Physicochemical conditions of the formation of beryl and aquamarine in Mufushan granopegmatite deposit, Hunan Province, China

Abstract

The formation of the Mufushan granopegmatite was closely related to the Late Yenshanian multiphase and multistage magmatic activities. More than one generation of beryl and aquamarine occur in different types of pegmatite in the granites. The presence of melt and melt-fluid inclusions strongly indicates a melt-solution character of the pegmatitic magma. Forming temperatures of the different generations of beryl in a Na⁺-K⁺-Ca²⁺-CO₃ ²⁻-Cl⁻-SO₄ ²⁻ solution ranges from 990 °C to 200 °C Aquamarine was formed at 720–180 °C. The contents of alkali metals (Na⁺ + K⁺) in the ore-formming solution of aquamarine are lower than those in the beryl, but the contents of alkali earths (Ca) and salinity are higher. The granite was generated by remelting of the basement formation (meta-sedimentary rocks of the Lengjiaxi Group) which also served as the source of ore-forming material. Beryllium in the pegmatite was transported mainly in the form of Na₂[Be(CO₃)₂], with part of it being complexed with Cl⁻ and SO₄ ²⁻. During the generation and evolution of the pegmatite, equilibrium might have been reached in the solid-melt-fluid or solid-fluid system. The intergranular solutions may have reacted with the early crystallized minerals, resulting in potash-feldsparization, albitization and muscovitization during which the ore-forming elements were mobilized and transported in favour of ore deposition.