To guide future exploration, this predominantly field based study has investigated the structural evolution of the central Kiruna area, the type locality for iron oxide-apatite deposits that stands for a significant amount of the European iron ore production. Using a combination of geologic mapping focusing on structures and stratigraphy, petrography with focus on microstructures, X-ray computed tomography imaging of sulfide-structure relationships, and structural 2D-forward modeling, a structural framework is provided including spatial-temporal relationships between iron oxide-apatite emplacement, subeconomic Fe and Cu sulfide mineralization, and deformation. These relationships are important to constrain as a guidance for exploration in iron oxide-apatite and iron oxide copper-gold prospective terrains and may help to understand the genesis of these deposit types. Results suggest that the iron oxide-apatite deposits were emplaced in an intracontinental back-arc basin, and they formed precrustal shortening under shallow crustal conditions. Subsequent east-west crustal shortening under greenschist facies metamorphism inverted the basin along steep to moderately steep E-dipping structures, often subparallel with bedding and lithological contacts, with reverse, oblique to dip-slip, east-block-up sense of shears. Fe and Cu sulfides associated with Fe oxides are hosted by structures formed during the basin inversion and are spatially related to the iron oxide-apatite deposits but formed in fundamentally different structural settings and are separated in time. The inverted basin was gently refolded and later affected by hydraulic fracturing, which represent the last recorded deformation-hydrothermal events affecting the crustal architecture of central Kiruna.
The central Kiruna area in northern Norrbotten hosts the largest underground iron mine in the world, the Kiirunavaara deposit. Together with the nearby Leveäniemi and Tapuli open pits in the Svappavaara and Pajala areas, respectively (Fig. 1), and the Malmberget underground mine near Gällivare (Fig. 1), the iron mines stand for the vast majority of the total iron ore production in Europe. Despite the relatively long tradition of mining in northern Norrbotten, the area is considered underexplored, and many fundamental geologic questions remain unanswered.
Several types of iron mineralization exist in the northern Norrbotten region (Frietsch, 1997). However, except for the reopened Tapuli skarn iron deposit (Bergman, 2018) in the Pajala area (Fig. 1), the only iron ore type in production today is iron oxide-apatite (IOA), also called Kiruna-type (Geijer, 1910). This ore type is characterized by the occurrence of apatite together with high Fe grades (dominated by magnetite) and relatively high contents of V and low contents of Ti (e.g., Frietsch, 1970; Parak, 1975). Since the first comprehensive study elaborating on the origin of the Kiruna-type ores (Geijer, 1910), debate on the genesis of IOA deposits has been intense with little consensus on their genesis. The largest controversy among the research community is whether the iron oxides crystallized from a melt or a hydrothermal fluid. Geijer (1910) first suggested that the Kiirunavaara deposit was formed from a magnetite-rich lava but later refined the model to an intrusive magmatic model based on the presence of an ore breccia in the hanging wall of the Kiirunavaara deposit (Geijer, 1919). Later, Parak (1975) proposed an exhalative hydrothermal model including many components similar to our recent understanding of volcanic massive sulfide (VMS) deposits (e.g., Franklin et al., 2005). Much of the argumentation by Parak (1975) relies on observations and chemical data from the Per Geijer iron ores, which form parts of the studied area in this paper. As the concept of hydrothermal or magmatic/hydrothermal iron oxide copper-gold (IOCG) was introduced, Hitzman et al. (1992) classified IOA deposits as a copper-gold–deficient end member under the loosely defined IOCG group of deposits. Recent studies from the Great Bear magmatic zone in Canada indicate that IOA and IOCG deposits represent different metasomatic facies in one single metasomatic system (e.g., Corriveau et al., 2016; Montreuil et al., 2016a, b). Broman et al. (1999) suggested a magmatic-hydrothermal process was responsible for the formation of the El Laco IOA deposit in Chile. Martinsson (2004) speculated that the IOA deposits in Kiruna formed from a process similar to that indicated for El Laco involving the immiscibility of volatile-rich iron oxide melts producing magmatic fluids giving rise to both magmatic and hydrothermal features. Recently, this view has gained support by workers studying Andean examples (e.g., Knipping et al., 2015; Valesco et al., 2016; Tornos et al., 2016), and a direct link between magmatic IOA and hydrothermal IOCG formation has been indicated as possible (Reich et al., 2016).
One rarely studied key parameter is the structural setting and subsequent structural evolution of IOA deposits, which has only been addressed by a few studies from the northern Norrbotten area (Vollmer et al., 1984; Wright, 1988; Bauer et al., 2018). In addition to some reported faults (Parak, 1969), only one detailed structural description of an ore locality has ever been published from the Kiruna area (cf. figs. 4.3, 6.3 in Wright, 1988). Regional- to semiregional-scale structural studies have been conducted in Kiruna or adjacent areas, but these studies provide a somewhat intermittent assessment of the characteristics and timing of the structural development (cf. Vollmer et al., 1984; Wright, 1988; Talbot and Koyi, 1995; Bergman et al., 2001; Grigull et al., 2018; Luth et al., 2018a).
In this predominately field based study, we aim to identify key aspects of the Orosirian (ca. 1.9–1.8 Ga) stratigraphic column in Kiruna in order to understand the geologic and tectonic conditions for the emplacement of IOA deposits. The subsequent tectonic reworking of the area is then described using regional- and deposit-scale key localities. By this approach, we aim to provide an up-to-date documentation and interpretation of the structural evolution of the type locality for IOA deposits.
The overall goal of this study is to establish geometry, relative age, and sense of shear of the larger brittle-ductile structures within the study area. We also aim to investigate the relationship between these structures, ore formation, and subsequent transposition, as well as to reevaluate the preshortening geologic setting responsible for the IOA emplacement.
Neoarchean granitoids and amphibolite rocks form the basement of the Fennoscandian Shield (Gaal and Gorbatschev, 1987; Bergman and Weihed, 2020). In northern Norrbotten (Fig. 1), the basement belongs to the Norrbotten nucleus, suggested to be one of three Neoarchean nuclei dispersed and reassembled during a rifting and collisional-accretionary cycle during the Paleoproterozoic (e.g., Lahtinen et al., 2005). Continental rifting during the Siderian to Orosirian (ca. 2.5–2.0 Ga: Bergman and Weihed, 2020) caused regional-scale rift-parallel fault systems, tholeiitic volcanism, and associated sedimentation generating a large greenstone province stretching from northern Norway to Russia (Pharaoh and Pearce, 1984; Martinsson, 1997; Lahtinen et al., 2005; Melezhik and Hanski, 2012; Hanski et al., 2014; Bingen et al., 2015). In northern Norrbotten, the greenstone belts occur as NNE- and NNW-trending belts (Fig. 1) and are host to a number of metal deposits (Martinsson, 1997; Bergman et al., 2001; Martinsson et al., 2016; Lynch et al., 2018).
During the Paleoproterozoic, the early Svecokarelian cycle (1.90–1.86 Ga) generated two suites of comagmatic plutonic-volcanic rocks (Fig. 2): Haparanda Suite-Porphyrite Group and Perthtite Monzonite Suite-Kiirunavaara Group (Bergman et al., 2001; Martinsson, 2004). Haparanda Suite-Porphyrite Group rocks predominate to the east and comprise calc-alkaline intermediate to felsic volcanic to volcaniclastic rocks and related dioritic to granodioritic intrusions. To the west, shoshonitic mafic to felsic Kiirunavaara Group volcanic and volcaniclastic rocks related to Perthite Monzonite Suite gabbroic, monzonitic, and granitic intrusions predominate (Bergman et al., 2001; Martinsson, 2004). Late Svecokarelian (1.81–1.78 Ga) magmatism comprises I- to A-type plutonic rocks (e.g., Edefors Suite; Fig. 2), that form part of the Transcandiavian igneous belt, which stretches from northwestern Norway to southern Sweden (Andersson, 1991; Åhäll and Larson, 2000; Weihed et al., 2002; Högdahl et al., 2004; Rutanen and Andersson, 2009), as well as associated S-type granite (Lina Suite; Fig. 2).
Mineralizing events during the Orosirian coincide with the early and late cycles of the Svecokarelian orogeny (e.g., Billström et al., 2010). For example, IOA and porphyry-style Cu-Au-Ag deposits (PCDs) were formed in the time interval from 1.89 to 1.87 Ga (Fig. 2; cf. Romer et al., 1994; Wanhainen et al., 2009; Martinsson et al., 2016; Westhues et al., 2016). IOCG deposits also formed during the early Svecokarelian, but the timing of the mineralization events is not as tightly constrained, with ages around 1.86 Ga (Fig. 2) or slightly younger (e.g., Smith et al., 2007; Martinsson et al., 2016). In contrast, ore deposits that formed during the late Svecokarelian cycle are primarily restricted to structurally controlled IOCG deposits with ages at ca. 1.80 to 1.78 Ga (e.g., Fig. 2; Edfelt, 2007; Billström et al., 2010; Martinsson et al., 2016). The only IOA deposits linked to the late Svecokarelian cycle in northern Norrbotten are the Tjårrojåkka and Saivo deposits dated at approximately 1780 and 1758 Ma, respectively (Edfelt, 2007; Martinsson et al., 2016). However, these deposits are anomalous in age, and the role of late-cycle IOA formation in Norrbotten remains an unresolved question.
The metamorphic evolution of the northern Norrbotten area is poorly constrained but is considered to be of low to medium pressure-temperature (P-T) Buchan style (Bergman et al., 2001; Tollefsen, 2014; Skelton et al., 2018). Metamorphic key mineral assemblages and limited geothermometry data (Bergman et al., 2001) suggest that the metamorphic grade increases from west to east from greenschist facies to upper amphibolite facies conditions, but age constraints are lacking. However, several studies indicate that regional metamorphic grades can be linked to the early Svecokarelian cycle, whereas low-P high-T conditions predominated during the late cycle. The effect of contact-metamorphic aureoles around early- and late-cycle intrusive rocks on the metamorphic systematics in Norrbotten is unknown but may be a significant contributor to the regional P-T variations (cf. Monro, 1988; Bergman et al., 2001; Tollefsen, 2014; Hellström, 2018; Skelton et al., 2018). In the Aitik Cu-Au-Ag deposit near Gällivare (Fig. 1), Monro (1988) reports amphibolite facies M1 conditions at 520° to 600°C and 3 to 5 kb based on garnet-biotite geothermometry and Si-Al content in hornblende associated with garnet. The M1 metamorphism was subsequently overprinted by a hydrothermal event at ca. 1.78 Ga estimated at 200° to 500°C and 1 to 2 kbar (Wanhainen et al., 2012). At the Malmberget IOA deposit, Bauer et al. (2018) observed a gneissic amphibolite facies S1 fabric folded without the development of an axial plane-parallel S2 cleavage in the resultant F2 synform, thus, interpreted as D2 taking place at higher crustal levels compared to D1 (Bauer et al., 2018). West of Kiruna, Andersson et al. (2020) observed syntectonic growth of porphyroblastic hornblende aligned with an S1 fabric indicating that peak metamorphism occurred during D1 deformation in that area. Caledonian (Fig. 1) overprinting processes do show up in some geochronological data sets as, e.g., lower intercept ages in U-Pb zircon data (Billström et al., 2019, and references therein) but are generally regarded as having a low impact on recorded metamorphic grade, structure, and alteration. U-Pb ages of stilbite from open fractures (D4 in Bauer et al., 2018) in the Malmberget IOA deposit yield 1730 ± 6.4 Ma, indicating that temperatures remained below 150°C from that time and that relatively stable tectonic conditions have remained up until today (Romer, 1996).
