I am not sure what kind of fireplace you have. But there is definately a legal clearance limit around the opening for combustable materials. Check with your local building codes or manufacturer if you have one of those "insert" types of fireplaces.
A wood mantle will act a heat shield. Even though wood burns it still acts as an insulator so it should be sufficent to block most of the heat. But it also have to be a certain distance away from the opening.
I am not sure if you have a masonry or "insert" type, depending on type will determine how far away from the opening the mantle will need to be.
You might have to move the TV higher after the mantle is installed, do some measuring first before making a decision because the TV may end up very close to the ceiling, or may not fit at all.
Overall I think the TV is too close to the opening, so consider a relocation if possible.
I've included hearth clearances from 2003 that you can use to determine if installing a mantle is feasible. Please use these only as a guide, check your local codes to obtain the exact distance required. But those should good enough to see how much farther up the wall the TV may have to moved.
FIREPLACE HEARTH CLEARANCESS 1/2003
R1003.9.1 Hearth Thickness - The minimum thickness of firepalce hearths shall be 4" thick. (102)
R1003.9.2 Hearth Extension Thickness - The minimum thickness of hearth extension shall be 2" (51 mm)
Exception; When the bottom of the firebox opening is rasised at least 8" (203mm) above the top of the hearth extension, ahearth extension of not less than 3/8" inch thick (9.5mm) brick, concrete,stone, tile,or other approved noncombustible material is provided.
R1003.10 Hearth Extension Dimensions - Hearth extensions shall extend at least 16 inches (406mm) in front of and at least 8 inches (203mm) beyond each side of the fireplace opening. Where the fireplace opening is 6 square feet (0.557m2) or larger, the hearth extension shall extend at least 20 inches (508mm) in front of and at least 12 inches (305mm) beyond each side of fireplace opening.
R1003.12 Fireplace Clearance - All wood beams, joists, studs and other combustible material shall have a clearance of not less than 2 inches (51mm) from the front faces and sides of masonry fireplaces and not less than 4 inches (102mm) from the back of masonry fireplaces. The air space shall not be filled except to provide fireblocking in accordance with Section R 1003.14.
Masonry fireplaces listed and label for use in contact with combustibles in accordance with UL 127 and installed in accordance with the manufactures installation instructions are permitted to have combustible material in contact with their exterior surfaces.
When masonry fireplaces are part of masonry or concrete walls, combustible materials shall not be in contact with the masonry or concrete walls less than 12 inches (305mm) from the inside surface of the nearest firebox lining.
Exposed combustible trim and edges of sheathing materials such as wood siding, flooring and drywall shall be permitted to abut the masonry fireplace sidewalls and hearth extension provided such combustible trim or sheathing is a minimum of 12 inches (305mm) from the inside surface of the nearest firebox lining.
Exposed combustible mantels and trim may be placed directly on the masonry fireplace front surrounding the fireplace opening providing such combustible materials shall not be placed within 6 inches (152mm) of a fireplace opening. Combustible material within 12 inches (306mm) of the fireplace opening shall not project more than 1/8 inch (3.2mm) for each 1-inch (25.4mm) distance from such opening.
2114.3 MASONRY FIREPLACES
2114.36 Fireplace hearth extensions shall be of approved noncombustible materials for all fireplaces. Where the fireplace opening is less than 6 sq.ft. (0.56m²), the hearth extension shall extend at least 16 inches (406mm) in front of the facing material and at least 8 inches (203mm) beyond each side of fireplace opening. Where fireplace opening is 6 sq.ft. (0.56m²) or larger, the hearth extension shall extend at least 20 inches (508mm) in front of the facing materials and at least 12 inches (305mm) beyond each side of the fireplace opening. Where a fireplace is elevated above or overhangs a floor, the hearth extension shall also extend over the area under the
2114.3.7 Fireplaces constructed of masonry or reinforced concrete shall have hearth extensions of brick, concrete, stone, tile, or other approved noncombustible material properly supported and with no combustible material against the underside thereof. Wooden form or centers used during the construction of a hearth and hearth extensions shall be removed when construction is completed. EXCEPTION: A header of combustible material may be used to support the hearth extension provided that it is located more than 12 inches (305mm) from the face of the fireplace.
2114.3.8 All combustible wood beams, joist, and studs shall be maintained a minimum of 2 inches (51mm) from the outside face of the chimney or fireplace masonry. Headers supporting trimmer arches at fireplaces shall be not less than 20 inches (508mm) from the face of the chimney breast. Trimmers shall be not less than 6 inches (152mm) from the inside face of face of the nearest flue lining.
2114.3.9 Woodwork shall not be placed within 4 inches (102mm) of the back of the fireplace, but shall not prevent plastering directly on the masonry or metal lath and metal furring.
2114.3.10 All combustible mantels and similar trim shall be kept at least 6 inches from fireplace opening. Parts of the mantel assembly located along the sides of the fireplace opening, which project more than 1-1/2 inches (38mm) from the face of the fireplace, shall have additional clearance equal to he projection. Parts of the mantel assembly located above and projecting more than 1-1/2 inches (38mm) from the fireplace opening shall not be placed less than 12 inches (305mm) from the top of the fireplace opening.