Gasikule Salt Lake Brine And Sediments Distribution And Uranium Enrichment Mechanism

Salt lakes have been extensively studied owing for its abundant mineralresources. Thus，many researchers pay high attention to the questions about thedistribution of U in the salt lake and its source. It has been found that the uranium inGasikule Salt Lake which located at the northwest of Qaidam Basin is not only higherthan the concentration of the average seawater but also exceed that of the averagebrines in the Qaidam Basin. Therefore，it is particularly significant to study thedistribution，sources，and enrichment factors of uranium in the Gasikule Salt Lake.When I started my this study，I collected all kinds of information about thewaters，brines and sediments of periphery area and eastern playa’s boreholes in theGasikule Salt Lake. Then，I measured the uranium concentration，hdyrochemicalcomposition，trace elements，chronology，uranium isotope. Based on the regionalgeology，hydrogeology，sedimentary characteristics，hydrogeochemistry，sedimentarygeochemistry， pollen assemblage characteristics， palaeoclimate since280ka， Ianalysed the factors which influenced the uranium distribution in the salt lake fromtectonic setting，sources，hydrodynamic conditions，sedimentary cycle，palaeoclimateto water evolution. I showed the uranium enrichment mode in the end. From all ofabove，the main results can be summaried as follows.1. In fresh water，uranium concentration in shallow water (stream water，swampwater，phreatic water) is higher than in deep water (artesian water，well water，oilfield water) for10times. The average uranium concentration of drillcores brines is78.08μg/L，which is the same as the superficial brines in Qaidam Basin. Horizontally，the distribution of uranium in drillcores brines was seriously affected by the freshwater supply，ie the uranium concentrations generally increase with farmer from thefresh water. Vertically，uranium concentrations in evaporite deposition’s brine higherthan debris deposition. Moreover，uranium contentations vertical differentiationsignificantly in a crystal brine layer and decreasing with increasing depth. Theconcentrations of uranium in brines range between13.90～933.00μg/L， averaging of274.17μg/L，which is not only the83.08times of the seawater，but also the2.83timesof the superficial brines in Qaidam Basin. Uranium concentrations increase from superficial brines，intercrystalline brines to salt-bond brines，while decrease in thebittern brines.2. The concentrations of uranium sedimentation range between0.19～15.80μg/g，averaging of3.37μg/g. There was large different uranium concentration indrillcores sedimentations. Horizontally，the distribution of uranium in drillcoressedimentations was seriously affected by the fresh water supply，ie the uraniumconcentrations generally increase with farmer from the fresh water. Vertically，uranium concentrations in debris deposition were higher than evaporite period.Moreover，uranium contentations vertical differentiation significantly in a crystalbrine layer or debris layer and increasing with deeper sedimentation. The averageuranium concentration in pure salts (halite，epsomite，gypsum) was1.02μg/g，whichwas approximately one third of sedimentations. Therefore，two thirds of uranium insedimentation was absorbed by clay while one third uranium was mingled with salineminerals. Chemical sedimentary differentiation in play such as capillary evaporationand leaching was very significant to uranium enrichment in brines andsedimentations.3. Through laboratory analysis，about7863terrestrial plant pollens were found inZK06drillcore sedimentations and an average of315per sample. According to plantecological habits，we divided into six pollen assemblage zones combined with itslithology from102.69m. Referring to pollen assemblage characteristics，Gasikule SaltLake’s paleo-vegetation changed from veld to desert steppe，and the paleo-climatechanged from cool and mid wet to cold and dry from280ka. Uranium concentration insediments was significantly correlated with total Sporo-pollen concentration，Shrubsand Artemisia，and also with herb concentration. The uranium concentration incool-mid climatical zones is very high，averagely three times as high as that in cold-dry zones. This climate evolution was benefit for uranium transform fromprovenance to salt lake and pre-enrichment.4. Based on the data of equilibration experiments and leaching experiments，uranium in the Salt Lake peripheral Quaternary sediments most easily to be leachedout，salt bearing rock in Pliocene was secondly，and provenance rocks were mostdifficult. Ability of uranium migrated from suspended particles to suspended liquid is normal. Moreover，the trend of migration coefficient (KU) between brines andsedimentations was consistent with uranium concentrations in suspended liquid.Through the leaching experiment and equilibration experiments，we could concludethat uranium in this salt lake come from Pliocene salt bearing rocks，provenance rocksand depth water. According to U-series nuclides calculation，uranium recharge ratiofrom Pliocene salt bearing rocks，provenance rocks and depth water account for about18.7%，80.4%and0.9%respectively.5. Uranium enrichment mechanism in Gasikule Salt Lake could be summarizedas a compound origin model which would be specific showed as follow. Uraniumfrom peripherial base rock，salt bearing rock and Quaternary sediments was leachatedby rain and superficial water and then enter salt lake. Uranium was foremore enrichedin salt lake after mingled with deep water. During the enrichment process，uraniumconcentrations were affected by the hydrodynamic conditions，deposition cycle，palaeoclimate and water evolution. In all influential factors，sources and waterevolution were dominance principles， tectonic setting， palaeoclimate andhydrodynamic conditions were secondary affecting factors，but deposition cycle wasminimum factor.