The group assembled for the morning session in the Meldon Aplite Quarry car park and Richard Scrivener of the BGS gave an introduction to the general area with reference to the local Geological Sheet (No 324 Okehampton).

The area in question lies about half a mile north of the Dartmoor granite boundary but well within the aureole of thermal metamorphism. The country rock is a folded series in the Lower Carboniferous series termed by Murchison the Culm Measures. These comprise the Meldon Chert Formation, Meldon Slates, quartzites and volcanic group and the Meldon slates with lenticles. Locally these are interspersed with large limestone outcrops, thought possibly to be pieces of continental margin 'olistolites' that slid down into the deep ocean floor. Further north the rocks grade into the mudstone and sandstones of the Upper Carboniferous Crackington formation. The whole form a syncline around the West Okement River with 2 anticlines overturned to the north by thrust faulting.

There are 3 quarries in the vicinity. A few 100 metres to the north of our vantage point is the site of the earliest, which exploited a limestone outcrop. The stone was used for both building and lime burning, with a succession of limekilns at the site. Cut off from a ready supply of beach sand and shells or seaweed for soil conditioning burnt lime was in great demand to raise the soil pH, consequently the quarry was worked to a depth of about 130 feet (40 metres). Documented from at least 1790 this activity had ceased by the late 1800s and the quarry is now flooded and known as 'Meldon Pool'.

The second quarry is the extensive BR aggregate quarry to the east. This has been in use since at least 1874 when the viaduct was built and supplies ballast for lines. The thermally metamorphosed hornfels from this quarry are well suited to this purpose as they have a very high crushing point and include Meldon Cherts with similarly high crush points, in the region of 75 to 80 000 lbs/square inch (cf Dartmoor granite 35 to 40 000 lbs/square inch).

The Aplite Quarry has been worked in 2 bays bisected by Red-a-ven Brook. The aplite (sometimes also known as granulite) found here is a unique feature in the south west of England. The earliest documented quarrying dates to the beginning of the 20th century and though still being worked in the 1940's it had closed by 1976.

The main interest in the aplite was for use in glass making. Initial efforts in the 1890s utilised material discarded by earlier quarrying. However, by 1920 there was a large investment and high hopes for establishing a major enterprise with up to 12 furnaces and 500 workers. In the event it seems that only 2 furnaces at most were ever built and the whole venture was closing down by early 1921. The output of this short lived venture seems to have been mostly medicine and cosmetic bottles. Little if anything of the remains on site today can be attributed to that enterprise.

In the west bay of the Aplite quarry are several notable features. The aplite has been intruded into the country rocks as a dyke of fine-grained white rock. The present exposure is approximately 10m wide, dipping at an angle of about 70° to the south to meet the granite and occupies most of the inner face of the quarry. At the hanging wall to the north the aplite contacts the dark Upper Devonian slates, which have been altered by thermal metamorphism to spotted hornfels and quartzite. In places where small aplite off shoots have occurred pegmatites have been formed. A feature of these pegmatites, or 'stockschieder', are the large feldspar crystals, which formed hanging down into the melt and may have a curved form described as being much like a bunch of bananas.

The west bay.

Pegmatite.

Whilst the main bodies of the Dartmoor granite and associated aplite dyke were intruded at a (relatively shallow) depth of 8 to 10 km approximately 280 Ma, local uplift was at a relatively high rate of about 2mm per year. Thus within 3 to 4 million years the aplite dyke was only at 2 to 3 km depth following surface erosion. Circulating groundwater facilitated extensive hydrothermal activity and as a result there are a number of rare minerals associated with this dyke. Most notably in this section the dark brown mineral axinite and purple fluorite are commonly found though rarer minerals such as fluorapatite, hedenbergite, albite and apatite also occur. In addition the conditions were conducive to polymetallic mineralization seen extensively in the Dartmoor aureole. Quantites of axinite were found, one piece forming a stringer in the hornfels with both pyrite and chalcopyrite inclusions.