The structural subsurface architecture of this part of the Fennoscandian Shield is the result of a polyphase deformation history with deformation events approximately coinciding with the early and late magmatic cycles of the Svecokarelian orogeny (Fig. 2). Prominent NW- to NE-trending crustal-scale deformation zones tend to spatially coincide with Rhyacian-Orosirian metasupracrustal belts that took up the majority of strain in northern Norrbotten (Fig. 1). On regional-scale aeromagnetic maps (Bergman et al., 2001), magnetic lineaments form an approximately N-directed undulating pattern wrapping around intrusive bodies. At least two phases of folding can be recognized (e.g., Wright, 1988; Bergman et al., 2001; Bauer et al., 2018; Grigull et al., 2018; Andersson et al., 2020). The early phase (D1) generated a heterogeneously developed, regional, penetrative continuous fabric. D2 is characterized by strong strain partitioning into deformation zones, whereas outside of these zones, tectonic foliation was only sparsely developed and, in Gällivare and west of Kiruna (Fig. 1), accompanied by brittle components (cf. Bauer et al., 2018; Andersson et al., 2020). Similar deformation systematics have been recorded from the Skellefte district (Bauer et al., 2011; Skyttä et al., 2012; Bauer, 2013) where the minimum age of D1 is constrained by a U-Pb zircon age at 1874 ± 4 Ma (D2 in Skyttä et al., 2012). In northern Norrbotten, Hellström (2018) reports a maximum age at 1878 ± 3 Ma for folding east of Kiruna, which was interpreted to represent the age of migmatization due to early Svecokarelian contact metamorphism. Other estimates of the timing of early deformation are broadly limited to field observations in relationship to geochronological data of early-cycle plutonic rocks (Bergman et al., 2001) and dikes (Cliff et al., 1990) indicating a minimum deformation age at 1.88 Ga; however, contradicting field relationships are present (Luth et al., 2018a). Timing of the late-cycle deformation is generally attributed to the intrusion of the approximately 1.8 Ga syntectonic Lina Suite (Fig. 2; Bergman et al., 2001). In the Gällivare area (Fig. 1), Wanhainen et al. (2005) report Re-Os ages at 1850 and 1765–1750 Ma for deformed and undeformed pegmatite-aplite dikes, respectively, representing maximum and minimum ages for late-cycle deformation, which agrees with field observations in nearby areas (Lynch et al., 2015; Bauer et al., 2018).
The stratigraphic column of the central Kiruna area
Since the discovery of the Kiirunavaara deposit, the Kiruna area has been mapped several times. The first 1:50,000 maps were produced by the Geological Survey of Sweden (SGU) during the mid-1960s (Offerberg, 1967), but mapping campaigns and petrographic studies provided a rather holistic understanding much earlier (e.g., Geijer, 1910; Lundbohm, 1910).
The central Kiruna area (Fig. 3A-C) constitutes the best-preserved, continuous Rhyacian-Orosirian sequence in Norrbotten (Figs. 2, 3A-C). The sequence was emplaced on top of the Neoarchean tonalitic-granodioritic Råstojaure Complex (Skiöld, 1979; Martinsson et al., 1999). The basement rocks cover vast areas north of Kiruna and constitute one of the least known geologic domains in Sweden. The depth to the basement increases toward the south and has been estimated based on reflection seismic investigations (Holmgren, 2013), as well as modeling of gravimetric, magnetic, and petrophysical data (Luth et al., 2018a) to range between 2 and 3.5 km in Kiruna.
In Kiruna, NE- to NNE-trending greenstone belts represent the basal parts of the Paleoproterozoic stratigraphic column. The Rhyacian pile is subdivided into a lower and an upper unit: the predominately sedimentary Kovo Group and the predominately volcanic Kiruna Greenstone Group, respectively (Martinsson, 1997). The latter hosts the syngenetic Viscaria Cu deposit west of Kiruna and the epigenetic Pahtohavare Cu-Au deposit (Figs. 1, 2) in the south (Lindblom et al., 1996; Martinsson, 1997).
The Orosirian stratigraphy in central Kiruna (Fig. 2) was broadly established during the early 20th century and constitutes an E-dipping sequence younging to the east (Lundbohm, 1910). The only fundamental disagreement raised during the subsequent 110 years of geologic research is regarding the position of the basal horizon, the Kurravaara conglomerate, and its relationship to the underlying Rhyacian Kiruna greenstone group (cf. Geijer, 1910; Ödman, 1957; Frietsch, 1979; Forsell, 1987; Wright, 1988; Martinsson et al., 1993; Kumpulainen, 2000). The Kurravaara conglomerate has been interpreted as a molasse that formed in an emergent thrust zone (Wright, 1988; Talbot and Koyi, 1995) or, alternatively, as an alluvial fan deposit (Kumpulainen, 2000). The clasts originate from the underlying Kiruna greenstone group (e.g., Forsell, 1987) and calc-alkaline intermediate-felsic volcanic rocks presumed to derive from the Porphyrite Group (Martinsson and Perdahl, 1993). On top of the Kurravaara conglomerate, the Kiirunavaara Group (Fig. 2) includes trachyandesitic subvolcanic rocks and lavas (Hopukka Formation) followed by rhyodacitic tuffs and subordinate breccia conglomerates (Luossavaara Formation) that constitute the footwall and hanging-wall rocks, respectively, to the Kiirunavaara and Luossavaara IOA deposits (Fig. 3A-C; Martinsson, 2004; Martinsson and Hansson, 2004). A ~5-km-long horizon of IOA mineralization, or sometimes apatite only (Geijer and Ödman, 1974), is found at the contact between the Luossavaara Formation and an overlying rhyolitic ignimbrite unit (Frietsch, 1979). Traditionally, the rhyolitic ignimbrite has been described as the “Rektor porphyry” (e.g., Geijer, 1950) and constitutes the lowest member of the Matojärvi Formation (Martinsson, 2004). The Matojärvi Formation (Figs. 2, 3A-C) marks a change in the geologic development of the area, as the stratigraphy turns into more volcano-sedimentary-sedimentary in character. Explosive bimodal volcanism, rapid erosion, and turbulent deposition gave rise to a volcanic-volcano-sedimentary-sedimentary sequence composed of rhyolitic tuffs and ignimbrites (Geijer, 1950; Frietsch, 1979), which are conformably overlain by basaltic agglomerates, tuffs, and lavas (Martinsson, 2004) and followed by heterolithic breccia conglomerates (Andersson et al., 2017), lithic graywackes, and a phyllitic uppermost horizon (Lundbohm, 1910; Frietsch 1979). The volcanic sequence of the Orosirian Kiirunavaara Group in Kiruna was formed in a short time interval of no more than 15 m.y. (Westhues et al., 2016). The volcanic period was followed by sedimentation of crossbedded arenites interrupted by horizons of sedimentary breccia conglomerates situated at its basal and middle parts (Figs. 2, 3A-C), together referred to as the Hauki quartzite (Martinsson, 2004). The Hauki quartzite can be correlated regionally to similar units (Bergman et al., 2001) and marks the end of the Orosirian in the study area.
The mineralogy and chemistry (e.g., Geijer, 1910; Parak, 1975; Perdahl and Frietsch, 1993; Westhues et al., 2016) and minimum crystallization ages (e.g., Cliff et al., 1990; Romer et al., 1994; Westhues et al., 2016) of the volcanic rocks are well studied in Kiruna, and many attempts have been made to correlate these volcanic rocks to the geologic and tectonic development of northern Norrbotten (e.g., Witschard, 1984; Perdahl and Fritsch, 1993; Martinsson, 2004; Martinsson et al., 2016). However, the development of the metasedimentary rocks (Ödman, 1972; Frietsch, 1979; Wright, 1988; Kumpulainen, 2000; Ladenberger et al., 2017) has not been given the same attention despite the information these rocks provide regarding the geologic evolution in one of the important mining districts in Europe. In the “Orosirian stratigraphy” section, we present field descriptions of the metasedimentary units that will serve as a background when the preshortening geologic history is discussed in the “Basin development” section under “Synthesis of the Orosirian Structural Evolution.” The volcanic rocks will only be briefly described here, as their mineralogical and chemical character has been covered previously (e.g., Geijer, 1910; Parak, 1975; Frietsch, 1979; Westhues et al., 2016).
The supracrustal rocks in Kiruna host a wide range of deposit types. A tuffitic unit of the middle part of the Rhyacian greenstones (Kiruna greenstones; Martinsson, 1997) hosts the syngenetic exhalative Viscaria Cu deposit (Martinsson, 1997; Martinsson et al., 1993; Masurel, 2011). Ten km south of Viscaria, the epigenetic Pahtohavare Cu-Au deposit occurs within the same formation (Viscaria Formation) as Viscaria (Martinsson et al., 1993; Lindblom et al., 1996; Martinsson, 1997). Both Viscaria and Pahtohavare were mined during the 1990s. The IOCG-style Rakkurijärvi iron oxide sulfide breccia mineralization (Smith et al., 2007) is an advanced exploration project located a few kilometers east of Pahtohavaare and hosted by Orosirian metavolcanic rocks. Beyond these deposits, several others are mentioned and/or described by, for example, Grip and Frietsch (1973), Frietsch (1997), Bergman et al. (2001), and Martinsson et al. (2016).
