Abstract: Leucogranites are widely distributed in the Himalayan orogenic belt and are closely related to rare-metal mineralization. This study focuses on the Nb and Ta mineralization and columbite-group minerals (CGMs) from the granitic pegmatite in the Lalong pluton, eastern Himalayas. Three zones (i.e., wall zone, intermediate zone, and core zone) can be identified from the Lalong pegmatite based on their lithology. The CGMs mainly exist in the intermediate zone (quartzfeldspar- muscovite zone). Major element, trace element, and U-Pb isotopic compositions of CGMs were systemically analyzed. Two types of CGMs are distinguished by occurrences and chemical compositions. Type I CGMs have euhedral occurence and uniform chemical compositions with low Ta^# [Ta/(Nb+Ta)] ranging from 0.08 to 0.34. Type II CGMs mainly distribute around crystals of the type I CGMs or as tiny veins cutting through the type I CGMs with higher Ta^# ranging from 0.45 to 0.60. Several structures (i.e., oscillatory structure, metasomatic structure, fissure-filling, and porous) are well developed in the type II CGMs. The normalized REE patterns of these CGMs are similar, showing a strong negative Eu anomaly with δEu ranging from 0.001 to 0.020. The LREE/HREE ratio of CGMs ranges from 0.016 to 0.044. Our results indicate a two-staged Nb-Ta mineralization in the Lalong pluton. The first-staged mineralization is related to the magmatic process with the crystallization of the type I CGMs, while the second-staged process is late-staged hydrothermal fluidrich magmatism with the formation of the type II CGMs. The REE pattern of the type II CGMs, however, is not convincing enough to indicate the magmatic-hydrothermal mineralization. The CGMs U-Pb isotopic analysis reveals the age of Nb- Ta mineralization at 22.3±0.3Ma, implying a rare-metal resource potential of the leucogranites formed in the Neohimalayan period (25-14 Ma).

Key words: Lalong leucogranite, rare-metal mineralization, Higher Himalaya, Lhozhag