The Variscan granite batholith of Devon and Cornwall is host to extensive deposits of primary kaolin, known in Britain as china clay, which support an industry supplying over 3 million t.p.a. of product. This is the principal source of kaolin for W. European paper and ceramic markets.
The genesis of these deposits took place in six stages. The initial stage was the intrusion of the main batholith at 290 Ma, a S-type biotite granite rich in heat producing radioelements; followed by limited Sn/W mineralisation (Stage 2). Stage 3a was the intrusion of an evolved Li-B-F rich magma at 270 Ma., low in colouring elements (Fe, Ti), which only reached the upper surface of the batholith in a significant way in the western part of the St. Austell granite intrusion, but probably underlies the batholith throughout most of its length. This was accompanied by the mainstage hydrothermal Sn/Cu mineralisation and associated greisenisation and tourmalinisation (Stage 3b). Intrusions of felsitic dykes (elvans) brought this episode to a close (Stage 3c). The ensuing Stage 4 cross-course mineralisation involved saline, lower temperature fluids without boron, and radiogenically driven convective circulation became the dominant hydrothermal mechanism. Argillation followed permeable zones established by tectonism and earlier hydrothermal activity. Because of the saline nature of the fluids, the clay mineral assemblage was dominated by smectite and illite, with only limited amounts of kaolinite. Flushing of the system by meteoric water, following the transition to a wetter climate in the Mesozoic allowed pervasive circulation of warm fresh water, which converted the clay minerals and feldspars to the kaolinite dominated assemblage we see today (Stage 5). Continuous solution and recrystallisation of the kaolinite led to leaching of colouring oxides and a steady increase in lattice order. Large authigenic curled stacks of kaolinite also formed in the matrix. Stage 5 merged into Stage 6, which is the deep weathering in Mesozoic and early Tertiary times which affected much of Europe. There is evidence that convective circulation and kaolinisation are still proceeding slowly today. Fortunately erosion has not stripped too much of the soft kaolinised granite away and the deposits, as seen today take the form of funnels or tabular (on edge) bodies with depths exceeding 200m in places. Most of the worthwhile china clay deposits are in or close to the intrusions of the Stage 3 lithium mica granite, notably in the western part of the St. Austell granite and the south-western edge of the Dartmoor granite.