The most famous deposit in Kiruna is the >2,500 million tonne (Mt) Kiirunavaara magnetite-apatite deposit. It constitutes a 5-km-long and up to 100-m-thick, moderately steep (60°–70°), E-dipping tabular body (Grip and Frietsch, 1973). The magnetite-apatite body is situated at the contact between the trachyandesitic Hopukka Formation (footwall) and the rhyodacitic Luossavaara Formation (hanging wall). Both formations are brecciated by the ore (Geijer, 1919). As of 2020, the magnetite-apatite body is mined from a main haulage level at 1,365-m depth, and mineral reserves are reported at 616 Mt at 42% Fe (proved and probable reserves; Luossavaara-Kiirunavaara AB, 2019). To the north along the same contact, the tabular-shaped Luossavaara magnetite-apatite deposit occurs (e.g., Geijer, 1910). The Luossavaara deposit is approximately 700 m long and up to 40 m thick and comprises 20 Mt of ore (Grip and Frietsch, 1973). It was mined in both open pit and underground until mining ceased in 1985 (Hallberg, 2005).
The Per Geijer iron ores comprise five hematite-magnetite-apatite deposits, namely Rektor, Henry, Nukutus, Haukivaara, and Lappmalmen. They form a rather large system of IOA mineralization (Geijer and Ödman, 1974). The orebodies are situated at the contact between the Luossavaara Formation and the Matojärvi Formation or hosted by the Matojärvi Formation (Figs. 2, 3B, C). Lappmalmen is a blind orebody known from drilling and possibly represents a deep-seated continuation of smaller deposits at the surface (Parak, 1969). During the 20th century, 9 Mt were produced in open pits and underground operations at 43.25% Fe from the Per Geijer iron ores (Grip and Frietsch, 1973).
Metamorphism and hydrothermal alteration
In detail, the metamorphic evolution of the Kiruna area is poorly understood and only covered by a few studies (cf. Frietsch et al., 1997; Bergman et al., 2001). No P-T data is currently available from central Kiruna, but key metamorphic minerals (actinolite, chlorite, albite, epidote, hornblende) in mafic rocks indicate greenschist facies or rarer lower amphibolite facies conditions (Bergman et al., 2001). Skiöld and Cliff (1984) present a Sm-Nd isochron age at 1932 ± 45 Ma of amphibole, titanite, and plagioclase in a greenschist mineral association hosted by the underlying greenstones, but the relevance of this age in respect to metamorphism further up in the stratigraphic column in central Kiruna remains unknown. In low-strain blocks, the supracrustal rocks in central Kiruna show a high degree of preservation of primary volcanic and sedimentary features (e.g., Geijer, 1910; Frietsch, 1979; Martinsson, 1997; Kumpulainen, 2000; Bergman et al., 2001). The high level of preservation makes the area particularly useful for geologic interpretations, which can be used as a proxy when similar areas in more high-grade metamorphic terrains are studied further east in Norrbotten.
The Kiruna area is characterized by alkali and calcic-iron alteration spatially and temporally related to IOA and IOCG mineralizations (e.g., Martinsson et al., 2016; Westhues et al., 2016). The stratigraphically lower Kiirunavaara and Luossavaara (magnetite-apatite) deposits are different from the Per Geijer iron ores (hematite-magnetite-apatite) in terms of their associated hydrothermal alteration (e.g., Geijer, 1910; Parak, 1975; Martinsson and Hansson, 2004; Martinsson, 2015; Martinsson et al., 2016; Westhues et al., 2016). The trachyandesitic footwall rocks to the Kiirunavaara and Luossavaara deposits are affected by sodic-calcic alteration in terms of volumetrically dominant, pervasive albite and more localized massive actinolite (Geijer, 1910; Martinsson, 2004; Martinsson and Hansson, 2004; Martinsson et al., 2016). Hydrobreccias dominated by magnetite-actinolite are common features of these deposits, extending tens of meters into the hanging wall (Martinsson and Hansson, 2004). In contrast, the Per Geijer iron ores show a potassic-dominated hydrothermal overprint (Martinsson, 2015; Martinsson et al., 2016; Westhues et al., 2016) represented by K-feldspar, quartz, sericite, calcite-ankerite, chlorite, and minor tourmaline, which is strongest developed in the hanging-wall rocks (Martinsson, 2015). Albite occurs only sparsely, and actinolite has never been reported from the Per Geijer iron ores.
Time constraints on hydrothermal mineral assemblages in Kiruna indicate that at least four temporally separated hydrothermal events can be recognized. Hydrothermal titanite-actinolite-calcite assemblages from the Luossavaara footwall rocks yield a titanite U-Pb age of 1876 ± 9 Ma (Romer et al., 1994) and represent the earliest documented fluid flow through the volcanic pile. This hydrothermal event can be temporally correlated with U-Pb zircon ages at 1875 ± 5 and 1876 ± 7 Ma from altered hanging-wall rocks at Kiirunavaara and Rektor, respectively (Westhues et al., 2016). Furthermore, it is possible that also the 1854 ± 18 U-Pb allanite age (Smith et al., 2009) and the 1859 ± 2 Ma rutile age (Martinsson et al., 2016) at the Rakkurijärvi Cu-Au (IOCG) deposit correlate to this early hydrothermal event but more likely indicate a younger and temporally separate event. However, much of the geochronological data from this geologic period overlaps within errors, leaving the actual number of hydrothermal-magmatic events during this early phase an unresolved question (cf. Cliff et al., 1990; Romer et al., 1994; Smith et al., 2009; Westhues et al., 2016).
Younger hydrothermal ages have been recorded at 1718 ± 12, 1623 ± 23 (Blomgren, 2015; Andersson et al., 2016), 1738 ± 19, and 1628 ± 12 Ma (Westhues et al., 2017) by U-Pb monazite data from Rektor and Kiirunavaara. Cliff and Rickard (1992) report a secondary Pb-Pb isochron age at 1540 ± 7 Ma for pyrite overprinting the Kiirunavaara orebody, which is similar to reset Sm-Nd isotope ages (Cliff and Rickard, 1992). Ages between 1600 and 1500 Ma are ambiguous in northern Norrbotten, with disturbed Rb-Sr ages reported regionally (Fig. 2; e.g., Welin et al., 1971). These much younger ages do not correlate to any known magmatic event in Norrbotten, and the meaning of these ages is poorly understood.
The structural grain in Kiruna is steeply east dipping and strikes north to northeast (Fig. 3A, B). It is dominated by a heterogeneously developed cleavage trending subparallel to lithological contacts. Bedding generally shows uniform attitudes over large distances and between rock units, and only few areas show clear evidence of folding (Fig. 3A; Wright, 1988; Grigull et al., 2018; Andersson, 2019). In low-strain blocks, cleavage is steeper than bedding and strikes counterclockwise from bedding, indicating westward vergence (Wright, 1988; Grigull and Jönnberger, 2014) of the E-dipping supracrustal stack. LS-tectonites characterize the tectonic structures in Kiruna (Wright, 1988).
The central Kiruna area is adjacent to a steep, NNE-trending, crustal-scale deformation zone, the Kiruna-Naimakka deformation zone (Fig. 1; Bergman et al., 2001). Based on SC fabrics near Naimakka (Fig. 1), Bergman et al. (2001) interpreted west-side-up kinematics for this zone (fig. 54C in Bergman et al., 2001). In contrast, ~20 km northeast of Kiruna, Luth et al. (2018a) reported east-side-up kinematics due to northeast-southwest to east-west shortening during the inversion of Rhyacian intracontinental rift basins. In central Kiruna and the open pits, shear zones with similar north-northeast orientations are present (Fig. 3A, B) and trend parallel to lithological contacts, but these structures have never been investigated in detail, and their relationship to the Kiruna-Naimakka zone remains uncertain.
The relationship between the IOA deposits and deformation in central Kiruna is an unresolved question. Early studies of the open pits did not recognize the ductile tectonic structures, and most geologic features were interpreted as primary magmatic (e.g., Geijer, 1950). When the role of ductile deformation was recognized, the orebodies were interpreted as megaboudins (Vollmer et al., 1984) or continuous sheets occupying limbs and hinge zones of overturned folds (Forsell, 1987). Faults have been mapped and subscribed as a controlling factor on shape and position of some ore lenses (Parak, 1969), but the character of these faults is unknown. Wright (1988) provides the only publicly available account of a deformed magnetite-apatite ore in Kiruna, where a boudinaged section of the Luossavaara deposit was described in detail (cf. figs. 4.3, 6.3 in Wright, 1988). Later studies describe the hanging-wall rocks to the Rektor deposit as strongly sheared (Bergman et al., 2001; Martinsson, 2015; Andersson, 2019) and folded (Grigull et al., 2018).
There is no generally accepted structural model for the central Kiruna area, but most models include approximately east-west shortening (Vollmer et al., 1984; Forsell, 1987; Wright, 1988; Talbot and Koyi, 1995; Grigull et al., 2018; Andersson, 2019). Wright (1988) introduced a WNW-directed fold-thrust model (D1-D2 in Wright, 1988) with a relative timing synchronous or slightly postemplacement of the IOA deposits followed by subsequent folding (D3) and shearing (D4) with unknown shortening direction and timing. The fold-thrust model presented by Wright (1988) gained support by Talbot and Koyi (1995) but was rejected by Bergman et al. (2001) based on the steep dips of the inferred thrusts. Vollmer et al. (1984) argued for one single episode of WNW-directed shortening manifested by folding around a fold axis plunging 60° to the south-southeast, a view supported by Grigull et al. (2018). Grigull et al. (2018) add an additional component of continued east-west shortening responsible for the inversion of the Hauki quartzite, interpreted as a graben structure in agreement with Witschard (1984). Both west-side-up (Bergman et al., 2001) and east-side-up senses of shear (Wright, 1988; Talbot and Koyi, 1995; Grigull et al., 2018) have been interpreted for the central Kiruna area; however, little evidence is at hand (cf. Wright, 1988, Grigull et al., 2018).
The timing of deformation in Kiruna is ambiguous. An 1880 ± 3 Ma U-Pb zircon age of an undeformed crosscutting granophyre dike at the Kiirunavaara IOA deposit has been suggested to represent the minimum age of the NS-oriented tectonic grain (Cliff et al., 1990). Field observations of the 1.88–1.86 Ga plutonic rocks (Perthite Monzonite Suite; Bergman et al., 2001) have been interpreted both as syn- (Vollmer et al., 1984) and late- to postregional D1 (Wright, 1988; Talbot and Koyi, 1995; Bergman et al., 2001), but components supporting both interpretations are present (Luth et al., 2018a).
