Browsing by Author "Tvauri, Artur"

The albite-spodumene pegmatites of the Kaustinen Li-province host one of the most significant lithium resources in Europe. Economic potential and petrogenesis of the Kaustinen Li pegmatites have been studied previously, however the magmatic hydrothermal processes behind the petrogenesis and lithium enrichment are not yet fully understood. Because of its wide P-T-X stability range, complex mineral chemical structure with abundance of varying trace elements, and high resistance to alteration, tourmaline is an ideal proxy for the study of the Kaustinen Li province. In this study, the major and trace element compositions of tourmaline from six albite-spodumene pegmatites and the Kaustinen pegmatite granite were analyzed in order to investigate: 1) the regional variations in tourmaline chemistry between the individual pegmatites and the assumed source pluton, and 2) how this reflects the origin, evolution, and fractionation of the pegmatite forming melts. Based on their texture and location within the pegmatites and the granite, the tourmalines were divided into wall rock tourmalines, contact tourmalines, core zone tourmalines and granite tourmalines. Tourmaline in the Kaustinen Li-province belongs to the alkali-group and shows significant compositional variation in the dravite – schorl – elbaite (Mg – Fe – Li+Al) ternary system. Chemical evolution of the melts is most notably displayed by a two-step trend of variation in the tourmaline Y-site, where first Mg is substituted for Fe, and then Fe is substituted for Li+Al. Tourmaline in the metasomatized wall rock shows chemical affinity to the metasedimentary or metavolcanic wall rock, especially in cases where the fluid/rock ratio is interpreted low during metasomatism. Major and trace element composition of tourmaline reveals differences in the internal evolution of three pegmatite dikes, with weak internal chemical zonation and contemporaneous crystallization throughout the dike in the Rapasaaret and Heikinkangas dikes, whereas in Matoneva, the chemical zonation is strong, indicating crystallization inwards from the dike walls. On a regional scale, tourmaline composition in these three pegmatites and the Kaustinen pegmatite granite form chemical trends expected if the pegmatite melts would be originated from the Kaustinen pegmatite granite, except for Sr, which would be expected to decrease during fractional crystallization, but has lower concentrations in the granite than in the pegmatites. Because of this, the pegmatite melts likely could not have fractionated from the melts represented by the two granite samples, but as the Kaustinen pegmatite granite shows significant heterogeneity, the parental melts of the pegmatites could have been derived from a different part of the pluton. No direct evidence for the pegmatite melts having stemmed from low-degree partial melts initially enriched in Li through staurolite melting reactions can be seen from the tourmaline major and trace element data. As the geochemical signatures produced by staurolite melting reactions are not fully understood and as the REE data from tourmalines of this study are limited, further assessment of this alternative petrogenetic model is difficult.