Structural Evolution of the Central Kiruna Area, Northern Norrbotten

Economic Geology, v. XXX, no. XX, pp. X–X

Structural Evolution of the Central Kiruna Area, Northern Norrbotten, Sweden: Implications on the Geologic Setting Generating Iron Oxide-Apatite and Epigenetic Iron and Copper Sulfides

Joel B. H. Andersson,† Tobias E. Bauer, and Olof Martinsson Division of Geosciences and Environmental Engineering, Luleå University of Technology, SE-971 87 Luleå, Sweden

Abstract 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.

Introduction ore breccia in the hanging wall of the Kiirunavaara deposit The central Kiruna area in northern Norrbotten hosts the (Geijer, 1919). Later, Parak (1975) proposed an exhalative largest underground iron mine in the world, the Kiirunavaara hydrothermal model including many components similar to deposit. Together with the nearby Leveäniemi and Tapuli our recent understanding of volcanic massive sulfide (VMS) open pits in the Svappavaara and Pajala areas, respectively deposits (e.g., Franklin et al., 2005). Much of the argumen- (Fig. 1), and the Malmberget underground mine near Gäl- tation by Parak (1975) relies on observations and chemical livare (Fig. 1), the iron mines stand for the vast majority of data from the Per Geijer iron ores, which form parts of the the total iron ore production in Europe. Despite the relatively studied area in this paper. As the concept of hydrothermal or long tradition of mining in northern Norrbotten, the area is magmatic/hydrothermal iron oxide copper-gold (IOCG) was considered underexplored, and many fundamental geologic introduced, Hitzman et al. (1992) classified IOA deposits as questions remain unanswered. a copper-gold–deficient end member under the loosely de- Several types of iron mineralization exist in the northern fined IOCG group of deposits. Recent studies from the Great Norrbotten region (Frietsch, 1997). However, except for the Bear magmatic zone in Canada indicate that IOA and IOCG reopened Tapuli skarn iron deposit (Bergman, 2018) in the deposits represent different metasomatic facies in one single Pajala area (Fig. 1), the only iron ore type in production today metasomatic system (e.g., Corriveau et al., 2016; Montreuil is iron oxide-apatite (IOA), also called Kiruna-type (Geijer, et al., 2016a, b). Broman et al. (1999) suggested a magmatic- 1910). This ore type is characterized by the occurrence of ap- hydrothermal process was responsible for the formation of atite together with high Fe grades (dominated by magnetite) the El Laco IOA deposit in Chile. Martinsson (2004) specu- and relatively high contents of V and low contents of Ti (e.g., lated that the IOA deposits in Kiruna formed from a process Frietsch, 1970; Parak, 1975). Since the first comprehensive similar to that indicated for El Laco involving the immiscibil- study elaborating on the origin of the Kiruna-type ores (Gei- ity of volatile-rich iron oxide melts producing magmatic flu- jer, 1910), debate on the genesis of IOA deposits has been ids giving rise to both magmatic and hydrothermal features. intense with little consensus on their genesis. The largest con- Recently, this view has gained support by workers studying troversy among the research community is whether the iron Andean examples (e.g., Knipping et al., 2015; Valesco et al., oxides crystallized from a melt or a hydrothermal fluid. Gei- 2016; Tornos et al., 2016), and a direct link between magmatic jer (1910) first suggested that the Kiirunavaara deposit was IOA and hydrothermal IOCG formation has been indicated formed from a magnetite-rich lava but later refined the model as possible (Reich et al., 2016). to an intrusive magmatic model based on the presence of an One rarely studied key parameter is the structural setting and subsequent structural evolution of IOA deposits, which †Corresponding author: e-mail, [email protected] has only been addressed by a few studies from the northern

ISSN 0361-0128; doi:10.5382/econgeo.4844; 29 p. Digital appendices are available in the online Supplements section. 1 Submitted: February 20, 2020 / Accepted: February 1, 2021

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Y Naimakka l e WA A r c t i c c i r c r Luleå NOR v e Karesuando FINLAND Luleå-Jokkmokk zone

KNDZ aledonian co C Fennoscandian shield Helsinki Stockholm Oslo

200km

Kiruna Silurian nappes (ca. 0.42 Ga) Orosirian plutonic rocks (ca. 1.89-1.78 Ga) Orosirian supracrustal rocks (ca. 1.89-1.86 Ga) Leveäniemi Rhyacian supracrustal rocks (ca. 2.2-2.1 Ga) Archean plutonic rocks (>2.5 Ga) Crustal scale deformation zones Fe-ore deposits and advanced prospects

PSZ Cu-Au deposits and advanced NDZ prospects

Pajala 40km Gällivare

Fig. 1. Generalized geology of northern Norrbotten highlighting Paleoproterozoic metasupracrustal belts. Modified after Andersson et al. (2020). Study area indicated by the black mark. Red box in the inset map shows the approximate outline of the geologic map. KNDZ = Kiruna-Naimakka deformation zone, NDZ = Nautanen deformation zone, PSZ = Pajala shear zone.

Norrbotten area (Vollmer et al., 1984; Wright, 1988; Bauer et Regional Geology al., 2018). In addition to some reported faults (Parak, 1969), Neoarchean granitoids and amphibolite rocks form the base- only one detailed structural description of an ore locality has ment of the Fennoscandian Shield (Gaal and Gorbatschev, ever been published from the Kiruna area (cf. figs. 4.3, 6.3 1987; Bergman and Weihed, 2020). In northern Norrbotten in Wright, 1988). Regional- to semiregional-scale structural (Fig. 1), the basement belongs to the Norrbotten nucleus, studies have been conducted in Kiruna or adjacent areas, but suggested to be one of three Neoarchean nuclei dispersed and these studies provide a somewhat intermittent assessment of reassembled during a rifting and collisional-accretionary cycle the characteristics and timing of the structural development during the Paleoproterozoic (e.g., Lahtinen et al., 2005). Con- (cf. Vollmer et al., 1984; Wright, 1988; Talbot and Koyi, 1995; tinental rifting during the Siderian to Orosirian (ca. 2.5–2.0 Bergman et al., 2001; Grigull et al., 2018; Luth et al., 2018a). Ga: Bergman and Weihed, 2020) caused regional-scale rift- In this predominately field based study, we aim to identify parallel fault systems, tholeiitic volcanism, and associated sed- key aspects of the Orosirian (ca. 1.9–1.8 Ga) stratigraphic col- imentation generating a large greenstone province stretching umn in Kiruna in order to understand the geologic and tec- from northern Norway to Russia (Pharaoh and Pearce, 1984; tonic conditions for the emplacement of IOA deposits. The Martinsson, 1997; Lahtinen et al., 2005; Melezhik and Hans- subsequent tectonic reworking of the area is then described ki, 2012; Hanski et al., 2014; Bingen et al., 2015). In northern using regional- and deposit-scale key localities. By this ap- Norrbotten, the greenstone belts occur as NNE- and NNW- proach, we aim to provide an up-to-date documentation and trending belts (Fig. 1) and are host to a number of metal de- interpretation of the structural evolution of the type locality posits (Martinsson, 1997; Bergman et al., 2001; Martinsson et for IOA deposits. al., 2016; Lynch et al., 2018). The overall goal of this study is to establish geometry, relative During the Paleoproterozoic, the early Svecokarelian cycle age, and sense of shear of the larger brittle-ductile structures (1.90–1.86 Ga) generated two suites of comagmatic plutonic- within the study area. We also aim to investigate the relation- volcanic rocks (Fig. 2): Haparanda Suite-Porphyrite Group ship between these structures, ore formation, and subsequent and Perthtite Monzonite Suite-Kiirunavaara Group (Bergman transposition, as well as to reevaluate the preshortening geo- et al., 2001; Martinsson, 2004). Haparanda Suite-Porphyrite logic setting responsible for the IOA emplacement. Group rocks predominate to the east and comprise calc-alka-

Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/doi/10.5382/econgeo.4844/5324457/4844_andersson_et_al.pdf by guest on 23 September 2021 IOA AND EPIGENETIC Fe AND Cu SULFIDES, KIRUNA, SWEDEN 3 ineralization M Andersson (2019), esthues et al. (2016), f et al. (1990), ? ? ? annhainen et al. (2005), rmation ? fo D2 D4 D3 D1 De eihed (2020), Sarlus et al. (2020). ? ? References Frietsch (1979), Skiöld (1986), Clif Romer et al. (1994), Martinsson (1997), Bergman (2001), Bergman et al. (2006), W Edfelt (2007), Smith, et al. (2009), W Sarlus et al. (2017), Bergman (2018), Bergman and W M2 M1 Metamorphism ) e iscaria-type Iron oxide-apatite Extension Iron oxide Cu-Au Isotope resetting of the Rb/Sr-system Metamorphic events (greenschist- to amphibolite facies) Brittle-ductile shortening Synexhalative Cu (V Porphyry Cu-Au-Ag-Mo Brittl trusive rocks In Mineralization Deformation Metamorphism ?

Supracrustal rocks

a t S Statherian rosirian Orosirian O yacian Rh a) Hauki quartzite Kiirunavaara group Porphyrite group Kiruna greenstone group Mafic dike intrusion Lina suite Edefors suite (TIB) Jyryjoki granite Perthite-monzonite suite Haparanda suite Supracrustal rocks Intrusive rocks ime (G 1.60 1.64 1.80 1.86 1.54 1.68 1.72 1.76 1.80 1.82 1.82 1.84 1.86 1.88 1.88 1.90 1.90 1.92 1.94 1.98 2.02 2.06 2.10 2.14 2.18 2.22 2.26 2.30 T

Kiirunavaara group Kiirunavaara oup gr rite hy rp Po Ca. 1.87 Ga e Ca. 1.85 Ga - a a e vi ka i amaa angas tzit ojär rmation rmation rmation rmation rmation rmation group Hauk Kiruna Hopuk fo fo fo fo Hosiovaar Quar Mat fo fo oussavaar Muotk Hosiok L Greenstone Kurravaara conglomerat e te at ra

nt e re te te s te ra ra va ra , agglomer ndesic , cohe ignimbrit te te eccia conglome a, tuff ra ra ac hya e e av e tu ff, ack e e e ack w lit dacic tuff w yllit ay enit enit yo yo eccia-Conglome eccia-Conglome lcanic, br ay eccia-conglome

Ar Br Ar Br Ph Conglome Gr Conglome Gr Br Basalc l Rh Rh Basalc, tr subvolcanic and la vo

N AI R ST HIGH Fig. 2. Chronostratigraphic summary of northern Norrbotten, modified after Andersson et al. (2020). Regional stratigraphy and central Kiruna as strati - igneous belt. graphic columns. TIB = Transscandinavian Orosirian central Kiruna strat. column

