Genetic Model of Na-Cabonate in Tamusu Trona Deposit,Bayingobi Basin,Inner Mongolia
The Lower Cretaceous Bayingobi Formation in the Hari sag,Bayingobi basin,Inner Mongolia,is a typical carbonate-type alkaline lake during the depositional period,and a large amount of Na-carbonate minerals developed during the salinization of the basin.In this paper,electron probe,X-diffraction,carbon and oxygen isotope and laser Raman spectroscopy were carried out to analyze the layered,speckled and vein-like Na-carbonate minerals in the trona layers of the Bayingobi Formation.The Na-carbonate minerals in the trona layer are mainly composed of trona,nahcolite,shortite,eitelite and bradleyite,with a small amount of hydrothermal minerals such as pyrite,chabazite-Na,searlesite and albite.The results of carbon and oxygen isotope testings show that the Lower Cretaceous trona layers in the Bayingobi basin were formed in a closed alkaline lake environment,the Na-carbonate minerals were formed at a temperature range of 34-80 ℃(average 57 ℃).Under control of 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,which is controlled by hydrothermal exhalation and evaporative deposition,it can provide a new idea for the exploration of trona deposit.