Genesis of precambrian sulfide ores, skellefte-district, Sweden

Some 80 stratabound pyrite deposits occur in Precambrian volcanic and sedimentary rocks in the Skellefte district of northern Sweden. These deposits have been considered previously to have originated by concentration of metals from the supracrustal rocks during the formation of a late-kinematic palingenetic granite.However, the ores show all the characteristics of deposits associated with submarine volcanic rocks from other parts of the world and in other geologic epochs. In particular, the deposits are stratabound, closely associated with keratophyric and dacitic volcanics, and contain lead isotopes which are coincident with the primary "growth curve". The Pb-Cu-Zn and Pb:Zn ratios are also characteristic of these deposits. Sulfur isotope ratios are remarkably homogeneous over the whole area and fall at about the meteoritic standard.Furthermore, the ores have been subjected to regional metamorphism which is related mainly to the formation of the late-kinematic granites. Macroscopic ore metamorphic features include tight folding, brecciation, and boudinage. The structural setting of many of the deposits is similar to that expected from the process of bedding transpositions during regional metamorphism. The pyrite of the ores has been studied microscopically using structure-etching techniques. The ores commonly show pyrite micromosaics and these have been examined statistically. It is concluded that the pyrite has been annealed and subsequently deformed to various degrees in different deposits. Relict textures have been studied in the etched pyrites. Rogenpyrit, framboidal, and colloform textures have been tentatively identified in these relicts.It is concluded that the ores were formed in association with volcanism in a shallow marine environment during the formation of a geosyncline and subsequently subjected to at least two phases of deformation