Try lifting the mantle up (with a partner). We removed our mantle years ago to have it refinished. After hours of trying to pry the thing off the wall (and nearly destroying it), we accidentally discovered it was attached simply by hanging from hooks on each side. With a partner on each side, we simply lifted it straight off. This was a no longer working coal fireplace, not a purely decorative mantle, so it might be different. But, worth a try, I think.