Cornubian Ore Field, Cornwall - South Crofty, Geevor, Mt Wellington, Wheal Jane, Hemerdon
Sn W Cu
New & Recent International |
Click on image for details.
The Cornubian Ore Field comprises a 180 x 40 km, south-west trending belt on the Cornish Peninsula of south-western England, UK which has been continuously exploited since the bronze age. The key deposits worked more recently have included the Geevor, South Crofty, Mount Wellington and Wheal Jane tin mines and the Drakelands mine that exploits the Hemerdon tungsten-tin deposit. Many of the deposits of the Peninsula were historically significant copper producers prior to the rise of tin.
The oldest rocks of the Cornish Peninsula are the Lizard Series quartzites, mica schists and hornblende schists which are intruded by serpentinite, gabbro, epidiorite and granite gneiss believed to be Cambro-Ordovician in age. These are restricted to the tips of the Lizard, Dodman and Start Peninsulas.
These older rocks are overlain to the north by Ordovician quartzites, which are in turn followed by Devonian sequences comprising a lower conglomerate grit, sandstone and shale sequence, the Gramscatho Beds that are found below the more widespread Mylor Series. The Mylor Series sedimentary rocks comprise strongly folded and faulted shales and mudstones with lesser grits, sandstones, cherts and calcareous rocks. Where within the aureoles of the Permo-Carboniferous granite contact, they have been converted to metasediments, hornfels and skarns, known locally as 'killas'.
Within the shale-mudstone sequence, a series of basic lava flows and tuffs and contemporaneous ultrabasic and basic intrusives are known, locally termed greenstones. These rocks are successively overlain to the north east by (or interfinger with) the Lower and Upper Old Red Sandstone and the Carboniferous Millstone Grit. Spilitic lavas are mapped in both the late Lower and Upper Old Red Sandstone.
The Devonian and Carboniferous sequences are intruded by a series of Permo-Carboniferous granitic bodies, comprising from east to west the Dartmoor Forest, Bodmin Moor, St. Austell, Carnmenellis, Godolphin and Lands End batholiths.
These bodies, in general, have thermal aureoles from 1 to 3 km in width outwards from their margins, marked by silicification in the immediate contact zone (the 'killas') and by spotting at a greater distance. There is evidence that these granitic bodies are the peaks of a single continuous mass running along the Cornish Peninsula, and in depth, the granite underlies the entire area occurs at increasingly greater depths northwards. Overall, the upper granite contact slopes NNW at angles of 40 to 50° but has rolls, ridges and domes, rather than being planar.
Intruding the Devonian sequences and these granites are swarms of dacite porphyry dykes, locally termed elvan dykes, oriented in a ENE-WSW to east-west direction, ranging from a few metres up to 100 m in thickness and up to 3 km in length. They are best developed to the north of, between and within the Godolphin and Carnmenellis Granites, and to a lesser extent in the vicinity of the other batholiths. They are believed to be related in some cases to the finer and slightly later phase of the granitic bodies.
Distribution of Mineralisation
The ore field is related to the Permo-Carboniferous (295 to 270 Ma), s-type ilmenite series high heat, two mica, tourmaline bearing monzogranite batholith intruding a series of folded and thrusted Devono-Carboniferous mud-rocks, sandstones and mafic volcanics.
Base metal deposits generally take the form of steeply dipping lodes emplaced within the roof zone of the steep sided botholith and in the hornfelsed enclosing sediments. These lodes are generally tabular and less than 2 m thick. Some have been followed to depths of 900 m and persist over lengths of as much as 6 km. Most dip at beteen 60° and vertical and have a gangue of quartz, usually with chlorite, tourmaline, feldspar, hematite or white mica. The main metallic mineral is usually cassiterite with associated arsenopyrite, with or without chalcopyrite, chalcocite, specularite, sphalerite and wolframite. Late pyrite is common, as are fluorite and siderite. Grades range from 0.6 to 2% Sn, with variable copper from 0.25 to more than 1% Cu.
The field is estimated to have historicaly produced the following:
Tin ....... 2.5 Mt of metal Copper ....... 2 Mt of metal Lead ....... 0.25 Mt of metal
Zinc ....... 0.025 Mt of metal
As2O3 ....... 0.25 Mt WO3 ....... 5600 t Uranium ....... 2000 t of ore Silver ....... 235 t Iron ....... 2 Mt in hematite & siderite
Manganese ....... 0.1 Mt Barite ....... 0.45 Mt Fluospar ....... 10 000 t Antimony ....... 1000 t Pyrite ....... 0.15 Mt
China clay ....... 100 Mt.
