| The rare metals Nb and Ta are widely used in various fields such as steel industry,superconducting material industry,electronics industry,atomic energy,aerospace and medical treatment because of their unique physical and chemical properties,and have an important strategic position in the international key mineral resources.In recent years,regional geological survey and research work show that some areas in the Great Xing’an Range have geological conditions for the mineralization of rare metals such as Nb and Ta.On the basis of pre-study of existing regional geological and mineral data,the paper selectes the Baerzhe of Jarud Banner,the Toudaoqiao of Ewenki Autonomous Banner typical Nb（Ta）deposits,and the Tuohelinchang,Wulukanggou and Jiyuanlinchang in Keyihe area of Oroqen Autonomous Banner Nb（Ta）ore occurrences as the research objects.Through detailed field geological investigation,sample collection,metallogenic geological characteristics,diagenetic and metallogenic chronology,rock,mineral and isotope geochemistry,the relationship between igneous emplacement age,magma source and petrogenesis and niobium-tantalum mineralization of typical deposits（occurrences）in the area has been identified.Based on this,the regional tectonic evolution and genesis of deposits（occurrences）have been discussed and the metallogenic model of Nb（Ta）rare metals in the study area is established.Baerzhe is a typical alkaline granitic-type Nb-Ta deposit,which is divided into east and west ore bodies.The east ore body has obvious vertical mineralization zoning,which is the main ore body of the Baerzhe Nb-Ta deposit.The mineralization is mainly related to the arfvedsonite-albite granite in the middle and the albite granite at the top.The industrial grade of the west ore body is not as good as that of the east ore body,and the mineralization is mainly related to the arfvedsonite granite and the arfvedsonitebearing granite.The ore mineral types of the two ore bodies are roughly the same,composed of columbite,manganocolumbite,samarskite,fersmite-（Ce）,polycrase and nioboaeschynite,most of these Nb-Ta ore minerals are disseminated among the mineral particles crystallized in the early stage of magma.The mineralization of Toudaoqiao alkaline volcanic-type Nb（Ta）deposit mainly occurs in alkaline rhyolite.The ore minerals are mainly columbite,followed by niobium-bearing magnetite and niobiumbearing ilmenite.The Tuohelinchang,Wulukanggou and Jiyuanlinchang in Keyihe area are all alkaline volcanic-type Nb（Ta）ore occurrences.Except that the Nb（Ta）mineralization in the Wulukanggou ore occurrence mainly occurs in the alkaline ignimbrite,the other two Nb（Ta）mineralizations are closely related to alkaline rhyolite.The Nb（Ta）ore minerals in Tuohelinchang are composed of niobium-bearing ilmenite and niobium-bearing zircon;Jiyuanlinchang are composed of mainly niobium-rutile and columbite,as well as a small amount of fergusonite and samarskite;Wulukanggou are composed of mainly niobium-bearing ilmenite,with a small amount of niobiumbearing zircon,fergusonite,euxenite,and trace amounts of formanite,samarskite,columbite and tantalite.The Nb（Ta）ore minerals in these volcanic rocks are mainly scattered in the edge of the main rock-forming mineral particles such as quartz or distributed in the felsic matrix.The results of LA-ICP-MS zircon U-Pb geochronology show that there are two stages of mineralization in the eastern and western ore bodies of Baerzhe: the magmatic zircon U-Pb ages（122.4 ± 2.6 Ma,132.6 ± 3.3 Ma and 130.3 ± 3.5 Ma）of the eastern（DBEZ1）and western（XBEZ1,XBEZ2）ore bodies of Baerzhe represent the crystallization age of highly evolved granitic magma in the eastern and western ore bodies of Baerzhe,respectively,which is the peak age of Nb-Ta rare metals mineralization.The hydrothermal zircon U-Pb ages（115.4 ± 1.8 Ma,123.5 ± 1.7 Ma and 122.2 ± 0.87 Ma）,which represent the age of fluid-melt interaction in the late stage of highly evolved granitic magma in the eastern and western ore bodies of Baerzhe,respectively.In addition,the zircon U-Pb age