Geochemical and boron isotopic compositions of tourmalines from selected gold-mineralised and barren rocks in SW Finland

Tourmaline-bearing rocks and a number of gold occurrences coexist in southwestern Finland, in a region where extensive magmatism and hydrothermal systems occurred during the Svecofennian orogeny, 1.9–1.8 Ga ago. Tourmalines investigated for this study were sampled from late-orogenic granite pegmatites (Erajarvi, Kietyonmaki), quartztourmaline veins from gold occurrences (Kutemajarvi, Riukka), and the tourmalinised contact between a synorogenic intrusion and supracrustal rocks (Lepomaki Au occurrence, Ylojarvi Cu-W-As-Ag-Au deposit). Major element compositions of the tourmalines are controlled by their host lithologies, whereas their boron isotopic compositions reflect the B reservoir(s) and the magmatichydrothermal processes of the B-bearing systems. The obtained δ 11 B values for granite pegmatite, vein, and intrusion-country rock contact-related tourmalines range between -10.2 ‰ and -13.2 ‰, -10.2 ‰ and -11.6 ‰, and -9.6 ‰ and -8.7 ‰, respectively, and are similar to tourmalines worldwide. The lighter B isotopic composition at Lepomaki gold and Ylojarvi polymetallic occurrences indicate different B reservoirs and magmatic-hydrothermal processes between synand late-orogenic tourmalines. Geological observations and geochemical data on tourmalines also suggest that B was a constituent in hydrothermal fluids at all localities, including the gold-mineralising fluids at Lepomaki, Ylojarvi and Riukka.