C13 St Mewan Beacon

[SW 985 534]

Highlights

This site displays a rare exposure of quartz–topaz–tourmaline rock of hydrothermal origin, formed immediately under the metamorphic rocks of the granite roof.

Introduction

St Mewan Beacon is situated on the southern margin of the St Austell Granite, 3 km WNW of St Austell and just outside the Blackpool china-clay pit (Figure 5.10).

The St Austell Granite was emplaced in three episodes, the second of which cuts across the first, near St Dennis (Figure 5.4). Both the first and the second consist of megacrystic biotite granite of typical Cornubian type (Type B, (Table 5.1); Exley and Stone, 1982). A third intrusion of Li-mica–albite–topaz granite (Type E, (Table 5.1)) was emplaced within the second boss, and this is now exposed between St Dennis and St Stephen and near Hensbarrow Beacon. It is believed to have been derived from biotite granite at depth (see 'Petrogenesis' section and site descriptions) and upon emplacement to have metasomatized much of the second intrusion, albitizing the oligoclase, converting biotite to zinnwaldite and introducing topaz. It is this type of granite (Type D, (Table 5.1)) which is adjacent to St Mewan Beacon. Accompanying and following these intru­sions, the introduction of boron gave rise to extensive tourmalinization which preceded greisening, metalliferous mineralization and kaolinization (Manning and Exley, 1984).

Field relations, textures and composition have, in the past, been used to suggest either a 'pneumatolytic' (Ussher et al., 1909) or 'magmatic' (Collins and Coon, 1914) origin for the rocks of the Beacon, but Manning (1981) and Pichavant and Manning (1984) have concluded, from fluid-inclusion and other experimental data, that the rock was formed by complex hydrothermal processes.

Description

The rocks exposed at St Mewan make up a line of low crags along the south-facing slope. Storage tanks now occupy a small quarry at the western end, from which rock was formerly taken to pave grinding mills for china stone.

For the most part, the rocks are equigranular, fine- to medium-grained and made up of quartz and topaz with subordinate tourmaline, but banded quartz–tourmaline rock occurs in the southern side of the quarry, the banding dipping at about 40° to the south. The suite forms a contact facies between the main part of the granite, which is very kaolinized here, and its country rock consisting of tourmalinized pelites, semipelites and psammites of the Lower Devonian Meadfoot Group (Collins and Coon, 1914).

Interpretation

In addition to the quartz, topaz and tourmaline, the rocks of the Beacon contain accessory muscovite (sometimes as a replacement for topaz), apatite and opaque ore. The proportions of the main minerals vary to give rocks which may be very quartz- or tourmaline-rich, especially near the margins of the outcrop, but the average composition is about 60% quartz, 25% topaz and 15% tourmaline. They therefore fit into the St Austell sequence after the main intrusions and metasomatism, and before the main post-magmatic tourmalinization (between Stage III and Stage IIIb of (Table 2.2)). However, not only are these unusual rocks very hard, but experiments on melting relations and fluid-inclusion composition suggest that they are too refractory to have been produced from a straightforward magmatic melt, although they could have crystallized in equilib­rium with saline hydrothermal fluid at about 620°C (Manning, 1981). The latter would link them to the hydrothermal (i.e. high-temperature, low-pressure) mineralization stage; occasional exposures of comparable rocks are found in clay workings and mines, although these are seldom long-lived enough and accessible enough to be examined. Manning (1981) concludes that 'multi­stage and complex processes' were involved, but that more work is required on stability relations in highly saline systems containing B, F and OH in order to advance knowledge of these systems and processes further. The commencement of such work is reported in Pichavant and Manning (1984), where it is concluded that in the H₂O-saturated system Q_z–Ab–Or–H₂O added B parti­tions into the vapour phase while added F partitions into the melt, and that added B effects little change in the minimum melt compositions.

St Mewan Beacon provides a rare chance to see an unusual topaz-rich rock of high-temperature hydrothermal origin arising in the change from late- to post-magmatic conditions and providing a link in the evolutionary continuum. It is unlikely to have crystallized from an ordinary melt, and is probably the result of interaction between magma and a volatile phase rich in F, B and OH.

Conclusions

St Mewan Beacon consists of an igneous rock made up predominantly of the minerals: quartz, tourmaline and topaz, believed to have formed within the topmost portion (roof) of part of the St Austell granite intrusion. It has been suggested that it formed by the modification of solidifying granite magma by hot (hydrothermal) solutions containing fluorine, boron and water, through the alteration and reorganization of the chemistry and mineral content of the crystallizing granite. The fluids were a legacy of the waning igneous activity which formed the Cornubian granites. The site provides a rare chance to see an unusual rock of high-temperature origin arising in the change from late- to post-magmatic conditions and providing a link in the evolutionary continuum.

References