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Geology of the country around Penzance. Memoir for 1:50 000 geological sheets 351 and 358 (England and Wales)

A J J Goode and R T Taylor

Bibliographical reference: Goode, A J J, and Taylor, R T. 1988. Geology of the country around Penzance. Memoir of the British Geological Survey, sheets 351 and 358 (England and Wales).

British Geological Survey. Natural Environment Research Council.

London: Her Majesty's Stationery Office 1988. © Crown copyright 1988 First published 1988. ISBN 0 11 884388 5. Printed in the United Kingdom for HMSO Dd 240430 C20 6/88 398 12521

(Front cover)

(Rear cover)

Preface

Work in the district by the Geological Survey was originally undertaken by its founder Sir Henry T de la Beche, whose mapping was published on Old Series Sheet 33 in 1839. A resurvey at the six-inch scale by E E L Dixon, D A MacAlister, C Reid and B S N Wilkinson in 1898–1903 was published in conjunction with a memoir in 1907. A second resurvey was carried out at the same scale by Mr A J J Goode, Dr R T Taylor and Dr A C Wilson in 1970–76 under Mr G Bisson, the District Geologist, and the map was published in 1984.

This memoir was compiled by Mr A J J Goode and edited by Dr P M Allen. Dr J Hawkes and Mr J Dangerfield provided petrographical descriptions of igneous and metamorphic rocks. Dr R C Scrivener contributed notes on fluid inclusions. Photographs of the district, now registered in the BGS photographic collections at Keyworth, were taken by C J Jeffery and H J Evans.

F G Larminie Director, British Geological Survey Keyworth, Nottingham NG12 5GG. 25 March 1988

Notes

Geology of the country around Penzance—summary

The district around Penzance (Sheets 351 and 358) has been exploited for tin and copper since the Bronze Age. Only the Geevor tin mine was still open in 1986, but the vigorous development of the mining industry, particularly in the 18th and 19th centuries, has always been the stimulus here for geological investigation. The district featured on Maton's geological map of Cornwall in 1797 and accounts, both of the minerals and granite, were published around that time. In this memoir, descriptions of the mineralisation, mineral deposits and other aspects of the economic geology are important features.

The oldest rocks in the district are the Devonian Gramscatho Beds and Mylor Slates. The sedimentary successions, and the metabasic igneous rocks (commonly known as greenstones) contemporaneous with the Mylor Slates, are described and an account is given of their folding and faulting during the Variscan orogeny. Geologically and topographically the district is dominated by the Land's End Granite. This and the Tregonning–Godolphin Granite were emplaced during the late Carboniferous. They are described in detail and an account is given of their metamorphic aureoles. Other intrusive rocks described include basic rocks of uncertain association, but which are older than the granites, and dykes of lamprophyre, aplite, quartz-feldspar-porphyry (elvan) and intrusive breccias. The youngest igneous rock in the area is the early Cretaceous phonolite of Wolf Rock. Small outcrops of Pliocene St Erth Beds are described and there is a full account of the Quaternary deposits.

(Geological succession)

Geological succession in the Penzance district

Chapter 1 Introduction

Geographical setting

This memoir describes the geology of the part of southwestern Cornwall covered by 1:50 000 sheets Penzance (351) and Land's End (358) (Figure 1). The area extends from Land's End to Godrevy Point on the north coast and to Trewavas Head on the south coast, and includes Wolf Rock, some 13 km to the south-west of Gwennap Head.

The area is dominated by the Land's End Granite, which occupies about 60 per cent of the district (Figure 2). The rest of the land area consists of Devonian sediments and minor contemporaneous igneous rocks, and a swarm of minor intrusions, Carboniferous to Permian in age.

The broadly reniform shape of the Land's End Granite determines the general form of the Penwith peninsula bounded by steep cliffs, commonly 50 to 90 m high. The development of three sets of joints in the granite has enabled it to part into cuboidal blocks from which are sculpted the castellated cliffs of Land's End (Plate 1), (Frontispiece); (Plate 2). Behind the peninsula, St Ives and Mount's Bays have been carved by marine erosion in the softer and less resistant sedimentary rocks. Inland, much of the area is a dissected and degraded plateau at about 130 m above OD. The plateau is clearly defined west of St Ives, abutting the granite ridge of rounded summits running south-westwards to St Just and rising to 200 m above OD. Watch Croft, on this ridge, forms the highest point, at 252 m above OD, in the district. Small, sporadic granite tors break an otherwise uninterrupted skyline in this area (Plate 3). Much of the land to the east of the Penwith peninsula is below 100 m above OD, but the twin hills of Godolphin and Tregonning rise to 162 m and 194 m respectively.

Streams draining the Land's End Granite flow mainly along NW–SE valleys following faults or joints. The short, north-westerly-flowing streams have steep, apparently ungraded thalwegs, whereas the longer streams flowing south-eastwards have some graded reaches. The area to the south and east of Hayle is drained by the River Hayle and its tributaries, which flow northwards into St Ives Bay. The Red River drains a considerable area farther east towards Camborne and Redruth and enters the sea 1 km to the south of Godrevy Point. Structural controls upon drainage patterns are not obvious on the outcrop of the Devonian rocks.

Man and industry

The Land's End peninsula is endowed with numerous prehistoric sites. People of the Palaeolithic (c.250 000–c.8000 BC) and Mesolithic (c.8000–c.3500 BC) periods are known only from scattered implements. In the Neolithic period (c.3500–c.2000 BC), however, settlements, walled fields, ritual structures and tombs were built from local granite slabs and other local stone was used to make polished axes. The latter were sufficiently prized to be traded outside the area along with pottery made from clay thought to have come from the Lizard peninsula. The building of large megalithic tombs ceased in the Bronze Age (c.2000–c.700 BC) and interment, usually preceded by cremation, took place in a cist of stone slab box concealed beneath a mound of earth known as a barrow or tumulus. It was also in the Bronze Age, probably after 1000 BC, that European immigrants are thought to have brought the knowledge of metal to the region and subsequently began the exploitation of local metallic ores, notably of copper and tin. The Romans (43–410 AD) undoubtedly traded for metal in Cornwall.

The production of copper, after its heyday between 1830 and 1860, began to slump in the 1860's followed by tin in the 1870's and since the last war Geevor Tin Mine at Pendeen has been the only producing mine in the district. More easily worked sources of metal overseas had caused prices to fall, putting the Cornish mines with their narrow lodes out of business, a situation which has again been highlighted by the international tin crisis of 1985–86 and the subsequent closure of Geevor mine.

In recent years some attention has been given to reworking the mine dumps, but as long ago as 1908 a company was formed to hand-pick pitchblende from the mine dumps at Wheal Trenwith where it had previously been discarded as an impurity in copper ore. Work lasted for four years and the ore was delivered to Madame Curie in France.

On Tregonning Hill, the earliest china clay workings in Cornwall were started in 1746 by William Cookworthy, a Plymouth apothecary attempting to simulate Chinese porcelain.

Beach sand is still worked from the towans at Gwithian but the practice of using shell sand and seaweed to improve the acid granite soils appears largely to have died out. Much quarrying has taken place over the years, mainly hard rocks for building and roadstone. Penlee Quarry remains open, producing aggregate, and Castle an Dinas Quarry is worked intermittently.

Unlike fishing and mining, farming, the third traditionally important industry in Cornwall, still flourishes. The sedimentary and volcanic rocks of the Penzance and Marazion areas have produced good soils, which in conjunction with a mild climate have given rise to a thriving market-gardening industry. Soils on the granite are not as good, but produce some cereal and some good pasture land.

Penzance is the principal town of this part of Cornwall and has its origins in a small fishing port offering protection from westerly gales. Early associations with mining led, in 1663, to Penzance being appointed a 'coinage' town, where smelted tin was sampled to check its purity. In practice a ''coin' or corner of the rectangular ingots was removed by officials for assay. In the nineteenth century Penzance became a stannary town. The requisites and produce of local mining passed through the port until well into the twentieth century. The Royal Geological Society of Cornwall was founded in 1814 and is still in existence, administering the local geological museum. Many specimens have been presented to the museum by 'Cousin Jack' miners who spread all over the world, 'wherever there was a hole in the ground', and sent geological specimens back home to Penzance.

St Ives has long been a fishing port, but tin and copper ore were shipped from the Pier, built in 1767–70 by Smeaton.

The landscape of a large proportion of the area has been dominated by the building of mine engine houses, now standing gaunt and roofless, their Cornish steam engines long since dismantled. Cornish engines succeeded earlier machines by Newcomen and Watt but it is unlikely that any engine houses of this period still survive. At the Crowns, Botallack, two of the most spectactularly situated examples cling to the cliffs, their tapered red brick chimneys contrasting with the drab hues of the cliff and the ocean. Most of the mining villages consist of small, randomly sited cottages constructed from the cheapest materials; granite boulders cleared from the field, cob made from local head deposits or stone from mine waste. In marked contrast are the well constructed cottages of the planned industrial village of Halsetown, St Ives, built in the 1830s. James Halse, MP, solicitor and mining venturer built it to house his workers and thus secure their franchise under the 1832 Reform Act and retain his Parliamentary seat of St Ives Borough. Roofs are mainly slated. Some cottage walls have been slate hung for additional waterproofing against the driving rain of the Penwith winters. The attraction of some villages lies in the colourwashing of the cottages which overpowers the dominant greyness of the stone-walls and the surrounding granite.

History of research

The growth of geological knowledge in Cornwall went hand-in-hand with progress in the mining industry. Early accounts include those of Borlase (1758) and Pryce (1778) on minerals, and there are geological maps of Cornwall by Maton (1797), Forbes (1822), Carne (1822) and Boase (1832). The Royal Geological Society of Cornwall was founded in 1814 with the Prince of Wales, later King George IV, as its patron. Sir Henry De la Beche, who founded the Geological Survey in 1835 and was its first Director, surveyed Cornwall, Devon and part of Somerset and described the geology in the Survey's first maps and memoir, published in 1839. At about that time, Sedgwick, Murchison and Lonsdale defined the Devonian System, which subsequently replaced the 'grauwacke' of De la Beche on the revised Geological Survey map of 1846. Smyth inserted additional lodes on a later (1866) edition of this map. The present sheet area was resurveyed on the six-inch scale by C Reid, B S N Wilkinson, E E L Dixon and D A MacAlister, and the one-inch scale map with memoir published in 1907. On this map the Devonian rocks were subdivided into the Mylor, Portscatho and Falmouth Series.

De la Beche (1839) was the first to make correlations between the Devonian successions in south-west England and the Harz Mountains in Germany. More recent attempts to understand the Devonian sedimentary histories and Variscan structures of these areas have been made by Hendriks (1937), Matthews (1977, 1978) and Holder and Leveridge (1986a,b). Hendriks, in particular, contributed much in publications between 1931 and 1970 towards the understanding of the structure and stratigraphy of southwest Cornwall.

The igneous rocks of the district have been studied by numerous workers. The Land's End Granite was described by Davy (1818), Carne (1822, 1828), and Forbes (1822) and was mapped by De la Beche (1839). Boase, (1832) recorded the fine-grained granite of Castle an Dinas, which Reid and Flett (1907) concluded had been intruded into the main granite. Bagchi (1947) suggested that the megacrysts of potassium feldspar in the Land's End Granite were porphyroblasts and resulted from growth in the solid state by alkali metasomatism. Stone and Austin (1961) confirmed this late potassium metasomatism.

Floyd (1976, 1983) has extensively examined the metabasic igneous rocks. The petrology and petrogenesis of the granite have been described by Exley and Stone (1966), Exley and others (1983) and Hawkes and Dangerfield (1978). Gravity and magnetic surveys by Bott (1958) have led to the interpretation of the shape of the Cornubian batholith.

Henwood (1843), Hunt (1884) and Collins (1912) wrote accounts of Cornish mineralisation. The Geological Survey memoir, 'The metalliferous mining region of south-west England', by Dines (1956) gives details of most of the known mines. More recent accounts of Cornish mineralisation include the works of Hosking (1966, 1969), and reviews by Jackson (1979) and Thorne and Edwards (1985).

A concise account of the geology of south-west England by E A Edmonds and others (1975) is published in the series British Regional Geology.

Geological setting

The sedimentary rocks and contemporaneous basic lavas of the district were folded during the Variscan orogeny in the late Devonian–early Carboniferous, and were intruded in the late Carboniferous by the Land's End and Tregonning–Godolphin granites.

The sedimentary rocks have yielded no dateable fossils. Palynological evidence from the adjacent Falmouth district however, indicates that they probably range in age from middle to late Devonian, and possibly into the early Carboniferous. The oldest rocks exposed in the district are the Gramscatho Beds, consisting of medium- to coarse-grained, turbiditic sandstone in beds up to 2 m thick within an argillaceous sequence. The succeeding Mylor Slates consist predominantly of dark grey slate, usually with silty laminae. Slump beds and sedimentary breccias are present locally and are indicative of sea-floor instability. Basic volcanism occurred during the deposition of the Mylor Slates, giving rise to pillow lavas and associated sills and dykes with minor amounts of tuff and agglomerate.

The district was subjected to several phases of deformation during the Variscan orogeny. The major phases post-date the deposition of the Mylor Slates and had ceased by the time the granites were emplaced.

The Land's End and Tregonning–Godolphin granites were emplaced about 290 to 300 million years (Ma) ago. Younger veins of fine-grained granite and aplite commonly occur near their margins. Alteration of the granites in the form of tourmalinisation, greisenisation and kaolinisation is fairly widespread.

Quartz-feldspar-porphyry (Elvan) dykes, emplaced mainly in the Mylor Slates at about 280 to 285 Ma, represent a later phase of acid igneous intrusion. They were intruded along a fracture system that formed as the granite cooled. Metalliferous mineralisation that gave rise to the economic deposits of tin and copper resulted from hydrothermal fluids circulating in the same fracture system. Mineralisation took place at intervals from the Permian to the Cretaceous.

No Mesozoic sedimentary rocks are preserved in the district. Triassic and Cretaceous rocks occur offshore and may formerly have covered the peninsula. The St Erth Beds consist of gravels, sands and clays with a marine fauna that suggests a late Pliocene age.

During the Pleistocene period, ice cover did not extend into this area though sea ice reached the Isles of Scilly and parts of the west Cornwall coast. Fluctuating climatic conditions at this time brought about constantly changing sea levels. Raised beaches were formed during interglacial periods and remnants of the 5 to 8 m beach are commonly seen around the coast beneath head deposits. Several examples of a 15 to 20 m beach are recorded and another occurs at about 30 m. Head deposits, which were formed under permafrost conditions from earlier weathering products by solifluction processes, are widespread. A generalised geological sequence of the district is shown on the inside front cover.

Chapter 2 Devonian rocks

Two sedimentary rock formations are distinguished on the Penzance sheet: the Gramscatho Beds of interbedded sandstone and slate occur in the north, whereas, the Mylor Slates, in which sandstone is rare, occupy the central and southern parts of the district.

The interbedded sandstone and slate of the St Ives Bay area have been called the Portscatho Series, the Falmouth Series and the Hayle Sandstones by the Geological Survey. Hendriks (1931) considered that the Portscatho Series was inseparable from the Grampound and Probus Series of the Falmouth district, and adopted the compound term Gramscatho to describe them. Early Survey workers thought that the rocks, now described as Gramscatho Beds, were Lower Palaeozoic in age. Lang (1929) dated plant remains (Dadoxylon hendriksi) from the Gramscatho Beds of the neighbouring district as Middle Devonian. Le Gall and others (1985) have described spores and acritarchs of Frasnian age from the Portscatho Series north of the Lizard, now included in the Gramscatho Beds.

The Mylor Slates were named by the Geological Survey in 1898 and were also thought to be Lower Palaeozoic in age. An early Devonian age was proposed by Hendriks (1939). Spores and acritarchs from near the base of the Mylor Slates in a borehole at Mount Wellington, St Day, indicate a Famennian age (Turner and others, 1979).

Rocks exposed in the coastal section from Navax Point [SW 593 437] westwards and southwards to the Red River were mapped in the 1984 survey as Mylor Slates. They are a variable succession of slate with siltstone, thinly bedded fine-grained sandstones and some coarse, graded, turbiditic sandstones (Plate 4).

After re-examination in the light of further work in the Falmouth district to the east, these rocks are now thought to be part of the Gramscatho Beds. It is possible that they represent a transition between the more sandy part of the Gramscatho Beds and the Mylor Slates.

Flett (1946) considered the Mylor Slates to be older than the Gramscatho Beds. The relationship between them is difficult to determine in the present district because their boundaries are usually either faulted or not exposed. To the south-east of Navax Point the sandstone succession structurally overlies the rocks interpreted above as transitional between the two formations, although the sandstones are inverted near the junction. Inverted sandstones are present in Fishing Cove [SW 5955 4285], which suggests that the predominantly pelitic part of the succession here and the overlying sandstone to the east is also inverted, but the full extent of possible folding is not visible.

The BGS borehole at Parbola [SW 6157 3637] (Beer and others, 1975), penetrated about 300 m of dark grey Mylor Slates, including several intrusions of elvan, before passing into a succession of fine-grained, thinly bedded sandstone and interlaminated sandstone and slate. Some graded sandstone beds are present but these indicate frequent inversion of bedding resulting from small-scale folding, and do not provide evidence for the way up of the succession as a whole. Quartzitic sandstone was recorded at the bottom of the hole. A second borehole, CTL26 [SW 5998 3206], drilled about 500 m south-east of Godolphin Bridge encountered sandy-laminated and silty-laminated slate and fine-grained sandstone, at a vertical depth of about 100 m beneath uniform slates. The sandstone successions encountered in both of these boreholes do not have a close resemblance to those exposed on the coast, but in the absence of any evidence of regional inversion it appears that the Mylor Slates are probably younger than the Gramscatho Beds.

