit is one of the important non-metallic mineral resources of Guizhou Province, it mainly distributed in the areas of Kaiyang- Xifeng and Wengan- Fuquan. For a long time, the goal of exploration survey of phosphorite deposit has been Doushantuo Formation of Sinian and Gezhongwu Formaiton of Cambrian. A new phosphorite deposit with the columnar stromatolite structure had discovered in the top of Dengying Formation of Sinian with the deepening of research degree in recent years. It is a new type of phosphorite deposit and a new stratum. The study of the phosphorite deposit geological characteristics and metallogenic mechanism has important guiding significance. Therefore, according to stromatolite phosphorite at the top of the Dengying Formation as the research object, through to its mineralogy, petrology and geochemistry characteristics of research, obtained the following results:1. The results of the stratigraphic sequence of phosphorite series shows that the phosphorite bedding existed in the top of Dengying Formation. The bedding of phosphorite is organic reef of stromatolite, and the columns of stromatolite are the ore-body of phosphorite, that thickness of ore bed from 1.48 m to 11.51 m, and there is a lenticular phosphoric dolomite in middle and upper place of ore-body of phosphorite.2. Mineralogy and petrology of dark laminae and white laminae research results indicate that mineral of dark laminae are mainly dolomite, and white laminae are mainly consisted of dolomite, cellophane and phosphatic algal pellet. It could be seen silkiness and Prismatic algae in dark laminae, and algal pellet are formed of silkiness algae. These results indicate that organic matter and acid which formed by the processing of algal growth changed physical-chemical condition of water, and it led to sedimentation of phosphate.3. Major elements analysis showed that the content of P2O5 of white lamina is significantly higher than the dark laminae, that explanation white lamina forming phosphatisation bedding. Phosphate mineral of phosphorite are carbonate fluorapatite mainly.4. Trace elements analysis showed that the phosphorite was significantly enriched in U, Mo, As, Sr, and Ba elements. Mo, As and Sr element are biological elements, that indicated mineralization of phosphorite related to algae.5. REE distribution patterns of phosphorite is similar to biogenic phosphorite, that indicate the formation of deposits had related to biology. Ceanom value(-0.25 ~-0.11) of phosphorite reflect that the environment of phosphorite may be secondary oxidizing environment. Ceanom value and content of Nd diagram shows phosphate precipitation process has a high deposition rate.6. Carbon-oxygen isotope analysis shows that carbon isotope of phosphorite and dolomite of alage have lower than the others dolomite of Dengying peroid, which indicating that biology could lead to degreed of carbon isotope. The ore-forming process of phosphorite is directly or indirectly relative to biology of algae. But at the same time, the water with retention characteristics of the deep hot seawater has been brought to the mineralization area by ascending current. It exacerbating ?13C negative bias of phosphorite. Ancient temperature by oxygen isotope calculation results shows that the forming temperature of matallogenic period from 19.28? to 77.89?, and white laminae is much higher than dark laminae. It indicates that the temperature is invalid by the fractional distillation of algae.7. Phosphorite deposit was formed in shallow shoal of open platform margin environment. The sedimentary environment of phosphorite had from subtidal zone to lower part of intertidal zone. Stromatolite was formed in the deeper water environmental conditions with low-energy. The formation of phosphorite associated with biogenic agency.8. According to the characteristics of biogenic agency, mineral deposit and facies of phosphorite, the exploration survey of phosphorite of Dengying Formation should distribute in the shallow shoal of open platform margin facies. There put forward two metallogenic prospect areas, which are Xiaoba – Yangaitang in the north of study area and Huanglianba of Meitan county.