Difficult to advise without a picture. Does it look as though the marble
mantle was fitted last? If so, it will be difficult to remove the cast iron
fireplace without damaging the marble.

Yes, I understand that without a picture. I was curious if there was a
'typical' way that these things were built.

Mine seems to be the iron fireplace sitting behind an overlapping
'picture frame' of rectangular section black painted hardwood. That
frame itself seems to go behind the marble surround. Hmm, tricky??