Clay Mineral Assemblages And Trace Element Characteristics In Dazi Section In Changdu Area And Study On Sedimentary-diagenetic Environmental

The Changdu-Lanping-Simao-Khorat Basins are located on the eastern Tethys structural zone.A very large ancient potash deposit was found in the Khorat Basin,while only small-scale ancient potash deposit was found in the Lanping-Simao Basin in China.The formation of potassium-containing evaporite deposits in the Lanping-Simao-Khorat Basin are related to the metamorphic seawater in the terrestrial environment,and the distribution characteristics of carbonate minerals,the age of mineralization,and the changes in Rb and Br content indicate that the seawater invaded from the northern Lanping Basin to the Khorat Basin.Therefore,the potassium salt mineralization model in the eastern Tethys is a model of seawater migration,metamorphism,and salt-to-potassium formed in the multi-stage basin,that is,the new Tethys ocean water entered the Changdu-Lamping-Simao-Khorat Basin in turn in a metamorphic and concentrated form,and salt deposits of varying thickness and scale have developed in different sedimentary environments.The Lanping-Simao-Khorat Basin lacks sulfate deposits formed by evaporation and concentration of normal seawater,while the Changdu Basin is rich in sulfates.Studies of S and Sr isotopes in gypsum in the Changdu Basin indicate that they are marine sediments,which are homologous to the evaporites in the Lanping-Simao Basin,suggesting that the material of the potassium salt deposit in the Simao Basin originates from metamorphic seawater in the eastern Qiangtang area,but the research on the sedimentary-diagenetic environmental characteristics of gypsum-bearing formations in Changdu Basin is lacking.This paper takes the late Jurassic gypsum-containing Dazi section in the Changdu area as the research object,through the analysis and research on the clay mineral assemblage and trace element geochemical characteristics,the sedimentary-diagenetic environment characteristics of the Dazi section were systematically summarized and studied.Combined with the profile sedimentary and structural features,an attempt was made to analyze the problem of brine concentration into salt.Thus,the data of the sedimentary-diagenetic environment of the salt-bearing strata in the Changdu area have been added,and it can provide certain clues and evidence for the model of the seawater migration,metamorphism,and salt-to-potassium formed in the multi-stage basin,and conducive to the work of potash exploration in the future.Analytical studies of clay minerals indicate the type of the illite/smectite?I/S?mixed layer mineral in the Dazi section is an IS?-type ordered I/S mixed layer mineral and the montmorillonite crystal layer appears every 9 to 19 illite crystal layers,which indicates the illite petrification degree of smectite is high and it is greatly affected by diagenesis.The Dazi section is characterized by a late diagenetic stage and its diagenetic temperature is more than 190?.The high content of illite and I/C mixed layer and low content of kaolinite indicate that its sedimentary-diagenetic environment was characterized by weak alkalinity.Besides,the characteristics of high content of Na⁺,K⁺,Al³⁺and low content of Fe²⁺,Mg²⁺have a certain promotion effect on the transformation process of smectite-illite/smectite-illite and a large amount of chlorite was inhibited in the late burial peocess at the same time.Studies of trace element geochemistry and profile lithology have shown that the evolution of the paleoclimate environment in Dazi section has gone through two stages:the first stage?85?65m?of the paleoclimate is generally dry and hot;the second stage?65?0m?of the paleoclimate is warm and humid in which there are three fluctuations of dry-hot and warm-humid and dry-hot climate trends.In the Dazi profile sedimentary period,the gypsum deposition stage was in a salt water environment,but the sedimentary environment after the completion of the thick gypsum deposition stage presented fresh water characteristics as a whole.And the water body of the Dazi profile mainly presents an oxidizing environment,and the water body presents a weak oxidizing environment only in a certain period of time.By analyzing the characteristics of the sedimentary-diagenetic environment of the profile,and combining the characteristics of the profile lithology and tectonic environment,which indicate the relatively low concentration of brine in the Dazi profile is not sufficient to promote the crystallization of potassium salts.The strong tectonic uplifting movement may cause the concentrated brine to migrate,resulting in a lack of potassium salt deposits during the period of Dazi profile deposition.Therefore,the Late Jurassic in the Changdu Basin should not be used as a prospecting layer for exploring potassium in the future.