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

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 concentrated in zones V and VI. The rareelements 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 degrees of fractional crystallization than many other Li-Cs-Ta (LCT-family) pegmatites worldwide, as evidenced by the relatively low Nb-Ta, Zr-Hf, and K-Rb fractional 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. The hydrothermal fluids were composed predominantly of Li, F, Cs, and Ta, which conformed to the evolved trend of pegmatitic melt. These late hydrothermal fluids exsolved from the magma as it differentiated, and may have the benefit with rare-elements metallogeny. (C) 2019 Elsevier B.V. All rights reserved