I'd say you don't need a guide to La Union, if you get there you can see nothing but mines. Check it out on Google Earth.
Most of them are easy to access. There are underground sites there too but don't mess with them unless there's a few of you.
2 of us descended one which consisted of a spiral staircase around the outside of a shaft (stones stuck out of the side) they stopped after 50 feet but the shaft kept on going - we didn't!
I've been looking at mines in Spain about 20 times but only in the south, west and east, never the centre and the north.
If you get bored with that a bit further to the south-west is Mazarron and then further on, the Sierra Almagrera which is like mining heaven.
I've never found any useful guides or truly useful websites. I started with a Michelin maps and worked from there.
Plenty of photos here...
http://www.bocamina.com/esp/tablasel.htm
The finest workers in stone are not copper or steel tools, but the gentle touches of air and water working at their leisure with a liberal allowance of time.
Henry David Thoreau