The tin mineralisation of the Cornish Peninsula is largely restricted to the portion to the west of Truro. In this area it is found mainly on the northern margin of the Carnmenellis and Godolphin Granites, from just to the east of Truro, through Redruth and Cambourne to Penzance, with lesser areas within and on the margins of the Lands End Granite. This area, it will be noted, also corresponds broadly to both the distribution of the elvan (dacite porphyry) dykes and the Devonian, basic lavas and tuffs. Near the Geevor Mine, basic volcanics carry disseminated sulphides with a significant tin content (0.1% Sn). Within the Truro to Lands End region, known tin mineralisation is largely confined to crosscutting lodes which occur either as quartz-tourmaline-chlorite veins within both sediments and granites or chlorite-tourmaline-sulphide-silica filled breccia zones within the 'killas' immediately below 'elvan' dykes. Some occurrences are known where cassiterite occurs as fracture coatings and disseminations within 'elvan' dykes. These yield grades of up to 1% Sn over lengths of 150 m and widths of a few metres. Alluvial tin deposits were also worked in the region, although many were partly fed by tailings from mills treating lode ore.
Copper mineralisation is widespread in the same area, often laterally removed from tin in parts of the same lode (e.g., the South Crofty area). As is outlined in the Wheal Jane description below, tin levels of 50 ppm to 0. 5% Sn have also been encountered within siltstones and pyritic or graphitic shales of the Mylor series in the Mt. Wellington and Wheal Jane area.
To the east and north of Truro tin is less important and tungsten assumes more prominence. Notable occurrences are at Cligga Head and Hemerdon where tin-tungsten mineralisation occurs as disseminations and within small Permo-Carboniferous granite cupolas as sub parallel sheeted and en echelon greisen veins.
Numerous minor tin, copper and tungsten bearing veins and iron occurrences are known between Truro and the Dartmoor Forest Granite. None of these is currently of much significance, although some supported important copper operations during the nineteenth century.
On the northern margin of the Dartmoor Forest Granite an anticlinal core of Lower Carboniferous limestones and cherts, interbedded with black shales, calcareous shales and siliceous sandstone, outcrop in an ENE-WSW direction. Sections of the carbonate sequence have zones of coarse grained grossularite-rich, wollastonite calcite, wollastonite-hedenbergite-andradite and diopside-grossularite-idocrase rocks which contain abundant massive pyrrhotite with lesser disseminated chalcopyrite, sphalerite, lollingite, arsenopyrite and bornite. These rocks are interpreted in the literature as skarns related to the Dartmoor Granite. In some areas they locally contain tin mineralisation as malayite. Tin mineralisation with up to 6.8% SnO2 centred on the Red-a-Ven Mine lies within a wide zone of iron 'metasomatism', with the resultant rocks containing up to 0. 2% SnO2. These rocks are associated with the "calc-flintas" of Devon and Cornwall.
Geevor Mine (#Location: 50° 9' 9"N, 5° 40' 34"W)
The Geevor Mine is located between the villages of Pendeen and Trewellard on the far western tip of Cornwall. It was operational between 1911 and 1990 during which time it produced about 50 000 tons of black tin.
Geological Setting - The Geevor tin mineralisation is located within a series of NW-SE trending veins cutting both contact metamorphosed sediments of the Devonian Mylor Series and Permo Carboniferous granites on the north-western margin of the Lands End Granite. The Lands End granite is a coarse grained, grey granite with plagioclase laths up to 6 x 2 cm, quartz crystals up to 5 to 6 mm in diameter, and biotite. In general the granite is completely unaltered except in the immediate vicinity of the mineralised veins, as described later.
The Mylor Series sediments are, in general, fine grained slates which have been contorted. Along the granite margin they have been heavily silicified and comprise a dark grey massive, amorphous, hard siliceous rock which shows no trace of bedding or other sedimentary features. Away from the contact the sedimentary features gradually become evident, although occasional similar siliceous bands are still found. Interbedded within the Mylor Series are "diabase" units which are basic lavas.
In the Geevor Mine area the granite contact dips westward at around 40° while the Mylor Series has a similar but steeper westerly dip and strikes at approximately 40°.
The following sequence of events is postulated:
• Emplacement of the Lands End Granite.
• The development of a series of north-south dykes of fine granite cutting both the Lands End Granite and the Mylor Series. These dykes are assumed to be related to the younger fine granite which intrudes the main batholith.
• The development of the NW-SE trending lodes following jointing and fracture directions. The lodes cut the granitic dykes.
• A series of north-south faults which cut and slightly displace the lodes. The faults resulted in fracturing with some associated hematite. These faults have some control over the mineralisation in the lodes. Two adjacent faults cutting a lode may limit the mineralised section of that lode. In some cases one mineralised lode may be limited by a particular fault, with the mineralisation of an adjacent lode being limited on the other side of the same fault giving a stepped appearance to the mineralisation.
• A series of ENE-WSW trending fractures known as "cross courses" cut all of the above features. These usually carry barren white quartz with occasional traces of hematite and minor sulphides.
Mineralisation The mineralisation exploited at the Geevor Mine occurs within NW-SE trending veins which, in general, dip steeply to the north east. The veins are not mineralised throughout. Within their total extent they carry smaller elongate shoots of payable tin mineralisation which plunge at 15° to the west. These economic accumulations occur in the sections of the veins cutting both the granite and sediments, although the majority is within the granite. Within individual veins the payable ore is bounded laterally above and on the NW ends by a narrow chalcopyrite-arsenopyrite sulphide zone, and on the lower extremities by a larger arsenopyrite rich zone.
