Area: 0.1 hectares.
Other Information:
A new site, listed as of national importance in the Geological Conservation Review, under the name Wheal Martyn.
Description and Reasons for Notification:
This locality shows megacrystic lithium-mica granite that constitutes one of the principal granite varieties of the Permo-Carboniferous St. Austell mass. This specially excavated site is significant in providing evidence that the original intrusion, a biotite granite of variable grain size, has been altered by two metasomatic events. The first introduced potassium, giving rise to alkali-feldspar megacrysts (sometimes up to 5cm long). The second event involved an influx of fluid carrying lithium, sodium, fluorine and boron into the rock system. The resultant recrystallisation saw the development of a new suite of minerals (zinnwaldite, albite, fluorite and tourmaline) that incorporated these elements. It is this granite variety which has undergone the most severe kaolinisation and in which most china-clay workings are sited.