Element Geochemical Characteristics And Mineralization Of Lijiagou Pegmatite Type Lithium Deposit In Ke’eryin Area,Western Sichuan

Lijiagou pegmatite type lithium deposit is located in the southeast ore gathering area of rare metal ore field in Ke’eryin area,and its geotectonic position is located in the middle of Songpan-Garze orogenic belt.It is a super large pegmatite type rare metal deposit discovered in recent years,which is of great significance.On the basis of previous studies on Ke’eryin pluton,combined with the geological characteristics of the region,mining area,pegmatite vein and deposit,this paper has carried out petrographic observation,whole rock element geochemical analysis and muscovite element geochemical analysis on two mica granite and different types of pegmatite.In this paper,the metallogenesis of Lijiagou pegmatite type lithium deposit has been discussed and we have obtained the following understanding:The pegmatite vein of Lijiagou deposit is divided into five zones in horizontal area.Micro plagioclase type（I）and micro plagioclase albite type（II）pegmatite dikes can be seen in the periphery of the mining area,which are in penetrating contact with the two mica granite.Different degrees of mineralization can be seen in each zone.The mineralization is mainly concentrated in albite type（III）and albite spodumene type（IV）pegmatite veins,while albite lepidolite type（V）is rare.The vein shape is mainly vein,big vein and a small amount of lenticular,which mainly occurs in the upper Triassic Zhuwo formation（T₃zh）.The ore minerals are mainly spodumene,followed by beryl,niobium tantalite and cassiterite,accompanied by quartz,feldspar（mainly microcline and albite）,muscovite and a small amount of garnet,tourmaline and other gangue minerals.The major elements of the rocks indicate that the element covariance relationship between the two mica granites and pegmatites and the DI-Si O₂and SI-Si O₂relationships are discontinuous;Among the trace elements,the two mica granites and pegmatites are characterized by abrupt changes of adjacent lithofacies elements.The relationship between Zr/Hf-Nb/Ta and K/Rb-Rb indicates that liquid immiscibility may be involved in the intrazonal differentiation;Among the rare earth elements,the∑REE,LREE/HREE and TheδEu values also showed the characteristics of mutation and fluctuation,and the∑REE-La _N/Yb _Nand La-La/Sm relationships also showed the characteristics of separation.The element characteristics of the two mica granites and pegmatites are consistent with the definition of Li-F granitic rocks.Based on the above characteristics,it is considered that there may be no continuous crystallization differentiation between the two mica granites and pegmatites,and the differentiation in the area may also contain liquid immiscibility.The major elements of muscovite show that muscovite in two mica granite and pegmatite is mainly common muscovite,while muscovite in albite pegmatite and spodumene pegmatite tends to be lithium rich muscovite,which are all secondary genesis muscovite and related to hydrothermal metasomatism;There is a positive correlation between n（Al）and n（Si）,and a negative correlation between n（Fe）and n（Si）,indicating that the substitution of Al³⁺by Fe²⁺is common;There is a positive correlation between Li₂O and F,indicating that Muscovite can be used as the carrier of volatile matter;Among the trace elements,Rb and Cs in muscovite tend to be enriched in spodumene pegmatite,indicating that K is generally replaced in the later stage,and muscovite can also be used as the carrier of dilute alkali metals;muscovite in the two mica granites and pegmatites is characterized by abrupt changes in Rb and Cs contents.The K/Rb-Li,K/Rb-Rb and K/Rb-Cs relationships are in reverse.The magmatic crystallization differentiation indexes K/Cs and Nb/Ta also show discontinuous characteristics,indicating that there may be no continuous crystallization differentiation relationship between the two granites.To sum up,chemical analysis shows that the two mica granite is high-K alkaline-calc alkaline strongly peraluminous granite.The tectonic discrimination map shows that it is a granite in the transition period from syn collision to post collision,and the source rocks are mainly sandstones dominated by greywacks and clastic rocks and partially melted argillaceous rocks.Lijiagou deposit is a typical LCT type pegmatite deposit.The pegmatites are characterized by alkaline-calc alkaline strongly peraluminous granitic pegmatites,which were formed in a post orogenic environment.The clay rich argillaceous rocks of the Triassic Xikang Group provided the initial material source for the pegmatite magma.Liquid immiscible differentiation of magma plays an important role in the diagenesis and mineralization of pegmatite.The late magmatic hydrothermal stage is of great significance to pegmatite mineralization.The gas-liquid separation in the fluid phase intensifies the precipitation of minerals and promotes the growth of pegmatite minerals,thus forming the Lijiagou pegmatite type lithium deposit.