The veins and mineralised selvages average from 30 to 45 cm in width and are often mineralised over a km in length and vertically for up to 300 m. As a rule of thumb, it is said that if the main vein centre exceeds 1 m in thickness, it is probably barren or uneconomic.
Veins occur as individuals or as pairs, several hundred metres from other vein developments, or as groups of 5 to 10 veins spread over widths of around 200 to 400 m. In general, the major veins are parallel and when close enough are connected by sub parallel veins. The spacing of major veins is very variable, ranging from 1 m up to 300 m. Individual veins are occasionally distorted by up to 2 m from their overall planer form. This tends to complicate their extraction.
During an underground visit in 1978, a characteristic Geevor vein was examined along a mine opening over an interval of 150 m. This vein nature along the opening as follows. At the first sighting the vein comprised a 20 cm quartz-tourmaline-hematite leader made up of alternating irregular bands, each of which was from 5 to 30 mm thick. The bordering granite is altered and carries high tin levels over a 30 cm width on either side of the leader. Thirty metres along, towards the granite contact the vein is 35 to 40 cm wide, but in a further 10 m has decreased to 10 to 15 cm. Here it is well banded and is notable for a 2 to 3 cm thick band of cassiterite at its centre and some discontinuous bands and blebs of chalcopyrite. A further 20 m along, the vein is 15 cm wide and has a central 2 cm thick band of tourmaline with a lattice work of brown cassiterite through it. The other constituents of the vein are as described previously, with quartz being about 25% of the total vein. Thirty metres further along, the vein has developed into what is apparently a typical high grade zone and comprises from the foot wall:
• Fresh un-altered granite.
• A single tourmaline band, 1 cm thick.
• Banded hematite-quartz with quartz being 75% of the rock. This zone is 5 cm thick.
• A single hematite band, 1 cm thick.
• Massive chlorite with cassiterite, 5 cm thick.
• Hematite band, 1 cm thick.
• Highly altered hematitic granite, 5 cm thick.
• Pink (flesh coloured) altered granite, 15 cm thick.
• Tourmalinised granite, 5 cm thick.
• Fresh granite.
About thirty metres further along the drive, the granite-sediment contact was encountered. About thirty metres further along the vein began to have blebs and bands up to 5 cm across of chalcopyrite and arsenopyrite indicating the approach of the margin of the mineralised section of the vein. Cassiterite bands were still seen in this section. The drive ended here.
Tin mineralisation is apparently usually associated with the quartz tourmaline and chlorite sections of the veins. A vein like that described above would assay between 5 and 10% Sn over the width of the vein and selvage.
The Lands End Granite has a regional background at 80 ppm Sn.
The production rate is of the order of 110 000 to 120 000 tpa of ore. The average grade for the 6 month period from April 1st to October 31st 1977 was 0.919% Sn, compared with a grade of 0.836% for the year ended Dec. 31st 1976. During this period, the mill recovery is of the order of 80%, occasionally being as high as 85%, of available tin in the ore.
South Crofty Mine (#Location: 50° 13' 20"N, 5° 16' 31"W)
The South Crofty Mine is within the township of Pool between Redruth and Cambourne in Cornwall. For several centuries, shallow workings exploited copper rather than the deeper tin mineralisation. References implying hard rock mining, rather than placer working, occur in the area as early as 1592, and clearly by the mid-17th century mining was well established. Mining for copper continued in bursts over the ensuing two centuries. Tin became increasingly important from the 1860s onwards as the price of metal improved and the mine deepened – although copper was still an important product up to the end of the nineteenth century when it was replaced by arsenic and, later, tungsten as the chief by-product of tin mining. Tin was first extracted as a major product in 1873. Tin mining has continued since then with periods of closure. It last closed in 1998, but since 2004 has been under development for further operaion. During the 20th century the mine was an amalgamation of 12 earlier mines worked in the past for tin and copper.
Geological Setting - The South Crofty tin mineralisation is contained within sub-vertical ENE-WSW trending veins dipping both to the north and south, cutting the Permo-Carboniferous Carnmenellis Granite. These veins occur within a ridge like irregularity on the contact of the batholith which outcrops as an elongate granitic exposure of some 5 x 1 km surrounded by intruded sediments on the north eastern margin of the main granite.
The granite contact dips at 20 to 25° to the north-west, below the metamorphosed, intruded Devonian Mylor Series slates and interbedded basic lavas.
The nearby Wheal Pendarves Mine, which was worked as part of the same operation in the late 20th century, is developed on the same granite ridge, where the crest has plunged below Mylor Series sediments to the south- west. In this region the granite ridge is covered by some 60 m of sediments at its crest.
