The Skellefte district in northern Sweden hosts many volcanogenic massive sulfide (VMS) deposits and is considered one of the most important European mining districts for Cu, Zn, Pb, Ag, and Au. The volcanic and sedimentary rocks that the VMS deposits are hosted in were deformed during the Svecokarelian orogeny, with three documented regional deformation phases. These events imparted a distinct attitude and geometry to the deposits, their host succession, and discordant zones of synvolcanic hydrothermal alteration. Few studies have investigated the detailed deformation effects on the sulfide minerals.

In this contribution, we document the structural characteristics and remobilization history of mineralization at the Rävliden North Zn-Pb-Cu-Ag deposit—one of the most important recent discoveries in the district consisting of 8.5 million tonnes (Mt) grading 1.01% Cu, 3.45% Zn, 0.53% Pb, 78.60 g/t Ag, and 0.23 g/t Au. At Rävliden, massive to semimassive sphalerite-rich mineralization with lesser pyrrhotite, galena, pyrite, and silver minerals occurs structurally above stringer-type mineralization dominated by chalcopyrite, pyrrhotite, and pyrite. These mineralization types exhibit evidence of deformation and remobilization such as (1) sulfide-alignment parallel to tectonic foliations; (2) rounded wall-rock tectonoclasts in a ductile deformed sulfide matrix (“ball ore” or durchbewegt ore); and (3) sulfides in tension gashes, strain shadows, piercement veins, and late, straight veinlets crosscutting tectonic fabrics. These features are attributed to polyphase deformation during the D1, D2, and D3 events at temperature ranging from 200° to 550°C. Remobilization of sulfides was mostly within the bounds of the main mineralization (i.e., 10–100 m), with few local external occurrences. A combination of solid-state and fluid-assisted remobilization processes are inferred.

Rare brittle veinlets and zeolite-cemented breccias with sphalerite, galena, and silver minerals occur in the stratigraphic hanging wall, where they crosscut all Svecokarelian structures. This mineralization type is highly reminiscent of Phanerozoic low-T vein- and breccia-hosted Pb-Zn deposits of the Lycksele-Storuman area west of Rävliden North, which have been linked to far-field effects associated with the opening of the Iapetus Ocean (0.7–0.5 Ga). We suggest that this Zn-Pb mineralizing event led to the formation of the late sulfide-zeolite veinlets and breccias at Rävliden North, and that elements such as Ag and Sb within this mineralization were locally remobilized from Rävliden.

The Rävliden North Zn-Pb-Cu-Ag volcanogenic massive sulfide (VMS) deposit is one of the most important recent discoveries in the Skellefte district, northern Sweden. As of 2022, total mineral resources are of 8.5 million tonnes (Mt) grading 0.23 g/t Au, 78.60 g/t Ag, 1.01% Cu, 3.45% Zn, and 0.53% Pb (Agmalm, unpub. report), and exploration is ongoing. The mineralization is located west of the Kristineberg mine in the westernmost part of the Skellefte district. Rävliden North is hosted in what has been informally named the “Rävliden ore horizon” (Hannington et al., 2003), at the transition from the Skellefte group metavolcanic rocks and the stratigraphically overlying metasedimentary rocks of the Vargfors group. This stratigraphic interval hosts other VMS deposits, such as Hornträsk, Rävliden, and Rävlidmyran, that occur in calc-silicate–and Mg chlorite-rich host rocks. The host rocks are the result of metamorphism of carbonate-bearing, hydrothermally altered volcanic and siliciclastic sedimentary rocks (Hannington et al., 2003).

Rävliden North and the other deposits in the Kristineberg area are hosted by a system of west-plunging anticlines (Årebäck et al., 2005). The deposits and their host rocks have been subjected to greenschist to lower amphibolite facies metamorphism and polyphase deformation at c. 1.9 to 1.8 Ga during the Svecokarelian orogeny (Skyttä et al., 2020). This has resulted in considerable change in the geometry of ore lenses, including remobilization of primary mineralization (Årebäck et al., 2005; Skyttä et al., 2013).

Remobilization processes are important since they can lead to the upgrading of metal tenors (Marshall and Gilligan, 1993). This upgrading can be through coarsening of grain sizes, providing potential for improved beneficiation. Moreover, remobilization can lead to the formation of minerals carrying precious elements (e.g., sulfosalts and tellurides) and their concentration in syntectonic structures (Marshall et al., 1998; Cugerone et al., 2020). Most extreme remobilization can lead to the formation of new orebodies (Marshall et al., 1998). Despite the importance of these processes, there are few studies addressing deformation and remobilization of VMS mineralization in the Skellefte district.

Årebäck et al. (2005) invoked remobilization to explain the origin of crosscutting massive sulfide mineralization in the Einarsson zone at the Kristineberg deposit. However, Årebäck et al. (2005) presented little textural evidence to support this hypothesis. Hence, a primary, synvolcanic origin for this ore lens could not be completely ruled out. Wagner et al. (2007) presented a mineralogical study of pyrite and arsenopyrite from the Boliden deposit. They concluded that progressive recrystallization of fine-grained arsenopyrite in the massive ore lenses occurred in conjunction with remobilization of gold into the crystal structure of pyrite and arsenopyrite hosted in quartz veins. Fluid-facilitated mass transfer was inferred as the main remobilization mechanism. The textural data presented by Wagner et al. (2007) is, however, insufficient to fully understand the mechanism of remobilization or to relate the timing to the different deformation events of the Svecokarelian orogeny. In a regional-scale structural study, Bauer et al. (2014) correlated the present-day geometries of the VMS deposits to the deformation styles of the host rocks. It was concluded that the morphology of the deposits records stretching, flattening, and transposition because of ductile strain, but this was not reconciled with structural observations at the macroscale in the ore lenses. Hence, the exact mechanisms involved in deformation and remobilization of the Skellefte district, especially at the deposit and macroscale, remain poorly characterized.

A study by Johansson (2017) indicates that the Rävliden North deposit is particularly favorable for such a study, as shown by an abundance of sulfide in different structural configurations, e.g., banded sulfides, sulfides filling tension gashes, and strain shadows occupied by galena and silver sulfosalts. Johansson (2017) also documented the presence of sulfides and silver sulfosalts in the stratigraphic hanging wall of Rävliden North, which were tentatively interpreted to have formed via remobilization of sulfides from the main ore lenses at depth. This, along with an abundance of drill core from ongoing exploration, means that Rävliden North is an excellent deposit to study to better understand the structural and metamorphic overprint of Skellefte district VMS deposits.

In this study, we provide a detailed mineralogical and textural characterization of the mineralization styles at Rävliden North with emphasis on the spatial distribution and relative timing of the deformation of the mineralized ore lenses. We also discuss possible remobilization mechanisms as well as constrain the deformation history of the different mineralization types. In addition, we discuss possible implications for the exploration of similar deposits in the Skellefte district and globally. This study will serve as a basis for future work on the understanding of the performance of the different mineralization types in a mineral processing circuit.

Brief history of discovery

The Rävliden area has presented both exploration and mining challenges due to its structural complexity and the lack of a clear geophysical contrast between mineralization and host rocks using conventional electromagnetic methods. Despite being explored for almost a century, initially by the Geological Survey of Sweden (SGU) and then by Boliden, geologic interpretations of the poorly cropped out Kristineberg area (Figs. 1, 2), including Rävliden, have primarily relied on drill core logging and off hole geophysical anomalies. The discovery of Zn-Cu mineralization in the Rävliden area in 1921 by the SGU (Lindberg, 1979) led to the future discovery and operation of the Rävlidmyran mine (north of Rävliden North) from 1950 to 1991. Despite the closure of the mine, mineralization areas to the south remained untapped (Jansson and Persson, unpub. report). In 2007, Boliden identified the Rävliden area as a key mineralized target for exploration, initially drilling the horizon below the conductive graphite- and pyrrhotite-rich metasedimentary rocks of the Vargfors group. Offhole anomalies from downhole electromagnetic (DH-EM) surveys were initially used as targets for drilling, but this did not return mineralized intervals, and the offhole anomalies were attributed to the conductive graphite- and pyrrhotite-rich metasedimentary rocks.

Around the same time, a new structural model for the Skellefte-Vargfors groups contact was developed, which led to a geologic model that recognized reactivated synvolcanic faults as primary targets. This was later supplemented by new seismic investigations (Dehghannejad et al., 2010; Bauer, 2013) and further refined in 3D by Skyttä et al. (2013). Combining this information with offhole anomalies, an intersection of tremolite marble, subeconomic disseminated pyrite, pyrrhotite, sphalerite, and chalcopyrite, associated with strong sericite and chlorite alteration, gave the first true indication of mineralization at Rävliden North. In 2010, an intersection containing 37 m of massive polymetallic mineralization, including 19.1 m grading 4% Zn, 0.87% Cu, 32 g/t Ag, and 0.46 g/t Au and 16.85 m grading 6.2% Zn, 1.23% Cu, 96 g/t Ag, and 0.42 g/t Au, was drilled. This discovery highlights the importance of ongoing exploration efforts in the metal-endowed Skellefte district, as the structural complexity and blind nature of deposits continue to pose significant challenges. Studies that provide better understanding of the metamorphic and deformation history of deposits, such as the present study in Rävliden North, are thus important in continued exploration success in the Skellefte district.

