Iron
On most mine sites there is a reference when the mine is built called MINE SITE NORTH found by a combination of planets and the sun.
This is so the builders can reference the plans to a North. This is not magnetic north. As the ore deposits usually disturb the earths magnetic field and a compass will read the wrong north.
You may be able to find the iron boxes by using a weak compass.
Get 3 compasses. Mechanically join two of the compasses back to back, north of one compass to south of the other compass.
This will make a weak compass that is not effected as much by the earths field.
When in the field look at the two compasses and see if there is any change between the weak compass and the earth compass. Then try and triangulate the changes to find your target.
rgds
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