Gramscatho Beds (GrO)

The Gramscatho Beds consist of interbedded sandstone and medium to dark grey slate and siltstone. The sandstone beds are light to medium grey. They vary in thickness mostly from 0.1 to 1.0 m, with some beds up to 2.0 m. The sandstone beds are commonly graded. In places, the bases are very coarse, with grains up to 3 mm, and locally mud-flake conglomerates. The beds grade up into fine-grained, laminated and ripple cross-laminated tops. In some cases the tops of the beds are convoluted. The thicker beds are usually composite, being made up of two or more graded units with little or no pelitic material separating them. The lower surfaces of the beds, which do not part readily because of the effects of folding and metamorphism, are seldom seen. Sediment injection structures and lode casts are seen in sections through the beds. The structures in these beds are typical of those formed during deposition from turbidity currents.

The sandstone beds are greywacke in composition. They are predominantly composed of irregular quartz grains, but also contain lithic fragments. Most commonly these are fine-grained quartzitic fragments with highly sutured grain boundaries indicative of a metamorphic source-rock. Occasional granophyric igneous fragments also occur. Plagioclase grains are common, but do not predominate over the lithic fragments in most of the rocks examined. A few grains of detrital brown tourmaline and opaque ore minerals are also found. The original muddy matrix has recrystallised to a fine-grained intergrowth of chlorite and white mica.

The Gramscatho Beds are particularly well exposed at Lelant Towans [SW 540 302] and Black Cliff [SW 554 388]. Very similar beds to these crop out at [SW 577 413] near Peters Point, where they are faulted against slates, and on Ceres Rock [SW 574 414].

The sandstone beds south-east of Navax Point generally resemble those at Black Cliff. They are mainly graded greywacke beds, usually less than 0.5 m thick. The tops of the beds are commonly laminated, but some show convolution. They are separated by thin pelitic partings. The sandstone beds are finer grained than those at Black Cliff and are lithic greywackes.

The triangular group of rocks centred around [SW 563 446], are exposed at low tide about 2 km north-west of Godrevy Point and are called 'The Stones' on Admiralty Charts. They were described on the previous edition of the Penzance sheet as 'Rocks of Hard Sandstone' and are probably an outcrop of the Gramscatho Beds.

Although current directions cannot be determined from the turbidite sandstones, Wilson and Taylor (1976) suggested that much of the detritus was derived from a massif that lay to the south of the district beneath what is now the English Channel.

A comparison of Devonian stratigraphy and Variscan structure in south-west England with parts of Europe has been made by Holder and Leveridge (1986b). They suggested that uplift during the Variscan orogeny in both of these regions gave rise to similar flysch-type successions. They argued that the uplifted massif to the south of Cornwall, the existence of which was inferred by Wilson and Taylor (1976), was created by a succession of northward overthrust nappes of crystalline basement and Lower Palaeozoic rocks. Holder and Leveridge (1986a) described it as the Normannian High and correlated it with the Mid-German Crystalline Rise, a similar belt of northerly overthrust, older rocks.

Mylor Slates (MrS1)

The Mylor Slates exposed on the coast are variable grey-green, blue-grey or dark grey to black slate, commonly with pale grey or white silty laminae up to a few millimetres thick (Plate 5). Rare impersistent sandstone beds up to 0.1 m thick also occur. Pale buff-weathering slates at the eastern end of Puffer Cliff [SW 5600 2817] contain much more white mica, and less chlorite and opaque minerals than the slates elsewhere.

The Mylor Slates are interbedded with basic lava, tuff and agglomerate. The highly vesicular and angular lava fragments of the agglomerate interbedded with slate on Great Hogus (p.11) suggest maximum water depths of 100 to 200 m. (Allen, 1980; Jones, 1970).

Sedimentary breccia is widespread in the southern part of the outcrop of the Mylor Slates. It is well exposed around Stackhouse Cove [SW 5454 2859] to [SW 5495 2815] in Mount's Bay, and is particularly prominent just west of Long Zawn [SW 5466 2850]. Sedimentary breccia also occurs in the slate north of St Michael's Mount [SW 5176 3042]. On the north coast, a slumped horizon about 100 m thick crops out in the cliffs on the east side of Fishing Cove [SW 5970 4285].

The breccia is characterised by the streaked and lensoid form of the silty component, and the isolated lenses of fine-grained and medium-grained sandstone, set in a silty pelitic matrix. Some sandstone lenses at Long Zawn are over 1.0 m long and 0.3 m in thickness. The sandstone clasts in these rocks contain meta-quartzite and igneous fragments similar to the Gramscatho Beds sandstones, but the proportions of lithic fragments, feldspar, and chlorite/white mica matrix are lower and the rocks approach lithic arenite in composition.

The sedimentary breccias are believed to have formed by slumping of the sediment soon after deposition rather than by the tectonic disruption of the beds. Several features of these rocks, when considered together, indicate that they are not of tectonic origin. Many of the sandstone clasts are isolated from one another in a random fashion, which cannot be related to tectonic disruption of a sandstone bed. Some clasts have an irregular shape, when viewed normal to the bedding, with no indications of tectonic alignment and horizons of undisrupted laminated slate occur within the slumped succession, which also points to the contem poraneous and episodic nature of the slumping. It is possible that some of the smaller, isolated masses of metabasic rock and pillow lava, such as those at Stackhouse Cliff [SW 5478 2843] and Piskies Cove [SW 5542 2776], have also been emplaced by slumping.

The Mylor Slates intercalated with the metabasic rocks along the northern margin of the Land's End Granite are intensely deformed and affected by later contact metamorphism. Faint traces of silty lamination define the folds. Pale silty lenses and flecks in cordierite-bearing hornfelsed slate on the cliff at Zawn a Bal [SW 3630 3332] and silty lenses in hornfelsed slate south of Porthmeor Point [SW 4244 3785] are similar to the sedimentary breccia in Mount's Bay. The isolated exposures of hornfelsed slate around the Land's End Granite at Cowloe [SW 350 266], on the Longships [SW 320 254] and on the Outer Bucks [SW 4428 2285] off Tater-du indicate that the granite is entirely emplaced within the Mylor Slates.

The slate exposed in the cliffs at Gwithian Towans [SW 578 415] is soft, medium to dark grey with fine silty lamination and some thin beds of laminated sandstone. Small lenses of fine sandstone appear to be starved ripples. In some places these appear to have foundered into the underlying layer. These relatively well preserved sedimentary features have not been recognised in other parts of the Mylor Slates. Although these rocks have undergone the same sequence of folding as the slates elsewhere, they are softer and less well cleaved and seem to be of a slightly lower metamorphic grade. The relationship of the slate at Gwithian to the rest of the Mylor Slates outcrop is obscured by blown sand. It is likely that faulting occurs in the vicinity of the mouth of the Red River separating these rocks from the Gramscatho Beds and the overlying transitional rocks around Navax Point (p.11). The Gwithian slates do not resemble the transitional beds and are of a lower metamorphic grade. It is possible, therefore, that they represent a different structural or even stratigraphical level within the Mylor Slates juxtaposed by thrusting.

The acritarch fauna from Mount Wellington indicates a marine origin for the Mylor Slates, but the sedimentary en vironment is poorly understood. The rocks were probably formed in an intracratonic basin. Thin silty beds may represent distal turbidites, but Holder and Leveridge (1986a) suggested that during the deposition of the Mylor Slates, the coarse-grained, southerly-derived detritus was trapped by a 'foredeep and bulge'. This warping of the sea floor was in response to tectonic crustal movement which also led to the formation of the sedimentary breccia.

The Devonian sedimentary rocks and their associated basic rocks were subjected to low-grade regional metamorphism during the Variscan orogenic episode and later to high-temperature contact metamorphism around the granites. Regional and contact metamorphic mineral assemblages are discussed on pages 11, 17 and 22.

Chapter 3 Metabasic rocks

Extrusive and intrusive rocks of basaltic composition that have undergone low-grade regional metamorphism during the Variscan orogeny have been called 'greenstone' since the late nineteenth century by the Geological Survey. Many derive their green colour from secondary minerals such as actinolite and chlorite. Harder and less altered rocks are locally referred to as 'blue elvan', 'ire-stone' or 'iron-stone'. The extrusive rocks include lavas, tuff and agglomerate, contemporaneous with the late Devonian Mylor Slates. The intrusive rocks are mainly dolerite, locally becoming gabbroic, and are thought to be of the same age as the lavas. In inland exposures it is rarely possible to differentiate between extrusive and intrusive greenstones.

The metabasic rocks are concentrated in a well defined zone extending from Penzance to Camborne, but there are coastal exposures to the south and north. Pillow lavas, agglomerates and metabasic intrusions are present on the south coast, to the south-east of Marazion and near Mousehole. On the north coast, between the Brisons and St Ives, a zone mainly of pillow lavas and contemporaneous sills may represent part of the Penzance to Camborne belt, displaced by the intrusion of the Land's End Granite.

The metabasic rocks contain a variety of minerals including sodic plagioclase, colourless and pale mauve pyroxenes, actinolitic amphibole, chlorite, epidote, ilmenite, sphene, calcite and apatite.

Since the work of Dewey and Flett (1911), all Cornish sodium-rich greenstones have been regarded as spilites. Floyd (1976) examined the geochemistry of the metabasic rocks and confirmed that outside the granite aureole they had undergone low-grade hydrous alteration ('spilitisation') causing the loss of Ca relative to Na, the loss of K, Ba, Rb and Sr and the uptake of H2O. Elements such as Ti, Y, Nb, Zr and P, which are not readily mobilised during alteration and metamorphism, indicate that the metabasic rocks of the Penzance area are predominantly continental tholeiites, although alkali basalt lavas may be present locally (Floyd, 1976). likely, therefore, that these sills are derived from the same magma source as the lavas, but were intruded into sea-floor sediment where they cooled slowly and developed a coarser texture. A large intrusion exposed in Penlee (formerly Gwavas) Quarry [SW 468 278] is uniform in appearance throughout its considerable thickness. It is a medium- to fine-grained dolerite, which has been contact metamorphosed and is now a hornblende-plagioclase (albite)-hornfels with some biotite. This body appears to thin to the south-west and south-east and the upper surface is broadly conformable with bedding and cleavage, which dip away from the Land's End Granite contact. The intrusion probably has a laccolithic form, and features near the upper contact, such as autobrecciation and traces of vesicularity, suggest that the magma was intruded into wet sediment. The fine grain-size also suggests relatively rapid cooling and higher level intrusion than the younger basic sills. It contrasts with the thinner gabbroic intrusions at Cudden Point and the coarser, apparently younger, gabbroic intrusions exposed between Penlee and Mousehole, against which it is faulted [SW 466 275] north of Paul. The Cudden Point intrusion and the smaller intrusion at the western end of Kenneggy Sands are sill-like in form and contain rafts of stoped sedimentary rocks.

At Temis Cove [SW 535 293] a sill appears to cut across pillow lava. Both are relatively undeformed and retain original igneous ophitic textures and relict clinopyroxene in places, although they may be sheared at the contacts.

Sedimentary rocks adjacent to the contacts of some of the metabasic rocks are greyish white and adinolised in zones that can exceed 1 m in width. These rocks are composed essentially of albite, quartz and chlorite. Adinoles are usually formed around the intrusive basic rocks, but are also found at the contacts of some lavas, where they result from the interactions between the sediment with contained salt water and the hot basic magma. The adinoles are enriched in Na and depleted in K, Fe and Mg compared with unaltered slates.

Intrusive rocks

Intrusive rocks fall into two main groups; those which appear to be broadly contemporaneous with the volcanic rocks and those with more clearly intrusive relationships, which appear to post-date the volcanic rocks. Both groups pre-date the last deformation of the Variscan orogeny and commonly have an associated penetrative cleavage.

Near St Ives there are many metabasic sills that were originally dolerite and gabbro. Although mainly uniform and massive, some of these sills appear to pass up into large crude pillow structures and have amygdaloidal, ragged or autobrecciated margins. Such features form when magma is injected into soft, wet sediments just below the sea floor. It is likely , therefore, that these sills are derived from the same magma source as the lavas, but were intruded into sea-floor sediment where they cooled slowly and developed a coarser texture. A large intrusion exposed in Penlee (formerly Gwavas) Quarry [SW 468 278] is uniform in appearance throughout its considerable thickness. It is a medium- to fine-grained dolerite, which has been contact metamorphosed and is now a hornblende-plagioclase (albite)hornfels with some biotite. This body appears to thin to the south-west and south-east and the upper surface is broadly conformable with bedding and cleavage, which dip away from the Land's End Granite contact. The intrusion probably has a laccolithic form , and features near the upper contact, such as autobrecciation and traces of vesicularity, suggest that the magma was intruded into wet sediment. The fine grain-size also suggests relatively rapid cooling and higher level intrusion than the younger basic sills. It contrasts with the thinner gabbroic intrusions at Cudden Point and the coarser, apparently younger, gabbroic intrusions exposed between Penlee and Mousehole, against which it is faulted [SW 466 275] north of Paul. The Cudden Point intrusion and the smaller intrusion at the western end of Kenneggy Sands are sill-like in form and contain rafts of stoped sedimentary rocks .

At Temis Cove [SW 535 293] a sill appears to cut across pillow lava. Both are relatively undeformed and retain original igneous ophitic textures and relict clinopyroxene in places, although they may be sheared at the contacts.

Sedimentary rocks adjacent to the contacts of some of the metabasic rocks are greyish white and adinolised in zones that can exceed 1 m in width. These rocks are composed essentially of albite, quartz and chlorite. Adinoles are usually formed around the intrusive basic rocks, but are also found at the contacts of some lavas , where they result from the interactions between the sediment with contained salt water and the hot basic magma. The adinoles are enriched in Na and depleted in K , Fe and Mg compared with unaltered slates.

Extrusive rocks

Pillow lavas are common along the north coast of the Land's End peninsula. They are amygdaloidal, with the vesicles now filled by calcite, chlorite and amphibole. The pillows were formed during submarine eruptions, when molten lava broke up into elongate, pillow-shaped bodies varying in size from 0.1 to 0.5 m wide and up to 2 m long. Individual pillows commonly remained plastic and were moulded upon underlying pillows, occasionally enclosing small amounts of sediment. The moulded lobes of lava thus provide evidence of the original way up (Plate 6).

Basaltic tuff and agglomerate have been recorded, usually in association with pillow lavas. They are best exposed on Great and Little Hogus rocks near Marazion [SW 5130 3040] and [SW 5115 3067] and they also occur on the foreshore of St Michael's Mount [SW 5165 3014]. On Great Hogus two agglomerate beds are interbedded with slate in a large westerly plunging anticline. The coarser grained of the agglomerate beds contains amygdaloidal basalt blocks 100 to 200 mm across, the largest up to 0.5 m long, which although flattened in the axial plane of the fold show irregular, angular outlines when viewed normal to the foliation. Beds of tuff and individual beds of smaller fragments (10 to 20 mm size range) within the agglomerates show grading and indicate that the fold is upward facing. The size of the largest clasts in the agglomerates suggests that they were not deposited far from a vent. Although the agglomerates appear to have been deposited in water, the high degree of vesicularity and angularity in some instances could indicate shallow water. On the south-east side of Great Hogus there is a lava formed of small pillows. Small pillows also occur at two localities around Zennor Head [SW 448 393] and [SW 451 393].

Thermal metamorphism and metasomatism of metabasic rocks

The metabasic rocks within the aureole of the Land's End Granite display a wide variety of thermal metamorphic assemblages. Early workers in this area had difficulty in deciding which hornfelses were of basic igneous origin and which were sedimentary. Although hornblende-bearing rocks were recognised, they were thought to have been derived from 'clay slates'. Tilley and Flett (1930) described the unusual anthophyllite/cordierite (Plate 7) and banded plagioclase-hornblende assemblages (Plate 8) in greenstones at Botallack. They initially believed that these rocks were weathered prior to contact metamorphism, but Tilley (1935) concluded that metasomatism associated with regional metamorphism accounted for these enigmatic rocks. Mobilisation of various elements, particularly Ca and Fe, occurred with the addition of water. The metabasic hornfelses of the Land's End aureole were considered by Floyd (1983) to be the result of essentially isochemical metamorphism of low-grade regional metamorphic assemblages, rather than metasomatism as suggested by Chinner and Fox (1974). At some localities, however, the emplacement of the granite was accompanied by the addition, under hydrous conditions, of K, Rb, Cs, B, F, U and the light rare earth elements resulting in the widespread development of biotite.

Thermally altered metabasic rocks of this area can be divided into three general groups:

i Hornblende-bearing rocks

ii Cummingtonite/anthophyllite-bearing rocks

iii Ca-rich pyroxene-bearing rocks

Hornblende-bearing rocks

Most greenstones belong to this group. Their spilitic mineralogy has been altered to hornblende-plagioclaseilmenite-sphene assemblages ± brown mica. Original igneous and pyroclastic textures remain largely intact except at localities in proximity to the granite.

