I stripped all the old brick off the fireplace and am replacing it with slate tile. This is a full masonry fireplace with all four sides exposed and a corner firebox.  The firebox has an angle iron over it that carried the load of the brick.  Since I am replacing with 1/2″ slate tile, I need to build the existing surface out since I can’t remove the angle iron over the firebox.  Would it be best to just build the face out with 4″ concrete block or brick to mantle height then apply the slate over that?  Would metal studs with cement board be an option?  I also need to carry the load of a mantle.  Any advice would be appreciated.  Thanks  -Ed