The Skellefte district belongs to the Bothnia-Skellefteå lithotectonic unit of the 2.0 to 1.8 Ga Svecokarelian orogen (Fig. 1) in the Fennoscandian shield (Skyttä et al., 2020; Stephens, 2020). The extent of the district is loosely defined by the distribution of Paleoproterozoic metavolcanic and siliciclastic metasedimentary rocks belonging to the Bothnian, Skellefte, Vargfors, and Arvidsjaur groups (Fig. 2), which have undergone regional metamorphism from greenschist to amphibolite facies grade (Skyttä et al., 2020). Metasedimentary rocks of the Bothnian supergroup have been interpreted to stratigraphically underlie the Skellefte group, at least in some parts of the district (Skyttä et al., 2020). The Vidsel-Röjnoret shear zone limits the surface distribution of the Vargfors and Skellefte groups to the east (Fig. 2; Skyttä et al., 2013, 2020). Farther east, there are more poorly constrained metasedimentary rocks belonging to the Bothnian supergroup.

Allen et al. (1996) defined the 1.89 to 1.88 Ga Skellefte group as a mix of juvenile volcanic rocks with predominant rhyolitic to dacitic compositions, with relatively minor amounts of andesite and basalt. Rock types include porphyritic intrusions, lavas, and intercalated sedimentary rocks, with the latter commonly being represented by gray to black mudstone, volcaniclastic rocks, sandstone, and breccia-conglomerate. Deposition of the volcanic rocks occurred as subaqueous porphyritic cryptodomes, lavas, and volcaniclastic rocks (Allen et al., 1996; Kathol and Weihed, 2005). The Skellefte group was deposited in a calc-alkaline continental arc that underwent extension (Vivallo and Claesson, 1987; Allen et al., 1996). The 1.88 to 1.87 Ga Vargfors group, which overlies the Skellefte group, consists of fine- to coarse-grained siliciclastic rocks, locally interbedded with volcanic rocks (Allen et al., 1996). Turbiditic black mudstones, graywackes, argillites, volcanogenic monomictic conglomerates, and polymictic conglomerates make up the sedimentary rocks of the Vargfors group. Volcanic rocks are typically Mg-rich, pyroxene- or hornblende-rich porphyritic basaltic lavas and subordinate rhyolite, dacite, and andesite (Kathol and Weihed, 2005). Mercier-Langevin et al. (2013) presented thermal ionization mass spectrometry (TIMS) U-Pb zircon ages of 1893.9 +2.0/−1.9 and 1890.8 ± 1 Ma for volcanic rocks at the Skellefte group and the Vargfors group in the Boliden deposit, respectively (Fig. 2). These ages overlap with ages of Skellefte group volcanic rocks elsewhere in the district, highlighting the diachronous nature of the Skellefte group-Vargfors group contact. Facies analyses on sedimentary and volcanic rocks by Allen et al. (1996) indicate variable depth of deposition (below wave base to deep water) over short distances (tens of km), which indicates changes in the environment during the deposition of the Skellefte and Vargfors groups. The short-distance variations of the water depth have been attributed to the development of crustal compartments during differential extension and subsidence of the Skellefte arc (Allen et al., 1996; Bauer et al., 2011; Skyttä et al., 2012). North of the Skellefte district, metavolcanic rocks belonging to the 1.88 to 1.86 Ga Arvidsjaur group overlie the Skellefte group. The Arvidsjaur group is considered largely coeval to the Vargfors group (Kathol and Weihed, 2005). Intrusive rocks span a range from early orogenic 1.89 to 1.88 Ga GI Jörn metagranitoids (Wilson et al., 1987; Bejgarn et al., 2013), including the Viterliden intrusion (Skyttä et al., 2011), to late, postorogenic 1.82 to 1.78 Ga Revsund-type and Skellefte-Härnö suite granitoids (Fig. 2; Kathol and Weihed, 2005).

The earliest known deformation episode (D1) in the Skellefte district is constrained to c. 1.88 Ga by Skyttä et al. (2020) based on ages by Lundström et al. (1997) (1877 ± 2 Ma) and Rutland et al. (2001) (1874 ± 3 Ma). The D1 deformation event is considered syntectonic to the Vargfors basin opening and attributed to northeast to southwest transpression or northwest to southeast transtension (Bauer et al., 2011; Skyttä et al., 2012). According to Skyttä et al. (2012), D1 divided the crust into an upper, unmetamorphosed domain (Vargfors and Skellefte group rocks) and a lower, strongly metamorphosed domain (Bothnian groups rocks), developing a compaction foliation (S1 in Allen et al., 1996) in (meta)altered volcanic rocks that is parallel to synextensional WNW-ESE–striking faults. A D2 event was constrained at minimum 1.86 Ga by dating a syntectonic granite dike (1861 ± 3 Ma; Rutland et al., 2001; Skyttä et al., 2020). The style of post-D1 deformation is inferred as coaxial in the upper domain and noncoaxial in the deeper domain (Skyttä et al., 2012). Skyttä et al. (2012) attributed such contrasting structural signatures to two alternative tectonic models: (1) a dynamic prograde south-southeast to north-northwest transpressional tectonic evolution (D2) with strain partitioning between the lower and upper crustal domains, which produced upright folding in the upper domain and penetrative ductile, subhorizontal southeast to northwest flow in the deeper domain or (2) alternatively, two separate events with a 1.87 Ga event producing upright folds in the shallower domain, and a 1.86 Ga event responsible for the noncoaxial deformation at the deeper domain (Skyttä et al., 2012). The latest known deformation event in the Skellefte district, D3, produced east-west shortening along north-south major shear zones (Fig. 2; Bergman Weihed, 2001). Bergman Weihed (2001) reported local deformation of the 1.82 to 1.78 Ga granitoids of the Revsund suite, suggesting a 1.80 Ga age for the D3 event. Weihed et al. (2002) presented titanite ages of 1.80 Ga for the Vidsel-Röjnoret N-S–trending shear zone, south of the Skellefte district. The 1.86 Ga D2 event and the 1.80 Ga D3 event in the Skellefte district correlate with the 1.88 to 1.86 Ga D1 and 1.80 Ga D2 deformation events in Norrbotten, respectively (Bauer et al., 2022; Andersson et al., 2022).

Most VMS deposits in the Skellefte district are hosted in the stratigraphic upper part of, or within, the Skellefte group (e.g., Kristineberg deposit) as well as in the basal part of the Vargfors group (e.g., Holmtjärn; Allen et al., 1996). Allen et al. (1996) suggested that this stratigraphic interval corresponds to periods of intense arc extension and volcanism where the mineralization is associated with the evolution of individual volcanoes or volcanic complexes. Although the mineralizations are associated with below-storm-wave-base to deep volcanosedimentary facies associations, the mineralizations occurred mainly as subseafloor replacement of volcaniclastic and sedimentary rocks (Allen et al., 1996). The biggest deposits in the Skellefte district are associated with high-strain zones. These deformation zones are interpreted as transfer faults that functioned as conduits for hydrothermal fluids during the formation of massive sulfide mineralization (Allen et al., 1996; Bauer et al., 2014). On the contrary, smaller deposits that are not associated with regional high-strain zones could have formed by replacement of water-saturated porous volcaniclastic deposits in the vicinity of faults (Bauer et al., 2014).

Bauer et al. (2011, 2014) and Skyttä et al. (2012) found that the geometry and attitudes of the VMS deposits in the Skellefte district directly mirror the subdivision of the crust. Deposits deformed at shallow structural levels exhibit parallel orientations to the main foliation, while massive sulfide deposits deformed at deeper structural levels exhibit plunges similar to mineral lineations (Bauer et al., 2014). The strong association with the Skellefte group volcanism and the shared history of deformation between the massive sulfides and their host rocks supports a synvolcanic origin for the VMS deposits in the Skellefte district.

The Deppis-Näsliden shear zone (Fig. 2) separates the Kristineberg area from the rest of the Skellefte district. Two E-W–trending anticlines in the Kristineberg area host the Kristineberg, Rävlidmyran, Rävliden, and Rävliden North mineralizations (Fig. 3). The fold axes plunge 15° to 20° toward the west and are separated by faults (Årebäck et al., 2005; Skyttä et al., 2013).

In the Kristineberg area, the Skellefte group is dominated by felsic metavolcanic rocks with minor mafic metavolcanic and metasedimentary rocks (Hannington et al., 2003). Rock types include (meta)quartz-feldspar porphyry, rhyolite, and felsic volcaniclastic rocks (Hannington et al., 2003; Årebäck et al., 2005). The Vargfors group comprises intercalations of metamorphosed sedimentary rocks and minor metavolcanic rocks. The former are (meta)breccia, conglomerate, sandstone, siltstone, and mudstone (now graphitic phyllite). The metavolcanic rocks are (meta)dacite, rhyolite, and andesite. The Skellefte group rocks are intruded by granitoids of the Viterliden intrusion and Revsund-type granitoids (Årebäck et al., 2005). The Viterliden intrusion is considered coeval to the Skellefte group volcanic rocks with emplacement at c. 1.89 Ga (U-Th-Pb zircon secondary ion mass spectrometry data; Skyttä et al., 2011).

The Kristineberg deposit comprises massive sulfide mineralization, predominantly pyrite spatially associated with sphalerite and chalcopyrite in Skellefte group volcanic rocks. The ore lenses are typically transposed into subvertical orientations adjacent to steep faults. The host rocks are chlorite-quartz and muscovite-quartz schist, interpreted by Barrett et al. (2005) as the metamorphic products of hydrothermally altered volcanic rocks with predominant rhyolitic and dacitic compositions.

The Rävliden North mineralization is hosted at the transition between the Skellefte and Vargfors groups (Fig. 4A), in an F2 anticline plunging 7° to 14° toward the west (Fig. 4B). The mineralizations in the Rävliden horizon are typically rich in Zn and Pb with dominant amounts of pyrite, hosted in calc-silicate rocks and schist with tremolite, chlorite, dolomite, and quartz (Hannington et al., 2003; Chmielowski et al., 2016). Chmielowski et al. (2016) argued that the current mineralogy of host rocks reflects amphibolite facies regional metamorphism of chlorite- and carbonate-altered rhyolitic to dacitic precursors. The exploration team at Boliden divides the ore lenses and mineralization types at Rävliden North into Cu- and Zn-rich types (Bjänndal, 2022). Johansson (2017) described the occurrence of silver-bearing sulfides and sulfosalts in the stratigraphic hanging wall, hosted in sulfide-cemented breccia, quartz veins, and calcite veins.

