Kaolinisation, Mineralisation and Structures in Biotite Granite at Bodelva, St

Read at the Annual Conference of the Ussher Society, January 1999 KAOLINISATION, MINERALISATION AND STRUCTURES IN BIOTITE GRANITE AT BODELVA, ST. AUSTELL, CORNWALL S. MUELLER, P.W. SCOTT AND M.J. EVANS Mueller, S., Scott, P.W. and Evans, M.J. Kaolinisation, mineralisation and structures in biotite granite at Bodelva, St. Austell, Cornwall. Geoscience in south-west England, 9, 310-317. Bodelva China Clay Pit until recently was providing kaolin for the paper and other industries and at closure was yielding around 40,000 tonnes per annum of product. It is entirely located within the medium to coarse grained biotite rich facies of the eastern part of the St. Austell granite. Four distinct stages of alteration have been delineated: slightly, medium, highly and completely altered biotite granite. These occur together with minor exposures of fine grained biotite granite and a few areas of primary greisening. Various types of mineralised veins and dykes are widespread throughout the china clay pit and follow mainly west-south-west — east-north-east and north-north-west — south- south-east trends respectively. Two major sets of iron oxide stained fracture zones, following the same trends are spatially related to completely altered biotite granite. There is strong evidence for the extreme form of kaolinisation within these areas being predominantly structurally controlled. The intense iron oxide staining along the southern rim of the pit is considered a result of de-ferruginisation of biotite. Analyses of micas show an almost continuous trend of compositions, represented by changes in the cation distributions in octahedral sites from tri-octahedral (Fe+Mn+Mg rich) biotite to di-octahedral (Al rich and Fe+Mn+Mg poor) types approaching stoichiometric muscovite. S. Mueller and P. W. Scott, Camborne School of Mines, University of Exeter, Redruth, Cornwall TR15 3SE M.J. Evans, Goonvean Ltd, St Stephen, St. Austell, Cornwall PL26 7QF INTRODUCTION lithological variation is now recognised within the pluton. Most recently, Manning et al. (1996) describe six granite types based on Bodelva China Clay Pit is situated on the south-eastern edge of differences in mineralogy and texture. These are biotite granite, the St. Austell granite, Cornwall [SX 049 548]. It covers an area of 2 lithium mica granite, globular quartz tourmaline granite, equigranular approximately 0.5 km . Until recently it was producing kaolin for the tourmaline granite, fine-grained tourmaline granite, and topaz granite. paper and other industries. Production ceased in September, 1998, and They describe the biotite granite as making up 70% of the outcrop and the site is currently being redeveloped for the Eden Project. This is a corresponding to the coarsely porphyritic types in the Lands End and series of large, super-strong, polymer covered, steel-framed domes Dartmoor granites. It is coarse grained with microperthitic K-feldspar containing flora from various climatic zones, along with associated megacrysts and a hypidiomorphic granular texture. The sequence of educational and visitor facilities. mineralisation and alteration affecting the St. Austell granite have Most of the working china clay pits in the St. Austell area are in been reviewed by Bristow and Exley (1994) and Psyrillos et al. the central and western parts of the granite (Bristow and Exley, 1994; (1998). These include greisening and tourmalinisation with associated Manning et al. , 1996) (Figure 1) which is made up mainly of lithium Sn, W and Cu mineralisation, Fe, U, Pb and Zn mineralisation mica, tourmaline and topaz granites. Bodelva China Clay Pit was associated with a first phase of argillic alteration, and a later extensive unique in being the only operating one in the eastern biotite granite. kaolinisation. Further mineralisation is associated with emplacement The objective of this work is to record the major geological features in of felsite dykes ('elvans'). Bristow and Exley (1994) and Psyrillos et the pit before it ceased to be available for study. This has included al. (1998) differ in favouring low and high salinity fluids, respec- establishing the variations in the granite, the different stages of tively, as responsible for the kaolinisation, although both sets of alteration in the formation of kaolinite, the other types of authors emphasise the low temperatures (30-200°C and <100°C mineralisation and structural features within the pit. The significance respectively) during this event. Isotopic evidence indicates that the of these to our understanding of the