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Sulphide mylonites are fine-grained massive sulphides which have deformed in a plastic manner. In the Renström Zn-Pb-Cu-Ag-Au VMS deposit, one of several operating mines in the Early Proterozoic Skellefte District in Northern Sweden, shear-zone metamorphism has resulted in the development of mylonitic fabrics within the sulphides. The massive sulphide ore is hosted in a shallow submarine to subaerial volcano-sedimentary sequence which has been variably metamorphosed and deformed. Initially, the sequence underwent burial metamorphism which was followed by an amphibolite grade regional metamorphic event at pressures of around 7.5 kbar and temperatures of 540-600°C This has been overprinted by a retrogressive metamorphic event at greenschist facies (at ca. 400°C with concomitant ductile deformation. Finally the area was uplifted to shallower crustal levels with associated cataclastic deformation.
Both the regional and dynamic metamorphic events have resulted in the development of specific textures in the sulphide ores. Textural evidence indicates that pressure solution has been mainly responsible for the plastic deformation in pyrite, while the weaker sulphide minerals such as pyrrhotite, chalcopyrite and galena have generally recrystallised in response to the high strains.
Sulphide mylonites are probably common rocks in many polydeformed massive sulphide deposits like Renström. They may have previously been misinterpreted as primary depositional textures.