I want to mount a 55" TV above the fireplace, preferably close to the top of the fireplace not too high up. I bought a mount (Sanus OLF18-B1) that attaches to the studs at 4 points in a rectangular pattern, supporting a horizontal spacing between 16" and 26". I used a magnetic stud finder to find the metal fasteners on my studs. See image below where I put tape on the locations found by the stud finder. But looking at the pattern, I'm not even sure whether I have vertical studs above the fireplace. Maybe there's something running horizontally instead? If I drill somewhere below those marked locations near the center, do you think there will be studs there?
It is hit or miss without knowing the original framing. Very often there is a horizontal member (beam or header) above the fireplace opening. This carries the wall load around the fireplace opening. It is not uncommon for that "header" to be elevated above the actual firebox. All fireplaces have a required "minimum distance to combustible materials" (wood being a very combustible one). These fireplace clearances will often put the "header" higher up on the wall (since heat rises).
So yes, there is most likely wood back there but it is impossible to tell you where with certainty.