Discussion On Magnesium-rich Source With High Mg/Li Ratio In Lithium-rich Salt Lakes In The Middle Qaidam Basin

Salt lake resources are the distinctive resources in the western China.The potassium,magnesium,lithium and boron resources occurred in salt lakes are widely used in high-efficiency agriculture,clean energy,new materials,and other industries,and play key roles in the development of the national economy.Abundant salts and brines are distributed in the Qaidam Basin in the northeastern Qinghai-Tibetan Plateau.The inorganic salt reserves in this basin are up to 378 billion tons,of which the reserves of potassium chloride?KCl?,magnesium salts and lithium chloride are 700million tons,4.78 billion tons,and 13.92 million tons,respectively.By comparing the TDS,Mg²⁺and Li⁺concentrations of brines in modern salt lakes in the QB,it is found that the TDS of brines take on slightly increasing trend along the reverse-“S”evolution path of salt lakes,while the Mg²⁺and Li⁺concentrations present irregular variations,indicating that high Mg²⁺concentrations in the Li-rich salt lakes in the middle QB are not derived from the long-term evolution of the ancient Qaidam lake from the west to the east.As one of the strategic resources,lithium has attracted much attention in the 21st century.With the vigorous development of new energy vehicles and rechargeable batteries,it is expected that global lithium products will exceed one million tons?in terms of lithium carbonate equivalent?in the next decade.The global lithium resources mainly include lithium-rich brines and hard rocks,accounting for 63%and37%of global lithium resources,respectively.Brine lithium resources are mainly distributed in Bolivia,Argentina,Chile,China and the United States.About 86.8%of brine lithium resources in China are reserved in salt lakes on the Qinghai-Tibetan Plateau.The Yiliping playa,Xitai and Dongtai salt lakes,and Bieletan section of the Qarhan Salt Lake in the central Qaidam Basin are rich in lithium with the proven reserves 230×10⁴t.Due to the similar ionic radius of lithium?0.60??and magnesium?0.66??,the lithium-rich salt lake brines often have high magnesium contents,and high Mg/Li ratios have become a prominent feature of salt lake brines in China and result in problems in lithium extraction.However,studies on the sources of magnesium in brines have been poorly reported.This study focused on the Nalenggele River and its terminal salt lakes,collected river waters,brines and surrounding rocks and analyzed the major and trace elemental compositions of waters,mineral assemblages and trace elemental contents of rocks,and H-O-Sr isotopes of river waters-brines-rocks in order to constrain the source of magnesium in these lithium-rich salt lakes.The main conclusions are as follows:1.Previous studies showed that the salt lakes in the Qaidam Basin were formed by the evolution of the ancient Qaidam Lake from west to east.By comparing the chemical compositions of the modern salt lake brines in the basin,it is found that the K⁺contents and salinity of salt lake brines from the west to east show a steady and slightly increasing trend,while the Mg²⁺and Li⁺contents are enriched in the middle of the basin,which does not conform to the evolution trend of salt lakes.This comparison indicates that high Mg in brines in the central part of the basin did not result from the long-term evolution of the ancient lake from west to east.2.The major and trace elemental compositions of river waters and brines show that these waters have similar ionic distribution,and Na⁺and Cl^-are main major ions,followed by SO₄^2-and Mg²⁺;B³⁺and Li⁺concentrations in brines are much higher than those of river waters.Combined with the data of evaporation experiment of the Qinghai Lake water and the hydrogen and oxygen isotopes of river waters and brines,it is found that the Mg²⁺concentrations of river waters and brines fall on the evaporation path of the Qinghai Lake water,and the isotopic values of brines plot on the local evaporation line,indicating that the brines in the terminal salt lakes are formed by evaporation and concentration of river water.3.The XRD results show that the surrounding rocks in the pre-mountain recharge area of the river include Mg-bearing carbonate minerals,including dolomite,actinolite and magnesium calcite.The magnesium contents in these rocks range from0.20%-11.53%.The strontium and oxygen isotopes of river waters-brines-rocks indicate that the river waters-brines-Mg-bearing carbonates have similar ⁸⁷Sr/⁸⁶Sr ratios,which are much lower than the ⁸⁷Sr/⁸⁶Sr ratios of granite,indicating the weathering and leaching of magnesium-bearing carbonates provides Mg to the rivers and terminal salt lakes.