Las Médulas (Spain), a historical gold mine

In NW Spain, on the border of Galicia, Asturias and Castile and Leon, in the Rio Sil River, the exploitation of gold placer deposits tooks place. Gold from alluvium was exploited by the Celts, but the Romans developed large-scale exploitation. The paper describes the attractive landscape, which is a relict ofa Roman gold mine. This mine was probably the largest in the Roman Empire. Gold occurs in the Neogene alluvium (~20 Ma years). The deposit is probably a weathering product of gold-bearing quartz veins. Three deposit series have been distinguished. They are composed of conglomerates with intercalations of finer-grained material - sand and clay. The top series, Las Medulas, has the thickness of >100 m, the middle series, Santalla, has the thickness of 0-30 m, and the most gold-bearing (20-100 mg/m3, at the base even 60-300 mg/m3) and intensely red gangue Orellan series found at the base is 0-25 m thick. The basement is represented by Devonian and Silurian limestones, shales and quartzites, Ordovician limestones, dolomites and shales, and Cambrian quartzites with slates at the top. The deposit is cut by numerous faults, which form blocks, and are the cause of variations in the level of the deposit series. The mining technique used by the Romans is also characterized. In the mine, where mining works were carried out on the surface of1228 ha, about 1626 tons of gold were exploited during the periodfrom ~25 BC to the early third century AD,. A technique ofruina montium was usedfor gold exploitation. It consists of drilling in excavations ofpoorly compacted rocks - tunnels, adits, shafts (up to a depth of 100 m). Through these excavations, the waterfallingfrom above was transferred. Water under pressure has burst and washed out the deposit. Presently, the remains of the Las Medulas deposit series are represented by the characteristic red rocks, which are poorly compacted. They create morphology of the mining landscape, which is inscribed on the UNESCO World Heritage Site list