The Geochemical Characteristics Of Gypsum Deposits In Hexi Area,Lanping Basin,Western Yunnan

The Lanping Basin in western Yunnan is an important salt-bearing basin in China.The thickness of salt-bearing strata is nearly 2km.It has been confirmed that there are three sets of salt-bearing construction.Previous studies on salt mineral resources in the basin mainly focused on potassium salts,and the degree of research on gypsum was low.This paper mainly discusses the gypsum material source,formation environment and gypsum genesis mechanism through the investigation of gypsum development characteristics in Hexi region,combined with the analysis of trace and rare earth elements and the study of isotope geochemical characteristics.Research indicates:?1?The gypsum in the Hexi area mainly exists in the Upper Triassic Sanhedong Formation and the Paleocene Yunlong Formation.Due to the influence of tectonic compression,the stratified gypsum of the original sediment is deformed and invades the overlying strata.The strength of the pressure affects the intrusion form of gypsum,such as salt mound,salt strain,salt anticline,salt puncture and so on.?2?By analyzing the ratio of trace elements in gypsum and surrounding rock in the Upper Triassic Sanhedong Formation,Sr/Cu,Sr/Ba,V/Cr,Ni/Co,U/Th,?U values show the gypsum is in dry and hot weather conditions that formed by evaporation in a salt water-anoxic reducing environment.?3?The rare earth element chondrite distribution curve of gypsum and limestone is generally characterized by the relatively enrichment of light rare earth elements?LREE?,the flatness of heavy rare earth elements?HREE?,and the presence of negative elements of Eu.?REE is generally lower.LREE/HREE and?La/Yb?_N show a moderate degree of fractionation between light and heavy rare earths,and the light and heavy rare earth fractionation of gypsum samples is higher than that of limestone.?La/Sm?N and?Gd/Yb?N of all the samples showed that the degree of fractionation inside the light and heavy rare earths in the sample was not significant.?4?The?¹⁸O-?¹³C correlation indicates that although the sample is affected by certain diagenesis,its original carbon and oxygen isotope characteristics have not changed.The Z value of paleo-salinity ranges from 123.22‰to 133.6‰,with an average of 129.33‰,Above 120,it shows a large salinity,which is judged as marine deposition;The tracer diagram of?¹⁸O_V-SMOW-?¹³C_V-PDB shows that CO₂ in the fluid mainly comes from the dissolution of marine carbonate rock.?5?The ⁸⁷Sr/⁸⁶Sr values of limestone and gypsum ranged from 0.707695 to 0.709014with an average value of 0.707987,which is similar to the seawater strontium isotope values,indicating that they are homologous and are marine sediments.?6?The relative enrichment of heavy sulfur in gypsum,the range of?³⁴S is12.9‰¹4.3‰,the average value is 13.6‰,by comparing and analyzing the?³⁴S values in the Late Triassic Sanhedong Formation,it is further proved that the gypsum in the upper Triassic Sanhedong Formation is the original gypsum.In summary,it is believed that the source of gypsum formation is mainly the mixture of seawater and deep brine of the Tethys Ocean during the Late Triassic.Under dry and hot weather conditions,the water body evaporates and precipitates to form the original gypsum layer.With the Himalayan movement,India and the Eurasian plate collide and squeeze,so that the original gypsum layer in the Sanhedong Formation is instructed by structural deformation and invaded into the overlying Cenozoic Yunlong Formation in the form of salt structure,forming a unique gypsum ore body in Hexi area.