Need suggestions on how to seal 1/8 to 1/4 inch cracks in the bottom of a fireplace.

Is this a fireplace that is made of real bricks that are a part of a wall? Or, is this fireplace a kind that sits out from a wall a little bit? Do the cracks run vertically or horizontally? Not quite sure what kind of fireplace it is that you are asking about. A little more information would be helpful here. Thank you!