Copper-rich Greisen - Gunheath (#22)
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Fact sheet
Description: 

This topaz greisen has been subjected to copper-bearing fluids which have left behind considerable chalcopyrite. The rock looks green in hand specimen because later fluids have altered the chalcopyrite and two green secondary copper minerals have been formed. They are both hydrous copper-aluminium phosphates - turquoise (light bluish-green) and libethenite (darker apple green). Both are difficult to see in thin section (you will have to zoom into the thin section view to see them) because they only form thin coatings. Instead you will see large areas of an opaque (black) mineral in the ppl view. That's mostly chalcopyrite.

Note that the plagioclase feldspar is considerably altered - much is now replaced by fine-grained sericitic mica. Testimony to an extensive hydrothermal system that was responsible for creating the china clay resource of this granite complex.

Gunheath china clay pit is situated on a small lobe of extremely evolved, granite - it's the source of many elements (boron, lithium, fluorite, phosphorus) present in this sample, and the driving force behind the hydrothermal system.

In Place
In Time
Location description: 
Gunheath china clay pit, near Stenalees, St Austell, Cornwall
Accuracy:
Location precision: 
Good
Timescale:
Timescale: 
Permian
Ma = Millions of years ago
In Focus
Collection:
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Type:
Type: 
igneous
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Accessory minerals:

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