You took a wrong turn. The mine is small, but confusing as ####. Go through once more, there is another passage that leads down, rather easy to miss, and the runekeeper is there. One passage terminates in a dead end (forcefield), the other has a large-ish room with more trolls and a runekeeper.
I also spent about 15 minutes in that place and was sure that I found a showstopper bug, but the passage is indeed there and so is the runekeeper.