There have recently been some new wide gravelled paths laid on the popular 'Precipice Walk' at Dolgellau.
Its not mining but spectacular views. There is courtesy parking at the end of the minor road directly opposite the toll bridge on the Barmouth road.
A steep ascent by vehicle, (presumably yours is adapted for wheelchair), and when the road forks take the right fork through a gate.
Courtesy parking at the very end, and follow the path.
EDIT
When you get your vehicle through the gate, a few yards on you will find somewhere to pull off the road to the left.
Follow that direction for about 50 yards, you may get the wheelchair there, and you will see something very interesting. Trust me.
My avatar is a poor likeness.