| The evaporite,which is made up of salt-forming elements in earth surface,is a special type of sedimentary rocks forming through the processes of sedimentation,diagenesis and epigenesis in supergene environment.Qarhan Salt Lake,with the potash evaporate forming nowadays,is a nature laboratory for studying the formation mechanism of nonmarine potash evaporate.Moreover,Qarhan Salt Lake deposits two hydrochemical types of brine: chloride type and magnesium sulphate subtype,which is the largest liquid potassium-magnesium salt deposits in China.Therefore,the study about the formation mechanism of Qarhan Salt Lake cannot only explain the formation of ancient potash evaporate,but also have great theoretical significance to learn the potassium genetic mechanism of inland salt lake in China.In order to interpret the origin of brine deposits in this playa,five profiles from Bieletan,Dabuxun,Qarhan and Huobuxun sections in Qarhan Salt Lake were selected in this study.The depth is in the range from 0 to 7.0 m and the stratigraphy of these salt sediments belongs to Holocene salt group.We collected samples in 10 cm interval from five porfiles and analyzed mineral assemblages of sediments by XRD method and elementary analyzer in the laboratory.Moreover,we measured the composition of hydrogen and oxygen isotope of different brines in Qarhan Salt Lake.Combining the distribution of different hydrochemical types,we hoped to provide the mineral evidence for the material source of the Qarhan Salt Lake.Through this study,we have reached these following conclusions:1?The results show that total average concentrations of halite,gypsum,bischofite and carbonate were more than 70%,4.7%,3.4% and less than 1%,respectively,which implies that the mineral assemblage was relatively simple and mainly deposited halite,poor gypsum and carbonate minerals.2?With contrasting major element content of different regions,we concluded that the content of K+ in the west was obviously higher than the east,the content of Mg2+,SO42-in the west was relative higher than the east,the content of Cl-,Na+,Ca2+,CO32-showed lower rangeability in Qarhan Salt Lake.On account of the Golmud River,the content of Mg2+ in DBXS profile is higher than others,but the content of Cl-,Na+ is far lower than others.3?Based on the correlation analysis of major elements,we found that there were three profiles having a good linear relativity between SO42-and Mg2+,which explained the fact of polyhalite and carnallite existing in the west of Qarhan Salt Lake.The profile of DBXS presented negative correlation between Ca2+ and Mg2+,which reflected that Golmud West River had dissolved the surface layer salt minerals.In addition,there were abundant bischofites to form under the function of great evaporation.4?On the basis of the distribution characteristics of hydrogen and oxygen isotopes,we found that the distribution of hydrogen and oxygen isotopes of natural water had relations with the supply of rivers/rain,deep water and evaporation.There were many rivers and lots of rain supplying to the brine lakes and intercrystalline brine in the west and southwest of Qarhan Salt Lake.There was some deep water and evaporation impacting the supply for the brine lakes and intercrystalline brine in the center and northeast of Qarhan Salt Lake.The adulteration gradually receded from the north to the south.5?The study found that the average content of gypsum in each section from west to east obviously decreased,the average content of Mg-bearing minerals from the southwest to the northeast part of Qarhan playa also decreased.Based on the sedimentary features of five profiles in this study,combining with the elemental characteristics of brines with enriching in Ca2+ and depleted Mg2+,SO42-in the north and northeast part of Qarhan playa,we concluded that the north and northeast of Qarhan Salt Lake should be received the supply from chloride type spring,which offers the mineral evidence for the mixing theory between river waters and deep spring. |
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