We went to Luxembourg last year. In the France/Lux/Belgium triangle there are more show mines per square inch than I have ever found before in one area. Each nation has it's own iron museum mine plus loads of private/group ones. Some friendly, some not. None speak English.
I met a Luxembourg explorer who took me to a couple of sites but most are blocked or being blocked. It is a criminal offence there to trespass in mines and the police are quite hot about it and visit laybys and 'usual haunts'.
The mines are very extensive, thousands of kilometres and navigation is a big problem. After we left them, my friend was doing a through trip of 25Km on mountain bikes!
Sadly France is catching up on the sealing of mines and I believe it is illegal there too. The famous Mines de Bentailliou in the Pyrenees and several more have been bulldozed after a fatality. Other areas have been re-sealed recently too. This year we visited the iron mines around Mt. Canigou in the eastern Pyrenees and all were newly sealed.
Spain appears a bit more laid back in the Pyrenees area at least.
Les