The fresh Carnmenellis Granite comprises a grey, coarse grained granite with plagioclase feldspars from 4 mm in diameter to laths of 6 x 15 mm, quartz crystals around 3 mm in diameter, and biotite. In general the granite is not altered, except on the margins of veins and in restricted areas where there are one or two thin (5 mm) thick quartz-tourmaline veins. In these cases, the granite is kaolinised over a width of up to 1 m straddling the veinlets.
In the South Crofty area there are a series of around five ENE trending vein systems, each only 100 to 200 m wide. They are distributed over an area some 5500 m in length and 1800 m in width. In the northern-central part of the area, a NW to W trending series of 'caunter' veins crosscuts the series, while a smaller zone is apparent to the south west. All of these systems fall within leases controlled by South Crofty.
The main series of veins being mined when visited in 1978 by South Crofty Mines comprised about 20 veins made up of 12 main veins with a number of offshoots. This vein system is within a length of some 2. 4 km and width of 800 m and is developed to a depth in excess of 750 m. To the west this group of veins is cut by a major cross-course fault, which, in addition to determining the western margin of the South Crofty Lodes, also delimits the neighbouring Dalcoath veins to the east.
The north eastern margin of the South Crofty Lodes is determined by their passage out of the granite into the Mylor Series sediments. The veins feather out and lose their tin content within the sediments.
Within the ore zone there is some faulting, but this is usually normal faulting and causes no problems with the extraction.
Structure - Mineralisation is largely present in the form of 'lodes' (i.e., fissure veins and associated mineralisation) within a series of fracture zones that are the result of complex normal and reverse faults related to stresses developed due to both intrusion within the killas and cooling within the granite. These fractures were pathways for the passage of magmatic mineralising fluids, and sites of deposition of the early hypothermal tin-tungsten
mineralisation, and for later mesothermal and epithermal phases related to convecting meteoric and connate fluids.
In the New Cook’s Kitchen Shaft, the complexity of the lode fissure relationships is demonstrated, with north and south dipping structures joined by bridging veins, giving an overall dendritic appearance. In section, many of the lodes have curved surfaces, e.g., south dipping veins in the granite, may reverse their dip, upon entering the overlying killas, to become subvertical or north dipping. There is a general increase in the number of structures upward, where major deep fractures, upon entering shallower levels of lower regional stress tend to branch and 'horsetail', with conjugate or ladder veins forming in the intervening ground.
Several of the lodes found within the contact zone in this area fault the granite-killas contact, emphasisinging their primary mode of formation as dislocating structures. These fracture zones now host complex multiphase lode structures, the strongest of which persist for up to 1 to 2 km along strike and for dip heights of up to 600 to 1000 m. This section is paraphrased from Western United Mines Limited website, 2016..
Mineralisation - Tin mineralisation at the South Crofty Mine is contained within a series of quartz-tourmaline-hematite veins±chlorite. High grade mineralisation is usually found within less siliceous veins in association with chlorite and/or tourmaline. Fluorite is a common ore associate.
The Western United Mines Limited website, 2016 describes the five following main phases of mineralisation that have been identified within the recent workings:
• An early black tourmaline (schorl) phase, with thin, tin bearing stringers of schorl emplaced throughout the fracture zones. The tungsten bearing (greisen type mineralisation) quartz floors and pegmatite veins (described below) are of similar age.
• A blue tourmaline phase, which carries the majority of the economic tin mineralisation in the form of fine‐grained cassiterite, and may be in discrete seams, veinlets or disseminated grains. This phase has signs of very rapid crystallisation, often displaying brecciation textures related to explosive decompression.
• A chlorite phase, which often overprints the previous phase, characterised by as dark green crystalline chlorite as the dominant gangue mineral. It often carries coarsely crystalline cassiterite, as disseminations and seams, which may form classic 'sparable' type crystals.
• A tin-barren fluorite phase, which occupies sections of the lodes with 'caunter orientation', where the lodes have been faulted by later tensional wrench faults. These intra-lode segments, which have the same strike as east-west trending caunter lodes, have been infilled with a fluorite-haematite-earthy chlorite-quartz paragenesis, substituting earlier tin rich phases of mineralisation.
• A 'caunter lode' phase, representing later mesothermal/epithermal mineralisation that was emplaced into east-west trending fractures. These lodes are typically poor in cassiterite, carrying a gangue of early amorphous chlorite-haematite-fluorite-quartz, with copper-lead-zinc-bismuth mineralisation. Where crossing the earlier lodes they fault them, often with considerable displacement.
• A 'cross-course' phase, which are infilled wrench faults. Most have final phases of displacement and mineralisation which post‐date all of the preceding phases. Many have a generally north-south orientation related to Permo-Triassic wrench faulting. These faults carry an epithermal assemblage of chalcedonic silica with earthy chlorite, hematite, minor amounts of marcasite, and occasional copper and bismuth sulphides. Displacements along
crosscourses vary from a few cm, but typically ~1 m, to >100 m in the case of the Great Crosscourse. Many lodes also show intralode shearing related to this
phase and carry the same paragenetic sequence as infilling/replacements within the lode.
