Deep Magmatic Process And Metallogenic Model Of Pegmatite-type Rare Metal Mineralization In Northeastern Hunan

Northeastern Hunan is located in the middle of Jiangnan orogenic belt.The area has experienced a long evolution history and multi-stage tectonic events,forming a series of rare metal deposits（points）.The giant Renli Nb-Ta deposit,as one of the top 10 geological exploration achievements in 2017,is one of the most significant rare metal metallogenic regions around the Mufushan composite batholith,and even for southern China.It is also considered as the concentration center of the rare metal mineralization in Mufushan area.Previous studies on rare metal deposits in northeastern Hunan have been done,however,inadequate studies have been performed on the evolution process of deep magma,the contribution of magmatism to rare metal mineralizaiton,and fertilizaiton mechanism of rare metal,which constrained studies on mieneralization and metallogenesis of the area.In this study,the spatial distribution characteristics and spatial occurrence of the pegmatite in the Renli deposit were determined,through detailed observation of the geological characteristics of the deposit,elemental geochemical analysis,micro-areas in-situ analysis of mineral,and radioactive isotope dating.The evolution history and evolution process of pegmatite diagenesis,the contribution of magmatic-hydrothermal evolution to pegmatite diagenesis and rare metal mineralization such as Nb and Ta,and the metallogenic model were established.The main achievements and understandings are as follows:（1）Through field geological survey and detailed microscopic observation of mineral associations,the granites closely related to rare metal mineralization are divided into three types,including biotite granite,two-mica granite and muscovite granite.The geological characteristics of130 pegmatite veins in the mining area are systematically sorted out,and the pegmatites from northeast to southwest are divided into biotite pegmatite belt,two-mica pegmatite belt,muscovite albite pegmatite belt,and spodumene muscovite albite pegmatite belt.The corresponding mineralization belts are non-mineralization,Be+Nb+Ta-Be+Nb+Ta,and Li+Be+Nb+Ta belt.（2）Through the whole rock analysis,it can be seen that the granite is a peraluminous high-K calc-alkaline series of S-type granite,and the pegmatite belongs to the calc-alkaline-low-K calc-alkaline series of S-type granite.The rare metal pegmatite and the late Yanshanian granite are the products of the same melting event magma undergoing high crystallization differentiation evolution.Based on this study and combined with regional related data,it is considered that the Renli Nb-Ta deposit is formed in the lithosphere extension environment of the late Mesozoic tectonic system in eastern China.The diagenetic temperature of ore-bearing pegmatite is 590°C～630°C.The parent rock of muscovite albite pegmatite enriched in niobium-tantalum ore is two-mica granite,which is’brother’to muscovite granite,and the magmatic evolution model of gas-liquid separation is established.（3）The trace elements of granite and pegmatite indicate that the source of ore-forming materials is mainly partial-melting mudstone..Hf isotope studies show that theεHf（t）value of pegmatite is-8.1～-6.0,with an average value of-7.2,and the two-stage model age T_DM2is 1695 Ma～1560 Ma.It is considered that the source rock is derived from the melting of Mesoproterozoic crustal material.（4）The apatite in the pegmatite is fluorapatite,and the two-mica pegmatite contains mainly magmatic apatite,and the muscovite albite pegmatite contains mainly hydrothermal apatite.Through EPMA and LA-ICP-MS analysis,it can be seen that apatite has relatively consistent Ca O and P₂O₅content,high F and low Cl,high Y/Ce ratio,low Th/U and La/Sm ratios,which are consistent with apatite in S-type granite.The Y/Ho ratios are mostly over 34,indicating that pegmatite may have experienced melt-fluid interaction or immiscibility of volatile-rich melt.Apatite in muscovite albite pegmatite has higher Fe O,Mn O,Si O₂and SO₃,relatively lower F and Cl contents,larger Ce anomaly,smaller Eu anomaly,lower Y/ΣREE ratios and larger Ce/Pb ratios than those in two-mica pegmatite,indicating that muscovite albite pegmatite is more oxidized and more mobile than two-mica pegmatite.Niobium-tantalum ore is formed in a higher oxidation environment.The mixing of post-magmatic hydrothermal fluids rich in Fe,Mn,Si and S elements promotes the further enrichment of Nb-Ta oxides.