How did these veins form?

To answer this, one has to go back about 450 million years to the Devonian period. At that time Cornwall was beneath an ocean where muds and silts were being deposited and were being disrupted by sub- sea volcanoes.

Some hundred million years later, in the middle of the Carboniferous Period, continental collision started to buckle the now hardened mud rocks causing fractures to occur along stress lines. A high mountain chain was formed. As this grew, so the underlying rocks were heated to such an extent that they melted and started to well upwards. The great pressures reopened the pre-existing fault fractures and where temperature, pressure, and chemical conditions were right the copper and tin ores were deposited.

At Levant the upwelling molten rock cooled to form the granite in the eastern part of the mine which lies against the baked shale's and volcanic rocks (collectively called 'killas' by the miners) in the western part. The granite - killas contact dips steeply to the west. The veins show evidence of repeated reopening by ground movements and the major phases of these openings are characterized by the different mineral content.

Cassiterite was an early mineral in the veins and it is found in both granite and killas.

Cassiterite (Tin Oxide)

Copper mainly in the form of Chalcocite, occurs in the killas and overlies the tin bearing parts of the vein.

Chalcocite (Copper Sulphide)

The copper and tin zones overlap where they meet in the middle levels of the mine. A little tungsten as scheelite (Ca WO4) is also present in the western part of the mine ( the undersea section).

Gangue materials which accompany the tin and copper ores are quartz, and tourmaline in the granite country and chlorite in the killas country; haematite occurs abundantly in most parts of the mine. It is reddish brown and gave the surrounding sea a characteristic red tinge when the mines were at work.

Thanks to Charles Smith (ex Geevor geologist) for this article.