Beryl as an indicator for elemental behavior during magmatic evolution and metasomatism in the large Shihuiyao Rb-Nb-Ta-Be deposit, Inner Mongolia, NE China

In the large Shihuiyao Rb -Nb -Ta -Be deposit, a beryl-bearing granite was identified with three texturally different zones that show an increasing degree of fractionation, including the albite granite (bottom zone), albite granite with pegmatite pockets (intermediate zone), and intercalation of granite and pegmatite layers (top zone). Beryl occurs both in the intermediate and top zones. Microtextural (SEM), Raman spectra, and trace-element (LA-ICPMS) data on beryl from these zones shed new light on the behavior of trace elements during magmatichydrothermal evolution. In the intermediate zone, magmatic beryl (Brl-1) has a homogeneous texture, while beryl in the top zone (Brl2) can be divided into magmatic (Brl-2a) and metasomatic subtypes (Brl-2b). The magmatic Brl-2a shows oscillatory zonation, whereas the metasomatic Brl-2b is darker in BSE images and highly porous. Fractional crystallization led to an increase in Na, Sc, Fe, Rb, and Cs contents from Brl-1 to Brl-2a. Comparing these data with those of beryl in other pegmatites worldwide reveals that Cs in beryl is an ideal trace element to monitor the magmatic evolution of the pegmatite host. Distinct textural and chemical features of Brl-2a and Brl-2b reflect metasomatic alteration by a dissolution-reprecipitation process for Brl-2b. In combination with published experimental and fluid inclusion data, we propose that metasomatic fluids exsolved from the magma during latemagmatic stages could transport large amounts of rare metals, such as Ta and Rb. The higher content of Ta in Brl2b compared to Brl-2a suggests that metasomatic fluids exsolved from the magma play a favorable role in Ta mineralization. In contrast, decreasing Rb contents in Brl-2b is attributed to the high mobility of Rb in the fluid. Such Rb-rich fluids are expected to generate Rb orebodies of economic value surrounding the ore-bearing granite, implying that more investigation into hydrothermal Rb mineralization in the Shihuiyao district is warranted.