C12 Tregargus Quarries

[SW 949 541]

Highlights

Tregargus Quarries are unusual in that they contain the two late-magmatic variants (non-megacrystic lithium–mica–topaz granite and flu­orite granite) of the St Austell sequence; examples of late- and post-magmatic mineralization and alteration are developed.

Introduction

The Tregargus Quarries (Figure 5.10) are sited in fluorite granite which contains patches of non-megacrystic Li-mica–albite–topaz granite (Types E and F, (Table 5.1)). These facies, which are surrounded by megacrystic Li-mica–albite–topaz granite (Type D, (Table 5.1)), are part of the western intrusion of the St Austell mass recog­nized by Richardson (1923), Exley (1959), and others subsequently. This intrusion was emplaced alongside an earlier biotite granite boss centred on Luxulyan (Figure 5.10), a distinction not identified by the early Geological Survey work (Ussher et al., 1909). The western intrusion was originally biotite granite also, but it was largely metasomatically altered by the incoming Li-mica–albite–topaz granite magma (Hawkes and Dangerfield, 1978; Dangerfield et al., 1980; Hawkes et al., 1987), which had itself been generated from biotite granite at depth (Stone, 1975; Manning and Exley, 1984; Pichavant and Manning, 1984).

Fluorite granite ( 'china stone' or 'Cornish stone') such as that at Tregargus, was thought by Ussher et al. (1909) and Howe (1914) to be an intermediate stage in the breakdown of granite to china clay, but Coon (1913a, 1913b) regarded it as a late variety modified by circulating water. Exley (1959) believed it to be the youngest member in a differentiation sequence, but the current view (Manning and Exley, 1984); Pichavant and Manning, 1984) is that it is a by-product of the complex metasomatic processes by which the Li-mica–albite–topaz granite altered its biotite granite host.

Post-magmatic greisening and tourmalinization are seen in the veins of the quarries, and there is considerable early kaolinization. The last process has given rise to much controversy. One school of thought, exemplified by Hickling (1908) and Sheppard (1977), argued that it resulted from extensive deep weathering, but a second (for example, Collins, 1878, 1887, 1909; Butler, 1908; Ussher et al., 1909; Howe, 1914; Exley, 1959, 1964, 1976) held that it was due to hydrothermal fluids of magmatic origin. A third, currently popular thought, attributes kaolinization to a combination of processes of both kinds (Coon, 1911, 1913a, 1913b; Tomkeieff, discussion in Exley, 1959; Bristow, 1977; Durrance et al., 1982; Bristow et al., in press).

All of these evolutionary concepts are put into historical context in the Petrogenesis' section of this chapter.

Description

The Tregargus Quarries are just under 1 km north-west of St Stephen, a village 7 km west of St Austell, and are towards the southern limit of an elliptical area, roughly 3.5 km from north to south and 1.25 km from east to west, which, for convenience, is here called the Nanpean area (Figure 5.10). Within it occur two of the four main varieties of St Austell Granite (Exley, 1959; Exley and Stone, 1982; Exley et al., 1983; Manning and Exley, 1984) which have been extensively worked for china stone, the latter formerly being used as a flux in pottery manufac­ture.

In the Nanpean area most of the rock contains lithium-mica (zinnwaldite – Stone et al., 1988), albite and topaz. Its texture is not megacrystic, and it encloses irregular patches, up to 400 m across and at least 70 m deep, of fluorite granite. The latter have rapidly transitional, non-intrusive contacts with the former, which the fluorite granite resembles in texture and composition apart from having no iron-bearing minerals (the mica is muscovite) and an average of 2% fluorite (Table 5.1). The two most important areas are im­mediately west of Nanpean and around Tregargus.

The rock in the Nanpean area escaped the potassium metasomatism which gave rise to large feldspar megacrysts elsewhere in the St Austell mass, but they do show, in varying degrees, greisening, tourmalinization and kaolinization. The first is both pervasive through the rock, but restricted to the replacement of some of the K-feldspar, and localized, where it occurs as true greisen borders a few centimetres thick alongside quartz and quartz–tourmaline veins. Tourmalinization, although pervasive in other places, having produced much (4%) tourmaline within the megacrystic Li-mica granite (Type D, (Table 5.1)), is localized in veins at Tregargus.

Kaolinization, and its relationship to joints and faults, is best seen in the non-megacrystic Li-mica granite, the fluorite granite largely remaining fairly fresh, although, where it has been affected, it used to be graded from fresh 'hard purple' to altered 'soft white' by quarrymen before being sold for pottery making (Exley, 1959; Keeling, 1961).

The predominant rock at the Tregarkus Quar­ries is fluorite granite, and all grades of china stone occur, although not much 'hard purple'. The non-megacrystic Li-mica granite (Type E, (Table 5.1); locally known as shellstone) is to be found in irregular areas, no larger than a few square metres, but in such a way that the relationships between the two types may be seen. All the rock has been altered and veined in the manner described above. Among less-usual min­erals found in the quarries are autunite, gilbertite and killingite.

