Did the front spindle touch the deck and cut into it when the mower was new? If not, something is worn and allowing excessive movement and you will have to find out what went wrong. Some pictures would be a great help.
Try this: engine off, parking brake set, deck height raised to maximum, blade belt disengaged - reach under the deck with leather gloves on and grab each cutting blade by the end and try to wobble it up and down to feel for movement. The blade should turn by hand but not have any up/down play. If there is play in the bearings or you hear grinding while turning the blade, you have a spindle bearing or bearings going bad and this could allow the spindle pulley to hit something causing the sparks. This is just a first guess based on the information you provided so make the checks, take some pictures and report back with your findings. You may have to remove the deck to check everything so be prepared to do some physical work.