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Field trip to Kit Hill, Callington, Cornwall 23/Jan/2005 Grid Ref.: SX375715
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The party of about 24 met in the summit car park of the Kit Hill Country Park near Callington at 10:00. It was gloriously sunny and promised to be a great morning despite the cold wind. Event leader Linda Fowler began with a description of the south-west granite batholith by reference to the BGS Lands End sheet, identifying the granite outcrops and highlighting how these sat in the syncline formed by Devonian rocks overlaid by rocks of Carboniferous age. Turning to the BGS 1:50 000 Tavistock sheet Linda illustrated how the geology was somewhat more complex at the local scale.
The local area had been subject to mountain building events at the time of the Variscan orogeny causing thrust faulting of the rocks. Subsequently there had been relaxation and reactivation of these faults as well as dextral north-west south-east faulting. This had led to areas of weakness possibly providing pathways for granite intrusion and probably providing the opportunity for the Tamar River to either cut its channel afresh or to work on existing drainage courses.
A lesson on granite

First stop was the old engine house stack in the corner of the car park. From here Linda pointed out the granite of Dartmoor forming the eastern horizon beyond the Tamar. Closer to us was the microgranite of Hingston Down, the granite of Kit Hill itself and then off to the west the granite edge of Caradon Hill and Bodmin Moor. Linda also gave a quick summary of the human occupancy of Kit Hill, mining possibly dating back into the 1500s right up to the early 1900s, then later a spa facility with its own reservoir and tearooms. Now only the ornate stack remains, crowned by a forest of radio and mobile phone aerials. The next stop was the granite quarry on the north-west side of the hill. At some point the track we were following crosses a massive elvan of pale fine-grained granite with quartz phenocrysts. A piece was eventually found and handed around. This was intruded at a late stage into the granite and though well preserved on Kit Hill it is heavily weathered to the east where it was worked for brick clay and also where it crops out in Hingston Down quarry.
The quarry is cut back into the hillside but is also cut down into the hill, though now flooded. Linda began by prising a description of granite from the typically reticent masses. Kit Hill granite is a textbook pale granite of quartz, biotite and feldspar. The grains are randomly oriented with only small phenocrysts of feldspar, which show little alignment. In places some of the granite shows rusty iron staining, a side effect of tourmalinisation, whereby the hot fluids bringing in the dissolved boron causes leaching of the iron from the biotite in the granite.
There's lots of it

Inspection of the quarry faces showed 3 distinct sets of joints in the stone. Well defined near vertical joints at more or less 90 degrees to each other and less well defined horizontal joints. Closer inspection showed that these horizontal joints followed the curve of the land surface. 2 mechanisms have been proposed for formation of these joints: contraction of the rock after it solidified and continued to cool and decompression as the overlying 3km or so of rock was eroded away. This is probably the reason for the quarrymen working down into the hill as the less weathered and jointed stone was to be found at greater depth.
At the entrance to the quarry we were able to observe 2 inclines. The first to be used cut diagonally across the hillside and joined the narrow gauge railway, which carried the stone to the quays on the Tamar. The later incline follows the slope of the hill to join the later standard gauge railway. The incline was operated on the gravity principal whereby filled trucks going down would haul empty trucks up by means of a hawser and pulley system some remains of which can still be seen.
During the climb back up to the car park we discussed the poly-metallic mineralisation of the area. Models proposed by Dewey and Dines describe processes of metasomatic mineralisation by circulating fluids whereby zoning occurs along a temperature gradient either with depth in close proximity to the granite or with distance along fault zones radiating from the granite. In these models the zones appear like layers of an onion, tin at highest temperature, copper at the lowest. However, this is further complicated by subsequent movement of the metals, for example copper, where the minerals are dissolved by meteoric water from above, the resulting copper oxide minerals in the surface regions being enriched in metal whereas the sulphides deposited below the water table are relatively poor in metal content.
Note the nice joint planes

On Kit Hill the metal mineralisation appears to have taken place quickly without an opportunity for zoning to occur. The principal minerals exploited have therefore been Cassiterite (tin) and Wolframite (tungsten). Arsenopyrite (arsenic) only occurs on the north side of the hill and was never exploited in sufficient quantity to warrant a calcining complex on site. Some minor copper and iron exploitation also took place.
Back at the car park attention again switched to the granite and the origin of granite. In general terms granite has an elemental composition close to continental crust. In the case of the Cornubian batholith the source of material is thought to have been sediments from the floor of the Rheic Ocean buried during the period of basin closure. A further suggestion is that these may have liquefied under what is now northern France and have been intruded laterally in a series of pulses to the present location. This would have led to both regional and contact metamorphism of the surrounding slates.
Consequently we strolled down the slope towards South Kit Hill mine looking for the contact boundary. This was seen as a sharp change in the material making up the path, from granite on top the hill to grey micaceous slates below. Unfortunately the composition of these is such that they show little of the spotted appearance often found in contact metamorphosed slate.
South Kit Hill Mine

We concluded our excursion at the site of the South Kit Hill mine with a summary of the formation of the hill and subsequent exposure and exploitation.
As a final note Linda suggested that as we headed down to the AGM we look back at the hill to observe the terraced nature of the profile. This has been brought about in the postglacial period by freeze thaw and water action leading to a series of slumps in the surface soils. We thanked Linda and adjourned for lunch and the AGM.
Wolframite (tungsten ore)