of the volcanic rocks in the study area shows that the related Nb（Ta）mineralization is mainly divided into two periods: the early Early Cretaceous is the age of the volcanic rocks of the Manketouebo Formation and their Nb（Ta）mineralization of the Toudaoqiao（132.1 ± 1.3 Ma）and Wulukanggou（142.5 ± 1.2 Ma）,while the age of the volcanic rocks of the Baiyingaolao Formation in Tuohelinchang（109.3 ± 1.2 Ma）and Jiyuanlinchang（104.5 ± 0.65 Ma）represents another period of Nb（Ta）mineralization occurred in the late Early Cretaceous.The granites and volcanic rocks related to Nb（Ta）mineralization in Baerzhe,Toudaoqiao,and Keyihe areas are characterized by rich Si,ALK,Fe and poor Mg,Ca.The geochemical composition of the whole rock（high 10000*Ga/Al and Zr+Nb+Ce+Y values）shows that it has typical A-type granitic rock properties.The tracings of zircon Lu-Hf isotopes of Baerzhe,Toudaoqiao,Tuohelinchang and Jiyuanlinchang,and whole-rock Sr-Nd isotopes of Wulukanggou,as well as the study of the electron probe composition of Baerzhe arfvedsonite,indicate that the Great Xing’an Range area was subjected to the retro-subduction of the ancient Pacific plate during the early Cretaceous period,and the lithosphere was stretched and thinned,which triggered the upwelling of the asthenosphere mantle material to heat the Neoproterozoic-Paleozoic newly generated potassium-rich intermediate-basic lower crust partially melted and formed the Baerzhe alkaline granite and the Toudaoqiao-Keyihe area alkaline volcanic rock.The mineralized rocks in Baerzhe,Toudaoqiao and Keyihe areas are rich in incompatible elements such as Rb,Nb,Ta,Zr and Hf,and poor in elements such as P,Ti,Ca,Mg,Ba and Sr.At the same time,they have high Rb/Sr ratios,significant negative Eu anomalies and M-type rare earth tetrad effect(TE1,3>1.10),indicating that the magmas forming these rocks have undergone a high degree of fractional crystallization.In the process of differentiation and evolution of granitic magma,volatiles such as F and H2O can make the magma highly differentiated and evolved by reducing the viscosity and solidus temperature of the melt.At the same time,they can also increase the number of non-bridging oxygen（NBO）in the melt,thereby increasing the solubility of Nb-Ta elements,so that they are continuously enriched in the residual melt and eventually mineralized.Most of the niobium-tantalum ore minerals in the mineralized rock mass are disseminated between the early rock-forming minerals,and the elements such as Nb-Ta,Zr-Hf and HREE-Y with similar geochemical properties show obvious linear correlation.Therefore,this paper proposes that the Nb-Ta element mineralization in Baerzhe,Toudaoqiao,and Keyihe areas is mainly controlled by the high degree of fractional crystallization in the late magmatic stage,and the hydrothermal fluid only has a superimposed effect on the Nb-Ta mineralization of the Baerzhe granite type.In addition,the magma source with significant rich in alkali,rare earth elements and volatiles is also another key factor in the Nb-Ta mineralization in the study area.The development of folds and secondary faults in the ore-forming area also provides a favorable metallogenic environment for the upward migration and differentiation evolution of ore-bearing magma in the area.Based on the study of the nature of magmatic source and magmatic evolution in Baerzhe,Toudaoqiao and Keyihe areas,this paper proposes for the first time that the formation of Nb（Ta）ore in the study area is mainly double controlled by the highly fractional crystallization（and hydrothermal metasomatism）of magma and the magmatic source rich in rare and rare earth elements.At the same time,it is found that niobium（tantalum）minerals are mainly in the form of disseminated occurrence between or at the edges of early mineral particles by means of mineral analysis.The above understanding has important guiding significance for the exploration and further development and utilization of rare metal minerals such as Nb-Ta in the region. |
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