CHEMICAL EVOLUTION OF TOURMALINE FROM A GRANITIC PEGMATITE IN THE NÁÁTS’IHCH’OH IGNEOUS COMPLEX, NORTHWEST TERRITORIES, CANADA

Tourmaline-bearing granitic pegmatite dikes of the Nááts’ihch’oh igneous complex (62°46’8.33”N, 128°56’9.07”W) located in the Sapper Range of the Selwyn Mountains in the Northwest Territories, Canada were investigated in order to determine the chemical composition of tourmaline as a function of its position within the pegmatite body. The chemical composition of tourmaline adjusts to a wide range of conditions and can therefore be used to interpret the evolution of the pegmatite system. The mineralogy of all observed components was investigated by combining X-ray powder diffraction and SEM analysis. Chemical analysis of tourmaline crystals was completed using electron microprobe analysis and the tourmaline species were named using the scheme developed by Henry et al. (2011). Tourmaline species occur in the parental porphyritic granite, the aplite dikes, the pegmatite dikes, the metasedimentary host rock, and the late-stage quartz veins. The chemical composition of the tourmaline varies due to the coupled substitution Si + Mg <-> Al + Fe + Ti. Different tourmaline species were distinguished as a result of the substitution Na + Ca <-> [] at the nine-coordinated X site. Crystals of tourmaline in the pegmatite dikes vary in composition from schorl at the rims of the crystals to foitite at the cores of the crystals. The sedimentary host rocks have been locally metasomatised to dravite and quartz. Crystals of dravite with uvite cores grow into the late-stage quartz veins. Na and F contents are highest in tourmaline crystals in the pegmatite dikes. Their concentrations increase from crystal core to rim due to increasing fractionation. The Mg content is highest in tourmaline crystals in the metasedimentary host rock. This is likely due to Mg-bearing minerals present in the original sedimentary host rock that provided the Mg necessary to crystallize the dravite. The Ca content is higher in tourmaline crystals in the metasedimentary host rock than in tourmaline crystals in the pegmatite dikes. This is likely due to fluoride complexes that caused Ca incompatibility in the late-stage melt and fluids. The overall mineralogy of all components of the system consists of common igneous minerals such as quartz, plagioclase, K-feldspar, biotite, and actinolite. There is no evidence of rare-element mineralization.