The nearest mine I can find to Wern Ddu (on the Corwen - Ruthin road) in Wilkinson's list is Ty Newydd at SJ073477.
However, I'm a bit dubious about this grid reference as there is no sign of anything on the modern 1:10,000 map at this location.
There are, however, a number of tips marked close to Wern Ddu, so one of them may be the mine in question.
Dave