| China is the world's largest potash consumer and potash resource importer.In order to overcome the high dependence of China's foreign potash resources,it is necessary to strengthen the investigation and research of domestic resources and increase the resource reserves.The paper started from the development of in situ micro-area analysis method of salt rock mineralogy.The attempt of Laser ablation inductively coupled plasma mass spectrometry?LA-ICPMS?analysis method for salt rock minerals was preliminarily attempted.Finally,a method was established for the determination of mineral elements content in halite and sylvine,by electron probe in situ micro-area analysis.Taking Mesozoic salt deposits in Simao basin of southwest Yunnan and Paleogene salt-bearing strata in Tarim Kuqa basin as research objects,using in situ micro-area observation and analysis methods of various minerals including microscope observation,inclusion recognition,scanning electron microscope observation and energy spectrum composition analysis?SEM-EDS?,X-ray diffraction qualitative and whole rock analysis,LA-ICPMS and EPMA.A systematic study of petrography and mineralogy was carried out on the core samples of well-L2 in the Mohei area of the Zhengdong salt belt in Simao basin and well-MK-1 in the Mengyejing area of the Jiangcheng salt belt,the sectional samples of the salt diapirs in Wensu Saltern and Quele Dome Quele-Awate area,Western Kuqa basin.On the basis of these studies,the contents of K and Br and the parameters of Br×103/Cl?w/w?value of the indicative trace elements were measured by the EPMA,which further discussed the salt source and metallogenic model of Simao basin,and preliminarily predicted the prospects of potassium exploration in Kuqa basin.The attempt of LA-ICPMS method for salt rocks was preliminarily tried,and the USGS synthetic basalt glass GSE-1G was selected as the standard sample.The experimental conditions suitable for the Laser system were:20?m,10Hz,130m J,50%T,8.30J/cm2,and the dwell time of trace elements in ICPMS should be as long as possible.The conditions and calibration standards showed good applicability to the K and Br tests of halite.However,due to the low ionization efficiency of Cl element in ICPMS,the Cl test of stone salt under the condition and calibration standard sample is not ideal,and I element lacks appropriate standard sample.Furthermore,the electron probe analysis method of halite and potassic halite was established,and the applicability of the method to the analysis of main contents and trace elements of halite and potassic halite was better by adjusting and optimizing the test conditions.However,due to the low ionization efficiency of Cl element by ICPMS,the conditions and calibration standards are not ideal for the Cl test of rock salt,and the I element lacks appropriate standard sample.The results of fine mineralogical observation of the bearing mud-gravel salt rock samples of the Lower Cretaceous Mengyejing Formation?K1m?from well-L2 in the Mohei area,Zhengdong salt belt in Simao basin show that the main salt minerals are halite,anhydrite,dolomite,calcite,etc..there are two occurrence types of halite in the bearing mud-gravel salt rocks.One is the principal part halite of cementing detrital particles with plastic rheological characteristics,the particle size is more than 100?m,with plastic rheological characteristics.The other is halite that have been precipitated into the clasts and wrapped in detritus of salt or clay minerals,the particle size is 5-100?m.The characteristics of both inclusions indicate secondary origin halite.By means of EPMA,the K content of principal part halite,halite wrapped in detritus in the K1m from well-L2,and potassium-bearing halite in the J2h from well-MK-1 were less than0.09wt%,less than 0.18wt%and less than 0.13wt%,respectively.The content of Br were less than 60×10-6,70×10-6?410×10-6 and 70×10-6?500×10-6,respectively.The value of Br×103/Cl?w/w?were?0.10,0.12?0.71 and 0.12?0.85,respectively.The content and value of principal part halite is significantly lower than the latter two,and corresponding to the halite stage of terrestrial source or sea-continent mixed source or recrystallized,and was subjected to posttectonics.The value of the latter two are close to each other,most of them are corresponding to the halite stage of marine source,and a small part are corresponding to the stage of potassium and carnallite crystallized by marine source mother liquor.According to the two typical occurrences of halite of the sample and K,Br content and Br×103/Cl?w/w?value parameters It is inferred that halites wrapped in detritus in the K1m from well-L2 were the evidence of the salt diapir of the deep paleosalt bodies.It is believed that the Mohei salt deposit in the Dongdong salt belt also develops mixed salt bodies penetrated into the shallow part of the Middle Jurassic salt diapir.These understandings have improved the exploration practice of"two-storey"metallogenic theory in the study area.Possible metallogenic model was as follows.After the deep marine paleosalt bodies of the Middle Jurassic were migrated to the K1m by salt diapir,part of the paleosalt bodies was leached,mixed,modified and combined by the Middle Jurassic residual seawater migrated from the inner part of the basin,continental source water flowing from the periphery of the basin and deep hydrothermal fluid,and with to form the new brine.Part of them was preserved in the form of solid paleosalt gravels.In the late Early Cretaceous,during brine evaporation of new mother liquor into salt,the solid paleosalt gravels were wrapped by formed salts and terrigenous clastic minerals.In the later period,sedimentary diagenesis and mineralization were carried out under the cementation of the principal part halite of mother liquor crystallization.The results of fine mineralogical observation and analysis of the sectional samples of the salt diapirs in Wensu Saltern and Quele Dome of Quele-Awate area,Western Kuqa basin show that the halite,anhydrite,dolomite,etc.are developed in the salt-bearing strata.Sylvite was found in the rock salt layer of Wensu Saltern field for the first time,showing that the ancient salt lake has evolved to the stage of potassium salt precipitation during the Paleocene-Eocene period,indicating a good prospect for potassium.On the basis of these studies,the contents of K and Br and the parameters of Br×103/Cl?w/w?value of the indicative trace elements were measured by the EPMA,which further discussed the salt source and metallogenic model of Simao basin,and preliminarily predicted the prospects of potassium exploration in Kuqa basin.The content of Br were 130×10-6?580×10-6 and the value of Br×103/Cl?w/w?were0.28?1.23 in the sylvite.However,the Br content of salt rock showed different characteristics in different test points and different samples.Part of the content of Br were below the detection limit,part of the content were between 60×10-6?220×10-6,Br×103/Cl?w/w?value were 0.10?0.37.They reflected that the salt rocks of the Kumugeliemu Group were formed under the mixed environment of terrestrial and continent source,and part of the halite were subjected to the late recrystallization or strong leaching of atmospheric water.The research in this paper provides new methods and ideas for determining the material source of salt rock and salt-forming environment,and provides theoretical support for the subsequent work of prospecting for salt and potassium in deep layers and the exploration of potassium resources in China. |
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