If there are 2x4 walls associated with this fireplace, then those walls would be subject to the receptacle rules. If this is a 100%, all four sides, masonry fireplace, then it depends. Are there sections of your fireplace with a dimension of 2 feet? IMO the same rules for a three piece sliding glass door would apply for the fireplace.(with the fireplace part(s) with the mantle being treated as the slider part). Floor mounted receptacle outlets may be the answer.
