Great Orme
Copper Mines

Geology and Mineralisation

The Great and Little Ormes comprise sequences of limestones, with thin shale-bands, underlain by a massive sandstone. These rocks were deposited in shallow, warm seas that covered the area in Lower Carboniferous times, about 350 million years ago. The sea-bed was later uplifted by earth-movements to its present positions overlooking the modern-day seaThe limestone is of a porous, brown appearance that is partly due to the replacement of its major constituent, calcite (calcium carbonate) by dolomite (calcium magnesium carbonate). This process is known as dolomitisation. The dolomitised limestones are intensively mineralised along fractures and in the abundant open cavities.

Typical Great Orme ore consists of saddle dolomite, forming crusts of the typically yellowish-white curved crystals, upon which brassy-yellow chalcopyrite crystals, typically 3-5mm across, are scattered. A few other ore-minerals, including galena, sphalerite, pyrite, and marcasite also occur, but only rarely.The copper mineralisation is developed within an area in which numerous steeply inclined faults cut the geological succession.

The faults, which run in a north-south direction, are all mineralised, but the most intense mineralisation, now worked-out, was developed along four particularly large faults. The mineralisation is strongly controlled by the types of rocks through which the faults pass, and it is best developed in the thickest beds of limestone.

The Great Orme Copper Mine is extremely important in geological terms because it is where the first UK example of the "copper-dolomite association", an important worldwide class of mineral deposits, was recognised. This type of mineral deposit ("copper-dolomite association") is often associated with massive lead-zinc deposits, as in parts of Ireland. Towards the end of the mineralising cycle responsible for the formation of the lead-zinc deposits, the hot saline mineralising waters (hydrothermal fluids) undergo a change in their chemistry. They become magnesium-rich and sulphur-poor, and, where they encounter limestone sequences, they deposit the distinctive saddle dolomite (dolomite with curved crystals). Should the fluids circulate through certain deeply-buried rocks (red marls or volcanics), they may take copper into solution, which then combines with the remaining sulphur to form the copper-iron sulphide chalcopyrite, which crystallises with the saddle dolomite. Sulphide ores of metals, including copper, tend to react with ground waters in areas close to surface. At Great Orme this reaction has been particularly intense, due to the extremely porous nature of the ground.

Chalcopyrite is often altered to the copper carbonate, malachite, which formerly occurred at Great Orme in sufficient amounts to be mined as a copper ore in its own right. Malachite is a conspicuous mineral, forming bright green crusts and thin stains. The more rare deep blue copper carbonate azurite also occurs at Great Orme, and forms small crystalline masses on saddle dolomite and small spheroidal nodules in the distinctive rock-band called the Azurite-Bearing Shale.

Dissolution of the limestone has also led to the formation of several generations of secondary calcite - some of which enclose green malachite. In places, when the mine was at work, copper as a native metal was reported, occurring as branched groups of crystals.In the post-1845 period, when official figures for mineral production were kept, the Great Orme copper mines produced well over 25,000 tons of ore. Since miners have dug away at the deposit for over 3500 years, it is anybody's guess as to what the total production was - but it would have significantly exceeded this amount.