THE TANCO PEGMATITE AT BERNIC LAKE, MANITOBA. XIII. EXOCONTACT TOURMALINE

The petalite-subtype Tanco granitic pegmatite, southeastern Manitoba, intrudes an amphibolite (metagabbro), which is metasomatically altered to tourmaline and magnesian annite close to the contact. The composition of exocontact tourmaline depends on the composition of the host rock and on the composition and amount of fluid injected from the pegmatite-forming melt into the host rock. Two compositional groups of tourmaline occur in the exocontacts at Tanco: (1) common feruvite – schorl – dravite (Ca- and Mg-rich), and (2) rare intermediate ternary tourmaline: elbaite – schorl – dravite (Na-, Al- and Li-rich). The Ca, Mg and Ti contents of group-1 tourmaline increase with distance from the contact between the pegmatite and the host rock. Group-1 and -2 tourmalines crystallize as a result of Na-, Al-, Li- and B-rich pegmatite-derived fluids infiltrating the Ca-, Mg-and Fe-rich host amphibolite, or as a chemical reaction between the pegmatite-forming melt and the host rock. The exocontact tourmaline is zoned, with a (Fe, Mg)-rich group-1 core and a Li-rich group-2 rim. Early consumption of Ca, Mg and Fe by crystallization of feruvite – schorl – dravite increased the chemical potential of Na, Al and Li in the fluid and promoted crystallization of elbaite – schorl – dravite. The rare ternary tourmaline of group 2 crystallized in the tourmaline aureole from fluids with high chemical potential of B, Al, Na and Ca, which are major constituents in tourmaline, but absent or minor in holmquistite. The influx of B-rich acidic fluids promoted crystallization of tourmaline and prevented crystallization of holmquistite.