Secondary beryl in cordierite/sekaninaite pseudomorphs from granitic pegmatites – A monitor of elevated content of beryllium in the precursor

Abstract

Cordierite-group minerals (cordierite and sekaninaite) from granitic pegmatites are often strongly to completely altered to a fine- or coarse-grained mixture of muscovite, chlorite and/or, biotite, along with several less common secondary minerals, including mainly paragonite, tourmaline, and secondary beryl. The mixture is a common product of early subsolidus hydrothermal alteration at the examined pegmatites of the beryl-columbite subtype – Věžná I and Drahonín (Moldanubian Zone, Czech Republic) and Mount Begbie (Shuswap Complex, Canada); of the beryl-columbite-phosphate subtype – Szklary (Góry Sowie Block, Poland); and of miarolitic intragranitic pegmatites – Zimnik (Massif Strzegom-Sobótka, Poland). We studied in detail (EPMA, LA-ICP-MS) relics of primary cordierite/sekaninaite: Věžná I (Crd77–72Sek27–22MnCrd2–1, Be = 0.39–0.25 apfu, Li = 0.06–0.04 apfu), Drahonín (Crd13–9Sek74–71MnCrd17–16, Be = 0.24–0.18 apfu, Li = 0.07–0.05 apfu), Szklary (Crd50–49Sek30–26MnCrd25–21, Be = 0.45–0.41 apfu, Li ≤ 0.02 apfu), Mount Begbie (Crd34–33Sek53–43MnCrd24–14, Be = 0.33–0.29 apfu, Li = 0.26–0.23 apfu), and Zimnik (Crd2–1Sek75–71MnCrd28–23, Be = 0.25–0.15 apfu, Li = 0.18–0.12 apfu). Secondary beryl has a similar Mg/(Mg+Fe) ratio to its cordierite/sekaninaite precursor but is Mn depleted. The mineral assemblages and textures of the pseudomorphs were examined with a focus on secondary beryl, which forms anhedral grains to subhedral elongated crystals, up to 0.3 mm in size, or aggregates of these in textural equilibrium with associated phyllosilicates and tourmaline. Tourmaline is known from Věžná I, Drahonín, Mount Begbie, and Zimnik, the last also with topaz and “zinnwaldite” (a mineral with chemical composition between siderophyllite and polylithionite). Secondary beryl in pseudomorphs after cordierite/sekaninaite from granitic pegmatites and more evolved granites may have been often overlooked; hence, we present its textures and morphology so that it can be recognized during routine EPMA study and to study the source of elevated concentrations of Be in primary cordierite/sekaninaite. The empirical limit of detection of secondary beryl in pseudomorphs is ∼500–1000 ppm Be, which corresponds to ∼1–2 vol.% of secondary beryl. The chemical composition of the secondary beryl and other minerals indicate that the fluids responsible for the alteration were exsolved from the residual pegmatite melt and were not contaminated by fluids from the host rocks.