The mountains surrounding Allihies consist predominantly of Old Red Sandstone dating from the Devonian Period, approximately 530 m.y. (million years). The rocks were formed from sediments produced under extreme desert conditions, such as now existing in the Sahara Desert.
Devonian sedimentary rocks form much of west County Cork, consisting largely of the compressed, sheared, contorted siltstones and sandstones of the Variscan mountain building episode. Around Allihies, they extend eastward from the coast, and inland into the mountains beyond the village.
Where veins of either direction intersect one another, an S or Z bend is sometimes formed, often enriched in copper minerals.
During the last two centuries, commercial interest lay in metalliferous (Quartz)veins containing copper minerals injected into fractures coursing variously east to west or north-north-east within the Devonian sediments.
The main ore mineral formerly extracted in course of working the Allihies mines was chalcopyrite (copper-iron-sulphide) together with as little as possible of worthless quartz (silicon dioxide). Other sulphides produced in lesser quantities were tetrahedrite (copper/antimony) and pyrite (iron) plus minor quantities of chalcocite (copper) and molybdenite (molybdenum).
Visitors to the sites of former mine waste dumps may notice remaining rock debris stained strongly red or orange. This is due to hydration of iron oxide and/or oxidation of iron sulphide, producing the hydrated iron oxide mineral, limonite. In other places waste rock is stained green by the oxidation of copper sulphides to form copper carbonate, malachite.