I would use a hammer drill and some hexhead tapcon screws to bolt a 2x4 ledger to the face of the rocks.
3/16" or 1/4" x 3 1/2" should do it in about 4 or 5 places across the width (every 12" - 16") Use bigger flat washers under the heads of the tapcons to help hold the 2x4 secure. Make your mantle to slip over the 2x4 and fasten with regular wood screws from above and below.
Save some of the stone dust from the hammer drill because if anyone changes their mind you could mix this with epoxy and fill the holes in the stone (3/16" holes are easier to fill and less noticeable)
Just some ideas for you...
Cheers! - Jim