| Abstract: | The magnetic susceptibility of the Helvetic Jurassic black shales, sampled around the Aar and Gothard massifs in the Swiss Alps, is due to iron-bearing silicates, magnetite or pyrrhotite. It exhibits a clear correlation with the Alpine metamorphism, from zeolite to amphibolite facies: it firstly decreases due to the breakdown of original magnetite, then sharply increases when pyrite is transformed into magnetic pyrrhotite. Using numerous field measurements of the susceptibility it was possible to carry out a fast and precise mapping of the pyrrhotite-in isograde, which appears to correspond closely to the stilpnomelane-out isograde. The matrix susceptibility, obtained by means of high-field measurements with a cryogenic magnetometer, is due to iron-bearing silicates and shows only smooth variations with metamorphism. Both monoclinic and hexagonal pyrrhotites are observed in the high-grade area. |
