EMPLOYEE WAS LOWERING THE DRILL MAST IN PREPARATION TO MOVE TO NEXT DRILL SITE. WHEN THE MAST WAS ABOUT 25% DOWN, WELDS LET LOOSE THAT WERE HOLDING MAST SUPPORT PIVOT. THE MAST FREE FELL THE REST OF THE WAY. THE MAST DECK CAME IN CONTACT WITH THE OPOERATORS CAB, LOOSENING IT AND SHAKING UP THE OPERATOR. HEAD CONTUSION AND NOSE LACERATION.