Mineralogical constraints on the magmatic–hydrothermal evolution of rare-elements deposits in the Bailongshan granitic pegmatites, Xinjiang, NW China

Abstract The Bailongshan pegmatite district in south Xinjiang Province, NW China, is a newly discovered area of rare-element pegmatites. It is a superlarge Li Rb reserve that includes an estimated 3.45 million tons of Li2O and 176,000 tons of Rb2O. On the basis of a detailed characterization of textures and minerals in the Bailongshan 2# and 5# pegmatites, six zones are identified: a saccharoidal albite zone (zone I), a graphic pegmatite zone (zone II), a blocky microcline zone (zone III), a muscovite–quartz zone (zone IV), a cleavelandite–spodumene zone (zone V), and a quartz–spodumene zone (zone VI). The detailed mineralogical characteristic is performed by EPMA for illustrating both the existent state of rare-elements and the magmatic-hydrothermal evolution of the Bailongshan pegmatite. Lithium, Be, Rb, Nb, Ta, and Sn are major economic rare-elements which are trend to enriched in zone V and VI. The rare-elements are hosted by minerals including spodumene, montebrasite, eucryptite, lepidolite, beryl, Rb-enriched K-feldspar, columbite-group minerals, cassiterite, and tourmaline, which formed from the fractional crystallization of pegmatitic magma. The Bailongshan pegmatites show lower crystallization differentiation degree than many other Li-Cs-Ta (LCT-family) pegmatites worldwide, as evidenced by the relatively low Nb Ta, Zr Hf, K Rb fractionation degree in the carrier minerals. In zones V and VI, the primary minerals record intense interaction with hydrothermal fluids, which caused the alteration of spodumene and mica, the development of Cs- or Na-enriched veinlets in beryl, and the formation of Mn- and Ta-enriched veinlets coupled with micropores within columbite-group minerals. These late hydrothermal fluids exsolved from the magma as it differentiated, and may have the benefit with rare-element metallogeny.