In the smaller east bay the footwall of the aplite dyke is well exposed. Here the aplite has been intruded into the cherts and mudstones. Thermal metamorphism has produced calc silicate minerals such as wolastonite and granite; collectively termed skarn from the Swedish meaning waste. As skarns form there is a decrease in the volume of the rock, which causes microfractures to form. It is in such fractures that hydrothermal fluids can begin to circulate and as a result skarns are often associated with tin and tungsten mineralisation both in south-west England and further a field. Axinite, fluorite and petalite were all observed as well as ripple marks in the mudstones.

In both sections, a small amount of manganese mineralization was also observed. The source of the manganese was nodules formed on the seabed, which have been incorporated by subsequent tectonic and hydrothermal processes.

A little further upstream on the Red-a-ven Brook is the site of Meldon mine (aka Okehampton Wheal Maria, Red-a-ven Mine or Devon Copper Mine). The copper occurred here as mineralised bands principally of chalcopyrite associated with arsenopyrite and pyrite: a sulphide skarn, and the mine is thought to lie on the same E.N.E. trending lode as Homerton and Forest Mines. However, the mine was originally intended as a tin prospect. Reworked in the 1920s on the basis of chemical analysis it became apparent that the high tin content occurred not as cassiterite but was incorporated in the garnet and malayaite (CaSn[O|SiO4] and was therefore commercially worthless.

The main feature of the site is the collar of engine shaft, which is enclosed by 2 spoil tips. Between this and the brook is the bed of a leat and a wheel pit, which is now in filled with rubble. Inspection of the dump in the bottom of the valley yielded samples of various sulphide ores: chalcopyrite (CuFeS2), sphalerite (Zn,Fe2+S), arsenopyrite (FeAsS) and pyrite (FeS2) but also lollingite (FeAs2) and green grossular (Ca3Al2(SiO4)3.



The afternoon walk began after lunch at the Warren House Inn on the Moretonhampstead to Princetown Road.

The Birch Tor, Vitifer and Golden Dagger mining complex was the largest and most productive of the Dartmoor tin works. The site is comprised of 5 setts, which have been worked individually and in conjunction over the years. Immediately across the road from the Inn is the West Vitifer sett, separated from the Vitifer sett to the east by a slight ridge of moorland. Beyond these to the south but still on the west side of the brook is the Golden Dagger sett. On the eastern hillside beyond the brook and below Birch Tor is the Birch Tor sett. Beyond the ridge on the crest of which sits Birch Tor itself is the East Birch Tor sett, incorporating Headland Mine.

Cassiterite (SnO2), the principal tin bearing mineral, is not only hard (Mohs 6-7) but is also very tough as the tetragonal structure is without cleavage. It also has a high specific gravity in the region of 7g cm-3. Consequently once it has weathered out of the host rock it suffers little abrasion in streams and frequently forms placer deposits.

Once the 'Old Men' found the black tin (cassiterite), either as placer deposits or shode stones, they would establish a streaming works and this was probably how the Vitifer mine began. The principal was simple though the layout of sites is often very difficult to elucidate. The first step was usually to divert the stream that had deposited the tin from its course by a series of leats so that the deposits could be worked. A jigging table was then set up at 90 degrees to the course of the streambed. This was in the form of a trough or chute slightly inclined, with transverse slats fitted to the bottom, to which water from the leat could be run using a launder. The ground upstream of the jigging table was then dug up and loaded onto the jigging table, large pieces of waste rock would be removed by hand and the lighter sand was washed away by the flow of water, leaving the cassiterite on the base of the jigging table caught by the transverse slats. Once the ground within reach had been worked out the jigging table and launder were moved to the next section.

As often as not as the streaming operation moved up the deposit the lode of cassiterite which had been weathered down to form the placer would be encountered. With little mechanisation these lodes were often little worked by the Old Men.

In the late 18th and early 19th centuries the application of waterpower greatly assisted the miners. Waterwheels were set up fed by complex networks of leats. The power was distributed around the sett by flat rods; a series of metal rods resting on rollers supported on tripods. The circular motion of the wheel drove a rocker bob, imparting a horizontal motion to the rods which could be turned either horizontally or vertically by corner bobs. Stamps to crush the ore, jigging tables and buddles for ore separation, and pumps for de-watering the workings then used the power.