（5）The Nb-Ta group minerals in the mining area mainly occur in the muscovite albite pegmatite in the inner and outer contact zones,mostly in the form of semi-automorphic-automorphic niobium iron ore and niobium manganese ore minerals.Through scanning electron microscopy,electron probe and LA-ICP-MS micro-area in-situ analysis,it can be seen that,characterized by Nb>Ta,the ratios of w（Mn）/（w（Mn）+w（Fe））ranges from 0.28 to 0.6,and the ratios of w（Ta）/（w（Ta）+w（Nb））ranges from 0.13 to 0.49,which is characterized by the evolution of niobite to niobite.The Nb-Ta mineral element substitution pairs mainly include W-Mo,Zr-Ti and Sn-Ti.The Ta/（Ta+Nb）ratios are positively correlated with the content of Li,Mo and W elements,and the Ta/（Ta+Nb）ratios are negatively correlated with Sc,Co,Zn and Sr.Magmatic columbite（Ct1）and magmatic-hydrothermal columbite（Ct2）coexist in the No.5 vein.The mineralization mechanism of Nb-Ta deposit is magma liquid immiscibility,and 138.3 Ma is the conversion time from the magmatic stage of Nb-Ta mineralization to the magmatic-hydrothermal stage.Based on the principal component analysis（PCA）of 15 elements such as U,Th,Pb,Ti,Mn,Fe,Zn,Y,Zr,Nb,Sn,Yb,Lu,Hf and W in No.5 vein Ct1 and Ct2 niobite,it is considered that the trace element data of Ct1 and Ct2 niobite are suitable for principal component analysis.The original 15 element indexes can be replaced by the first two principal components to verify the trace element characteristics of magmatic period and magmatic-hydrothermal period.The total score equation of Ct1=0.838×principal component score 1+0.162×principal component score2.The total score equation of Ct2 is Y=0.736×component score 1+0.264×component score 2.It is considered that U,Th,Pb,Ti,Y,Nb,Ta,Yb,Lu and W may be niobium tantalum iron ore group minerals（AB2X）.Y,Yb,and Lu can be used as indicators for magmatic,magmatic-hydrothermal columbite.（6）LA-ICP-MS zircon U-Pb,apatite U-Pb,monazite U-Pb and niobium-tantalite U-Pb isotopic dating were carried out.The zircon U-Pb isotopic ages of two-mica pegmatite and muscovite albite pegmatite are147.1±1.4 Ma and 144.3±0.3 Ma,respectively.The monazite U-Pb isotopic ages of those pegmatites are 139.2±0.4 Ma and 135.2±0.4 Ma,respectively.The apatite U-Pb isotopic ages of those pegmatites are 135±8 Ma,130±2 Ma and 134±3 Ma,respectively.The magmatic niobate（Ct1）U-Pb isotopic age of No.5 vein is 138.5±1.1 Ma,the magmatic-hydrothermal niobate（Ct2）U-Pb isotopic age is 138.3±2.1 Ma,and the niobate U-Pb isotopic age of No.36 vein is 137.8±0.7 Ma.Combined with previous studies,it is believed that the emplacement age of pegmatite in the Renli deposit is ca.147.1 Ma and the cooling age is ca.130 Ma.The whole process of pegmatization lasts about 17 Myr,of which 147 Ma～138.3 Ma is the magmatic stage of pegmatite,lasting about 8.7 Myr,and 139.2 Ma～135 Ma is the magmatic-hydrothermal stage of pegmatite,lasting about 4.2 Myr,and 134 Ma～130 Ma is the hydrothermal stage of pegmatite,lasting～4 Myr.The metallogenic age of niobium tantalum ore is that the metallogenic age of niobium tantalum ore in magmatic stage is 140.7 Ma～138.3 Ma,the metallogenic age of magmatic hydrothermal stage is 138.3 Ma～137.8 Ma,and the metallogenic age of hydrothermal stage is～133 Ma,in the later stage,a hydrothermal mineralization event of～125 Ma was superimposed,and four mineralization events of magmatic stage,magmatic-hydrothermal stage,hydrothermal stage and late hydrothermal stage formed the Renli super-large niobium-tantalum deposit.（7）Based on the study of geological background,geological characteristics of the deposit,source of ore-forming materials,element geochemical analysis,stable isotope and radioactive isotope dating,the enrichment mechanism of rare metal elements such as Nb and Ta in Renli is put forward.It is considered that the ore-forming elements have experienced the initial enrichment of the source area,the pre-enrichment of granite differentiation,the mineralization of pegmatization precipitation and the re-enrichment of post-magmatic hydrothermal solution.The metallogenic process,then reveals the key ore-controlling factors and prospecting criteria and metallogenic regularity,and establishes the metallogenic model of rare metals such as niobium and tantalum.