Evolution Of Fluids And Significance Of Prospecting Of The Lijiagou Pegmatite Spodumene Deposit In Sichuan Province

Lijiagou pegmatite spodumene deposit in the southwest of Ke’eryin orefield,Sichuan Province is a newly discovered pegmatite-type spodumene deposit within the central Songpan-Garze fold belt（SGFB）.A systematic study of inclusions in the deposit has been carried by Petrography and microthermometry,Laser Raman Spectra,thermodynamic calculation to discussed the characters and ource of ore-forming fluid and the mineralization,we gain the achievements and judgements.（1）The Upper Triassic Zhuwo Formation（T₃zw）is the main ore-bearing stratum of Lijiagou deposit.Spodumene（max）,coltan and cassiterite（Second）,beryl（rare）,are the main ore mineral.Microcline,quartz,albite,muscovite,tourmaline,garnet are the gangue mineral.（2）There are eight distinguishable types of fluid inclusions in the Lijiagou deposit:crystal-rich inclusions,CO₂-rich CO₂-H₂O inclusions,liquid-rich vapor-liquid H₂O inclusions,H₂O-rich CO₂-H₂O inclusions,vapor-rich vapor-liquid H₂O inclusions,pure liquid H₂O inclusions,pure liquid CO₂ inclusions and pure vapor CO₂ inclusions.（3）From microcline pegmatite zone（I）to albite spodumene pegmatite zone（IV）in the Lijiagou deposit,the homogenization temperatures and pressure of fluid inclusions are Similar,the salinity and the density values value of fluid inclusions are enhancing.This paper employs the Hydrothermal Diamond Anvil-Cell（HDAC）to test the initial dissolution temperature of daughter minerals and homogenization temperature in the crystal-rich inclusions.Results show that the homogenization temperature of crystal-rich inclusions in the Lijiagou deposit is 658～741℃,the initial dissolution temperature of daughter minerals in the crystal-rich inclusions is402～512℃.Ore-bearing pegmatite in the albite spodumene pegmatite zone,the homogenization temperature of inclusions mostly range from to 288.9～393.4℃,the salinity range from to 6.1～22.6 wt%,the density range from to 0.72～0.94 g/cm³,the pressure range from to 181.1～280.4 MPa,the mineralization depth range from to6.7～10.4 km.This results show that the ore-forming fluid has the characteristics of the low to middle salinity,the middle to high density and rich CO₂.The Lijiagou deposit is high temperature and middle-depth type deposit.（4）The results of laser Raman analysis of fluid inclusion in lijiagou deposit show that the composition of the fluid inclusion in all pegmatite zone is similar,and the composition of the ore-forming fluid includes H₂O,CO₂,N₂,CH₄ and so on.Laser Raman spectroscopic analyses showed the occurrence of silicate minerals and carbonate minerals,in the crystal-rich inclusions hosted in spodumene and quartz.This show that the ore-forming fluid of pegmatite in Lijiagou deposit is rich in volatile silicate fluid and has carbonate composition.（5）The crystal-rich inclusions and the medium-high salinity CO₂-rich CO₂-H₂O inclusions are coexistent.The medium-high salinity vapor-liquid H₂O inclusions and low salinity CO₂-H₂O inclusions are coexistent and their homogenization temperatures are approximate.It is suggested that the ore fluid underwent immiscibility and resulted in phase separation and rapid precipitation of spodumene,which could be the main cause of formation of Lijiagou deposit.（6）The ore-forming fluids evolution have been discussed in this paper.Variation of inclusion types from crystal-rich inclusions to fluid inclusion indicates that the ore-forming medium varies from magma-hydrothermal transition stage to hydrothermal solution stage.（7）It is found that pegmatite formation in Lijiagou deposit does not possess the characteristics of magmatic crystallization differentiation.Combined with chronology,it is considered that the Lijiagou pegmatite spodumene deposit may be a pegmatite deposit with anatectic melting.