EE WAS BACKFILLING AN OLDER, NON-WORKING AREA OF THE MINE USING A 915 LHD. AFTER DUMPING A LOAD OF MUCK, EE WAS IN THE CENTER OF THE ROOM STARTING TO BACK DOWN THE ALLEYWAY WHEN HE HEARD A ""SN AP"" & SAW A MOVEMENT ALONG THE ROOFLINE. A PIECE OF SALT APPEARS TO HAVE STRUCK THE TRACTOR, W/SOME OF IT EITHER HITTING THE EE DIRECTLY OR DEFLECTING OFF THE TRACTOR & SUBSEQUENTLY STRIKING