Skyttä et al. (2013) describes three overall types of foliations in the Kristineberg area. A compaction-related foliation, SC, subparallel to S0 bedding, (termed S0/C in our study); a main foliation, S2, parallel to E-W–trending F2-axial planes; and a crenulation S2L foliation with a dominant east to west trend with fanning patterns around the Kristineberg and Kimheden (northeast of Kristineberg) deposits. Comparable foliations have been identified in the Rävliden North deposit (Fig. 4B) presenting similar attitudes. Both Skyttä et al. (2013) and Allen et al. (1996) interpret the SC foliation as the result of compaction of altered volcanic rocks during the 1.88 Ga D1 extensional phase of the Svecokarelian orogeny. In addition, Skyttä et al. (2013) only identified the SC foliation in the Skellefte group (meta)volcanic rocks. Coaxial deformation during the 1.86 Ga D2 event formed S2, which is interpreted by Skyttä et al. (2013) as the main foliation within the Vargfors group rocks. Identification of S2 and SC relationships were only possible in intertectonic chlorite porphyroblasts with SC foliation trails and in the hinges of rootless early folds in the altered metavolcanic rocks around the Kristineberg deposit (Skyttä et al., 2013). The late S2L foliation affects both Vargfors and Skellefte group rocks and is attributed to noncoaxial deformation during the D2 event.

During the D2 event, south-southeast to north-northwest transpression along high-strain zones produced the complex geometry of the anticlines in the Kristineberg area (Skyttä et al., 2013). This overprinted and established the current geometry of the VMS deposits with increased strike-slip shearing and subhorizontal flow at deeper crustal levels. The regional deformation events at 1.88 to 1.86 Ga (D1 and D2; Skyttä et al., 2013) led to both internal remobilization (i.e., within the main mineralization zone) and, to a certain extent, external remobilization (i.e., outside the main mineralization zone) of sulfides at the Kristineberg deposit (Årebäck et al., 2005). However, most of the geometry of the massive sulfides in the Kristineberg deposit (and the Skellefte district) exhibits a parallelism between their long axes and the surrounding structural pattern (Skyttä et al., 2012; Bauer et al., 2014). Skyttä et al. (2013) attributed this to original (syngenetic) shape and subsequent deformation rather than remobilization.

Drill cores were selected from profiles that were constructed by Boliden geologists prior to the start of this project. Data collected from drill core observations included host-rock lithology, structures, modal mineralogy, mineral associations, and alteration mineralogy. Sampling focused on the main economic mineralizations defined by Boliden (Bjänndal, 2022) and occurrences of sulfides in the stratigraphic hanging wall. Fifty-nine samples were selected from 2,461 m of logged core for polished thin section preparation. Sampled locations and meters logged are shown in Figure 5A-B. Lists of samples and meters logged are provided in Appendix Tables A1 and A2.

Microscopy was performed using a Nikon ECLIPSE E600 POL and a ZEISS Axioscope 7 with a motorized stage. Sulfide modal abundances (vol %) were based on visual estimations using modal percentage charts. Some samples were etched with NaClO to enhance textures in sulfides, such as twins that would otherwise not been recognizable. A drop of NaClO was added, and after about 1 minute the surface was rinsed with tap water. The microscale characterization was complemented with scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) at Luleå University of Technology. This was done in two SEM labs equipped with a high-resolution (~0.8 nm) Zeiss Merlin SEM (Merlin lab) and ZEISS Gemini Sigma 300 VP (QanTMin lab). Analyses were carried out with an accelerating voltage of 20 KeV and an emission current of 1.0 nA. Additional photomicrographs can be found in Appendix Figures A1 through A3.

Vein structural measurements were performed using a methodology described by Holcombe et al. (2017). The measurements were conducted using an oriented-core printable wraparound protractor marked with beta and alpha angles, with the bottom of the core serving as the baseline for all measurements. The data set was imported to Leapfrog for transformation to dip and dip direction and then to ioGAS for data visualization and calculation of best fit great circles.

Detailed mineralogy of the Rävliden North deposit

Based on hand-specimen mineralogy and lithological associations, four major mineralization types can be distinguished in Rävliden North:

  1. Sphalerite + pyrrhotite + galena ± pyrite (Sp + Pyh + Gn ± Py) mineralization;

  2. Chalcopyrite + pyrrhotite + pyrite (Ccp + Pyh + Py) mineralization;

  3. Pyrrhotite + pyrite ± arsenopyrite (Pyh + Py ± Apy) mineralization in the stratigraphic hanging wall; and

  4. Sphalerite + galena ± sulfosalts (Sp + Gn ± Ss) mineralization in the stratigraphic hanging wall.

Table 1 summarizes the main characteristics of these mineralization types, where the first two types constitute the main mineralizations and the last two represent barren and subeconomic mineralization.

The mineralization types are hosted in volcanic rocks that underwent alteration, deformation, and metamorphism. The Sp + Pyh + Gn ± Py and Ccp + Pyh + Py mineralizations are commonly hosted in three main host rock types: (1) calc-silicate rock (i.e., tremolite- and calcite-bearing marble), (2) graphitic phyllite, and (3) chlorite-quartz schist (Fig. 6; Table 1).

Main mineralization types: In the Sp + Pyh + Gn ± Py mineralization (Figs. 7A-D, 8A-C), brown to reddish sphalerite (~5–35 vol %) occurs in cm- to m-scale intervals (~5–8 m average thickness; Fig. 7A, C), associated with pyrrhotite (~2–20 vol %) and galena (~1–15 vol %), and locally pyrite (~1–20 vol %; Table 1). Sulfides account for up to 70 to 80 vol % to nonsulfide gangue. When sulfides are below ~30 vol %, the mineralization is termed “semimassive” (Fig. 7A), and when it is above ~30 vol %, it is labeled massive (Fig. 7C).

At the micro-scale, sphalerite is commonly anhedral, equigranular, and twinned, with approximately 120° intergranular boundaries (Fig. 7B; App. Fig. A1). Subhedral pyrrhotite grains are intergrown with sphalerite, locally outlining sphalerite grain boundaries (Fig. 7D). Galena and sulfosalts occur as boudin neck vein infill with sphalerite, and pyrite is intergrown with pyrrhotite and sphalerite. Locally, pyrite exhibits overgrowths delimited by inclusions of galena, sulfosalts, sphalerite, and nonsulfide minerals. Sulfosalts include freibergite ((Ag,Cu,Fe)12(Sb,As)4S13), boulangerite (Pb5Sb4S11), hessite (Ag2Te), nisbite (NiSb2), dyscrasite (Ag3Sb), gudmundite (FeSbS), and bismuthinite (Bi2S3; App. Fig. A1). Rare (<0.1 vol %) minerals such as diaphorite (Pb2Ag3Sb3S8), zoubekite (AgPb4Sb4S10), native bismuth, and altaite (PbTe) were also found locally (App. Figs. A1-A2). In foliated samples (Fig. 7A; App. Fig. A1), the foliation planes are composed of chlorite, sericite, tremolite, graphite, muscovite, and minor talc. Quartz, anorthite, and calcite are major nonsulfide constituents.

The Ccp + Pyh + Py mineralization (Fig. 7E-H) is located structurally below the Sp + Pyh + Gn ± Py mineralization (Figs. 4, 6). Chalcopyrite and pyrrhotite occur in similar modal concentrations: approx. 5 to 20 vol %, with pyrite in the range approx. 1 to 15 vol %. Sphalerite (0.5–5 vol %) associated with chalcopyrite (Fig. 7F) occurs in close proximity to Sp + Pyh + Gn ± Py mineralization, and the transition between both mineralization types is locally gradational. The sulfides are hosted in tension gashes (Fig. 7E), straight veinlets, and locally in strain shadows of quartz and anorthite porphyroblasts, which are locally deformed to porphyroclasts (Fig. 7G, H). Chalcopyrite exhibits exsolution lamellae of cubanite and is commonly in intergrowth with sphalerite forming “chalcopyrite disease” (Fig. 7F); minor (<1 vol %) arsenopyrite occurs as euhedral to subhedral porphyroblasts. The Ccp + Pyh + Py mineralization has a lower concentration of sulfosalts, as evidenced by the reduced amount of galena (< 1 vol %). Ag-Hg amalgam (e.g., eugenite, Ag9Hg2; App. Fig. A2) was found only in this mineralization type.

Hanging-wall mineralization: Subordinate mineralizations hosted in the stratigraphic hanging wall of the Rävliden North deposit do not show evident alteration. The most common mineral association is Pyh + Py ± Apy (Figs. 8D-F, 9A-B), hosted by graphitic phyllite and metasiltstone of the Vargfors group rocks (Fig. 6). Pyrrhotite, pyrite, and arsenopyrite modal abundances are in the range of approx. 5 to 20, 5 to 15, and 1 to 5 vol %, respectively (Table 1). Pyrite and pyrrhotite commonly occur in veins (Fig. 8D), locally as massive pyrrhotite (Fig. 8E), and disseminated pyrite mineralization (Fig. 8F).