Two representative mined veins were inspected during a visit in 1978, neither being high grade lodes. The first was made up as follows, from the footwall:
• Altered muscovite to sericite rich granite with only quartz crystals remaining of the original granite.
• A 20 cm band of fine chlorite.
• White quartz 10 to 15 cm thick.
• Black tourmaline rich quartz with crosscutting and parallel white quartz veinlets from l to 10 mm thick.
• White quartz over a 10 cm width.
• Black tourmalinitic quartz, 2 cm wide.
• White quartz.
• Altered granite.
There is some disseminated pyrite within the vein, which, along with the altered granite selvage, is heavily stained with hematite. This section of the vein is massive and siliceous. In higher grade sections it is usually more haematitic and friable.
A second vein visited comprised the following from the footwall :
• Fresh granite.
• A 30 cm selvage of pink (flesh coloured) altered granite with only the quartz crystals remaining of the original texture.
• Black tourmalinised quartz with stringers of white quartz cross cutting and parallel to the vein. The stringers vary from 1 to 20 mm in thickness.
• Alternating white quartz and hematite over a 10 cm width. The individual bands are from 5 to 20 mm thick, with 30% of the rock being quartz.
• Black tourmalinitic quartz, 30 cm thick.
• Pink altered granite about 30 cm thick.
• Fresh granite.
In the lower levels of the mine some 'pegmatite' veins are encountered. These dip shallowly to the south at around 10 to 20° and comprise thin quartz veins (2 to 10 cm thick) with a few associated similarly sized pink feldspar veins. The quartz veins occasionally carry 2 cm wide aggregate of 3 mm wolframite crystals. Wolframite is also found in the vertical veins and is extracted in the mill.
In some areas the main veins plus their mineralised selvages locally carry mineable grades over widths of 10 m, but these are only relatively minor in comparison with the total reserves.
Copper was mined in the South Crofty and neighbouring vein systems during the last century and in the early 1900s. Copper occurs within the lode systems where they cut the Mylor Series slates above the granite, while tin occurred below the granite contact. A mixed copper-tin zone was mined near the contact.
At the Wheal Pendarves Mine only two tin bearing vein systems are worked. The first of these is called the Tryvhena vein and comprises a hematitic lode system while the other, the Harriet, is a tourmaline quartz vein series.
Annual production in 1977 was 218 000 t @ 0.99% Sn, with a mill recovery of ~75%.
Reserves quoted in 1977 were
Probable ore - 317 800 tonnes @ 1.69% Sn.
Indicated ore - 676 750 tonnes @ 1.69% Sn.
Inferred ore - 1 250 000 tonnes.
Mount Wellington Mine (#Location: 50° 13' 58"N, 5° 8' 31"W)
The Mount Wellington tin mine is located near Bissoe, to the south east of Redruth in Cornwall, and just over 1 km to the SW of the Wheal Jane Mine. Construction of the mill and surface installations was completed in early 1976 and production began later in the year.
Geological Setting - The Mount Wellington tin mineralisation is contained within crosscutting lode zones which cut the Devonian Mylor Series, some 3.5 km to the NE of the Permo-Carboniferous Carnmenellis Granite. 'Elvan' dykes, which parallel the strike of the Mylor Series but cut the bedding down dip, are parallel to, and immediately above, the main No. 1 lode.
In the mine area, the Mylor Series comprises ENE-WSW striking grey siltstones and shales. They dip at around 10 to 15° to the north and comprise a relatively monotonous sequence. These sediments are cut in the mine area by a single large, and a number of smaller 'elvan' dykes. The main 'elvan' dyke outcrops sporadically over an interval of around l.5 km and is of the order of 20 m thick. It trends parallel to the strike of the Mylor Series but dips northwards at from 35 to 40°.
'Elvan' is a local term for a dacite porphyry. Within the Mount Wellington Mine two types of 'elvan' are known, one is a kaolinitic (or white) variety, the other siliceous. The kaolinitic elvan comprises kaolinised feldspar phenocrysts from 2 to 3 mm across, set in a fine, pale grey-green, kaolinitic ground mass. The siliceous 'elvan' is a variety of the same in which feldspar laths from 2 to 4 cm wide, and 1 mm diameter quartz crystals, are set in a grey siliceous matrix. The main 'elvan' is predominantly the siliceous variety.
The No. 1 Lode occurs immediately below the main 'elvan', and consequently strikes ENE and dips at 35 to 40° to the north. This lode is worked over a strike length of around 200 m and down dip for a distance of up to 300 m. The average thickness is from 2 to 3 m. The lode is cut and terminated to the west by the Mount Wellington fault which strikes north-south and dips to the west at 80°. About 150 m to the west, a second parallel fault, the Adit Fault, is developed. The Mount Wellington Fault is a relatively thin structure, less than 1 m thick, characterised by a crush zone. It has a throw of 20 to 25 m. The Adit Fault in contrast is represented by a 4 m wide white quartz filled interval. The throw on this fault is of the order of 10 m.
