| Mengyejing potash deposit in the Simao Basin of Yunnan is the first continental ancient potash deposit, which was discovered in 1963, is also the most promising area to looking for potassium. As a continental potash deposit, the Mengyejing potash deposit is different from liquid potash such as Qaidam Basin in Qinghai or Lop Nur in Xingjiang, and this two liquid potash accounted for 98% of China potash reserves.Against the background of potash resource shortage and a large potash deposit, which has similar mineralization conditions with Mengyejing, was found in Vientiane Basin and Khorat Basin, the Mengyejing potash deposit is particularly important.Since the deposits were detected, great deals of basic research have been carried out, such as structural features, gravity geophysical survey and chemical characteristics of water. These works provide significant basic research data for our further study. But few researches in geochemistry had been developed, especially isotopic characteristics which are too unitary. This study is focused on the samples in salt sections(68-244m) of SHK4 drilling in Jiangcheng, we analyzed its mineral assemblages, element distribution and Boron, Chlorine isotope variation, then discussed the sedimentary environment, material sources and other issues.1. XRD analysis indicates that the main minerals are halite, anhydrite and quartz,similar in all strata. But the content is different in the individual layers. Several samples contain a small amount of magnesite, dolomite, calcite and clay mineral(chlorite), also there found some ankerite and bischofite in individual samples.According to the lithologic and mineral combination characteristics of this sample, we divided the salt segment of SHK4 drilling into 4 sub-salt layers. In each sub-layer,halite is basically has a mutual relative to anhydrite; the layers in which muscovite and ankerite were founded are consistent with the segment which has a sudden decrease of halite content, instructions that there has several times of terrestrial water or freshwater added in the deposition period.2. The variation of the main and trace element and characteristics of coefficient with depth might indicate that this characteristic has a strong correlation with depositenvironment and material source; Due to trace elements(e.g. Li, Br and B) are easy to enrich in the brine, these trace element contents is increasing with depth in general,but the dramatic changes in depositing sequence is also indicated that the terrigenous influenced to the salt segment.3. In general, the δ11B value of SHK4 drilling is from-4.30‰ to +35.58‰, the mean value is lower than the sea’s δ11B value(39.5 ‰), and individual sample’s value is close to modern seawater’s mean value; the δ37Cl value of SHK4 drilling is from-0.18‰ to +2.79‰, the mean value is 1.08‰, in addition to individual samples(Cl21, Cl23, Cl25 and Cl 28), the rest of the samples have positiveδ37Cl value, which is higher than seawater. With the decrease of depth, δ11B and δ37Cl values both have an obvious cyclicity, indicating the salt segment has been repeatedly affected by different levels of exogenous substances. According to the trend of boron and chlorine isotope, this exogenous substance with a low δ11B value and high δ37Cl value, as the river meets these two features are more likely to be the major factors.4. Boron and Chlorine isotopes showed the opposite trend, which relate to their own fractionation mechanisms. Boron and Chlorine isotopic compositions exhibit these trends in evaporating deposition also shows that these two isotopes changes are able to determine the evaporation stages and division the sedimentary cycles.In summary, the deposits formed in the salt segment of SHK4 drilling are controlled by many factors. For revealing the evolution of brine and material sources in this region, the mineral assemblages, element distributions and isotopic variation has an important role. |
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