Occurrences of sphalerite, galena, and minor sulfosalts in modal concentrations below 10 vol % (Table 1) in the hanging wall are grouped as Sp + Gn ± Ss mineralization (Fig. 9C-F). This mineralization type occurs exclusively in breccia (Fig. 9C-D) and veinlets (Figs. 9E-F, 10AE), commonly proximal to deformation zones (Figs. 4A, 6). Sp + Gn ± Ss have a similar mineralogy to Sp + Pyh + Gn ± Py, with approx. 5 to 10 vol % more silver sulfosalt concentration (Table 1). This mineralization can be further subdivided depending on the association to pyrite (Sp + Gn ± Ss + Py; Fig. 9C-D), quartz (Sp + Gn ± Ss + Qz; Fig. 9E-F), and zeolite/calcite (Sp + Gn ± Ss + Zeo[Cal]; Fig. 10A-E). The sulfide-cemented breccia in the Sp + Gn ± Ss + Py mineralization exhibits a matrix composed of sulfides and sulfosalts (Fig. 9D) and clasts composed of cm-sized subangular to subrounded graphitic phyllite or metavolcanic sandstone (Fig. 9C). In the Sp + Gn ± Ss + Qz mineralization, sulfosalts are hosted by galena (Fig. 9F) and are spatially associated with sphalerite.

In the Sp + Gn ± Ss + Zeo(Cal) mineralization, zeolite-cemented breccia and veinlets occur (Fig. 10A-B). The breccia matrix is composed of laumontite, heulandite, wairakite, and apophyllite (Fig. 10C). Clasts are composed of an earlier sulfide-cemented breccia and graphitic phyllite (Fig. 10B). Unlike the main mineralization types, sphalerite in the Sp + Gn ± Ss + Zeo(Cal) mineralization exhibits growth zonation defined by variable color (Fig. 10B, F), caused by variable iron content as confirmed by SEM-EDS analyses. Veinlets crosscut folded host rocks (Fig. 10E). Ullmannite (NiSbS), pyrargyrite (Ag3SbS3), pyrostilpnite (Ag3SbS3), argentopyrite (AgFe2S3), sternbergite (AgFe2S3), stephanite (Ag5SbS4), cobaltite (CoAsS), and breithauptite (NiSb) were observed in Sp + Gn ± Ss + Cal veinlets (App. Fig. A2). Etching of sphalerite reveals intergranular boundaries at roughly 120° with local grain boundary migration (App. Fig. A2).

Structural relationships within the mineralizations of the Rävliden North deposit

The mineralization at Rävliden North exhibits structural and textural features similar to the host rock, with sulfide and gangue minerals defining the same penetrative and spaced foliations as are observed regionally in the Kristineberg area. Sulfide minerals also occur in syndeformational features that crosscut these foliations, such as veinlets and tension gashes. In this section, we will relate these different structural relationships to the polyphase deformation history described by Skyttä et al. (2013) using crosscutting relationships.

An example of sulfide-bearing cleavage domains belonging to the penetrative (S0/C), penetrative to spaced (S2), and, locally, late spaced crenulation foliation (S2L) in the Sp + Pyh + Gn ± Py mineralization is presented in Figure 11A. The foliation domains are defined by chlorite and muscovite, with sphalerite, galena, pyrrhotite, and minor chalcopyrite. Figure 11B and C depict the SC foliation crenulated by S2 and further deformed by S2L foliation. The S2L foliation in Figure 11A is pseudoparallel to the SC orientation and shows a dextral sense of shear on S2. Muscovite aggregates locally overgrow S2 (Fig. 11B) and are then deformed by S2L, forming an S-C structure (Fig. 11C).

In order to differentiate between S0/C, S2, and S2L, all three foliations must be observed simultaneously (Fig. 11A). In areas near the limbs of the Rävliden North anticline and where structural data is missing, composite foliations with parallel SC and S2 are expected due to the locally heavily transposed nature of the stratigraphy in the mineralized zone. While crenulation foliations are classified as S2L by Skyttä et al. (2013), subsequent folding of S0/C during the D2 event means that S2 can also occur as a crenulation foliation.

Pre- to syn-D1mineralization: The complex tectonic overprint at Rävliden North makes identifying relict mineralization textures related to pre- to syn-D1 events challenging in hand specimens and locally ambiguous. However, examples in the Pyh + Py ± Apy mineralization are parallel pyrite and pyrrhotite following a penetrative graphite-defined foliation, potentially representing SC (Figs. 8F, 12A). At the microscale, flattened and S0/C-aligned pyrite nodules in Figure 9B illustrate mineralization associated with the D1 event, which formed during diagenesis and was later deformed during tectonic overprinting.

Mineralization aligned subparallel to D2-related structures: Mineralization deformed during D2 is demonstrated in the main mineralization types via alignment with S2 and S2L foliations. In Sp + Pyh + Gn ± Py mineralization, sphalerite and pyrrhotite are subparallel to a foliation defined by tremolite (Fig. 8A; S2), graphite (Fig. 12B; S2), or chlorite (Fig. 7A; S2 and Fig. 11; S2L). Figure 12B illustrates the presence of Sp + Pyh + Gn ± Py mineralization, where sphalerite and pyrrhotite exhibit a spaced foliation parallel to a penetrative foliation defined by graphite. The sample could be in a fold limb or a D2-related shear zone with subparallel foliation orientations. In addition, a slight crenulation of the foliations could be related to the S2L foliation (blue lines in Fig. 12B). In Ccp + Pyh + Py mineralization, chalcopyrite and pyrrhotite crosscut S0/C and S2/2L foliation (see next section for details), in strain shadows (Fig. 7G-H) or as boudin neck vein infill (Fig. 12C) subparallel to S2 foliation defined by chlorite.

In Pyh + Py ± Apy mineralization, pyrite and pyrrhotite are oriented subparallel to spaced S2 foliation defined by graphite (Fig. 8F) and chlorite in graphitic phyllite and siltstone. In the proximity of shear zones or within S2 foliation, pyrite grains exhibit micaceous inclusions delimiting curved foliation subparallel to the S2 foliation outside the grain (Fig. 12D). Moreover, these syn-S2 pyrite porphyroblasts exhibit growth regions with quartz fringes aligned subparallel to the stretching direction (App. Fig. A1). Post-S2 pyrite porphyroblasts overgrow folded SC foliation trails delimited by micaceous minerals (Fig. 12E).

S2- and S2L-parallel boudinage, characterized by sulfide-rich (commonly sphalerite, pyrrhotite, chalcopyrite) and silicate-rich (commonly chlorite, sericite) layers enveloping boudins of more competent host rock (commonly tremolite-, quartz-rich) and pyrite, can be seen locally (Figs. 7A, 11A, 12C). The interboudin zone displays vein infill and host inflows (i.e., scar folds), with mineralogy of the infill varying based on the mineralization type. Galena-rich infill is common in Sp + Pyh + Gn ± Py mineralization, chalcopyrite-pyrrhotite-rich in Ccp + Pyh + Py, and pyrrhotite-rich in Pyh + Py ± Apy. Following the boudinage classification proposed by Goscombe et al. (2004), these examples can be classified as symmetric tapering boudins (Fig. 7A) and necked boudins (i.e., pinch and swell; Fig. 12C).

Durchbewegt ore: Marshall and Gilligan (1989) coined “durchbewegt ore” in the specific case of deformation by strain partitioning of rigid material and ductile massive sulfide groundmass that experienced vorticity in noncoaxial shear zone environments. The deformation process is called durchbewegung, and the resulting texture, durchbewegt ore. The clasts may be angular to rounded, with random orientations due to clast rotation, composed of one or more rock types, or single crystals of silicate composition, and ore minerals such as pyrite. The groundmass must be composed of one or more sulfides (Marshall and Gilligan, 1989).

Durchbewegt ore was identified in two different mineralizations within D2-related deposit-scale shear zones: Sp + Pyh + Gn ± Py and Ccp + Pyh + Py. The main difference between the two mineralizations is in the matrix composition, with sphalerite being dominant in Sp + Pyh + Gn ± Py (Fig. 13A-B) and chalcopyrite and pyrrhotite in Ccp + Pyh + Py (Fig. 13C, D). In the Sp + Pyh + Gn ± Py mineralization, etching of sphalerite reveals subrounded boundaries and twinning (App. Fig. A3). In some cases, bulging between sphalerite grains is observed. Galena, pyrrhotite, and minor chalcopyrite are present in low-pressure sites and strain shadows around quartz, tremolite, and anorthite porphyroclasts (Figs. 7G, 13B). In the Ccp + Pyh + Py mineralization, pyrite occurs as porphyroclasts (Fig. 13D). Etching reveals bulging recrystallization of the chalcopyrite-pyrrhotite matrix (Fig. 13E) and deformation twins (Fig. 13F; App. Fig. A3). In pyrite-rich durchbewegt ore, cataclastically deformed pyrite porphyroblasts are intergrown with chalcopyrite and pyrrhotite (App. Fig. A3).

Subrounded, cm-size clasts of nonsulfide gangue include chlorite quartz schist, graphitic phyllite, and tremolite marble. Quartz, anorthite, tremolite, and minor zoisite exhibit corroded and irregular boundaries (Fig. 13B). Some chlorite schist clasts exhibit S2 foliation (Fig. 13A), indicating a post-D2 and possibly D3-related timing for the latest formation of durchbewegt ore, which will be discussed further in the discussion section.

