EMPLOYEE WAS WORKING AT 49 BLOCK MAIN NORTH TRACK ENTRY. HE WAS IN THE PROCESS OF LOADING DEBRIS INTO MINE CAR. EMPLOYEE HAD PICKED UP AN OLD 5 FOOT BOLT WHICH HAD A TROLLEY HANGER ATTACHED TO THE END. EMPLOYEE THREW BOLT INTO EMPTY MINE CAR. END OF BOLT BOUNCE BACK AND STRUCK EMPLOYEE ON LEFT CHEEK CAUSING A LACERATION.