Pyrrhotite from the Sanbagawa pelitic schists of the Shiraga-yama area, central Shikoku, Japan

Abstract

Sulfides and oxides in pelitic schists of the Sanbagawa metamorphic terrain in the Shiraga-yama area, central Shikoku, Japan, are pyrrhotite, pyrite, chalcopyrite, ilmenite, and rutile. The most abundant assemblage of opaque minerals is pyrrhotite+pyrite+chalcopyrite in the chlorite zone and a lower-grade part of the garnet zone, and pyrrhotite+chalcopyrite+ilmenite+rutile+ (pyrite) in a higher-grade part of the garnet zone and the biotite zone. In this area, in addition to monoclinic Fe7S8 pyrrhotite which is common, intermediate pyrrhotite with Fe9S10 does occur, and also Fe9Sio+pyrite assemblage is observed. This fact suggests that the Sanbagawa schists were metamorphosed above 310°C and that, although the majority of hexagonal pyrrhotite reacted with pyrite as cooling of metamorphic rocks proceeded, some pyrrhotite persisted to keep high temperature chemistry. It was also revealed by this study that pyrrhotites in the samples collected at the surface exposures are more or less oxidized but those collected from the drill cores are little oxidized.