Cummingtonite/anthophyllite-bearing rocks

These rocks are rare compared with the hornblende-bearing variety. The best exposures are at Kenidjack Cliff [SW 356 327] (Plate 7), and on the promontory between De Narrow Zawn and Zawn a Bal [SW 362 333], Botallack. Gradation into hornblendic varieties is evident at these localities.

Ca-rich pyroxene-bearing rocks

These rocks are slightly more common than those bearing cummingtonite/anthophyllite. They occur sporadically within the hornblendic types and as discordant sheet-like and vein-like bodies. The discordant nature and relatively coarse grain-size implies that they are of late origin. Alderton and Jackson (1978) supported a late age for the veins, suggesting that they formed during an early phase of hydrothermal mineralisation. Some of the vein minerals, notably grossular, amphibole, pyroxene and epidote, contain concentrations of tin, varying from 144 to 2233 parts per million (ppm). The sporadic occurrences are best seen at Great Cliff, Tater-du [SW 440 230], and the sheets and veins in the cliffs between The Crowns [SW 362 336] and Trewellard North Cliff [SW 374 352].

Chapter 4 Granite and contact metamorphism

The Land's End Granite and the smaller granites of Tregonning–Godolphin and St Michael's Mount are part of a chain of intrusions, interconnected at depth, stretching from the Isles of Scilly to Dartmoor. The whole plutonic complex is commonly referred to as the Cornubian batholith. Its emplacement post-dates the major tectonic events of the Variscan orogeny. Intrusions of similar age occur in Brittany, central and eastern France, Corsica, Sardinia, western Spain and Portugal. Gravity anomalies suggest that the granites of south-west England are joined by subsurface granite ridges (Bott and others, 1958), which have been confirmed locally by mineral exploration boreholes.

Field evidence from the well exposed granite contacts in the Penzance area indicates that the granite passively intruded the Devonian rocks by veining and block stoping. Partly digested xenoliths, common in some areas, together with sharply defined contacts suggest that the granite began to lose its ability to stope through the country rocks when it reached the relatively high crustal level now exposed at the surface.

There are two main types of granite in the district: biotite-granite and lithium-mica-granite, the latter being present only in the Tregonning intrusion.

Biotite-granite

Biotite-granite in the district has been divided into coarse-grained, medium- to coarse-grained, and fine-grained types with respective mean minimum matrix grain-sizes of 2 mm, 1 mm and less than 1 mm. Coarse-grained granite has megacrysts of potassium feldspar which vary in size and number (Figure 3). Fine-grained granite may have abundant megacrysts of biotite, feldspar and quartz or be deficient in megacrysts. The fine-grained granites have previously been described as intruding the coarse-grained granite (Reid and Flett, 1907), but limited field evidence suggests that the former occur as flat-lying, sheet-like enclaves.

Pegmatite and aplite are uncommon, mainly occurring as small sheets near the margins of intrusions. A small domelike offshoot of the Land's End Granite at Porthmeor is capped by sheets of pegmatite 0.5 to 1 m thick with potassium feldspar megacrysts measuring 0.30 X 0.12 m.

The principal minerals in all biotite-granite types are quartz, potassium feldspar, plagioclase and biotite. The potassium feldspar is orthoclase perthite, but in areas affected by mineralisation small amounts of microcline have been recorded. Both albite and oligoclase occur.

Schorlite is the most common accessory mineral, sporadically developed by complex partial replacements of the main minerals excluding quartz. Other accessory minerals include: monazite, zircon, apatite, topaz, anda lusite, rutile, anatase, brookite, fluorite, ilmenite, hematite, titanomagnetite, uraninite, lollingite, xenotime and garnet. Corundum (sapphire) and cordierite have been found in a xenolith below Carn Veslan [SW 4190 3715] (Hawkes, 1961). Wilson (1975b) described andalusite and sapphire from a xenolith on Trencrom Hill [SW 5176 3631].

Megacrystic coarse-grained granite

The Land's End Granite is formed exclusively of biotite-granite of which more than 80 per cent is coarse grained, with a variable proportion of orthoclase megacrysts up to 120 mm long (Figure 3). Crystals of greater than 15 mm in length range from sporadic individuals at Porth Nanven [SW 355 308] to 30 per cent of the granite on parts of Rosewall Hill [SW 494 392]. There is a simple linear relationship between the increasing mean length of megacrysts and mean volume per cent in the Land's End Granite.

Solid-state growth of quartz has produced numerous anhedral megacrysts, typically 5 to 10 mm and up to 20 mm in diameter. The mean minimum grain-size of the matrix is 2.4 mm. Slightly finer-grained variants show no clear field relationships and cannot be mapped separately.

Coarse- to medium-grained granite

From Godolphin Hill [SW 593 313] southwards to Rinsey Croft [SW 602 280] the biotite-granite is medium grained with small potassium feldspar megacrysts. The St Michael's Mount biotite-granite is coarse- to medium-grained and poorly megacrystic with relatively small megacrysts and much textural variation near the granite margin.

The Godolphin Granite differs texturally from the Land's End Granite in that potassium feldspar megacrysts are rare and are only about 20 mm long. There is a close resemblance to a fine-grained, marginal variety of the Land's End Granite found in the Mousehole area.

Fine-grained granite with megacrysts

The feldspar megacrysts, mainly of euhedral orthoclase perthite up to 125 mm long, typically form 1 to 12 per cent of the rock. Quartz megacrysts are anhedral, commonly 5 to 8 mm, but range up to 10 mm across. Biotite megacrysts are clots 4 to 7 mm across. The minimum mean matrix grain-size is 0.47 mm. In thin section the matrix quartz grains have a lobate, almost ramifying form, similar to that of the quartz megacrysts. The mean proportion of potassium feldspar is 4 or 5 per cent less than in the coarse-grained granite due to a larger proportion of mica. Allowing that 3 per cent muscovite was originally biotite, the biotite content was significantly higher than any of the other varieties of biotite-granite.

Biotite-rich fine-grained granite with megacrysts of biotite, feldspar and quartz occurs in parts of the Castle an Dinas and Chun enclaves, and also forms small bodies at Kenidjack Cam, Land's End and to the north of Treen [SW 395 240].

Some varieties of fine-grained granite are generally poor in megacrysts. These include aplogranite with feldspar megacrysts, ranging from 15 to 110 mm across and making up 1 to 2 per cent of the rock. Megacrysts of quartz up to 5 mm, biotite from 2 to 4 mm and pinitised cordierite, are fairly common. The minimum mean matrix grain-size is 0.54 m.

The largest enclave of fine-grained granite, some 12 square kilometres in extent, known as the Castle an Dinas Granite, is largely a biotite-poor variety with a few potassium feldspar megacrysts (Figure 3). Records from Wheal Georgia Consols c.[SW 487 365] suggest that 'the newer (fine-grained) granite was passed through at 60 fathoms from surface in Flatrod Shaft' (Reid and Flett, 1907). This supports other evidence that many of the fine-grained granites have a flat-lying form. Similar bodies have been recorded at Chun Castle [SW 405 339], Bartinney [SW 392 295] and Polgigga [SW 375 236].

An exposure of fine-grained, poorly megacrystic granite, previously mapped as elvan, appears to intrude the Land's End Granite at Pednvounder Beach [SW 393 224]. It is sheet-like with a sharp upper contact dipping 15° to the east and apparently encloses small masses of the surrounding megacrystic granite. The base of the sheet is not clearly defined, there being gradation into the main coarse-grained granite.

Lithium-mica-granite

Lithium-mica-granite has a hypidiomorphic texture and consists largely of anhedral quartz, euhedral to anhedral potassium feldspar, plagioclase and lithium mica. Accessory minerals include topaz, schorlite-tourmaline, amblygonite (Stone, 1975), apatite, fluorite, cassiterite, rutile, triplite and lollingite.

In the Tregonning intrusion the lithium-mica-granite forms a V-shaped outcrop enclosing the southern part of the Godolphin biotite-granite. It is a coarse, even-grained rock containing a pale brown, lithium-bearing mica. The potassium feldspar is orthoclase with minor amounts of microcline, and the composition of the plagioclase is close to albite. Aplite and pegmatite sheets are developed in the roof of the Tregonning Granite and are well exposed in cliffs near the contact with the surrounding slate. The contact between the Tregonning and Godolphin granites is not exposed. Granite debris on mine dumps [SW 5882 3006] shows diffuse veining of fine- to medium-grained biotite-granite by the coarser lithium-mica-granite. A quarry [SW 5998 3044] on Tregonning Hill shows an inclusion of Godolphin biotite-granite within lithium-mica-granite. Further evidence of lithium enrichment in biotite-granite from Lloyds Shaft [SW 5840 3125] also suggests that the lithium-mica-granite postdates the biotite-granite. Quartz-porphyry veins from Legereath Zawn [SW 607 267], said by Stone (1975) to be chemically similar to the Godolphin Granite, are cut by lithium-mica-granite.

Alteration

Tourmalinisation

Tourmaline is widespread in the Penzance district. A variety, yellow to brown in thin section, is present in all granite types as a primary mineral enclosed within biotite. Secondary tourmaline is invariably acicular and blue to blue-green in thin section; it occurs with quartz in hydrothermal veins or pervasively replacing feldspar and mica in granite (e.g. Trevalganite).

Greisenisation

Greisens are formed by the hydrothermal alteration of feldspar and biotite to quartz, muscovite, tourmaline and topaz. On St Michael's Mount steeply dipping greisen veins, consisting of aggregates of muscovite and mosaics of quartz with minor plagioclase, tourmaline and topaz, replace the granite.

Kaolinisation

The alteration of plagioclase to secondary mica or kaolinite and mica aggregates is widespread, but extensive alteration is restricted to four areas. The largest is a zone 1.5 km long and up to 200 m wide, trending NW–SE, located some 2 km east of St Just. It is worked at Bostraze, the only operating china clay pit in the district.

Intrusive age of the granite

In the Penzance district the granites intrude the late Devonian Mylor Slates, which were deformed by the Variscan orogeny. Late phase folds (F4) occur in slate xenoliths in granite veins associated with the Tregonning Granite. The relationship of contact metamorphic minerals to cleavages suggests that the granite post-dates the major orogenic episodes. After cooling the granite was cut by elvan dykes.

Isotope dating of granites in south-west England has yielded a wide range of ages. Dodson and Rex (1971) summarised the data, which indicate a spread of ages from 252 to 309 million years (Ma), with a concentration of dates between 276 and 286 Ma. Hawkes (1981) suggested that the Cornubian granites were intruded between 310 and 300 Ma, on the basis that they preceded lamprophyres of south-west England dated by the potassium-argon method at 296 ± 5 Ma (Rundle, 1980). Darbyshire and Shepherd (1985) using the rubidium-strontium method, gave a possible age of emplacement of the Cornubian granites of 290 to 280 Ma, but certainly no earlier than 295 million years. The lack of agreement among the isotopic ages may indicate that the emplacement of the Cornubian batholith was a multiphase process with the main granite intruded near the Carboniferous/Permian boundary, which is dated at 290 million years (Snelling, 1985).

The rubidium-strontium isotope dating of the Land's End Granite by Darbyshire and Shepherd (1985) has proved indecisive. A rubidium-strontium whole rock age of 268 ± 2 Ma and a mineral age of 272 ± 2 Ma are incomputable with other dates; notably 279 ± 4 Ma on muscovite from a greisen at Bostraze, 282 ± 6 Ma from the Wherry elvan and the potassium-argon date of 292.9 ± 3.4 Ma for the phlogopite from the Chyweeda lamprophyre dyke. The Chyweeda dyke belongs to a suite that probably post-dates granite emplacement. The similarity between granite age and the age of the main mineralisation may indicate that the isotopic ratios in the Land's End Granite has been adjusted by the mineralising fluids.

Origin of the granites

High initial 87Sr/86Sr ratios (0.710–0.716) (Darbyshire and Shepherd, 1985) and a high ratio of K2O to Na2O indicate that the Cornish granites as a whole are S-type granites derived from remobilised crustal rocks. Granitoid xenoliths with a pronounced gneissose texture have been recorded in a metabasic body at Botallack and provide a rare, albeit limited, insight into the basement of the Land's End area (Goode and Merriman, 1987). Floyd and others (1983), suggest that partial melting of a granulitic crust would produce large volumes of granitic magma. The fine-grained granite generally forms enclaves within the coarse-grained granites, but to the east of the district in the Carn Marth Granite, near Redruth, an irregular fine-grained enclave undoubtedly predates the main coarse-grained granite. Rounded xenoliths of fine-grained rock occur within the coarse-grained granite, veins of coarse-grained granite cut the fine, and volatiles trapped below enclaves of fine-grained granite have given rise to localised pegmatites. Near Porthcurno, a sheet of fine-grained granite appears to cut the main coarse-grained granite, but the lower margin appears to be gradational. It is suggested, therefore, that the rising granite plutons may have pushed out advance sheets and veins. Rapid chilling of these small bodies produced fine-grained granite bodies, only partially assimilated by the rising pluton.

The origin of lithium-mica-granite is not well understood. Stone (1975) believed that the evolutionary step from biotite-granite to lithium-mica-granite magma is not readily acceptable. Conventional in-situ differentiation within a magma chamber seems impossible on chemical grounds. If, however, an accumulation of volatiles occurred in the heated roof region of a biotite-granite mass, partial remelting might take place. Stone's concepts are supported by the experimental work of Manning (1981), who suggested that accumulations of lithium, boron and fluorine in a residual magmatic fluid would assist remelting of biotite-granite to react with the lithium to form lithium-mica-granite.

Contact or thermal metamorphism

The emplacement of the Cornubian granite batholith thermally metamorphosed the adjacent country rocks. The shape and size of the aureole depends on the subsurface extent of the batholith and on the chemical compositions of the country rocks. In the Penzance area the aureole varies from about 0.75 to 1.25 km in thickness. Thermal spotting occurs in the slates over much of their outcrop between the Land's End, Tregonning/Godolphin and Carnmenellis granites, indicating the presence of interconnecting ridges of granite at relatively shallow depths. The aureole of the St Michael's Mount Granite is very restricted, reflecting perhaps its size and the steepness of its margins.

In the Penzance area the dominantly pelitic rocks of the Mylor Slates were regionally metamorphosed during the Variscan orogeny to low greenschist facies with a typical mineral assemblage of chlorite-white-mica-quartz-albite. The first contact metamorphic mineral to appear in the pelites is andalusite, commonly accompanied by biotite. Closer to the contact, cordierite appears and forms much of the rock, which loses its fissility and becomes a hornfels. Corundum has been recorded from highly aluminous hornfels south of Carbis Bay in the Land's End Granite aureole.

The metamorphic mineral assemblages of the thermal aureole of the granites in this district suggest that these rocks now belong to the albite-epidote-hornfels-facies and the hornblende-hornfels-facies of contact metamorphism (Turner, 1948; 1968).

Xenoliths of sedimentary rocks are not uncommon in the Land's End Granite, but may be concentrated in swarms, as in the vicinity of Sennen. They tend to be biotite-rich and sedimentary structures are still clearly recognisable in some. Wilson (1975b) described sapphire (corundum) and andulusite from a xenolith on Trencrom Hill [SW 5176 3631] presumably formed from silica-depleted, aluminous country rock. Other minerals recorded elsewhere include sillimanite, cordierite and spinel. These minerals may have formed at higher temperatures at depth before rising to their present position and it is possible that some xenoliths may belong to the pyroxene-hornfels-facies.

Chapter 5 Minor igneous intrusions

Igneous activity continued at intervals in the Penzance district after the emplacement of the granites and gave rise to minor intrusions of aplite and lamprophyre, quartz-feldsparporphyry dykes locally known as 'elvans' and the phonolite forming the Wolf Rock. The intrusive breccias associated with some elvans are also described here, although they are not strictly igneous.

Aplite

Aplite veins occur in the Land's End Granite and penetrate the country rocks close to its contact. They are usually considered to represent a late magmatic phase related to the main granite intrusions. They commonly have sharp contacts with the granite, but some narrow aplite-like veins trending NW–SE in the Land's End Granite along the southern coast have transitional margins and appear to result from a reduction in grain-size of the granite during metasomatism associated with quartz-tourmaline veining.

Aplite veinlets occur in the hornfelsed dolerite of Gwavas Quarry [SW 468 278]. Tourmalinised aplite dykes are present on the south coast and at Boscawen Cliff [SW 422 230]. One such dyke, which is vertical, trends E–W and is 2.5 to 3.5 m wide. It has pegmatitic margins 0.1 to 0.5 m wide and contains dark bands of tourmaline. At Zawn Gamper [SW 4375 2315] steeply dipping dykes, 0.3 to 0.5 m wide, trending E–W, are completely tourmalinised although the surrounding granite is unaltered. Tourmaline disappears where the dykes pass into the Tater-du greenstone to the east. The dyke between Cam Barges [SW 446 233] and Lamorna Point [SW 450 236] trends approximately NE–SW and dips at 62° to the north-west. The upper, northerly margin contains euhedral feldspar megacrysts.

A vertical aplo-granite dyke aligned NNW–SSE intrudes the Tregonning Granite [SW 5986 2994] below Tregonning Hill. It is composed of albite, quartz and muscovite with accessory topaz.

Lamprophyre

Lamprophyre dykes occur sporadically in Cornwall and west Devon, commonly north-trending, although some dykes have other trends controlled by the local structure. The most westerly dyke, and the only one of this suite in the Penzance district, is a mica-lamprophyre about 0.5 m wide and trending 009° that crops out at Chyweeda [SW 612 326], near Binner Bridge. The dyke appears to have steeply inclined contacts and cuts across the structural trend of the slates.

