In true armchair explorer style, I was having a look at Vernon and Bennet Volume 7. It mentions a sulphur mine in the Afon Ddu valley. I had a poke up the gorge up the tourist climb a few years ago, but did not see any adits. Preumably these are further up the valley from the footbridge ?
There's nothing shown on the OS map in the Afon Ddu valley.
On a separate note, Derwen Deg is mentioned in the same volume, but deemed to be inaccessible due to being on private land. I suppose this is still the case almost 20years since the book was published?