Metallogenisis Of Carbonatite In Qinling Orogenic Belt

Rare earth elements（REEs）and niobium（Nb）are important strategic and critical metals,which are crucial for the economy and national security.In recent years,many carbonatite-related polymetallic deposits have been found in the northern and southern margins of the Qinling Orogenic belt in China,whereas REE and Nb mineralization mechanism in these deposits remains unclear.The Miaoya carbonatite complex,located in the South Qinling belt,is the second largest carbonatite-hosted REE-Nb deposit in China after Bayan Obo.The light and heavy REE resources of Jialu carbonatite both reach industrial grade.This thesis aims to conduct combined mineralogy,U-Pb geochronology,chemical and C-Sr-Nd isotopic compositions of the dominant and ore minerals,to decipher the distribution of ore-forming elements and the genetic history of these carbonatite hosted multi-metal deposits.REE and Nb mineralization in Miaoya deposit are often independent,i.e.,Nb-rich samples generally lack REE mineralization.Monazite and bastn?site are the most dominant REE minerals.Monazite grains mainly occur at the rims of fluorapatite,whereas bastn?site together with some monazites form aggregates in ankerite veins.Ferrocolumbite and pyrochlore-group minerals are the most dominant Nb minerals at Miaoya,and are abundant in carbonatite containing fluorapatite aggregate bands,which show complex chemical compositions.Miaoya carbonatite samples are divided into equigranular and inequigranular carbonatite.Equigranular carbonatite mainly occurs in massive rock body,with calcite showing typical magmatic and homogranular texture.Calcite in these carbonatites has the highest contents of Sr and REE,and shows a near flat REE patterns with slightly LREE enrichment.In situ Sr and C isotope analyses of these calcites show relatively homogeneous compositions,with ⁸⁷Sr/⁸⁶Sr ratios of approximately 0.7035 andδ¹³C values of 6.0‰.Inequigranular carbonatites are usually found in the area with structural fractures.Calcite usually shows hetrogranular texture,and some displays feature of plastic deformation and alteration.It has lower Sr and REE contents,and shows LREE-depleted chondrite normalized REE patterns.In addition,C-Sr isotopic compositions of these calcites display an elevated trend with level of alteration.This study shows that REE are mainly hosted in calcite and apatite during Early Paleozoic magmatism.During Triassic,the metasomatism induced by tectonic thermal events caused the dissolution-reprecipitation of REE in calcite and apatite to form new REE minerals including bastn?site and monazite.Magmatic Nb-bearing minerals in Miaoya（e.g.,uranopyrochlore,betafite and fersmite）are altered during Triassic metasomatism.They are commonly replaced in situ by secondary ferrocolumbite,uraninite,ilmenorutile and other Nb-bearing minerals.Secondary ferrocolumbites inherit geochemical features of their precursors,including Ta content.The earliest formed uranopyrochlore contains the highest Ta₂O₅ content（2.33～4.97 wt.%）,and its alteration product ferrocolumbite also shows the highest Ta₂O₅ content（2.7～6.2 wt.%）.Based on the trace element composition of Nb-bearing minerals and associated calcite,we speculate that the degree of columbitization is related to the fluorine activity in the fluid-rock interaction.Pyrochlore-group minerals first release Na⁺during hydrothermal alteration,which can maintain the valence-state balance of REE³⁺in apatite and thus hinder the dissolution-reprecipitation process,resulting in the independence of REE and Nb mineralization at Miaoya deposit.To sum up,Triassic metasomatism is beneficial for REE migration and deposition,whereas it plays an insignificant effect on Nb mineralization.The Jialu ore body dominantly contains coarse-grained Mn-rich calcite carbonatite,brecciated and banded ore.The dominant LREE minerals are parisite,bastn?site and monazite,while the dominant HREE minerals are xenotime.In situ U-Pb dating of LREE and HREE minerals all show a similar age,with the ages of parisite,bastn?site,monazite and xenotime at 231.4±7.2 Ma,234.7±6.5 Ma,217±5.0 Ma and 227±1.6 Ma,respectively.It indicates that the LREE and HREE mineralization occurred together during Triassic.The primary Mn-rich calcite has the lowest Sr isotopic composition(⁸⁷Sr/⁸⁶Sr=0.7057～0.7067)and the highest HREE content（HREE+Y=663～1497ppm）,whereas REE minerals are absent.Combined with the in-situ Nd isotopic composition of primary and altered REE minerals,we assumed that the Jialu carbonatite originated from low-degree partial melting of the EM1 mantle with contributions of eclogite facies bearing metamorphic oceanic crust.The Sr-Nd isotopic compositions of hydrothermal calcite and altered bastn?site indicated that late hydrothermal alteration involved crustal materials.LREE and HREE mineralization in Jialu is mainly related to post-magmatic hydrothermal alteration.The migration and precipitation of REEs are controlled by various ligands.In the carbon-hydrothermal fluids rich in sulfate and fluorine,and the migration of REEs is mainly controlled by SO₄^2-.Barite crystallization results in the precipitation of REE-rich fluorocarbonate minerals and xenotime,forming the brecciated ore.Hydrothermal fluids rich in F and Mg metasomatized the country-rock（Taihua Group）containing apatite and extracted P,Nb,Ti and other elements to form a banded ore dominated by monazite.To sum up,this thesis has obtained new understandings on the distribution,age,source and metallogenic processes of Miaoya and Jialu deposits.Miaoya carbonatite magma originated from the partial melting of the mantle with mixed EM1,EM2 and HIMU features during Early Paleozoic,forming carbonatites enriched in LREE and Nb.LREE is dominantly hosted in calcite and apatite during magmatism,and the formation of bastn?site and monazite is mainly related to Triassic metasomatism.Jialu carbonatite originated from the EM1 dominated mantle with the contribution of eclogite-bearing metamorphic oceanic crust,generating a unique carbonatite magma rich in HREEs.At the hydrothermal stage,xenotime,fluorocarbonate and monazite are formed.Different enrichment of LREE and HREE in the south and north of Qinling Orogenic belt is mainly due to the carbonatite magma source compositions that formed by partial melting of different mantle sources,and the later hydrothermal process promoted the enrichment and mineralization of REEs.Despite the significant roles that the hydrothermal process plays in the generation of carbonatite-associated REE mineralization,Nb mineralization in Miaoya carbonatite mainly occurred in the magmatic stage dominated by pyrochlore,while Triassic metasomatism only results in columbitization of the magmatic niobium minerals.