Distribution Characteristics Of Boron Contents In Water And Surrounding Rock And Provenance Analysis Of Boron-rich Fluids In The North-central Margin Of Qaidam Basin

Boron is an important nonmetallic material,which is widely used in many fields.With the exploitation and utilization of boron mineral resources in China,boron mineral resources with high grade are mainly concentrated in the salt lakes on the Qinghai-Tibetan Plateau.Among of them,the modern salt lakes in Qaidam Basin?QB?accounts for about 60%in China.On the basis of previous studies,the source of boron-rich fluids in the QB has been preliminarily carried out in this work.Based on the systematic and comprehensive sampling of river waters,springs,surrounding rocks from the catchment of Qilian Mountains in the north-central margin of the QB,in combination with published data of river waters and terminal salt lakes from the catchments of Altyn and Kunlun Mountains in the northwestern and southern QB,the following works have been completed:firstly,distribution characteristics of boron contents in river waters in the QB were concluded;Secondly,boron contents of surrounding rocks and strata,and mineral assemblages of different rocks were analyzed;hydrogen and oxygen isotopic compositions and Na-K-Mg equivalent values of thermal springs in the Da Qaidam town,and Gonghe-Guide basins with similar geological background,were also determined;Finally,the model of boron-rich source in the north-central margin of QB is put forward for understanding some analogues in the research area.Some following conclusions have obtained:?1?The boron and lithium contents in the catchments in the Qilian,Altyn and Kunlun Mountains in the QB are higher than average value of rivers in the world.These values are the highest in the Nalenggele River from Kunlun catchment,and then Qilian and Altyn cathments,respectively.On the whole,there are great differences among different rivers in the same catchment.Rivers and terminal salt lakes,fed by thermal springs,take on a high boron values in the QB.?2?The analyses of Piper and Gibbs diagram according to main elementary compositions of river waters in the QB indicate that the main solutes in these surface waters are controlled by weathering of surrounding rocks.Similarly,the water types of main solutes in these rivers are obviously different.Most rivers in the catchments in the Qilian and Altyn Mountains are the Ca-Mg-Cl type,and those in the Kunlun Mountains are the Ca-HCO₃ type.However,the water type of hot springs and salt lakes from Da Qaidam Town and Guide-Gonghe subbasins is the Na-Cl type,implying that Boron enrichment in the QB are fed mainly by hot springs.?3?The boron contents of surrounding rocks in the Qilian Mountains are the highest,especially the boron-rich granite occurring in feldspar minerals.Abundant granite distributed in the north-central margin of the QB provides originally boron source for terminal lakes.?4?The?D and?¹⁸O values of river waters and hot springs in the Da Qaidam Town and Guide-Gonghe subbasins indicate that the hot spring originated from the ice-snow meltwater in the high-altitude areas.The Na-K-Mg equivalent valent of hot springs show that reservoir temperature of Dachaidam hot spring is 160?,corresponding a burial depth of 6.2 km,while that of Gonghe-Guide Basin is 170?230?,corresponding a burial depth of 6.6?9 km.The comparisons between B contents and Cl^-concentrations,Na/K-Na/Cl and?¹¹B values,and B contents and?¹¹B values of hot springs in the Da Qaidam Town,Guide-Gonghe and Tibet show that the high boron source of hot spring in the Da Qaidam Town is from the both contribution of magma degassing and water-rock interaction.