My folks have a plasma above the fireplace with no issues, of course, it is a gas fireplace with blower, does not harm the tv if you have a mantle.
Try to have the tweeters of the right/left/center as close as possible. Ideally, you want them at ear level when seated, that may not be possible in this situation. You don't want your center way high and your other speakers low, that will screw up the soundstage.