Excuse addition from someone who knows nothing about mining but who knows, and cherishes, the area.
I believe that the Natural Retreats development is dead in the water. They have just the one site in the UK - Yorkshire Dales - which is outside the National Park, but have now opened sites in the Irish Republic. I guess they met with opposition to all their proposed developments in U.K National Parks.
Personally I've always been in favour of something being made out of the original mill site ruins by Rhaeadr Mawddach, possibly by tidying up the ruins and providing some interpretative info on the site. Bit of a seating or picnic area perhaps. Given popularity with walkers to the falls here, it might retain something of the heritage.
At one stage there was an idea of a 'Gold Mine Centre' in Dolgellau but this fell flat - for what reason I don't know - and the building proposed for this is now a car showroom.
Unfortunately there is not a great deal of public interest in the old gold mines but I personally feel that some focal point of interest should be provided.
Incidentally, recent walks have shown that all the abandoned gold workings around Foel Ispri - above Llanelltyd - have been fenced off and provided with warning notices, presumably as a statutory requirement.
Someone touched on pollution and I'm given to understand that in the Gwynfynydd area it's not from the mine, but from disused army ordnance on the old Bronaber ranges which is seeping into the Afon Gain.