The purpose of this study is to reevaluate the structural setting responsible for the emplacement of the IOA deposits in central Kiruna and the subsequent tectonic overprint. The stratigraphy is reexamined to delineate between the inferred fold-thrust belt (Wright, 1988; Talbot and Koyi, 1995) and Orosirian graben development (Witschard, 1984; Grigull et al., 2018). Structural mapping and microstructural analysis are used to constrain the number of deformation events (cf. Vollmer et al., 1984; Wright, 1988; Grigull et al., 2018), kinematics (cf. Wright, 1988; Bergman et al., 2001; Grigull et al., 2018), and the role of deformation on the shape of the orebodies (cf. Vollmer et al., 1984; Wright, 1988). The Per Geijer iron ore field and the Luossavaara orebody are used as case examples because they provide excellent exposures in the open pits. The open pits are not in production, which allows for detailed structural investigation and sampling but provides limited exposures under the pit lakes. By using this approach, we hope to show the most probable preshortening setting in Kiruna and to clarify the relative timing and kinematics of the subsequent deformation as well as the role of deformation on shape of the orebodies.
Geologic mapping was conducted between 2016 and 2018 covering the study area as well as adjacent areas in order to provide regional background data for comparison. One oriented 1,173-m drill core (PG81619) intersecting the Matojärvi Formation was mapped in order to obtain information on contact relationships to and within the Matojärvi Formation as well as structural variations. A total of 847 outcrop observations were performed, and 1,127 structural elements were measured. All structural measurements were collected using a Brunton Geo Pocket Transit, and all data were digitized in field on a ruggedized iPad mini device using the Field Move application (formerly Midland Valley Exploration Ltd., currently Petroleum Experts Ltd.). All lineations were measured as the pitch on planes and recalculated into true orientation using the Geo Calculator software (Holocombe, Coughlin, Oliver, Valenta Global). For magnetic rocks, the strikes of planar structures were estimated using known points in the terrain. Structural analysis was performed using the Move 2017 software package (Petroleum Experts, Ltd.), whereas maps were constructed using ArcMap 10.1 (ESRI). Stereographic plots were produced as lower-hemisphere, equal-area stereographic projections using Move 2017 and Dips 7.0 (Rocscience). Sixty-eight oriented samples were collected and cut perpendicular to foliation and parallel with lineation in order to allow for kinematic interpretations. The samples were sent to Vancouver Petrographics Ltd. for thin-section preparation. Petrography and microstructural investigations were performed using a conventional petrographic polarization microscope equipped with a digital camera (Nikon Eclipse E600 POL). Kinematic interpretations on macroscopic structures were performed in parallel with lineation.
Two-dimension-forward modeling using Move 2019 software package (Petroleum Experts Ltd.) was performed. Different fault geometries and shear styles were tested using the 2D Move-on-Fault functions applying 800 m of reverse simple shear with 90° shear angle displacing a synthetic layer cake model with estimated layer thicknesses. The results were then compared to the conceptual cross sections based on surface data to verify that subsurface geometries can be reconstructed from modeling.
A 40-cm core sample (drill core PG81619) with a diameter of 51 mm was analyzed by X-ray computed tomography (CT) and X-ray fluorescence (XRF) in tandem at the Orexplore AB facility in Kista, Sweden. The CT images are presented with enhanced grayscales to increase visibility of the structural features.
In the following section, the prefix “meta” for metamorphic rocks is avoided in order to simplify the text and to emphasize protoliths. The stratigraphic column is summarized in Figure 2.
The Kurravaara conglomerate constitutes the lowermost Orosirian stratigraphic unit in central Kiruna. Based on drill core observations, the contact between the Kurravaara conglomerate and the overlying Hopukka Formation is gradational (Martinsson et al., 1993), whereas the lower contact, where exposed, is sharp (App. Fig. A1a). The clasts consist of Rhyacian basalt and intermediate to felsic porphyric volcanic rocks, and the conglomerate is moderately to poorly sorted (Fig. 4A); however, local clast size gradations are present (App. Fig. A1b). Within the Kurravaara conglomerate, phyllonite horizons (App. Fig. A1c) as well as graywackes with developed bedding (App. Fig. A1d) do occur but are volumetrically subordinate. The same lithic graywacke forms the matrix in the volumetrically dominant gravelly horizons of the unit, but phyllosilicate-dominated matrices also occur in altered high-strain sections. Both clast-supported (Fig. 4A) and matrix-supported (Fig. 4B) conglomerates are common.
The Hopukka Formation consists of basalt and trachyandesite that form the footwall to the Kiirunavaara and Luossavaara deposits. Amygdules (App. Fig. A1e) are common as well as albite alteration, either by common selective-pervasive (patchy) reddish albite together with magnetite (Fig. 4C) or pervasive whitish albite alteration (App. Fig. A1f). Farther up in stratigraphy, the Loussavaara Formation is generally defined by a reddish fine-grained rock carrying reddish feldspar phenocrysts and xenoliths of the Hopukka Formation (Fig. 4D), but grayish groundmasses carrying whitish feldspar phenocrysts also occur frequently along with aphyric tuffite (App. Fig. A1g). Within the lower part of the Luossavaara Formation, poorly sorted polymict breccia conglomerate occurs (Fig. 4E) in a ~30- × 100-m lensoid geometry (Fig. 3B). The clasts consist of aphyric, light-colored, and darker intermediate volcanic rocks (App. Fig. A1h), most probably derived from the Hopukka Formation, suggesting a local origin. The clast shape is angular to subrounded ranging from pebbles to boulders up to 30 cm in size. The unit is both clast and matrix supported (cf. App. Fig. A1h, i) varying from outcrop to outcrop. The matrix is composed of lithic graywacke grading into a grayish feldspar-phyric felsic volcanic rock.
The Matojärvi Formation (Figs. 2, 3A-C) constitutes the hanging wall to the Per Geijer iron ores. The rocks are generally tectonized and define a heterogeneous sequence of volcanic, volcano-sedimentary, and sedimentary rocks. The stratigraphically lowest unit of the Matojärvi Formation is composed of a red aphyric quartz-rich rhyolite tuff (App. Fig. A2a). A mylonitized compositional banding is generally present in the rhyolite interpreted as bedding S0 (Fig. 4F), which locally grades into a quartz matrix affected by a characteristic selectively pervasive (patchy) K-feldspar alteration (App. Fig. A2b). On top of the rhyolite tuff, basaltic agglomerate (Fig. 4G), tuffite (App. Fig. A2c), and subordinate coherent basalt (App. Fig. A2d) are deposited. Within the Matojärvi Formation, several polymict breccia conglomerates (Fig. 4H) occur that display a wide range of clast compositions. Hematite, felsic feldspar-phyric and aphyric volcanic rocks, intermediate feldspar-phyric and aphyric volcanic rocks, basalt, jasper, chert, and quartz occur as clast material in different amounts. Locally, portions of the clast material consist of a rheologically weak aggregated whitish and reddish material (App. Fig. A2e). In spatial association with the Rektor orebody (Fig. 3B, C), hematite dominates the clast material (App. Fig. A2f), whereas in other parts, hematite clasts are normally present in subordinate amounts (App. Fig. A2g). The clasts are generally stretched with aspect ratios of up to 1:8 (App. Fig. A2h), and in highly strained sections the matrix is often altered into sericite schist. The clast shape is subangular to subrounded with sizes ranging from pebble to boulder with sizes up to 30 cm. The breccia conglomerate is moderately to very poorly sorted and generally matrix supported (Fig. 4H; App. Fig. A2e, g, h), but clast-supported sequences (App. Fig. A2f) are locally present. The matrix is composed of grayish to reddish lithic graywacke, but in some localities, hematite constitutes the matrix (App. Fig. A2i). The stratigraphic top of the Matojärvi Formation is composed of lithic graywacke that shows a weak to intense schistosity and a selective-pervasive bedding-parallel carbonate alteration (Fig. 4I) followed by a thin horizon of intrafolial phyllite showing abundant drag folds related to flanking structures (Fig. 4J).
The Hauki quartzite (Figs. 2, 3A-C) is the youngest unit in the central Kiruna area. It varies from reddish arkosic to light-gray quartz-arenite, and the two variants grade into each other. However, locally, sharp contacts between the two types are present (App. Fig. A3a). The unit is characterized by a well-developed bedding marked by iron oxides (App. Fig. A3b), and crossbedding is widespread (Fig. 4K). Two horizons of poorly sorted alluvial breccia conglomerate (Fig. 4L) occur at the base and middle positions. The clast sizes range from ~0.5 to 50 cm; they are locally derived from the Hopukka, Loussavaara, and Matojärvi Formations and are generally matrix supported (App. Fig. A3c) with minor clast supported sections (App. Fig. A3d). Erosional contacts indicate that the Hauki breccia conglomerate is in general semiconformable with the surrounding bedding (~15°; App. Fig. A3e), but local erosional discordances of up to 60° (App. Fig. A3f) have been measured.
In the following section, the character of recognized tectonic structures is described. Relative timing of these structures, their tectonic relevance, and their role for mineralized systems are interpreted and clarified in the “Synthesis of the Orosirian Structural Evolution” section.
Foliation: In low-strain blocks, a heterogeneously developed, continuous and steeply E-dipping cleavage is present (Fig. 3A). In competent volcanic or arenitic rocks, the low-strain cleavage is defined by feldspar ± quartz ± biotite ± amphibole ± calcite. Where strain increases, chlorite ± biotite ± albite ± quartz ± calcite defines mylonitic fabrics in mafic volcanic rocks (Fig. 5A). In felsic to intermediate volcanic rocks as well as in volcano-sedimentary and breccia conglomerate units, the mylonitic cleavage is defined by sericite + quartz ± calcite ± chlorite (Fig. 5B). Mylonite zones show bulging or subgrain rotation dynamic quartz recrystallization (Fig. 5C; Passchier and Tourow, 2005). In the north, the cleavage of the Hauki quartzite is oriented axial plane parallel to folded bedding planes (Fig. 3A) associated with bulging recrystallization quartz textures. In the south, the bedding orientation is uniform, and the mean principal orientation of the low-strain cleavage (76/114°) as well as the high-strain cleavage (75/107°) is slightly steeper and rotated anticlockwise in respect to the mean principal orientation of the bedding (56/127°). However, in outcrop scale, the high-strain cleavage often subparallels the bedding, indicating transposition of bedding in association to shearing.
