At their peak they mined magnetite, which is a higher grade ore, but the northern deposits were depleted long ago, leaving mostly taconite, which is only 25-30% iron, the rest being mostly silica. After a lot of processing, the taconite is turned into pea sized nodules that are then sent to steel plants.
They were until very recently. The Chinese have shut down most iron ore and steel production in the US.
At some point soon, every American manufacturing job will have been free traded to China.