Follow my logic. Huge ammounts of ore were mined from Minnesota and Upper Michigan. This was/is being shipped to Detroit to build cars and to smelters in and around the great lakes. Think of all the steel, like that manufactured in Bethleham, PA that was in turn shipped elswhere to build bridges, skyscrapers, etc. Then there are the cars and the junk yards full of dead cars. That displaced metal must, by reason, effect the magnetosphere.
The magnetic field of Earth is generated by the molten core of iron and nickel. Iron ores near the surface have a much smaller mass than the core; they can only affect local perturbations (magnetic anomalies)
I follow your logic, and it is not good. If this were to be effective, it would have shown up a long time ago. You are also leaving out the deposits else where in the world. The net effect would be zero based on all of the deposits every where else, and the gyroscopic effect of the mass of the earth in it's rotation.
It may have affected it very slightly, but not anywhere near to where it would cause a problem or large disruption. Even though it seems like that is a lot of iron ore, it's really only a tiny amount compared to what's still in the earth.