Cleavage subparallels the orebodies. The strain intensity in the host rock is normally high in association with the ores but drops sharply in the ores (Fig. 5D). Only in rare cases, the IOA orebodies show a developed foliation, and in those cases, it is defined by flattened iron oxides (Fig. 5E), dynamically recrystallized quartz showing subgrain rotation or grain boundary migration textures (Fig. 5F), or apatite bands (Fig. 5G) that tend to show geometries resembling boudins (Fig. 5H). Pinch-and-swell structures are occasionally associated to highly strained rocks in central Kiruna. In the hanging-wall rocks of the Henry deposit, a sericite + quartz + calcite + chlorite mylonite fabric wraps around a quartz vein, showing pinch and swell parallel to steep stretching lineation (Fig. 5I).
Lineation: Stretched minerals and clasts define the most pronounced ductile lineation in central Kiruna. Clasts may show an aspect ratio of up to 1:8 (App. Fig. A2h) in highly strained units, but the stretching lineation is more commonly seen as a subtle stretching of feldspar or quartz on foliation surfaces. The mineral lineation parallels the stretching lineation and is defined by biotite ± chlorite or sericite on foliation planes. Along the Matojärvi Formation, the stretching lineation steepens from south to north. Within the Rektor open pit and southward (Fig. 3B), the stretching lineation plunges moderately steeply toward the south, whereas north of the Rektor open pit, the stretching lineation shows steep to vertical plunges (Fig. 6A). This indicates a kinematic change from an oblique-slip-dominated system in the south to a dip-slip-dominated system toward the north. However, the stretching lineation in the Luossavaara open pit is steep and plots similar to the stretching lineation north of Rektor (Fig. 6A) implying that the kinematic change from north to south is only valid for the structures within the Matojärvi Formation and does not reflect the area as a whole.
Slickensides on fracture planes are common throughout the area. They are commonly developed on chlorite-altered fracture planes in competent volcanic units. Stepped surfaces show calcite ± malachite ± azurite in pressure shadows but only rarely provide high-confidence kinematic information. Slickensides plot in poorly defined clusters (Fig. 6B) in respect to the stretching lineation (Fig. 6A), but they show a steepening trend toward the north along the Matojärvi Formation similar to that of the stretching lineation and also plot similar to the stretching lineation in the Loussavaara open pit (cf. Fig. 6A, B).
Shear fabrics and kinematics: Proto- to ultramylonites are developed throughout the central Kiruna area forming centimeter-scale to tens-of-meters-wide shear zones commonly oriented largely concordant to lithological units. Interconnecting discordant splay structures linking up with the larger structures are present in the Matojärvi Formation (Fig. 3A, B). The most prominent shear zones are developed at lithological contacts and within the Kurravaara conglomerate, Matojärvi Formation, and the breccia conglomerates of the Hauki quartzite (Fig. 2) where rheologically weak rocks dominate. Strain partitioning is significant producing sharp contacts between highly strained rocks and low-strain blocks in centimeter (Fig. 7A) to district scale. Competent volcanic rocks and orebodies adjacent to high-strain rocks commonly show weakly developed or no fabrics. Instead, brittle deformation dominates in the low-strain units, occasionally developing Riedel fractures mimicking the kinematics of spatially related shear zones (Fig. 7B).
Only one major stratigraphic repetition occurs in the area, evident by the superposition of a Rhyacian basaltic tuffite adjacent to the Orosirian Hauki quartzite (Fig. 3A-C). This deformation zone is poorly exposed at the surface but has been intersected by drill holes, showing the contact as mylonitic. The discordant orientation of this structure is supported by a vertical (88/296°) cleavage measurement in the basaltic tuffite as well as a steep E-dipping (76/136°) high-strain cleavage south of the study area indicating the orientation of the basaltic tuffite as discordant in relationship to the mean principal bedding orientation (56/127°) of the Hauki quartzite. In the south, the Loussavaara Formation is brought into contact with the Hauki quartzite. At that contact (Fig. 3B), protomylonitic rocks of the Loussavaara Formation show dip angles (38/107°) parallel with the bedding of the Hauki quartzite (38/078°). Minor stratigraphic repetitions also occur at the contact between the Hauki quartzite and the Matojärvi Formation where meter-scale sequences of mylonitic phyllite of the Matojärvi Formation occurs sandwiched between brecciated arenite of the Hauki quartzite.
The kinematics of the shear zones are consistently reverse oblique- to dip-slip-dominated showing east-block-up sense of shear from north to south. This is indicated by kinematic indicators such as rotated delta (Fig. 7C) and sigma (Fig. 7D, E) porphyroclasts/sigmoids, SCC´ fabrics (Fig. 7F, G), and oblique foliation (Fig. 7H). We have also observed microstructures resembling magnetite fish (Fig. 7I) showing a contrasting sense of shear relative to the area as a whole. These structures are not well established (Passhier and Trouw, 2005), and it is unclear whether any reliable kinematic information can be obtained from them in the Kiruna area. The sigmoidal shape of the clast as well as parts of the quartz in pressure shadows in Figure 7I indicates dextral sense of shear, but a significant pure shear component seems to be present, which complicates the interpretation of this microstructure.
Folds: A relatively early fold sequence is present in the northernmost part of the mapped area (Fig. 3A), where pillow basalt of the Rhyacian Kiruna greenstone group and the Orosirian Kurravaara conglomerate form an open synclinal synform cored by the Hauki quartzite. Mesoscale folds with a related axial plane-parallel amphibole + biotite + plagioclase continuous cleavage is present (Fig. 8A, B) within the pillow basalt, showing measured fold axes plunging to the north-northeast (75/020°) or moderately steeply to the southwest (45/220°). The form line defining the overturned anticlinal fold shape in the northeast (Fig. 3A) is interpreted based on ground magnetic data, and the fold shape is constrained by bedding orientations and stratigraphic-up indicators. The northernmost part of the Hauki quartzite (Fig. 3A) gives access to a series of near-symmetrical upright folds with a calculated S0/S0β-axis plunging moderately steep to the south-southwest (55/213°). Where exposure is high enough for fold characterization, synclinal Ramsey class 1C folds (Fig. 8C) have been recorded. Parts of the topography are controlled by bedding, allowing the use of high-resolution LIDAR data to support the extrapolation of some of the bedding form lines (Fig. 3A) in line with Grigull and Jönnberger (2014).
Late gentle folding deforms the tectonic foliation and shear fabrics in central Kiruna. Crenulation of mylonitic sericite (Fig. 9A) and (Fig. 9B) chlorite domains affects the lithostructural boundaries, where the Haukivaara and Loussavaara deposits are located (Fig. 3B), as well as the breccia conglomerates of the Hauki quartzite (Fig. 9C). The related crenulation cleavage is spaced (Fig. 9B, C), and the resultant microfolds are open with a bluntness ranging from concentric in sericite-dominated parts (Fig. 9C) to kinked in chlorite-dominated parts (Fig. 9B). The related crenulation lineation plunges shallowly to steeply toward the east-northeast with a mean principal orientation of 39/070° (Fig. 9D), which we interpret as the fold axis (Fig. 3A, B). The fabric of the shear zone affecting the Per Geijer iron ores is undulating, and both the Rektor and Henry orebodies exhibit a gently bent geometry (Fig. 3B; Geijer, 1950; Parak, 1969). The β-axis of the cleavage measurements from the Rektor open pit plots subparallel to the crenulation lineation measured throughout the study area (Fig. 9D). The cleavage-cleavage intersection point of a gently folded 1-m section of the Nukutus hanging wall also subparallels the crenulation lineation (Fig. 9E, F), implying that the fold pattern repeats in different scales.
Hydraulic breccias: A characteristic structural feature of the Nukutus open pit consists of a mosaic breccia (Jébrak, 1997). In competent low-strain blocks, the mosaic breccia is forming a classic jigsaw puzzle pattern defined by veins filled by a hydrothermal mineral cement with little or no rotation of fragments (Fig. 10A). However, the vein network is transposed into alignment with the tectonic grain as strain increases (Fig. 10B, C) farther into the hanging wall because of east-block-up shearing. Horizontal (Fig. 10D) and steep (Fig. 10E) sections of the Nukutus breccia show that two sets of veins (V1 and V2 in Fig. 10D, E) dominate, and both sets affect each other. V1 veins probably represent extensional veins, whereas V2 veins probably developed as shear fractures, subsequently offsetting and being offset by V1 veins as new V1 veins formed during progressive deformation and vein development.
In the footwall to the Loussavaara deposit, a vertical monolithic magnetite breccia occurs in between parallel meter-wide magnetite dikes (Fig. 10F, G). Adjacent to the breccia and magnetite dikes, a high-strain chlorite zone occurs broadly concordant with the lithological contacts (Fig. 10F, G). The fragment sizes are rather uniform at ~3–4 cm, but some fragments reach up to ~30 cm; they are angular to subrounded and pervasively whitish albite-altered footwall rocks of the Hopukka Formation (Fig. 10H). The fragments show a subvertical orientation (Fig. 10H) and are often transected by magnetite veins.
Late quartz + calcite veins carrying subrounded fragments of both the ore and wall rocks crosscut all other fabrics and are best developed at the Loussavaara (Fig. 11A) and Rektor (Fig. 11B) deposits. Discordant reddish apatite veins developed at the Rektor and Nukutus deposits show crosscutting relationships to the ore and the dominant structural grain (Fig. 11C) and are also interpreted to be of a late timing. However, the apatite veins are earlier than the quartz + calcite hydrothermal breccia based on crosscutting relationships. The apatite veins carry ore xenoliths and specularite veins, and the apatite shows abundant monazite inclusions.
Faults: In the Haukivaara open pit, a moderately E-dipping reverse fault has thrusted the Hauki quartzite over steep E-dipping mylonitic rocks of the Matojärvi Formation, giving rise to a structural discordance (Fig. 12A). The brittle deformation mainly occurred along the bedding planes of Hauki quartzite, producing a several-meter-thick fault gouge. A meter-thick zone of the gouge is poorly lithified and consists of sand- to gravel-sized particles mixed with hematite-rich clay, indicating recent reactivations and groundwater flows within the fault zone. Bedding and/or fault-parallel ductile components are present in the form of mylonitic intercalations (Fig. 12B), indicating that the faulting occurred under brittle-ductile conditions; however, brittle components dominate. This is supported by drill core observations 3.5 km northward along the same structure, where smaller stratigraphic repetitions are present and where there is an interplay between mylonitic rocks and brittle fracturing.
