Right........ assuming its an original casting and not a repro.
First... At the back of the piece an at either end towards the top there should/will be either a small protusion like a hook or an eye or something similar These are there so that you can get a heavy gauge wire attached to the casting......... When I say heavy gauge I mean something the thickness of coathanger wire or thicker. Anything less is not worth doing .
You will need about 4 inches of a loop of this wire sticking out after uts fastened to the casting.
Next. Offer the casting up to the wall in its final position. Mark a spot in line with where the wire is attached and just above the top of the casting on the wall. If its a cast top that bolts on do this with the top unbolted and removed. If its an unfinished top meant to be finished with a wooden or other surround just use the top of the casting as a guide.
Remove the casting and get a scutch chisel or an SDS bit and make a hole about inch and a half diameter and about five inches deep.............So that the bottom of the hole is level with the top of the casting.. The next bit is where the skill comes in. You have to plug the hole with lead.. Strips of lead, full depth of the hole but lining the sides, so that there is room for the wire to go in the middle of the hole to its full depth. with the lead in place, offer up the casting again and feed the wire into the lead lined cavities........
Make sure the casting is level and plumb. Feed more strips of lead in to the holes until you cannot feed any more in, the get a drift or piece of steel rod and begin to hammer the lead into the hole. As the lead is hammered in more room will appear and more lead must be fed in until ot is absolutely packed in, the wire will be locked into the lead and the lead will have taken up shape with the irregularities in the hole in the masonry so if its hammered in good and tight it has locked the casting against the wall.
The mantle top. either a cast one or a timber/plaster/ stone one will cover the hole although you can skim it over with plaster anyway if you wish..... The bottom of the casting is held to the wall either by brackets if its a cast not meant to have a surround to finish it or if it is meant to be finished with a surround then when the surround is fixed to the wall this will retain the casting.
If you prefer to use bonding as suggested then go ahead..But first ask yourself how it is going to fix a cast iron piece to a wall? I know it is done this way but is inherently unsafe unless there is a substantial mechanical fixing also..
You will be aware of how much these things weigh. Imagine that tipping forward on to the hearth rug..........
PS yes I have fitted these, and the above is the traditional way. If its a repro then it may even have drilled lugs so you can fix it with rawl plugs !
Good luck anyway, and work safely