Extremely Fe-rich beryl from Zhangji deposit: Implications for the unique volcanic fluid environment of beryllium mineralization in Southeast China

Abstract

Beryl group minerals and bertrandite have been identified in the altered rocks of the Zhangji deposit in the volcanic intrusion complex of Southeast China. Complex internal textures and compositional variations characterise two groups of beryl (primary magmatic beryl and secondary hydrothermal beryl). The primary beryl (Brl-I) with low FeO^T (<1.52 wt%), Na₂O (< 0.27 wt%), and minor MgO (<0.07 wt%) showed a regular hexagonal crystal shape, compositional rhythmic annulus, and a planar contact relationship with primary K-feldspars. With the action of hydrothermal fluids, two types of secondary beryl can be distinguished: (1) The Fe-, Mg-, and Na-rich beryl (Brl-II-1), with up to 5.74 wt% FeO^T, 2.60 wt% MgO, and 2.31 wt% Na₂O occurs along fractures in beryl crystals. (2) The extremely Fe-enriched beryl (Brl-II-2) has with up to 11.59 wt% FeO^T, 1.70 wt% MgO, and 1.53 wt% Na₂O and is related to extended metasomatism of Fe-rich fluids. Furthermore, the detailed textural study revealed that beryl evolved through reactions to form bertrandite, K-feldspar, and muscovite. Compared with primary K-feldspar which exhibited a clear contact boundary with surrounding minerals, secondary K-feldspar was adularia and lacked complete crystal form. In addition, compared with primary muscovite which was a long lamellar crystalline shape, secondary muscovite was distributed in clusters in beryl and showed as small grains with higher Fe content. Fe-rich beryl is rarely reported, to our knowledge, the extremely Fe-, Mg-, and Na-rich beryl in Zhangji deposit contains the second highest Fe content in the world, only lower than that found in Latium (Italy), of the metallogenic environment formed as a result of iron-rich and strong oxidation fluids. The strong volcanic magmatic activity during the Late Mesozoic formed a volcanic intrusion complex in Southeast China, this extremely Fe-rich beryl was formed in the special volcanic hydrothermal environment of the volcanic intrusion complex along Southeast China.