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line intermediate to felsic volcanic to volcaniclastic rocks and et al. (2020) observed syntectonic growth of porphyroblas- related dioritic to granodioritic intrusions. To the west, sho- tic hornblende aligned with an S1 fabric indicating that peak shonitic mafic to felsic Kiirunavaara Group volcanic and vol- metamorphism occurred during D1 deformation in that area. caniclastic rocks related to Perthite Monzonite Suite gabbroic, Caledonian (Fig. 1) overprinting processes do show up in monzonitic, and granitic intrusions predominate (Bergman et some geochronological data sets as, e.g., lower intercept ages al., 2001; Martinsson, 2004). Late Svecokarelian (1.81–1.78 Ga) in U-Pb zircon data (Billström et al., 2019, and references magmatism comprises I- to A-type plutonic rocks (e.g., Edefors therein) but are generally regarded as having a low impact on Suite; Fig. 2), that form part of the Transcandiavian igneous recorded metamorphic grade, structure, and alteration. U-Pb belt, which stretches from northwestern Norway to southern ages of stilbite from open fractures (D4 in Bauer et al., 2018) Sweden (Andersson, 1991; Åhäll and Larson, 2000; Weihed et in the Malmberget IOA deposit yield 1730 ± 6.4 Ma, indicat- al., 2002; Högdahl et al., 2004; Rutanen and Andersson, 2009), ing that temperatures remained below 150°C from that time as well as associated S-type granite (Lina Suite; Fig. 2). and that relatively stable tectonic conditions have remained Mineralizing events during the Orosirian coincide with the up until today (Romer, 1996). early and late cycles of the Svecokarelian orogeny (e.g., Bill- The structural subsurface architecture of this part of the ström et al., 2010). For example, IOA and porphyry-style Cu- Fennoscandian Shield is the result of a polyphase deforma- Au-Ag deposits (PCDs) were formed in the time interval from tion history with deformation events approximately coincid- 1.89 to 1.87 Ga (Fig. 2; cf. Romer et al., 1994; Wanhainen ing with the early and late magmatic cycles of the Svecok- et al., 2009; Martinsson et al., 2016; Westhues et al., 2016). arelian orogeny (Fig. 2). Prominent NW- to NE-trending IOCG deposits also formed during the early Svecokarelian, crustal-scale deformation zones tend to spatially coincide with but the timing of the mineralization events is not as tightly Rhyacian-Orosirian metasupracrustal belts that took up the constrained, with ages around 1.86 Ga (Fig. 2) or slightly majority of strain in northern Norrbotten (Fig. 1). On region- younger (e.g., Smith et al., 2007; Martinsson et al., 2016). In al-scale aeromagnetic maps (Bergman et al., 2001), magnetic contrast, ore deposits that formed during the late Svecokare- lineaments form an approximately N-directed undulating pat- lian cycle are primarily restricted to structurally controlled tern wrapping around intrusive bodies. At least two phases IOCG deposits with ages at ca. 1.80 to 1.78 Ga (e.g., Fig. 2; of folding can be recognized (e.g., Wright, 1988; Bergman et Edfelt, 2007; Billström et al., 2010; Martinsson et al., 2016). al., 2001; Bauer et al., 2018; Grigull et al., 2018; Andersson et The only IOA deposits linked to the late Svecokarelian cycle al., 2020). The early phase (D1) generated a heterogeneous- in northern Norrbotten are the Tjårrojåkka and Saivo deposits ly developed, regional, penetrative continuous fabric. D2 is dated at approximately 1780 and 1758 Ma, respectively (Ed- characterized by strong strain partitioning into deformation felt, 2007; Martinsson et al., 2016). However, these deposits zones, whereas outside of these zones, tectonic foliation was are anomalous in age, and the role of late-cycle IOA formation only sparsely developed and, in Gällivare and west of Kiruna in Norrbotten remains an unresolved question. (Fig. 1), accompanied by brittle components (cf. Bauer et al., The metamorphic evolution of the northern Norrbotten 2018; Andersson et al., 2020). Similar deformation systemat- area is poorly constrained but is considered to be of low to me- ics have been recorded from the Skellefte district (Bauer et dium pressure-temperature (P-T) Buchan style (Bergman et al., 2011; Skyttä et al., 2012; Bauer, 2013) where the minimum al., 2001; Tollefsen, 2014; Skelton et al., 2018). Metamorphic age of D1 is constrained by a U-Pb zircon age at 1874 ± 4 Ma key mineral assemblages and limited geothermometry data (D2 in Skyttä et al., 2012). In northern Norrbotten, Hellström (Bergman et al., 2001) suggest that the metamorphic grade (2018) reports a maximum age at 1878 ± 3 Ma for folding east increases from west to east from greenschist facies to upper of Kiruna, which was interpreted to represent the age of mig- amphibolite facies conditions, but age constraints are lacking. matization due to early Svecokarelian contact metamorphism. However, several studies indicate that regional metamorphic Other estimates of the timing of early deformation are broadly grades can be linked to the early Svecokarelian cycle, whereas limited to field observations in relationship to geochronologi- low-P high-T conditions predominated during the late cycle. cal data of early-cycle plutonic rocks (Bergman et al., 2001) The effect of contact-metamorphic aureoles around early- and and dikes (Cliff et al., 1990) indicating a minimum deforma- late-cycle intrusive rocks on the metamorphic systematics in tion age at 1.88 Ga; however, contradicting field relationships Norrbotten is unknown but may be a significant contributor to are present (Luth et al., 2018a). Timing of the late-cycle de- the regional P-T variations (cf. Monro, 1988; Bergman et al., formation is generally attributed to the intrusion of the ap- 2001; Tollefsen, 2014; Hellström, 2018; Skelton et al., 2018). proximately 1.8 Ga syntectonic Lina Suite (Fig. 2; Bergman In the Aitik Cu-Au-Ag deposit near Gällivare (Fig. 1), Monro et al., 2001). In the Gällivare area (Fig. 1), Wanhainen et al. (1988) reports amphibolite facies M1 conditions at 520° to (2005) report Re-Os ages at 1850 and 1765–1750 Ma for de- 600°C and 3 to 5 kb based on garnet-biotite geothermometry formed and undeformed pegmatite-aplite dikes, respectively, and Si-Al content in hornblende associated with garnet. The representing maximum and minimum ages for late-cycle de- M1 metamorphism was subsequently overprinted by a hydro- formation, which agrees with field observations in nearby ar- thermal event at ca. 1.78 Ga estimated at 200° to 500°C and eas (Lynch et al., 2015; Bauer et al., 2018). 1 to 2 kbar (Wanhainen et al., 2012). At the Malmberget IOA deposit, Bauer et al. (2018) observed a gneissic amphibolite Local Geology facies S1 fabric folded without the development of an axial The stratigraphic column of the central Kiruna area plane-parallel S2 cleavage in the resultant F2 synform, thus, interpreted as D2 taking place at higher crustal levels com- Since the discovery of the Kiirunavaara deposit, the Kiruna pared to D1 (Bauer et al., 2018). West of Kiruna, Andersson area has been mapped several times. The first 1:50,000 maps

were produced by the Geological Survey of Sweden (SGU) tion gave rise to a volcanic-volcano-sedimentary-sedimentary during the mid-1960s (Offerberg, 1967), but mapping cam- sequence composed of rhyolitic tuffs and ignimbrites (Geijer, paigns and petrographic studies provided a rather holistic 1950; Frietsch, 1979), which are conformably overlain by ba- understanding much earlier (e.g., Geijer, 1910; Lundbohm, saltic agglomerates, tuffs, and lavas (Martinsson, 2004) and 1910). followed by heterolithic breccia conglomerates (Andersson et The central Kiruna area (Fig. 3A-C) constitutes the best- al., 2017), lithic graywackes, and a phyllitic uppermost hori- preserved, continuous Rhyacian-Orosirian sequence in Nor- zon (Lundbohm, 1910; Frietsch 1979). The volcanic sequence rbotten (Figs. 2, 3A-C). The sequence was emplaced on top of of the Orosirian Kiirunavaara Group in Kiruna was formed in the Neoarchean tonalitic-granodioritic Råstojaure Complex a short time interval of no more than 15 m.y. (Westhues et al., (Skiöld, 1979; Martinsson et al., 1999). The basement rocks 2016). The volcanic period was followed by sedimentation of cover vast areas north of Kiruna and constitute one of the crossbedded arenites interrupted by horizons of sedimentary least known geologic domains in Sweden. The depth to the breccia conglomerates situated at its basal and middle parts basement increases toward the south and has been estimated (Figs. 2, 3A-C), together referred to as the Hauki quartzite based on reflection seismic investigations (Holmgren, 2013), (Martinsson, 2004). The Hauki quartzite can be correlated re- as well as modeling of gravimetric, magnetic, and petrophysi- gionally to similar units (Bergman et al., 2001) and marks the cal data (Luth et al., 2018a) to range between 2 and 3.5 km end of the Orosirian in the study area. in Kiruna. The mineralogy and chemistry (e.g., Geijer, 1910; Parak, In Kiruna, NE- to NNE-trending greenstone belts rep- 1975; Perdahl and Frietsch, 1993; Westhues et al., 2016) and resent the basal parts of the Paleoproterozoic stratigraphic minimum crystallization ages (e.g., Cliff et al., 1990; Romer column. The Rhyacian pile is subdivided into a lower and an et al., 1994; Westhues et al., 2016) of the volcanic rocks are upper unit: the predominately sedimentary Kovo Group and well studied in Kiruna and many attempts have been made the predominately volcanic Kiruna Greenstone Group, re- to correlate these volcanic rocks to the geologic and tectonic spectively (Martinsson, 1997). The latter hosts the syngenetic development of northern Norrbotten (e.g., Witschard, 1984; Viscaria Cu deposit west of Kiruna and the epigenetic Pahto- Perdahl and Fritsch, 1993; Martinsson, 2004; Martinsson et havare Cu-Au deposit (Figs. 1, 2) in the south (Lindblom et al., 2016). However, the development of the metasedimentary al., 1996; Martinsson, 1997). rocks (Ödman, 1972; Frietsch, 1979; Wright, 1988; Kumpu- The Orosirian stratigraphy in central Kiruna (Fig. 2) was lainen, 2000; Ladenberger et al., 2017) has not been given the broadly established during the early 20th century and consti- same attention despite the information these rocks provide tutes an E-dipping sequence younging to the east (Lundbohm, regarding the geologic evolution in one of the important min- 1910). The only fundamental disagreement raised during the ing districts in Europe. In the “Orosirian stratigraphy” sec- subsequent 110 years of geologic research is regarding the po- tion, we present field descriptions of the metasedimentary sition of the basal horizon, the Kurravaara conglomerate, and units that will serve as a background when the preshortening its relationship to the underlying Rhyacian Kiruna greenstone geologic history is discussed in the “Basin development” sec- group (cf. Geijer, 1910; Ödman, 1957; Frietsch, 1979; Forsell, tion under “Synthesis of the Orosirian Structural Evolution.” 1987; Wright, 1988; Martinsson et al., 1993; Kumpulainen, The volcanic rocks will only be briefly described here, as their 2000). The Kurravaara conglomerate has been interpreted as mineralogical and chemical character has been covered previ- a molasse that formed in an emergent thrust zone (Wright, ously (e.g., Geijer, 1910; Parak, 1975; Frietsch, 1979; West- 1988; Talbot and Koyi, 1995) or, alternatively, as an alluvial hues et al., 2016). fan deposit (Kumpulainen, 2000). The clasts originate from This paper follows the nomenclature used by Martinsson the underlying Kiruna greenstone group (e.g., Forsell, 1987) (2004) and accepted by the Geological Survey of Sweden and calc-alkaline intermediate-felsic volcanic rocks presumed (Bergman, 2018). We aim to keep descriptions in accordance to derive from the Porphyrite Group (Martinsson and Per- with the recommendations by the Committee for Swedish dahl, 1993). On top of the Kurravaara conglomerate, the Ki- Stratigraphic Nomenclature (Kumpulainen, 2016; Kumpulai- irunavaara Group (Fig. 2) includes trachyandesitic subvolcanic nen et al., 2017). rocks and lavas (Hopukka Formation) followed by rhyodacitic tuffs and subordinate breccia conglomerates (Luossavaara Ores Formation) that constitute the footwall and hanging-wall The supracrustal rocks in Kiruna host a wide range of de- rocks, respectively, to the Kiirunavaara and Luossavaara IOA posit types. A tuffitic unit of the middle part of the Rhyacian deposits (Fig. 3A-C; Martinsson, 2004; Martinsson and Hans- greenstones (Kiruna greenstones; Martinsson, 1997) hosts son, 2004). A ~5-km-long horizon of IOA mineralization, or the syngenetic exhalative Viscaria Cu deposit (Martinsson, sometimes apatite only (Geijer and Ödman, 1974), is found 1997; Martinsson et al., 1993; Masurel, 2011). Ten km south at the contact between the Luossavaara Formation and an of Viscaria, the epigenetic Pahtohavare Cu-Au deposit occurs overlying rhyolitic ignimbrite unit (Frietsch, 1979). Tradition- within the same formation (Viscaria Formation) as Viscaria ally, the rhyolitic ignimbrite has been described as the “Rektor (Martinsson et al., 1993; Lindblom et al., 1996; Martinsson, porphyry” (e.g., Geijer, 1950) and constitutes the lowest mem- 1997). Both Viscaria and Pahtohavare were mined during the ber of the Matojärvi Formation (Martinsson, 2004). The Ma- 1990s. The IOCG-style Rakkurijärvi iron oxide sulfide brec- tojärvi Formation (Figs. 2, 3A-C) marks a change in the geo- cia mineralization (Smith et al., 2007) is an advanced explo- logic development of the area, as the stratigraphy turns into ration project located a few kilometers east of Pahtohavaare more volcano-sedimentary-sedimentary in character. Explo- and hosted by Orosirian metavolcanic rocks. Beyond these sive bimodal volcanism, rapid erosion, and turbulent deposi- deposits, several others are mentioned and/or described by,

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6 ANDERSSON ET AL. 7535000 h A

tion of dip , saw teet verse shear al size: 1.22 Re zones in direc and on upthrown block rv te L n: 146 Max: 34 In 732000 syncline 0 S Late crenulation L n: 11 tion ical rt with inclination with plunge 0 with inclination S turned anticline er ounging direc Cleavage Bedding Early syncline Cleavage ve Anticline Lineation L Ov High strain/bedding Y F n: 12 58 77 40 55 50 55 75 80 A1b 70 g.

