Deformation and metamorphism of massive sulphides at Sulitjelma, Norway

Abstract

The copper-bearing stratabound pyritic massive sulphide bodies contained in metamorphosed basic eruptives of Ordovician age at Sulitjelma in Nordland County, Norway, form one of the important fields of sulphide mineralisation within the Köli Nappe Complex. The sulphide bodies and their enclosing rocks were subject to successive stages of penetrative deformation and recrystallisation during the cycle of metamorphism and tectonic transport caused by the Scandian Orogeny. Textures within the ores and the immediate envelope of schists show that strain was focused along the mineralised horizons. The marked contrast in competence between the massive pyritic sulphides and their envelopes of alteration composed dominantly of phyllosilicates, and the metasediments of the overlying Furulund Group, led to the formation of macroscale fold and shear structures. On the mesoto microscale, a variety of textures have been formed within the pyrite-pyrrhotite-chalcopyrite-sphalerite sulphide rocks as a result of strain and recrystallisation. Variations in pyrite:pyrrhotite ratios and in the texture and proportions of associated gangue minerals evidently governed the strength and ductility of the sulphide rocks so that the same sulphide mineral can behave differently, displaying different textures in different matrices. In massive pyritic samples there is evidence of evolution towards textural equilibrium by recrystallisation, grain growth and annealment during the prograde part of the metamorphic cycle. Later, brittle deformation was superimposed on these early fabrics and the textural evidence is clearly preserved. By comparing published data on the brittle-ductile transformation boundaries of sulphide minerals with the conditions governing metamorphism at Sulitjelma, it is concluded that most of the brittle deformation in the sulphides took place during or after D3 under retrograde greenschist conditions. Grain growth of pyrite in matrices of more ductile sulphides during the prograde and early retrograde stages of metamorphism produced the coarse metablastic textures for which Sulitjelma is well-known. In some zones of high resolved shear stress, pyrite shows ductile behaviour which could be explained by a dislocation flow mechanism operating at conditions close to the metamorphic peak. In those horizons in which pyrrhotite is the dominant iron sulphide, the contrast in ductility between silicates, pyrite and pyrrhotite has led to the development of spectacular tectonoclastic textures in which fragments of wall rock have been broken, deformed, rolled and rotated within the ductile pyrrhotite matrix.