Tension gashes and piercement veins: The Sp + Pyh + Gn ± Py and Ccp + Pyh + Py mineralizations locally contain tension gashes (Figs. 7E, 8A, 14A-E) and piercement veins (Fig. 7A; App. Fig. A2). Piercement veins consist of a sulfide-rich, vein-shaped body that transgresses foliation at high angles (Marshall and Gilligan, 1989). The mineralogy varies based on the type of mineralization and host rock. Galena with sulfosalts is predominant in the Sp + Pyh + Gn ± Py mineralization (Figs. 7A, 8A, 14A-B), and chalcopyrite and pyrrhotite are predominant in the Ccp + Pyh + Py mineralization (Figs. 7E, 14C-D). The presence of calcite and quartz depends on the host-rock mineralogy. Tension gashes crosscut foliation planes and occur as vein infill in interboudin zones. En echelon arrangements with pseudosigmoidal shapes are common (Figs. 7E, 14A). Acicular tremolite aggregates overgrowing a foliation (seemingly penetrative SC/2) defined by chlorite are locally crosscut by tension gashes (Fig. 7E). Piercement veins are commonly found at the interface between sulfide-rich and silicate-rich layers in the Sp + Pyh + Gn ± Py, Ccp + Pyh + Py, and Pyh + Py ± Apy mineralizations. The strike of tension gashes and piercement veins in Rävliden North show a general east-to-west orientation (Fig. 14E), which is subparallel to the east-to-west orientation of the axial plane of the Rävliden North anticline (Fig. 4B). Figure 14E shows additional strikes of sulfide veins in the north-northwest to south-southeast and south-southwest to north-northeast orientations, which can form either due to D2-related F2 folding or as post-D2, i.e., D3-related vein formation (see discussion for details).

The Sp + Pyh + Gn ± Py mineralization contains tension gashes with galena intergrown with sulfosalts minerals (Fig. 14B; App. Fig. A2), tellurides, and native bismuth. Backscattered electron imaging also reveals “graphic”-like textures between galena and boulangerite (Fig. 14B), as well as “blebs” of eugenite within tension gashes and piercement veins (App. Fig. A2). Etching reveals anhedral equigranular shapes with local subgrains in chalcopyrite and pyrrhotite in the Ccp + Pyh + Py mineralization. Chalcopyrite also exhibits growth twinning (App. Fig. A2), whereas pyrrhotite deformed polysynthetic twins showing lenticular shape with chevron-like appearance (Fig. 14D).

Post-D2mineralization: Mineralization hosted by brittle structures was observed in the main mineralization styles and the hanging wall of Rävliden North, occurring as breccia and veinlets.

In both Sp + Pyh + Gn ± Py and Ccp + Pyh + Py mineralizations, subangular breccia clasts (Fig. 8C) are composed of S2-foliated host rock (quartz-chlorite schist, calc-silicate rock). The matrix is sphalerite-rich in the former and chalcopyrite-pyrrhotite–rich in the latter. In the hanging wall, the sulfide-cemented breccia in the Sp + Gn ± Ss + Py mineralization (Fig. 9C) has subangular clasts of S2-foliated graphitic phyllite. Etching of sphalerite revealed subrounded, twinned grains with bulging recrystallization. Galena fills spaces between sphalerite grains following a subparallel trend.

Sulfide-bearing veinlets in Rävliden North are found in all mineralization types (Fig. 8D, 9E-F, 10D-F, 14C) and are straight without apparent relationship to the formation of boudinage or tension gashes. They crosscut both Sp + Pyh + Gn ± Py and Ccp + Pyh + Py mineralization (Fig. 14C), with mineralogy reflecting the crosscut mineralization type. In the Ccp + Pyh + Py mineralization, veinlets locally crosscut pyrite, anorthite and tremolite porphyroblasts, and quartz porphyroclasts in durchbewegt ore. The veinlets also host galena, sulfosalts, Ag-Hg amalgams, hessite, and native bismuth (App. Fig. A2).

In the hanging wall, veinlets in the Pyh + Py ± Apy (Fig. 8D), Sp + Gn ± Ss + Qz, and Sp + Gn ± Ss + Zeo(Cal) mineralizations (Fig. 9E, 10D-F) occur locally in intervals discordantly crosscutting folded foliation at a high angle adjacent to shear zones (Fig. 10E). Sp + Gn ± Ss + Zeo veinlets have sharp to irregular contacts with the host rock (Fig. 10D).

Synthesis of metamorphism, deformation, and remobilization

Via the integration of drill core and microscale observations, we identified several lines of evidence for polyphase syntectonic remobilization of sulfides at Rävliden North, starting during the 1.88 to 1.86 Ga D1 and D2 ductile deformation events and continuing during late brittle events.

Ductile deformation events: During the D1 phase of coaxial deformation as described by Skyttä et al. (2013), sphalerite and pyrrhotite acted incompetently and were transposed parallel to a penetrative SC foliation (Fig. 15A). The presence of sulfides oriented parallel to the S2 foliation, commonly penetrative and parallel to SC (SC/2), along the limb zones of the Rävliden North anticline, shows evidence of ductile deformation and internal solid-state remobilization during the D2 event. In the hinge zone, the S2 foliation forms a spaced crenulation of SC (Fig. 15B). Locally, sulfides are oriented parallel to S2L, which commonly crenulates both S2 and SC. Although some examples of mineralization were deformed during the D1 event, the D2 event was more influential, rendering the identification of unequivocal examples of the early-deformed mineralization types challenging. Nevertheless, deformed pyrite nodules found in Pyh + Py ± Apy mineralization represent textures that likely formed during diagenesis and survived deformation due to the high competency of pyrite during deformation (Craig and Vokes, 1993; Lafrance et al., 2020).

Acicular aggregates of tremolite crystals overgrow SC/2 penetrative foliation and SC/2-parallel sulfides (Fig. 15B). The unoriented nature suggests growth occurred between the formation of the S2 foliation and a later deformation phase that caused boudinage of the tremolite aggregates. This means that tremolite growth outlasted the main phase of D2 ductile deformation and likely represents the metamorphic peak (Bucher and Grapes, 2011b) at Rävliden North. The regional metamorphic peak in the central and southern parts of the Skellefte district reached amphibolite facies (600°C and 200 to 250 Mpa; Kathol and Weihed, 2005) and is constrained as post-D2 deformation (Weihed et al., 2002; Skyttä et al., 2012) due to crustal thickening by stacking of the crust during D2 (Skyttä et al., 2012). In the western Skellefte district, post-D1 andalusite and post-D2 cordierite porphyroblasts in the Kristineberg deposit suggest a post-D2 metamorphic peak (Årebäck et al., 2005).

The relationship between the acicular tremolite aggregates and S2L foliation in Rävliden North is unknown. Nevertheless, the dominant east to west orientation of crenulation foliations at Rävliden North (Fig. 4B) could be related to the late-D2 event as described by Skyttä et al. (2013). A D3-related origin for the S2L foliation in Rävliden North is possible as it is post-S2 and composed of graphite, chlorite, and sericite, i.e., a retrograde metamorphic greenschist facies association. Furthermore, the discontinuous, locally brittle S2L crenulation foliation in Rävliden North is consistent with descriptions by Bergman Weihed (2001) of D3-related crenulation foliation (S3) in the central Skellefte district.

Ductile-brittle deformation events: The competency contrast between sulfides and host rocks is expressed via boudinage parallel to S2 and S2L (Fig. 15B) and the formation of durchbewegt ore (Fig. 15B). The latter is associated with deposit-scale synvolcanic faults that underwent inversion as shear zones during the D2 deformation event (Skyttä et al., 2013). Clasts with S2 foliation (Fig. 13A), however, suggest that the shear zones were reactivated, possibly during the D3 event (Bergman Weihed, 2001). This suggests that the synvolcanic faults that formed during the rifting of the Skellefte arc were reactivated at least once during the D2 basin inversion event, and again during the D3 event. During both events, durchbewegt, and potentially tension gashes, and piercement veins formed.

The interpretation of a tectonic origin for durchbewegt ore has been noted in many VMS deposits, among others those in the Bathurst and Flin Flon camps in Canada (van Staal and Williams, 1984; Park, 1996; de Roo and van Staal, 2003; Jonasson et al., 2009); the ABM deposit, Canada (Denisová et al., 2023); the Copperton deposit, South Africa (Bailie and Gutzmer, 2011); the Ashele VMS deposit, China (Zheng et al., 2016); Cerro Maimon, Dominican Republic (Torró et al., 2016); the Iberian Pyrite Belt and northwest Iberia (Castroviejo et al., 2011); and the Urals (Vikentyev et al., 2017). However, it is worth noting that synvolcanic transported breccias, with clasts from collapsing domes and block faulting, may be mistakenly identified as durchbewegt ore when deformed (Lafrance et al., 2020). Similarly, subseafloor replacement processes that create host-rock windows can also lead to misinterpretations of durchbewegt ore postdeformation. Lafrance et al. (2020) emphasizes the possibility of durchbewegt ore being mistakenly assigned a solely tectonic origin, when dual origins are a possibility. In Rävliden North, the close spatial association of durchbewegt ore to shear zones warrants a tectonic origin, as no association to primary breccias has been observed.

The occurrence of tension gashes, piercement veins, and boudin neck vein infill in the main mineralization types (Fig. 15D) provides compelling evidence for ductile-brittle deformation and internal remobilization of sulfides. Boudinage vein infill, tension gashes, and piercement veins are dilatational sites formed by rheology contrast (Passchier and Trouw, 2005; Lafrance et al., 2020) between incompetent (e.g., galena, chalcopyrite, pyrrhotite) and competent material (e.g., calc-silicate rocks). These structures show various stages of deformation from ductile (boudinage) and brittle-ductile (tension gashes) to brittle (piercement veins). In particular, the mineral composition of tension gashes is highly reflective of the hosting mineralization (Ciobanu et al., 2006, 2011), with galena and associated sulfosalts commonly occurring in Sp + Pyh + Gn ± Py mineralization, and chalcopyrite, pyrrhotite, calcite, minor galena, and sphalerite being prevalent in Ccp + Pyh + Py mineralization. This clear correlation between the mineralogy of the hosting mineralization and the mineral composition of the tension gashes strongly supports that remobilization occurred within the bounds of preexisting mineralization (Marshall and Gilligan, 1987; Marshall et al., 1998; Lafrance et al., 2020). Additionally, the fact that these structures cut SC foliation, acicular tremolite aggregates, and S2 foliation planes indicates that they likely formed post-D1 and are associated with either the late D2 event as in Skyttä et al. (2013) or the D3 event as described by Bergman Weihed (2001).

