This place is pretty much out of the way (the nearest large town is Ponferrada), but it is certainly worth going to see. It is the most remarkable testimony to the effects of the Roman occupation of Spain. An entire mountain was blown apart over two and a half centuries to extract the tiny amount of gold in the rock (about 6 grammes per tonne of rock): it is an extraordinary story
Plan to spend several hours at least there to make it worth the trip and enjoy the area and the countryside.
There are morning and afternoon guided tours (1200 and 1630 the Friday that we visited) that cost a ridiculously cheap 3€ for a two hour amble around the site.
A typical itinerary might be to arrive at midday and take in one of the walks (the Visitors' Centre will suggest suitable itineraries): down to the lake, up to the viewing point where you get an amazing view from the mountains in the north around the whole mining area. The walk up to the "mirador" (viewing area) is a four and a half kilometre round trip on wide, easy paths, with one steeper section that will leave the less fit out of breath, but can be handled easily by anyone in reasonable shape. Take photos at the top and enjoy the magnificent view before walking back down.
Almost every other house in the small village around the mine seems to be a restaurant or a gift shop (WATCH OUT! Internet access in the village is sporadic at best, so shops and restaurants do not accept credit cards) so then sit down and enjoy a relaxed meal and a look around the gift shops before going to the Visitors' Centre for the 1630 tour.
The tour takes you on a loop through the milllenary chestnut trees and the mined area, taking in couple of caves and galleries. However, although the public are allowed to enter the cave and the tunnels, it is very much at their own risk. The cave is not deep, but is an obligatory stop for a photo. The galleries are a later stop and require some gentle climbing that requires care and sensible shoes. Go up and you will find two short tunnels cut into one of the few remaining sections of mountain that come out above where the group is waiting below: children especially will love this.
The final section of the walk, which is around 3km with plenty of stops, is a short, steeper downhill section over a much rougher path: one family had a pushchair with them and struggled a bit on this segment, although it is doable with a bit of effort.