Fractionation in Li-tourmaline from the Lhenice lepidolite pegmatite - "darrellhenryite loop" in F-saturation

The lepidolite-subtype pegmatite at Lhenice, South Bohemia, Czech Republic, is a moderate-sized, highly fractionated pegmatite with abundant lepidolite, elbaite, amblygonite, and cassiterite. As the first mineralogical study at the locality, we acquired EMPA and LA-ICP-MS data of pink and colorless tourmaline from the most fractionated pegmatite units. When looking in detail at the tourmaline assemblages, we can conclude that the primary tourmaline (Tur 1) crystallized from Na-rich melt that produced abundant albite. The tourmaline crystallization overlaps with emergence of the first mica n the albite zone; we suggest that the sudden drop in the tourmaline F content was caused by start of mica crystallization. The F-depleted tourmaline (darrellhenryite) in the lepidolite core is also either coeval or slightly older than the associated lepidolite. The evolution path of the early tourmaline therefore follows a “loop” of Na,F decrease and increase, turning at darrellhenryite composition. Late development of the primary tourmaline is characterized by an increase in Na, OH, F and Fe contents. The new tourmaline generation (Tur 2) is associated by late K-feldspar and muscovite, suggesting hydrothermal crystallization. Textures of latest Fe-enriched (but not Mg- or Ca-bearing) fluor-elbaite (Tur 3) indicate hydrothermal origin; we suggest that its high Fe content might reflect composition of primary hydrothermal fluid in a closed system.