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Qongjiagang pegmatite-type Li deposit / Mineral inclusion / Accessory minerals / Plagioclase / Melt-fluid characteristics
Abstract:
Qongjiagang pegmatite-type Li deposit in Tibet is the first discovered pegmatite-type deposit with economic value in the Himalayan region, which confirms that the Himalayan region has the potential to become a strategic base of rare metal in China, and provide indications to find pegmatite-type Li deposit in the Himalayan region. In this study, we use SEM to identify the type, frequency and occurrence (relationship with cracks) of mineral inclusions in the three main accessory minerals, monazite, apatite and zircon from granite and pegmatite of Qongjiagang Li deposit, combining with the EPMA analysis of feldspar inclusions in apatite to comprehensively trace the property and evolution of the melts and fluids. Our study indicates that: (1) the main mineral inclusions in monazite, apatite and zircon from Qongjiagang Li deposit include silicate, oxide, phosphates and a small amount of sulfide, not only the REE-rich monazite and apatite filling or intersecting cracks are formed by hydrothermal alteration, but also the uraninite and thorianite isolated from cracks occur in the self-irradiation region of zircon are related to fluids; (2) the appearances of columbite and pyrochlore inclusions in the apatite from tourmaline-muscovite granite demonstrate that the initial melt is enriched in Nb and Ta, the amount and type of rare metal mineral inclusions can be used as an indicator for rare metal mineralization in highly evolved granite and pegmatite; (3) the plagioclase inclusions with high and a large range of An values in apatite from spodumene pegmatite represent the capture of less-differentiated melt and continuously fractional crystallization. Our results indicate that the types and compositions of mineral inclusions in accessory minerals can be good tracers for the characteristics and evolution of melts and fluids in the highly evolved granite-pegmatite system
