Hydrochemical Of Brines And Their Geological Significances From Southern Qinghai,China

Through the complicated geological history,it produced extremely rich mineral resources in Qinghai-Tibet Plateau.A series of continental red clastic rocks and gypsum-bearing salt rocks,numerous small and medium sized salt lakes,saline waters,saline springs and saline rivers are extensively developed in Hoh Xil and Nangqen Basin,which located at southern part of Qinghai Province.Each Basin has different forms of brine.Brines has characteristics of small size and high salinity in Tuotuohe Basin,high salinity saline springs in Nangqen Basin,and high degree of evolution and large scale in Lexiewudan Lake.The research on brine resources in southern Qinghai is less in the past,especially the first discovery of high concentrations of saline springs in Nangqen Basin??150 g/L?.Therefore,this study mainly using brine water chemistry characteristics,B and Cl isotopic as tracer to explore the formation of salt substances and material sources,and using elemental analysis?ion content?method,coefficients characteristic and comparing brine resources in the Qaidam Basin and North Tibet Plateau to analyze salt formation conditions and predict favorable salt-forming mineralization areas.In this matter we carried out field visits to Saline springs in the Nangqen Basin for two times in summer 2014 and winter 2016.Another field investigation?and sampling?has been done around the Tuotuohe Basin during 2014.In total of 21samples of brine samples from Tuotuohe Basin and 14 samples from Nangqen Basin were collected.Due to the poor natural conditions,sampling from Lexiewudan Lake can't be carried out this time,so the data used in this study was obtained from research team in 2010?in total of 12 samples?.The research content and results include the following aspects:1.ThehydrochemistryresearchofLexiewudanLakeshowsthat,hydrochemistry is chloride type.The B₂O₃ content is usually 36.00-255.0 ppm,the Li⁺content is 47.00-103.1 ppm,and the K⁺content is 0.82-1.67 g/L.On the spatial distribution,the degree of salinity on the west side of Lexiewudan Lake is the lowest,presumably due to diluting by the infiltration of inflows of Liusha and Lexiewudan rivers,some springs and snowmelt interaction to the western and northwestern parts of the lake.Vertically upwards,affected by dilution effect caused by ice layer,the TDS first increases with depth and then stabilizes from the surface to the bottom of the lake.2.Through the research of brine hydrochemistry in Tuotuohe Basin,it is found that K⁺,Ca²⁺,Na⁺,Mg²⁺,Cl-,SO₄^2-are distributed in salt lake brine in Tuotuohe area,which is the main chemical composition of brine in this area.The evidences of coefficient of sodium chloride?n?Na?/n?Cl??,Boron chloride?n?B?/n?Cl??,bromine chloride?Br×10³/Cl?and chlorine isotope show that,all type of brine resources in this area belongs to leached brine,which formed by atmospheric precipitation or groundwater leached from the Mesozoic/Cenozoic saline strata in Tuotuohe Basin.At the same time,brine resources are affected by geological and tectonics of surrounding area.Cluster analysis,correlation between Boron and Lithium and B isotope analysis show that,B and Li in this area have homology and caused by dissolving the widespread high B,Li contents of volcanic rocks in this area.In addition,based on the evidence of B isotopes,dissolution of marine carbonate?widely distributed?in this area is the main reason of the characteristic high value of?¹¹B in brines.3.Based on the research of brine hydrochemistry in the Nangqen Basin,it is found that flows of saline springs varies greatly.The smallest spring outlet time is less than 0.01L/S and the maximum is 1.7L/S.Spring water mostly flow along the fault or near the fault line and salinity are very high,with an average of 228.30g/L.Trace elements such as Li⁺,B₂O₃ and Br^-in various saline springs have different levels of enrichment.Through the characteristic values of coefficient of sodium chloride?n?Na?/n?Cl??,Boron chloride?n?B?/n?Cl??,bromine chloride?Br×10³/Cl?,H-O isotopes and chlorine isotopes,it is inferred that saline springs formed by leaching underground salt strata,during atmospheric precipitation or groundwater upwelling it became exposed to the surface by underground faults.That is,the saline springs belong to leached brine.The salinity of saline springs vary little with the seasons,indicating that salt spring water comes from underground confined water.Based on all the evidences,it is considered that the saline springs are artesian in nature and come out from fault system.The analysis of B isotopes shows that there are two main sources of B source in this area,one of them is the dissolution of marine carbonate layer,results are higher B isotopic values,and the other is the presence of high B content salts in Duolunduo saline spring caused by the surrounding volcanic rocks.Compared with the geothermal system in Tibet,the Nangqen Basin has a similar source of material B belong to the shallow crust.4.Through the analysis of brine characteristics and hydro-chemical types,we concluded as follow.a)The trend of salinization degree of brines in Tuotuohe Basin increased significantly from south to north.Basin in the south is carbonate type?Dangqu,Yaxicuo,etc.?,the middle is sodium sulfate subtypes?Bucha,Yanhe,etc.?,while the brine in north has evolved to magnesium sulfate subtypes belong to brine self-analysis?Goulucuo,Chacuo,Duoxiu,etc.?.b)The variation trend of salt mineralization degree in the Nangqen Basin is gradually increasing from west to east,with the lowest being 150.28 g/L of Gayang spring and the highest being 332.39 g/L of Duolunduo spring,and all of brines belong to Sodium sulfate subtype.c)Lexiewudan Lake has salinity of 61.08 g/L,which has evolved to chloride type salt lake.d)The hydrochemical types of all the brine resources in the South of Qinghai Province show that the spatial distribution of hydration types gradually changes from the southeast to the northwest by the carbonate and sodium sulfate subtypes,short evolutionary history to the magnesium sulfate subtypes with longer evolutionary stages and deeper metamorphism,and then chlorinated brine characteristics.e)Compared with the Qaidam Basin and the salt lake area in Northern Tibet,Southern Qinghai area is located in the middle of two salt lake areas.In brine evolution stage,the chemical type of brine water is composed of chloride type with high degree of evolution in Qaidam Basin,Magnesium sulfate subtype and sodium sulfate subtype in southern Qinghai area,then to the northern Tibet Plateau with less evolution of carbonate-based salt lake.It shows that the history of salt lake brine evolution in northern Tibet is relatively short,that of salt lake brine in Qaidam Basin,however,the evolution history of brine in Nangqen Basin and Tuotuohe Basin in southern Qinghai is between the two areas.5.According to the study of coefficient of potassium content,Potassium salinity?K×10³/?salt?,potassium chloride?K×10³/Cl?,bromine chloride?Br×10³/Cl?,potassium bromine?K/Br?,and chlorine isotope composition,It is concluded that Cangcuo and Daqiao Lake both have two anomalous characteristic coefficient values and can be used as a favorable target of potash mineralization.The B and Li contents in Gomucumudi,Donghu,Xihu salt lakes of Tuotuohe Basin,and Duolunduo saline spring of Nangqen Basin are high,which can be regarded as favorable B and Li mineralization areas.