Roman gold mining

Pliny: "Gold in our part of the world ... is found in three ways: first, in river deposits. ... No gold is more refined, for it is thoroughly polished by the very flow of the stream and by wear. The other methods are to mine it in excavated shafts or to look for it in the debris of undermined mountains." Placer deposits are the easiest and first to be exploited. Where the Romans recognized ores on the surface, they followed them into the ground by strip-mining. Opencast was used for many metals. Deep-vein mining was the most difficult and dangerous. Only gold and silver were valuable enough to justify this kind of mining.

'Ruina montium' was the Roman method to systematically dismantle entire mountainsides.

Las Médulas is an otherworldly landscape in northwestern Spain that served as the largest and most important open-pit gold mine in the Roman Empire. Located near the town of Ponferrada in the El Bierzo region (province of León), the site is listed as a UNESCO World Heritage Site.

The Las Médulas mine operated for about 250 years, from 25 BC until the early 3rd century AD. Roman engineers built over 700 kilometers of canals to divert water into massive, elevated reservoirs. Upwards of 60,000 slaves tunneled into the core of the mountains. Water was abruptly released through sluice gates into the networks of cavities. The immense hydraulic pressure caused catastrophic landslides, tearing the mountain apart. The collapsed material was washed down inclined channels to isolate the gold.

Rock-cut aqueduct in La Cabrera.

Pliny the Elder witnessed and described the site in full operation around 74–77 AD. The Romans abandoned the mine in the early 3rd century AD as the gold ran out. Las Médulas yielded an estimated 1,500 tons of gold. Pliny stated that 20,000 Roman pounds (6,560 kg) of gold were extracted each year.

Today a kilo of Au is $135k giving $855,600,000

Removing rock was a difficult and time-consuming process in Roman mines. Iron was used for most tools, though stone hammers and wedges have been recovered. Wood was used for buckets to remove ore. Remains of wooden ladders have been found. Leather sacks, miners' sandals and caps have been recovered.

It was labour intensive work: "those individuals of outstanding physical strength break up the quartz rock with iron hammers, applying to the work not skill, but force". Shafts were vertical or inclined passages that provided access, ventilation, and a path for ore removal. They were normally square, small (1-2 meters square), and braced with wood to prevent collapse.

The deep mine workings created problems with ventilation, lighting, and drainage. The Romans knew the dangers. The huge state-controlled operations relied heavily on slaves, prisoners of war, and condemned criminals. Life expectancy of a Roman miner was short. Most enslaved laborers sent to the mines were essentially given a death sentence. Virtually none survived 5 years of brutal conditions marked by physical exhaustion, poor diet, harsh punishments, and regular fatal accidents.