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 Krli 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-chalcopyritesphalerite 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.

Mineralogy, geochemistry and sulfur isotope characterization of Cerro de Maimón (Dominican Republic), San Fernando and Antonio (Cuba) lower Cretaceous VMS deposits: Formation during subduction initiation of the proto-Caribbean lithosphere within a fore-arc

“…This correspondence is almost systematic in Cerro de Maimón and overlaps with the complex deformation history that certainly involved brittle deformation during retrograde metamorphic conditions (e.g. Cook et al, 1993). By means of orientation contrast (OC) imaging and electron backscatter diffraction (EBSD), Barrie et al (2010) concluded that the preservation of annealing textures in greenschist-facies metamorphosed massive sulfides of the Norwegian Caledonides potentially represented surface-driven processes (re-arrangement by a dissolution/syntaxial overgrowth) during metamorphism This conclusion contrasts with the classical assumption that foam textures result from recrystallization (e.g.…”

“…pyrite alternate with intense microfracturing and blow-apart and porphyroclastic textures, locally developing pressure shadows. Very similar textural relationships and gradations are described in other VMS-bearing districts worldwide that underwent low-grade metamorphism, such as the Iberian Pyrite Belt (Velasco et al, 1998), the Appalachians (Brueckner et al, 2015) or the Norwegian Caledonides (Cook et al, 1993;Barrie et al, 2010;Lockington et al, 2014).…”

“…By means of orientation contrast (OC) imaging and electron backscatter diffraction (EBSD), Barrie et al (2010) concluded that the preservation of annealing textures in greenschist-facies metamorphosed massive sulfides of the Norwegian Caledonides potentially represented surface-driven processes (re-arrangement by a dissolution/syntaxial overgrowth) during metamorphism This conclusion contrasts with the classical assumption that foam textures result from recrystallization (e.g. Vokes, 1969;Cook et al, 1993;Velasco et al, 1998;Craig, 2001), which certainly occurs at higher than greenschist facies metamorphic grade (McClay and Ellis, 1983). This conclusion is in good agreement with the limited mobilization of gold during metamorphic upgrading observed in Cerro de Maimón ores (discussed in section 9.4).…”

Intergrowths of hexagonal and monoclinic pyrrhotites in some sulphide ores from norway

“…Most of the stratabound massive sulphide deposits in the Norwegian Caledonides were metamorphosed to greenschist or amphibolite facies (Table 2). Pyrrhotite in these deposits has been deformed, recrystallized and annealed during prograde metamorphism ( Table 2 and Vokes, 1968;Cook et al, 1993). Annealing of pyrrhotite is generally thought to be important only above 450~ which is well above 254~ the transformation temperature from mpo to hpo (Fig.…”

Effects of greenschist-facies metamorphism and related deformation on the Mobrun massive sulfide deposit, Qu�bec, Canada

“…However, the behaviour of individual sulfide minerals during metamorphism and related deformation of massive sulfide deposits may vary greatly, depending on temperature, strain rate, confining pressure, fluid pressure, differential stress, permeability within the zone of deformation, and the nature of adjacent or matrix-forming sulfides (Ashby and Verrall 1973;Kelly and Clark 1975;Knipe 1989;Cook et al 1993;Duckworth and Rickard 1993). Cox (1987) subdivided deformation mechanisms in sulfide minerals into the following categories: 1) brittle failure and cataclastic flow; 2) low-temperature plasticity (dislocation glide and mechanical twinning); 3) dislocation creep; 4) solution-precipitation creep; 5) solid-state diffusion creep; and 6) grainboundary sliding.…”