Two important sulphide ore-bearing formations have been recognized in the Cambro-Silurian succession of the Røros district. One is composed of submarine greenstones and quartz keratophyres (the Hersjø Formation). The other one, which is studied here, consists of a heterogeneous series of banded meta-argillites and meta-greywackes with subordinate thin green interlayers of probable volcanogenic origin (the Røsiø Formation). These two formations are embedded in a thick succession of various clastic sedimentary rocks which have undergone metamorphism and folding during the Silurian orogeny. The numerous sulphide deposits which occur in the Kjøli area are largely confined to the Røsiø Formation. The sulphide bodies are chiefly composed of massive pyrite and/or pyrrhotite showing variable chalcopyrite and sphalerite grades. The massive sulphides often occur as thin, extensive sheets, locally as irregular, small lenses which rarely exceed one metre· in thickness. It is probable that the sulphides have been precipitated primarily on the sea-floor, but there is no clear evidence of a volcanogenic origin. Lo????al discordant relationships between the massive sulphides and the wall rocks are ascribed to metamorphic mobilization of the sulphides. The thicker sulphide lenses may themselves represent mobilized portions of thinner sulphide-rich units.