I was recently reading a post on Apartment Therapy on someone asking how to hang a picture in the middle of a slate fireplace. I cringed when I read that her friends "suggested drilling into the grout which would be an easy fix".
Here's the thing : As a general rule, if the slate on a fireplace goes all the way to the ceiling, my advice is leave it alone. It was designed to be a focal point and stand alone.
I would also avoid adding a wood mantle on it. Most of the time, it just doesn't fit and very often it's just going to accumulate clutter, causing the fireplace to "disappear" behind it.
Not every vertical surface in your home need something on it. In the case of a slate fireplace, less is more.