Words: Chris Popham, Photos:

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The party of about 24 met in the summit car park of the Kit Hill Country Park near Callington at 10:00. It was gloriously sunny and promised to be a great morning despite the cold wind. Event leader Linda Fowler began with a description of the south-west granite batholith by reference to the BGS Lands End sheet, identifying the granite outcrops and highlighting how these sat in the syncline formed by Devonian rocks overlaid by rocks of Carboniferous age. Turning to the BGS 1:50 000 Tavistock sheet Linda illustrated how the geology was somewhat more complex at the local scale. The local area had been subject to mountain building events at the time of the Variscan orogeny causing thrust faulting of the rocks. Subsequently there had been relaxation and reactivation of these faults as well as dextral north-west south-east faulting. This had led to areas of weakness possibly providing pathways for granite intrusion and probably providing the opportunity for the Tamar River to either cut its channel afresh or to work on existing drainage courses.	A lesson on granite
First stop was the old engine house stack in the corner of the car park. From here Linda pointed out the granite of Dartmoor forming the eastern horizon beyond the Tamar. Closer to us was the microgranite of Hingston Down, the granite of Kit Hill itself and then off to the west the granite edge of Caradon Hill and Bodmin Moor. Linda also gave a quick summary of the human occupancy of Kit Hill, mining possibly dating back into the 1500s right up to the early 1900s, then later a spa facility with its own reservoir and tearooms. Now only the ornate stack remains, crowned by a forest of radio and mobile phone aerials. The next stop was the granite quarry on the north-west side of the hill. At some point the track we were following crosses a massive elvan of pale fine-grained granite with quartz phenocrysts. A piece was eventually found and handed around. This was intruded at a late stage into the granite and though well preserved on Kit Hill it is heavily weathered to the east where it was worked for brick clay and also where it crops out in Hingston Down quarry. The quarry is cut back into the hillside but is also cut down into the hill, though now flooded. Linda began by prising a description of granite from the typically reticent masses. Kit Hill granite is a textbook pale granite of quartz, biotite and feldspar. The grains are randomly oriented with only small phenocrysts of feldspar, which show little alignment. In places some of the granite shows rusty iron staining, a side effect of tourmalinisation, whereby the hot fluids bringing in the dissolved boron causes leaching of the iron from the biotite in the granite.	There's lots of it
Inspection of the quarry faces showed 3 distinct sets of joints in the stone. Well defined near vertical joints at more or less 90 degrees to each other and less well defined horizontal joints. Closer inspection showed that these horizontal joints followed the curve of the land surface. 2 mechanisms have been proposed for formation of these joints: contraction of the rock after it solidified and continued to cool and decompression as the overlying 3km or so of rock was eroded away. This is probably the reason for the quarrymen working down into the hill as the less weathered and jointed stone was to be found at greater depth. At the entrance to the quarry we were able to observe 2 inclines. The first to be used cut diagonally across the hillside and joined the narrow gauge railway, which carried the stone to the quays on the Tamar. The later incline follows the slope of the hill to join the later standard gauge railway. The incline was operated on the gravity principal whereby filled trucks going down would haul empty trucks up by means of a hawser and pulley system some remains of which can still be seen. During the climb back up to the car park we discussed the poly-metallic mineralisation of the area. Models proposed by Dewey and Dines describe processes of metasomatic mineralisation by circulating fluids whereby zoning occurs along a temperature gradient either with depth in close proximity to the granite or with distance along fault zones radiating from the granite. In these models the zones appear like layers of an onion, tin at highest temperature, copper at the lowest. However, this is further complicated by subsequent movement of the metals, for example copper, where the minerals are dissolved by meteoric water from above, the resulting copper oxide minerals in the surface regions being enriched in metal whereas the sulphides deposited below the water table are relatively poor in metal content.	Note the nice joint planes
On Kit Hill the metal mineralisation appears to have taken place quickly without an opportunity for zoning to occur. The principal minerals exploited have therefore been Cassiterite (tin) and Wolframite (tungsten). Arsenopyrite (arsenic) only occurs on the north side of the hill and was never exploited in sufficient quantity to warrant a calcining complex on site. Some minor copper and iron exploitation also took place. Back at the car park attention again switched to the granite and the origin of granite. In general terms granite has an elemental composition close to continental crust. In the case of the Cornubian batholith the source of material is thought to have been sediments from the floor of the Rheic Ocean buried during the period of basin closure. A further suggestion is that these may have liquefied under what is now northern France and have been intruded laterally in a series of pulses to the present location. This would have led to both regional and contact metamorphism of the surrounding slates. Consequently we strolled down the slope towards South Kit Hill mine looking for the contact boundary. This was seen as a sharp change in the material making up the path, from granite on top the hill to grey micaceous slates below. Unfortunately the composition of these is such that they show little of the spotted appearance often found in contact metamorphosed slate.	South Kit Hill Mine
We concluded our excursion at the site of the South Kit Hill mine with a summary of the formation of the hill and subsequent exposure and exploitation. As a final note Linda suggested that as we headed down to the AGM we look back at the hill to observe the terraced nature of the profile. This has been brought about in the postglacial period by freeze thaw and water action leading to a series of slumps in the surface soils. We thanked Linda and adjourned for lunch and the AGM.	Wolframite (tungsten ore)