The exposed rock is sheared and chloritised. A relatively fresh fragment shows the rock to be a medium-grained minette, consisting mainly of megacrysts of phlogopite mica with darker zones, probably biotite, at the edges. The groundmass is altered orthoclase, phlogopite and quartz.

A potassium-argon age determination on the phlogopite has given a mean age of 292.9 ± 3.4 Ma (Rundle, 1980), similar to ages from other lamprophyres in south-west England (p.16).

Quartz-feldspar-porphyry (elvan)

Acidic dyke rocks closely associated with the Cornubian granite batholith are commonly referred to by the Cornish mining term 'elvan'. The word has been suggested to originate from the Celtic–el = rock, van = white (Johannsen, 1938)–or, more probably, from the Cornish–elven = a spark (Arkell and Tomkeieff, 1953). The rocks are mostly quartz-feldspar-porphyries, with textural varieties described as felsite, rhyolitic elvan, granophyre or microgranite. The distribution of elvan dykes is broadly similar to that of the hydrothermal cassiterite-bearing veins.

The main trend of elvan dykes in the Penzance district is ENE–WSW with minor trends E–W and WNW–ESE (Figure 4). The Praa Sands elvan trends NW–SE. Dykes vary in width from 1 to 20 m; 10 m dykes are common. They usually dip to the north or north-west at 25–80°, but a few dip to the south. Many elvans have been traced for 1 to 2 km; a few may extend considerably further.

Fresh quartz-feldspar-porphyry is usually pale to medium grey in colour, but commonly weathers to creamy buff or pale brown. In mineralised areas, elvans are altered by hematitisation or chloritisation to shades of red and green respectively. The texture of the elvans varies from strongly porphyritic, with phenocrysts of potassium feldspar up to 30 mm long, to rhyolitic with flow-banding and folding. Some dykes, like the Praa Sands elvan, appear to be composite with an earlier fine-grained, flow-laminated phase at the margins, penetrated by a later, coarser megacrystic phase (Stone, 1968). Others show a transition between these two phases. The matrix grain-size of the elvans is variable with the centres ranging from 0.1 to 0.2 mm and the finer grained flow-laminated margins from 0.003 to 0.03 mm. The chief constituent minerals are quartz, orthoclase, albite, sericite/muscovite and biotite, the last commonly altered to chlorite. Some specimens contain pinite, a micaceous alteration of cordierite. Most elvans contain about 40 per cent quartz and 1 to 2 per cent biotite, but other constituents are more variable. The quantities of feldspar and muscovite/ sericite appear to vary inversely. Modal analysis of five specimens from the Wherry Elvan [SW 294 470], which contains more albite than most elvans, is:

Vol. %

Quartz

40

Orthoclase

28.5

Albite

17.5

Sericite/Muscovite

13

Chloritised biotite

1

Accessories

trace

The areal association of the elvans with the granites has in the past lead to their being considered as late-stage intrusions of residual granite magma. The elvans, however, are younger than the mantle-derived lamprophyres.

It has been suggested that the fissure systems controlling the elvan dykes and later mineral veins were related to the stress fields produced by the intrusion of the granites. However, the intervention of the lamprophyres, which requires a different regional stress field, means that the earlier fracture system would have had to be reactivated to allow the emplacement of the elvans and mineral veins.

Chemical analyses of the elvans show a range of Na2O/K2O ratios, with some dykes having normal granitic compositions but many having very low ratios of Na to K compared with the granites. The marginal phase of the Trenow elvan [SW 5265 3015], which invaded brecciated slate, has a very high Na2O/K2O ratio. Some elvans contain xenoliths of granite and small composite grains of feldspar and quartz, which also appear to be derived from granite. These compositional features have led to the view (Henley, 1972) that the elvans have been formed by the action of potassium-rich fluids on solid granite either at depth or during intrusion in a fluidised state. Despite the association of the elvans with intrusive breccias (see below), the field evidence indicates that the dykes were emplaced as magma, which may have been modified at depth by the same volatiles that produced the intrusive breccias.

A rubidium-strontium age determination of the dyke at Wherry Rocks [SW 4705 2940] has given 282 ± 6 Ma (Darby-shire and Shepherd, 1985), an age apparently older than the Land's End Granite (p.16). In a borehole at Holmans Quarry, Carwynnen [SW 6585 3661] in the neighbouring Falmouth district, an elvan dyke emplaced in the Carnmenellis Granite is cut by a 2 cm vein of pegmatitic granite. Although this vein may be a late-stage granite intrusion, it demonstrates the nearness in age of the elvans and the granite.

Near Legereath Zawn [SW 607 267] narrow veins and dykes of quartz-feldspar-porphyry are cut by veins of the Tregonning Granite (Stone, 1975), which is itself intruded by an elvan dyke on Tregonning Hill [SW 599 300]. These early porphyries resemble elvans but contain fresh, pink andalusite. The microscopic texture of the biotite also indicates thermal metamorphism by the granite. An almost identical porphyry dyke occurs in a raft of hornfelsed slate [SW 5945 2675] northwest of Trewavas Head. It is possible that these rocks represent an early phase of elvan magmatism, but they are chemically similar to the Godolphin Granite which predates the Tregonning Granite and extends to about 1 km north of Legereath Zawn, where it is fine grained and porphyritic in part.

Intrusive breccia

A distinctive breccia-filled fissure crops out at Kenneggy Sands [SW 5620 2827], but most examples of intrusive breccia have been located in borehole cores and in mine dump material (Figure 4). The rock type has also been identified from mining records. Where present within the slate country rock, the breccias commonly contain some fragments of granite among those of locally derived slate (Plate 9). The fragments, some of which are well rounded, are set in a comminuted rock matrix which may be tourmalinised or chloritised.

The breccias appear to occupy the same fissures as the elvan dykes and may be closely linked in origin with the phase of elvan magmatism (Goode, 1973; Goode and Taylor, 1980). Some breccia-filled fissures were subsequently invaded by the elvan dykes, leaving breccia at the margins or as screens within the elvan. In others, angular and rounded elvan fragments are included in the breccias. A few specimens have been found in which elvan magma appears to have been incorporated in the breccia at the time of formation, enveloping some of the rock fragments.

The breccias are interpreted as being formed by the rapid discharge of gases, mainly steam, through fissures, carrying granite fragments to higher levels and jostling and abrading dislodged wallrock fragments, causing rounding and the formation of the matrix of comminuted rock. The system may have been fluidised.

Phonolite

Wolf Rock, lying 13.5 km south-west of Gwennap Head, Land's End, and the Epson Shoal about 8 km to the east of Wolf Rock are composed of fine-grained, microporphyritic, soda-rich phonolite with fluxion texture. The phonolite was intruded into ?Devonian rocks. The general fine-grained character, fluxion texture and roughly circular outcrops suggest that the phonolite forms the remains of volcanic necks.

The phenocrysts are of sanidine feldspar and the feldspathoids nepheline and nosean (as pseudomorphs). The groundmass contains aegirine-augite, sanidine, nepheline and pseudomorphous nosean. Minor constituents include apatite, pyrite, tourmaline and rare fluorite. Micro-xenoliths composed mainly of biotite and magnetite have an inner rim of chlorite and an outer green-brown hornblende rim.

Potassium-argon and rubidium-strontium age determinations from the Wolf Rock and Epson Shoal indicate a Lower Cretaceous intrusive age of about 131 Ma.

The intrusions probably took place during the early phase of the opening of the North Atlantic when the Western Approaches trough opened as a rift (Harrison and others, 1977).

Chapter 6 Structure and regional metamorphism

Deformation of the Devonian rocks of the Penzance district began in the late Devonian and continued until the late Carboniferous. There are similarities in structural style and age of deformation with parts of the Variscan orogenic belt, or Variscides, of Belgium, Germany, eastern Europe, Ireland, Newfoundland and the Appalachians. Bailey (1929) first suggested a structural continuity between the European fold belts and those of America. Throughout the belt only remnants of the Variscides are preserved, in part tectonically reworked by the Alpine orogeny and commonly isolated by deposits of younger sedimentary rocks. The orogeny was initiated by the differential movement of crustal plates, but the peninsular nature of south-west England has made the task of defining former plate margins difficult (Bromley, 1975). Mitchell (1974) suggested that the south-west England granites and the associated tin mineralisation were attributable to a collision between two continental plates. This concept has been supported by Floyd (1983), whose examination of the basic rocks of south-west England show them to be of continental rather than of oceanic affinity.

The onset of Variscan crustal movements created instability of the sea-floor during the deposition of the Upper Devonian Mylor Slates, giving rise to extensive slump brec cias between Long Zawn [SW 547 285] and the area east of Prussia Cove [SW 559 280]. The first major phase of deformation did not occur until after the deposition of the Mylor Slates.

Folding

Polyphase deformation is readily demonstrable in the Penzance district. Each succeeding phase of folding is characterised by different fold geometry, illustrated in (Figure 5), though stereographic plots show that the major fold phases are co-axial.

The scale of folding varies with the lithologies involved. In the thinly bedded Mylor Slates fold amplitudes rarely exceed a few metres, whereas in the interbedded sandstone and slate of the Gramscatho Beds 20 to 30 m amplitudes are common. In the vicinity of St Agnes Head, to the east of this area, folds with an amplitude of 100 m or more occur in the Gramscatho Beds. Way-up evidence suggests that large-scale inversion of bedding by folding in the Penzance district is unlikely. Much of the incompletely exposed section along the west bank of the Hayle River near the mouth, though inverted, consists of a series of isolated inverted limbs of first-phase folds; the right-way-up limbs are either not exposed or have been sheared out.

Faulting

The history of faulting in the Penzance district is complex and it is difficult to ascribe faults to specific events largely because of the limited stratigraphical range of the rocks present. The three most active periods of faulting were during the Variscan orogeny and the intrusion and cooling of the Cornubian granite batholith, and in Tertiary times.

ENE–WSW thrust faulting accompanied the earliest Variscan deformation, giving rise to the northerly transported Lizard and Carrick Nappes to the south and east of the Penzance area. Seismic profiles indicate that the southerly dipping Carrick Thrust crosses the southern part of the offshore map area beneath Mounts Bay.

Crustal compensation during the intrusion of the granite batholith resulted in block faulting parallel to its WSW–ENE axis. Further block faulting occurred during the cooling and contraction of the batholith. Many of the numerous ENE–WSW tin-bearing lodes are ENE–WSW faults that formed in this way. Faults with an ENE–WSW trend are also seen in the coastal sections between Hoe Point [SW 572 277] and Kenneggy Sands [SW 563 283], and at Magow Rocks [SW 582 424] and Godrevy Point [SW 579 434]. Because they tend to be parallel to the strike of the Devonian sedimentary rocks they are not apparent inland.

The most prominent fault trend in both granite and Devonian rocks is NW–SE. Movement on this set of fractures indicates that they are wrench faults, commonly with a dextral displacement. Dearman (1963) considered them to be of Tertiary age. Alternatively, it has been suggested by Coward and Smallwood (1984) that the NW–SE faulting was intimately associated with Variscan thrusting. Low-angle fault movements have been recorded in some mines, where they displace lodes, and were locally termed slides. Faults of this style with an inclination to the south-south-east occur near Navax Point [SW 592 436] and Hudder Down [SW 600 429]. Similar faults with an inclination to the north-north-east occur at Black Cliff [SW 554 387].

Regional metamorphism

The Variscan regional metamorphism of Cornwall has generally been said to be of greenschist facies (Phillips, 1966), but metabasic and sedimentary rocks in the Roseland- Meneage area of south Cornwall are attributable to the pumpellyite-actinolite facies (Barnes and Andrews, 1981). Higher grade rocks, located in the Lizard, Start and Eddystone areas, are now regarded as nappes from deeper crustal levels, which have been tectonically transported northwards.

The regional metamorphism appears to be synchronous with the first deformation, which is closely associated with the northward thrusting of the Lizard (Barnes and Andrews, 1981). A typical mineral assemblage in sedimentary rocks is chlorite + illite + potassium feldspar + quartz, which is converted to chlorite + muscovite or phengite + quartz. The meta-chlorite of the latter assemblage is more aluminous and poorer in Fe, Mg and Si than the former (Velde, 1965; Primmer, 1983).

The study of white mica crystallinity from pelites in west Cornwall has shown values mostly in the range 0.20 to 0.33°2θ (CuKα). The 2M1 potassium-mica polytype predominates in the less-than-2-micron fraction, but about one in three pelite samples also contains sodium-micas both as a discrete phase and interlayered with potassium-mica. These crystallinity values suggest that the metamorphic grade in this region is high anchizone to low epizone, equivalent to the pumpellyite-clinozoisite zones in Snowdonia (Roberts, 1981; Merriman and Roberts, 1985).

The regional metamorphism is overprinted by contact or thermal metamorphism in the vicinity of the granites (p.17).

Chapter 7 Mineralisation

Hydrothermal metalliferous mineralisation in south-west England has a close spatial relationship with the Cornubian batholith. De la Beche (1839, p.286) suggested that 'the presence of granite or elvan has had considerable influence in promoting the presence of tin or copper ores which either occur in them or in other rocks in their vicinity… ' Mineralisation is most commonly developed along joints or fractures known as lodes, either as an infilling or as replacement in the wallrock, both in country rocks adjacent to the granite bosses and in the granite margins.

The country rocks near concealed granite ridges were subjected to stress during the emplacement and cooling of the batholith, which resulted in the formation of normal and reverse faults trending WSW–ENE parallel to the long axis of the batholith, which subsequently became mineralised.

Crosscourses cut the WSW–ENE veins throughout Cornwall; they are commonly quartz or clay-filled faults, usually with a north or north-west trend. Some carry low-temperature mineralisation but, more rarely, high-temperature ores such as cassiterite are present. Several crosscourseg can be traced across the peninsula.

The main primary ore-minerals that make up the higher temperature lodes of the district are cassiterite, SnO2, wolframite, (FeMn) WO4, and arsenopyrite, FeAsS. Lower temperature ores include chalcopyrite, CuFeS2, sphalerite, ZnS and galena, PbS.

Gangue minerals associated with ores include quartz, present in veins of all types, and feldspar, mica, tourmaline, chlorite and hematite, restricted to higher temperature deposits. Fluorite occurs in both high and low-temperature veins, whereas baryte, chalcedony, dolomite and calcite are low-temperature gangue minerals.

Most veins have been subjected to leaching of primary ores in their upper parts. The zone of leaching is capped by a gossan of manganese and iron oxides. Beneath this, in the leached zone of oxidised enrichment above the water table, the red and black oxides of copper, cuprite, Cu2O, and melaconite, CuO, form. Supergene enrichment of leached metals, particularly copper, occurs at lower levels, and in the zone of 'secondary sulphide enrichment' below the water table, copper is redeposited as the sulphides, bornite, Cu5FeS4, and chalcocite, Cu2S.

Nearly 80 per cent of the tin-tungsten deposits in the world are associated with granite. Although the mechanisms of transport and deposition are fairly well known, the source of the metals is not. In view of the generally accepted crustal origin for the Cornish granites, a primary mantle source may be discounted. It is speculated that anatexis of crustal rocks served to concentrate the metals of mineralisation, some of which found their way into magmatic minerals during granite crystallisation. Biotite, for example, contains a high proportion of tin, which can be released during the formation of chlorite by post-magmatic alteration. The tin is then scavenged by hydrothermal fluids and concentrated before being deposited in lodes.

The origin and composition of ore-forming hydrothermal fluids and the physical condition under which they were transported has been the subject of considerable recent research, especially by the analysis of fluid inclusions trapped in quartz and other minerals and stable isotope analysis of mineralised wallrock and vein material. In the St Just area Jackson and others (1982) recognised five types of mineralisation: metasomatic alterations (skarns), pegmatites, sheeted vein systems, fissure veins and replacements. Stable isotope studies indicate that only the first two were associated with fluids of magmatic origin. A meteoric origin is suggested for the fluids responsible for metalliferous mineralisation in the other three, though the salinities quoted, especially for the Bostraze sheeted vein complex, together with the absence of evidence of boiling might presuppose a magmatic component. In normal lodes minimum temperatures for cassiterite, for example from Simms Lode of Geevor Mine, fall in the range 320 to 380°C and for cassiterite from Levant Mine in the range 330 to 360°C with salinities from 17 to 19 wt% NaCl equivalent in intergrown fluorite. Both minimum-trapping temperatures and salinites from quartz-fluorite-sulphide assemblages, however, fall in lower ranges (200 to 270°C and 5 to 11 wt% NaCl equivalent).

In the St Michael's Mount sheeted vein deposit, (Jackson and Rankin, 1976; Moore and Moore, 1979) minimum-trapping temperatures for quartz-cassiterite and quartz-stannite were in the ranges 340 to 400°C and 280 to 300°C respectively. Salinites ranged from 8 to 14 wt NaCl equivalent.

In both the areas studied there is evidence for the early deposition of cassiterite from low to moderately saline fluids at relatively high temperatures followed by a cooler, lower salinity stage of sulphide mineralisation. Magmatic fluids would appear to be important only at the earliest stages of mineralisation, after which dilution by meteoric or formation waters becomes predominant.

