The County Archive in Dolgellau has some stuff, but not an awful lot.
There are lots of workings dotted around the area, many of which were no more than unsuccessful trials. But be careful WRT Gwynfynydd - there is an ochre issue there - has been for years. So far as I know Clogau has seen odds and ends of work, amount unclear, so it might be regarded as private, but the notion that other mines have been blocked-off recently to stop people after gold is news to me. I doubt this to be the case. the Crown did fence-off lots of shafts and adits in the district - not just at the gold mines but also those that worked the manganese - more of a liability thing. Unless they were trying to stop people nicking manganese, too!