It is possible that the approximately E-W–oriented D3 compressional event, as described in Bergman Weihed (2001), induced strike-slip movement in the axial-parallel WNW-ESE–striking shear zones in the Rävliden North anticline. The D3 event, under brittle-ductile conditions, could have produced internal remobilization of sulfides via ductile and fluid-assisted deformation mechanisms, with tension gashes and piercement veins forming bounded by mineralization oriented subparallel to the F2 axial plane of Rävliden North. This is consistent with the overall west-northwest to east-southeast orientation, as well as the north-northwest to south-southeast and south-southwest to north-northeast for veins in Rävliden North (Fig. 14C).

Metamorphosed VMS deposits commonly display mineralization parallel to fold hinges and stretching lineations (Marshall and Gilligan, 1993; Bauer et al., 2014; Lafrance et al., 2020). Despite remobilization at Rävliden North, we note that the deposit occupies a similar stratigraphic setting as most deposits in the district, and a zonation from lower Cu-rich stringer-type mineralization to upper, Zn-rich massive mineralization. Such zonation is a common feature in unmetamorphosed VMS deposits, such as the TAG hydrothermal mound, Mid-Atlantic Ridge, 26°N, as documented by Petersen et al. (2000). The process responsible for well-zoned VMS deposits is zone refining (cf. Knuckey et al., 1982; Eldridge et al., 1983; Petersen et al., 2000). Zonation is preserved through greenschist and lower amphibolite facies metamorphism and deformation, aiding in the reconstruction of deformed ore lenses (Lafrance et al., 2020). Examples, among others, include the ABM VMS deposit, Canada, which preserved primary textures as well as zone-refining related chemical zonation (Denisová et al., 2023), and the Copperton deposit, South Africa, which kept mineral zonation with Cu enriched in the center of the ore-body and Zn + Pb in the fringes and top of the orebody (Bailie and Gutzmer, 2011). In the Urals, Metal zoning was preserved in most VMS deposits regardless of metamorphic grade (Vikentyev et al., 2017). Zheng et al. (2016) explain that deformation and metamorphism at greenschist facies in the Ashele VMS deposit, China, produced foliated ore, durchbewegt ore, and late postdeformation veins. Despite extensive remobilization, primary zonation was preserved (Zheng et al., 2016). In Rävliden North, the metal zoning through zone refining was largely preserved. This suggests that most of the remobilization during the D1, D2, and D3 events was internal, within the limits of the primary element zonation in the deposit.

Brittle deformation events: The presence of sulfide and quartz breccias, and micro- to mesoscale veinlets in the main mineralization lenses (Fig. 15D), indicates post-D2, possibly D3, brittle deformation. The chalcopyrite veinlets in Figure 14F show a slip displacement of S2 foliation in calc-silicate rocks and reveal a sinistral movement towards the center. The veinlets are oblique to the foliation and are consistent with slip movement and subsequent chalcopyrite (Ccp) veinlet formation during the D3 regional shortening (Bergman Weihed, 2001). In the hanging wall, sulfide-cemented breccias and sulfide-bearing quartz veins crosscutting ductile deformed rocks are evidence for several stages of brittle deformation. The sulfide-cemented breccias could have resulted from reactivation of the approximately E-W–oriented shear zones subparallel to the axial plane of the Rävliden North anticline during the D3 event as described by Bergman Weihed (2001). While most of the brittle deformation affecting rocks with S2 and S2L foliation or folded S0 is likely related to the post-D2, possibly D3 event, local brittle deformation regimes could explain the sulfide-cemented breccias in the main mineralized lenses (Fig. 8C).

The late phase of mineralization at Rävliden North is characterized by the presence of calcite- and zeolite-hosted sulfide veins and zeolite-cemented breccias in the hanging wall (Sp + Gn ± Ss + Zeo(Cal) mineralization). Zeolites are late, low-temperature minerals (approx. 50°–300°C), and the structures that host them are likely the latest phases of significant brittle deformation. The zeolite minerals belong to the zeolite facies metamorphism, being stable at temperatures below 200°C and pressure below 250 Mpa at a geothermal gradient of 30°C/km–1 (Bucher and Grapes, 2011a).

Drake et al. (2009) reported multiple generations of fractures filled with low-temperature minerals in the Fennoscandian shield. These fractures may have formed during the waning stages of the Svecokarelian orogeny (>1.75 Ga), or as far-field effects of the Danopolonian orogeny (c. 1.47–1.44 Ga), the Sveconorwegian orogeny (c. 1.1–0.9 Ga), or the Caledonian orogeny (c. 0.5–0.4 Ga) (Drake et al., 2009). Zeolites and calcite were only identified in fractures from the Danopolonian, Sveconorwegian, and Caledonian orogenies (Drake et al., 2009).

Billström et al. (2012) described Zn-Pb mineralization associated with calcite and fluorite veinlets and breccias in the Lycksele-Storuman ore district, west of Rävliden North. They suggest that mineralization could have occurred during the opening of the Iapetus ocean (0.7–0.5 Ga; Robert et al., 2021), based on a tentative sphalerite Rb-Sr isochron age of 534 ± 13 Ma, which probably dates the mineralization. Given the proximity of the Skellefte district to the Caledonian orogen (Fig. 1) and the similarity of mineralogy and mineralization style to the Lycksele-Storuman deposits, the calcite and zeolite veins and breccias at Rävliden North could have originated due to far-field effects from the Caledonian orogenic cycle—perhaps associated with the same Cambrian event responsible for the Lycksele-Storuman Zn-Pb deposits.

Pyrite porphyroblasts were also observed in sulfide-cemented breccias in the hanging wall. The presence of pyrite porphyroclasts and porphyroblasts in different deformation phases opens the possibility for more accurate timing and constraining of the deformation by using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) methods. However, this is outside the scope of the current contribution.

The origin of sulfosalts, tellurides, amalgams, and antimonides

Silver sulfosalts and tellurides associated with galena in tension gashes, piercement veins, and pressure shadows in the Sp + Pyh + Gn ± Py and Ccp + Pyh + Py mineralizations may have formed synmetamorphically. Minerals of Ag, Bi, and Sb can form via exsolution from α-galena (Chutas et al., 2008) during the retrograde phase of metamorphism, or directly via hydrothermal processes as observed in modern sea-floor hydrothermal systems and less-deformed VMS deposits (Hannington et al., 1986, 2005; Hannington, 2014; Revan et al., 2014; Günay et al., 2019). Another possibility is an origin related to partial melting of low-melting-point chalcophile elements (LMCEs) as reported by previous studies (Ciobanu et al., 2006; Denisová et al., 2023).

Ciobanu et al. (2006) proposed remobilization of bismuth melts in the Soimus Ilii vein Cu prospect and the Highis Cu-Au occurrence, Romania. Ciobanu et al. (2006) documented evidence of the infiltration of metamorphogenic hydrothermal fluids at upper greenschist facies peak temperature of 400°C. The fluids remobilized and precipitated Bi minerals associated with quartz, chlorite, and biotite, which infill dilational sites including gashes, cleavage planes, pores in chalcopyrite, and lamellae in chlorite. The Bi minerals exhibited blebs/patch morphology with curvilinear and cuspate low-angle mutual boundaries as well as symplectitic textures, interpreted as evidence of rapidly cooled or quenched crystallization from a melt (Ciobanu et al., 2006). Even so, Ciobanu et al. (2006) argues that similar textures could result from exsolution. Similarly, Denisová et al. (2023) related graphic textures between galena, sulfosalts, tellurides, and antimonides to partial melting of sulfides. Denisová et al. (2023) argued, in line with Tomkins et al. (2007), that elevated Bi, Hg, Sb, and/or As along with coexistence of multiple minerals (galena, chalcopyrite, arsenopyrite, sulfosalts) can lead to sulfide anatexis. Such partial anatexis would have occurred, in the ABM VMS deposit, Canada, on a small scale (clusters <1 mm in size), with symplectitic intergrowths accounting for less than 1 vol % (Denisová et al., 2023).

The graphic-like textures formed by galena and boulangerite in Rävliden North (Fig. 14B) appear unrelated to metamorphogenic fluids, as no associated silicates are present in the same dilational site. Nevertheless, tension gashes, piercement veins, and strain shadows containing sulfides, sulfosalts, and silicates (or calcite) may have experienced fluid-mediated remobilization. Moreover, the texture in Figure 14B does not exhibit a quenched-symplectitic texture as the ones shown in Denisová et al. (2023) and Ciobanu et al. (2006). Therefore, in Rävliden North, it is not entirely clear whether sulfide anatexis did occur. However, if it occurred, the scale seems to have been very local (<1 mm), in line with Denisová et al. (2023). A quenched-symplectitic-like texture in Rävliden North is found in Appendix Figure A2 between pyrrhotite and eugenite (Ag-Hg amalgam) and is the only example that could be related to partial melting (Tomkins, 2007). Nevertheless, considering multiple hypotheses for the origin of galena-sulfosalts associations, including synvolcanic, fluid-mediated remobilization (including melting of LMCEs) during metamorphism, or exsolution from galena postmetamorphism, is a sound approach, until further evidence that supports one of these possibilities.