Mineral zoning

The metalliferous deposits of Cornwall have been widely considered as textbook examples of primary thermal zoning in which the temperature gradient that existed between the roof of the granite and the land surface correlated with broadly defined mineral zones. Minerals that crystallise at relatively high temperatures, such as cassiterite, wolframite and arsenopyrite, were deposited in the marginal parts of the granite plutons or in the adjacent country rocks. Minerals that crystallised at relatively low temperatures, for example sulphides of copper, zinc, lead and antimony, were deposited farther away from the granite. Dewey (1925) collated a wealth of information regarding the distribution of minerals recorded from Cornish mines in the nineteenth century and constructed thicknesses for individual mineral zones. Dines (1934 and 1956), aware that mineral deposits did not fit neatly into the recognised pattern, proposed that mineralisation was located about emanative centres. He envisaged localities where pressure/temperature conditions and fissures were well suited for the passage of fluids and the transport and deposition of minerals. Other factors now regarded as important in controlling mineral deposition are the reaction of the hydrothermal fluids with the wallrock and the composition of these fluids, particularly their salinity.

The concept that mineral zones are sharply defined and contain limited suites of ore and gangue minerals is now regarded as an over simplification. Repeated pulses of hydrothermal activity under varying conditions allow a wide range of minerals to be deposited in successive stages in the same vein. Hence, it is possible to find cassiterite in conjunction with galena, apparently representing an overlap of mineral zones.

Dines believed that the zonation and mineralisation in the St Just area was indicative of an emanative centre. The tin zone in Geevor–Levant was thought to be relatively narrow (Dines, 1956), but recent exploration at Geevor Mine has proved significant tin mineralisation below the lowest mine levels.

East of the Land's End Granite, tin mineralisation occurs within or adjacent to the St Michael's Mount and Tregonning–Godolphin granites and above concealed granite ridges that link these outcrops at depth to the Carnmenellis Granite. Copper sulphides occur widely in this area, occasionally associated with tin, or, in ESE–WNW lodes, with abundant sphalerite. Galena with some sphalerite and silver minerals occurs in late N–S crosscourses at Gwithian Towans.

Age of mineralisation

Mineralisation followed the intrusion of the elvan dykes, but there are no other stratigraphical controls useful for dating it.

Metallised pegmatites have been dated at 285 to 280 Ma, greisenisation at about 280 to 275 Ma and the main phase of lode deposition at 270 Ma. At least three subsequent phases of mineralisation have been recognised by Jackson and others (1982) at 225 to 215 Ma, 170 to 160 Ma and 75 Ma.

Types of mineral deposits

Four main types of metalliferous mineral deposits occur in the district. In order of decreasing economic importance they are lodes; replacement ore bodies including carbonas, floors and mineralised elvans; sheeted vein complexes and greisens (Figure 6).

Lodes

The term lode may have been derived from 'lead' which early miners used to describe structures leading to ore. The lodes usually dip at 70° or more and vary considerably in thickness, the average being 0.3 to 0.6 m.

The overall fracture pattern varies within the district, as does the trend of lodes. Tin and copper lodes of the St Just–Pendeen area trend NW–SE and WNW–ESE in contrast with the generally ENE–WSW trend of those between Penzance and Redruth. Individual lodes vary considerably in dip. Repeated movements and brecciation along the fractures in conjunction with repeated episodes of mineralisation have given rise to complex lode structures and ore textures (Dines, 1956).

Garnett (1966) described four phases of fracture formation and accompanying styles of mineralisation from Geevor Mine. The full sequence is:

  1. Very little movement of lode walls.
    1. Wallrock alteration: tourmalinisation, greisenisation, reddening and chloritisation.
    2. Impregnation of the granite wallrock by arsenopyrite, pyrite and chalcopyrite, and some replacement by cassiterite.
    3. Intense tourmalinisation of the wallrock and some infilling of the fissure with tourmaline, with further wallrock alteration.
  2. Reverse and normal faulting of the lode walls.
    1. a)Introduction of feldspar and arsenopyrite.
    2. b)Introduction of cassiterite in association with quartz, tourmaline, chlorite, hematite, sulphides (pyrite and chalcopyrite) and green fluorite.
  3. Stress relaxation, usually resulting in normal faulting or simple parting.
    1. Introduction of sulphides (pyrite, chalcopyrite, sphalerite and chalcocite).
    2. Introduction of quartz and purple fluorite.
    3. Introduction of calcite and siderite.
  4. Shear movement followed by relaxation. Horizontal displacement of the lode walls followed by infilling of tension fractures with quartz and jasper.

Replacement ore bodies

Replacement mineralisation is common in the district and is usually found close to lodes. Carbonas are irregular, pipe-like replacement mineralised bodies, commonly developed in granite near lodes and accompanied by tourmalinisation and kaolinisation. The term carbona is believed to have been derived from the Aramaic word meaning a 'treasure house' or 'place rich in good things' (Hunt, 1884), although Jackson (1975) suggested that it is a Cornish term. Carbonas may have been formed in a vugh or pre-existing void (Collins, 1912) or at the intersection of two or more fractures, or by the extraordinary development of mineralisation in fracturing associated with a lode. The best known carbonas occurred in the mines of the St Ives' area. Providence Mine [SW 524 386] worked six major carbonas for tin, and to a lesser extent for copper, in the vicinity of Standard Lode (Dines, 1956, pp.122–124). The detailed mineralogy of these carbonas is not known, but quartz, feldspar, tourmaline, cassiterite and pyrite have been recorded. St Ives Consols [SW 507 399] was noted for the Great Carbona, an ore body developed in the hangingwall of Standard Lode and measuring some 18 m X 18 m X 230 m in length. It was irregular in shape and consisted of a network of veinlets, bunches and pipes in altered granite containing pink and green feldspars. Cassiterite and copper ore were associated with pyrite and tourmaline and parts of the structure were rich in fluorite (Dines, 1956, pp.114–116). Other carbonas have been recorded in Goole Pelas [SW 496 396], and Rosewall Hill and Ransome United [SW 496 392].

Disseminated cassiterite/sulphide mineralisation within a granite sheet complex in Levant Mine has been described as a carbona by Jackson (1975). This occurrence is unlike those of the St Ives area and is associated with widespread albitisation.

Floors are flat-lying, cassiterite-bearing structures and the term was originally used to described ore bodies near the granite contact in the St Just–Pendeen area. At Grylls Bunny [SW 3644 3344] an irregular series of tin-bearing floors, 1 to 4 m thick, occur near the surface in metasomatised metabasic rocks. The combined thickness of the floors is about 35 m and they measure about 75 m X 75 m. A pillared cavern there represents eighteenth century workings in floors with a 20° northerly dip. The metabasic rocks of this area have undergone iron and calcium metasomatism. Subsequent boron-rich hydrothermal fluids brought about the replacement of some hornblende-biotite-hornfels horizons by tourmaline. Tourmaline-rich layers, known to the miners as 'cockle', were subsequently mineralised by cassiterite (Jackson, 1974). Carne (1822) said that there is evidence for these floors being connected to normal lodes nearby. The mineralisation at Trewidden Bal [SW 443 296] is likened to floors by Hawkins (1822). Here tin ore occurs in irregular veins 0.01 to 0.25 m thick, which impregnate 'elvans', the latter probably being gently dipping granite sheets in the killas near the granite contact. Floors have also been recorded in Trewellard [SW 377 399], Herland [SW 595 371], Reeth [SW 593 304] and Speed [SW 566 288] mines.

Some elvan dykes are well jointed and acted both as channelways and host rocks for hydrothermal fluids, which deposited cassiterite. Several well documented examples in the Penzance area have yielded rich concentrations of cassiterite, which occurs in pockets invariably near lodes intersecting the elvan.

The elvan cropping out south-west of Wherry Rocks [SW 469 297] and extending north-eastwards to the South Pier of Penzance harbour [SW 478 300] was worked for tin during the 18th century. Wherry mine, located on offshore reefs, was reached by means of a trestle bridge from the shore. A contemporary account of this mine was given by Hawkins (1818). The elvan, a pinkish quartz-feldspar-porphyry, is approximately 6 m wide and was worked across a width of about 5 m. The rock was impregnated with irregular masses and veinlets of cassiterite where the elvan is intersected by Black Lode (Dines, 1956).

Parbola Mine (or Wheal Jennings) [SW 6132 3623] is sited on an E–W trending elvan dyke, 12 to 18 m in width and dipping 40 to 60° south (Dines 1956). Exploration boreholes drilled in 1971 indicated that the thickness of the elvan varied from 16 m some 300 m west of the mine to 54 m, 200 m east of the mine. The elvan is fine grained throughout with scattered small phenocrysts of quartz and feldspar near the centre. It is traversed by thin vertical veins of quartz, tourmaline and chlorite with irregular patches of cassiterite and associated kaolinisation. The veins are believed to follow a joint pattern trending 020°; few of the joints pass into the surrounding slates.

At Relistian Mine [SW 6035 3680] there is a similar occurrence where South Lode cuts an E–W trending elvan. Intrusive breccias are associated with this elvan and were described in nineteenth century literature as conglomerates. The paucity of matrix in some breccias allowed them to act as a channel for mineralising fluids. This type of deposit was worked for cassiterite in Relistian and other nearby mines (Goode and Taylor, 1980). The workings of Wheal Vor [SW 623 301] also encountered cassiterite-impregnated elvans (Carne, 1818; Henwood, 1843).

Sheeted vein complexes and greisens

Commonly found in the apical regions of granite intrusions, sheeted vein complexes form near the granite/country rock contact and extend for a limited distance into the aureole. Sheeted vein complexes may provide indications of minor hidden granite cupolas. Typically, the mineralogy of the veins and associated alteration consists of quartz and white mica with topaz, fluorite and a variety of ore minerals.

Greisenisation is essentially an alteration process brought about by the passage of fluorine-rich hydrothermal fluids through a well developed joint system. Alteration is common adjacent to the veins. Where it is more pervasive it may be associated with albite or microcline enrichment, depending on whether the fluids were potassium or sodium rich. Cassiterite-wolframite-bearing greisen veins are considered to have formed in the earliest stages of mineralisation after the emplacement of the granite plutons.

The granite of the southern foreshore of St Michael's Mount is cut by a complex of steeply dipping, subparallel quartz veins up to 0.15 m thick and up to 50 m in continuous strike length, persisting into the slates for about 20 m. The majority strike at 080° and the remainder at about 060°. Greisenisation extends for about 0.1 to 0.2 m on either side of the veins and locally the greisen veins coalesce. Typically, greisen-bordered veins contain quartz, orthoclase, albite, lithium-mica, tourmaline, topaz, beryl, apatite, fluorite, chalcopyrite, sphalerite, cassiterite, wolframite, stannite, bismuthinite, molybdenite and secondary uranium minerals. On the west of St Michael's Mount, cassiterite and wolframite occur, but on the eastern side only wolframite.

Sheeted veins are present in the china-clay workings of Bostraze [SW 383 317] and Balleswidden [SW 392 310]. Many quartz-tourmaline-sericite veins occur in the centre of the zone of kaolinisation and some carry cassiterite. Miners in nearby Balleswidden Mine used to refer to the veins as gry (plural gries), which may have evolved from the Cornish 'gwry' which in turn was probably derived from the German 'greisen'. A zone of sheeted veins some 650 m wide and 750 to 1000 m in length has been intersected in mine workings in the area of Wheal Reeth [SW 590 306], Lady Gwendolen Mine [SW 590 302] and Great Work Mine [SW 595 307]. Dump material and records from these mines confirm the presence of greisen veins up to 0.1 m wide carrying cassiterite, wolframite, chalcopyrite, sphalerite and traces of scheelite veined by later fluorite and potassium feldspar (Halliday, 1980).

In the Conqueror Branch Lodes of Great Wheal Fortune, Breage [SW 626 290], narrow greisen-bordered veins contain cassiterite, wolframite, sulphide, topaz and quartz. The wallrock has been extensively altered to quartz and sericite. (Dines, 1956; Hosking, 1966).

Chapter 8 Pliocene

Relatively little is known of the Mesozoic and early Tertiary history of the district. Permo-Triassic and Cretaceous rocks that overlie the Palaeozoic rocks in Devon have only submarine outcrops around west Cornwall. The development of erosional surfaces in south-west England has been widely studied (Gullick, 1936; Balchin, 1966; Everard, 1977). Several, resulting from subaerial and marine erosion, have been identified, ranging in height from below sea level to over 300 m above OD. The suggested ages of these surfaces range from Permo-Triassic through Cretaceous to Pleistocene.

The most striking surface in the district occurs at about 130 m above OD (Plate 10). The higher parts of the Land's End Granite and the Tregonning and Godolphin hills stand above it like 'islands', their margins representing a degraded cliff line. The only recorded indications of the presence of possible beach deposits at the landward edge of the surface are granite boulders recorded at 130 m above OD in a mine shaft [SW 371 340] at Huel Carn (Came, 1828) and pebbles of local origin at a height of about 125 m above OD on a shaft dump [SW 6006 3050] at Great Work Mine. The cutting of the 130 m surface has commonly been linked to the deposition of the late Pliocene St Erth Beds (Reid and Flett, 1907; Wilson, 1975a). However, fluviatile sands on the 130 m surface at St Agnes Beacon [SW 710 502], north-east of the present district, are of probable Oligocene age (Atkinson and others, 1975). Consideration of the tectonic history of submarine erosion surfaces around south-west England has led to the suggestion that the 130 m surface is either Oligocene or Miocene in age. However, its history may be complex and it may have been submerged on more than one occasion. The ages of the higher (presumably older) surfaces are even more problematical.

St Erth Beds

The St Erth Beds occur as four isolated outcrops at St Erth [SW 556 352], Splattenridden [SW 535 358] and [SW 536 363], Tregurtha Farm [SW 530 317] and Angew Farm [SW 5910 4085] at 27 to 45 m above OD. These deposits consist of fluvial gravels, beach sand with a wind-blown component, marine sand and clay and provide evidence of a marine transgression onto a terrain with a pre-existing topography. At St Erth and Splattenridden, clay, sand and gravel lie on the sides of a broad valley between the Land's End Granite and high ground to the east. At Tregurtha, sand and clay occupy the remnants of an E–W to NW–SE tributary to this valley. At Angew, clay lies in a NW–SE aligned valley.

The clay at St Erth has yielded an abundant molluscan fauna which, together with ostracods and foraminifera, suggests a late Pliocene age (Mitchell and others, 1973). The deposits at Splattenridden and Tregurtha have yielded no palaeontological evidence of age, but are lithologically and sedimentologically similar.

The St Erth Beds were worked in the past for moulding sand. Present exposures in the sandpits are limited to the sands, but a generalised succession at St Erth consists of 0.4 m of blue clay with marine fossils on 0.3 m of pebbly sand which in turn rests on 5 m of fine-grained sand, ferruginous sand and gravel.

The molluscan fauna includes 35 species of gastropod and 20 species of bivalve: none of these are of littoral types, but one species of freshwater bivalve has been identified. This fauna suggests deposition in a shallow sea slightly warmer than at the present day. Assessments of depth based on benthonic foraminifera are between 20 and 100 m (Mitchell and others, 1973) The planktonic foraminifera indicate depths much less than 100 m and probably in the photic zone (Jenkins, 1982).

The deposits near Splattenridden are not exposed. Surface debris over the southern deposit consists of gravel, iron-cemented sand and pebbly sandstone in bands parallel to the contours of the hill slope and apparently reflecting the underlying stratigraphy (Wilson, 1975). The lowest horizon consists of pebble gravels formed from local rocks. These pass up into pebbly sandstone and sandstone. Analysis of the grain-size of the sandstones suggests that they pass up from alluvium into beach sands. The sediments at Splattenridden are probably equivalent to the pebbly basal deposits and sands at St Erth and similarly indicate a marine transgression. Surface debris over the northern deposit consists of pebble and cobble gravel, and coarse-grained and pebbly sandstones with a ferruginous cement. The gravel, which consists of locally derived rocks, including granite, occupies the western and northern part of the outcrop and the coarse sandstones and pebbly sandstones tend to occur over the south-eastern part.

At Tregurtha Farm the deposit is not exposed. Up to 6 m of stiff brown clay were recorded in a shaft [SW 5274 3165] which was being sunk in 1901. Rounded sand grains occur within the local soil and head. Shallow drilling has proved the presence of clayey sand, sandy clay, gravel and coarse sand. The generalised succession at [SW 5300 3169] is:

Thickness m

HEAD

2.30

ST ERTH BEDS

Sandy clay and sand

1.30

Possibly cryoturbated clay with some sand and silt; evidence of lamination

3.10

Coarse sand with fine sand and clay matrix

0.80

Gravel and sand, both angular and rounded, with silty matrix passing down into coarse sand

1.00

Coarse and medium sand with some fine gravel

> 2.20

At [SW 5242 3167] the generalised succession is:

Thickness m

HEAD

2.00

ST ERTH BEDS

Fine and medium sand with clay laminae

3.95

Fine and medium micaceous sand

0.50

Sand with clay laminae

0.15

Fine and medium micaceous sand becoming clayey downwards

0.50

Clay with sand and silt laminae

3.65

Coarse clayey ferruginous sand with some gravel

> 0.40

Much of the medium- and coarse-grained sand is well rounded and polished and its marine origin is supported by the sparse presence of glauconite. The coarsest material, particularly at the base of the succession, is poorly sorted and includes fluviatile material. Rounded grains with frosted surfaces near the base of the section may be of aeolian origin. No fossils have been found, but the succession, which passes upwards from coarse sand and gravel into clays and fine sand and resembles that at St Erth, may indicate a marine trangression. The Geological Survey map at a scale of six inches to one mile of 1906 recorded 7 ft (2.13 m) of buff clay and 10 ft (3.05 m) of 'candle clay' in a well at Angew Farm [SW 5910 4085], Gwithian, in a small north-westerly trending valley, opening into the valley of the Red River. The height of about 45 m above OD and similar lithology suggest this deposit may be related to the St Erth Beds.

