In the present report copper ore samples from three different sulphide deposits in Bergslagen, central Sweden, were investigated; the Kaveltorp and Ljusnarsberg mines, near Nya Kopparberg and the Tunaberg mine, near Nyköping. The samples, which represent reduced ore, i.e. they contain the assemblage chalcopyrite + cubanite rather than the more common assemblage chalcopyrite + pyrite, were provided by the Museum of Evolution in Uppsala.
Ore microscopy shows intricate intergrowth textures between chalcopyrite and cubanite in ore from Kaveltorp and Ljusnarsberg. Occasional small stars of sphalerite in both the copper phases suggest annealing during metamorphism. Pyrrhotite, occurring as medium grained “soap‐bubble” masses together with the copper minerals, shows two modes of appearences; (i) a flame‐like variety, which the microscopy investigation shows to consist of two phases closely intergrown with one another. A microprobe traverse shows indeed two distinct compositions with a Fe‐rich variety being close to stoichiometric FeS, while a S‐rich variety has a composition close to Fe7S8, (ii) occasionally around minute parallel cracks in “normal” homogenous pyrrhotite, a slightly lighter colored variety indicates late‐stage oxidation. The Specimen from Tunaberg is most spectacular, with straight lamellae of chalcopyrite and cubanite in one another. The size of these lamellae in fact suggests that the museum specimen is part of a composite single crystal at least 5‐10cm in size. Formation of such composite Cu‐Fe‐sulphide single crystals is enigmatic.
Arkivexemplar i Uppsala universitets arkiv