The Differentiation Mechanism Of Rare Earth Elements In Parent Rocks Of Ion Adsorption-type Rare Earth Element Deposits

Ion adsorption-type rare earth element(REE)deposits supplies 80% of the world's heavy rare earth resources.The genesis of these deposits has attracted much attention from geologists worldwide.Nanling region is a famous resource base of ion adsorption type rare earth deposits in the world,which is an ideal place to study the genesis of such deposits There are many types of ion adsorbed rare earth deposits in this region,but the mechanism of rare earth element differentiation in mineralized pluton is remain unclear.Therefore,we present systematic petrological,mineralogical,chronological and geochemical study of the Yijiang high europium-rich scandium light rare earth deposit and systematically compare the geochemical characteristics of the mineralized pluton of the ion adsorption type rare earth deposits in Nanling area,revealing the mechanism of controlling the differentiation of rare earth elements in the mineralized pluton.Based on these study,the following conclusions can be drawn:(1)The Yijiang quartz diorite yield a zircon LA-ICP-MS U-Pb age of 436.9 ± 3.3Ma(MSWD = 1.7,n = 20),indicating that the mineralized pluton was formed in Caledonian.(2)The occurrence state of Sc-Eu-LREE in Yijiang pluton has been identified: Sc is mainly hosted by Zr and Mg-Fe-enriched minerals(e.g.hornblende);Eu is mainly hosted to Ca-enriched minerals(e.g.plagioclase)and LREE are mainly hosted by accessory minerals(e.g.apatite).(3)It is clear that the rock genesis,magmatic properties and the restriction mechanism of the enrichment of Sc-Eu elements in Yijiang pluton:a.The Yijiang quartz diorite mainly formed by partial melting of the lower crustal material,and there may be a small amount of mantle-derived material mixed in,forming magma with high Sc,Fe and Mg content,which is facilitates to the formation of scandium-rich hornblende and pyroxene minerals;b.The nature of the magma reduction and the low degree of differentiation make the Eu element remain in plagioclase.(4)The restriction mechanism of REE differentiation in mineralized pluton of ioadsorption type REE deposit is preliminarily clarified:a.The source materials of HREE-type mineralized rocks are mature crust,which facilitates generate a melt with high content of Al,Si,F and P,and high degree of fractional crystallization.LightREE-bearing minerals(e.g.monzonite)crystalized at the early stage during the evolution of the magma.The content of alkali elements and fluorine increase with the increasing degree of fractional crystallization,which HREE and Y elements remained in the residual magma.The HREE-bearing minerals crystalized at late stage due to the decreasing of temperature and crystallization of fluorite;b.Light-REE related magma derived from mixed source of crustal and mantle material,which facilitates to generate a metaluminous to peraluminous and calc-alkaline to alkaline magma with high Si and P content,and medium degree of fractional crystallization.During the evolution of the magma,both LREE-and HREE-bearing minerals coexist in the magma,which prevents the differentiation of the REE elements;c.The magmatic source for LREE-Eu deposit related rocks are dominated by low mature crustal-derived material with mantle magma,which facilitates the generation of metaluminous and calc-alkaline magma with low Si and high P content,and low degree of fractional crystallization.During the evolution of the magma,both LREE-and HREE-bearing minerals coexist in the magma,which prevents the differentiation of the REE elements.