Tourmaline and muscovite chemistry as a proxy for the origin and evolution of Paleoproterozoic lithium pegmatites in Kaustinen, western Finland

The Paleoproterozoic Kaustinen lithium province in western Finland hosts numerous Li deposits in Li-Cs-Ta (LCT) pegmatites of the albite-spodumene type. As for many rare-element pegmatite populations, the source of the pegmatite-forming melts in Kaustinen is not fully understood. In this study, we use major and trace element chemistry of tourmaline and muscovite to test whether a genetic link exists between the Kaustinen albite-spodumene pegmatites and the nearby Kaustinen pegmatite granite, and to explore the internal evolution of albite-spodumene pegmatites. The compositional variation in tourmaline and muscovite from crystal to regional scales reveal differences in the internal evolution and probable melt sources of individual albite-spodumene pegmatites. We interpret differences in intra- and inter-sample variation between individual pegmatites to reflect differences in solidification vectors. Contamination from wall rocks during the solidification of pegmatites is shown to affect mineral chemistry even in the inner zones of some of the pegmatites, evidenced by the elevated concentrations of Mg, Ca, Ti and Sr in the rims of the sharply zoned tourmaline crystals. Large-Ion Lithophile Element (LILE) concentrations and ratios in tourmaline and muscovite vary between pegmatites and do not agree with fractional crystallization trends calculated for the Kaustinen pegmatite granite. The major differences in LILEs between individual pegmatites suggest multiple melt sources for the albite-spodumene pegmatites of Kaustinen, with the variations likely reflecting the mineral proportions involved in the melting reactions and subsequent fractionation of the nascent melts. • Multiple melt sources for albite-spodumene pegmatites in the Kaustinen Li province • Differences in solidification vectors between pegmatites • Subtle wall rock contamination observed in some pegmatites