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

Abstract

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 (Eräjärvi, Kietyönmäki), quartztourmaline veins from gold occurrences (Kutemajärvi, Riukka), and the tourmalinised contact between a synorogenic intrusion and supracrustal rocks (Lepomäki Au occurrence, Ylöjärvi 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 δ11B 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 Lepomäki gold and Ylöjärvi 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 Lepomäki, Ylöjärvi and Riukka.

Keywords

• Tourmaline
• Chemical composition
• Boron isotopic composition
• Gold
• Finland