The No. 2 Lode is developed below the No. 1 Lode and is cut by both faults. To the east it is terminated at its intersection with No. 1 Lode, and to the west it feathers out in most areas, but may be connected in others, with small lodes developed below minor 'elvan' dykes. This lode strikes NE-SW and dips to the north at 45°. The intersection with No. 1 Lode plunges to the east. No. 2 Lode is developed over a strike length of 200 m, thickness of 2 m and down dip extent of at least 450 m.
No. 3 Lode, which is thin (averaging 1.5 m ) and erratic is developed only to the east of the Mount Wellington Fault. It strikes parallel to the No. 1 Lode, but dips more steeply at 50° to the north. Its upper margin is at the intersection with the overlying No. 1 Lode, 90 m below the surface.
A series of other minor lodes (eg. the Hot Lode) which were largely worked out last century, were developed to the west of the Adit Fault.
Mineralisation - Tin mineralisation at the Mount Wellington mine occurs as invisible cassiterite within crosscutting pyrite lodes. There are two main lodes and a number of less significant developments, as outlined previously.
The No. 1 Lode was inspected at five different localities during a visit in 1978. In general, it varies from 1 to 3 m in thickness, but averages between 2 and 3 m. It occurs immediately below the main 'elvan' dyke. The base of the 'elvan' dyke is usually underlain by a low grade zone from 10 cm to 3 m in thickness. This comprises either a highly siliceous grey rock with 10 to 15% pyrite as a stockwork of veinlets and disseminations, or as chloritic shales of the Mylor Series with a similar pyrite content in a similar form. Below this, the main extractable portion of the lode is found over a thickness of from one to two metres. The upper half is usually more pyritic than that below. In some places this section has up to 50 to 60% pyrite and is a semi-massive to massive sulphide band. The lower part varies from 10 to 15%, up to 30% pyrite. There is a general tendency for there to be more pyrite in the lower levels of the mine.
The lode zone is usually heavily chloritised with brecciated ex-white quartz veins making up 5 to 15% of the rock. The pyrite is present as fine to medium grained disseminations and as irregular veinlets. In general these veinlets are present in three erratic but mutually normal directions.
The No. 1 Lode is relatively consistent in thickness and grade, with from 0.6 to 0.7% Sn. Unlike the other lodes it also carries 1 to 2% Zn. The lower margin of the lode is usually fairly sharp, marked by a sudden decrease in the chlorite content. The rock below is a chloritic but recognisable shale with 5 to 15% pyrite. This pyrite zone extends for 1 to 2 m below the main lode.
No. 1 Lode in contrast is harder, more brittle, more siliceous and more erratic in grade and thickness. This lode was seen at two widely separated parts of the mine. It ranges from 1 to 3 m in thickness but averages about 2 m, but often branches into two or more veins, particularly on its margins.
Mineralisation is present as 3 roughly mutually perpendicular, irregular directions of fine pyrite veinlets from 1 to 3 mm thick with accompanying disseminations. The lode horizon is siliceous and tourmalinitic. When chlorite is present it is usually coarsely crystalline, in contrast to the fine chlorite masses of No. 1 Lode.
White quartz often occurs within the lode and comprises 5 to 10% of the rock. Pyrite usually totals 15 to 20% of the lode. No. 2 Lode carries grades of between 0. 5 and 2% Sn.
No. 2 Lode always has accompanying quartz chlorite leaders from 10 to 50 cm thick either at the centre, hanging wall or footwall margins. This lode tends to be thickest in pockets adjacent to the major faults suggesting they had a secondary effect on its development.
Annual production in 1977 was ~156 000 tpa @ 0.6 to 0.7% Sn, with a mill recovery of 45 to 50%.
Wheal Jane Mine (#Location: 50° 14' 32"N, 5° 7' 37"W)
The Wheal Jane Mine is located near the village of Baldhu, some 7 km due east of Redruth, and just over 1 km NE of the Mt. Wellington Mine. The present day mine is set amongst twelve old mines, some worked as early as 1700 AD, while one, Wheal Widden, dates back to 1684. These were mined with continuing profitably until 1873 when competition from Australian producers led to a severe depression in Cornish tin mining. The first Wheal Jane survived until 1884. It was re-opened in 1906, but closed again after only seven years. In the mid 1960s, Consolidated Gold Fields commenced exploration in the area. By 1969 sufficient reserves had been confirmed to allow a decision to mine. Development and construction was completed and production begun by late 1971. As indicated above, the Wheal Jane lodes were known prior to Consolidated Gold Fields interest in the area, but fell under the category of well developed lodes rendered uneconomic to previous workers by unfavourable metallurgical properties, in this case the fineness of the cassiterite.
