Mineralogical And Geochemical Characteristics Of Hydrothermal Sedimentary Dolomite

"Dolomite problem"has attracted much attention in the field of sedimentology and even geology.Its formation mechanism is still one of the scientific problems to be solved in the field of carbonate rock research.Taking the dolomite of Bayingebi Formation in Yingejing depression of Bayingebi Basin and the dolomite of Fengcheng Formation in Mahu sag of Junggar Basin as the research object,this paper studies the dolomite by means of rock mineralogy and geochemical characteristics,compares the dolomite of Bayingebi Formation and Fengcheng Formation,and discusses their genetic mechanism.（1）Bayingebi Formation as a whole is dominated by dark gray,gray and gray white dolomitic zeolite rocks.Typical hydrothermal sedimentary structures such as stockwork,spotted,massive,laminar and syngenetic deformation structures are common,and show obvious laws in the longitudinal direction.It is rich in hydrothermal minerals such as dolomite,analcite and pyrite,and enriched in elements such as Ca,Mg and Mn.It is characterized by LREE enrichment,hree loss and E_U negative anomaly.（2）The dolomites of Fengcheng Formation in Mahu sag are mainly composed of micrite,silty dolomite,tuffaceous dolomite and silty dolomite.According to the particle size and crystal form of dolomite,its microstructure can be divided into four categories:micrite structure,silty structure,fine medium crystal structure and medium coarse crystal structure;There are many types of dolomite,which can be divided into dispersion type,laminar type and strip lump aggregate type according to the degree of aggregation.（3）Through the analysis of carbon and oxygen isotopes and V/（V+Ni）,U/TH,V/Cr and Ce_anom ratio,it is inferred that the lake basin in the sedimentary period of Yingejing depression and Mahu sag has certain sealing,high water salinity and is in an anoxic reduction environment as a whole.The water temperature during dolomite deposition in yingejing sag is 36～73℃,with an average of 52℃.It belongs to low-temperature"white chimney"hot water deposition;The lake water temperature during the dolomite deposition period of Fengcheng Formation is 21～113℃,with an average of 68℃.The temperature is relatively high and varies widely.（4）Strontium in the hydrothermal sedimentary rocks of Bayingebi Formation mainly comes from the crust derived aluminosilicate deposited on the basement,and a small amount from magmatic eruption mixed with mantle derived strontium,showing the source nature of mixed thermal fluid.The strontium isotope of dolomite of Fengcheng Formation retains the characteristics of residual seawater to a certain extent,and is affected by mantle derived fluid,which is significantly lower than that of Bayingebi Formation.（5）The sedimentary period of Bayingebi Formation is controlled by multiple activities of Altyn Tagh fault zone.The basin has frequent tectonic movement and magmatic activity,with developed hydrothermal or fluid migration channels and high paleogeothermal background.The formation of dolomite is mainly affected by hydrothermal sedimentation.（6）The dolomite of Fengcheng Formation in Mahu sag is affected by many factors,and its genesis is complex,mainly including hydrothermal sedimentation,microbial induction and metasomatism.