Sedimentary Geology Of The Phosphorite Deposits And Phosphogenic Event From Ediacaran Doushantuo Formation In Central Guizhou Provinve

The Doushantuo phosphorite in central Guizhou Province having high P₂O₅ grade and great economic value is one of the important phosphate resources in the world,which is regarded as the first true phosphorite giant in Earth history during the Neoproterozoic-Cambrian Phosphogenic Event.Researches for the depositional conditions and mineral deposit formation of the Doushantuo phosphorite in central Guizhou Province interpret the formation processes of phsohorite deposits and then provide the significant theoretical foundation for the mineral exploration as well as further insighs for the study of global changes in geological history.This study is suppoted by the project of the National Natural Science Foundation of China?U1812402?,the China Geological Survey?12120114016501?and the Bureau of Geology and Mineral Exploration and Development of Guizhou Province?NO.[2013]56?.Based on the detail works of field survey,investigation and catalog of drill cores,sampling and comprehensive analysis of petrology,mineralogy,sedimentology,stratigraphy,geochemistry for the Doushantuo phosphorite in the Kaiyang,Weng'an,Fuquan,Xifeng,Zunyi and Danzhai region,this study have achieved the research results for the sedimentary geology of the phosphorite deposits and Phosphogenic Event from Ediacaran Doushantuo Formation in Central Guizhou Provinve.These theoretical achievements are also combined with practical application,yielding the following results:?1?Two phosphorite facies,pristine phosphorite and reworked phosphorite,have been identified on the basis of their sedimentary structures and petrographic characterisitics in the Doushantuo Formation.Pristine phosphorites are precipitates composed of apatite minerals formed physicochemically or biochemically in situ in the marine environment,reflecting the locus of phosphogenesis in sediment.They occur as micritic phosphorites and biochemical phosphorite in the Doushantuo Formation,which developed in the low-energy deep-water or barrier coast environment.Reworked granular phosphorites are allochthonous high-energy deposits produced by the hydraulic concentration of francolite peloids,grains and fragments derived from pristine phosphorites,characterizing the Kaiyang Doushantuo economic phosphorite deposits.Rework phosphatic grains are cementedby francolites and dolomites in early diagenesis process,and commonly experienced weathering or silicification in late diagenesis stage.?2?Reconstructing the paleogeography and sedimentary environment of Doushantuo Formation in central Guizhou Province.Kaiyang region was located in north marginof central Guizhou Highland which was a high-energy phosphorus rich beach during the Doushantuo period.The economic phosphorite deposits in Kaiyang were mainly composed of granular phosphoriteswhich went through multi-stages broken,exposed and reworked during the process of sedimentation diagenesis.Weng'an region is a semi-restricted shallow bay where phosphate-rich water was euphotic flow into the euphoticzone,providing the favourable environment for Ediacaran radiation of life,resulting in the acculumation of pristine biochemical phosphorite deposits with abundant fossils.The micritic phosphorite interbedded with shale,dolomicrite layers developed in deep-water shelf such as Zunyi and Danzhai region is a pristine facies with weak reworking,which is difficult to concentrate the P₂O₅ content.?3?The Doushantuo phosphorite in central Guizhou Province is the upwelling-related deposits,and its formation and expansion are coupled with the Neoproterozoic Oxygenation Event.Comprehensive analysis of lithofacies characteristics and geochemical data behavior indicates that the phosphorite of the Doushantuo Formation developed in an upwelling-related phosphogenic systems.The Neoproterozoic Oxgenation Event paved the way for the evolution of multicellular animals and produced the conditions necessary for the accumulation of economic phosphorite in the Doushantuo Formation.Phosphatic lithofacies,formed as the result of the concentration of phosphate near the redox interface through microbial mediation or“Fe-redox pump”in the Doushantuo Formation developed along the entire Yangtze epeiric platform,recording a change in the marine phosphorus cycle linked to the progressive oxygenation of the ocean.Ventilation permitted the expansion of phosphogenic environments into deeper and deeper environments.The phosphorus removal from water column conversely controlled the conversely primary productivity and ocean-wide oxygen demand,enhancing the oxygenation of the deep ocean and then further reinforcing eukaryote evolution.Thus,the Neoproterozoic Oxygenation Event have regulated the coevolution of phosphogenesis expansion and life radiation,which was well recorded in Doushantuo phosphorite.?4?The dynamic sedimentary and metallogenic processes of high-grade phosphorite in Doushantuo Formation of Central Guizhou provide could be divided into three stages.The first stage is the pristine phosphogenesis:upwelling of the bottom waters might have brought P-enriched waters to the shallow photic zone,stimulating the supersaturation of phosphate in seawater through the bio-chemical dynamism and then the precipitation of francolite.The second stage is winnowing and reworked process:the syndepositional wave-and storm-induced erosion,winnowing and reworking continuously restructured pristine phosphatic facies and removed the lighter terrigenous grains,forming assemblages of economic granular phosphorites.The last stage is expose and leaching process:the exposure event caused by the sea-level fall led to the weathering and leaching of phosphatic successions,the slightly acidic meteoric water dissolved carbonate minerals creating carbonate-rich solutions which travelled through subsurface drainage,resulting in secondary enrichment in P₂O₅ content.The three-stage sedimentary and metallogenic processes regulated by the paleogeography and frequent sea-level changes dynamically and repeated worked on the phosphatic deposits,finally forming the high-grade and economically significant phosphorites.