Geochemical Characteristics Of Yanshanian Granite In Southeastern Guangxi And Its Implications For Rare Earth Mineralization

Ion-adsorbed rare earth ore is the dominant ore type in China,and the initial exploration of this type of deposit is mainly distributed in seven southern provinces of China（Jiangxi,Fujian,Zhejiang,Sichuan,Guangdong,Guangxi and Yunnan）.In recent years,with the detailed study of ion-adsorbed rare earth ore and the continuous upgrading of exploration technologies,some ion-adsorption rare earth ore mineralization potential areas have been delineated in the area of Darongshan-Shiwandashan in Southeastern Guangxi.In order to explore the mineralization potential of ion-adsorbed rare earth ore in Southeastern Guangxi,detailed field geological survey,petrology,elemental geochemistry,geochronology,zircon Hf isotope,and rare earth mineralogy have been carried out for the granites in the Cenxi area.Furthermore,comparative study with the metallogenic parent rocks of typical ion-adsorbed rare earth ore in South China is also carried out.The main achievements are as follows:（1）Zircon U-Pb chronology studies have shown that the formation age of granite in the Cenxi area of Southeastern Guangxi is 161 Ma～155 Ma,which belongs to the production of late Jurassic magmatic activity and is consistent with the era of the first diagenetic mineralization in South China druing the early Yanshanian.（2）Elemental geochemical studies have shown that the granite is characterized by high silicon（Si O₂=74.5%～75.3%）,potassium-rich（K₂O/Na₂O=1.43～1.51）.Their Rittmann index（σ）is 2.19～2.29,and the aluminum saturation index（A/CNK）is 1.00～1.03,belonging to high differentiation,high potassium calcium alkaline,and quasi-aluminous–weakly peraluminous rocks.The total amount of rare earth elements in granite was high(ΣREY=260×10^-6～349×10^-6)withΣLREE/ΣHREY of 2.18～2.66 and La_N/Yb_N of 5.97～7.58,indicating that they are enrichment of light rare earth.They have high（Zr+Nb+Ce+Y）contents and 10,000Ga/Al values,showing similar geochemical features to A-type granite.Zircon Hf isotope studies have shown that theirε_Hf（t）values are-1.44～+1.49 with two-stage mode age(T_DM2)of 1046 Ma～901 Ma.The above data suggest that the granite mainly derived from the melting of metamorphic mudstone and/or metamorphic greywacke,and formed in an extension background.（3）Rare earth mineralogy studies have shown that granite is rich in rare earth minerals such as apatite,zircon,monazite and parisite,among which the total amount of rare earth oxides in monazite and parisite are 61.4%～67.6%and 62.5%～65.1%,respectively.Combined with the exogenous conditions such as high temperature and humidity,abundant rainfall and strong weathering in this area,it is inferred that this area has the potential to form ion-adsorbed rare earth ore.（4）Comprehensive comparative studies have shown that the material composition and crystallization of granite are important influencing factors for the formation of ion-adsorbed rare earth ore.For Southeastern Guangxi,the addition of more mantle-derived materials may be an unfavorable factor for the formation of ion-adsorbed rare earth ore in this area,and strong differentiation is more conducive to the formation of ion-adsorbed rare earth ore in this area.