| The worldwide demand for the K,B and Li mineral resources is increasingly vigorous and competitive.At present,China’s own K,B and Li mineral raw materials are mainly of brine types,and the available resources are mostly concentrated in the Qaidam Basin of the Qinghai-Tibet Plateau.The brine resources in this basin have great advantages in terms of the location and development condition,the grade and reserve potential and therefore,are the backbone of the development of K,B and Li industries in China.The Qaidam Basin is unique in China and even in the world in terms of the salt lake deposition and mineral resources enriched in brine.Many researches have been performed on the regulation of the salt lake formation and the genesis and evolution of brine,and abundant data have been accumulated.However,due to the limitations of technical conditions and research materials at that time,the theory of salting mechanism and brine mineralization of some typical salt lakes in Qaidam Basin are still debated.For example,the Qarhan Salt Lake,which is a super large Quaternary potassium salt brine deposit,is still controversial in the mechanism of solid salt deposition and brine potassium salt aggregation.There are three theories of origin,namely,the migration of the ancient lake in the west,the brine recharge of the northern fault zone and the capture of the ancient lake in the Kunlun Mountains.At the same time,the metallogenic mechanism of K,B,and Li of the Tertiary palaeobrines has not been thoroughly revealed.With the large-scale development of brine resources and the increasing national demand for K,B,and Li mineral supply and storage,the existing salt-forming theory in the Qaidam Basin can no longer fully meet the requirements of resource exploration,development,and utilization.It is necessary to pay attention to both the Tertiary palaeobrines and Quaternary modern salt lake brines,and to introduce multiple techniques of the isotope analysis to conduct comprehensive studies on the brine type deposits of the basin.In this study,the brines including the Tertiary palaeobrine and Quaternary modern salt lakes in the Qaidam Basin are taken as an interconnected system,and the key metallogenic mechanism of K,B,and Li resource enrichment in the basin is comprehensively revealed by the means of hydrochemistry and geochemistry of multiple isotopes,including H,O,B,Li,S,and Sr elements(δD,δ18O,δ11B,δ7Li,δ34S,87Sr/86Sr).The main achievements and understandings are as follows:(1)The results show that the palaeobrines enriched in K,B,and Li are mainly concentrated in the anticlinal structural areas,such as in Nanyishan,Dafengshan and Jiandingshan,which has a good development and utilization prospect.The Great lake in the Tertiary acted as the material base of the palaeobrine.Under the controling of the later tectonism,a variety of fluid sources jointly supplied the ancient brines and determined the enrichment and mineralization of K,B,and Li elements in the palaeobrines.δD and δ18O isotopes datas indicate that the palaeobrines in the Kunbei fault-step belt are of meteoric origin,and the palaeobrines enriched in K,B,and Li elememts in the Yingxiong Ridge uplift in the central part of the western Qaidam Basin are the result of the deep magmatic fluid recharge controled by the active structures on the basis of the ancient lake deposits with high evaporation concentration.The δ11B and δ7Li isotopes data provide further evidence for that the magmatic fluid recharge might be regional and irregular,and there is a "point recharge" in the Xiaoliangshan anticline,resulting in the diverse distribution of K,B,and Li minerals.In the same area,there can also be a variety of recharge water bodies,such as in the Shizigou anticline,the origin of palaeobrine includes ancient salt lake deposition and geothermal spring water.(2)The salting process of the super-large potassium salt deposit in the Qarhan Salt Lake and the genetic evolution of potassium-rich brine are systematically revealed.Firstly,it is proposed that the CaCl2 type karst water originated from the Kunlun Mountains and migrated to the Sebei area,and then migrated along the stratum to the northern edge of the Qarhan Salt Lake.Secondly,the origin of solute liquor in three major salting periods of the Qarhan Salt Lake was determined.The first period was dominated by karst water and brines from the Qaidam ancient lake,the second period was dominated by geothermal fluid,and the third period was dominated by karst water and ancient lake water from the Kunlun area.Finally,it is suggested that the potassium-rich brine was the result of mixing K-rich liquor with river water and karst water in the south and north of the salt lake after multiple phases of the NaCl precipitation.(3)Based on hydrochemical evidence of the potassium-rich salt lakes,such as the Kunteyi,Chahansilatu and Dalangtan lakes in the Qaidam Basin,and the data ofδD,δ18O,δ11B,δ34S and 87Sr/86Sr values of several brines and salt sediments from the drill holes,it can be found that the Quaternary brines in the northwest of the Qaidam Basin are related to the evaporative and concentrated paleo-lake bodies.In addition,most of them were recharged by palaeobrine,which resulted in the differences of Li and Rb contents in brine and the deposition of a large amount of polyhalite deposits.(4)A basin scale hydrochemical survey and H-O-Li(δD,δ18O,and δ7Li)isotope study were conducted in the Dongtai,Xitai,Yiliping and DaQaidam salt lakes in the central Qaidam Basin.It was confirmed that the abnormal enrichment of B and Li in the salt lake brine was directly related to the high B and Li contents in the recharge water.The input of geothermal fluid,related to the highly differentiated granites that are enriched in B and Li elements,along with the river is the key factor to the B and Li mineralization.(5)The brine-type K,B,and Li metallogenic model of the Qaidam Basin is proposed,and the key mechanism is the recharge of magmatic hydrothermal,high-temperature geothermal fluids.Under the orogenic environment of the continental collision of the Qinghai-Tibet Plateau,Partial melting and highly differentiation of the magmas in the thickened crust took placed,resulting the output of melts enriched K,B,and Li elements,such as alkali metal elements of the melt.Hydrothermal fluids of magmatic origin through the emanation or along the fault zone then upwelled and were mixed with deep circulation of groundwater recharge to the ancient strata in brine.This process caused the supernormal enrichment in K,Li,Rb,and B of the brines in the Qaidam Basin brine.Under the influence of large-scale tectonic movements,such as the continuous strike-slip in the Altun fault zone,the ancient brines could further supply to the Quaternary surface salt lake in the northwest of the basin along the faults,which promoted the K,B,and Li brine mineralization of the modern salt lake on the basis of concentrated salt materials deposited in the ancient Great Lake in the west.Magmatic rocks of high differentiation and their hydrothermal fluids as the major material sources of B and Li minerals,migrated along with the large streams to the closed lake basins,and eventually in the arid climate,they were evaporated into salt,B,Li,and other mineral elements were concentrated in the salt lake brines.In the end,the mineralization might have been controlled by multiple factors coupling of structure,source,climate,and hydrological conditions. |
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