Study On The Vanadium Enrichment Mechanism In The Lower Cambrian Black Shale In Sansui, Southeast Guizho

The Ediacaran-Cambrian transition is an extremely important geological period in the long history of earth evolution.A series of major global geological events have occurred,including the paleomarine chemical conditions changes,the emergence and rapid differentiation of metazoans（Cambrian life explosion）,large-scale metallogenic events（Ni-Mo-V-Ba polymetallic mineralization）.These major geological events have been recorded and preserved by the lower Cambrian black shales,which widely distributed all over the world.Generally speaking,how polymetallic elements are generally enriched and mineralized on a global scale is one of the most remarkable scientific questions.A large number of molybdenum,nickel,vanadium barite and other polymetallic deposits are developed in the lower Cambrian black shales on the southern margin of the Yangtze platform,especially in Guizhou,Hunan provinces,which provides an important window for exploring the lower Cambrian polymetallic mineralization.Indeed,compared with molybdenum,nickel and barite deposits,the study on lower Cambrian black shales vanadium deposits is relatively weak,and the research on the source of vanadium,the evolution of metallogenic environment and analysis of metallogenic model needs to have further exploration.Sansui vanadium deposit in Southeastern Guizhou is an important content of lower Cambrian black shales polymetallic deposit in the southern margin of Yangtze platform.The ore body mainly occurs in black carbonaceous and siliceous mudstone at the bottom of Jiumenchong formation in Early Cambrian.Taking Sansui vanadium deposit in Southeastern Guizhou as the research object,this paper systematically carried out a comprehensive study of petrography,element geochemistry,organic carbon composition and Hg isotope geochemistry.Through SEM experiments,we found that there are a large number of barite and its banded surrounding structure in the V-enriched samples and vanadium mainly occurs in clay minerals such as illite,which can be supported by the good covariant relationship between vanadium concentration and TOC in Sansui vanadium samples,and reveals that the enrichment of vanadium is closely related to the adsorption of organic matter.Element geochemical analysis shows that the samples indicate hydrothermal signals in the Lg Th-Lg U binary diagram,U/Th value and Y/Ho value.The Hg isotopic study of black shales samples further shows that the most of theΔ¹⁹⁹Hg values are near～0（volcanic Hg）.Considering that a 5-10cm thickness volcanic ash layer is developed at the boundary between Liuchapo and Jiumenchong formation in Sansui vanadium deposit,we infer that the material source of vanadium mineralization is likely to be related to volcanic/hydrothermal activities in the same period.The Mo-TOC binary diagram shows that the study area is in an strong restricted basin environment.Through analyze the binary diagram of Mo _EF-U_EFand the relevant redox sensitive elements,we found that there is an obvious fluctuation of redox conditions（oxic at the bottom of Liuchapo formation,anoxic/euxinic at the top of Liuchapo formation and the bottom of Jiumenchong formation）.Furthermore,the samples in Sanshui vanadium deposits have high CIA index（>50）as a whole,and the Al₂O₃value（18.83%）,Mo Concentration（89.3ug/g）and CIA（>80）also reach the highest value at the vanadium enriched layer of Jiumenchong formation,suggesting that strong continental weathering may promote the input of a large number of terrestrial materials and provide necessary material sources for mineralization,which can be supported by theΔ¹⁹⁹Hg-δ²⁰²Hg binary diagram as well(relatively lowerΔ¹⁹⁹Hg).In conclusion,we support that the hydrothermal/volcanic activity and enhanced continental weathering on the Yangtze block during the Ediacaran-Cambrian transition period provided the main sources for the Sansui vanadium deposit in Southeast Guizhou.Considering that the polymetallic deposits temporal and spatial distribution characteristicsin and tectonic evolution background in the Yangtze platform,which result in the strong seafloor expansion and other related geological processes leading to the continuous upraise of uplift facies region.Then,an ideal place for mineralization in a protected basin isolated from the deep-water basin facies region is formed in the transitional slope facies region.The initial ore-forming materials further migrated to the transitional slope facies region（especially strong restricted basin）and accumulated on a large scale.In addition,with the strong continental weathering and hydrothermal activities,a large amount of organic matter enters the ocean and the productivity of ancient ocean is improved,which jointly promotes the water environment in the limited basin to show the anoxic/sulfide condition,in such extremely limited basin anoxic/sulfide condition,a large amount of preserved organic matter further adsorbs and controls vanadium for mineralization.