Roman gold mines of Os Biocos

The Os Biocos mine is one of the largest gold mines built directly on rock in Galicia, and the heritage project opened up avenues for understanding the intricacies of ancient gold mining practices in a visually engaging way.

The mining technique was known as ruina montium (biocos in Galician, hence the name of the site), a technique by which, as Roman naturalist Pliny the Elder explains, the rock was extracted mechanically, weakening and breaking it down with mallets, wedges and picks, followed by alternating heat (fire) and cold (water) to fracture the rock from thermal shock.

Once the mass of rock had been broken down, at the foot of the mining faces, a selection of the gold-bearing material would be made. The ore thus selected was subjected to an enrichment process, probably nearby the mine, in which it was crushed, calcined, washed and ground. This resulted in a very fine concentrate, which was washed in a pan to obtain the tiny particles of gold.

Hydraulic power was essential to flush all unsorted spoil or waste material out of the mine. This was done through drainage channels carved through the rock, which could be underground when necessary, like tunnels through a mass of unmined rock. These drainage channels carried the residue to the valleys, where it would be washed away by the rivers.

An analysis of the mine gives an estimate of the amount of rock extracted: between 1,290,000 - 1,935,000 m3. The average gold grade at the mine cannot be identified due to the lack of specific calculations. However, considering the typical amounts of gold found in primary settings (within rock formations), Roman mining activities at Os Biocos probably yielded between 774 kg and 1,161 kg of gold (assuming an average richness of 600 mg/m3).

The Os Biocos gold mine exemplifies the processes of historical transformation that took place after the Roman conquest of Northwest Spain. The Roman Empire extended to the northwest of the peninsula under the rule of Augustus, and after subjugating its populations, Rome established a new organisation and exploitation of the territory and of the local communities, which completely broke with the previous way of life. Almost a century later,
the construction of the Via Nova in the late 1st century AD reflects a new intervention to rearrange the same territories and populations to serve the interests of the state. Rome also had a particular interest in mineral wealth in the northwest of the peninsula, and in order to exploit the gold resources, it imposed a system of taxation in which the local communities had to pay the Roman administration with their work in the mines.