Compositional evolution of tourmaline in granitic pegmatites
Selway, Julie Beryl
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: La_tovic_ky, Dobra Voda, Dolni Bory, Roz_na and Radkovice pegmatites, Czech Republic, and lepidolite pegmatites at Red Cross Lake, Manitoba; (2) petalite-subtype: Tanco pegmatite, Manitoba, Uto pegmatite, Sweden, and Marko's and Pegmatite maline 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.