Study On The Genesis Of Na-carbonate Of Tamusu Trona Deposit In Bayingobi Basin,Inner Mongolia

A super-large trona deposit was explored in the Hari Sag of Bayingobi Basin in Inner Mongolia,which belongs to a super-large trona deposit.Among them,the paleo-lakes during the depositional period of the Lower Cretaceous Bayingobi Formation in the Hari Sag were typical carbonate-type soda lakes.During the basin’s salinization process,a large number of Na-carbonate minerals formed,which are similar to the mineral types,structures,and combination features of the Fengcheng Formation carbonate rocks in the Mahu depression of the Junggar Basin,both are located in active tectonic movements areas.The formation mechanism of the carbonate rocks remains one of the urgent scientific issues to be addressed in the field of carbonate rock research.In this paper,the Na-carbonate minerals of the Lower Cretaceous Bayingobi Formation in the Hari Sag of the Bayingobi Basin and the Fengcheng Formation in the Mahu Sag of the Junggar Basin were studied.Analytical tests such as electron probe,X-ray diffraction,carbon and oxygen isotopes,strontium isotopes and laser Raman spectroscopy were carried out to study the rock mineralogy and geochemical characteristics of Na-carbonate,to summarize the sedimentary characteristics of Na-carbonate minerals in Bayingobi Formation and Fengcheng Formation,and to explore the genetic mechanism.（1）The layered,speckled and veined Na-carbonate minerals in the trona deposits of the Bayingobi Formation in Inner Mongolia are mainly trona,nahcolite,shortite,eitelite and bradleyite,etc.In addition to Na-carbonate,the rocks contain a small amount of hydrothermal minerals such as pyrite,sodium chabazite,borosilicate reedmergnerite and albite.Minerals such as trona,nahcolite,shortite,eitelite and borosilicate reedmergnerite also appeared in the Fengcheng Formation of Xinjiang.（2）The carbon and oxygen isotope research results of thermal water sedimentary rocks in the Bayingobi Formation of the Hari Depression in Inner Mongolia and the Fengcheng Formation of the Mahu sag in Xinjiang show that the alkaline mineral layers in the sedimentary basin were formed in a closed alkaline lake environment with high water salinity during the sedimentation period.The formation temperature of carbonate minerals is 34-80℃（average57℃）,and they belong to the"white chimney"type of hydrothermal sedimentary rocks.During the deposition of dolomite in the Fengcheng Formation of the Mahu Sag in Xinjiang,the lake water temperature ranged from 31 to 115℃（average 65℃）,with high and fluctuating temperatures.（3）The sedimentary periods of the Bayingobi Formation and the Fengcheng Formation were controlled by multiple phases of activity in the fault zone,with frequent basin tectonic movements and magmatic activity.These formations have well-developed hydrothermal or fluid migration channels and a relatively high geothermal background.Strontium isotope analysis testing results show that both formations are influenced by mantle-derived fluids.Na+primarily comes from hydrothermal fluids and lake-bottom sediments,and the formation of Na-carbonate rocks is mainly controlled by hydrothermal sedimentation.（4）Under control of hydrothermal exhalative deposition and evaporation,silicate minerals were first formed in the trona layers,and then dolomite and calcite were formed,after Ca²⁺and Mg²⁺were almost consumed,trona and nacholite were precipitated.With sufficient Na⁺brought by the hydrotherm,the early formed dolomite and calcite underwent metasomatism to form dawsonite and dawsonite.Based on the comprehensive analysis of mineralogy and geochemistry,the Na-carbonate sedimentary model of alkaline lake is established.（5）The carbonate minerals deposits show the sequence of formation of dolomite/calcite-trona-nahcolite-shortite-eitelite.Shortite and eitelite were formed in the last stage,and the development of such minerals in hydrothermal sedimentary strata represents the sufficient Na⁺during the mineralization process,it means there may be trona deposit layers in the formation.Therefore,shortite and eitelite can be used as landmark minerals for trona deposit prospecting,the carbonate mineral genesis model of the Tamusu trona deposit can guide the exploration of the trona deposit,...