For iron gun mounts, whether European or American in the period I believe we are talking about forged iron. "Cast Steel" did exist, but if I understand correctly, that was a process for making steel ingots in crucibles, to be forged into other items, not directly cast into the shape of the finished items.
There are many examples of finely finished, sometimes even chiseled and engraved, forged iron mounts on European guns, going back into the 1600s. And there are examples of American iron mounts that are highly finished out. Take a look at some of the work by the Honakers. There was also an early iron mounted gun discussed on here a few months back, shown in one of Dr. Whisker's books and Jerry Noble's book, that appears to be from the 1780-90 period and has a finely finished guard, nice incised carving, and no buttplate. Some have proposed it to be associated with the Bryan family of gunmakers. There are others.
I guess what I am getting at is that once you have the basic shape there, there, you could dress out an iron mount to the same degree as you would a brass mount if you wanted.
Guy