Genetic Relation Between Granite and Pegmatite and Their Metallogenic Significance in Renli Nb-Ta Deposit， Northeast Hunan， China: Evidence from Coltan U-Pb Dating， Hf Isotopes and Geochemistry

The Renli niobium-tantalum polymetallic deposit in Hunan province is a super-large pegmatitic rare metal deposit recently discovered in China. The purpose of this paper is to evaluate the magmatic origin and metallogenic process of granites and pegmatites in Renli ore field， as well as the genetic relationship between these rocks by using petrography， whole-rock major and trace element analysis， in situ LA-ICP-MS U-Pb dating of coltan and Hf isotope. In the orefield， pegmatite is commonly found in Mufushan composite batholith or near the contact zone of the Lengjiaxi Group， and has a close spatial relationship. In terms of diagenetic and metallogenic ages， the U-Pb isotope ages of magmatic coltan （Ct1） and magmatic hydrothermal coltan （Ct2） are 138.5±1.1 Ma and 138.3±2.1 Ma， respectively. The U-Pb isotope ages of coltan of two-mica pegmatite are 137.8±0.7 Ma. The formation age of pegmatite is slightly later than that of various types of granite in the mining area. In terms of provenances， ε_Hf（t） values of zircon in the muscovite albitite pegmatite range from -6.39 to -7.38， and the two-stage model age(T_DM2) ranges from 1695 to 1604Ma， which are roughly consistent with the ε_Hf（t） and the two-stage model age(T_DM2) values of granitic intrusions in the complex rock mass. The granite and pegmatite are produced by partial melting of the Lengjiaxi Group. The two-mica granite and pegmatite have similar rare earth element and trace element partitioning patterns， indicating that there is a genetic relationship between pegmatite and two-mica granite. In summary， it is considered that pegmatite is the product of high crystallization differentiation of two-mica granite. The enrichment mechanism of Nb， Ta and other rare metal elements is proposed. It is believed that ore-forming elements have undergone the ore-forming process of initial enrichment in source area， granitic magmatic differentiation enrichment and later hydrothermal reenrichment. Additionally， the replacement mechanisms of coltan group minerals mainly includes W-Mo， Zr-Ti and Sn-Ti. U， Th， Pb， Ti， Y， Nb， Ta， Yb， Lu， W may be the B cations in the coltan group minerals (AB₂X₆)， and Y， Yb， Lu can be used as the indicator elements of coltan.