EMPLOYEE WAS BOLTING IN THE HEADGATE ENTRY. HE WAS DRILLING FOR A RIB BOLT WHEN A ROCK APPROX. 2'X 2'X 3"" FELL FROM THE TOP & STRUCK HIM ON THE LEFT ARM & THE FORCE FROM THE WEIGHT CAUSED HIS ARM TO HIT PART OF THE PLATFORM ON THE BOLTER. THIS CAUSED A LACERATION TO HIS LEFT ARM & SUTURES WERE REQUIRED. NO TIME LOST.