Geological Setting - The Wheal Jane orebodies are found within sediments of the Devonian Mylor Series, closely associated spatially with intrusive dacite porphyry dykes, or 'elvans', some 5 km to the north-east of the Permo-Carboniferous Carnmenellis granite. In the Wheal Jane area the Mylor Series comprises, in general, well laminated grey to black siltstones to shales. The laminations vary from 1 to 5 mm in thickness and are often slumped. Minor tuff bands are found within the mine area. The sediments dip to the south at a shallow angle to the south of Wheal Jane, although in the vicinity of the orebody they flatten. In the mine area the sequence is intruded by a group of en echelon 'elvan' dykes, rising both vertically and laterally to the east. These dykes cap 90% of the economic tin lodes and dip to the north at between 35 and 60°. They average around 20 m in thickness but vary from less than one to 30 m.
The 'elvan' comprises feldspar phenocrysts and laths from 1 to 3 mm in width, and clear quartz crystals around l mm in diameter, set in a fine grey matrix. In places xenoliths of coarse-grained granite are found within the 'elvan'. Chilled margins are usually obvious, both at the top and bottom of the dyke. In these margins the 'elvan' is a fine-grained siliceous rock with little texture other than the quartz crystals remaining, although in some areas, what is interpreted as flow banding is obvious.
The tin mineralisation within the mine occurs as a series of lodes branching off the upper margin of the Main or Moors Shaft (M) Lode. The M Lode varies from 1 to 3 m in thickness, but averages around 2 m. It dips at from 50 to 60° to the north, locally being as steep as 80°. The branching lodes dip at around 35 to 45° to the north, except M Lode North which is the lowest and western-most. It also dips at between 50 to 65°, but towards its upper margin flattens to intersect the main M Lode.
The main branching lodes, like the 'elvan' dykes which cap them, are developed in an en echelon pattern, rising both vertically and laterally to the east. These are from west to east and from bottom to top of the 'en echelon stack' M Lode North, C, Ca, B2 and B1 Lodes. This group of en echelon lodes plunge to the west at between 10 and 15°. The lodes are extremely variable in thickness, ranging from say 3 m, to less than l m in 2 or 3 m in some cases. The branching lodes are in general from 1 to 4 m thick. B lode, the best developed lode, averages 3.6 m in thickness.
The M Lode at Wheal Jane corresponds to the No. 2 Lode at the neighbouring Mt Wellington Mine, while the branching lodes correspond to No. 1 Lode at Mt Wellington. A series of north-south trending faults (or cross courses) have been mapped, cutting the lodes. These have a throw of the order of l m or so. Those seen during the visit were around 70 cm thick and were filled with white quartz and lesser puggy clay. These appear to have had some influence on the mineralisation as they often mark the margin of economic sections of a lode they cut, as also occurs at Geevor. Each of the branching lodes is capped by an 'elvan' dyke which steepens at the hinge with the M Lode and follows the latter upwards. The M Lode tends to flatten slightly above the hinge line. Below the hinge zone, the M Lode often cuts the sediments without an accompanying elvan dyke.
In some sections of the mine, groups of offshoots splay off the hanging wall of the M Lode below the main branching Lodes. These are generally around 1 m in thickness but can locally be as thick as 2 or 3 m, and are of limited lateral extent. In the hinge zones with these hanging wall offshoots the M Lode is often as thick as 6 m. To the south-west, the main lode system is cut by a steeply dipping east-west copper bearing fault zone known as the Nangiles Lode. This displaces the Wheal Jane Lodes by 5 to 10 m, and is then believed to pass into the Mt Wellington Leases across the boundary pillar. The main mineralised lode system extends over a length of 3 km. To the east, the lodes thin and become lower grade. In some areas, B lode in particular is cut by 'elvan' dykes associated with adjacent lodes. The lode can be traced through the 'elvan' as an unmineralised chlorite zone which develops into ore when it emerges
Mineralisation - The thickness, grade, texture, mineralogy and grade of the lodes within the Wheal Jane Mine are all very variable, often changing laterally within a few metres. The lode often comprises strongly brecciated, altered and sulphidic sediments occurring immediately below the 'elvan' dykes. Typical ore types, of which there are a large number recognised, are silicified, tourmalinised, chloritic, banded pyrite and sphalerite in a chloritic gangue or pyritic, brecciated sediments. An example of brecciated ore seen comprised breccia fragments from 2 or 3 mm to 1.5 cm composed of tourmalinised siltstone, chloritised siltstone and siliceous siltstone in a fine, chlorite pyrite-arsenopyrite matrix with later cross-cutting fine pyrite veinlets. On the lower levels of the western section of the mine, the M Lode often has associated central quartz veins of from 10 to 30 cm in thickness which are usually barren, although occasionally they are high grade. The remainder of the lode is composed of brecciated, tourmalinised, chloritised, siliceous sediments which have been invaded by sulphides which, with chlorite, form the matrix of the breccia. Sulphides comprise pyrite with slightly less sphalerite and arsenopyrite, and form up to 30% of these lodes. The branching lodes are characterised by fluorite, arsenopyrite, sphalerite, pyrite and trace wolframite with a little pyrite in places.
