In the Spanish province of Leon, 20 km from Ponferrada, is Las Medulas, the Roman gold digging site. Before the Romans started mining, the local Celtic tribes obtained gold dust from the rivers. Poseidonius of Rhodes, a teacher of Cicero, wrote in the first century BCE about the fact that this precious metal is here.
The origin of the name Las Medulas is debatable. Many scholars speculate that it came from Mount Mons Medulius, where the Battle of the Cantabras and Asturias took place in 22 BCE, but we cannot say for sure.
The mine began its operation from the reign of Octavian Augustus, around 25 BCE and was used until the beginning of the 3rd century CE Our main written source is Pliny the Elder in 72-74 CE who is a public prosecutor in the Spanish province of Tarraconiensis. A technique he called Ruina Montium was used to extract gold.
For many months, slaves, Roman prisoners and workers from the conquered tribe of Asturians (whose total number was estimated at 60,000 by Pliny, in fact, the estimate of 10,000 are more likely), orbits diverging from the top tunnels in the rocks. Due to the working conditions, their mortality was high.
On the other hand, work was underway on the canals to supply water. In many places, the rock was excavated taking care of the angle of inclination thanks to the diseases device, equipped with a spirit level, thanks to which it was possible to carry out levelling. Apart from these tunnels, the water was brought in by 7 aqueducts, the total length of which is estimated at about 300 kilometres. The decline in them ranged from 0.6 to 1%. They were 90 centimetres deep, 1.28 cm wide, and 1.60 cm at the corners. Artificial water reservoirs have also been created to increase pressure and the effect of water on the rock. The entire system allowed to use approx. 16 million m3 of water annually. Which gave 190 thousand. m3 per day. Building this network was the most expensive part of the project.
The water led to the prepared tunnels washes the rock and weakened it, and when the right amount of it accumulated, the side of the mountain collapsed under the influence of the weight.
Then the water was reused. The output was rinsed through channels lined with stems, twigs of, among others, heather and rosemary bushes, which caught grains, husks and gold dust. The bushes were then burned and the gold was recovered from the ashes.
The beginning of mining in Las Medulas coincided with the monetary reform of Octavian Augustus, which permanently introduced a golden aureus weighing 7.79 g into the monetary system. Aureus had previously appeared to be beaten by Sulla, Pompey, and Caesar. This coin functioned until the time of Constantine the Great, who in 309 CE replaced it with a solid weighing 3.89 g. A large inflow of ore allowed for the continuous issuance of these gold coins.
During the two and a half centuries of the mine’s operation (1st century BCE – early 3rd century CE) about 5 million Roman pounds of gold were mined, which is about 1,600 tons. Annual production is estimated at 6.5 tons. From one ton of excavated material, it was possible to obtain 3 grams of valuable ore. Over time, mining became unprofitable. Gold amounts have fallen and its prices have fallen. This led to the closure of the mine.
Pliny the Elder, describing Las Medulas, concluded that the achievements of people surpassed the work of the Giants and that purple and pearls can be obtained from the bottom of the sea more easily than gold from these mountains.