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The nature and significance of illite associated with quartz-hematite hydrothermal veins in the St. Austell pluton, Cornwall, England

¹ Department of Earth Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK

^* E-mail: psyrillo{at}otenet.gr

(Received 26 June 2000; revised 8 March 2001)

Quartz-hematite veins in the St. Austell pluton feature an assemblage of authigenic kaolin, illite, chlorite and hematite. Similar assemblages occur in altered granites adjacent to the veins. Complex textures in the veins and altered granites show that kaolin was an initial precipitate from hydrothermal fluids that was subsequently replaced by illite, hematite and chlorite. The sequence of mineral precipitation reflects a kinetic control on crystal growth, with early non-equilibrium precipitation of kaolin from fluids with compositions appropriate for the equilibrium formation of illite. Hematite formed under relatively oxidizing conditions that subsequently became more reducing to permit the late precipitation of chlorite. Illite associated with the quartz-hematite veins differs texturally, mineralogically and chemically from the coarsely crystalline hydrothermal muscovite (sericite or gilbertite) associated with quartz-tourmaline veins and greisen. Quartz-hematite veins are thus considered to record a distinct mineralization event that is not related to the early greisenization or the economic kaolinization.

KEYWORDS: illite, hematite, kaolin, chlorite, authigenesis, Cornwall, England

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