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 analysisof tourmaline from a fault breccia of <2 cm width withinmassive quartz–tourmaline rocks at Roche. Brecciated tourmalinegrains have overgrowths of <400 µm width [Fe/(Fe +Mg) = 0·31–0·99] with four chemically distinctzones (1–4, towards the margins). Variations in overgrowthcomposition were caused by episodic mixing between Mg-, Al-richmagmatic hydrothermal fluids (dominant in zone 1), with an increasingcomponent of more oxidizing, Fe-rich formation waters (zones2 and 4). More oxidizing conditions are supported by high Sncontents in zone 2 (<0·35 wt %), with Sn probablypresent as Sn⁴⁺ rather than Sn²⁺, the usual form in hydrothermalfluids. From X-ray maps, zones 1 and 3 occur exclusively asovergrowths on pre-existing grains, indicating that overgrowthformation was kinetically favoured over tourmaline nucleation.In zones 2 and 4, nucleation and growth occurred, possibly asa result of supersaturation with respect to tourmaline duringincreased mixing with formation waters. Tourmaline is associatedwith the main episode of mineralization in many important mineraldeposits, often unaffected by alteration. This method of studyinghydrothermal fluid evolution may therefore have uses in exploration,particularly for tourmaline-breccia-hosted ores in Cu-porphyrydeposits. KEY WORDS: breccia; Cornwall; hydrothermal; tin; tourmaline