. Fi

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irunavaar Ki β A 60 717000 oliation al size: 1.07 onic f rv Iron oxide apatite-deposit ct te Te g n: 123 Max: 25 In Beddin f, agglomerate fs 3 km Fig. 3. A) Structural/lithological map of the central Kiruna area. Lithological contacts modified after Offerberg (1967) and Martinsson and Perdahl (1993). Coordinates: (1967) and Martinsson Perdahl modified after Offerberg area. Lithological contacts map of the central Kiruna Fig. 3. A) Structural/lithological Sweref99. B) Geologic map of the area Loussavaara/Rektor to Nukutus. Lithological contacts modified after Martinsson and Perdahl (1993). Coordinates: C) Conceptual cross sections based on surface data along profiles A-B and C-D in Figure 3A, B. f, ignimbrite al size: 0.99 Arenite, breccia-conglomerate Phyllite Graywacke Breccia-conglomerate Basaltic lava, tuf Rhyolitic tuf Rhyodacitic pyroclastic and coherent, breccia-conglomerate Basalt, trachyandesite Conglomerate, graywacke Basaltic lavas, tuf rv te

n: 654 Max: 123 In 7545000 Hauki quartzite Matojärvi formation Hopukka formation Kurravaara conglomerate Kiruna greenstone group Luossavaara formation Orosirian Rhyacian

C B 720000 75 722000 Fig. 5A, D, 13B 80 Fig. 10A-E D 7540800

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Fig. 6A 7537600

a 40 Fig. 12A, B App. Fig. A2c Fig. 9A

Fig. 7G 500 m

Mineralization Orosirian Bedding S0 with inclination Stratigraphic up Hauki quartzite Iron oxide-apatite Hauki hematite Breccia-conglomerate Luossavaara formation Cleavage S2, inclined/vertical Brittle fault Arenite Rhyodacitic pyroclastic and coherent volcanic Matojärvi formation rocks, breccia-conglomerate Reverse shear zones, Hopukka formation Lineation L2, with plunge saw teeth in direction Phyllite Andesite, basalt of dip and on upthrown Graywacke F3 synclinal with plunge Kurravaara conglomerate block Breccia-conglomerate Conglomerate, graywacke

Basaltic lava, tuff, agglomerate Rhyacian Formline high strain S2/ bedding S0 Rhyolitic tuff, ignimbrite Basaltic lavas, tuffs

Fig. 3. (Cont.)

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AB C 0m

? ?

-2000m

0m CD

? ? ?

-2000m Orosirian Hauki quartzite Hopukka formation Main shear zones Breccia-conglomerate Basaltic lava, tuff, agglomerate Andesite, basalt Cleavage Arenite Rhyolitic tuff, ignimbrite Kurravaara conglomerate High strain Matojärvi formation Mineralization Conglomerate, graywacke Bedding Phyllite Iron oxide-apatite Rhyacian Sense-of-shear Graywacke Luossavaara formation Basaltic lavas, tuffs Breccia-conglomerate Rhyodacitic pyroclastic and coherent volcanic rocks, breccia-conglomerate

Fig. 3. (Cont.)

for example, Grip and Frietsch (1973), Frietsch (1997), Berg- tabular-shaped Luossavaara magnetite-apatite deposit occurs man et al. (2001), and Martinsson et al. (2016). (e.g., Geijer, 1910). The Luossavaara deposit is approximately The most famous deposit in Kiruna is the >2,500 million 700 m long and up to 40 m thick and comprises 20 Mt of ore tonne (Mt) Kiirunavaara magnetite-apatite deposit. It consti- (Grip and Frietsch, 1973). It was mined in both open pit and tutes a 5-km-long and up to 100-m-thick, moderately steep underground until mining ceased in 1985 (Hallberg, 2005). (60°–70°), E-dipping tabular body (Grip and Frietsch, 1973). The Per Geijer iron ores comprise five hematite-magnetite- The magnetite-apatite body is situated at the contact between apatite deposits, namely Rektor, Henry, Nukutus, Haukivaara, the trachyandesitic Hopukka Formation (footwall) and the and Lappmalmen. They form a rather large system of IOA rhyodacitic Luossavaara Formation (hanging wall). Both for- mineralization (Geijer and Ödman, 1974). The orebodies mations are brecciated by the ore (Geijer, 1919). As of 2020, are situated at the contact between the Luossavaara Forma- the magnetite-apatite body is mined from a main haulage level tion and the Matojärvi Formation or hosted by the Matojärvi at 1,365-m depth, and mineral reserves are reported at 616 Formation (Figs. 2, 3B, C). Lappmalmen is a blind orebody Mt at 42% Fe (proved and probable reserves; Luossavaara-Ki- known from drilling and possibly represents a deep-seated irunavaara AB, 2019). To the north along the same contact, the continuation of smaller deposits at the surface (Parak, 1969).

During the 20th century, 9 Mt were produced in open pits and through the volcanic pile. This hydrothermal event can be underground operations at 43.25% Fe from the Per Geijer temporally correlated with U-Pb zircon ages at 1875 ± 5 and iron ores (Grip and Frietsch, 1973). 1876 ± 7 Ma from altered hanging-wall rocks at Kiirunavaara The timing of IOA emplacement in Kiruna is constrained and Rektor, respectively (Westhues et al., 2016). Furthermore, with reported crystallization ages at 1888 ± 6 Ma (U-Pb ti- it is possible that also the 1854 ± 18 U-Pb allanite age (Smith tanite; Romer et al., 1994), 1878 ± 4 Ma (U-Pb titanite; Mar- et al., 2009) and the 1859 ± 2 Ma rutile age (Martinsson et tinsson et al., 2016), and 1877 ± 4 and 1874 ± 7 Ma (U-Pb al., 2016) at the Rakkurijärvi Cu-Au (IOCG) deposit corre- zircon; Westhues et al., 2016) that overlap within error. late to this early hydrothermal event but more likely indicate a younger and temporally separate event. However, much of Metamorphism and hydrothermal alteration the geochronological data from this geologic period overlaps In detail, the metamorphic evolution of the Kiruna area is within errors, leaving the actual number of hydrothermal- poorly understood and only covered by a few studies (cf. Fri- magmatic events during this early phase an unresolved ques- etsch et al., 1997; Bergman et al., 2001). No P-T data is cur- tion (cf. Cliff et al., 1990; Romer et al., 1994; Smith et al., rently available from central Kiruna, but key metamorphic 2009; Westhues et al., 2016). minerals (actinolite, chlorite, albite, epidote, hornblende) Younger hydrothermal ages have been recorded at 1718 in mafic rocks indicate greenschist facies or rarer lower am- ± 12, 1623 ± 23 (Blomgren, 2015; Andersson et al., 2016), phibolite facies conditions (Bergman et al., 2001). Skiöld and 1738 ± 19, and 1628 ± 12 Ma (Westhues et al., 2017) by U-Pb Cliff (1984) present a Sm-Nd isochron age at 1932 ± 45 Ma monazite data from Rektor and Kiirunavaara. Cliff and Rick- of amphibole, titanite, and plagioclase in a greenschist min- ard (1992) report a secondary Pb-Pb isochron age at 1540 ± 7 eral association hosted by the underlying greenstones, but Ma for pyrite overprinting the Kiirunavaara orebody, which is the relevance of this age in respect to metamorphism further similar to reset Sm-Nd isotope ages (Cliff and Rickard, 1992). up in the stratigraphic column in central Kiruna remains un- Ages between 1600 and 1500 Ma are ambiguous in northern known. In low-strain blocks, the supracrustal rocks in central Norrbotten, with disturbed Rb-Sr ages reported regionally Kiruna show a high degree of preservation of primary volcanic (Fig. 2; e.g., Welin et al., 1971). These much younger ages and sedimentary features (e.g., Geijer, 1910; Frietsch, 1979; do not correlate to any known magmatic event in Norrbotten, Martinsson, 1997; Kumpulainen, 2000; Bergman et al., 2001). and the meaning of these ages is poorly understood. The high level of preservation makes the area particularly use- ful for geologic interpretations, which can be used as a proxy Structural geology when similar areas in more high-grade metamorphic terrains The structural grain in Kiruna is steeply E-dipping and strikes are studied further east in Norrbotten. north to northeast (Fig. 3A, B). It is dominated by a heteroge- The Kiruna area is characterized by alkali and calcic-iron neously developed cleavage trending subparallel to lithologi- alteration spatially and temporally related to IOA and IOCG cal contacts. Bedding generally shows uniform attitudes over mineralizations (e.g., Martinsson et al., 2016; Westhues et al., large distances and between rock units, and only few areas 2016). The stratigraphically lower Kiirunavaara and Luos- show clear evidence of folding (Fig. 3A; Wright, 1988; Grigull savaara (magnetite-apatite) deposits are different from the et al., 2018; Andersson, 2019). In low-strain blocks, cleavage Per Geijer iron ores (hematite-magnetite-apatite) in terms is steeper than bedding and strikes counterclockwise from of their associated hydrothermal alteration (e.g., Geijer, bedding, indicating westward vergence (Wright, 1988; Grig- 1910; Parak, 1975; Martinsson and Hansson, 2004; Martins- ull and Jönnberger, 2014) of the E-dipping supracrustal stack. son, 2015; Martinsson et al., 2016; Westhues et al., 2016). LS-tectonites characterize the tectonic structures in Kiruna The trachyandesitic footwall rocks to the Kiirunavaara and (Wright, 1988). Luossavaara deposits are affected by sodic-calcic alteration in The central Kiruna area is adjacent to a steep, NNE-trend- terms of volumetrically dominant, pervasive albite and more ing, crustal-scale deformation zone, the Kiruna-Naimakka localized massive actinolite (Geijer, 1910; Martinsson, 2004; deformation zone (Fig. 1; Bergman et al., 2001). Based on Martinsson and Hansson, 2004; Martinsson et al., 2016). Hy- SC fabrics near Naimakka (Fig. 1), Bergman et al. (2001) drobreccias dominated by magnetite-actinolite are common interpreted west-side-up kinematics for this zone (fig. 54C features of these deposits, extending tens of meters into the in Bergman et al., 2001). In contrast, ~20 km northeast of hanging wall (Martinsson and Hansson, 2004). In contrast, Kiruna, Luth et al. (2018a) reported east-side-up kinematics the Per Geijer iron ores show a potassic-dominated hydro- due to northeast-southwest to east-west shortening during the thermal overprint (Martinsson, 2015; Martinsson et al., 2016; inversion of Rhyacian intracontinental rift basins. In central Westhues et al., 2016) represented by K-feldspar, quartz, ser- Kiruna and the open pits, shear zones with similar north- icite, calcite-ankerite, chlorite, and minor tourmaline, which northeast orientations are present (Fig. 3A, B) and trend par- is strongest developed in the hanging-wall rocks (Martinsson, allel to lithological contacts, but these structures have never 2015). Albite occurs only sparsely, and actinolite has never been investigated in detail, and their relationship to the Kiru- been reported from the Per Geijer iron ores. na-Naimakka zone remains uncertain. Time constraints on hydrothermal mineral assemblages in The relationship between the IOA deposits and deforma- Kiruna indicate that at least four temporally separated hy- tion in central Kiruna is an unresolved question. Early studies drothermal events can be recognized. Hydrothermal titanite- of the open pits did not recognize the ductile tectonic struc- actinolite-calcite assemblages from the Luossavaara footwall tures, and most geologic features were interpreted as primary rocks yield a titanite U-Pb age of 1876 ± 9 Ma (Romer et magmatic (e.g., Geijer, 1950). When the role of ductile de- al., 1994) and represent the earliest documented fluid flow formation was recognized, the orebodies were interpreted as

