Sedimentary pyrite and its metamorphism in the Oslo Region

Occurrence, morphology, and metamorphism of sedimentary pyrite of the lower part of the Cambro-Silurian sequence (black shales, marls, and limestone) are described. Pyrite and trace amounts of sphalerite are the only sulfide minerals in the unmetamorphosed areas. Within the thermal aureole of the Permian Drammen granite, pyrite has been completely replaced by homogeneous monoclinic pyrrhotite. This replacement is found up to 3 km away from the granite contacts; an outer zone of the aureole can be defined in which pyrrhotite replaces pyrite, but no other contact-metamorphic minerals appear. The transformation probably took place below 320°C and as the consequence of a very low partial sulfur pressure. The latter is thought to be due to the intrusion at high level, which enabled gases generated in the homfelsed inner aureole to escape in a steady stream removing sulfur at the same time. The use of metamorphic pyrrhotite as an indicator of the depth of metamorphism is suggested.