Sulfide deformation mechanisms

Foliation: The alignment of sulfide minerals in a subparallel orientation to the penetrative SC and S2 foliations could have been attained by intergranular and intragranular mechanisms. A combination of dislocation flow and dislocation glide (Marshall et al., 1998; Lafrance et al., 2020) would explain such alignment during the formation of SC and S2 foliations producing dynamic recrystallization (Fig. 15A). Aside from pyrite, all common sulfides (sphalerite, galena, pyrrhotite, and chalcopyrite) would have experienced these processes since the foliation is formed by mica, graphite, and sulfides that can accommodate strain because they largely behave in a ductile manner, whereas pyrite behaves in a cataclastic manner (Marshall et al., 1998; Lafrance et al., 2020). Pyrite ductile deformation starts at temperatures of up to 500°C (Cox et al., 1981). In semimassive mineralization the interaction between host rocks and mineralization (i.e., polymineralic rocks) may allow solution transfer processes (Marshall et al., 1998). Therefore, grain boundary sliding cannot be ruled out as a possibility for sphalerite to be translocated parallel to SC and S2 foliations.

The formation of spaced S2L crenulation foliation is controlled by rocks that can accommodate strain, such as mica-rich rocks. Passchier and Trouw (2005) suggest that pressure solution and solution transfer processes influence the formation of spaced secondary foliations in non-sulfide–bearing rocks. However, sulfides such as sphalerite, pyrrhotite, chalcopyrite, and galena exhibit ductile behavior and high strain rates, limiting the influence of these processes on their alignment parallel to S2L (Marshall and Gilligan, 1987). Nonetheless, fluid-assisted remobilization may play a large role in sulfides as recognized by Marshall et al. (1998).

Etched sphalerite reveals twinned grains in S2 domains. The shape is stepped rather than tapered, suggesting that the twinning is not related to deformation (Passchier and Trouw, 2005). The absence of grain shape preferred orientation may indicate sphalerite was deformed, then recrystallized post-metamorphic peak, erasing evidence of intragranular deformation (observable through light and SEM microscopy; Fig. 7B). This process may have occurred after the metamorphic peak, due to synkinematic and static recrystallization (Fig. 15B; Marshall and Gilligan, 1987; Passchier and Trouw, 2005).

Durchbewegt ore: The durchbewegt ore in the Sp + Pyh + Gn ± Py and Ccp + Pyh + Py mineralizations in Rävliden North exhibits twinned and subrounded, equigranular grains of sphalerite, chalcopyrite, and pyrrhotite in the massive sulfide matrix. Deformation twins in pyrrhotite and chalcopyrite suggest that the mechanism of deformation for the durchbewegt ore may have been a combination of cataclastic flow (clasts and pyrite) and dislocation flow (chalcopyrite, pyrrhotite, sphalerite, galena; Marshall et al., 1998). These mechanisms produce dynamic recrystallization such as bulging, subgrain rotation, and grain boundary migration (Fig. 15E). Galena, pyrrhotite, and minor chalcopyrite filling strain shadows around host-rock clasts may have formed via a combination of dislocation flow and pressure solution (Marshall et al., 1998). The formation of durchbewegt ore is largely due to ductile solid-state remobilization processes. Nevertheless, de Roo and van Staal (2003) suggested solution mass transfer as an assisting process after observing dilational jogs along major shear zones defined by sulfide mylonites at the Heath Steele and Brunswick mines, Canada. Veins and infilling of strain shadows are common textures in durchbewegung structures that suggest solution mass transfer (Lafrance et al., 2020).

Piercement veins and tension gashes: Previous studies (Maiden et al., 1986; Tourigny et al., 1993; Marshall et al., 1998; Lafrance et al., 2020) suggest mixed fluid-assisted and solid-state remobilization as the origin of piercement veins, which form from extensional delamination of host rocks infilled with sulfides. The presence of calcite or quartz in veins—as occur locally at Rävliden North—may suggest hydrofracturing of the host rock with translocation of sulfides via dislocation flow and fluid-assisted diffusion (Maiden et al., 1986; Marshall et al., 1998; Lafrance et al., 2020).

Quartz- and calcite-bearing tension gashes may be filled by minerals from the local surroundings and fluid from regional metamorphism, which cause minerals to precipitate in dilatational sites through pressure solution and diffusion during progressive noncoaxial deformation (Passchier and Trouw, 2005). Little research exists on sulfide-filled tension gashes (e.g., Andersson et al., 2016). However, the translocation mechanism likely involves dislocation flow, fluid-induced dislocation flow, and fluid-assisted diffusion. This may explain tension gashes in the Sp + Pyh + Gn ± Py and Ccp + Pyh + Py mineralizations, where galena, chalcopyrite, and pyrrhotite are translocated from surrounding SC-, S2-, and S2L-parallel mineralization. The presence of calcite associated with sulfides can be accounted for by dissolution and precipitation of calcite in the open spaces before sulfide translocation via fluid-induced dislocation flow or fluid-assisted diffusion. The role of fluid-assisted processes seems to be more dominant in the Sp + Gn ± Ss mineralization in the hanging wall than in the main mineralized lenses. Sulfide remobilization in quartz veins (Sp + Gn ± Ss + Qz mineralization) may have occurred through fluid-induced dislocation flow (Marshall et al., 1998).

The content of pyrite relative to more incompetent common sulfides determines the rheology of the VMS deposit during greenschist facies metamorphism (Lafrance et al., 2020), and the resulting sulfide texture/structure. Pyritic ore tends to preserve primary features such as framboids, nodules, and chimney fragments (Lafrance et al., 2020). Examples include framboidal pyrite in the Barika VMS deposit, Iran (Tajeddin et al., 2019); Cerro Maimon, Dominican Republic (Torró et al., 2016); ABM deposit, Canada (Denisová et al., 2023); and Dry Creek, United States (Dusel-Bacon et al., 2012), among others. Orebody morphology is also influenced by the pyrite content, which produces stacking or transposition (Lafrance et al., 2020). De Roo and van Staal (2003) explain that strain partitioning between pyrite-rich ore and pyrite-poor ore produced tectonic layering (i.e., stacking) in the Heath Steele and Brunswick deposits, Bathurst Mining Camp, Canada. In the Iberian Pyrite Belt, stacking produced thicker orebodies, whereas in deposits in northwest Iberia, transposition produced flattened, thinner, and elongated orebodies (Castroviejo et al., 2011). Tectonic stacking can be beneficial since thicker orebodies can be anticipated.

In Rävliden North, transposition was dominant; mineralized lenses are flattened, elongated, and follow the fold axis of the anticline that hosts the mineralization. This is a result of lower pyrite content relative to more ductile sulfides. Comparing with other deposits worldwide, the Whalesback VMS deposit, Canada, shows elongated and flattened ore lenses as well as sulfides in foliation, durchbewegt ore, piercement veins as a result of ductile deformation (dislocation flow), and, locally, dissolution, solution transport, and precipitation (Cloutier et al., 2015). Torró et al. (2016) documented that, at the Cerro Maimon VMS deposit, Dominican Republic, transposition of mineralization occurred due to strain partitioning that produced cataclastic deformation in pyrite and ductile deformation in sphalerite, galena, and chalcopyrite. The dominant remobilization mechanism seems to have been solid-state mechanical transfer as in Marshall and Gilligan (1987) and Marshall et al. (1998). Post-metamorphic–peak remobilization infilled veins with galena, tellurides, sulfosalts, arsenopyrite, and electrum, the latter remobilized from pyrite crystal lattice (Torró et al., 2016). Vikentyev et al. (2017) argues that ductile deformation/mechanical remobilization with combination of fluid-assisted flow strain partitioning forms boudinage, durchbewegt, and foliated/banded mineralization in VMS deposits in the Urals, producing flattened, elongated, and ribbon-like orebodies. The Barika VMS deposit, Iran, shows polyphase deformation with sulfide strain partitioning that produced boudinage, foliation, strain shadows, and folding (Tajeddin et al., 2019). Tajeddin et al. (2019) invoked chemical internal remobilization of refractory gold in pyrite through solution-precipitation creep (solution transfer; Cox, 1987) to visible gold and electrum phases associated with sulfosalt minerals in late veins developed during the shear brittle-ductile phase of deformation of the Barika deposit.

Constraints for P-T conditions during deformation

The textural evidence in Rävliden North can provide insight on the pressure and temperature conditions in which the sulfides were remobilized during the 1.88 to 1.86 Ga D1 and D2 deformation events of the Svecokarelian orogeny. The weaker nature of sphalerite, pyrrhotite, and chalcopyrite relative to pyrite and silicates (e.g., quartz, amphibole, feldspar) caused them to become aligned subparallel to the tectonic foliations. The dominant cataclastic deformation in pyrite and the presence of low-temperature sulfosalts associated with galena (Chutas et al., 2008) suggest a sulfide deformation temperature range of 200° to 450°C at Rävliden North.

The temperature and pressure determinations from upper greenschist facies metavolcanic rocks in the Kristineberg area are approximately 550°C and 200 to 250 Mpa, respectively (Kathol and Weihed, 2005). Meanwhile, temperature and pressure estimates from amphibolite facies metavolcanic rocks are approximately 600°C and 200 to 250 Mpa (Kathol and Weihed, 2005). The transformation of chlorite and tremolite to hornblende at 550°C marks the shift from upper greenschist to lower amphibolite facies metamorphism in the Skellefte district, as indicated by Kathol and Weihed (2005). Tremolite overgrowing the penetrative S2 foliation at Rävliden North suggests the possibility of a metamorphic peak at a temperature of 550°C (Bucher and Grapes, 2011b) after D2. At these conditions, sulfide rocks and typical host rocks with quartz or carbonate are ductile and weak (Etheridge et al., 1983; Marshall et al., 1998), with pyrite possibly being ductile (Marshall and Gilligan, 1987). The lack of ductile-deformed pyrite in the deposit can be explained by the largely and dominantly brittle behavior (Graf and Skinner, 1970) and high competence of pyrite during prograde metamorphism (Craig and Vokes, 1993).

