Study Of Characteristics Of Trona And Nahcolite And It's Indication Of Ancient Environment In Permian Fengcheng Formation,Wuerhe Area,Junggar Basin

Hydrothermal sedimentary rock is a hot topic in geology research in the world today.Related research on lacustrine hydrothermal sedimentary rocks is still weak.The study on special halite minerals is of great significance to the deep analysis of the mechanism of hydrothermal deposition,diagenesis and salify.In this paper,research on the salt minerals such as trona and nahcolite stored in the"white smoke"hydrothermal sedimentary rocks in the Permian Fengcheng Formation,Werhe area of the Junggar basin has been constructed.The petrological analysis means include core observation,thin slice identification,XDR analysis,scanning electron microscope analysis and electron probe analysis.Geochemical means include C/O isotopes analysis,principal elements analysis and Sr isotopes analysis.Trying to solve the source of salt forming substances,the properties of salt forming fluids,salt forming models and their internal relations with the paleoenvironment and paleoclimate of trona and nahcolite,The results obtained are as follows:?1?The eruption rocks with trona and nahcolite of the Permian Fengcheng Formation in the Junggar basin have a large number of abnormal sedimentary structures and constructions,such as the lamellar and banded rhythmic interbedded structures formed by volcanic component,dolomitic,cryptocrystalline siliceous and complex salt minerals,mesh-vein structure et.al.The characteristics of hydrothermal sedimentary minerals such as searlesite and calcas suggest that they are the origin of hydrothermal deposition at the bottom of the lake.?2?The crystals of trona in Fengcheng Formation are large plate columnar,parallel or oblique crossing,and represent the characteristics of rapid growth in static brine.The characteristics of nahcolite are needle like,radial aggregate like,and the crystal floating on the surface of trona,indicating that they are later than trona.?3?Because of the drought,the evaporation is strong,the lighter ¹²C and ¹⁶O escape through evaporation,and with the effect of the hydrothermal,it causes?¹³C and?¹⁸O in trona and nahcolite positively blased.The C/O isotope characteristics show that the sedimentary environment of the two minerals has a relatively closed higher temperature and higher salinity,and the Sr isotope results show that it is possible that the thermal fluid that can form two kinds of minerals has the mantle source composition and is affected by the mixing of lake water.?4?The salt forming substances of trona and nahcolite may be closely related to the volcanic activity in the study area and the sedimentary environment of the lake basin.The water-rock reaction of the tuff and the hydrothermal fulid at the bottom of the lake may provide material sources for the two kinds of salt minerals,and the volcanic activity and the fracture system in the sedimentary environment at the bottom of the lake provide the conditions for the hydrothermal activity.The tuff premierly deposited in the water,and then take water-rock reactions,make the brine at the bottom of the lake concentrate HCO₃^-and CO₃^2-ions and Na+ions.,poor Ca²⁺and Mg²⁺ions,alkaline,which is suitable for the precipitation of trona and nahcolite.?5?Trona and nahcolite precipitate in the arid and hot paleo lake environment of the early Permian Fengcheng Formation.Because of the early Permian atmospheric carbon dioxide concentration can not meet the precipitating demand of the nahcolite,it is presumed that the paleo climate in study area was not affected by the global climate at that time,but the regional volcanic eruption and the existence of the hydrothermal fluid in the area.They provides a carbon source and a large amount of CO2 gas and ash,resulting in a rising concentration of carbon dioxide in the study area,a hot regional paleoclimate,and the salinization of paleo lake and then precipitate alkali salt.