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megaboudins (Vollmer et al., 1984) or continuous sheets oc- for detailed structural investigation and sampling but provides cupying limbs and hinge zones of overturned folds (Forsell, limited exposures under the pit lakes. By using this approach, 1987). Faults have been mapped and subscribed as a control- we hope to show the most probable preshortening setting in ling factor on shape and position of some ore lenses (Parak, Kiruna and to clarify the relative timing and kinematics of the 1969), but the character of these faults is unknown. Wright subsequent deformation as well as the role of deformation on (1988) provides the only publicly available account of a de- shape of the orebodies. formed magnetite-apatite ore in Kiruna, where a boudinaged section of the Luossavaara deposit was described in detail Methodology (cf. figs. 4.3, 6.3 in Wright, 1988). Later studies describe the Geologic mapping was conducted between 2016 and 2018 hanging-wall rocks to the Rektor deposit as strongly sheared covering the study area as well as adjacent areas in order to (Bergman et al., 2001; Martinsson, 2015; Andersson, 2019) provide regional background data for comparison. One ori- and folded (Grigull et al., 2018). ented 1,173-m drill core (PG81619) intersecting the Mato- There is no generally accepted structural model for the cen- järvi Formation was mapped in order to obtain information on tral Kiruna area, but most models include approximately east- contact relationships to and within the Matojärvi Formation west shortening (Vollmer et al., 1984; Forsell, 1987; Wright, as well as structural variations. A total of 847 outcrop obser- 1988; Talbot and Koyi, 1995; Grigull et al., 2018; Andersson, vations were performed, and 1,127 structural elements were 2019). Wright (1988) introduced a WNW-directed fold-thrust measured. All structural measurements were collected using model (D1-D2 in Wright, 1988) with a relative timing syn- a Brunton Geo Pocket Transit, and all data were digitized in chronous or slightly postemplacement of the IOA deposits field on a ruggedized iPad mini device using the Field Move followed by subsequent folding (D3) and shearing (D4) with application (formerly Midland Valley Exploration Ltd., cur- unknown shortening direction and timing. The fold-thrust rently Petroleum Experts Ltd.). All lineations were measured model presented by Wright (1988) gained support by Talbot as the pitch on planes and recalculated into true orientation and Koyi (1995) but was rejected by Bergman et al. (2001) using the Geo Calculator software (Holocombe, Coughlin, based on the steep dips of the inferred thrusts. Vollmer et al. Oliver, Valenta Global). For magnetic rocks, the strikes of pla- (1984) argued for one single episode of WNW-directed short- nar structures were estimated using known points in the ter- ening manifested by folding around a fold axis plunging 60° to rain. Structural analysis was performed using the Move 2017 the south-southeast, a view supported by Grigull et al. (2018). software package (Petroleum Experts, Ltd.), whereas maps Grigull et al. (2018) add an additional component of contin- were constructed using ArcMap 10.1 (ESRI). Stereographic ued east-west shortening responsible for the inversion of the plots were produced as lower-hemisphere, equal-area ste- Hauki quartzite, interpreted as a graben structure in agree- reographic projections using Move 2017 and Dips 7.0 (Roc- ment with Witschard (1984). Both west-side-up (Bergman et science). Sixty-eight oriented samples were collected and cut al., 2001) and east-side-up senses of shear (Wright, 1988; Tal- perpendicular to foliation and parallel with lineation in order bot and Koyi, 1995; Grigull et al., 2018) have been interpreted to allow for kinematic interpretations. The samples were sent for the central Kiruna area; however, little evidence is at hand to Vancouver Petrographics Ltd. for thin-section preparation. (cf. Wright, 1988, Grigull et al., 2018). Petrography and microstructural investigations were per- The timing of deformation in Kiruna is ambiguous. An formed using a conventional petrographic polarization micro- 1880 ± 3 Ma U-Pb zircon age of an undeformed crosscutting scope equipped with a digital camera (Nikon Eclipse E600 granophyre dike at the Kiirunavaara IOA deposit has been POL). Kinematic interpretations on macroscopic structures suggested to represent the minimum age of the NS-oriented were performed in parallel with lineation. tectonic grain (Cliff et al., 1990). Field observations of the Two-dimension-forward modeling using Move 2019 soft- 1.88–1.86 Ga plutonic rocks (Perthite Monzonite Suite; Berg- ware package (Petroleum Experts Ltd.) was performed. Dif- man et al., 2001) have been interpreted both as syn- (Vollmer ferent fault geometries and shear styles were tested using the et al., 1984) and late- to postregional D1 (Wright, 1988; Talbot 2D Move-on-Fault functions applying 800 m of reverse sim- and Koyi, 1995; Bergman et al., 2001), but components sup- ple shear with 90° shear angle displacing a synthetic layer cake porting both interpretations are present (Luth et al., 2018a). model with estimated layer thicknesses. The results were then The purpose of this study is to reevaluate the structural compared to the conceptual cross sections based on surface setting responsible for the emplacement of the IOA depos- data to verify that subsurface geometries can be reconstructed its in central Kiruna and the subsequent tectonic overprint. from modeling. The stratigraphy is reexamined to delineate between the in- A 40-cm core sample (drill core PG81619) with a diameter ferred fold-thrust belt (Wright, 1988; Talbot and Koyi, 1995) of 51 mm was analyzed by X-ray computed tomography (CT) and Orosirian graben development (Witschard, 1984; Grigull and X-ray fluorescence (XRF) in tandem at the Orexplore AB et al., 2018). Structural mapping and microstructural analy- facility in Kista, Sweden. The CT images are presented with en- sis are used to constrain the number of deformation events hanced grayscales to increase visibility of the structural features. (cf. Vollmer et al., 1984; Wright, 1988; Grigull et al., 2018), kinematics (cf. Wright, 1988; Bergman et al., 2001; Grigull Results et al., 2018), and the role of deformation on the shape of the orebodies (cf. Vollmer et al., 1984; Wright, 1988). The Per Orosirian stratigraphy Geijer iron ore field and the Luossavaara orebody are used as In the following section, the prefix “meta” for metamorphic case examples because they provide excellent exposures in the rocks is avoided in order to simplify the text and to emphasize open pits. The open pits are not in production, which allows protoliths. The stratigraphic column is summarized in Figure 2.

The Kurravaara conglomerate constitutes the lowermost C), hematite dominates the clast material (App. Fig. A2f), Orosirian stratigraphic unit in central Kiruna. Based on drill whereas in other parts, hematite clasts are normally present core observations, the contact between the Kurravaara con- in subordinate amounts (App. Fig. A2g). The clasts are gener- glomerate and the overlying Hopukka Formation is grada- ally stretched with aspect ratios of up to 1:8 (App. Fig. A2h), tional (Martinsson et al., 1993), whereas the lower contact, and in highly strained sections the matrix is often altered into where exposed, is sharp (App. Fig. A1a). The clasts consist of sericite schist. The clast shape is subangular to subrounded Rhyacian basalt and intermediate to felsic porphyric volcanic with sizes ranging from pebble to boulder with sizes up to 30 rocks, and the conglomerate is moderately to poorly sorted cm. The breccia conglomerate is moderately to very poorly (Fig. 4A); however, local clast size gradations are present sorted and generally matrix supported (Fig. 4H; App. Fig. (App. Fig. A1b). Within the Kurravaara conglomerate, phyl- A2e, g, h), but clast-supported sequences (App. Fig. A2f) are lonite horizons (App. Fig. A1c) as well as graywackes with locally present. The matrix is composed of grayish to reddish developed bedding (App. Fig. A1d) do occur but are volu- lithic graywacke, but in some localities, hematite constitutes metrically subordinate. The same lithic graywacke forms the the matrix (App. Fig. A2i). The stratigraphic top of the Mato- matrix in the volumetrically dominant gravelly horizons of the järvi Formation is composed of lithic graywacke that shows a unit, but phyllosilicate-dominated matrices also occur in al- weak to intense schistosity and a selective-pervasive bedding- tered high-strain sections. Both clast-supported (Fig. 4A) and parallel carbonate alteration (Fig. 4I) followed by a thin hori- matrix-supported (Fig. 4B) conglomerates are common. zon of intrafolial phyllite showing abundant drag folds related The Hopukka Formation consists of basalt and trachyan- to flanking structures (Fig. 4J). desite that form the footwall to the Kiirunavaara and Luos- The Hauki quartzite (Figs. 2, 3A-C) is the youngest unit in savaara deposits. Amygdules (App. Fig. A1e) are common as the central Kiruna area. It varies from reddish arkosic to light- well as albite alteration, either by common selective-pervasive gray quartz-arenite, and the two variants grade into each other. (patchy) reddish albite together with magnetite (Fig. 4C) or However, locally, sharp contacts between the two types are pervasive whitish albite alteration (App. Fig. A1f). Farther up present (App. Fig. A3a). The unit is characterized by a well- in stratigraphy, the Loussavaara Formation is generally de- developed bedding marked by iron oxides (App. Fig. A3b), fined by a reddish fine-grained rock carrying reddish feldspar and crossbedding is widespread (Fig. 4K). Two horizons of phenocrysts and xenoliths of the Hopukka Formation (Fig. poorly sorted alluvial breccia conglomerate (Fig. 4L) occur at 4D), but grayish groundmasses carrying whitish feldspar phe- the base and middle positions. The clast sizes range from ~0.5 nocrysts also occur frequently along with aphyric tuffite (App. to 50 cm; they are locally derived from the Hopukka, Lous- Fig. A1g). Within the lower part of the Luossavaara Forma- savaara, and Matojärvi Formations and are generally matrix tion, poorly sorted polymict breccia conglomerate occurs (Fig. supported (App. Fig. A3c) with minor clast supported sections 4E) in a ~30- × 100-m lensoid geometry (Fig. 3B). The clasts (App. Fig. A3d). Erosional contacts indicate that the Hauki consist of aphyric, light-colored, and darker intermediate vol- breccia conglomerate is in general semiconformable with the canic rocks (App. Fig. A1h), most probably derived from the surrounding bedding (~15°; App. Fig. A3e), but local erosional Hopukka Formation, suggesting a local origin. The clast shape discordances of up to 60° (App. Fig. A3f) have been measured. is angular to subrounded ranging from pebbles to boulders up to 30 cm in size. The unit is both clast and matrix supported Structural characteristics (cf. App. Fig. A1h, i) varying from outcrop to outcrop. The In the following section, the character of recognized tectonic matrix is composed of lithic graywacke grading into a grayish structures is described. Relative timing of these structures, feldspar-phyric felsic volcanic rock. their tectonic relevance, and their role for mineralized sys- The Matojärvi Formation (Figs. 2, 3A-C) constitutes the tems are interpreted and clarified in the “Synthesis of the hanging wall to the Per Geijer iron ores. The rocks are gen- Orosirian Structural Evolution” section. erally tectonized and define a heterogeneous sequence of Foliation: In low-strain blocks, a heterogeneously devel- volcanic, volcano-sedimentary, and sedimentary rocks. The oped, continuous and steeply E-dipping cleavage is present stratigraphically lowest unit of the Matojärvi Formation is (Fig. 3A). In competent volcanic or arenitic rocks, the low- composed of a red aphyric quartz-rich rhyolite tuff (App. Fig. strain cleavage is defined by feldspar ± quartz ± biotite ± am- A2a). A mylonitized compositional banding is generally pres- phibole ± calcite. Where strain increases, chlorite ± biotite ± ent in the rhyolite interpreted as bedding S0 (Fig. 4F), which albite ± quartz ± calcite defines mylonitic fabrics in mafic vol- locally grades into a quartz matrix affected by a characteristic canic rocks (Fig. 5A). In felsic to intermediate volcanic rocks selectively pervasive (patchy) K-feldspar alteration (App. Fig. as well as in volcano-sedimentary and breccia conglomerate A2b). On top of the rhyolite tuff, basaltic agglomerate (Fig. units, the mylonitic cleavage is defined by sericite + quartz ± 4G), tuffite (App. Fig. A2c), and subordinate coherent basalt calcite ± chlorite (Fig. 5B). Mylonite zones show bulging or (App. Fig. A2d) are deposited. Within the Matojärvi Forma- subgrain rotation dynamic quartz recrystallization (Fig. 5C; tion, several polymict breccia conglomerates (Fig. 4H) occur Passchier and Tourow, 2005). In the north, the cleavage of that display a wide range of clast compositions. Hematite, the Hauki quartzite is oriented axial plane parallel to folded felsic feldspar-phyric and aphyric volcanic rocks, intermedi- bedding planes (Fig. 3A) associated with bulging recrystalliza- ate feldspar-phyric and aphyric volcanic rocks, basalt, jasper, tion quartz textures. In the south, the bedding orientation is chert, and quartz occur as clast material in different amounts. uniform, and the mean principal orientation of the low-strain Locally, portions of the clast material consist of a rheologi- cleavage (76/114°) as well as the high-strain cleavage (75/107°) cally weak aggregated whitish and reddish material (App. Fig. is slightly steeper and rotated anticlockwise in respect to the A2e). In spatial association with the Rektor orebody (Fig. 3B, mean principal orientation of the bedding (56/127°). Howev-

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Fig. 4. Field characteristics of the supracrustal rocks in central Kiruna. A) Poorly sorted clast-supported conglomerate, Kurravaara conglomerate, X721289 Y7542973. B) Matrix-supported conglomerate, Kurravaara conglomerate, X722854 Y7544000. C) Albite + magnetite-altered trachyandesite, Hopukka Formation, X720060, Y7540349. D) Coherent rhyodacitic rock carrying reddish plagioclase phenocrysts and a xenolith of the Hopukka Formation, X719292 Y7537862. E) Sedimen- tary breccia conglomerate carrying angular clasts of albite-altered rocks, Loussavaara Formation, X719906 Y7539676. F) Compositional banding in rhyolite tuff indicating bedding S0. Parallel mylonitic cleavage produces a composite S0/Scleavage fabric, Matojärvi Formation, X719913 Y7537868. G) Basaltic agglomerate, Matojärvi Formation, X720625 Y7540755. H) Poorly sorted mixture of felsic to mafic rheologically weak and competent clasts, Matojärvi Formation, X720673 Y7540413. I) Graywacke showing selective-pervasive bedding-parallel carbonate alteration, drill core: PG81619. J) Intrafolial sericite- chlorite-quartz phyllite forming a series of drag folds related to flanking structures, drill core: PG81619. K) Crossbedding in arenite, Hauki quartzite, X721310 Y7540105. L) Polymict breccia conglomerate at the middle position, X721276 Y7539954. Coordinates: Sweref99. Ab = albite, Mgn = magnetite.

