Mineralogical characteristics and significance of beryl from the rare-element pegmatites in the Lushi County, East Qinling, China

Beryl which is one of the most important beryllium minerals records the rock-forming process and rare-element (REL) mineralization of granitic pegmatites. East Qinling Lushi pegmatite district is a famous rare-element producing area in China. This study investigated the internal zonation patterns of the pegmatite dykes from the East Qinling, including No. 703 Li-mineralized pegmatite and No. 302 Be-mineralized pegmatites in Nanyangshan, Qiantai Li-mineralized pegmatite in Qiligou-Qiantai, Daxigou and Jiucaigou Li-mineralized pegmatites in Caijiagou, Xishan'gou and Wayaogou Be-mineralized pegmatites in Wayaogou. These pegmatites belong to peraluminous LCT-family and REL-Li subtype. The Nanyangshan No. 703, Jiucaigou and Daxigou pegmatites belong to complex type and spodumene subtype, while the Qiantai pegmatite belong to albite-spodumene type. The Nanyangshan No. 302, Wayaogou and Xishan'gou pegmatites belong to beryl type and beryl-columbite subtype. The EMPA results show that beryls are enriched in alkali and poor in iron and magnesium. The substitution mechanisms of beryl from the REL pegmatites in the East Qinling are channel-octahedral, channel-tetrahedral and alkali cations in channel. The substitutions of beryl from the Xishan'gou and Wayaogou are Na(Fe2+, Mg) square(-1) Al-1 and NaLi square(-1) Be-1, respectively. The beryls from the No. 302, Qiantai, Daxigou and Jiucaigou pegmatites mainly display substitution of (Na, Cs) Li square(-1) Be-1, while the substitutions: (1) NaFe2+ square(-1) Al-1, (2) NaCs-1 and (3) (Na, Cs) Li square(-1) Be-1 were revealed for the No. 703 pegmatite. The Cs2O contents and Na/Cs values indicate a sequence of the decreasing degree of fractionation and evolution of pegmatites, which is No. 703 -> Qiantai -> (Daxigou and Caijiagou) -> No. 302 -> (Xishan'gou and Wayaogou). Li-mineralized pegmatitic magma is more highly fractionated than those of beryllium mineralization. The internal zonation patterns of beryls are homogeneous grain, layered zoning, alteration borders, patches and complex irregular zoning in BSE images. Compared with the Be-mineralized pegmatites, the beryls from the Li-mineralized pegmatites show more complex internal zonation patterns, reflecting strong liquid immiscibility and replacement. With the increase of degree of fractionation and evolution of pegmatitic magma, the FeO contents of beryl decrease and the internal structures of beryl become more complex, such as alteration border and irregular zoning. The FeO content and internal structure of beryl could be potential indicators of the degree of fractionation and evolution of pegmatites. The chemical compositions and internal structure features of beryl suggest that Li-mineralized pegmatitic magma is highly fractionated REL pegmatitic magma when it is emplaced. The Daxigou, Jiucaigou and Qiantai Li-mineralized pegmatites did not undergo obvious fractionation, while the No. 703 pegmatites experienced fractionation and evolution after emplacement to finish further REL-enrichment. The lithium mineralization mechanisms are fractional crystallization and liquid immiscibility.