Wheal Benny was a smallish tin, wolfram, and yet, of course arsenic mine. It goes right down to the riverbank and had a waterwheel. To the left of Rosie, the wall of a wheelpit, and to her right, the river.
This is probably the most visible bit of it, with the gate across the adit entry to the mine at the back.
I think this was probably a wheel pit for one of the waterwheels. It's quite deep (and completely unfenced: note to self, keep careful track of location of both dogs at this point).