Fig. 5. A) Calcite + chlorite + sericite-dominated high-strain fabric, Nukutus hanging wall, X720588 Y7540948. B) SCC´ fabric, Henry hanging wall, X720622 Y7540475. C) Micrograph of bulging (BLG) dynamic quartz recrystallization, Nuku- tus hanging wall, X720623 Y7540756. D) Nukutus hanging-wall ore contact with abrupt change in strain intensity, X720683 Y7540957. E) Weakly developed cleavage in the Loussavaara ore, X719540 Y7538570. F) Micrograph of subgrain rotation (SGR) and grain boundary migration (GBM) dynamic recrystallization of quartz in the Rektor ore, X719863 Y7537886. G) Apatite bands in the Rektor ore, X719806 Y7537991. H) Deformed apatite in the Rektor ore, X719867 Y7537866. I) Quartz vein showing pinch-swell, Henry hanging wall, X720622 Y7540475. Coordinates: Sweref99. Apt = apatite, Mgn = magnetite.

er, in outcrop scale, the high-strain cleavage often subparallels Lineation: Stretched minerals and clasts define the most the bedding, indicating transposition of bedding in association pronounced ductile lineation in central Kiruna. Clasts may to shearing. show an aspect ratio of up to 1:8 (App. Fig. A2h) in highly Cleavage subparallels the orebodies. The strain intensity strained units, but the stretching lineation is more commonly in the host rock is normally high in association with the ores seen as a subtle stretching of feldspar or quartz on foliation but drops sharply in the ores (Fig. 5D). Only in rare cases, surfaces. The mineral lineation parallels the stretching linea- the IOA orebodies show a developed foliation, and in those tion and is defined by biotite ± chlorite or sericite on foliation cases, it is defined by flattened iron oxides (Fig. 5E), dynami- planes. Along the Matojärvi Formation, the stretching linea- cally recrystallized quartz showing subgrain rotation or grain tion steepens from south to north. Within the Rektor open boundary migration textures (Fig. 5F), or apatite bands (Fig. pit and southward (Fig. 3B), the stretching lineation plunges 5G) that tend to show geometries resembling boudins (Fig. moderately steeply toward the south, whereas north of the 5H). Pinch-and-swell structures are occasionally associated Rektor open pit, the stretching lineation shows steep to verti- to highly strained rocks in central Kiruna. In the hanging-wall cal plunges (Fig. 6A). This indicates a kinematic change from rocks of the Henry deposit, a sericite + quartz + calcite + an oblique-slip-dominated system in the south to a dip-slip- chlorite mylonite fabric wraps around a quartz vein, showing dominated system toward the north. However, the stretching pinch and swell parallel to steep stretching lineation (Fig. 5I). lineation in the Luossavaara open pit is steep and plots similar

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A B

Lstretching Lslickensides Rektor and southward

North of Rektorn

Luossavaara open pit n: 74 n: 121

Fig. 6. Lower hemisphere, equal-area stereographic projections of lineations of the central Kiruna area. A) Stretching lineation colored by area. B) Slickensides colored by area.

to the stretching lineation north of Rektor (Fig. 6A) implying as well as a steep E-dipping (76/136°) high-strain cleavage that the kinematic change from north to south is only valid for south of the study area indicating the orientation of the ba- the structures within the Matojärvi Formation and does not saltic tuffite as discordant in relationship to the mean princi- reflect the area as a whole. pal bedding orientation (56/127°) of the Hauki quartzite. In Slickensides on fracture planes are common throughout the south, the Loussavaara Formation is brought into contact the area. They are commonly developed on chlorite-altered with the Hauki quartzite. At that contact (Fig. 3B), protomy- fracture planes in competent volcanic units. Stepped surfaces lonitic rocks of the Loussavaara Formation show dip angles show calcite ± malachite ± azurite in pressure shadows but (38/107°) parallel with the bedding of the Hauki quartzite only rarely provide high-confidence kinematic information. (38/078°). Minor stratigraphic repetitions also occur at the Slickensides plot in poorly defined clusters (Fig. 6B) in re- contact between the Hauki quartzite and the Matojärvi For- spect to the stretching lineation (Fig. 6A), but they show a mation where meter-scale sequences of mylonitic phyllite of steepening trend toward the north along the Matojärvi For- the Matojärvi Formation occurs sandwiched between brecci- mation similar to that of the stretching lineation and also plot ated arenite of the Hauki quartzite. similar to the stretching lineation in the Loussavaara open pit The kinematics of the shear zones are consistently reverse (cf. Fig. 6A, B). oblique- to dip-slip-dominated showing east-block-up sense Shear fabrics and kinematics: Proto- to ultramylonites are of shear from north to south. This is indicated by kinematic developed throughout the central Kiruna area forming cen- indicators such as rotated delta (Fig. 7C) and sigma (Fig. 7D, timeter-scale to tens-of-meters-wide shear zones commonly E) porphyroclasts/sigmoids, SCC´ fabrics (Fig. 7F, G), and oriented largely concordant to lithological units. Intercon- oblique foliation (Fig. 7H). We have also observed micro- necting discordant splay structures linking up with the larg- structures resembling magnetite fish (Fig. 7I) showing a con- er structures are present in the Matojärvi Formation (Fig. trasting sense of shear relative to the area as a whole. These 3A, B). The most prominent shear zones are developed at structures are not well established (Passhier and Trouw, 2005), lithological contacts and within the Kurravaara conglomer- and it is unclear whether any reliable kinematic information ate, Matojärvi Formation, and the breccia conglomerates of can be obtained from them in the Kiruna area. The sigmoidal the Hauki quartzite (Fig. 2) where rheologically weak rocks shape of the clast as well as parts of the quartz in pressure dominate. Strain partitioning is significant producing sharp shadows in Figure 7I indicates dextral sense of shear, but a contacts between highly strained rocks and low-strain blocks significant pure shear component seems to be present, which in centimeter (Fig. 7A) to district scale. Competent volcanic complicates the interpretation of this microstructure. rocks and orebodies adjacent to high-strain rocks commonly Folds: A relatively early fold sequence is present in the show weakly developed or no fabrics. Instead, brittle defor- northernmost part of the mapped area (Fig. 3A), where pillow mation dominates in the low-strain units, occasionally devel- basalt of the Rhyacian Kiruna greenstone group and the Orosi- oping Riedel fractures mimicking the kinematics of spatially rian Kurravaara conglomerate form an open synclinal synform related shear zones (Fig. 7B). cored by the Hauki quartzite. Mesoscale folds with a related Only one major stratigraphic repetition occurs in the area, axial plane-parallel amphibole + biotite + plagioclase continu- evident by the superposition of a Rhyacian basaltic tuffite ad- ous cleavage is present (Fig. 8A, B) within the pillow basalt, jacent to the Orosirian Hauki quartzite (Fig. 3A-C). This de- showing measured fold axes plunging to the north-northeast formation zone is poorly exposed at the surface but has been (75/020°) or moderately steeply to the southwest (45/220°). intersected by drill holes, showing the contact as mylonitic. The form line defining the overturned anticlinal fold shape in The discordant orientation of this structure is supported by a the northeast (Fig. 3A) is interpreted based on ground mag- vertical (88/296°) cleavage measurement in the basaltic tuffite netic data, and the fold shape is constrained by bedding ori-

Fig. 7. A) Strong strain partitioning in Matojärvi basalt, Rektor hanging wall, X719977 Y7537830. B) Reverse brittle-ductile shear zone, Loussavaara footwall, X719214 Y7537725. C) Delta porphyroclast, Hauki breccia conglomerate, X721083 Y7534873. D) Sigma porphyroclast, Luossavaara footwall, X719433 Y7538499. E) Sigma porphyroclast, Kurravaara conglomerate, X722857 Y7544205. F) SCC´ fabric, Luossavaara footwall, same locality as Figure 7D. G) SC fabric, Luossavaara Formation near the contact to the Hauki quartzite, X720485 Y7536368. H) Oblique foliation in calcite domain, Matojärvi basalt, Henry hanging wall, X720648 Y7540460. I) Magnetite fish? Kurravaara conglomerate, same locality as Figure 7E. Coordinates: Sweref99.

entations and stratigraphic-up indicators. The northernmost cleavage is spaced (Fig. 9B, C), and the resultant microfolds part of the Hauki quartzite (Fig. 3A) gives access to a series of are open with a bluntness ranging from concentric in sericite- near-symmetrical upright folds with a calculated S0/S0 β-axis dominated parts (Fig. 9C) to kinked in chlorite-dominated plunging moderately steep to the south-southwest (55/213°). parts (Fig. 9B). The related crenulation lineation plunges Where exposure is high enough for fold characterization, syn- shallowly to steeply toward the east-northeast with a mean clinal Ramsey class 1C folds (Fig. 8C) have been recorded. principal orientation of 39/070° (Fig. 9D), which we interpret Parts of the topography are controlled by bedding, allowing as the fold axis (Fig. 3A, B). The fabric of the shear zone af- the use of high-resolution LIDAR data to support the extrap- fecting the Per Geijer iron ores is undulating, and both the olation of some of the bedding form lines (Fig. 3A) in line with Rektor and Henry orebodies exhibit a gently bent geometry Grigull and Jönnberger (2014). (Fig. 3B; Geijer, 1950; Parak, 1969). The β-axis of the cleavage Late gentle folding deforms the tectonic foliation and shear measurements from the Rektor open pit plots subparallel to fabrics in central Kiruna. Crenulation of mylonitic sericite the crenulation lineation measured throughout the study area (Fig. 9A) and (Fig. 9B) chlorite domains affects the lithostruc- (Fig. 9D). The cleavage-cleavage intersection point of a gently tural boundaries, where the Haukivaara and Loussavaara de- folded 1-m section of the Nukutus hanging wall also subparal- posits are located (Fig. 3B), as well as the breccia conglomer- lels the crenulation lineation (Fig. 9E, F), implying that the ates of the Hauki quartzite (Fig. 9C). The related crenulation fold pattern repeats in different scales.

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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

10cm 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 0

S 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 brec- C cia 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

10c m 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) Scleavage 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.

0 Faults: In the Haukivaara open pit, a moderately E-dipping S

B 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 me- relationships at fold bend of regional synclinal early fold, Rhyacian basalt, X722966 Y7545142. C) S-plunging upright parasitic

10cm ter-thick zone of the gouge is poorly lithified and consists of

cleavage sand- to gravel-sized particles mixed with hematite-rich clay, -S 0 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-duc-

tile conditions; however, brittle components dominate. This is

e supported by drill core observations 3.5 km northward along

S avag

cle the same structure, where smaller stratigraphic repetitions are present and where there is an interplay between mylonitic S0 rocks and brittle fracturing. South of the Haukivaara open pit, rocks of the Luossavaara Fig. 8. Field characteristics of relatively early folds observed in the northern part of the map sheet (Fig. 3A). A) NNE-plunging asymmetric parasitic fold, Rhyacian pillow asymmetric early folds observed in the northern part of map sheet (Fig. 3A). A) NNE-plunging of relatively Fig. 8. Field characteristics basalt, X723750 Y7545244. B) S synformal fold, Hauki quartzite, X722706 Y7543333. Coordinates: Sweref99. 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°) protomy-

A lonitic felsic volcanic rocks yield reverse dip-slip-dominated sense of shear, thrusting the Luossavaara Formation on top of

W E E

S cleavage crenulatio S

ion

t Screnulatio nula n n n

crenulatio

Scleavage

cleavage S0/S

A 2cm B 50µm C W 100µm

N S

SRektor

β SFig. 9E Lcren Lcren

n:12 n:114 D E 10cm F

Fig. 9. A) Field characteristics and orientation of relatively late folds and related crenulation. A) Crenulation of S0/Scleavage composite fabric in Matojärvi phyllite, Haukivaara hanging wall, X720324 Y7537170. B) Crenulation with kinked bluntness in a chlorite domain, Luossavaara footwall, X719286 Y7538023. C) Crenulation with concentric bluntness in sericite domain, Hauki breccia conglomerate, X721083 Y7534873. D) Relationship between the cleavage-cleavage β-axis at Rektor open pit and crenulation lineation. E, F) Relationship between the cleavage-cleavage intersection point of undulating cleavage (Nuku- tus hanging wall, X72060 Y7540747) and crenulation lineation. Coordinates: Sweref99.

the stratigraphically overlying Matojärvi Formation (Figs. 3B, and concentrated into axial planar fracture planes offsetting 7G) along dip angles subparallel to the bedding in the Hauki the bedding (Fig. 13F). The highest concentration of pyrite is quartzite. Similar orientation correlations for ductile defor- observed in the associated fold hinge (Fig. 13E) and a shear mation and brittle faulting are also present in the Rektor and band (Fig. 13G) bounding the fold. Henry open pits. In the Rektor hanging wall, a smaller stratigraphic repetition is caused by a fault paralleling oblique-re- Synthesis of the Orosirian Structural Evolution verse mylonitic rocks of the Matojärvi breccia conglomerate of the Kiruna area ~100 m east of the pit (Fig. 3B). Associated oblique structures resembling reverse duplex structures (Fig. 12C) would yield Basin development reverse kinematics of the fault, mimicking the overall sense of The Orosirian stratigraphy in central Kiruna is dominated by shear of the area; however, the associated structures may also conglomeratic rocks at its base followed by bimodal mafic- represent Riedel structures, implying that the kinematics of felsic volcanic rocks, subsequently changing character to the Rektor fault remains unresolved. volcano-sedimentary/sedimentary rocks followed by a final Sulfide-bearing structures: In competent volcanic rocks stage of sandstone deposition (Figs. 2, 4). Such a stratigraphic and the ore, secondary copper carbonates and visible Fe and record indicates a volcanic-sedimentary basin changing from Cu sulfides are restricted to brittle structures or rounded a predominantly volcanic to a predominantly sedimentary amygdule-like infills in different mineral associations. For character with time. The volcanic rocks occupying the lower- example, quartz + calcite + hematite + magnetite + chalco- middle sequence of the basin host magnetite-apatite ores, pyrite + malachite (Fig. 13A) or malachite (Fig. 13B) occur whereas the ores at higher stratigraphic levels are more oxi- in fractures and veins. In the Henry hanging wall, bornite + dized magnetite-hematite-apatite ores. High-strain intensities hematite + magnetite is hosted by a vein crosscutting perva- are spatially associated with competent orebodies (Fig. 5D), sively K-altered Matojärvi rhyolite parallel with the dominant whereas cleavage is only sparsely developed in the ores (Fig. NS-trending tectonic grain in the area (Fig. 13C). 5E). This, together with dynamic recrystallization of quartz X-ray CT imaging of a 40-cm section of mylonitized Ma- (Fig. 5F), indicates that deformation affected the competent tojärvi phyllite in drill core shows a variety of structural orebodies (Passchier and Trouw, 2005), hence providing field traps possible for sulfides in rheologically weak rocks. In the evidence for a preshortening timing of IOA emplacement in studied sample, the bedding is openly to tightly folded into central Kiruna (Fig. 14). antithetic flank folds (Fig. 13D, E) with an orientation that The Orosirian contact to the underlying Rhyacian green- accords with the overall structural grain in the area. In low- stones is sharp (App. Fig. A1a) and weakly discordant (Mar- strain sequences, pyrite is distributed along bedding planes tinsson, 1997). The timing of deposition of the Kurravaara

