D. A. C. MANNING, P. I. HILL & J. H. HOWE*
Abstract: Geological mapping of existing and redundant kaolin workings within the St Austell Granite has identified a suite of granitic rocks which show evidence of complex late-stage magmatic and hydrothermal processes. Coarse porphyritic biotite granites. like those which predominate in southwest England, occur much more widely than previously acknowledged, and are intruded by an apparently cogenetic suite of lithium-mica granites and tourmaline granites. The tourmaline granites characteristically exhibit very variable textures, with coarse quartz grains set within a fine grained, tourmaline-rich matrix. A highly evolved fine-grained tourmaline granite represents the most evolved of this suite. Topaz granites intrude the earlier granite varieties, and all are intruded by rhyolite porphyry dykes (elvans).
Major and trace element chemical data suggest that the biotite granite-lithium-mica granitetourmaline granite suite represents the product of crystallization of a granitic magma within which B (but not F) became progressively enriched until water saturation was achieved. Water exsolution effectively quenched any remaining granitic melt, resulting in the very variable textures shown by the tourmaline granites. The topaz granites are chemically distinct from their predecessors, showing marked enrichment in F, Li and P20O (but not B). Instead of being products of differentiation of biotite granite magma, the topaz granite melts may have been derived separately in a later episode of partial melting of the same source.
Kaolinization is widespread throughout the western part of the St Austell Granite, and deposits worked at present tend to be located in granite varieties other than biotite granite. The geochemical parameters used to distinguish the primary granite types (particularly Nb v. Zr and Ga-Nb-Zr plots) are sufficiently robust to permit the parent granitic rock type to be identified for heavily kaolinized material.
Keywords: Cornwall England, granites, kaolinization.
The St Austell Granite is of major importance as a source of high quality kaolin (china clay), supplying up to 3 million tonnes of clay per annum for the paper, ceramics, paints, plastics and rubber industries. Production is supplied from a large number of open pits scattered throughout the western part of the granite outcrop within a complex landscape of spoil heaps and mica lagoons (Fig. 1).
Although the St Austell Granite is so well exposed in the kaolin workings (especially when compared with the other granites of SW England), it has been neglected in regional studies. This is in part due to the need to work on largely unaltered material for petrogenetic research. However, field relationships are well exposed in working pits, and despite the pervasive nature of kaolinization several granite varieties can be distinguished and mapped on the basis of textural evidence. It is the purpose of this paper to report the results of an extended period of mapping and field observation (1985-1995), together with supporting basic petrological information, and so to provide an integrated lithological framework from which to build in future studies of kaolinization. For a comprehensive review of the china clay deposits of SW England, Bristow & Exley (1994) should
The granites of SW England share several basic characteristics, summarised in detail in a number of reviews (Alderton 1990: Floyd et al. 1993; Manning 1996). They are dominated by a rather homogeneous biotite granite, which may be coarsely porphyritic with K-feldspar phenocrysts up to 20 cm in length (as in the Lands End, St Austell and Dartmoor granites) or with a less coarse porphyritic texture in which K-feldspar phenocrysts reach 3 cm in length (characteristic of the Carnmenellis and Bodmin Moor granites). The total volume of the batholith is estimated to be 68000 km3, …