geological history of the St. fluids responsible for the kaolinisation are meteoric in origin Austell granite is discussed. (Sheppard, 1977). The St. Austell granite has attracted many researchers over the last half century (e.g. Exley 1959; Exley and Stone, 1982; Hill and HISTORY OF MINERAL EXTRACTION AT BODELVA Manning, 1987; Bristow and Exley, 1994) and considerable Little is recorded of the earliest clay workings in the Bodelva area but there were probably several small pits working in the early part of the 19th century. The mining records of the Royal Geological Society of Cornwall in 1858 mention a Wheal Carlyon Pit as producing 500 tons of clay a year. In the early 20th century cassiterite was extracted as a by-product along with china clay. Hard vein material, left behind after the hydraulic mining of the clay, was crushed using water-driven 'stamps' and the ore separated by gravity methods. Apparently, the revenue from the sale of the tin concentrate paid for the wages of the clay workers. In the 1970s, several mining companies made sampling surveys for cassiterite in the pit, and although ore was found, particularly in quartz-rich veins in the west of the pit, the grade was low and mineralisation too sporadic for commercial extraction, even with the high price of tin pertaining at that time. Figure 1. Distribution of granite varieties in the St. Austell Granite For the last 30 years Bodelva pit produced china clay mainly for (after Manning et al. 1996) and location of Bodelva China Clay Pit. the paper industry, particularly filler grades for magazine papers such 310 Kaolinisation, mineralisation and structures in biotite granite at Bodelva, St. Austell, Cornwall as 'weekend colour supplements'. In the mid-1980s a special clay was % 1 2 3 4 5 developed for the coating of rotogravure printing paper. This was produced by partially delaminating the 'book' structure shaped kaolin SiO 2 36.58 40.76 39.05 41.61 46.44 TiO 2.44 1.15 1.82 1.3 0.59 particles which are fairly abundant in the kaolinised granite. The 2 Al 2O3 21.4 23.92 32.22 24.14 32.03 special property this clay gives to the paper coating formulation is FeO 20.88 16.3 17.88 15.99 3.68 considered to be due to the relatively high aspect ratio of the MnO 0.55 0.75 0.55 0.51 0.22 delaminated clay particles for their relatively coarse particle size. MgO 4.14 2.29 2.52 2.15 1.74 During the last few years of operation, the annual clay production rate Na 2O 0.41 0.28 nd nd 0.78 K2O 9.75 10.33 9.8 10.1 10.34 reached 40,000 tonnes. Cl 0.32 nd nd nd nd For mineral specimen collectors, Bodelva China Clay Pit is well Total 96. 47 95.78 95.5 95.8 95.82 known for its veins of amethystine quartz, sometimes found on the Number of ions on the basis of 22 oxygen east side of the pit, for its honey coloured cassiterite, which occurred Si 5.546 5.9751 5.8171 6.0503 6.245 sporadically in quartz-tourmaline veins, and particularly for its 'pigs AI(Tet) 2.454 2.0249 2.1829 1.9497 1.755 eggs'. The latter are large (up to 10 cm in length) K-feldspar Al(Oct) 1.3699 2.1077 1.8937 2.1872 3.3213 megacrysts which often show various forms of twinning. They could Ti 0.2782 0.1268 0.2039 0.1421 0.0597 be extracted whole from the partly kaolinised granite. Elongate black Fe 2+ 2.6474 1.9983 2.2848 1.9444 0.1438 tourmaline crystals ('shorl') also have been found. Mn 0.0706 0.0931 0.0934 0.0628 0.0251 Mg 0.9356 0.5004 0.544 0.466 0.3488 Na 0.1205 0.0796 - - 0.2034 GEOLOGY AND CLASSIFICATION OF ALTERATION K 1.8857 1.9317 1.8622 1.8734 1.7737 Cl 0.0822 - - - - STAGES ∑Tet 8 8 8 8 8 ∑Oct 5.3017 4.8263 5.0198 4.8025 4.1687 The two major geological features at Bodelva are the variable ∑inter 2.0062 2.0113 1.8622 1.8734 1.9771 degree of kaolinisation, and the large area of reddish-brown iron oxide stained completely altered granite along the southern margin of the % 6 7 8 9 10 pit. Other features of importance are small areas of greisening, a few SiO 46.6 47.42 46.54 48.14 47.88 patches of fine grained biotite granite and a widespread distribution of 2 TiO 2 1.24 0.75 0.26 nd nd mineralised veins. At the time of survey there were large areas of Al 2O3 29.2 32.2 33.27 34.57 34.64 exposure, but significant parts of the pit were covered by sand tips or FeO 6.15 4.01 2.92 0.96 0.83 overburden and there had been some back-filling. MnO 0.28 nd nd nd nd MgO 1.85 0.88 1.31 0.29 0.37 Four stages of alteration have been recognised. These are slightly Na 2O 0.34 0.34 0.55 nd 0.26 altered biotite granite, medium and highly altered, and completely K2O 10.46 10.81 10.58 9.7 9.84 altered biotite granite.

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