South of the Haukivaara open pit, rocks of the Luossavaara Formation form a wedge extending into the younger Hauki quartzite (Fig. 3B). Close to the eastern boundary of the Luossavaara wedge, moderately steep (40/104°) protomylonitic felsic volcanic rocks yield reverse dip-slip-dominated sense of shear, thrusting the Luossavaara Formation on top of the stratigraphically overlying Matojärvi Formation (Figs. 3B, 7G) along dip angles subparallel to the bedding in the Hauki quartzite. Similar orientation correlations for ductile deformation and brittle faulting are also present in the Rektor and Henry open pits. In the Rektor hanging wall, a smaller stratigraphic repetition is caused by a fault paralleling oblique-reverse mylonitic rocks of the Matojärvi breccia conglomerate ~100 m east of the pit (Fig. 3B). Associated oblique structures resembling reverse duplex structures (Fig. 12C) would yield reverse kinematics of the fault, mimicking the overall sense of shear of the area; however, the associated structures may also represent Riedel structures, implying that the kinematics of the Rektor fault remains unresolved.
Sulfide-bearing structures: In competent volcanic rocks and the ore, secondary copper carbonates and visible Fe and Cu sulfides are restricted to brittle structures or rounded amygdule-like infills in different mineral associations. For example, quartz + calcite + hematite + magnetite + chalcopyrite + malachite (Fig. 13A) or malachite (Fig. 13B) occur in fractures and veins. In the Henry hanging wall, bornite + hematite + magnetite is hosted by a vein crosscutting pervasively K-altered Matojärvi rhyolite parallel with the dominant NS-trending tectonic grain in the area (Fig. 13C).
X-ray CT imaging of a 40-cm section of mylonitized Matojärvi phyllite in drill core shows a variety of structural traps possible for sulfides in rheologically weak rocks. In the studied sample, the bedding is openly to tightly folded into antithetic flank folds (Fig. 13D, E) with an orientation that accords with the overall structural grain in the area. In low-strain sequences, pyrite is distributed along bedding planes and concentrated into axial planar fracture planes offsetting the bedding (Fig. 13F). The highest concentration of pyrite is observed in the associated fold hinge (Fig. 13E) and a shear band (Fig. 13G) bounding the fold.
Synthesis of the Orosirian Structural Evolution of the Kiruna area
The Orosirian stratigraphy in central Kiruna is dominated by conglomeratic rocks at its base followed by bimodal mafic-felsic volcanic rocks, subsequently changing character to volcano-sedimentary/sedimentary rocks followed by a final stage of sandstone deposition (Figs. 2, 4). Such a stratigraphic record indicates a volcanic-sedimentary basin changing from a predominantly volcanic to a predominantly sedimentary character with time. The volcanic rocks occupying the lower-middle sequence of the basin host magnetite-apatite ores, whereas the ores at higher stratigraphic levels are more oxidized magnetite-hematite-apatite ores. High-strain intensities are spatially associated with competent orebodies (Fig. 5D), whereas cleavage is only sparsely developed in the ores (Fig. 5E). This, together with dynamic recrystallization of quartz (Fig. 5F), indicates that deformation affected the competent orebodies (Passchier and Trouw, 2005), hence providing field evidence for a preshortening timing of IOA emplacement in central Kiruna (Fig. 14).
The Orosirian contact to the underlying Rhyacian greenstones is sharp (App. Fig. A1a) and weakly discordant (Martinsson, 1997). The timing of deposition of the Kurravaara conglomerate is unknown but is bracketed between the crystallization of the calc-alkaline clasts (probably locally derived from the 1.90–1.87 Ga Porphyrite Group; Martinsson et al., 1993, 2016) and the overlying ~1.88 Ga Hopukka Formation (Martinsson et al., 2016). The contact between the Kurravaara conglomerate and the overlying volcanic rocks is poorly exposed. However, the overlapping ages reported for the Porphyrite Group (1.90–1.87 Ga) and Kiirunavaara Group (1.88–1.86 Ga) rocks (Fig. 2), together with the gradational upper contact of the Kurravaara conglomerate (Martinsson et al., 1993), indicate a geologic continuum in the basal part of the Orosirian Kiruna stratigraphy. The deposition of the bimodal mafic-felsic volcanic units was fast (<15 m.y.; Westhues et al., 2016) including the emplacement of IOA orebodies mainly at two stratigraphic positions. The volcanic pile was subsequently buried under lithic graywacke and siltstones and a rather thick package of crossbedded sandstones. The timing of deposition of the final arenitic sequence is critical because it determines the duration of the basin development. Ladenberger et al. (2017) presents detrital U-Pb zircon data for the Hauki quartzite suggesting a wide time span between 1917 and 1752 Ma for a probable maximum deposition age, making conclusions hard to draw at the current stage of research.
The supracrustal pile contains sedimentary polymict breccia conglomerates that occur repetitively in the stratigraphic record. These horizons are, from stratigraphic bottom to top, the Kurravaara conglomerate (Fig. 4A, B), the Luossavaara breccia conglomerate (Fig. 4D), the Matojärvi breccia conglomerate (Fig. 4H), and the Hauki breccia conglomerates (Fig. 4L). The volumetrically largest of these units is the lowermost unit, the Kurravaara conglomerate. The character of these epiclastic rocks varies in respect to clast and matrix composition as well as geometry. For example, Rhyacian greenstone clasts are restricted to the Kurravaara conglomerate, and the matrices of the Hauki breccia conglomerates are quartz arenitic in contrast to the other breccia conglomerates with lithic graywacke as a matrix. Furthermore, in contrast to the general absence of developed bedding planes in these deposits, bedding is locally well developed in the Kurravaara conglomerate (App. Fig. A1d). Geometries of the epiclastic deposits vary from small (30 × 100 m) and lensoid-shaped to concordant (or weakly discordant) horizons (Fig. 3A, B). However, many commonalities are present that indicate similar processes generating these epiclastic rocks. For example, (1) they are all polymict with clasts of diverse but local origin with sizes ranging from pebble to boulder (up to 30–50 cm in diam). (2) The shape of the clasts ranges from angular to subrounded, suggesting short but variable water transport distances. (3) They are in general poorly sorted. Bedding and clast gradations are developed in most units, but these features are rare and broadly suggest rapid deposition and burial. (4) The breccia conglomerates show both clast- and matrix-supported characteristics, indicating both high- and lower-energy depositional processes. On this basis, we interpret these breccia conglomerates as having been deposited in similar alluvial settings and suggest these deposits represent erosional peaks driven by synvolcanic normal faulting during the basin development.
During subsequent crustal shortening, the basin environment hosting the IOA orebodies was inverted (Fig. 14). Based on the orientation and kinematics of the tectonic structures in Kiruna, an east-west to northwest-southeast crustal shortening is inferred—a shortening direction typically subscribed to the late ca. 1.80 Ga Svecokarelian cycle (e.g., Andersson, 1991; Bergman et al., 2001; Weihed et al., 2002; Lahtinen et al., 2005; Bauer et al., 2018; Andersson et al., 2020). The most prominent deformation in the area constitutes moderate to steep NNE-striking reverse oblique to dip-slip brittle-ductile high-strain zones developed at lithological contacts, as well as in favorable lithologies of the Kurravaara conglomerate, Matojärvi Formation, and the Hauki quartzite (Fig. 3A-C). In a broader context, the Matojärvi Formation constitutes one single second-order high-strain zone (Fig. 3C) in the larger inversion architecture. The orebodies served as rigid bodies, and the sections that reach economic thicknesses are probably lineation-parallel boudins of the mineralized horizon (Fig. 3C), as was suggested by Vollmer et al. (1984), and in line with mesoscale boudinage and pinch-and-swell structures (Fig. 5I). In this respect, the structural style of the ore deposits investigated in this study shows similarities to the boudinaged Malmberget IOA deposit (Bauer et al., 2018).
Two-dimension-forward modeling of a synthetic layer cake model (Fig. 15A) with estimated layer thicknesses shows that parts of the interpreted subsurface geometries in cross sections (Fig. 3C) can be reconstructed. Reverse simple shear reactivation of listric faults with 800-m displacement produces a steepening and thinning of layers toward the fault due to transposition of horizontal layers into the fault orientation (Fig. 15B, C). In order to produce the dip angles and a juxtaposition of Rhyacian rocks adjacent to the Hauki quartzite, several listric faults are required (Fig. 15D), indicating that a series of faults were reactivated during the basin inversion. The modeling results accord with our observations of reverse layer-parallel shearing and bulging layer thicknesses (Fig. 3C) and suggest that the controlling structures originated as normal listric faults.
In low-strain blocks in the north (Fig. 3A), a heterogeneously developed, steeply E-dipping, penetrative, continuous cleavage is oriented axial plane parallel to folds formed under an overall east-west crustal shortening (Fig. 8). The folds in the northern part of the study area are open to tight. Associated parasitic folds are closed to tight, slightly asymmetrical, and west verging (Fig. 8A) or classified as symmetrical and upright Ramsey class 1C type (Fig. 8C). The series of folds developed in the northern part of the Hauki quartzite occur in an area that is squeezed between shear zones at the lithostructural boundaries (Fig. 3A). We interpret these folds to have developed as a response to reverse shearing at the lithostructural boundaries where most strain accumulated during the inversion.
Based on microstructures and orientation of stretching lineations (Figs. 6, 7), shear zone structures show systematic reverse dip-slip or reverse oblique-slip movements thrusting the east block up toward the west. East-block-up sense of shear is also indicated by the only major stratigraphic repetition in the study area, where a slice of a Rhyacian basaltic tuffite is juxtaposed on top of the younger Hauki quartzite (Fig. 3A, B) as well as a minor stratigraphic repetition juxtaposing the Loussavaara Formation above the younger Matojärvi Formation (Fig. 3B). Oblique movements predominate to the south, whereas dip-slip movements predominate to the north within the Matojärvi Formation and Kurravaara conglomerate. However, the steep movements recorded by the lithostructural boundary between the Hopukka and Loussavaara Formations at similar latitudes as oblique-slip movements in the Matojärvi Formation do not accord with a systematic northward steepening for all the shear zones and need to be explained otherwise. The kinematic changes along and across the shear zone system is ambiguous, and we suggest two alternatives: (1) strain partitioning, caused by the competent hematite-magnetite orebodies, gave rise to both dip-slip and oblique-slip movements in different parts of the system synchronously or (2) the deformation event is characterized by a cyclic activity activating different structures and parts of structures individually, with different kinematics.
