| Emeishan basalt widely distributes in Sichuan,Yunnan and Guizhou provinces in Southwest China,forming Emeishan igneous province.Emei igneous province erupts in many stages,and the main eruption period is about 0?258?259 ma.After the eruption,a large area of basalts is formed in the three provinces,which are slightly different.The basalts in eastern Yunnan and Western Guizhou are mainly high titanium basalts.Influenced by weathering and other geological processes,Emeishan basalt often forms different types of weathering crust between Permian Emeishan basalt formation and Xuanwei formation in Southwest China.Since the report of Nb rich polymetallic layers in eastern Yunnan,Nb rich polymetallic layers have been found in the clay rocks at the bottom of Permian Xuanwei formation in Sichuan,Chongqing and Guizhou.Over the years,previous studies on petrology,mineralogy and geochemistry of Nb rich polymetallic layers in eastern Yunnan have been carried out,but the research on Nb rich polymetallic layers in Northwest Guizhou is relatively backward.Therefore,this paper selects the representative NB rich polymetallic section in Weining Yulong of Northwest Guizhou as the research object,and studies the enrichment geological characteristics and occurrence state of Nb and other key metal elements in NB rich polymetallic layer of Northwest Guizhou by combining with the methods of electron microscope,SEM,TIMA,LRS and petrochemistry,and systematically discusses its enrichment mechanism.The Nb-rich polymetallic section in northwest Guizhou is generally composed of Emeishan basalt at the bottom,iron bearing rock series in the middle and aluminous clay rock series at the top.Element geochemical analysis shows that Nb content is the highest in the top bauxite clay rock,reaching 571×10-6.In addition,Zr and Ga also show obvious enrichment trend.However,the content of rare earth elements in aluminous clay rock is not high,but it shows high content of rare earth elements in the formation of iron bearing rock series.Meanwhile,the content of Ga in the iron bearing rock series is relatively high.The experiment of SEM shows Nb only exists in anatase in aluminous clay rock.Nb is closely related to Ti in nature.Nb can replace Ti in titanium bearing minerals in the form of isomorphism.Therefore,we think Nb in the aluminous clay rocks in northwest Guizhou is hosted in niobium rich polymetallic in the form of isomorphism.Emeishan basalts are mainly derived from partial melting of mantle plume.The magma formed by partial melting of mantle plume has higher REE,Nb,Zr,GA and other incompatible elements.At the same time,compared with basalts in other areas,Emeishan high titanium basalts in eastern Yunnan and Western Guizhou show lower Mg O and more enrichment of Nb,REE,Zr and other insoluble elements.In recent years,the research also shows that niobium rich minerals such as sphene,anatase and apatite are found in the high titanium basalt matrix in the East Yunnan West Guizhou area.Through detailed mineralogical study,it finds that the bauxite clay rock is mainly kaolinite,and also contains a small amount of feldspar minerals,pyroxene,anatase,etc.The mineral composition types are similar to the underlying Emeishan high titanium basalt.At the same time,Nb-Ta?Zr-Hf of aluminous clay rock are obviously correlated with those of the underlying Emeishan basalts.The normalized REE chondrite distribution patterns of the two are also very similar,showing obvious right dip,suggesting that Nb-rich aluminous clay rocks are probably the weathering products of Emeishan high titanium basalts.During the Carboniferous-Permian,the South China plate was located in the east of the paleo Tethys ocean and close to the ancient equator.In addition,the South China plate was not a unified continent,but a continent surrounded by the ancient ocean.Therefore,the South China plate maintains a humid climate due to its proximity to the marine wet source.In addition,the study area is located in the low latitude zone of the southern hemisphere,with hot environment and abundant precipitation.In this special environment,the weathering of Emeishan basalt is obviously promoted.From Emeishan basalt to aluminous clay rock,Nb,Zr and Al are obviously enriched,and REE is relatively poor.Therefore,based on CIA,CIW,mass balance calculation and other factors,it is considered that Nb enrichment is not only caused by weathering of Nb rich sphene in Emeishan basalt,but also caused by supergene weathering of Nb rich anatase and other minerals. |
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