Geochemical Characteristics And Diagenetic Fluid Analysis Of The Lower Cretaceous Hydrothermal Sedimentary Rocks In Tamusu Area,Bayingobi Basin

Bayingobi basin is located in Alashan area,Inner Mongolia.The mudstone of Bayingobi formation in the lower Cretaceous of tamusin in the basin has unique sedimentary structure and mineral composition.Based on the comprehensive study of petrology,mineralogy and Geochemistry(isotope geochemistry),this paper describes the characteristics of lake facies hydrothermal sedimentary rocks of Bayingobi formation in detail,and obtains the following understandings:(1)The hot water sedimentary rocks in the study area can be divided into the following five types:vein mudstone,mottle mudstone,massive mudstone,laminated mudstone and syngenetic deformed mudstone.The mineral composition is mainly analcite,dolomite,ferruginous dolomite,albite and illite.There is a good correlation between the mineral content and mudstone structure.It is consistent with the mineral assemblage and structural characteristics of typical low temperature"white chimney"hydrothermal sedimentary rocks.(2)The main oxides of the hydrothermal sedimentary rocks in the study area are Si O₂,Al₂O₃,Cao and Mg O,and the major elements are rich in Ca,Na,Mn and Fe.The trace elements are relatively enriched in large ion lithophile elements and relatively deficient compatible elements.LREE is relatively enriched,hree is relatively deficient,?Eu shows obvious negative anomaly,?Ce has no obvious positive and negative anomaly.Combined with the analysis of Ni-Co-Zn trigonogram and Fe Mn-(Cu+CO+Ni)×10 trigonogram,it is shown that the sedimentary rocks in the study area are of hot water sedimentary origin and are influenced by long-term heating fluid activity.(3)The hydrothermal sedimentary rocks in the study area are rich in trace elements such as Li,Sr,Cs and U;The negative correlation between?²⁶Mg and?¹⁸Ov_PDBindicates that there are fluids from different sources of Mg isotopes involved in the formation of dolomite;Combined with Nb/Ta and Zr/HF values,it shows that the main hydrothermal fluid in the sedimentary period of the hot water sedimentary rocks in the study area is from the continental crust,and also influenced by the mixing of mantle derived fluid and other fluids.(4)Based on the analysis of carbon and oxygen isotopes,V/(V+Ni),Ni/Co,V/Cr,U/TH,Ce_anom and the ratio of Sr/BA to Rb/K,it is indicated that the lower Cretaceous hot water sedimentary rocks in the tamusu area were formed in the deep salt water reducing lake environment under closed-semi closed condition.(5)The early Cretaceous in the study area is in the stage of extensional basin.During this period,magmatism was frequent,which provided the main thermal energy and kinetic energy for hydrothermal activity.Through the Altun fault zone,it provides a good convection channel for hydrothermal and lake water,and provides a continuous stream of thermal energy and ore-forming ions for the lake basin.Therefore,the Altun fault is the main controlling factor of hydrothermal sedimentation in the study area.(6)There are abundant analcite and dolomite in the mineral assemblage of Bayingobi formation.There are abundant analcite and dolomite in the mineral assemblage of Bayingobi formation.The water content of the target layer(400-800m)is 1-6%,which is low.The unique geological conditions and mineral composition make the rocks of Bayingebi formation have many advantages as the surrounding rock of high-level radioactive waste repository.