Geochemical constraints from zoned hydrothermal tourmalines on fluid evolution and Sn mineralization: an example from fault breccias at Roche, SW England

Hydrothermal fluid evolution north of the St Austell granite, southwest England, has been studied through geochemical analysis of tourmaline from a fault breccia of <2 cm width within massive quartz–tourmaline rocks at Roche. Brecciated tourmaline grains have overgrowths of <400 µm width [Fe/(Fe + Mg) = 0·31–0·99] with four chemically distinct zones (1–4, towards the margins). Variations in overgrowth composition were caused by episodic mixing between Mg-, Al-rich magmatic hydrothermal fluids (dominant in zone 1), with an increasing component of more oxidizing, Fe-rich formation waters (zones 2 and 4). More oxidizing conditions are supported by high Sn contents in zone 2 (