Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Zbyněk Buřival, Milan Novák

Secondary blue tourmaline after garnet from elbaite-subtype pegmatites; implications for source and behavior of Ca and Mg in fluids

Journal of Geosciences, volume 63 (2018), issue 2, 111 - 122

DOI: http://doi.org/10.3190/jgeosci.257

Secondary blue tourmaline (schorl to Fe-rich fluor-elbaite to very rare Fe-rich fluor-liddicoatite) with quartz partially replace spessartine-almandine garnet and albite in elbaite-subtype pegmatites cutting pyroxene gneisses and calcite or dolomite marbles (at Tamponilapa and Tsarafara-Nord, Sahatany Valley, Madagascar) and paragneisses (at Ctidružice, Moldanubian Zone, Czech Republic). Only garnet from the albite adjacent to an unit with Li-bearing minerals (Li-micas, Li-tourmalines) underwent this alteration, whereas associated primary tourmaline (schorl to Mg-bearing schorl) remained unaltered. Textural relations and chemical composition of the individual minerals suggest the following equation of replacement for averaged and simplified (Ca-free) compositions of garnet and secondary tourmaline from Tsarafara:
25 NaAlSi₃O₈ + 2 (Mn₂Fe₁)Al₂Si₃O₁₂ + 12 H₃BO₃ + 4 LiF → 4 (Na_0.75 0.25)(Al_1.25Li_1.00Fe_0.50Mn_0.25)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃F + 46 SiO₂ + 11 Na₂SiO₃ + 3 MnO + 12 H₂O
Elevated contents of Al, Fe, Na and Mn in secondary tourmaline were likely sourced from the replaced garnet and albite, whereas the residual fluids supplied B, F, Li, and H₂O. Garnet and associated primary tourmaline are rather Mg-rich but the secondary tourmaline is typically Mg-free. The contents of Ca are high in both tourmaline generations, but the secondary one is occasionally even more enriched compared to the associated primary tourmaline. Such a behavior of Mg and Ca suggests that no Mg was externally supplied during primary crystallization of primary tourmaline from the moment when pegmatite melt was sealed off the host rock. Negligible to none concentrations of Mg in secondary tourmaline show that the pegmatite system was closed to the host rocks during the hydrothermal alteration producing the secondary tourmaline generation. Evident absence of external Mg-contamination from host Ca, Mg-rich rock rules out also any contamination by Ca. High contents of Ca in primary tourmaline and garnet are related to originally Ca-enriched pegmatite melt contaminated before emplacement.