As a professional installer for the past 8 years, I have done hundreds of mountings on brick fireplaces. You would want to drill into the mortar using a concrete anchor and a standard lag bolt. Most of the mounts we use come with these anchors but I guess it depends on which mount you get.
The anchors that come the mounts are like these:
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The reason for mounting into the mortar is when you insert the concrete anchor and use the lag, it will expand and push and hold against the adjacent bricks. A hammer drill will work the best and go the quickest, but I guess work with what you got.
Using this method, I have never had any problems wall a TV falling or coming loose and I have mount some pretty big and heavy TV's on brick. When it comes to really heavy TV's, I sometimes turn to RedHeads.
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