Abstract:

Basement rocks occur in the northwestern corner of the Qaidam Basin and piedmont deposits and alluvial and lacustrine sediments prevail in the western basin.Several NWSE trending anticlines of different sizes occur in the Cenozoic sediments in the western basin.K+rich brines have been found in the Neogene sediments in the western Qaidam Basin.In this paper, statistic methods are used to examine the enrichment of TDS, K+ and B2O3 of the subsurface brines.The results show that the TDS of the brines are of bipeak values and the brines are rich in K+ and B3+ with enrichment probabilities of 628% and 6129%, respectively.When the Piper diagram is used to study brine samples, the data points are usually too concentrated to show the changes of the main ions.The previous methods are modified to express hydrochemistry of subsurface brines.Ratios of Na+ to Cl-, Cl- to Br-, Cl- to I-, K+ to Cl-, SO2-4 to Cl- and Ca2+ to Mg2+ are employed to analyze the brine samples, indicating that most of the brine samples are not saturated with respect to halite and only a small number of the brine samples are saturated with respect to halite.The δ18O and δD of the brine samples in the study area show both the 18O shift and the D shift, indicating a long time evaporation and concentration of the brines.The subsurface brines originated from the paleometeoric water of Neogene age.The Qaidam Basin uplifted and its subsidence center moved eastwards due to the quick uplift of the QinghaiTibet Plateau after Pliocene.The basin received sediments and the caprocks of the brinebearing formations formed.In the meanwhile, the change in atmospheric circumfluence led to an arid climate.As a result, the subsurface brines become the presentday brines after a long time evaporation, concentration and under a sealed state.

Key words: brine, hydrochemistry, ion ratio, origin and evolution, Qaidam Basin