Multi-index Study On The Source,Migration And Enrichment Of Lithium In The Nalenggele River Drainage And Terminal Lakes

Lithium（Li）,known as the"Green High-energy Metal of the 21st Century",plays an important role in energy storage materials.It is also one of the strategic mineral resources in China.About 82%of the world’s Li resources are brine type.The terminal salt lakes of Nalengele River in the Qaidam Basin in the northeastern Qinghai-Tibetan Plateau contain the largest Li brine deposit in China.Previous studies focused on evaluating resource potential,studying metallogenic process and mining technology of Li in the region.However,there are still disputes about the sources of the Li deposits in this area（whether from weathering of surrounding rocks,remnants of ancient lakes,leaching of saline formations,oilfield water,or deep hydrothermal fluids,etc.）,and solid geochemical evidence is lacking;Meanwhile,previously reported researches can not explain why there is a huge gap between Li supply（～10.26 million tons Li）and the reserves in the terminal lakes（2.305 million tons Li）,which highlights better understanding the migration and enrichment behavior of Li from source to sink.To remedy this,this study collected 92 water（including river,spring,brine,well water,etc.）and 47 sediment samples in the Nalenggele River Basin and its terminal lakes,and analyzed the major and trace（Li）ionic concentrations and Sr–S isotopes,and completed evaporation experiments of river and lake waters,and leaching experiments of sediments from the alluvial fans and the Xitaijinaier Salt Lake.Combined with previously reported data,the sources of Li and its migration and enrichment processes were discussed.The conclusions are as follows:（1）The chemical compositions of the Nalenggele,Chuakealagan,Hongshui and Golmud rivers,originated from the eastern Kunlun Mountains,are obviously different.Waters from the Nalenggele River are mainly Na-Cl-HCO₃-SO₄type.Lithium contents of Nalenggele river waters（0.46-0.65 mg/L）are an order of magnitude higher than those of Chuakealagan and Golmud rivers.The Li in the Nalenggele River is supplied by the Hongshui River which is recharged by the hot springs with high Li contents.The hydrochemistry,high Li content,high Sr and low S isotopes of hot springs in the study area are similar to those on the Qinghai-Tibetan Plateau;（2）The similar Sr isotopes of the Nalenggele River waters and the terminal lake brines,coupled with the comparable trend of the sulfur isotopic variation of the waters in the study area and those during the evaporation process,verified that the Li in the terminal lakes in the study area was originated from the recharge of Nalenggele River water.Compared with the waters in the study area,the residual brines from the ancient Qaidam lake,the leaching solutions from the salt-bearing strata or the oilfield waters in the western Qaidam Basin have significantly different hydrochemical characteristics,and Sr-S isotopes.This comparison indicates that the contribution of Li from the above three sources is negligible;（3）During the runoff of the Nalenggele River from the mountains to the terminal lakes,the chemical compositions of the river water were affected by mixing of groundwater and river water,water-rock interaction,and evaporation process,and the river water evolved from Cl·HCO₃·SO₄-Na·Ca type to Cl·SO₄-Na type;the chemical compositions of groundwater are controlled by water-rock interaction,evaporation and desulfurization,and the groundwater gradually evolved from Cl·HCO₃·SO₄-Na·Ca type to the Cl-Na type;（4）On the alluvial fan I of the Nalenggele River,the hydrochemical characteristics of the waters were dominated by water-rock reaction.The river water has stable hydrochemical compositions and Li contents.The reverse reaction of cation exchanges,carbonate precipitation and weak sulphate reduction were occurred in the groundwater.Weathering and leaching of rocks and desorption of Li from clay minerals provided a certain amount of Li to the groundwater;（5）On the second alluvial fan II of the Nalenggele River,the hydrochemical characteristics of the waters were mainly controlled by evaporation-crystallization process,and the TDS of waters increased rapidly.The comparison of the results of evaporation experiments and natural waters showed that the Li supplied by groundwater from the alluvial fan I was depleted in the river water.The groundwater has undergone the positive reaction of cation exchanges,the dissolution and precipitation of salt minerals,and the evaporation process,and at the margin of the plain area,nearly two-thirds of the Li loss from the groundwater;（6）The enrichment of Li in brines in the Xitaijinaier Salt Lake was dominated by the impulse replenishment of Nalenggele River under the long-term regional dry climate.This process had also resulted in an abnormal increase in magnesium（Mg）and potassium（K）concentrations in the brine;the inconsistency of the K:Mg:Li ratio between Yahu and Xitaijinaier lakes may be related to the ionic decoupling process caused by the climate.In addition,a large amount of Li still exists in the sediments in the form of residual brine,adsorption on clay minerals,and halite inclusions,etc.About 90%of Li can be released during the water-supplied mining process.