The interplay between ductile and brittle structures during deformation is important in Kiruna. The shear zones show both brittle and ductile features (e.g., Fig. 7B). Faults and mylonites with mimicking orientations and kinematics (cf. Figs. 3B, 7G, 12A, B) indicate a spatial-temporal association between brittle faults in competent units and shearing developed in rheologically weak units. Slickensides plot in poorly defined clusters (Fig. 6B) with respect to the stretching lineation (Fig. 6A). However, slickensides show a steepening trend toward the north in accordance with the stretching lineation, indicating that brittle and ductile linear structures were formed under the same stress regime and related in space and time. The poorly defined clusters in the slickenside plot can be explained on the basis of the varying orientations of the fracture planes along which the movements occurred. The orientation of the foliation planes showing stretching lineations is more consistently oriented, resulting in a tighter clustering of the stretching lineation.
Hydraulic breccias at the Nukutus (Fig. 10A-E) and Loussavaara (Fig. 10F-H) open pits are spatially associated to structures formed during the basin inversion and indicate the importance of hydraulic fracturing during the deformation event. The observed breccias probably reflect different stages of the breccia evolution. The occurrence in Nukutus is interpreted to have formed by fluid-assisted brecciation during the propagation stage (Jebrak, 1997), whereas the Luossavaara occurrence formed by corrosive wear during the later dilation stage (Jebrak, 1997), allowing fragment transportation in the evolved hydrothermal breccia. In Nukutus, the internal vein configuration with two sets of veins deforming each other (Fig. 10D, E) can be explained on the basis of reverse shearing in the hanging wall (Fig. 10A-C). In the case of the breccia in the footwall to the Luossavaara deposit (Fig. 10F-H), the timing of brecciation relative to plastic deformation is more ambiguous because of the absence of defined vein networks. However, the Luossavaara breccia pipe is bounded by a parallel chlorite high-strain zone (Fig. 10F), making a tectono-hydrothermal origin and a syndeformational timing of the breccia as the likely scenario.
In Kiruna, Fe and Cu sulfides associated with Fe oxides are hosted by structures formed by crustal shortening. In competent rocks, sulfides occur in brittle structures such as fractures and veins (Fig. 13A-C), and copper deposits close to the study area (Bergman et al., 2001; Smith et al., 2007; Martinsson et al., 2016) show similar structural associations. In rheologically weak rocks, the structural traps are more varied in style, as revealed by CT images showing different structural sulfide relationships in the Matojärvi phyllite (Andersson et al., 2019; Fig. 13D-G). The CT results suggest that pyrite was remobilized and transported along brittle axial planar fracture/cleavage planes and accumulated in a fold hinge and a shear band syncrustal shortening. Furthermore, the Nukutus fluid-assisted breccia (Fig. 10A-E) carries sparse pyrite and chalcopyrite hosted by veins interpreted in this study to have been formed in response to reverse shearing. Because of the very low sulfide content of the hematite-magnetite orebodies, we suggest that much of the visible Fe and Cu sulfide was introduced into the system during east-west crustal shortening. This would imply that a superimposed mineralizing event responsible for epigenetic Fe and Cu sulfides and Fe oxides can be linked to the inversion phase of the basin evolution, hence, in contrast to the IOA deposits linked to the synextensional basin development.
The inverted basin was gently refolded during a later deformation event, and the resultant structures are in accordance with an overall north-northwest–south-southeast to north-south crustal shortening (Fig. 14). The orientation of the associated crenulation lineation and the calculated cleavage-cleavage β-axis of the Rektor deposit (Fig. 9D), exhibiting a bent geometry, indicate that the undulating character of the ore field is the result of later refolding modifying the crustal architecture, including ore geometries. The resultant structures are subtle and are best developed in sericite and chlorite domains of the shear zones (Fig. 9A-C), whereas regionally penetrative fabrics are lacking. The lack of penetrative structures and/or strain accumulations into associated high-strain zones as well as the gentle fold hinges associated to the refolding indicate weak deformation, probably during the veining stages of the crustal shortening. However, no strike-slip reactivations of the NNE-striking shear zones have been recorded in this study, implying that the rotation of the stress field was abrupt rather than continuous and that the refolding event was probably separated by a tectonic pause from the dominant basin inversion event.
In the Kiirunavaara underground mine, Berglund and Andersson (2013) identified several NS-trending strike-slip fracture planes explained by an overall NS-directed crustal shortening. In the Pajala area (Fig. 1), D3 fault structures are compatible with a north-northwest–south-southeast shortening direction (Luth et al., 2018b). It is possible that the brittle strike-slip movements in the Kiirunavaara underground mine and the faulting in Pajala represent the same regional deformation event as the gentle refolding of the central Kiruna area indicated by this particular study, but further research is needed to determine the possible temporal linkage.
The inverted and refolded basin was further fractured during a last identified deformation event associated with hydrothermal activity (Fig. 14). Quartz + calcite hydrothermal breccias in the Rektor and Loussavaara open pits (Fig. 11A, B), as well as discordant reddish apatite veins at the Rektor (Fig. 11C) and Nukutus deposits, crosscut all other fabrics. The quartz + calcite hydrothermal breccias overprint the apatite veins; hence, the apatite veins are the earlier feature, implying that two separate deformation events may be reflected in the observed structures. We interpret this hydrothermal breccia stage as the last major geologic event based on crosscutting relationships and because the veins do not show any clear signs of tectonic reworking.
The age of the last hydrothermal fracturing is of importance because it brackets the time frame of the geologic evolution generating the present crustal architecture in central Kiruna. The hydrothermal U-Pb monazite ages at 1718 ± 12, 1623 ± 23 (Blomgren, 2015; Andersson et al., 2016), 1628 ± 12, and 1738 ± 19 Ma (Westhues et al., 2017) obtained from samples at the Rektor and Kiirunavaara deposits constitute candidates to bracket the age of this hydrothermal fracturing event(s). At the Malmberget IOA deposit, ~120 km southeast of Kiruna, Romer (1996) obtained U-Pb titanite and monazite ages at ca. 1740 and 1620 Ma from open fractures carrying low-temperature mineral assemblages involving apatite, stilbite, calcite, and biotite. The open fractures at Malmberget represent the last deformation event of the Malmberget IOA deposit (Bauer et al., 2018). Rb-Sr whole-rock data collected regionally (e.g., Welin et al., 1971) show ages similar to those of the hydrothermal titanite-monazite from Kiruna and Malmberget, which highlights the regional significance of this last deformation event dominated by hydraulic fracturing.
Basin development in Kiruna during the Orosirian is not a new hypothesis; however, earlier workers (Witschard, 1984; Grigull et al., 2018) have only tentatively suggested it without presenting evidence. Witschard (1984, p. 292) suggested that the Hauki quartzite was deposited “in a tectonically active graben” and Grigull et al. (2018) briefly suggested a hypothesis of inversion of this same graben structure, interpreted to have been eroded down into the Matojärvi Formation. We have not found evidence in support of or against the existence of an erosional contact between the Matojärvi Formation and the Hauki quartzite, as suggested by Grigull et al. (2018). Instead, we have only observed tectonic contacts between these units, including smaller tectonic repetitions and breccias related to shearing using the uppermost phyllite horizon as a shear plane. We agree that the development of grabens during Orosirian extension is the most likely scenario. However, we argue that the entire Orosirian stratigraphic record in central Kiruna, ranging from ≤1.88 Ga (Cliff et al., 1990; Romer et al., 1994; Martinsson et al., 2016; Westheus et al., 2016) to the deposition of the Hauki quartzite (Ladenberger et al., 2017), reflects the development of a basin environment.
The tectonic setting during the earliest geologic evolution in central Kiruna has been the subject of contrasting interpretations. Wright (1988) and Talbot and Koyi (1995) argue for a classic fold-thrust model synchronous with the IOA emplacement, implying that the area represents the foreland sector of a larger subduction system at approximately 1.9 Ga. We see several problems with this interpretation; the most obvious is that large volumes of volcanic rocks developed in this part of northern Norrbotten during this period of the geologic evolution. In Gällivare (Fig. 1), the Dundret mafic to ultramafic layered intrusion is temporally related to the same volcanic rocks as found in Kiruna (Sarlus et al., 2017, 2018), indicating an extensional setting at this time. Regionally, volcanic rocks similar to those found in Kiruna have a shoshonitic character in further support of an extensional setting (Perdahl and Frietsch, 1993). Southwest of Kiruna, Martinsson (2004) points out the importance of a thick basaltic unit at the basal parts of the volcanic pile, the mafic-felsic bimodality of the volcanic rocks, and the tendency to a within-plate basalt character—chemical and petrological characteristics further indicative of an extensional setting. In a subduction context, we argue that the crustal thinning during the early Svecokarelian cycle is best explained to have occurred in the back arc rather than the foreland of the subduction system. However, we note the paradoxical tectonic models that come from structurally based studies (Wright, 1988; Talbot and Koyi, 1995) at one hand and geochemical/petrological (Perdahl and Frietsch, 1993; Martinsson, 2004; Sarlus et al., 2018) studies on the other hand. We add here a stratigraphic argument in favor of an extensional setting generating a basin in Kiruna synchronously with IOA emplacement during the early Svecokarelian orogeny.
The generation of rift basins due to continental breakup during the Rhyacian is rather well established in the Fennoscandian Shield (e.g., Pharaoh and Pearce, 1984; Martinsson, 1997; Lehtinen et al., 2005; Melezhik and Hanski, 2012; Hanski et al., 2014; Bingen et al., 2015). The normal faults associated with this early phase of rifting constitute candidates for being the controlling structures also in the case of later extensional events during the Orosirian by normal fault reactivation. Hence, we further suggest that the Orosirian volcanic rocks were deposited in a rift environment already developed during the Rhyacian and that the succeeding Orosirian crustal thinning, manifested by, e.g., mafic intrusions of this age (Sarlus et al., 2017, 2018), occurred in the intracontinental back-arc regions during the Svecokarelian early-cycle orogeny. Such a tectonic setting and evolution shares many similarities to other IOA/IOCG districts of widely different ages. For example, the Bafq district in central Iran has been indicated as a back-arc basin, coeval with convergence along the proto-Tethyan margin during the Neoproterozoic to early Cambrian, and Zn-Pb and IOA deposits formed in this environment (e.g., Rajabi et al., 2015, 2020; Eslamizadeh, 2016). However, the Bafq district has also been indicated to represent the magmatic arc region of the orogeny (e.g., Ramezani and Tucker, 2003; Majidi et al., 2020); hence, consensus on the tectonic setting of central Iran has not been reached. The importance of back-arc extension and subsequent onset of crustal shortening have been highlighted as key aspects for IOCG formation in the Jurassic-Cretaceous Coastal Cordillera of northern Chile and southern Peru (e.g., Mpodozis and Ramos, 1989; Sillitoe, 2003) as well as the Paleo- to Mesoproterozoic Mt. Isa Inlier in eastern Australia (e.g., Giles et al., 2006; Tiddy and Giles, 2020). Together with the widespread sodic and potassic alteration, the bimodal character of host volcanic rocks, and proximity to deformation zones of the IOA and IOCG deposits in northern Norrbotten, the area also shares petrological, alteration, and structural characteristics with other IOCG/IOA prospective terrains including Brazil (e.g., deMelo et al., 2017; Craveiro et al., 2019), Canada (e.g., Corriveau and Mumin, 2010; Corriveau et al., 2016), and Mauritania (e.g., Kolb et al., 2008).
