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.
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Kaolin is an important industrial mineral in several world markets including uses in paper coating and filling, ceramics, paint, plastics, rubber, ink, fiberglass, cracking catalysts and many other uses (Murray, 1991). The kaolin minerals kaolinite, halloysite, dickite, and nacrite have essentially similar chemical composition but each has important structural and stacking differences. The most common kaolin mineral and the one that is the most important industrially is kaolinite [Al2Si205(OH)4]. Kaolinite can be formed as a residual weathering product, by hydrothermal alteration, and as an authigenic sedimentary mineral. The residual and hydrothermal occurrences are classed as primary and the sedimentary occurrences as secondary. Primary kaolins are those that have formed in situ usually by the alteration of crystalline rocks such as granites and rhyolites. The alteration results from surface weathering, groundwater movement below the surface or action of hydrothermal fluids. Secondary kaolins are sedimentary which were eroded, transported and deposited as beds or lenses associated with other sedimentary rocks. Most kaolin deposits of secondary origin were formed by the deposition of kaolinite which had been formed elsewhere. Some secondary deposits were formed from arkosic sediments that were altered after deposition, primarily by groundwater. There are far more deposits of primary kaolins in the world than secondary kaolin deposits because special geologic conditions are necessary for both the deposition and preservation of secondary kaolins.