I think this should read "Magnetics vs Electromagnetics". In the industry, "magnetic or mag" refers to a survey which detects the magnetic signatures/properties of the country rocks. "Electromagnetic or EM" refers to a survey which detects the presence of conductors, which may include concentrations of sulphides, graphite (all too common!) or other concentrations of other metallic minerals which are also conductors. Some minerals are naturally more "conductive" than others.