Geochemical Characteristics Of The Rich Strontium Stratum In Western Qaidam Basin And It's Geological Significance

Strontium is widely used in various fields such as photocells,alloy manufacturing,medicine,and military industry.With the continuous development of the world's industry,the application of strontium is gradually expanding.China is rich in strontium ore resources,and its reserves rank first in the world,mainly in the seven provinces?cities?of Qinghai,Yunnan,Shanxi,Hubei,Sichuan,Chongqing and Jiangsu.Among them,the Qaidam Basin in Qinghai is the most abundant,accounting for 48.4%of the total national reserves.Since after the discovery of large strontium deposits in the Dafengshan and Jiandingshan areas in the west of the Qaidam Basin in the 1950s,more than 10 small deposits and mineralization points have been discovered,including the Jianshan,Youquanzi,Nanyishan,Mangya,Huangfengshan,and Cunji sites.From the1980s,some works about the strontium deposits in the western Qaidam Basin has been done,and the results showed that there are still two major deficiencies in understanding the strontium deposits in the western Qaidam Basin:One is that the most of the research work is limited to the study of the Dafengshan mining area.The researchs about other deposits and mineralization sites are still blank,and there is a lack of overall understanding of the strontium deposits in the western Qaidam Basin.The second is little geological research on strontium deposits been carried out,and the understanding of the causes and sources of strontium ore is still not uniform.Therefore,the deep study of the strontium deposits in the western Qaidam Basin will provide a valuable theoretical basis for the exploration and development of strontium deposits in the area,which has important practical significance.Based on systematically collecting and organizing the geological exploration data and scientific research results in the study area,this paper focuses on the weak parts of predecessors'research works,mainly on the sedimentary environment,the material source,and cause of the strontium deposit in the western Qaidam Basin,in order to summarize the metallogenic regularity of the strontium deposits in the western Qaidam Basin,which could provide the scientific basis for the exploration and development of strontium deposits in this area.The study focused on the 9 sections of Lenghu anticline,the Jianshan,the Dafengshan,the Youdunzi,the Qingmingshan,the Jiandingshan,the Xiaoliangshan,the Nanyishan and the Shizigou in the western Qaidam Basin.The carbon,oxygen,and strontium isotope methods,combined with comprehensive analysis of multiple indicators are applied to explore the paleoenvironment,material sources,and genesis types of mineralization.The following understanding has been obtained:The major elements in the three formations of Qigequan Formation,Shizigou Formation,and Youshashan Formation in the western Qaidam Basin are basically the same,all of which are characterized by a high content of SiO₂,CaO,and Al₂O₃,and a low content of Na₂O,MgO,and K₂O.The average chemical elemental index?CIA?calculated from major elements is between 47.36 and 56.81,which is equivalent to fresh granite in the world,indicating that during deposition of the Qigequan Formation,Shizigou Formation and the Youshashan Formation in the western Qaidam Basin,the chemical weathering in the basin was weak?in the primary weathering stage?,reflecting the cold and dry climate at that time.Through the analysis of elemental geochemistry and isotope geochemistry,it has been found that from the Paleocene Youshashan Formation and the Shizigou Formation to the Pleistocene Qigequan Formation,their paleoclimate presents with the gradual drought trend,meanwhile the salinity of the lake water also increased,that the water body showed weak brackish characteristic during the deposition of the Youshashan Formation,and semi-brackish characteristic during the deposition of the Shizigou Formation and the Qigequan Formation.During the deposition of above three formations,the sedimentary environment of the lake basin was relatively stable,that all of them showed a semi-closed state or a dynamically changing state between open and close.The sedimentary environment was dominated by a weak oxidation.The trace elements,rare elements,and strontium isotope characteristics all reflect the same material sources for the deposition in the three different periods of the Youshashan Formation,the Shizigou Formation,and the Qigequan Formation in the western Qaidam Basin.The rare earth elements show the enrichment of light rare earth?LREE?,the stable source of heavy rare earth elements?HREE?,and the negative source of Eu elements,indicating that the source of sediments may be mainly the crustaceous felsic rocks.The Altun Mountains and Kunlun Mountains around the Qaidam Basin are distributed with volcanic rocks,granites,and quartz diorites,which are rich in strontium,which are the main source of strontium deposits in the study area.For the origins of strontium deposits in the western Qaidam Basin,scholars hold different opinions.There are mainly two viewpoints:the mineralization of underground hot water,and the chemical sedimentary mineralization.In this study,based on the comprehensive analysis of judging indictors for hot water deposition,characteristics of strontium isotopes,and elemental distribution characteristics,it is suggestted that strontium deposits in the western Qaidam Basin should be inland lacustrine deposits instead of the hot water mineralization.As the climate gradually dried up,the evaporation effect increased,and the basin in the state of close or semi-close,the evaporation caused the lake water to be concentrated and salinized,which leaded to the increase of the concentration of strontium in the water,and when it reaches a certain level,under which the celestite precipitates,and the strontium deposits was formed.The type of strontium deposits is celestite?SrSO₄?.From the perspective of the enrichment level,the main metallogenic age of strontium deposits should be Qigequan Formation,followed by the Pliocene Shizigou Formation in the western Qaidam Basin.