Multiple fluid interactions recorded in tourmaline from the Dorothy China Clay Pit, St. Austell, Cornwall, UK

Abstract Elaborately zoned blue–grey tourmaline from the Dorothy China Clay Pit, St. Austell, Cornwall, UK reveals a history of hydrothermal activity in an open system. At least five distinct generations of tourmaline are identified within a single crystal. They are characterised by complex replacement textures displaying dissolution and reprecipitation features and compositional variations identified using optical microscopy, BSE-imaging and EPMA. The Li-rich, alkali-poor rossmanite core of the grain is considered as generation 1 tourmaline. Generation 2 tourmaline comprises a more Na-rich species, especially elbaite. Generation 3, partially replacing the core, is richer in Fe, and is mostly schorl and foitite. Generation 4, primarily Fe-rich dravite, forms an Mg-enriched rim around generations 1–3. Generation 5 fluor-schorl replaces all previous tourmaline generations and parts of the quartz matrix. Each generation corresponds to a chemically distinct fluid event suggested by dissolution textures and compositionally differing overgrowths. Infiltration of B-bearing, neutral-to-acidic fluids facilitated the growth of tourmaline. These fluids contained varying amounts of major elements reflected in the changing tourmaline composition. Dissolution probably occurred because of an increase in fluid pH or a change in major cation abundances. Generation 1 tourmaline crystallised in equilibrium with a Li-rich, Na-poor granitic host rock. From generations 1 to 3, fluids generally increased in Na and Fe while decreasing in Al and Li. Fluids increased in F at generation 3, followed by the influx of more oxidising, Mg-enriched fluids at generation 4. The final generation 5 represents a return to compositions richer in Fe and F. The episodic changes in fluid composition preserved by each generation of tourmaline records fluid infiltration. These differing compositions might reflect, in part, progression of kaolinisation of the host granites or changes in the magmatic–hydrothermal fluids. The St. Austell kaolinite deposits formed from hydrothermal alteration of the preexisting granite through multiple stages of reactive fluid infiltration as recorded in tourmaline.