Not sure where to ask this so I ask it here. If there is a better place for it please move it there. Has anyone done analysis (chemical, polished section, XRD of the pyrite from Spain? https://www.mindat.org/photo-831794.html Unlike other pyrite crystals they are very conductive and register on a metal detector. There has to be a reason for this.
A paper in The American Mineralogist many years ago suggested it has to do with certain impurities like cobalt, copper, and nickel, as well as deviations in the stoichiometry of pyrite. Not sure if it will answer your question Reiner, but below is a link to that paper.