Characteristics And Genesis Of Salt Minerals, And Potash Formation In The Lop Nur Playa

The Lop Nur playa, one of the largest playa lakes in the world, is located in the eastern part of the Tarim Basin, Xinjiang. The Lop Nur gradually evolved into a salt lake from the late Middle Pleistocene to the Late Pleistocene. Huge amounts of glauberite minerals were deposited, accompanied by large-scale potash-bearing brines gathered. However, some key issues, such as the genesis of glauberite minerals and the relationship between potash-bearing brines gathered and huge amounts of glauberite minerals, remain less well known. Study on them will have important theoretical significance to search for potash resources further in the Lop Nur playa. Core samples from LDK01(781.5m in depth) in Luobei depression were studied on by identification of salt minerals in this study. Characteristics and distribution of the major minerals were presented. The genesis of huge amounts of glauberite minerals in the Lop Nur playa were analysed based on brine evaporation experiments and uranium-series dating conducted. It is vital that fluid inclusions in glauberite minerals were "opened", for the first time, and the chemical composition information were obtained. Moreover, the metallogenic environment and material sources were studied. The relationship between massive accumulation of glauberite minerals and potassium-rich brine formation was discussed eventually.Twenty-nine salt minerals were identified using thin section identification method, X-ray diffraction, scanning electron microscope and energy spectrum analysis methods. Fourteen new minerals, such as kalistrontite, celestite and glaserite, were firstly discovered in the Lop Nur area. The distribution characteristics of salt mineral in core LDKO1indicate that lake water experienced three stages:fresh water, saline water and high-salinity brine during the period of dry-wet climate oscillations. New discovery of kalistrontite in the glauberite rocks suggests that the strontium-rich water maybe had recharged the salt lake of Lop Nur. It will have important significance to genesis study of the glauberite minerals.Most single glauberite crystals were found to have experienced late reformation with loss of uranium by mass spectrometry U-series dating. It indicated that the glauberite crystals varied constantly during interaction with the brines. The huge amounts of glauberite deposition in the Lop Nur is a continuous process. The first stage:gypsum crystals precipitated by evaporation in the early ancient lake. The second stage:While the brines type changed, some glauberite crystals directly precipitated from the brines, others were formed by replacement of gypsum. The third stage:glauberite crystals grew constantly during interaction with the brines. The glauberite rock with "honeycomb pores", made up of coarse grains and megacrysts, formed finally.The analytical procedure of fluid inclusion composition by laser ablation ICP-MS has initially been established. Fluid inclusions in salt minerals in the Lop Nur playa were measured in this method. The results showed that contents of elements in fluid inclusions in gypsum, glauberite, bloedite and halite increased successively. The pattern of variation was consistent with the normal brine evolution trend. The composition of fluid inclusions indicated that the K-rich brines during the glauberite intergranular and the huge amounts of glauberite minerals were not formed in the same period. The potassium average content in glauberite inclusions is5.85g/L, which revealed that potassium in the brine was preliminary riched while the Lop Nur evolution to the glauberite deposition stage. Therefore, the relationship between massive accumulation of glauberite minerals and potassium-rich brine formation could be that:Brines containing potassium were reserved in the "honeycomb" pores. Then, the brines in glauberite intergranular occurred strong evaporation. It caused that shallow brine declined because of larger density and the light brine moved upward, forming the vertical convection system. The brines and glauberite crystals varied constantly under the interaction. In such vertical convection system, potassium ions were continuously enriched due to the concentration by evaporation. The current K-rich brines during the glauberite intergranular were formed eventually.