Cligga Head Mineralisation EarthCache
Cligga Head Mineralisation
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An Earthcache looking at mineralisation at this famous location. Cache is on the Southwest Coast Path.Terrain is off road. Parking in the industrial estate, or walk along the coast from either direction.
The location is a SSSI becasue of the biology and geology of the site. Therefore do not stray from the paths and do not hammer or collect samples of the rock. There is no need for either to complete the earthcache
Mineralisation is the process by which elements are concentrated and are deposited within a rock. The Earth contains various different elements, however these are not evenly distributed. At Cligga Head, tin and tungsten have been concentrated to levels where it becomes economic to extract and process the rock.
Cligga Head is an intrusion of granite, intruded around 277-274 million years ago.The granite is comprised of phenocrysts (larger than average crystals) of orthoclase feldspar. These are contained within a groundmass (crystals of average size for the rock) of quartz with small traces of biotite mica
There are a wide variety of different forms of mineralisation.
At Cligga head three are present.
The most obvious is a series of dark (compared to the rock) zones. These were granite, but during the time since emplacement,a hot fluid, most likly a salt water, flowed through the rocks. This broke down the feldspar and the black mica, forming quartz and muscovite (white) mica. This produces a rock known as Greisen. This was the first mineralisation present.
The second mineralisations, occurs within the Greisen. look closly and at the center of the Greisen is a thin dark line around half a centimetre across. This is the remains of the phase that deposited the tungsten and tin in the granite. A careful look at the veins will show crystals growing within them. Most are quartz crystals, but some are cassiterite (Tin ore Tin oxide) and wolframite (Iron manganese tungstanate) The wolframite is needle like with a length of up to 1cm. The Cassiterite is dark black and often reflective.
The final mineralisation is more prevelent, occuring throughout the granite. It is also less obvious. This is Kaolinisation or the breakdown of the feldspars into china clay. This can be seen on the outside of the feldspars that remain as a powdery coating.
Logging requirements
1) Which way do the Greisen veins dip and how much do they dip by?
2) What is the average width of the greisen veins?
3) Why are the Greisen veins aligned?
4) Why do the mineral veins run through the Gresien?
You do not need to leave the coast path to answer this question
Email me through my profile with the answers. Do not post the answersa in your log. Feel free to log at the same time. Photos would be nice but are not a requirement of logging this cache.
Cache placed with kind permission of Natural England
Additional Hints
(Decrypt)
Guvax nobhg ubj gurfr irvaf sbezrq naq jurer gur ebvpx jbhyq unir orra jura vg unccrarq