I have a (unsupported personal) theory that the Romans, who had their recently-discovered first-century fort just up the hill and left a bit from here, may have got there first on any easily-accessible silver that was near the surface.
There may come a day when I do a post showing a mine that isn't one I just happened to wander past this morning. But this is not that day, since it was a lovely morning for a walk. This is a mile or so below Gunnislake weir, which is the point at which the tide stops coming in. So, this is the river at high tide, 18 miles or so inland, and busy with canoes.