At temperatures above 500°C, plastic deformation in pyrite occurs via dislocation flow (Cox et al., 1981). Barrie et al. (2009) indicate that ductile deformation in pyrite grains can occur at temperatures as low as ~200°C, and at geologic strain rates (10–12 to 10–16 s−1) at temperatures as low as ~260°C (Barrie et al., 2011). This contrasting observation may reflect differences in strain rates (Lafrance et al., 2020). Barrie et al. (2011) caution against using their studies as a guide, as the flow laws used to model deformation mechanisms (e.g., dislocation glide, creep, diffusion creep) carry inherent errors, and extrapolation from experimental to geologic strain rates is necessary. Other factors such as grain size distribution, modal abundance, recrystallization processes, stress differences, strain heterogeneity, and matrix composition also influence pyrite deformation. Until further evidence (e.g., via electron backscatter diffraction) is obtained, further speculation regarding potential ductile behavior in pyrite during the deformation of Rävliden North is deferred.

During nonretrogressive cooling, pyrite porphyroblasts could have grown by sulfidation of pyrrhotite (Marshall and Gilligan, 1987) in a temperature range of 300° to 600°C (Craig and Vokes, 1993). At 350°C, annealing recrystallization would have given rise to local 120°C junctions in sphalerite without erasing previous textures or preferred distributions attained during prograde metamorphism (Marshall and Gilligan, 1987). Macro- and mesoscale remobilization textures such as veins, piercements, and rectangular boudinage can form due to the brittle/ductile contrast of silicate/massive sulfide (respectively) assemblages (Gilligan and Marshall, 1987; Marshall and Gilligan, 1987; Lafrance et al., 2020). Marshall and Gilligan (1987) proposed that the main reason is the low H2O pressure, inhibiting water-assisted ductile deformation processes in silicate-bearing rocks, whereas sulfide-rich rocks remain ductile. During retrogressive cooling at temperatures ranging from 650° to 350°C, sulfides and carbonate-silicate or phyllosilicate rocks would be ductile, enabling similar deformation mechanisms as in prograde metamorphism. Most of the macro- and mesoscale remobilization, with similar textures/structures as in prograde metamorphism, would occur within the 500° to 350°C range. This suggests that D2 did not immediately follow D1, since pre-D2 structures are preserved.

The post-Svecokarelian mineralization event formed sulfide- and sulfosalt-bearing zeolite breccia and veins as well as calcite veins in the hanging wall, likely at temperatures below 300°C. In the zeolite-bearing mineralization, laumontite and wairakite are stable at temperatures below 260°C, with lower stability fields for both minerals at 180° and 220°C, respectively (Bucher and Grapes, 2011b). Apophyllite, which is also found in the zeolite-bearing mineralization, is commonly stable at temperatures above 68°C, replacing laumontite (Weisenberger et al., 2012). Temperatures below 200°C can be inferred by the Ag minerals occurring in druses in the calcite veins. Pyrargyrite transforms to pyrostilpnite at temperatures below 192 ± 5°C (Keighin and Honea, 1969). Argentite and pyrargyrite form from stephanite, in the absence of sulfur, at temperatures below 197 ± 5°C (Keighin and Honea, 1969). Finally, sternbergite and argentopyrite are stable at temperatures below 152°C (Taylor, 1970).

The wider context of results

This study has important implications for the wider understanding of deformation and metamorphism of VMS deposits and their exploration. The study confirms that metamorphism can lead to favorable changes in mineral deposits, in this case with relative enrichment and remobilization of silver minerals to shallower depth that allows for vectoring to deeper mineralization. Even though grain size coarsening has been documented in VMS deposits like Kristineberg (Årebäck et al., 2005), Boliden (Wagner et al., 2007), Rosebery, and Mount Lyell (Huston et al., 1992), among others, in Rävliden North it occurs locally.

Polyphase deformation can substantially modify massive sulfide deposits, leading to complex paragenetic and crosscutting relationships. These can in turn lead to erroneous conclusions on the timing of mineralizing events if the full structural history of the deposit is not considered (Marshall and Spry, 1998). We emphasize careful attention to unequivocal crosscutting relationships that can be contextualized using regional deformation events, concurring with the ideas of Marshall and Gilligan (1987, 1993), Marshall and Spry (1998), and Lafrance et al. (2020), who discuss common ambiguities in the formation of sulfide-rich structures in metamorphosed sulfide deposits.

In Rävliden North, clear crosscutting relationships are, for example, veins crosscutting acicular aggregates of amphibole that formed at the metamorphic peak and veins crosscutting tectonic foliations (SC, S2, S2L) that can be correlated to regional deformation events. Hence, despite extensive evidence of remobilization, the macroscale, pretectonic mineral zonation and its spatial relation to premetamorphic hydrothermal alteration was not significantly modified at Rävliden North. Indeed, the present-day mineral zonation is similar to classic VMS deposits, with Zn + Pb-rich massive sulfide lenses located stratigraphically and structurally above Cu-rich stringer-type mineralization (Hannington, 2014). This suggests that, whereas the relative proportion and structural disposition of the mineral zonation was substantially modified, the gross deposit-scale premetamorphic relationships were largely preserved. Thus, most of the mineralization at Rävliden North can be explained by a single, synvolcanic mineralization event, followed by remobilization. An exception is the late vein and breccias of the Sp + Gn ± Ss + Zeo(Cal) mineralization, which may reflect truly crosscutting, juxtaposed, epigenetic mineralization. This can be recognized by integrating paragenetic and structural data. Future studies should aim to further test this model through various methods such as isotopic analysis and radiometric dating.

The Rävliden North deposit shows excellent examples of sulfide deformation, which can be contextualized in the D1, D2, and D3 deformation events of the Svecokarelian orogen in northern Sweden. Structural evidence shows that deformation caused mainly internal sulfide remobilization within the bounds of the main economic mineralization and only local occurrences of external remobilization.

Sulfides are aligned subparallel to tectonic structures that can be correlated with the main regional tectonic features of the Kristineberg area. In addition, features such as boudinage, durchbewegt ore, sulfide-filled tension gashes, and piercement veins are indicative of a transition from ductile (via brittle-ductile) to brittle conditions during the D2 and D3 deformation events. Late brittle D3-related occurrences are documented in the form of sulfide-cemented breccia and veinlets.

The range of temperatures that seemingly acted during the D1 event were below 300°C; at this temperature, sulfides (except pyrite) may have locally arranged subparallel to a penetrative compaction-related foliation. In conjunction with the D2 event, peak metamorphic temperatures likely reached 550°C, at which most sulfides become ductile, hence their predisposition in ductile structures such as foliation, boudinage, and durchbewegt ore.

The main sulfide deformation mechanism acting in the deposit is inferred to have been dislocation flow, acting in the temperature range from 300° to 600°C. Evidence for fluid-assisted dislocation flow is provided by apical quartz and calcite in association with sulfides in tension gashes as well as sulfides hosted in strain shadows.

Late veinlets in the stratigraphic hanging wall of the deposit contain a mineralogy and style of mineralization comparable to Zn-Pb mineralization in the nearby Lycksele-Storuman area west of Rävliden North. These deposits have been dated at 534 ± 13 Ma by Billström et al. (2012), who interpreted them to have formed during the opening of the Iapetus Ocean. We propose that a similar, if not the same, mineralizing event is responsible for the late sphalerite-bearing veins associated with zeolites in the hanging wall of Rävliden North. However, the Ag and Sb occurring in the veins at the Rävliden North deposit is seemingly lacking in the veins studied by Billström et al. (2012) and may constitute a local contribution via remobilization from the adjacent VMS deposit at Rävliden North.

This project is jointly funded by Boliden and the Geological Survey of Sweden (SGU RnD grant 36-2031/2018: Textural and chemical characterization of sulfide minerals for improved beneficiation and exploration, Skellefte district, Sweden). Boliden is thanked for providing access to their facilities and drill core, and the staff at the core archive are thanked for help in getting the right core palettes, even on short notice. Tobias Bauer at Luleå University of Technology (LTU) is thanked for continuous discussion about the structural geology of the Skellefte district and the Kristineberg area and help with interpretations of macro- and microstructures. Joel Andersson (LTU) and Leslie Logan (LTU) are acknowledged for structural geology discussions that helped during the interpretation of textures and structures. Lena Albrecht (Boliden) is thanked for guidance on structural measurements in core and discussions related to the structural geology of Rävliden North. We extend our gratitude to Stefan Andersson (SGU) and Yousef Ghorbani (University of Lincoln) for their valuable contribution in reviewing a presubmission version of this article. ChatGPT v. 3.0 was used to refine the grammar of the submitted version of the manuscript, and its use is elaborated in the Appendix. Yvonne DeWolfe and Andrew Martin are thanked for their thorough reviews, which significantly elevated the quality of the manuscript. We thank Lawrence Meinert for the editorial handling.

Jonathan Rincon is currently a Ph.D. candidate in ore geology at the Luleå University of Technology. His research focuses on the mineralogy, structural, and geochemical features of the Rävliden North VMS deposit and its behavior in a mineral processing circuit. He has a B.S. in geological engineering from the National University of Colombia and earned a triple M.S. in georesources engineering from the Universities of Liege, Lorraine, and Luleå (Emerald program). Previously, he worked as an engineering geologist in Colombia and later as a research engineer in Belgium, collaborating on geometallurgy projects with Bulgaria and Germany.

Gold Open Access: This paper is published under the terms of the CC-BY license.

Supplementary data