Compositional evolution of tourmaline in granitic pegmatites

Tourmaline is the most common B-bearing mineral in rare-element granitic pegmatites. The extensive substitutions in its crystal structure, which relate directly to the composition of the host rock, makes it a useful petrological and geochemical indicator mineral. The compositional evolution of tourmaline was examined from three pegmatite subtypes: (1) lepidolite-subtype: Laˇtovičky, Dobrá Voda, Dolní Bory, Rožná and Radkovice pegmatites, Czech Republic, and lepidolite pegmatites at Red Cross Lake, Manitoba; (2) petalite-subtype: Tanco pegmatite, Manitoba, Utö pegmatite, Sweden, and Marko's and Pegmatite #5, Separation Rapids pegmatite group, Ontario; (3) elbaite-subtype: Belo Horizonte pegmatite, California, and Řečice, Pikárec, Ctidružice and Vlastějovice pegmatites, Czech Republic.; The composition of exocontact tourmaline depends on the composition of the host rock and the amount plus composition of fluid injected into the host rock from the pegmatite melt. There are two compositional groups of tourmaline in the exocontact: (1) feruvite-uvite-schorl-dravite (Ca- and Mg-rich); (2) intermediate ternary tourmaline: elbaite-schorl-dravite (Na-, Al-, and Li-rich).; When a pegmatite intrudes a metapelite or a marble, the composition of the endocontact tourmaline depends on the composition of the host rock regardless of the pegmatite subtype. When an elbaite-subtype pegmatite intrudes a mafic rock, the composition of the endocontact tourmaline is Ca- and Mg-rich, but when a lepidolite- or petalite-subtype pegmatite intrudes a mafic rock, the endocontact tourmaline is Mg-rich and Ca-poor.; Each genetic subtype has characteristic tourmaline compositions. Lepidolite-subtype pegmatites are characterized by common foitite and rossmanite, absence of Na-rich schorl, low Mn and no Ca in the primary tourmaline, and late-stage enrichment of Fe and Mn in elbaite. Petalite-subtype pegmatites are characterized by common Na-rich schorl, low Mn and Ca in tourmaline, and late-stage enrichment of Ca and F in elbaite. Elbaite-subtype pegmatites are characterized by common Na-rich schorl in the massive pegmatite, common Mn-rich elbaite and rare liddicoatite in pockets, and late-stage enrichment of Ca and F in elbaite and liddicoatite.; Influx of Fe-rich fluids from, Fe-rich host rock produces rare Fe-bearing elbaite or foitite rims or terminations on late-stage tourmaline in lepidolite- and petalite-subtype pegmatites and elbaite-schorl to Fe-bearing elbaite or foitite rims, terminations or zones in elbaite-subtype pegmatites.