YB,

Yes, magnetic disturbance of compasses has long been seen/noted by folks on the iron range.

And in fact, that's how the Mesabi range was 1st discovered by the Merritt brothers of Duluth, who were also known as the "Seven Iron Men".  They basically used a compass, to locate and map the iron deposits of the Mesabi in the ground. They would walk over an area in a systematic pattern, taking compass readings at given locations, I believe both direction and strength, and then mapped it later.

At the time most mining folks didn't believe them but after they dug a few test pits and got the dirt tested the "run" was on.

Dan