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W E W E W E

Progressive transposition A 2.5cm B 2.5cm C 2.5cm

SSW NNE WNW ESE

V2 V1

2 V 1 V

D 1 cm E 1 cm

Chlorite high strain zone

Mgn dike

Breccia F 1m G 1m H 5 cm

Fig. 10. A-C) Progressive transposition of a hydraulic breccia carrying Fe and Cu sulfides, Nukutus hanging wall, X720627 Y7540845. D) Horizontal section of the hydraulic breccia in A-C). E) Vertical section of the hydraulic breccia, same sample as in Figure 10D. F, G) Monolithic magnetite breccia bounded by magnetite dikes and a high-strain chlorite zone, X719514 Y7538501. H) Close-up of the breccia zone in Figure 10F, G. Coordinates: Sweref99. Mgn = magnetite.

conglomerate is unknown but is bracketed between the crys- subsequently buried under lithic graywacke and siltstones and tallization of the calc-alkaline clasts (probably locally derived a rather thick package of crossbedded sandstones. The timing from the 1.90–1.87 Ga Porphyrite Group; Martinsson et of deposition of the final arenitic sequence is critical because al., 1993, 2016) and the overlying ~1.88 Ga Hopukka For- it determines the duration of the basin development. Laden- mation (Martinsson et al., 2016). The contact between the berger et al. (2017) presents detrital U-Pb zircon data for the Kurravaara conglomerate and the overlying volcanic rocks is Hauki quartzite suggesting a wide time span between 1917 poorly exposed. However, the overlapping ages reported for and 1752 Ma for a probable maximum deposition age, making the Porphyrite Group (1.90–1.87 Ga) and Kiirunavaara Group conclusions hard to draw at the current stage of research. (1.88–1.86 Ga) rocks (Fig. 2), together with the gradational The supracrustal pile contains sedimentary polymict brec- upper contact of the Kurravaara conglomerate (Martinsson cia conglomerates that occur repetitively in the stratigraphic et al., 1993), indicate a geologic continuum in the basal part record. These horizons are, from stratigraphic bottom to top, of the Orosirian Kiruna stratigraphy. The deposition of the the Kurravaara conglomerate (Fig. 4A, B), the Luossavaara bimodal mafic-felsic volcanic units was fast (<15 m.y.; West- breccia conglomerate (Fig. 4D), the Matojärvi breccia con- hues et al., 2016) including the emplacement of IOA orebod- glomerate (Fig. 4H), and the Hauki breccia conglomerates ies mainly at two stratigraphic positions. The volcanic pile was (Fig. 4L). The volumetrically largest of these units is the

cleavage

/S 0 S n Mg Ap t C C W 5cm

1.5c m

0 vage

l c S z+Ca Qt n

B B Mylonitic Mg E W l 1m Ca 2.5cm z e Qt Mylonit e Fig. 11. A) Late hydrothermal overprint manifested as quartz + calcite hosted by veins, Loussavaara open pit, X719432 Y7538382. B) Quartz breccia carrying xenolith, Rektor open pit, X719860 Y7537883. C) Apatite vein crosscutting the orebody and deformation, X719864 Y7537877. Coordinates: Sweref99. Apt = apatite, Cal calcite, Mgn magnetite, Qtz quartz. hanging Haukivaara quartzite, Hauki planes, fault bedding-parallel between Mylonitization Y7537219. B) X720377 wall, hanging Haukivaara fault, Reverse 12. A) Fig. wall, X720404 Y7537264. C) Fault with associated second-order structures, Rektor hanging X719926 Y7537885. Coordinates: Sweref99. zit i qua rt A A ault goug e F E Hauk

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Qtz+Cal+Hem+Mgn+Ccp+Mal Mgn Matojärvi rhyolite

Hem Mal

Bn+Hem

AB1cm 1cm C 4cm

Bedding

Cleavage old-axis F Quartz + sericite Pyrite Chlorite

D 1cm E 1cm

Bedding e Shear band

ture plan ac Quartz + sericite Fr Pyrite Chlorite Quartz + sericite Pyrite Chlorite F 1cm G 1cm

Fig. 13. A) Quartz + calcite + hematite + magnetite + chalcopyrite + malachite in brittle fracture, Rektor orebody, X719849 Y7537928. B) Fracture-hosted malachite, Nukutus orebody, X720678 Y7540955. C) Vein-hosted bornite + hematite, Mato- järvi rhyolite, Henry hanging wall, X720571 Y7540524. D) Flank fold bounded by shear band in Matojärvi phyllite, drill core: PG81619. E) Computed tomography (CT) image (enhanced grayscale) showing the distribution of pyrite (red) in relationship to the fold in Figure 13D. Modified after Andersson et al. (2019). F) CT image (enhanced grayscale) showing pyrite (red) in relationship to axial planar fracture planes, same sample as in Figure 13D, E. Modified after Andersson et al. (2019). G) CT image (enhanced grayscale) showing the distribution of pyrite (red) in relationship to a shear band, same sample as Figure 13D, F. Modified after Andersson et al. (2019). Bn = bornite, Cal = calcite, Ccp = chalcopyrite, Hem = hematite, Mal = malachite, Mgn = magnetite, Qtz = quartz. lowermost unit, the Kurravaara conglomerate. The charac- subrounded, suggesting short but variable water transport ter of these epiclastic rocks varies in respect to clast and ma- distances. (3) They are in general poorly sorted. Bedding trix composition as well as geometry. For example, Rhyacian and clast gradations are developed in most units, but these greenstone clasts are restricted to the Kurravaara conglomer- features are rare and broadly suggest rapid deposition and ate, and the matrices of the Hauki breccia conglomerates are burial. (4) The breccia conglomerates show both clast- and quartz arenitic in contrast to the other breccia conglomerates matrix-supported characteristics, indicating both high- and with lithic graywacke as a matrix. Furthermore, in contrast lower-energy depositional processes. On this basis, we inter- to the general absence of developed bedding planes in these pret these breccia conglomerates as having been deposited in deposits, bedding is locally well developed in the Kurravaara similar alluvial settings and suggest these deposits represent conglomerate (App. Fig. A1d). Geometries of the epiclastic erosional peaks driven by synvolcanic normal faulting during deposits vary from small (30 × 100 m) and lensoid-shaped the basin development. to concordant (or weakly discordant) horizons (Fig. 3A, B). However, many commonalities are present that indicate simi- Basin inversion lar processes generating these epiclastic rocks. For example, During subsequent crustal shortening, the basin environment (1) they are all polymict with clasts of diverse but local origin hosting the IOA orebodies was inverted (Fig. 14). Based on with sizes ranging from pebble to boulder (up to 30–50 cm the orientation and kinematics of the tectonic structures in in diam). (2) The shape of the clasts ranges from angular to Kiruna, an east-west to northwest-southeast crustal shorten-

+ + + ? + + ? + + +

Basin development Basin inversion * Extension. * E-W crustal shortening. * Synvolcanic normal faults generating alluvial * Brittle-ductile reverse oblique to dip-slip reactivation of breccia-conglomerates. normal faults. * Deposition of bimodal volcanic rocks and sedimentary * East-side-up sense of shear. successions. * Brittle-ductile shear zone development along concordant * IOA emplacement along synvolcanic faults and lithological contacts mimicking the overal sense of shear. lithological contacts. * Hydraulic fracturing and transpostion of horizons * Shallow crustal conditions. and modification of layer thicknesses. * Slightly assymetric west-verging folds with moderately steep S- and N-plunging fold-axes. Or, near- symmetric upright folds with moderately steep S-plunging fold-axes. * Modification of ore geometries. * Structural entrapment of Fe- and Cu-sulfides. * Greenschist facies metamorphism.

+ + + + ? ? + + + +

Refolding Fracturing * NNW-SSE to N-S crustal shortening. * Remobilization of apatite producing apatite veins. * Gentle folds with steep to shallow * Calcite + quartz hydraulic fracturing east-plunging fold axes. * Crenulation of sericite and chlorite domains. * Modification of ore geometries.

Fig. 14. Summary of the deformation scheme presented in this study.

ing is inferred—a shortening direction typically subscribed prominent deformation in the area constitutes moderate to to the late ca. 1.80 Ga Svecokarelian cycle (e.g., Andersson, steep NNE-striking reverse oblique to dip-slip brittle-ductile 1991; Bergman et al., 2001; Weihed et al., 2002; Lahtinen et high-strain zones developed at lithological contacts, as well al., 2005; Bauer et al., 2018; Andersson et al., 2020). The most as in favorable lithologies of the Kurravaara conglomerate,

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Matojärvi Formation, and the Hauki quartzite (Fig. 3A-C). In lithostructural boundaries where most strain accumulated a broader context, the Matojärvi Formation constitutes one during the inversion. single second-order high-strain zone (Fig. 3C) in the larger Based on microstructures and orientation of stretching lin- inversion architecture. The orebodies served as rigid bodies, eations (Figs. 6, 7), shear zone structures show systematic re- and the sections that reach economic thicknesses are proba- verse dip-slip or reverse oblique-slip movements thrusting the bly lineation-parallel boudins of the mineralized horizon (Fig. east block up toward the west. East-block-up sense of shear 3C), as was suggested by Vollmer et al. (1984), and in line with is also indicated by the only major stratigraphic repetition in mesoscale boudinage and pinch-and-swell structures (Fig. the study area, where a slice of a Rhyacian basaltic tuffite is 5I). In this respect, the structural style of the ore deposits in- juxtaposed on top of the younger Hauki quartzite (Fig. 3A, vestigated in this study shows similarities to the boudinaged B) as well as a minor stratigraphic repetition juxtaposing the Malmberget IOA deposit (Bauer et al., 2018). Loussavaara Formation above the younger Matojärvi Forma- Two-dimension-forward modeling of a synthetic layer cake tion (Fig. 3B). Oblique movements predominate to the south, model (Fig. 15A) with estimated layer thicknesses shows that whereas dip-slip movements predominate to the north within parts of the interpreted subsurface geometries in cross sec- the Matojärvi Formation and Kurravaara conglomerate. How- tions (Fig. 3C) can be reconstructed. Reverse simple shear ever, the steep movements recorded by the lithostructural reactivation of listric faults with 800-m displacement produc- boundary between the Hopukka and Loussavaara Formations es a steepening and thinning of layers toward the fault due at similar latitudes as oblique-slip movements in the Mato- to transposition of horizontal layers into the fault orientation järvi Formation do not accord with a systematic northward (Fig. 15B, C). In order to produce the dip angles and a jux- steepening for all the shear zones and need to be explained taposition of Rhyacian rocks adjacent to the Hauki quartzite, otherwise. The kinematic changes along and across the shear several listric faults are required (Fig. 15D), indicating that zone system is ambiguous, and we suggest two alternatives: a series of faults were reactivated during the basin inversion. (1) strain partitioning, caused by the competent hematite- The modeling results accord with our observations of reverse magnetite orebodies, gave rise to both dip-slip and oblique- layer-parallel shearing and bulging layer thicknesses (Fig. 3C) slip movements in different parts of the system synchronously and suggest that the controlling structures originated as nor- or (2) the deformation event is characterized by a cyclic activ- mal listric faults. ity activating different structures and parts of structures indi- In low-strain blocks in the north (Fig. 3A), a heteroge- vidually, with different kinematics. neously developed, steeply E-dipping, penetrative, continu- The interplay between ductile and brittle structures dur- ous cleavage is oriented axial plane parallel to folds formed ing deformation is important in Kiruna. The shear zones show under an overall east-west crustal shortening (Fig. 8). The both brittle and ductile features (e.g., Fig. 7B). Faults and folds in the northern part of the study area are open to tight. mylonites with mimicking orientations and kinematics (cf. Associated parasitic folds are closed to tight, slightly asymmet- Figs. 3B, 7G, 12A, B) indicate a spatial-temporal association rical, and west verging (Fig. 8A) or classified as symmetrical between brittle faults in competent units and shearing devel- and upright Ramsey class 1C type (Fig. 8C). The series of oped in rheologically weak units. Slickensides plot in poorly folds developed in the northern part of the Hauki quartzite defined clusters (Fig. 6B) with respect to the stretching linea- occur in an area that is squeezed between shear zones at the tion (Fig. 6A). However, slickensides show a steepening trend lithostructural boundaries (Fig. 3A). We interpret these folds toward the north in accordance with the stretching linea- to have developed as a response to reverse shearing at the tion, indicating that brittle and ductile linear structures were