Chapter 9 Quaternary

During the Pleistocene Epoch in Britain, cold to temperate climatic fluctuations took place over 1.6 million years. In northern and central Britain the later cold periods were associated with the development of ice sheets. There is evidence that one of these, probably the penultimate, reached as far south as the Isles of Scilly and the north Cornish and Devon coasts (Mitchell and Orme, 1967; Edmonds, 1972; Kidson and Wood, 1974). Within the district the only observable influence of this glaciation is the possible introduction of erratic rock material to raised and modern beaches (Kellaway and others, 1975; Kidson and Bowen, 1976). The Giant's Rock [SW 6236 2570], just east of the district at Porthleven, has long been considered a glacial erratic. However, it is possible that this block of gneiss may have been derived from the Normannian High (Holder and Leveridge, 1986a) by means of slumping during the deposition of the Mylor Slates.

High sea levels due to the melting of polar ice during the temperate climatic phases produced raised beaches at heights of 5 to 8 m, 15 to 20 m and possibly at about 30 m above OD. There is no evidence of their ages, but the 8 m beach has been correlated with the last interglacial (Shackleton and Opdyke, 1973; Keen and others, 1981).

The raised beaches consist of boulders, gravel and sand resting on erosion surfaces which slope up to the maximum height. The 8 m beach grades down into the top of the modern beach in places and it is likely that it was occupied by the sea several times. Large erratics, probably related to earlier glaciations, occur at this height and it has been suggested that the beach gravels may have been reworked (Davies, 1983). The initial cutting of the beach platform is earlier than the last interglacial (Edmonds, 1972; Kidson and Wood, 1974).

At the time of the last (Devensian) glaciation, the district was subjected to a periglacial climate. As a result, permafrost structures and solifluction deposits (head) are widespread. During the Pleistocene cold phases, the removal of the water to the expanded polar ice caps lowered sea level by up to 150 m. The lower reaches of the rivers in the district became graded to the lower sea levels. Since the last glacial maximum, about 18 000 years ago, sea level has gradually risen, submerging river channels and extensive afforested areas resulting in submerged forests offshore. Substantial areas of coastal sand dunes were formed in the late Pleistocene and were driven landward to their present positions by the rising sea (Kidson, 1977; Mottershead, 1977).

Raised beaches

Occurrences of pebbles recorded in trenches along the Hayle Road north of Marazion, on St Michael's Mount and near Trenow [SW 528 303] at about 30 m above OD are possible evidence of the 30 m raised beach (Round, 1944).

A deposit of cobbles resting on a water-worn surface, the 20 m raised beach, was formerly exposed at Penlee Quarry [SW 4729 2686] near Mousehole (Reid and Flett, 1907). Pebbles have been recorded at about this height at Morrab Place in Penzance, and traces of gravel and sand, some including flints, have been described from St Michael's Mount and around Marazion at between 15 and 20 m above OD (Round, 1944). An excavation [SW 5185 3122] north of Marazion exposed gravel composed mainly of local rocks, but with small polished pebbles of brown flint in a matrix of fine- to medium-grained sand at about 20 m above OD. A nearby borehole [SW 5086 3217] proved 1.5 m of sand and rounded gravel resting on Mylor Slates at 17.97 m above OD. Concentrations of pebbles and a weak bench feature in the fields north-west of these localities indicate the extent of the beach deposits.

A small patch of cryoturbated head with rounded pebbles and cobbles of local metabasic rock rests on a water-worn surface on the south-east side of the northern pinnacle of the Brisons [SW 3405 3115] at an estimated height of 20 m above OD.

Exposures of the 8 m raised beach and its associated wave-cut platform occur at many localities along the present coastline. Beach deposits vary in height according to the coastal topography, which also influences modern beach deposits. The larger deposits, therefore, occur in bays, but small pockets are preserved in caves and clefts in the higher cliffs.

The raised beach at Porth Nanven [SW 356 309] (Plate 11) was first described in 1758 by Borlase. Here 7 to 8 m of granite boulders, cobbles and pebbles lie beneath head and rest on a wave-cut surface of granite. The beach and a considerable area of the original wave-cut platform are also well exposed between Magow Rocks [SW 582 424] and the Cleaders [SW 580 431] south of Godrevy Point. Typical sections show 0.5 m of pebbles and cobbles, mainly of local rocks but with some farther-travelled material, such as pebbles of gneiss, resting on a rock platform. The pebbles are overlain by 1 to 2 m of beach sand with a few pebbles. This sand is locally cemented by calcium carbonate derived from its original shell content. The sand, about 6 m thick, may be windblown in part. The beach deposits are invariably overlain by head.

Inland sections through the 8 m beach occur in road cuttings [SW 507 319] to [SW 512 319] north-west of Marazion, where it consists of sand with some pebbles, including examples composed of flint (Taylor and Beer, 1981). The raised-beach platform is well developed around Basore Point [SW 532 295], where raised-beach deposits can be seen beneath head in Trevean Cove [SW 545 288], and at the western end of Praa Sands [SW 574 281].

Head

Head deposits are composed of the weathering products of the local rocks, and possible reworked earlier head deposits, which accumulated as a result of repeated freeze-thaw action and solifluction. The deposits are poorly lithified and consist of varying proportions of local rock fragments dispersed in an ochre to buff coloured silty and sandy matrix with some clay. The degree of sorting, always poor, is greatest in the material that has been transported farthest and least in the more or less in-situ, cryoturbated material.

A general mantle of 1 to 2 m of head (blanket head or regolith–not shown on 1:50 000 map) covers most of the district. In hollows, valleys and coastal embayments accumulations of basinal head (Goode and Wilson, 1976) may be up to 30 m thick. The thicker deposits commonly show some crude stratification with varying proportions of coarser material.

Head is seldom well exposed inland, but good exposures, particularly of basinal head, occur on the coast from the mouth of the Red River to Godrevy Point [SW 583 423] to [SW 580 434], at St Loy [SW 423 231], east of Marazion [SW 525 306], at Perran Sands [SW 542 292] and at Praa Sands [SW 583 278].

The valley above Lamorna Cove [SW 451 241] is incised in head. The present stream is progressively excavating the deposits, cutting steep-sided channels and meanders. Remnant patches of head occur on the Brisons [SW 340 311], Godrevy Island [SW 576 436] and St Clements Isle [SW 474 261].

Buried channels

The over-deepened river channels that formed in the Devensian cold phase were subsequently filled by alluvial and estuarine deposits. Offshore exploratory boreholes for alluvial tin in Mount's Bay have proved alluvial deposits in channels incised in a wave-cut platform. Peat, containing mainly herbaceous pollen, from a depth of approximately 32 m below OD in a borehole at [SW 506 273] has been radiocarbon-dated at 12 070± 80 BP (BP = before present, defined as AD 1950). These alluvial deposits were laid down by streams flowing into Mounts Bay, such as the Trevaylor Stream, which flowed across the low coastal platform prior to the Flandrian transgression. Boreholes in the Hayle Estuary have proved deposits of sand, gravel, clay and clay with boulders. One [SW 5547 3749] proved 34 m of these deposits without reaching bedrock. Other boreholes proved that the buried channel gradually shallowed upstream to a depth of about 5.5 m [SW 549 345] south of St Erth. A borehole [SW 5871 4199] in the valley of the Red River penetrated 10.6 m of sand and peat underlain by sand and gravel reaching bedrock at a depth of 10.6 m (Hosking and Camm, 1980). Former workings [SW 512 317] for alluvial tin in the Marazion River [SW 512 317] recorded about 10 m of sand, gravel, peat and fallen trees underlain by tin-bearing gravel (Carrie, 1846).

Submerged forest

Remains of the extensive forests that were submerged by the post-glacial rise in sea level occur at several localities in the district, mostly in the intertidal area. From time to time the remains of trees are exhumed during low tides when storms have scoured away the overlying beach sands. Some of the best examples occur in Mount's Bay, between Long Rock and Chyandour [SW 499 309] to [SW 471 309] and between Wherry Rocks and Larrigan Rocks [SW 470 294] to [SW 467 293] (Carne, 1846). These include fallen trunks and stumps rooted in peat and an extensive bed of peat 1.7 m thick. The trees include oak, hazel, birch and alder. A patch of small trees, possibly hazel, occurs beneath the beach at Praa Sands [SW 5760 2811]. Wood from Long Rock has been radiocarbon-dated at 4278± 50 BP.

Blown sand

Sand dunes known locally as towans, are well developed around St Ives Bay, and less so at Whitesand Bay [SW 358 265], north-west of Marazion [SW 511 312] and at Praa Sands [SW 580 280]. Records indicate that the dunes west of Marazion were formerly more extensive but have undergone erosion by the sea. Their remains, stretching west to Chyandour, are now mostly concealed. Sand dunes were once visible at Western Green [SW 472 297] to [SW 464 291] (Boase, 1828) but these are now concealed by the esplanade.

At Praa Sands [SW 5785 2802] the dunes overlie a thin layer of peat which has been radiocarbon-dated at 1805± 100 BP (Wellin and others, 1973). The peat was probably laid down in a small lagoon that was overidden by the dunes.

The blown sand of the district is now mostly stabilised by vegetation although some 'blow-outs' occur and small amounts of sand move in high winds.

Alluvium

The alluvial deposits of the district are composed of cobbles and gravel, with some sand, silt, clay and peat usually in the upper parts of the deposits. Alluvium has been an important source of detrital tin ore, and few alluvial tracts have remained undisturbed (Henwood, 1873; Dines, 1956). Where streams flow through mining areas, sand and silt tailings from the crushing of ore have contributed to the alluvium. The alluvium is variable in thickness but commonly 2 to 5 m thick along the larger streams.

The alluvial tract between Trevarrack [SW 480 312] and the Marazion River [SW 512 313] probably owes its origin to the barring of southward-flowing streams by sand dunes and beach deposits. A silty clay, generally more than 1.3 m thick, is underlain by 0.9 to 2.4 m of peat that rests on marine sand (Carne, 1846).

Chapter 10 Economic geology

Metalliferous minerals have been exploited in the Penzance district since the Bronze Age, when tin and copper ores were worked to make bronze weapons and utensils. The widely held concept that Cornwall may have been the Cassiterides, 'tin islands' off the western coast of mainland Europe, from which the Phoenicians obtained their tin, is refuted by Penhalurick (1986). The island of Ictis, a source of Roman tin, may have been St Michael's Mount.

The earliest tin workings were in cassiterite-bearing alluvial sands and gravels. Detrital tin, liberated solely by natural erosional processes, is fairly coarse grained and tends to be pure and uncontaminated by other metallic ores. Copper ore was probably obtained from lodes exposed in cliffs.

Later, ores of both metals were worked from shallow pits sunk on lode outcrops and by adit mining on the coast. Underground mining for tin, copper and lead may have been practised in west Cornwall during the Roman occupation, but repeated reworking of old mines has removed much of the evidence of earlier operations. Shafts did not become commonplace until the 14th century, when the richer alluvial ground had been worked out. During this period there was an influx of European miners, mainly German, bringing with them much expertise. The introduction of rock blasting with gunpowder in the late 17th century and steam-driven pumps in the late 18th century allowed a rapid expansion of Cornish mining, which continued into the 19th century. Copper production declined from a peak of 15 500 tons of metal in 1860 to almost nil in 1900. Tin production peaked in 1870 at 10 000 tons of metal, but fell to 4000 tons in 1900 and to a few hundred tons in 1920. Sites of mineral workings and recent exploration are shown in (Figure 7).

Alluvial tin deposits

Little detail is known of the workings in alluvial tin deposits in the area and there are no known production figures. Cassiterite commonly occurred in gravel resting immediately on bedrock 'shelf' below alluvial silt, sand and gravel. In some areas there was more than one tin-bearing horizon but the basal gravel was usually the richest. The alluvial tracts on the Land's End Granite are not extensive but many have been 'turned over' for their tin content, perhaps more than once. Broad basins such as Trevider Moor [SW 435 265], Drift Moor (now a reservoir) [SW 433 293] and Coldharbour Moor, Towednack [SW 489 377] were the largest areas worked (Henwood, 1874). Marazion Marsh and the streams draining into it also have a long history of alluvial tin workings (Henwood, 1874). The presence of alluvial deposits in submarine channels in. Mount's Bay, as suggested by Ong (1966), has been confirmed by acoustic profiling. Offshore drilling in 1982 proved up to 2 m of basal gravel in the channels. These channels are probably former extensions of the southerly flowing streams which now end in Marazion Marsh. The alluvium of the River Hayle was worked for tin between Leedstown and St Erth during the 1939–45 war; some 86 tons of 70 per cent tin concentrate were produced in the period 1943–45 (Dines, 1956). The sediments of the Hayle estuary also contain detrital tin, and recent prospecting has indicated economic grades and reserves.

Marine alluvial tin deposits

The sediments of St Ives Bay and Mount's Bay contain cassiterite derived from tailings slimes as well as naturally eroded detrital material. The Red River flows into St Ives Bay after passing through the Camborne–Redruth mining area and, despite stream tin works designed to recover the mine tailings, some fine-grained cassiterite has been carried out to sea. The Union Corporation subsidiary, Coastal Prospecting, investigated the tin content of the offshore sediments from 1963 to 1966, and in 1967 the mv Baymead commenced a dredging operation. The sand was upgraded on board ship and transferred to a processing plant at Lelant. Low tin content, problems of contamination of reserves with discharged waste sand and rough weather contributed to the abandonment of this enterprise.

For many years beach sand was worked for tin at Gwithian, and the remains of the processing plant can still be seen on the north bank at the mouth of the Red River. In more recent years the upgraded beach sand has been taken to Tolgus Tin Ltd.

Marine Mining (Cornwall) Limited prospected three offshore areas between St Ives Bay and Cligga Head in the 1970's. It was proposed to offload the initial concentrates by means of a floating pipeline to be further upgraded at the former treatment plant near the mouth of the Red River. Work commenced in 1985 to replace the equipment and storage facilities at this site.

Mining

There have been four main concentrations of copper and tin mining activity in the district: the St Just, St Ives, Gwinear and St Hilary/Godolphin–Tregonning areas.

Dines (1956) has summarised the geology, the nature of the workings and the mineral production of each mine or group of mines. A summary of copper end tin production from the more important mines of the district is given in (Table 1). Early accounts of specific aspects of the mining have been given by De la Beche (1839), Henwood (1843), Collins (1912) and Hamilton Jenkin (1961–65).

Geevor Mine

Early in 1986, Geevor was the sole working mine in the district. It incorporates North Levant, Wheal Came and Wheal Bal. The present company has operated under the name of Geevor since 1911. More recent additions include Boscaswell Downs Mine, acquired in 1936, the inland Levant lease, acquired in 1967, and the offshore lease of Levant Mine, acquired from the Crown in 1971.

The mine straddles the Land's End Granite–country rock contact. The contact lies near the collar of Victory Shaft [SW 3755 3450] where locally it dips at about 25° to the northwest. The main lodes have steep dips and trend WNW–ESE and NNW–SSE. They have been worked for copper and tin in granite, and copper in the killas offshore. The workings reach a depth of nearly 750 m below surface and extend under the sea for approximately 1500 m. There are indications that within the lodes the top of the mineralised zone becomes progressively deeper north-westwards. Details of lodes and workings are recorded in Dines (1956) and Garnett (1966).

From 1965 to 1971 the Boscaswell Downs section was dewatered and Treweeks Shaft [SW 3816 3470] rehabilitated to 14-Level. Simms Lode was developed over a length of 1.5 km and a crosscut driven on 14-Level, linking Victory and Treweeks sections. Examination of the lode below the old workings of Pendeen Consols was carried out by northerly crosscuts from the Treweeks section, Treweeks Shaft being deepened to 15-Level in 1979.

It has long been recognised that continuations of lodes below the old copper workings of Levant were some of the most promising prospects adjacent to Geevor. The major task of sealing a sea-floor breach into the uppermost levels of Levant failed in 1961. A second attempt began in 1964 and was successful in 1965. Levant was drained to the 40 fm level by March 1966. Boreholes were drilled south of Levant in 1968 to examine the Botallack lodes and the area between Levant and Botallack. Boreholes were also drilled to the south of Cape Cornwall to examine the extensions of lodes in the Porth Nanven area. In the same year, horizontal boreholes were drilled from Geevor 14-Level to dewater Levant to the 180 fm level. On completion in 1969, the two mines were joined by a crosscut at this level. Skip Shaft [SW 3683 3452] was rehabilitated to the 120 fm level below adit in 1969 and the 190 fm level below adit in 1970. With Levant partly dewatered it was feasible to look at the lodes below the old workings. A decision was made in 1974 to sink the Sub-incline Shaft from 15-Level near Victory Shaft, to run beneath Levant (Figure 8). Work started in 1975 and was completed to 19-Level in 1979, including the deepening of Victory Shaft. Levant was dewatered to the 278 fm level in 1980 and to the 302 fm level early in 1982. The Sub-incline Shaft will also permit the examination of some of the Geevor lodes below existing workings. It has been said by many geologists that the lodes in Geevor were becoming so poor at about 15-Level that deeper exploration would not prove useful. Results obtained from the sinking of the Sub-incline Shaft show that mineralisation improves in some structures at depth.

