柴达木盆地西部地下卤水水化学特征及其起源演化

柴达木盆地西北角分布有基岩，盆地西部分布山麓堆积及河、湖相沉积地层，新生界发育有多个北西—南东走向的规模不一的背斜，新近系分布有富钾地下卤水。利用统计方法分析卤水中TDS、K+和B2O3的富集情况。结果显示：TDS值出现双峰，卤水样品富K+和B3+的概率分别为628%和6129%。使用Piper图研究地下卤水时，经常出现样品点过于集中而不易反映主要离子含量的变化。本次研究改进了前人的图示方法，绘制适合于表示地下卤水的水化学图。利用钠氯系数、氯溴系数、氯碘系数、钾氯系数、脱硫系数、钙镁系数对研究区卤水水样进行分析，结果显示卤水样品多数未达到石盐沉积，少数达到石盐沉积。研究区水样氢、氧稳定同位素数据显示新近系地下卤水δ18O值和δD值均发生漂移，表明卤水经历长时间的蒸发浓缩。研究区地下卤水起源于早上新世的古大气降水，上新世以后由于青藏高原快速隆升，柴达木盆地抬升和沉降中心东移，接受沉积形成良好的盖层，同时大气环流的改变致使气候变得干燥，地下卤水经历蒸发浓缩封存，形成现今的卤水。

Hydrochemical Characteristics,Origin and Evolution of the Subsurface Brines in Western Qaidam Basin

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.