Interpretation

As has been indicated in the 'Petrogenesis' section, the St Austell Granite, which is unique in Britain, has had a more complicated evolution than the other Cornubian granite masses, starting with the intrusion of a boss of coarse-grained megacrystic granite characterized by biotite and zoned oligoclase (An_25–30) (Type B, (Table 5.1); Exley and Stone, 1982), centred on Luxulyan. Subsequently, a second intrusion was emplaced to its west, cutting across both the earlier granite and its metamorphic aureole. The second intrusion also consisted of biotite granite, and it is thought to have included several facies (Hawkes and Dangerfield, 1978; Manning and Exley, 1984; Hill and Manning, 1987). The later intrusion, however, did not rise as far as the first so that a large, but thin, area of its thermally metamorphosed Lower Devonian roof still remains north of St Dennis, penetrated by granite in several places (Figure 5.10).

Much of the biotite in the second intrusion has been replaced by lithium-mica (now known to be zinnwaldite; Stone et al., 1988), and the four main varieties of St Austell Granite were thought at one time to constitute a magmatic differentia­tion series of biotite granite (Type B) megacrystic Li-mica granite (Type D) non‑ megacrystic Li-mica granite (Type E) –+ fluorite granite (Type F) (Richardson, 1923; Exley, 1959). However, subsequent work has shown that this is not the case. It is now believed that, as a consequence of fluorine and water concentration, a residual 'magma' separated at depth from the biotite granite magma and that this contained relatively high concentrations of Li and Na rather than Fe and Ca, similar to the state, of affairs in the Tregonning–Godolphin area in west Cornwall (Stone, 1975; Manning, 1982; Pichavant and Manning, 1984). This residual magma intruded the second St Austell boss to give the non-megacrystic Li-mica–albite–topaz granite now seen in the Nanpean area (and also near Hens-barrow Beacon; see (Figure 5.10)). Under the influence of highly volatile fluids (particularly of fluorine and water) emanating from this intru­sion, biotite and oligoclase of the earlier rocks were altered. Iron was replaced by Li to give zinnwaldite, and Ca was replaced by Na to give albite (An₇); other adjustments, especially of Al, took place concurrently and gave rise, inter alia, to topaz. Excess Ca combined with F to produce fluorite, and hence local patches of fluorite granite, and excess Fe combined with B to form tourmaline (Manning and Exley, 1984; Pichavant and Manning, 1984). The megacrystic Li-mica granite (Type D, (Table 5.1)) in most of the western intrusion is thus interpreted now as a metasomatic aureole of the Nanpean (Type-E) Li-mica granite.

Some mineralization was associated with the intrusion of the coarse biotite granites in Devon and Cornwall, but the main period followed about 10 Ma later, at about 270 Ma BP (Jackson et al., 1982). It is believed, on field and other evidence, to have been consequent on the intrusion of the Li-mica–albite–topaz granite (Manning and Exley, 1984; Darbyshire and Shep­herd, 1987; Bristow et al., in press). The existence of both pervasive and vein-concentrated greisening and tourmalinization throughout the western intrusion attest to the circulation of volatiles, especially water, F and B, and to a sequence of metasomatism, alteration and replace­ment continuing through the late-magmatic and into the post-magmatic stages (Figure 5.4).

The fact that kaolinization is often found beneath unaltered rocks and to great depths effectively seemed to have ruled out weathering as a cause, until Sheppard (1977) described stable isotope ratios which indicated clearly that meteoric water was of the greatest importance. This led to a revival of interest (Bristow, 1977) in a combined magmatic/hydrothermal and weather­ing mechanism such as had been proposed by Hickling (1908), Coon (1911, 1913a, 1913b) and, more positively, by Tomkeieff (discussion in Exley, 1959). These authors envisaged an initial hydrothermal alteration, breaking down feldspar to mica and increasing porosity, followed by weathering to complete the process. The recog­nition of convection cells, circulating meteoric water driven by radiogenic heat (Figure 5.8), and the probability of such activity during the Mesozoic and Cenozoic (Durrance et al., 1982), has enabled a detailed chronology of the intrusion–mineralization–kaolinization continuum to be built up (Bristow, 1987; Bristow et al, in press; (Table 2.2)). During this, the original magmatic water was progressively replaced by meteoric water from the country rocks, bringing elements which, at appropriate temperatures, formed the various mineral deposits and finally brought about low-temperature processes such as kaolinization which have continued through to the present. Evidence of all the chronological stages of (Table 2.2), except stages 1 and 2, is present at Tregargus.

Conclusions

This site contains the only examples now readily accessible of the two youngest varieties of the chief members of the St Austell suite. The granite here originally contained the dark mica biotite, but because of concentrations of certain elements in the final granite magma and its associated solutions, reactions were set up which converted the original mineral composition and produced a different granite enriched in new elements, including lithium and fluorine. Formerly variously attributed to hydrothermal alteration and mag­matic differentiation, the Li-mica–albite–topaz granite is now thought to have been derived from a late-stage magma originating at depth, and the fluorite granite to have been the result of complex metasomatic exchanges with earlier biotite granite. Also to be seen are greisen and quartz–tourmaline veins and kaolinized rock. These have spatial relationships with both the fracture system and fresh rock which show that they have resulted from solutions, now known to be largely of meteoric origin, which have travelled up from deeper levels and penetrated the walls of the fractures.

References