A B C D

Fig. 15. 2D-forward modeling including estimated layer thicknesses and reverse reactivation of synthetic listric faults. A) Horizontal layer cake model and listric fault with no displacement. B) 400-m reverse offset along the listric fault. C) 800-m offset along the listric fault. D) 800-m offset along a second listric fault.

formed under the same stress regime and related in space and ometry, indicate that the undulating character of the ore field time. The poorly defined clusters in the slickenside plot can is the result of later refolding modifying the crustal architec- be explained on the basis of the varying orientations of the ture, including ore geometries. The resultant structures are fracture planes along which the movements occurred. The subtle and are best developed in sericite and chlorite domains orientation of the foliation planes showing stretching linea- of the shear zones (Fig. 9A-C), whereas regionally penetrative tions is more consistently oriented, resulting in a tighter clus- fabrics are lacking. The lack of penetrative structures and/or tering of the stretching lineation. strain accumulations into associated high-strain zones as well Hydraulic breccias at the Nukutus (Fig. 10A-E) and Lous- as the gentle fold hinges associated to the refolding indicate savaara (Fig. 10F-H) open pits are spatially associated to weak deformation, probably during the veining stages of the structures formed during the basin inversion and indicate the crustal shortening. However, no strike-slip reactivations of the importance of hydraulic fracturing during the deformation NNE-striking shear zones have been recorded in this study, event. The observed breccias probably reflect different stages implying that the rotation of the stress field was abrupt rather of the breccia evolution. The occurrence in Nukutus is inter- than continuous and that the refolding event was probably preted to have formed by fluid-assisted brecciation during the separated by a tectonic pause from the dominant basin inver- propagation stage (Jebrak, 1997), whereas the Luossavaara sion event. occurrence formed by corrosive wear during the later dila- In the Kiirunavaara underground mine, Berglund and tion stage (Jebrak, 1997), allowing fragment transportation Andersson (2013) identified several NS-trending strike-slip in the evolved hydrothermal breccia. In Nukutus, the inter- fracture planes explained by an overall NS-directed crustal nal vein configuration with two sets of veins deforming each shortening. In the Pajala area (Fig. 1), D3 fault structures are other (Fig. 10D, E) can be explained on the basis of reverse compatible with a north-northwest–south-southeast shorten- shearing in the hanging wall (Fig. 10A-C). In the case of the ing direction (Luth et al., 2018b). It is possible that the brittle breccia in the footwall to the Luossavaara deposit (Fig. 10F- strike-slip movements in the Kiirunavaara underground mine H), the timing of brecciation relative to plastic deformation and the faulting in Pajala represent the same regional defor- is more ambiguous because of the absence of defined vein mation event as the gentle refolding of the central Kiruna networks. However, the Luossavaara breccia pipe is bounded area indicated by this particular study, but further research is by a parallel chlorite high-strain zone (Fig. 10F), making a needed to determine the possible temporal linkage. tectono-hydrothermal origin and a syndeformational timing of the breccia as the likely scenario. Fracturing In Kiruna, Fe and Cu sulfides associated with Fe oxides are The inverted and refolded basin was further fractured during hosted by structures formed by crustal shortening. In com- a last identified deformation event associated with hydrother- petent rocks, sulfides occur in brittle structures such as frac- mal activity (Fig. 14). Quartz + calcite hydrothermal breccias tures and veins (Fig. 13A-C), and copper deposits close to the in the Rektor and Loussavaara open pits (Fig. 11A, B), as well study area (Bergman et al., 2001; Smith et al., 2007; Martins- as discordant reddish apatite veins at the Rektor (Fig. 11C) son et al., 2016) show similar structural associations. In rheo- and Nukutus deposits, crosscut all other fabrics. The quartz logically weak rocks, the structural traps are more varied in + calcite hydrothermal breccias overprint the apatite veins; style, as revealed by CT images showing different structural hence, the apatite veins are the earlier feature, implying that sulfide relationships in the Matojärvi phyllite (Andersson et two separate deformation events may be reflected in the ob- al., 2019; Fig. 13D-G). The CT results suggest that pyrite was served structures. We interpret this hydrothermal breccia remobilized and transported along brittle axial planar frac- stage as the last major geologic event based on crosscutting ture/cleavage planes and accumulated in a fold hinge and a relationships and because the veins do not show any clear shear band syncrustal shortening. Furthermore, the Nukutus signs of tectonic reworking. fluid-assisted breccia (Fig. 10A-E) carries sparse pyrite and The age of the last hydrothermal fracturing is of importance chalcopyrite hosted by veins interpreted in this study to have because it brackets the time frame of the geologic evolution been formed in response to reverse shearing. Because of the generating the present crustal architecture in central Kiruna. very low sulfide content of the hematite-magnetite orebodies, The hydrothermal U-Pb monazite ages at 1718 ± 12, 1623 ± we suggest that much of the visible Fe and Cu sulfide was in- 23 (Blomgren, 2015; Andersson et al., 2016), 1628 ± 12, and troduced into the system during east-west crustal shortening. 1738 ± 19 Ma (Westhues et al., 2017) obtained from samples This would imply that a superimposed mineralizing event re- at the Rektor and Kiirunavaara deposits constitute candidates sponsible for epigenetic Fe and Cu sulfides and Fe oxides can to bracket the age of this hydrothermal fracturing event(s). At be linked to the inversion phase of the basin evolution, hence, the Malmberget IOA deposit, ~120 km southeast of Kiruna, in contrast to the IOA deposits linked to the synextensional Romer (1996) obtained U-Pb titanite and monazite ages at ca. basin development. 1740 and 1620 Ma from open fractures carrying low-temperature mineral assemblages involving apatite, stilbite, calcite, Refolding and biotite. The open fractures at Malmberget represent the The inverted basin was gently refolded during a later deforma- last deformation event of the Malmberget IOA deposit (Bau- tion event, and the resultant structures are in accordance with er et al., 2018). Rb-Sr whole-rock data collected regionally an overall north-northwest–south-southeast to north-south (e.g., Welin et al., 1971) show ages similar to those of the hy- crustal shortening (Fig. 14). The orientation of the associated drothermal titanite-monazite from Kiruna and Malmberget, crenulation lineation and the calculated cleavage-cleavage which highlights the regional significance of this last deforma- β-axis of the Rektor deposit (Fig. 9D), exhibiting a bent ge- tion event dominated by hydraulic fracturing.

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Discussion associated with this early phase of rifting constitute candidates for being the controlling structures also in the case of later Basin development extensional events during the Orosirian by normal fault reac- Basin development in Kiruna during the Orosirian is not a tivation. Hence, we further suggest that the Orosirian volcanic new hypothesis; however, earlier workers (Witschard, 1984; rocks were deposited in a rift environment already developed Grigull et al., 2018) have only tentatively suggested it with- during the Rhyacian and that the succeeding Orosirian crust- out presenting evidence. Witschard (1984, p. 292) suggested al thinning, manifested by, e.g., mafic intrusions of this age that the Hauki quartzite was deposited “in a tectonically active (Sarlus et al., 2017, 2018), occurred in the intracontinental graben” and Grigull et al. (2018) briefly suggested a hypoth- back-arc regions during the Svecokarelian early-cycle oroge- esis of inversion of this same graben structure, interpreted to ny. Such a tectonic setting and evolution shares many similari- have been eroded down into the Matojärvi Formation. We ties to other IOA/IOCG districts of widely different ages. For have not found evidence in support of or against the existence example, the Bafq district in central Iran has been indicated of an erosional contact between the Matojärvi Formation and as a back-arc basin, coeval with convergence along the proto- the Hauki quartzite, as suggested by Grigull et al. (2018). In- Tethyan margin during the Neoproterozoic to early Cambri- stead, we have only observed tectonic contacts between these an, and Zn-Pb and IOA deposits formed in this environment units, including smaller tectonic repetitions and breccias re- (e.g., Rajabi et al., 2015, 2020; Eslamizadeh, 2016). However, lated to shearing using the uppermost phyllite horizon as a the Bafq district has also been indicated to represent the mag- shear plane. We agree that the development of grabens dur- matic arc region of the orogeny (e.g., Ramezani and Tucker, ing Orosirian extension is the most likely scenario. However, 2003; Majidi et al., 2020); hence, consensus on the tectonic we argue that the entire Orosirian stratigraphic record in cen- setting of central Iran has not been reached. The importance tral Kiruna, ranging from ≤1.88 Ga (Cliff et al., 1990; Romer of back-arc extension and subsequent onset of crustal short- et al., 1994; Martinsson et al., 2016; Westheus et al., 2016) ening have been highlighted as key aspects for IOCG forma- to the deposition of the Hauki quartzite (Ladenberger et al., tion in the Jurassic-Cretaceous Coastal Cordillera of northern 2017), reflects the development of a basin environment. Chile and southern Peru (e.g., Mpodozis and Ramos, 1989; The tectonic setting during the earliest geologic evolution Sillitoe, 2003) as well as the Paleo- to Mesoproterozoic Mt. Isa in central Kiruna has been the subject of contrasting interpre- Inlier in eastern Australia (e.g., Giles et al., 2006; Tiddy and tations. Wright (1988) and Talbot and Koyi (1995) argue for a Giles, 2020). Together with the widespread sodic and potassic classic fold-thrust model synchronous with the IOA emplace- alteration, the bimodal character of host volcanic rocks, and ment, implying that the area represents the foreland sector of proximity to deformation zones of the IOA and IOCG depos- a larger subduction system at approximately 1.9 Ga. We see its in northern Norrbotten, the area also shares petrological, several problems with this interpretation; the most obvious is alteration, and structural characteristics with other IOCG/ that large volumes of volcanic rocks developed in this part of IOA prospective terrains including Brazil (e.g., deMelo et al., northern Norrbotten during this period of the geologic evolu- 2017; Craveiro et al., 2019), Canada (e.g., Corriveau and Mu- tion. In Gällivare (Fig. 1), the Dundret mafic to ultramafic min, 2010; Corriveau et al., 2016), and Mauritania (e.g., Kolb layered intrusion is temporally related to the same volcanic et al., 2008). rocks as found in Kiruna (Sarlus et al., 2017, 2018), indicating an extensional setting at this time. Regionally, volcanic rocks Crustal shortening and basin inversion similar to those found in Kiruna have a shoshonitic character D1 structures are recognized regionally in northern Nor- in further support of an extensional setting (Perdahl and Fri- rbotten and are generally subscribed to an overall northeast- etsch, 1993). Southwest of Kiruna, Martinsson (2004) points southwest crustal shortening resulting in the accretion of the out the importance of a thick basaltic unit at the basal parts Skellefte VMS district to the south onto an Archean craton of the volcanic pile, the mafic-felsic bimodality of the volcanic to the north (assigned D2 in the Skellefte district; e.g., Allen rocks, and the tendency to a within-plate basalt character— et al., 1996; Bauer et al., 2011; Skyttä et al., 2012). The S1 chemical and petrological characteristics further indicative foliation is continuous and heterogeneously developed and as- of an extensional setting. In a subduction context, we argue sociated with epidote-amphibolite facies (Edfelt et al., 2005; that the crustal thinning during the early Svecokarelian cycle Andersson et al., 2020) to amphibolite facies (Bergman et al., is best explained to have occurred in the back arc rather than 2001; Bauer et al., 2018) peak metamorphism (Andersson et the foreland of the subduction system. However, we note the al., 2020). S1 foliation is folded into F2 folds west of central paradoxical tectonic models that come from structurally based Kiruna (Andersson et al., 2020) as well as to the east (Grigull studies (Wright, 1988; Talbot and Koyi, 1995) at one hand and et al., 2018) and to the southeast (Bauer et al., 2018). The geochemical/petrological (Perdahl and Frietsch, 1993; Mar- F2 fold axes are either south or southwest plunging (Bauer tinsson, 2004; Sarlus et al., 2018) studies on the other hand. et al., 2018; Andersson et al., 2020) and in accordance with We add here a stratigraphic argument in favor of an exten- an overall east-west crustal shortening. Furthermore, west of sional setting generating a basin in Kiruna synchronously with Kiruna reverse dip-slip reactivations of NNW-SSE–trending IOA emplacement during the early Svecokarelian orogeny. shear zones as well as strike-slip shearing along EW-trending The generation of rift basins due to continental breakup D2 structures indicate east-west crustal shortening during a during the Rhyacian is rather well established in the Fen- regional D2 event that probably occurred at slightly lower noscandian Shield (e.g., Pharaoh and Pearce, 1984; Martins- temperatures compared to M1/D1 (Andersson et al., 2020). son, 1997; Lehtinen et al., 2005; Melezhik and Hanski, 2012; The mineral associations chlorite ± biotite ± albite ± Hanski et al., 2014; Bingen et al., 2015). The normal faults quartz ± calcite in mylonitic volcanic rocks of mafic com-

