Abstract

In northwestern Sardinia, a Tertiary calc-alkaline volcanic succession, linked to the subduction of Insubric oceanic crust and subsequent post-collisional slab break-off, crops out and hosts several kaolindeposits. We have undertaken field and laboratory studies to establish how the Tertiary geodynamic setting of the western Mediterranean, and the composition and texture (especially porosity) of the volcanic rocks, control the geometry and quality of the deposits. Massive andesites, dacites, and mildly welded rhyodacitic ignimbrites are the precursors of the kaolin. Kaolindeposits from the andesites contain pyrite and jarosite, and the deposits which formed from the dacites contain up to 5% alunite. The amounts of pyrite and alunite are negligible in deposits which formed from ignimbrite. Fracture-controlled fluid-dominated systems (Izawa, 1986), responsible for the formation of the deposits, are linked to NNW–SSE-trending normal faults which formed during Burdigalian to Serravallian extension following on from collision between Adria and Europe (Corsica–Sardinia crust). Four types of deposit morphology (bedform, mushroom, fault parallel and funnel) are recognised, based on the attitude, fracturing, and porosity of the precursor rock. The remarkable funnel morphology was generated by repeated hydrothermal eruptions.

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