Crustal shortening and basin inversion
D1 structures are recognized regionally in northern Norrbotten and are generally subscribed to an overall northeast-southwest crustal shortening resulting in the accretion of the Skellefte VMS district to the south onto an Archean craton to the north (assigned D2 in the Skellefte district; e.g., Allen et al., 1996; Bauer et al., 2011; Skyttä et al., 2012). The S1 foliation is continuous and heterogeneously developed and associated with epidote-amphibolite facies (Edfelt et al., 2005; Andersson et al., 2020) to amphibolite facies (Bergman et al., 2001; Bauer et al., 2018) peak metamorphism (Andersson et al., 2020). S1 foliation is folded into F2 folds west of central Kiruna (Andersson et al., 2020) as well as to the east (Grigull et al., 2018) and to the southeast (Bauer et al., 2018). The F2 fold axes are either south or southwest plunging (Bauer et al., 2018; Andersson et al., 2020) and in accordance with an overall east-west crustal shortening. Furthermore, west of Kiruna reverse dip-slip reactivations of NNW-SSE–trending shear zones as well as strike-slip shearing along EW-trending D2 structures indicate east-west crustal shortening during a regional D2 event that probably occurred at slightly lower temperatures compared to M1/D1 (Andersson et al., 2020).
The mineral associations chlorite ± biotite ± albite ± quartz ± calcite in mylonitic volcanic rocks of mafic compositions (Fig. 5A) and sericite + quartz ± calcite ± chlorite (Fig. 5B) in mylonitic sedimentary and volcanic rocks of felsic compositions indicate greenschist facies conditions during deformation in central Kiruna. The orientation of the cleavage is compatible with an E-W– to NW-SE–directed crustal shortening consistent with the regional D2 event. The same cleavage is axial plane parallel to folded bedding planes with an S0/S0β-axis plunging moderately steeply to the south-southwest (55/213°: Fig. 3A), which is similar to S1/S1β-axes of F2 folds west of central Kiruna consistent with east-west crustal shortening (cf. Andersson et al., 2020). This indicates that the earliest tectonic fabric recorded in central Kiruna formed during the regional D2 event. We suggest two alternative explanations for the absence of recognizable D1 structures in the study area: (1) the central Kiruna area was subjected to high D2 strain, and S1 was transposed into alignment with the D2 structures or (2) ductile D1 structures were never recorded in central Kiruna, because the area represents crustal levels that were too shallow during D1 for plastic deformation to occur.
The second explanation is favored in this study for several reasons. If the regional S1 would have been recorded in central Kiruna, it would be expected to be recognizable, at least in distal parts of the high-strain zones where S1 would be folded in accordance with regional results (Bauer et al., 2018; Grigull et al., 2018; Andersson et al., 2020). The areas where S1 is recognizable in northern Norrbotten show higher metamorphic grades than the central Kiruna area (Bergman et al., 2001) and show retrograde alteration of peak metamorphic mineral associations (Andersson et al., 2020). No retrograde mineral products replacing earlier metamorphic peak minerals have been identified in this study, and our results indicate that neither regional M1 nor D1 were recorded in central Kiruna. The regional implication of this is that lower crustal levels around Kiruna must have been uplifted to the higher crustal levels of the central Kiruna area, an explanation that is supported by contrasting structural kinematic evidence reported regionally (cf. Lynch et al., 2015; Bauer et al., 2018; Luth et al., 2018a; this study). To the west and southeast of central Kiruna, steep and W-dipping mylonitic structures record west-side-up kinematics (Lynch et al., 2015; Andersson et al., 2020), which contrast the E-dipping mylonitic structures with recorded east-side-up kinematics presented in this study, as well as northeast of central Kiruna (Luth et al., 2018a). The effect is a juxtaposition of different crustal levels that also explains the exceptionally well preserved stratigraphy and primary rock features of central Kiruna compared to adjacent areas with disturbed stratigraphies and metamorphic overprint.
Relative time constraints on Fe and Cu sulfides
The occurrence of epigenetic Fe and Cu sulfides associated with Fe oxides hosted by structures that formed in response to crustal shortening provides field evidence of a syn- to post-deformational timing of much of the sulfides in central Kiruna. Brittle structures in competent rock units (Fig. 13A-C) as well as fracture planes, fold hinges, and shear bands in rheologically weak rocks (Fig. 13E-G) are examples of sulfide-bearing structures in central Kiruna that accord with an overall east-west crustal shortening during the regional D2 event. Similar relationships are present at the Gruvberget (Bergman et al., 2001) and Malmberget (Bauer et al., 2018) IOA deposits southeast of Kiruna, where structurally controlled epigenetic copper occurrences are present in the host rocks to the iron ores. This style of structurally controlled sulfide mineralization overprinting IOA deposits in northern Norrbotten can be linked in time to Svecokarelian late-cycle synorogenic magmatism at ca. 1.80 Ga, indicating a distinct time gap between IOA emplacement and structurally controlled copper mineralization (Martinsson et al., 2016; Bauer et al., 2018; Sarlus et al., 2020).
Similar IOA-IOCG relationships in northern Norrbotten are indicated by U-Pb titanite data on mineral associations interpreted as the age of mineralization in some breccia-hosted IOCG-style deposits linked to the early-cycle Svecokarelian orogeny (Smith et al., 2009; Martinsson et al., 2016). Ages are not as tightly constrained for the IOCG deposits as for the IOA deposits in Kiruna but also indicate Svecokarelian early-cycle IOCG formation as younger compared to IOA emplacement (Fig. 2; cf. Cliff et al., 1990; Romer et al., 1994; Smith et al., 2009; Martinsson et al., 2016; Westhues et al., 2016). Andersson et al. (2020) present a model including initial stages of basin inversion during a regional M1/D1 event west of Kiruna, and we tentatively suggest that the shift from an extensional setting to crustal shortening and the onset of basin inversion generated these early orogenic breccia-hosted IOCG-style deposits. During regional D2 crustal shortening, the basin continued to invert, and Cu-Au ± Fe was remobilized into reactivated and newly formed structures producing shear zone-hosted IOCG deposits (e.g., Nautanen and Kiskamavaara; Martinsson et al., 2016) and subeconomic sulfide occurrences trapped in association with older IOA deposits (e.g., central Kiruna, Gruvberget, Malmberget; Bergman et al., 2001; Bauer et al., 2018; this study).
The stratigraphic column in central Kiruna is suggested to result from intracontinental back-arc basin development. The basin is characterized by bimodal volcanic rocks in the basal to middle parts changing to a volcano-sedimentary and finally a sedimentary character in the upper parts of the basin. The IOA deposits are located at steep, E-dipping lithological contacts in the lower to middle parts of the basin stratigraphy; they formed under shallow crustal conditions, and they formed before crustal shortening.
The basin was inverted as a response to east-west crustal shortening under greenschist facies metamorphic conditions. Strong strain partitioning focused noncoaxial strain into lithological contacts and rheologically weak rocks, which gave rise to reverse, oblique to dip-slip, east-block-up sense of shearing along moderate to steep E-dipping structures. Competent volcanic rocks between the shear zones record limited finite strain. Instead, brittle fracture planes accounted for the deformation in these blocks during the basin inversion. The competent iron orebodies were probably boudinaged but generally lack penetrative fabrics, giving rise to local strain partitioning that may explain local differences in sense of shear recorded by different shear zones. Fe and Cu sulfides associated with Fe oxides are hosted by structures formed during the basin inversion, implying that IOA emplacement and epigenetic sulfide mineralization are spatially related but formed during different times and in fundamentally different structural settings.
The inverted basin was later refolded in response to approximately north-south crustal shortening, giving rise to gentle folds with steep E-plunging fold axes controlling the geometry of at least some of the orebodies. Hydraulic fracturing crosscuts all other fabrics and represents the last recorded geologic deformation event(s) affecting the crustal architecture of central Kiruna.
This study was financed by the Centre of Advanced Mining and Metallurgy (CAMM), which is thanked for the financial support. Luossavaara Kiirunavaara AB (LKAB) supported this project and is thanked for sharing data, granting permission to enter the open pits, and allowing us to publish this study. In particular, Laura Lauri, Lisa Klemo, Monika Sammelin, Ulf B. Andersson, Josefine Johansson, and Jan-Anders Perdahl at LKAB are thanked for reviewing the manuscript or helping the project during the mapping campaigns. Orexplore AB is thanked for performing the X-ray CT-XRF scanning. We are grateful to the reviewers Dr. Laurent Ailleres and Dr. Gustav Nortje, who contributed with constructive criticism and suggestions that improved the manuscript. Dr. Stefan Luth at the Swedish Geological Survey and Leslie Logan at Luleå University of Technology (LTU) are thanked for valuable inputs and discussions and/or field assistance. Prof. Thorkild Maack Rasmussen at LTU is thanked for the processing of magnetic data and for compiling magnetic maps. The authors acknowledge the use of the MOVE Software Suite for data collection and subsequent structural analysis granted by Petroleum Experts Limited.
Joel Andersson has spent the last nine years working on the structural geology of ore deposits in northern Norrbotten with an emphasis on iron oxide-apatite deposits both in industry and academia. In 2012 he graduated with a master’s degree in ore geology from Luleå University of Technology (LTU) where he specialized in the geochemistry and stable isotopic composition of granites associated with an orogenic gold deposit. He is currently a Ph.D. candidate in ore geology at LTU studying the structural evolution of mineralized systems in the Kiruna area by conducting regional- to deposit-scale investigations.