Work also began in 1980 on the rehabilitation of Allen's Shaft [SW 3646 3333], Botallack, with the aim of crosscutting at 9-Level to look at the seaward extensions of Wheal Edward and West Wheal Owles.

Work began in 1984 on deepening the Sub-incline Shaft to 24-Level. When operations were suspended in the spring of 1986 due to the tin crisis, the shaft was below 21-Level.

In recent years, Geevor Tin Mine has been reprocessing dump material from the West Penwith area. The waste has a similar mineralogy to the Geevor lodes, allowing it to be milled with Geevor ore.

A summary of tin production from Geevor Mine is given in (Table 2).

Recent mineral exploration

World demand for tin led to a high level of mineral exploration in the area between 1961 and 1973 (see Annual Reports of the Cornish Chamber of Mines for details). Attempts to re-open the clifftop mine at Carnelloe [SW 443 388] near Zennor, in 1961 and 1963, failed after a public enquiry. Between 1965 and 1972 Penwith Minerals prospected the area near Land's End Airport and the Nanquidno Valley, including Wheal Hermon [SW 356 306] where some underground development was carried out.

Cornish Land Ventures formed in 1963 and drilled 30 boreholes to prove extensions to known lodes around Crowan, Leedstown and Marazion. Baltrink Tin Ltd., in 1964, examined the easterly extensions of the main lode of Giew Mine [SW 449 371], south-west of St Ives. Extensions of the Wheal Sisters lodes were sought near Mennor [SW 526 366] in 1971 by Kappa Exploration. The Wherry mineralised elvan was drilled by Amalgamated Roadstone Corporation in 1967 and a similar elvan at Parbola [SW 614 365] was tested by Consolidated Goldfields in 1971.

Finally, in 1973 the Institute of Geological Sciences (now BGS) drilled boreholes near Bosworgy [SW 5809 3367] and Parbola [SW 6157 3633] in search of hydrothermal alteration associated with buried granite cupolas.

Building stone

Coarse- and fine-grained granites have been used extensively, both separately and in association for decorative effect in the district. The coarse-grained granite was chiefly worked in the Lamorna–Sheffield area, but other quarries occur throughout the peninsula. The quality of Land's End granite was generally not as good as that of Carnmenellis, and because of transport limitations much of the stone was used only locally, for gateposts, lintels and paving stones. Fine-grained granite, described variously as 'elvan', 'freestone', 'sandstone' or 'whetstone', was quarried at several localities south-east of Castle an Dinas. Elvan dykes provided a widespread and highly variable source of stone which, apart from the construction of some fine domestic dwellings, was used extensively for mine buildings and farm walls. Most of the small narrow quarries in the elvans have long since been filled in.

Metasedimentary rocks have been quarried for buildings, and in the east of the area the grey and olive-buff weathering sandstones of the Gramscatho Beds have also been used. Greenstone was unpopular with builders owing to its toughness to work, and was used chiefly in random walling at Newlyn, St Just and St Ives. An interesting building material found near Hayle and other areas of smelting is the residual slag from the smelter cast into heavy, rather scoriaceous, rectangular blocks. Parts of the Hayle canal have been constructed of this material.

Aggregate

Crushed stone is produced at Penlee metadolerite quarry at Newlyn; the production, 120 000 tonnes in 1982, is now about 300 000 tonnes annually. Stone is shipped fiom the adjoining harbour mainly to south-east England; some is used locally. The rock from Penlee is noted for its high crushing strength and has been used on the continent for roadmaking, but a low polished-stone value precludes its use for road surfaces in Britain. It is also specified as an aggregate to be used in concrete for the construction of bank vaults and security premises.

Until recently, Castle an Dinas fine-grained granite quarry near Penzance produced 130 000 tonnes per year of crushed stone, but is now worked only intermittently. Some of this was also shipped to south-east England; the rest used locally.

Sand

There are few sources of building sand in the Penzance district. The dune sand at Hayle has a high proportion of shell debris and is only suitable for mortar. Because of its high calcium content this sand has long been used as an agricultural conditioner for acid soils in conjunction with seaweed, which was commonly carted from the beaches of Mount's Bay for use as a fertiliser. Two pits remain open, one at Loggans, near Hayle and one at Gwithian.

The washed residue from the china clay working at Lower Bostraze is used for concrete-block making at the nearby Balleswidden works.

China clay

Since the first discovery of china clay on Tregonning Hill in 1746 by William Cookworthy, there have been numerous small workings in the granite of the Penzance district. Most were located near the prominent NNW–SSE fault or joint zones which cross the Land's End peninsula. The largest of these pits were Balleswidden [SW 392 310], Tredinney, [SW 394 288], Bohemia [SW 480 355] and Porthia [SW 495 377]. Lower Bostraze [SW 383 317], near St Just, is the sole china clay pit in work; about 80 to 100 tonnes per week are produced mainly for fire bricks for the steel industry and as a general purpose filler clay.

Water supply

The water supply of the Penzance district comes mainly from surface sources. Of these, the most important is the Drift reservoir [SW 434 293], impounding water from the Newlyn River and yielding about 4.0 million cubic metres per annum. Small concentrations of arsenic (e.g. 0.01 mg/l as As) are not uncommon in rivers that flow through mineralised areas and gather water from mine adits. Levels of arsenic exceeding 0.05 mg/l in drinking water have been recorded.

Geothermal energy potential

Considerable interest in the Cornubian granites as potential dry heat sources for geothermal power has culminated in drilling experimental deep wells in the Carnmenellis granite by Camborne School of Mines. Trials are proceeding to extract heat by circulating water through fractures in the granite and recovering the heated water by pumping.

Measurements of the geothermal gradient and heat flow are about 40°C km−1 and 126 mWm−2 compared to average crustal figures of around 30°C km−1 and 62 mWm−2 (Tammemagi and Wheildon, 1977). The main radiogenic contribution comes from U235 and U238 decay chains with lesser quantities from those of Th232 and K40. Mean elemental abundances are uranium 10.4 ppm, thorium 17.9 ppm and potassium 4.3 per cent (Tammemagi and Smith, 1975). Potassium feldspar, biotite and muscovite account for the bulk of the potassium, but opinion differs as to which minerals contains the uranium and the thorium. Simpson and others (1976) suggested zircon, apatite and sphene, whereas Ball and Basham (1979) believed uraninite and monazite to be the principal sources.

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FLOYD, P A. 1976. Geochemical variation in the greenstones of SW England. Journal of Petrology, Vol.17, 522–545.

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GOODE, A J J. 1973. The mode of intrusion of Cornish elvans. Report of the Institute of Geological Sciences, No. 73/7.

GOODE, A J J, and WILSON, A C. 1976. The geomorphological development of the Penzance area. Proceedings of the Ussher Society, Vol.3, 367–372.

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HAWKES, J R. 1961. The geology of the sector of the Land's End granite aureole between Cam Moyle and St Ives. PhD thesis, University of Birmingham.

HAWKES, J R. 1981. A tectonic 'watershed' of fundamental consequence in the post-Westphalian evolution of Cornubia. Proceedings of the Ussher Society, Vol.5, 128–131.

HAWKES, J R, and DANGERFIELD, J. 1978. The Variscan granites of south-west England: a progress report. Proceedings of the Ussher Society, Vol.4, 158–171.

HAWKINS, J. 1818. On submarine mines. Transactions of the Royal Geological Society of Cornwall, Vol.1, 127–142.

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HENDRIKS, E M LIND. 1931. The stratigraphy of south Cornwall. Report of the British Association for the Advancement of Science, (Bristol 1930), 332.

HENDRIKS, E M LIND. 1937. Rock succession in south Cornwall. Quarterly Journal of the Geological Society, Vol.93, 322–367.

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HOLDER, M T, and LEVERIDGE, B E. 1986(b). Correlation of the Rhenohercynian Variscides. Journal of the Geological Society, London, Vol.143, 141–147.

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HOSKING, K F G. 1969. The nature of primary tin ores of the south-west of England. Vol.3. 1155–1244 in A Second Technical Conference on Tin, (London: International Tin Council.)

HOSKING, K F G, and CAMM, G S. 1980. Occurrences of pyrite framboids and polyframboids in west Cornwall. Journal of the Camborne School of Mines, Vol.80, 33–42.

HUNT, R. 1884. British mining. (London: Lockwood.) JACKSON, N J. 1974. Grylls Bunny, a tin floor at Botallack.

Proceedings of the Ussher Society, Vol.3, 186–188.

JACKSON, N J. 1975. The Levant Mine carbona. Proceedings of the Ussher Society, Vol.3, 220–225.

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JACKSON, N J, and RANKIN, A H. 1976. Fluid inclusion studies at St Michael's Mount. Proceedings of the Ussher Society, Vol.3, 430–434.

JACKSON, N J, HALLIDAY, A N, SHEPPARD, S M F, and MITCHELL, J G. 1982. Hydrothermal activity in the St Just mining district. In Metallization associated with acid magmatism. EVANS, A M (editor). (Wiley.)

JENKINS, D G. 1982. The age and palaeoecology of the St Erth Beds, southern England, based on planktonic foraminifera. Geological Magazine, Vol.119, 201–205.

JOHANSSEN, A. 1938. Petrography of the igneous rocks. Vol.2, 300. (University of Chicago Press.)

JONES, J G. 1970. Intraglacial volcanoes of the Laugarvatn region, Southwest Iceland, II. Journal of Geology. Vol.28, 127–140.

KEEN, D H, HARMON, R S, and ANDREWS, J T. 1981. U series and amino acid dates from Jersey. Nature, London, Vol.289, 162–164.

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40 REFERENCES

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Appendix 1 BGS publications and other information on the area

Maps at 1:10 000 scale, solid editions

Manuscript copies of all the sheets are available for consultation only, but dyeline copies of the drawn-up standards may be purchased. The area was mapped between 1970–76, by A J J Goode, R T Taylor and A C Wilson. All maps are within the 100 kilometre square SW.

32 NE

AJJG

32 SE

AJJG

33 NE

AJJG, ACW

33 SE

AJJG, ACW

42 NW

AJJG, RTT

42 NE

RTT

42 SW

AJJG, RTT

43 NW

AJJG, ACW

43 NE

AJJG, ACW

43 SW

AJJG, RTT

43 SE

44 SE

AJJG, RTT, ACW AJJG, ACW

52 NE

RTT

53 NW

ACW

53 NE

ACW, RTT

53 SW

ACW, RTT

53 SE

ACW, RTT

54 SW

ACW

54 SE

RTT, ACW

62 NW

RTT–Dyeline copy in preparation

63 NW

RTT, ACW–Dyeline copy in preparation

63 SW

RTT, ACW Dyeline copy in preparation

64 SW

RTT–Dyeline copy in preparation

Maps at 1:50 000 scale

Maps at 1:253 440 (quarter inch to one mile) scale

Maps at 1:250 000 scale

Memoirs

British Regional Geology

IGS Report Series

Fluid Processes Research Group Reports

FLPU/84–5 The geomicrobiology of used and disused mines in Britain. N Christofe, J M West, J C Philp and J E Robbins, 1984.

Regional Geophysics Research Group Reports

86/8 A review of geophysical results in south west England. A J Burley, 1986.

Open File Reports

The geology, petrology and geophysics of the Land's End, Tregonning–Godolphin and St Michael's Mount Variscan granites. A J J Goode, J R Hawkes, J Dangerfield, A J Burley, R T Taylor and A C Wilson, 1987.

The minor intrusive rocks and intrusive breccias of the Penzance district including the Wolf Rock Phonolite. R T Taylor, R K Harrison and J Dangerfield, in preparation.

Rock samples

A range of thin sections has been cut and used to describe the district. They are available for examination on request.

Boreholes

Some deep boreholes have been sunk mainly for the purpose of locating mineral veins. BGS (IGS) drilled two boreholes for research purposes. The names and BGS reference numbers are:

Bosworgy (SW53SE/8) NGR [SW 5806 3367]

Parbola (SW63NW/51) NGR [SW 6157 3633]

A limited amount of shallow drilling results are also available for consultation.

Key sections

Devonian

Gramscatho Beds: Folded, coarse-grained, turbiditic sandstones interbedded with slate: Black Cliff [SW 554 388]; Peters Point [SW 577 412]; Godrevy Point [SW 580 433]

Mylor Slates:

Metabasic rocks

Granite

Minor igneous intrusions

Quartz-feldspar-porphyry (Elvan): Praa Sands [SW 573 281] Intrusive breccia: Kenneggy Sands [SW 5620 2827] (A narrow vein that may be difficult to locate)

Structure

Most of the fold phases are represented in Devonian rocks near Godrevy Point [SW 580 432] and also near Peters Point [SW 577 411]

Mineralisation

Lodes: The economic nature of tin and copper lodes has ensured that few good examples remain for study. It is possible to see something of lode structures in the cliffs and foreshore, where accessible, between Cape Cornwall and Pendeen Watch. Replacement ore bodies: Workings in the 'tin floors' of Grylls Bunny [SW 3644 3344] are accessible with care, the pillars consisting of the cassiterite-bearing material that was originally worked. Sheeted veins: Good examples of sheeted vein complexes occur at Bostraze and St Michael's Mount. Bostraze, near St Just, is a working china-clay pit operated by ECC International, (formerly English Clays Lovering and Pochin Ltd), John Keay House St Austell. The abandoned clay pits on Tregonning Hill [SW 604 296] are accesible with care. The St Michael's Mount vein complex is exposed on the southern foreshore which is not normally accessible to the public. Although collecting is forbidden, permission for bona fide parties to examine these rocks may be obtained from:

The Manager, St Michael's Mount, Manor Office, Marazion, Cornwall.

Quaternary

Economic geology

Evidence of mining is widespread throughout much of this area. Abandoned mine buildings, remains of equipment and waste tips are commonplace, usually set in a landscape of industrial dereliction. The engine houses at Botallack [SW 362 335] (Plate 12) and Trewavas [SW 600 265] are worth mentioning specifically, but other examples abound.

Consultation

Enquiries regarding the geology of the district should initially be directed to the British Geological Survey, Exeter Office, St Just, 30 Pennsylvania Road, Exeter, Devon, EX4 6BX (Exeter (0392) 78312). Material may be inspected, by appointment only, at the Exeter Office or at the headquarters of BGS, Keyworth, Nottingham, NG12 5GG. (Plumtree (06077) 6111).

Appendix 2 List of British Geological Survey photographs Penzance (351/358) Sheet

Copies of these photographs may be seen in the Library of the British Geological Survey, Keyworth, Nottingham NG12 5GG and at the British Geological Survey, Exeter Office, St Just, 30 Pennsylvania Road, Exeter, EX4 6BX. Colour or black and white prints and 35 mm slides can be supplied at a fixed tarrif.

Devonian rocks

A255

Quartz veins cutting killas, Priest's Cove, St Just

A260

Contortions in killas, Priest's Cove

A12385

Mylor Slates with greenstone and elvan intrusions

A12386

St Michael's Mount and Mylor Slates with greenstone intrusions

A12387

St Michael's Mount and Chapel Rock

A12391

Sandstone beds faulted against slate

A12395

Godrevy Island and coast south of Godrevy Cove

A12396

Polyphase folding (F3 and F4 folds) in silty banded slates

A12397

Large F2 folds in slates and sandstones

A12398

Polyphase folding (F2 and F3 folds) in silty banded slates

A13122

Folding in Mylor Slates, Foreshore east of Marazion.

A13123

Folding in Mylor Slates, Foreshore east of Marazion

A13124

Slumped sediments, foreshore south of Marazion

A13140

Folding in the Hayle Sandstones (Gramscatho Beds) Black Cliff, Hayle

A13141

Folding in Mylor Slates, Gwithian Towans

A13143

Folding in Mylor Slates, Magow Rocks

A13144

Folding in Mylor Slates, Magow Rocks

A13146

Folding in Mylor Slates, Gwithian Towans

A13209

Folds in Mylor Slates, Priest's Cove, Cape Cornwall

A13218

Slumped structure in hornfelsed slate, Zawn a Bal

A13235

Folding in hornfelsed slates, Mill Zawn

A14066

Folding in Mylor Slates, cliffs north-west of Hoe Point.

