Evolution of beryllium minerals in granitic pegmatite Maršíkov D6e, Czech Republic: Complex breakdown of primary beryl by internal and external hydrothermal-metamorphic fluids

Abstract

Beryllium mineralization was studied by EPMA and XRD techniques in the beryl-columbite pegmatite D6e from the Maršíkov District, Bohemian Massif, Czech Republic. A detailed study of microtextures in BSE images revealed a complex formation of fine-grained secondary Be-silicates at the expense of primary beryl and earlier secondary Be-minerals in the following proximal and distal assemblages: (A) primary magmatic beryl; (B) proximal secondary beryl; (C) proximal bertrandite + K-feldspar and minor muscovite, chamosite, gismondine-Ca and quartz; (D1) proximal assemblages of milarite + gismondine-Ca and bavenite-bohseite + epidote; (D2) distal assemblages on brittle tectonic cracks including milarite, bavenite-bohseite, albite, K-feldspar, quartz and rare phenakite, and (D3) epidote, bavenite-bohseite, quartz, albite, K-feldspar and minor milarite. A formation of secondary Mg,Fe,V,Na-enriched beryl (B) is connected with a mixing of residual (pegmatite) and external Ca,Mg,Fe,V-enriched fluids from the host amphibole gneiss at T ~ 300–400 °C and P ~ 200–400 MPa. The assemblage (C) formed due to an income of K,Mg,Ca-enriched fluids (residual + external) at T ~ 150–300 °C. The subsequent proximal (D1) and distal (D2, D3) assemblages formed during an moderate to strong income of Ca-rich external fluids from the host rocks related to retrograde hydrothermal-metamorphic overprint manifested by the Alpine-type hydrothermal veins. A common presence of epidote in the assemblages with bavenite-bohseite suggests crystallization at T < ~200–300 °C. Detailed textural and paragenetic study of primary and secondary Be-minerals is a useful tool to recognize and study various processes proceeded during subsolidus evolution of granitic pegmatites.