It is possible to detect tiniest human influences

on the magnetic fields that surround us.

http://en.wikipedia.org/wiki/Magnetometer

But big changes in the earth's magnetic field

have other causes, like plate tectonics and

the flow of liquids in the interior of the earth.

http://phys.org/news/2011-10-plate-tectonics-reversals-earth-magnetic.html

http://physicsworld.com/cws/article/news/2006/mar/21/geomagnetic-flip-may-not-be-random-after-all

Related:

http://physicsworld.com/cws/article/news/2002/jul/19/mobile-nmr-leaves-the-field-behind

http://physicsworld.com/cws/article/news/2004/oct/14/earthquake-shakes-up-gravity

http://www.newscientist.com/article/dn3251-magnetic-effect-of-ocean-flow-revealed.html

Regards,

Joachim

Earth's magnetic field is mainly due to the motion of the molten outer core of iron alloys (geodynamo) and is not too much dependent on the crust I guess. The field changes over time due to the changes in geodynamo mainly. The magnetic poles move too and even reverse in random intervals of the order of several hundred thousand years. The mining has no appreciable effect I guess. The is the standard model of geomagnetism but some alternative model (involving motions of ions in sea water) has recently been proposed but not generally accepted. But I guess in the vicinity of large deposits of magnetic materials (ferro- and ferri-magnets but not antiferromagnetic materials) the local magnetic field will definitely be slightly modified. Excessive mining will certainly cause even slighter changes. The theory of geomagnetism is still open to alternative models.

I would think that the amount of iron being mined or the total iron in the crust is insignificant (compared to the iron in the mantle and core) to make a difference on the magnetic field of the Earth.