In which part of their empire did the Romans mine the most gold?

Britain was rich in resources such as copper, gold, iron, lead, salt, silver, and tin. (...) The abundance of mineral resources in the British Isles was probably one of the reasons for the Roman conquest of Britain.

Archaeologists have long thought that the Romans were mining for gold in Spain in the first century B.C., and researchers from the University of Salamanca have now located the vast ancient gold mines in the province of León, and hidden under Eria Valley vegetation and crops. The gold mine complex, known as Las Médulas, is believed to be the largest gold pit of the Roman empire. It featured channels and reservoirs, as well as intricate hydraulic systems which diverted high-pressure water to the site for processing.