positions (Fig. 5A) and sericite + quartz ± calcite ± chlo- tionships are present at the Gruvberget (Bergman et al., 2001) rite (Fig. 5B) in mylonitic sedimentary and volcanic rocks and Malmberget (Bauer et al., 2018) IOA deposits southeast of felsic compositions indicate greenschist facies conditions of Kiruna, where structurally controlled epigenetic copper oc- during deformation in central Kiruna. The orientation of the currences are present in the host rocks to the iron ores. This cleavage is compatible with an E-W– to NW-SE–directed style of structurally controlled sulfide mineralization overprint- crustal shortening consistent with the regional D2 event. ing IOA deposits in northern Norrbotten can be linked in time The same cleavage is axial plane parallel to folded bedding to Svecokarelian late-cycle synorogenic magmatism at ca. 1.80 planes with an S0/S0 β-axis plunging moderately steeply to Ga, indicating a distinct time gap between IOA emplacement the south-southwest (55/213°: Fig. 3A), which is similar to and structurally controlled copper mineralization (Martinsson S1/S1 β-axes of F2 folds west of central Kiruna consistent with et al., 2016; Bauer et al., 2018; Sarlus et al., 2020). east-west crustal shortening (cf. Andersson et al., 2020). This Similar IOA-IOCG relationships in northern Norrbotten indicates that the earliest tectonic fabric recorded in cen- are indicated by U-Pb titanite data on mineral associations in- tral Kiruna formed during the regional D2 event. We suggest terpreted as the age of mineralization in some breccia-hosted two alternative explanations for the absence of recognizable IOCG-style deposits linked to the early-cycle Svecokarelian D1 structures in the study area: (1) the central Kiruna area orogeny (Smith et al., 2009; Martinsson et al., 2016). Ages was subjected to high D2 strain, and S1 was transposed into are not as tightly constrained for the IOCG deposits as for alignment with the D2 structures or (2) ductile D1 structures the IOA deposits in Kiruna but also indicate Svecokarelian were never recorded in central Kiruna, because the area early-cycle IOCG formation as younger compared to IOA represents crustal levels that were too shallow during D1 for emplacement (Fig. 2; cf. Cliff et al., 1990; Romer et al., 1994; plastic deformation to occur. Smith et al., 2009; Martinsson et al., 2016; Westhues et al., The second explanation is favored in this study for sever- 2016). Andersson et al. (2020) present a model including ini- al reasons. If the regional S1 would have been recorded in tial stages of basin inversion during a regional M1/D1 event central Kiruna, it would be expected to be recognizable, at west of Kiruna, and we tentatively suggest that the shift from least in distal parts of the high-strain zones where S1 would be an extensional setting to crustal shortening and the onset of folded in accordance with regional results (Bauer et al., 2018; basin inversion generated these early orogenic breccia-hosted Grigull et al., 2018; Andersson et al., 2020). The areas where IOCG-style deposits. During regional D2 crustal shortening, S1 is recognizable in northern Norrbotten show higher meta- the basin continued to invert, and Cu-Au ± Fe was remobi- morphic grades than the central Kiruna area (Bergman et al., lized into reactivated and newly formed structures producing 2001) and show retrograde alteration of peak metamorphic shear zone-hosted IOCG deposits (e.g., Nautanen and Kis- mineral associations (Andersson et al., 2020). No retrograde kamavaara; Martinsson et al., 2016) and subeconomic sulfide mineral products replacing earlier metamorphic peak miner- occurrences trapped in association with older IOA deposits als have been identified in this study, and our results indicate (e.g., central Kiruna, Gruvberget, Malmberget; Bergman et that neither regional M1 nor D1 were recorded in central al., 2001; Bauer et al., 2018; this study). Kiruna. The regional implication of this is that lower crustal levels around Kiruna must have been uplifted to the higher Conclusion crustal levels of the central Kiruna area, an explanation that The stratigraphic column in central Kiruna is suggested to re- is supported by contrasting structural kinematic evidence result from intracontinental back-arc basin development. The ported regionally (cf. Lynch et al., 2015; Bauer et al., 2018; basin is characterized by bimodal volcanic rocks in the basal Luth et al., 2018a; this study). To the west and southeast of to middle parts changing to a volcano-sedimentary and final- central Kiruna, steep and W-dipping mylonitic structures re- ly a sedimentary character in the upper parts of the basin. cord west-side-up kinematics (Lynch et al., 2015; Andersson The IOA deposits are located at steep, E-dipping lithological et al., 2020), which contrast the E-dipping mylonitic struc- contacts in the lower to middle parts of the basin stratigra- tures with recorded east-side-up kinematics presented in phy; they formed under shallow crustal conditions, and they this study, as well as northeast of central Kiruna (Luth et al., formed before crustal shortening. 2018a). The effect is a juxtaposition of different crustal levels The basin was inverted as a response to east-west crustal that also explains the exceptionally well preserved stratigra- shortening under greenschist facies metamorphic conditions. phy and primary rock features of central Kiruna compared to Strong strain partitioning focused noncoaxial strain into litho- adjacent areas with disturbed stratigraphies and metamorphic logical contacts and rheologically weak rocks, which gave rise overprint. to reverse, oblique to dip-slip, east-block-up sense of shearing along moderate to steep E-dipping structures. Compe- Relative time constraints on Fe and Cu sulfides tent volcanic rocks between the shear zones record limited The occurrence of epigenetic Fe and Cu sulfides associated finite strain. Instead, brittle fracture planes accounted for with Fe oxides hosted by structures that formed in response the deformation in these blocks during the basin inversion. to crustal shortening provides field evidence of a syn- to post- The competent iron orebodies were probably boudinaged but deformational timing of much of the sulfides in central Kiruna. generally lack penetrative fabrics, giving rise to local strain Brittle structures in competent rock units (Fig. 13A-C) as well partitioning that may explain local differences in sense of as fracture planes, fold hinges, and shear bands in rheologically shear recorded by different shear zones. Fe and Cu sulfides weak rocks (Fig. 13E-G) are examples of sulfide-bearing struc- associated with Fe oxides are hosted by structures formed tures in central Kiruna that accord with an overall east-west during the basin inversion, implying that IOA emplacement crustal shortening during the regional D2 event. Similar rela- and epigenetic sulfide mineralization are spatially related but

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formed during different times and in fundamentally different Bauer, T.E., 2013, The crustal architecture of the central Skellefte district, structural settings. Sweden: Ph.D. thesis, Luleå, Sweden, Luleå University of Technology, 142 p. Bauer, T.E., Skyttä, P., Allen, R.L., and Weihed, P., 2011, Syn-extensional The inverted basin was later refolded in response to ap- faulting controlling structural inversion: Insights from the Paleoprotero- proximately north-south crustal shortening, giving rise to zoic Vargfors syncline, Skellefte mining district, Sweden: Precambrian gentle folds with steep E-plunging fold axes controlling the Research, v. 191, p. 166–183. geometry of at least some of the orebodies. Hydraulic fractur- Bauer, T.E., Andersson, J.B.H., Sarlus, Z., Lund, C., and Kearney, T., 2018, ing crosscuts all other fabrics and represents the last recorded Structural controls on the setting, shape, and hydrothermal alteration of the Malmberget iron oxide-apatite deposit, northern Sweden: Economic Geol- geologic deformation event(s) affecting the crustal architec- ogy, v. 113, p. 377–395. ture of central Kiruna. Berglund, J., and Andersson, U.B, 2013, Kinematic analysis of geological structures in block 34, Kiirunavaara: Luossavaara Kiirunavaara AB (LKAB) Acknowledgments Investigations, 13-746, 78 p. Bergman, S., 2018, Geology of the northern Norrbotten ore province, north- This study was financed by the Centre of Advanced Mining ern Sweden: Geological Survey of Sweden, Reports and Bulletins, no. 141, and Metallurgy (CAMM), which is thanked for the financial 432 p. support. Luossavaara Kiirunavaara AB (LKAB) supported this Bergman, S., and Weihed, P., 2020, Archean (>2.6 Ga) and Paleoproterozoic project and is thanked for sharing data, granting permission to (2.5–1.8 Ga), pre- and syn-orogenic magmatism, sedimentation and miner- enter the open pits, and allowing us to publish this study. In alization in the Norrbotten and Överkalix lithotectonic units, Svecokarelian orogeny: Geological Society, London, Memoir 50, p. 27–82. particular, Laura Lauri, Lisa Klemo, Monika Sammelin, Ulf Bergman, S., Kübler, L., and Martinsson, O., 2001, Description of regional B. Andersson, Josefine Johansson, and Jan-Anders Perdahl at geological and geophysical maps of northern Norrbotten County (east of LKAB are thanked for reviewing the manuscript or helping the Caledonian orogen): Geological Survey of Sweden, Ba56, 110 p. the project during the mapping campaigns. Orexplore AB is Bergman, S., Billström, K., Persson, P.-O., Skiöld, T., and Evins, P., 2006, thanked for performing the X-ray CT-XRF scanning. We are U-Pb age evidence for repeated Palaeoproterozoic metamorphism and deformation near Pajala shear zone in the northern Fennoscandian Shield: grateful to the reviewers Dr. Laurent Ailleres and Dr. Gustav GFF: Journal of the Geological Society of Sweden, v. 128, p. 7–20. Nortje, who contributed with constructive criticism and sug- Billström, K., Eilu, P., Martinsson, O., Niiranen, T., Broman, C., Weihed, P., gestions that improved the manuscript. Dr. Stefan Luth at the Wanhainen, C., and Ojala, J., 2010, IOCG and related mineral deposits of Swedish Geological Survey and Leslie Logan at Luleå Univer- the northern Fennoscandian Shield, in Porter, T.M, ed., Hydrothermal iron oxide copper gold and related deposits: A global perspective, v. 3. Advances sity of Technology (LTU) are thanked for valuable inputs and in the understanding of IOCG deposits: Adelaide, PGC Publishing, p. discussions and/or field assistance. Prof. Thorkild Maack Ras- 283–296. mussen at LTU is thanked for the processing of magnetic data Billström, K., Evins, P., Martinsson, O., Jeon, H., and Weihed, P., 2019, and for compiling magnetic maps. The authors acknowledge Conflicting zircon vs. titanite U-Pb age systematics and the deposition of the use of the MOVE Software Suite for data collection and the host volcanic sequence to Kiruna-type and IOCG deposits in northern Sweden, Fennoscandian Shield: Precambrian Research, v. 321, p. subsequent structural analysis granted by Petroleum Experts 123–133. Limited. 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geochemical, petrographic, and fluid inclusion data from the Aitik Cu-Au- Ag deposit, northern Sweden: Ore Geology Reviews, v. 48, p. 306–331. Weihed, P., Billström, K., Persson, P.-O., and Bergman-Weihed, J., 2002, Rela- tionship between 1.90–1.85 Ga accretionary processes and 1.82–1.80 Ga oblique subduction at the Karelian craton margin, Fennoscandian Shield: GFF: Journal of the Geological Society of Sweden, v. 124, p. 163–180. Welin, E., Christiansson, K., and Nilsson, Ö., 1971, Rb-Sr radiometric ages of extrusive and intrusive rocks in northern Sweden: Geological Survey of Sweden, C Series, no. 666, 38 p. Westhues, A., Hanchar, J.M., Whitehouse, M.J., and Martinsson, O., 2016, New constraints on the timing of host-rock emplacement, hydrothermal Joel Andersson has spent the last nine years alteration, and iron oxide-apatite mineralization in the Kiruna district, Nor- working on the structural geology of ore deposits rbotten, Sweden: Economic Geology, v. 111, p. 1595–1618. in northern Norrbotten with an emphasis on iron Westhues, A., Hanchar, J.M, Voisey, C.R., Whitehouse, M.J., Rossman, G.R., oxide-apatite deposits both in industry and aca- and Wirth, R., 2017, Tracing the fluid evolution of the Kiruna iron oxide demia. In 2012 he graduated with a master’s degree apatite deposits using zircon, monazite, and whole rock trace elements and in ore geology from Luleå University of Technology isotopic studies: Chemical Geology, v. 466, p. 303–322. Witschard, F., 1984, The geological and tectonic evolution of the Precam- (LTU) where he specialized in the geochemistry brian of northern Sweden, a case for basement reactivation?: Precambrian and stable isotopic composition of granites associated with an orogenic gold Research, v. 23, p. 273–315. deposit. He is currently a Ph.D. candidate in ore geology at LTU studying the Wright, S.F., 1988, Early Proterozoic deformational history of the Kiruna dis- structural evolution of mineralized systems in the Kiruna area by conducting trict, northern Sweden: Ph.D. thesis, University of Minnesota, 170 p. regional- to deposit-scale investigations.

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