Niobium-Tantalum Minerals Decipher Evolution of the Qiongjiagang Li-Rich Granitic Pegmatites in the Himalayan Orogenic Belt, Tibet

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Abstract The Qiongjiagang giant pegmatite lithium deposit, located in the central section of the Himalayan orogenic belt, mainly comprises the spodumene-bearing pegmatite type, marginally accompanied by a petalite-bearing leucogranite dike and a lepidolite-bearing pegmatite. Existing uncertainties around the niobium (Nb) and tantalum (Ta) mineralization characteristics and their genetic ties among three types of Li-rich dikes justify further research. To enhance comprehension, backscattered electron imaging, energy dispersive spectrometry mapping, and electron microprobe analyses were employed. Microscopic features suggest that the Nb-Ta mineralization from Qiongjiagang spodumene-bearing pegmatite appears as the result of saturation of early magmatic columbite after lithium (Li) quenching arising from poikilitic spodumene crystallization. Subsequently, autometasomatism of hydrosilicate liquid caused partial dissolution of magmatic columbite as well as replacement of early primary minerals, forming fluid-induced Ta-rich overgrown and interstitial microcrystals (metasomatic columbite and pyrochlore) in microfractures. Muscovite crystallization and high Ta solubility may cause Nb-Ta element fractionation of individual zoned columbite and declining Nb/Ta ratios between discrete columbite and microcrystals. Both the continuous whole-rock compositional evolution from granite to lepidolite-bearing pegmatite and the gradual manganese (Mn) enrichment and titanium (Ti) decline of columbite geochemistry imply that the three types of dikes originated from three batches of sequential pulses of Li-rich magmas, demonstrated by progressive tourmaline or biotite fractionation of parent magma. The distinct Mn/Fe variation of columbite and whole-rock geochemistry also suggests that the three types of Li-rich magmas were already highly evolved in the upper part of their parental granitic magma chamber, instead of only fractionating once they escaped to the host rocks. Consequently, the textures and composition of columbite not only provide valuable insights into the magmatic-hydrothermal evolution of spodumene-bearing pegmatite, they also emphasize columbite's potential as a tracer for the degree of differentiation of magma.