I made a wooden cleat. One side is mounted to the back of the mantel and the other to the wall. You can also purchase similar mounts. I would use strong wall anchors into the brick.
http://www.homedepot.com/p/OOK-Hangman-200-lb-French-Cleat-Picture-Hanger-with-Wall-Dog-Mounting-Screws-55316/202341629
Foundre: Made