Study On Geochemical Characteristics Of Paleobrine In Kunteyi Playa Based On Fluid Inclusions Hosted In Halite

Polyhalite is an important K-bearing evaporite mineral,which is widely distributed in marine and Modern Salt Lake evaporite deposits of different geological ages.Kunteyi Salt Lake is located in the northern part of the Qaidam Basin,which is rich in polyhalite resources and is a good carrier for studying the genesis of Modern Salt Lake polyhalite.Although the formation mechanism of polyhalite in Kunteyi Salt Lake has been studied by some scholars,there is still much controversy.The composition of fluid inclusions in halite is an important method for reconstructing the evolution of paleobrine,which has been widely used in the process of paleoseawater evolution and reconstruction in different geological times.In this paper,we take ZK3608 core of Kunteyi Salt Lake as the research object,and use LA-ICP-MS technology to analyse the fluid inclusions hosted in halite,combined with lithological stratigraphic division and XRD facies analysis of drilling section,trying to clarify the geochemical characteristics of paleobrine and discuss the genesis of polyhalite.The results of mineral composition and assemblage analysis reveal that there are nine distinct chemical sedimentation-clastic sedimentary cycles in the sedimentary profile,reflecting the periodicity of the alternation of salt and light in lake water and the change of climate between dry and wet;the salt layer is formed in acidic oxidation environment and dry and cold climate,while the clastic sediment layer is formed in relatively warm and humid climate and alkaline reduction environment.The composition of fluid inclusions in primary halite shows that K increases gradually from bottom to top in the salt bed,indicating that brine is gradually concentrated in each evaporite cycle.There are good correlations among Mg,K and Rb.The changes of Ca and Sr are basically the same,but contrary to the changes of other elements.The cyclic changes of these ions with depth reflect that the evolution of paleobrine is a fluctuating process of relative concentration and desalination.The variation of polyhalite content with depth is consistent with that of K and Mg in fluid inclusions.When K and Mg content is high,polyhalite is enriched,but there is no obvious corresponding relationship between polyhalite and Ca content in halite inclusions.The polyhalite in clastic layer is highly enriched,and the original halogen can not provide so much Ca.Therefore,it is presumed that Ca in these polyhalogens is exogenous;K and Mg are enriched in salt layer while the polyhalite is less,so the original mother halogen can provide Ca source completely.Mineralogy and composition analysis of fluid inclusions in salt beds reveal that polyhalite in salt beds should be the product of primary sedimentation,while polyhalite in clastic beds is mainly of metasomatic origin.