Clark,
I haven't found the reference yet, but I will post it here when I do. Some of my saved links on that topic are dead. There has been some discussion of the SE MN deposits on the Orerail group in the past, and I recall that I found the info you are interested in while researching an answer to a question there. It may have been in a state or federal minerals yearbook or a similar publication. Virtually every cement plant receives some sort of iron-bearing material, so the Mason City plants had to get it somewhere. I understand your desire as a prototype modeler to have hard documentation.
Cheers, Rick