The Mineralogy,Geochemistry And REE Mineralization Of Carbonatite In Bachu,Xinjiang

Carbonatite is commonly mantle-derived associated with ultramafic-alkaline mafic rocks,and rarely exposed on the surface,which shows enrichment of light rare earth elements(LREEs),strontium,thorium and phosphorus,so carbonatite is one of the rock types which have potential mineralization of REEs.Vein-type carbonatites with ring distribution are widely exposed in the Bachu Wajilitage area,Northwest Tarim Basin,Xinjiang.Field observation and geological survey reveal carbonatite invaded the contact zone of layered basic complex(?282 Ma)and Devonian sedimentary strata,intersected with gabbro(?278 Ma),lamprophyre(?281 Ma)and syenite dikes.The prospecting engineering determines that REE mineralization in some sections reaches the boundary grade of industrial ore bodies.Nowadays,there is still some controversy about the age,rock types classification,petrogenesis and REE mineralization mechanism of carbonatite in this area.In this contribution,we present systemic study of field occurrence,petrography,mineralogy,lithogeochemistry and zircon U-Pb chronology,in order to reveal the rock type classification and diagenetic age of Bachu carbonatite and discuss the genesis,diagenesis mechanism and rare earth mineralization effects thoroughly,which could provide insights for the formation of Permian plume magmatism in Tarim.The main research results are as follows:1.Based on the field geological survey,the Bachu carbonatite invaded into the contact zone of the Based basic layered complex(?282 Ma)with the Devonian Yimugangtawu and Keziertag formations,spreading in a semi-circular arc shape,and there is a symbiotic relationship between Bachu carbonatite and syenite,which is cut through by lamprophyre(278-280 Ma)and diabase subsequently,so its formation age is 280-282 Ma.2.According to the rock structure,structural characteristics and mineral composition identification under the microscope,Bachu carbonatite can be divided into two categories,namely,porphyritic carbonatite and crystalline carbonatite.3.The analysis results of major and trace elements in the whole rock show that the Bachu carbonatite is magnesian carbonatite.Porphyritic carbonatite and crystalline carbonatite are both characterized by enrichment of large ion lithophile elements(Ba,Sr,Th)and depletion of high field strength elements(Nb,Ta,Zr,Hf,Ti),and two kinds of carbonatite both have right-leaning rare earth distribution pattern,but there are differences in the total amount of REEs between them.The total amount of REEs in porphyritic carbonatite is 10439?g/g?108496?g/g,and that in crystalline carbonatite is 505.8?g/g?2792?g/g.4.According to the composition analysis of porphyritic,matrix and crystalline carbonatite,the rare earth elements in Bachu carbonatite are mainly enriched in the matrix of porphyritic and crystalline carbonatite.Microscopic observation,backscattered images and EMPA analysis show that the porphyritic carbonatite matrix contains a large number of bastnasites,apatites,as well as barites and chlorites,indicating that REEs in Bachu carbonatite are enriched in the form of single minerals.5.The Sr-Nd isotopic composition and REE distribution pattern of carbonatite are similar to co-existent gabbros and lamprophyres,and have mantle-derived properties.Specifically,the?_Nd(t)value of carbonatites is relatively homogeneous(0.1-3.4),this range is lower than the depleted mantle(DM),between HIMU and EMI,showing the characteristics of the enriched mantle.Together with previous studies on Mg-C isotopic composition of Bachu mafic-ultramafic complex,it originates from enriched lithospheric mantle metasomatized by carbonaceous fluid.6.The low Si property of carbonatite indicates zircons separated from carbonatites are all inherited zircons,and the geochronology and zircon O isotopes also suggest that the peak ages of26 captured zircons are 2526 Ma,755 Ma and 550 Ma,and 10 of them are between 2498 and 2613Ma,the average?¹⁸O ratio of zircon is between 6‰and 8‰.It is speculated that Bachu carbonatite originates from the partial melting of the ancient lithospheric mantle,and the carbon-containing thickened lithosphere remelted by Permian plume at the bottom of the craton has some contribution.To sum up,carbonatite is derived from the remelting of the ancient carbonaceous lithosphere by the Permian Tarim plume magmatism in Bachu Wajilitag,The rapid intrusion of porphyritic carbonatite veins(pipes)formed by crystallization of carbonate melts which demixed with silicate melts,it has the characteristics of obvious enrichment of REEs,which is the key target of rare-earth deposits exploration in this area.