From the Inn the Birch Tor part of the sett can be seen, dominated by Lean's lodes climbing the hillside south of Birch Tor itself. The track through the Vitifer sett winds down the hillside from a car park on the far side of the road and a little east from the Inn. The layout of the sett can be seen best from Birch Tor. To the south of the curving track is the largest excavation along the line of Lance's Lode, to the north are Wall's and Paul Lode and closest to the road Hambley's and North Lode. These lodes were worked both open cast but also by shafts: Wall's, Bather's, Henry's, Jenkin's etc. and adits.

Vitifer Sett

At the bottom of the footpath that forks off to the south from the track there is an open space, which is the site where the main mine buildings once stood. The base of the chimney can still be seen. It was expensive to transport coal to the middle of the moor especially as there was so much free waterpower available. Consequently there was no blowing house on the Vitifer sett; the cassiterite was transported off the moor for smelting and the chimney was probably for a forge. Upstream (north) of the chimney base are a series of low walls enclosing rooms: forge and miners dries. Downstream are the dressing floors. The foundations of the counting house stand on the far side of the stream.

Further upstream beside the path close to where the main track curves down to the stream is an open wheel pit and directly across the path is the stonework for an angle bob. Further back up the hillside close to Walls lode is a further wheel pit where the stone arch of the tailrace can still be seen amongst in the heather.

At Vitifer cassiterite occurs in granular form in the granite associated not only with the gangue mineral tourmaline (black schorl: NaFe3Al6(BO3)3[Si6O18](OH)3(OH)) but also with large quantites of micaceous hematite (Fe2O3). The cassiterite is readily distinguished from both of these minerals. Cassiterite here is usually dark red-brown with a metallic lustre when wetted. Schorl is also usually black but has a vitreous lustre and needle like habit whilst micaceous hematite has a high silvery metallic lustre.

Vitifer Cassiterite

The early paragenesis of the lodes was mineralisation by tourmaline and some quartz. Extensive faulting with horizontal slickensides particularly apparent in underground mine workings and brecciation of the wallrock followed. Tormaline and cassiterite were then deposited. Further movement along the faults was accompanied by chlorite formation and subsequently hematite and quartz.

Hematite is abundant in all of the workings at this site both as large platy flakes most often associated with quartz but often as huge granular masses.

The difference in specific gravity between cassiterite and schorl is sufficient to separate the 2 minerals on a jigging table but the hematite remains associated with the cassiterite. In the early years of the 20th century when Golden Dagger mine was reworked, the separation of these 2 minerals was carried out at the works by means of electromagnets, using a separator, turbine and generator removed from Vitifer. After chimming; where the wet ore is loaded into kieves: wooden barrels with iron hoops, which are knocked by mechanically powered hammers causing the ore to settle allowing residual water and any lighter sand to be decanted from the top of the barrel, the final stages of ore preparation were drying and electromagnetic separation.

Output of cassiterite was sporadic from the middle of the 19th century totalling 1177 tons up to 1928 by some accounts. Unfortunately the hematite was unsuited to paint making and was discarded as waste.

The trip ended with thanks to Richard Scrivener and a walk back to the car park.



Additional Information and photos supplied by Mr C. Kelland.

References:

Perkins J.W., Geology Explained: Dartmoor and the Tamar Valley, David & Charles, Newton Abbot, 1972.

Fletcher M. et al, report on the Meldon Industrial Landscape, Devon, for the Royal Commission on the Historical Monuments of England, Exeter, 1997.

Dines H.G., The Metalliferous Mining Region of South-West England, HMSO, London, 1956.

For extensive information on cassiterite follow this link http://www.mindat.org/min-917.html

Richardson P.H.G., Mines of Dartmoor and the Tamar Valley After 1913, The Northern Mine Research Society, Sheffield, 1992.

Click here for extra photos.

Words: Chris Popham

Photos: Chris Popham

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