Metabasic rocks

A240

Greenstone coast scenery, Zennor

A241

Greenstone coast scenery, Zennor

A242

Greenstone coast scenery, St Just

A243

Greenstone coast scenery, St Just

A244

Greenstone coast scenery, St Just

A245

Greenstone coastal scenery, St Buryan

A246

Greenstone coastal scenery, St Buryan

A257

Junction of granite and greenstone, St Buryan

A258

Junction of greenstone and Killas, St Buryan

A259

Alternations of sheared greenstone and killas, Botallack Head, St Just

A261

Greenstone tor, The Devil's Rock, Penzance

A273

Greenstone quarry, Newlyn

A274

Crushing plant, Newlyn

A12383

Mylor Slates with greenstones forming the offshore rocks of the Greeb

A12384

The coast east of Perranuthnoe

A12390

Volcanic agglomerate

A12401

The coast south of St Ives Head

A12402

Spotted adinole around greenstone

A13126

Volcanic agglomerate, Great Hogus

A13127

Volcanic agglomerate, Great Hogus

A13130

Pillow lavas, Gulval Carn

A13131

Penlee Quarry, Newlyn

A13132

Penlee Quarry, Newlyn

A13166

Adinolised Slate, St Ives Island

A13167

Adinolised Slate, St Ives Island

A13168

Adinolised Slate, St Ives Island

A13169

Pillow lavas, Burthallan Cliff

A13170

Pillow lavas, Burthallan Cliff

A13171

Pillow lavas, Zennor Cliff

A13173

Greenstone cliffs, Carnelloe Cliff viewed across Porthglaze cove

A13180

View of Cudden Point, Trevean

A13182

Cliffs formed of pillow lavas, Carrick Du, St Ives

A13183

Pillow lavas in sea stack, Carrick Du, St Ives

A13184

Gurnard's Head

A13211

Pillow lavas, Kenidjack Castle

A13212

View from Kenidjack to Botallack Head, Kenidjack Cliff

A13213

Cordierite-hornfels, Kenidjack Cliff

A13216

Cordierite-hornfels, Zawn a Bal

A13217

Cordierite-hornfels, Zawn a Bal

A13219

An early fault in hornfelsed rocks, Zawn a Bal

A13220

Deformed basic metasomatic hornfels, Roscommon

A13221

Folded basic metasomatic hornfels, Roscommon

A13222

Banded garnet-magnetite-skarn, The Crowns

A13223

Dyke-like body of garnet, The Crowns

A13224

Massive development of garnet, The Crowns

A13225

Natural arch, Stamps and Jowl Zawn

A13226

Boulders of garnetiferous skarn, Cam Vellan

A13227

Pillow lavas, Cam Du

A13229

Banded hornblende-plagioclase rock, Trewellard North Cliff

A13230

Banded hornblende-plagioclase rock, Trewellard North Cliff

Granite and elvan

A234

Killas, coastal scenery, St Just

A235

Granite coastal scenery, Land's End

A236

Granite coastal scenery, Land's End

A237

Granite coastal scenery, Land's End

A238

Granite coastal scenery, Sennen

A239

Granite coastal scenery, St Just

A248

View of St Michael's Mount

A249

View of St Michael's Mount

A250

Granite veins in killas, Zennor

A251

Granite veins in killas, Zennor (nearer view)

A252

Granite veins in killas, Zennor

A253

Granite veins in killas, Zennor

A254

Granite veins in killas, Zennor

A256

Junction of granite and killas, Priest's Cove

A270

Granite quarry, Paul

A271

Granite quarry, Paul

A272

Granite quarry, Paul

A7505

Granite coastal scenery, Land's End

A7506

Granite coastal scenery, Land's End

A7507

Granite coastal scenery, Land's End

A7508

Granite coastal scenery, Land's End

A7509

Granite coastal scenery, Land's End

A7510

Granite coastal scenery, Land's End

A7511

Granite coastal scenery, Land's End

A7512

Granite coastal scenery, Land's End

A7513

Granite coastal scenery, Land's End

A7514

Granite coastal scenery, Land's End

A7515

Granite coastal scenery, Land's End

A7516

Granite coastal scenery, Land's End

A7517

Granite coastal scenery, Land's End

A8030

Lanyon Quoit. Constructed of granite blocks, northwest of Penzance

A8067

Granite scenery, Zennor village, Land's End area

A8074

Coastal scenery, Land's End area

A8075

Coastal scenery, Land's End area

A8076

Coast scenery, Land's End area

A8077

Coast scenery, Land's End area

A8079

Coast scenery, Land's End area

A8087

Geevor Mine, granite tongue in killas

A12373

Beach boulder of layered granite, aplite and pegmatite

A12374

Quartz-porphyry and slate intruded by granite

A12375

Granite sheets in Mylor Slates

A12376

Quartz-porphyry intruding slates cut by a later granite veins

A12377

Granite sheets in folded Mylor Slates

A12378

Granite sheet with slate inclusions

A12379

Granite and quartz-porphyry intruding Mylor Slates

A12380

Eastern contact and coastal scenery of the Tregonning Granite

A12388

St Michael's Mount

A12403

Trengwainton Carn

A12407

Castle-an-Dinas granite quarry

A12408

New Mill granite quarry

A13125

Layering in granite, St Michael's Mount

A13128

Brecciated elvan, Penzance foreshore

A13129

Elvan dyke, Chimney Rocks, Penzance

A13133

Land's End Granite, Lamorna Cove

A13134

Quarry in Land's End Granite, Sheffield

A13135

Feldspar megacryst-rich layer in granite, Lamorna Cove

A13136

Tourmalinised aplite dykes, Zawn Gamper

A13137

Granite contact, Zawn Gamper

A13138

Aplite dyke, Boscawen Cliff

A13139

Aplite dyke, Boscawen Cliff

A13151

Jointing in granite, Greeb Zawn

A13152

Granite coastal scenery, Logan Rock

A13153

Granite coastal scenery, Logan Rock

A13154

Fine-grained granite sheet in cliff at Pednvounder Beach

A13155

Granite relationships. Cliff near Daw Zawn, Pednvounder Beach

A13156

Logan Rock, Treen

A13158

St Loy's Cove, Paynters Cove, Boscawen Point

A13160

Veins at working end of Bostraze china clay pit, St Just

A13161

Granite tor, Cam Kenidjack, Carnyorth Common

A13162

Granite tor, Rosewall Hill

A13163

Blocks of Trevalganite in wall. Fields above Trevalgan Cliff

A13164

Zennor Valley, Foage Farm and Zennor Hill

A13165

Intrusive breccia in Hayle Sandstones (Gramscatho Beds). Porth Kidney Sands, Lelant

A13178

The Bishop and Trewavas Head, Trewavas Cliff

A13186

Fine-grained granite dykes, Treen

A13187

Granite cupola, Treen

A13188

Pegmatite and aplite sheets in roof zone of a granite cupola, Treen

A13189

Banding in granite, Treen

A13191

Granite veins cutting hornfelsed slate, Wicca Pool, near Zennor

A13192

Biotite-rich lens with flow-aligned feldspar in granite vein, Wicca Pool

A13193

Pegmatitic graphic intergrowth, Wicca Pool

A13194

Granite tors at Trendrine Hill

A13195

Granite tors at Trendrine Hill

A13198

Hornfelsed slate raft in granite, Great Zawn

A13199

Brandys, Bosigran Cliff, Porthmoina Island and Commando Ridge, Porthmoina Cove

A13200

Zennor Hill

A13201

Zennor Hill

A13202

Zennor Hill

A13205

Granite–hornfels contact, Porth Ledden, St Just

A13206

Granite apophysis in Hornfels. Porth Ledden, St Just

A14063

Elvan dyke, Praa Sands

A14064

Pegmatite and Aplite Sheets, Praa Sands

A14065

Pegmatite and Aplite Sheets, Praa Sands

A14067

Greisen veins, St Michaels Mount

A14068

Greisen veins, St Michaels Mount

A14069

Granite coastal scenery, nr Zawn Wells

A14070

Granite coastal scenery, nr Pordenack Point

A14071

Granite coastal scenery, nr Pordenack Point

A14072

Granite coastal scenery, Cam Sperm and Cam Boel

A14073

Granite coastal scenery, Porthgwarra

A14074

Intrusive breccia vein, Base of cliff south of Kenneggy

Pliocene, Quaternary rocks and features

A247

Blown sand, Whitesand Bay

A262

Raised beach platform, Whitesand Bay

A263

Raised beach, Penlee Quarry, near Mousehole

A264

Raised beach, Priest's Cove

A265

Raised beach, St Just

A266

Cliff or head, east of Marazion

A267

Cliff or head, east of Marazion

A268

130 m surface, south-east of St Just

A269

View showing large size of beach pebbles

A8066

130 m surface, Land's End area

A8067

130 m surface, Land's End area

A8069

Raised beach and head

A8070

Raised beach and head

A8071

Raised beach and head

A8072

Raised beach

A8073

Raised beach head

A12382

Peat resting on head and overlain by blown sand

A12392

Raised beach

A12393

Head deposit

A12394

Raised shore platform

A12399

Involution? in weathered slates

A13142

Head deposit resting on raised beach, Magow Rocks

A13145

Raised beach, Godrevy Point

A13147

Cliff composed of raised beach and head, Porth Nanven

A13148

Head terraces and the Brisons, Pen Nanven

A13149

Head terraces, Pen Nanven

A13150

Blown sand, Whitesand Bay, Sennen

A13157

Head, St Loy's Cove

A13159

Terrace features, Kelynack, near St Just

A13174

Moorland in a valley bottom, Trink

A13175

Fault-controlled valley, Drannack Mill

A13176

Fault-controlled valley, Nanpusker

A13185

Head and alluvium at Bussow Moor

A13208

Head overlying folded Mylor Slates, Priest's Cove, Cape Cornwall

A13240

Head and raised beach deposits, Porth Nanven, St Just

A14075

Recent sand dunes of towans, Gwithian

A14077

Head and raised beach deposits, Porth Nanven

Mines

A275

General view of Levant Mine, St Just

A8039

Local water supply from mine adit, St Just

A8080

Botallack Mine, St Just

A8081

Bellan Mine, St Just

A8082

Wheal Hermon, St Just

A8083

Geevor Mine, general view

A8084

Geevor Mine, general view

A8085

Geevor Mine, No. 1 Branch Lode

A8086

Geevor Mine, No. 1 Branch Lode

A8088

Lode faulted by a cross-course, Geevor Mine

A8089

Geevor Mine, Wethered Shaft

A8090

Geevor Mine, Victory Shaft

A8092

Geevor Mine, Mill

A8093

Geevor Mine, Mill

A8094

Geevor Mine, Mill. Holman-Mitchell table

A8095

Geevor Mine, Mill. The slimes plant

A8096

Geevor Mine, Mill. Tin dressing floor

A8097

Geevor Mine, Mill. Tin dressing floor

A8098

Geevor Mine, Mill. Tin dressing floor

A12381

Wheal Trewavas

A12400

Old china clay pit

A12406

Ding Dong Mine, engine house

A13177

Trewavas Mine, Trewavas Cliff

A13178

The Bishop and Trewavas Head, Trewavas Cliff

A13214

Botallack Mine, Crowns Section above Zawn a Bal

A13215

Botallack Mine and the Crowns, De Narrow Zawn

A13228

Levant Mine, Trewellard South Cliff

A13223

Tailings of Geevor Mine, Boscaswell Cliff

A13233

Levant Mine, Trewellard Zawn

A13234

Geevor Mine, Trewellard North Cliff

General views and miscellaneous

A12389

Sunset over St Michael's Mount and Marazion

A12404

Lanyon Quoit

A12405

Men-an-Tol ('stone of the hole')

A12409

Chysauster Iron Age village

A12410

Chysauster Iron Age village

A13181

Mount's Bay, Castle Gate

A13196

Zennor Quoit, Tremeader Common, Zennor

A13196

Mulfra Quoit, Mulfra Hill, near New Mill

A13203

Boswedden Valley and Cape Cornwall

A13204

Boswedden Valley and Cape Cornwall

A13207

General view of Cape Cornwall

A13210

General view of Cape Cornwall

A13231

Coast south-west of Pendeen Watch, Carn Ros

A13236

View of the coast from Portheras Cove to Gurnard's Head, Pendeen Cliff near Pendeen Cove

A13237

View of the coast from Pendeen New Cliff to Gurnard's Head

A13238

Geonor piston sampler, Tregurtha Farm

A13239

Geonor piston sampler, Tregurtha Farm

A14076

Chan Quoit (Chamber tomb), south of Morvah Church

Figures plates and tables

Figures

(Figure 1) Sketch map showing the topography of the Penzance district.

(Figure 2) Sketch map showing the geology of the Penzance district.

(Figure 3) Map showing the distribution of potassium feldspar megacrysts and enclaves of fine-grained granite.

(Figure 4) Map showing the distribution of elvan dykes and intrusive breccias.

(Figure 5) Diagram showing the styles of folding of the main phases of deformation.

(Figure 6) Sketch section through the Penzance area showing different styles of mineralisation.

(Figure 7) Sketch map showing sites of mineral workings and recent exploration. 1. Levant Mine. 2. Geevor Mine. 3. Botallack Mine. 4. West Wheal Owles. 5. Wheal Edward. 6. Bostraze. 7. Balleswidden. 8. Wheal Hermon. 9. Polpry Cove. 10. Tredinney. 11. Drift Moor. 12. Wherry. 13. Penlee Quarry. 14. Trevider Moor Carnelloe. 15. St Ives Consols. 16. St Ives Bay. 17. Gwithian. 18. Loggans. 19. Coldharbour Moor. 20. Porthia. 21. Giew Mine. 22. Wheal Sisters. 23. Mennor. 24. Hayle Estuary. 25. Mellanear. 26. Wheal Alfred. 27. Wheal Herland. 28. Parbola. 29. Bohemia. 30. Castle an Dinas. 31. Mounts Bay. 32. Marazion Marsh. 33. Wheal Virgin. 34. Wheal Prosper. 35. Marazion Mines. 36. Tremayne. 37. Halamanning. 38. Bosworgy. 39. Binner Downs. 40. Trewavas Mine.

(Figure 8) Section through the workings of Levant and Geevor mines along North Lode, No. 2 Branch Lode, North Pig Lode and Great Wheal Carne Lodes.

Plates

(Front cover)

(Rear cover)

(Geological succession) Geological succession in the Penzance district

(Plate 1) Pordenack Point, south of Land's End. Castellated cliffs formed from megacrystic coarse-grained biotite-granite with well defined joints. (A14071).

(Plate 2) Gwennap Head viewed from Pordenack Point. Rectilinear jointing in granite cliffs. (A14072).

(Plate 3) Trendrine Hill. Granite tors. (A13194).

(Plate 4) Cliffs, south of Godrevy Point. Large, open F2 folds in interbedded sandstone and slate of the Gramscatho Beds. (A 12397).

(Plate 5) Foreshore, east of Marazion. Silty banded Mylor Slates with tight to isoclinal F1 folds refolded by F4 folds with a flat-lying axial-planar cleavage. (A13123).

(Plate 6) Burthallan Cliff, north-west of St Ives. Pillow lava overlain by Mylor Slates; moulding of some pillows indicates that the succession is the right way up. (A13169).

(Plate 7) Kenidjack Cliff. Large knotted porphyroblasts of cordierite. (A13213).

(Plate 8) Trewellard North Cliff. Dark bands of hornblende segregated from pale, weathered plagioclase-rich bands. (A13230).

(Plate 9) Cornish Land Ventures Borehole 28, near Leedstown. Part of an intrusive breccia vein with a prominent rounded fine-grained granite clast, intruded into Devonian sedimentary rocks X 0.95 (MN25707).

(Plate 10) Zennor Hill. The 130 m erosional surface to the north of the Land's End Granite with part of a granite for in the foreground. (A13200).

(Plate 11) Cliff at Porth Nanven, west of St Just. Rounded boulders of granite, up to 2 m across, form part of the raised beach which is overlain by crudely stratiform head, which comprises angular granite debris. (A14077).

(Plate 12) The Crowns, Botallack, north of St Just. The chasm that separates these precariously perched engine houses is the worked-out back of Corpus Christi Lode (MN26829).

Tables

(Table 1) Coppers and tine production of the major mines of the district.

(Table 2) Tin production of Geevor Mine.

Tables

(Table 1) Copper‡1  and tin‡2  production of the major mines of the district

Name of mine

Dates of production

Tons of copper ore, % metal

Tons of black tin3

Alfred Consols, Wheal

1901–65

c.150 000

Balleswidden

1837–73

11 828

Binner Downs

1819–38

51 000

Botallack

1815–1905

22 465 (10–12%)

16 413

Halamanning ( + Retallack)

1781–1859

> 30 000

Herland

1755–74

> 30 000 (9–11%)

Levant

1820–1930

130 000 (10%)

2400

Marazion

1820–1930

30 000

Mellanear

1815–88

70 892 (6% )

Prosper

1836–72

14 600 (6%)

Prosper United

1836–72

22 500 (4%)

St Ives Consols

1827–1915

18 150

Sisters, Wheal (group)

1825–1900

10 696 (6–15%)

c.16 000

Tremayne

1815–68

29 520 (6–9%)

15 942

Virgin, Wheal

1821–47

22 974
  • Figures above are mostly generalisations, for Cornish mine records are inconsistent and usually incomplete. Many mines have no record of production before 1800, and where mines have amalgamated it is difficult to assess total production.
  • 3 Black tin is a general term for a concentrate of cassiterite

(Table 2) Tin production of Geevor Mine

1854–1911

4190 tonnes black tin (65 % Sn) (Includes North Levant, Wheal Geevor, and North Levant and Geevor Ltd)

19121–1984

4 835 855 tonnes ore treated; 51268 tonnes black tin (65% Sn)

Tonnes ore treated

Tonnes ore treated

Tonnes black tin

grade; kg black tin per tonne

range

mean

range

mean

1945–1968

45664–78662

61245

538–759

669

11

1969–1980

91949–118570

105933

794–1110

990

9.5

1981–1984

162558–226693

19510

1254–1508

1352

6.95
  • Since 1945 the amount of ore treated has risen steadily and the output of tin concentrate has almost doubled. The grades of ore treated have slowly fallen, accentuated by the addition of 14 942 tonnes of low-grade dump material in 19802 rising to 119 797 tonnes treated in 1984.
  • 1 Financial years ending
  • 2 Calendar years ending