St Piran's Day

St Piran is patron saint of Cornwall and tin miners. St Piran’s Day is celebrated every year in Cornwall on 6th March.

Legend has it St Piran rediscovered tin smelting when he arrived in Cornwall in the 5th Century. Whether he did or not is up for debate, but we do know that tin mining and the technology for smelting tin has existed in Cornwall since the Bronze Age.

The story of St Piran is still a great one. What is known of him is that he was a 5th Century abbot from Ireland. After being banished from Ireland for preaching Christianity, he was reputedly thrown over a cliff tied to a millstone into a stormy sea.

By means unbeknownst to science the millstone floated (perhaps the millstone was made of pumice!) and the sea went calm. St Piran drifted across the Irish Sea, and he landed at Perranporth beach, although it was likely called something different back then as it is now named after him!

After building an oratory in the sand dunes above the beach (the remains of which can still be seen today), St Piran noticed the black rocks he used for hearthstones around the fire began to ooze a white metallic liquid when hot. This is, of course, molten tin that is being smelted out of rock containing tin ore called cassiterite that has a black to dark brown colour.

It is these colours that are used for Cornwall’s flag today. The black background represents tin ore, and the white cross represents tin metal. Therefore, the Cornish flag has its roots in geology!

So, why do we get tin in Cornwall? This is all thanks to geological processes that have enriched and mineralised metals as ore minerals to levels that we as humans deem economically viable to extract.

Most mineralisation in Cornwall formed as a consequence of the Permian granites.

Vast amounts of water are dissolved in granitic magmas. As those magmas crystallise their chemistry evolves, becoming enriched in H2O and elements that don’t like joining minerals forming out of the melt, like metals such as tin. Eventually that water exsolves from the melt, known as magmatic-hydrothermal fluids.

Heat from the granite magmas also radiated through the host rock, mobilising connate and meteoric waters with different chemistries and causing them to circulate through the surrounding sedimentary and igneous rocks. These also mixed with the magmatic-hydrothermal fluids.

These aqueous fluids, sometimes known as volatiles, exist under high pressure and can be in excess of 500°C. They aren’t a gas or a liquid, but a supercritical fluid that can pass through solid rock and dissolve elements within them, including enriched metals from the residual magma or host rock.

The chemical and thermodynamic diversity of hydrothermal fluids leads to a complex mineral paragenesis and distinct geospatial and temporal variations in mineralisation across the region.

If a rock fractures it can cause fluids to rush in to re-equilibrate the pressure, altering the ability for volatiles to carry certain elements. Elements begin to bond together and precipitate minerals that fill the void and form a mineralised vein.

Gangue (waste) minerals can include quartz, tourmaline, fluorite, and chlorite. More importantly ore minerals containing metals can also form.

Metals mined in Cornwall include tin, copper, tungsten, arsenic, zinc, lead, iron, silver, gold, uranium, nickel, cobalt, antimony, and bismuth.

Repeated fracturing can create multiple episodes of veins. This, plus any alteration zones containing ore minerals is called a lode. They are often sub-vertical, up to several metres thick, and hundreds of metres in length and depth.

So there you have it. This important day in Cornwall is fundamentally controlled by the geology beneath our feet!

St Piran’s Day celebrates Cornwall and its special culture and heritage. Join in the festivities by eating some saffron cake or a pasty, and most importantly spending some time to appreciate this amazing place!