Geochemical Characteristics And Metallogenic Mechanism Of The Phosphorites In Weng’an, Guizhou Province

Guizhou ranks the second of all China’s provincial administerial units in the phosphate reserves.Weng’an phosphorus deposit is part of the Wengfu phosphorite deposit which is one of the top three regions of phosphorite deposits,and it is a typical marine sedimentary phosphorite deposit.Several phosphate ore explorations had been conducted in Weng’an region,which provided abundant geological data for phosphorite deposit study here.Therefore,Weng’an phosphorite deposit is chosen as the research subject.Constrained by paleogeographic conditions,Weng’an phosphorite deposits mainly occur in Wengzhaoba bay,Dawan bay and phosphorus accumulating platforms.The ore bodies,divided into two cycles of upper and lower phosphate-bearing layers,are thick and stable.Exploring at many sites suggests that there is only upper or lower cycle of phosphate-bearing layer occurs in the near Wengfu ancient island area,the contact zone of bays and the extensive sea.Ore species and mineral compositions of Weng’an phosphorite are complex,and sediment size variations exist in longitudinal profiles.By exploring valuble geological information possessed in the above-mentioned facts,we intend to study the sdimentary environment of diagenetic period and the metallogenesis.Migration forms and occurrence states of elements can reflect paleoclimate of diagenesis and oxidation-reduction conditions of sedimentary circumstances.In addition,the contents of mineral elements are related to their sources.By studying the components,the occurrence states and the contents of elements of phosphorite,we attempt to reveal the sources and accumulation of phosphorus,explore the metallogenic mechanism and metallogenic model of phosphorite deposits,ultimately,provide information for the exploration of similar phosphorite deposits.Based on the conclusion of the previous research on the idea of phosphorus rock,this thesis focus on the structure,stratigraphic unit,the typical geological profile,deep drilling core,appraisals phosphate rock ore,analyzes that the characteristics of the longitudinal profile of the tuo formation in the various mineralization zones,mineral geological characters;analyzes the content of elements in phosphorus rock,and the microelements and rare earth elements are standardized respectively with the average crust and the North America shale,explor the correlation between elements,the form of the various components,and the phosphorus is concentrated in the mining process,the effect of each component on phosphorus accumulation;besides,based on the geochemical characteristics of elements,explored the source of phosphorus,the ancient climate conditions,analyzed the sedimentary environment of weng,discussed its metallogenic mechanism.（1）Mineral compositions of Weng’an phosphorus ore is mainly collophane and dolomite,then comes quartz,purite,chlorite,hydromica,carbon.Types of ore give priority to intraclast phosphate rock,coal shale phosphate rock and compact phosphorite.Microcrystalline-colloidal structure,pelitic texture,fine crystallinecrumb structure,there are three kinds of structure,such as massive,stratiform and banding.Among them,microcrystalline-colloidal is that carbonate directly separate out from medium in relatively low energy environment,formation in colloid coagulation and algae binding.（2）Phosphorite are rich in P₂O₅,CaO,a positive correlation relationship with them,proving that carbonate platform is the condition of phosphatic gathered,value of Ca/Ca+Fe is a positive correlation relationship with P₂O₅,high salinity of the marine environment is more conductive to phosphatic deposits.Clay minerals are constituted by SiO₂,K₂O,TiO₂ and Al₂O₃,which is a negative correlation relationship with P₂O₅,reflecting clay materials that are in the form of cementing in pore between particles,hinders the phosphorous qualitative cementation,high levels of SiO₂ reflect that diagenetic stage has experienced strong silicification,shows a slight negative anomaly of δCe,value of Ceanom and Na content parameters a scatter diagram reflect that carbonate with high deposition rate forms in allow sea the Redox transitional environment,and sedimentary system of the up cycle have stronger oxidability than the down cycle of phosphate strata.（3）Volcano at the bottom of the sea erupts a mass of phosphorous,which are rose loop current to superficial,flow in carbonate platform from east to west,with shallow sea oxygen-rich environment,algaes gather phosphorus,lack of oxygen,are dead,transfer and coagulation to seafloor,phosphorus form phosphorus ore by enrichment firstly.Thereafter,occuring large-scale transgression and regression,waves screen phosphorite mechanically,which lead to phosphatic graining,physical and mechanical enrichment,energy recedes,and deposit in favorable places such as shallow,bay,form high-quality industry phosphorite,and the later part of forming rocks occurs stronger silicification.