In some sections of the western lower levels of the mine, the interval between the M and branching lodes has sulphide-cassiterite stringers with tourmalinised margins which yield ore grade widths. Cassiterite within the Wheal Jane lodes is very find grained, 50% being less than 50µm. Visible cassiterite is extremely rare. Visual indicators of tin mineralisation are quartz, chlorite, tourmaline and arsenopyrite. There is apparently a low order alteration zone below some of the lodes over widths of up to 30 m, forming a buff coloured sericitic rock. The margins of the lodes are sometimes sheared, but in many cases comprise less altered sediments with some disseminated sulphides and subeconomic to very low order tin levels. On the eastern margins of the orebody some small stannite bearing zones have been encountered. These areas however, have high associated silver levels of up to 1000 g/t.
Within the orebody there is a good correlation between Sn grades and Cu and As levels, but not with Zn values. The Moors Shaft Lode is regarded as being the feeder for the sulphides and tin mineralisation, rising from the west. South dipping zinc rich, but tin poor lodes are also known within the mine area.
In the Mt Wellington-Wheal Jane area some pyritic shale units are known that are from 5 to 6 m thick carrying 50 to 100 ppm Zn. These are found, in particular, on the eastern margin of the Wheal Jane property where thin 10 to 20 cm thick graphitic shale beds carry 0.1% to 0.2% Sn. In some localities within the mine, well laminated shales with no veining, sulphides or alteration have yielded levels of 0.5% Sn over limited widths of 1 to 2 m).
Production in 1977 totalled 164 236 t @ 0.88% Sn, 0.26% Cu, 2.73% Zn, with a mill tin recovery of 68.4%.
Hemerdon Deposit (#Location: 50° 24' 31"N, 4° 0' 35"W)
The Drakelands Mine (formerly known as the Hemerdon Mine, or the Hemerdon Ball or Hemerdon Bal Mine) is a tungsten and tin mine, located 11 km NE of Plymouth, in Devon. It lies to the north of the villages of Sparkwell and Hemerdon and adjacent to the large china clay pits near Lee Moor. The mine was out of operation since 1944, except for the brief operation of a trial mine in the 1980s, but went into large scale production in 2014.
At Hemerdon tin-tungsten mineralisation occurs within a small outlying Permo-Carboniferous granite cupola as disseminations and in sub-parallel and en echelon greisen veins. Fractures in the granite and 'killas' have been penetrated by mineralising fluids. Two vein-types are evident, with three different orientations. Quartz and quartz-feldspar veins form a stockwork with minor mineralisation, whilst greisen bordered veins are found in a sheeted vein system with wolframite and minor cassiterite mineralisation. The host cupola is a dyke-like granite body, extending from the Hemerdon Ball towards the Crownhill Down granite. Although principally within the granite the mineralised veins also occur in the flanking 'killas', but with lower grades of wolframite and cassiterite.
The mineralisation, which is present largely as wolframite and/or cassiterite, is accompanied by arsenopyrite, pyrite and locally stannite, occurs in quartz veins and stringers with greisen margins within kaolinised granite. Veining is at sufficient density to give a bulk mining grade of ~0.2% combined Sn and W. Surficial weathering of the granite has altered the primary quartz, feldspar, muscovite assemblage to clay minerals, mainly kaolinite, and remnant quartz to depths of up to 50 m in the granitic body.
The Hemerdon deposit is >1 km along strike, 600 m wide and persists to a depth of ~500 m.
JORC compliant measured + indicated + inferred resources at Hemerdon in 2015 were 145.2 Mt @ 0.15% WO3, 0.02% Sn, including proved + probable reserves of 35.7 Mt @ 0.18% WO3, 0.03% Sn (Wolf Minerals Limited, Annual Report, 2015).
Note: These descriptions, except where otherwise specified, are drawn from visits to these mines in 1978 while they were still in operation, and reflect the knowledge at that stage. More recent information is available from the references listed below.
The most recent source geological information used to prepare this summary was dated: 2014.
Record last updated: 9/2/2016
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
References to this deposit in the PGC Literature Collection:
Clayton R E, Spiro B 2000 - Sulphur, carbon and oxygen isotope studies of early Variscan mineralisation and Pb-Sb vein deposits in the Cornubian orefield: implications for the scale of fluid movements during Variscan deformation: in Mineralium Deposita v35 pp 315-331|
Jackson N J, Willis-Richards J, Manning D A C, Sams M S 1989 - Evolution of the Cornubian ore field, southwest England: Part II. Mineral deposits and ore-forming processes: in Econ. Geol. v84 pp 1101-1133|
Muller A, Seltmann R, Halls C, Siebel W, Dulski P, Jeffries T, Spratt J and Kronz A, 2006 - The magmatic evolution of the Lands End pluton, Cornwall, and associated pre-enrichment of metals : in Ore Geology Reviews v28 pp 329-367|
Willis-Richards J, Jackson N J 1989 - Evolution of the Cornubian ore field, southwest England: Part I. Batholith modeling and ore distribution: in Econ. Geol. v84 pp 1078-1100|
Top | Search Again | PGC Home | Terms & Conditions