Geochemical Constraints from Zoned Hydrothermal Tourmalines on Fluid Evolution and Sn Mineralization: an Example from Fault Breccias at Roche, SW England

Abstract

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 (<0·35 wt %), with Sn probably present as Sn4+ rather than Sn2+, the usual form in hydrothermal fluids. From X-ray maps, zones 1 and 3 occur exclusively as overgrowths on pre-existing grains, indicating that overgrowth formation was kinetically favoured over tourmaline nucleation. In zones 2 and 4, nucleation and growth occurred, possibly as a result of supersaturation with respect to tourmaline during increased mixing with formation waters. Tourmaline is associated with the main episode of mineralization in many important mineral deposits, often unaffected by alteration. This method of studying hydrothermal fluid evolution may therefore have uses in exploration, particularly for tourmaline-breccia-hosted ores in Cu-porphyry deposits.