The Nautanen North deposit in the Gällivare-Malmberget area of Norrbotten, Sweden, currently contains a resource of 21 million tonnes (Mt) at 1.46% Cu, 0.78 g/t Au, 6 g/t Ag, and 99 g/t Mo and remains open at depth and along strike. This study, based on extensive examination of drill core, geochemical data, and petrographic analyses, represents the first comprehensive description of the structural controls, hydrothermal alteration facies and paragenetic sequence of mineral precipitation, and styles and relative timing of iron oxide and sulfide mineralization at the deposit. The deposit is localized between bounding shear zones within the Nautanen deformation zone. High grades of Cu occur within discrete zones of brecciation and veining and as mineralized shear bands. Breccias in the northern portion of the deposit developed within a vertically stacked, relay-like zone in response to late deformation. Hydrothermal alteration of the host rocks was initially dominated by Na facies alteration, which was subsequently overprinted by Na-Ca-Fe, HT (high-temperature) Ca-Fe, HT Ca-K-Fe, HT K-Fe, and LT (low-temperature) K-Fe facies alteration. Magnetite mineralization occurred in at least two phases: an early phase during Na and Na-Ca-Fe facies alteration accompanied by apatite that is interpreted to reflect a distal signature of formation of the proximal Malmberget magnetite-apatite deposit and a later phase coincident with HT Ca-Fe to K-Fe alteration, which overlapped with the onset of Cu and Fe sulfide mineralization under HT Ca-K-Fe and K-Fe and LT K-Fe conditions. The Nautanen North deposit is shown to meet key criteria to be classified as an iron oxide copper-gold deposit.

Northern Norrbotten, Sweden, is one of the major mining districts of Europe. The area has hosted historic production of Fe and Cu since the mid-17th century (Tegengren, 1924), and hosts the Kiruna deposit, a type example of an iron oxide-apatite (IOA) deposit. Similar, but more strongly metamorphosed and deformed, IOA deposits are mined to the east of Kiruna at Svappavaara and to the south at Malmberget, collectively containing reserves and resources of 3,836 million tonnes (Mt) at 50.5% Fe (LKAB, 2021). In the south of the district, close to the town of Gällivare, are the Cu-Au-Ag-Mo deposits of Aitik and Liikavaara, which display porphyry deposit characteristics (Wanhainen, 2005; Warlo et al., 2020) and have combined reserves and resources of 2,278 Mt at 0.20% Cu, 0.13 g/t Au, and 1 g/t Ag (Boliden, 2022). Fifteen kilometers north-northwest of Aitik, the Nautanen North Cu-Au-Ag-(Mo) deposit contains a newly defined mineral resource of 21 Mt at 1.46% Cu, 0.78 g/t Au, 6 g/t Ag, and 99 g/t Mo (Boliden, 2022).

Nautanen North was discovered by New Boliden in 2012 while targeting ground electromagnetic anomalies within a strongly magnetized and hydrothermally altered portion of the Nautanen deformation zone (Fig. 1) and to date has remained undescribed. Previous research conducted in the Gällivare area has determined that the deformation zone is host to hydrothermal alteration and mineralization with clear iron oxide copper-gold (IOCG) affinities (Lynch et al., 2018), and portions of the Aitik deposit are recognized to have been overprinted by IOCG-style alteration and mineralization (Wanhainen et al., 2012). IOCG-style mineralization is also known to have occurred elsewhere within Norrbotten at the relatively small Rakkurijärvi (Smith et al., 2007) and Tjårrojåkka (Edfelt, 2007) deposits.

This paper is the first detailed geologic description of the Nautanen North deposit. It provides insights into the deposit’s host rock protolith sequence, hydrothermal alteration mineralogy, alteration facies, alteration paragenesis, and oxide and sulfide minerals and their modes of occurrence as well as illustrating the structural controls on Cu sulfide mineralization. The paper also discusses the timing of alteration and mineralization at Nautanen North. This detailed description of the Nautanen North deposit is then utilized to compare it to other mineral deposits within the region (and selected deposits globally) and propose that it should be classified as a sensu stricto IOCG deposit, making it the largest IOCG deposit yet discovered in Scandinavia.

The Norrbotten craton of northern Sweden constitutes a basement of Archean granitoids and gneisses with an overall tonalitic to granodioritic composition, with dating of granitic components providing a minimum age of 2.8 to 2.7 Ga for the basement rocks (Skiöld, 1979; Skiöld and Page, 1998; Bergman and Weihed, 2020). Siderian to Rhyacian greenstone rocks record the commencement of extension in the area at ca. 2.5 Ga that resulted in the deposition of basal metaconglomerate and quartzites of the Kovo group north of Kiruna on the Archean basement (Martinsson, 1997). This basal sequence was overlain by tholeiitic metabasalt and calc-alkaline volcaniclastic rocks deposited within a marine extensional basin (Martinsson, 1997; Kumpulainen, 2000; Bergman and Weihed, 2020). Tholeiitic pillow basalts and massive flows as well as tuffs, komatiitic lavas, andesitic to dacitic tuffs, conglomerates, carbonaceous shales (now graphitic schists), and carbonate rocks of the Kiruna Greenstone group (ca. 2.3–2.0 Ga) were deposited disconformably above the Kovo group rocks in the Kiruna area (Martinsson, 1997; Bergman and Weihed, 2020). These Rhyacian-age volcanic and sedimentary rocks record continued extension with accompanying siliciclastic sediment deposition followed by volcanism and later deposition of evaporites within a shallow marine basin experiencing cyclical or periodic desiccation (Martinsson, 1997). Evaporitic sedimentary rocks are not preserved or exposed at the current erosion level but are evidenced by the widespread scapotilization of the rocks within Norrbotten (Frietsch et al., 1997; Martinsson, 1997).

Overlying the Karelian-age rocks are synorogenic Paleoproterozoic supracrustal rocks of Orosirian age (ca. 1.9–1.88 Ga) represented by the Porphyrite and Kiirunavaara groups and younger sediments of the Hauki quartzite (Offerberg, 1967; Martinsson, 2004). The Porphyrite group is composed of basaltic to intermediate volcanic and volcaniclastic rocks and intercalated siliclastic sediments (Martinsson and Perhdahl, 1995) thought to represent a phase of continental arc magmatism related to an eastward-dipping subduction zone along the edge of the Norrbotten craton (Lahtinen et al., 2009).

Stratigraphically overlying the Porphyrite group are rocks of the Kiirunavaara group (Martinsson, 2004) (previously referred to as the Porphyry group; Bergman et al., 2001a); these are a sequence of tholeiitic basaltic lavas, pyroclastic to porphyritic dacite, andesitic lava flows, ignimbritic rhyolitic tuffs, conglomerate, sandstone, and siltstone (Frietsch, 1979; Martinsson, 2004). At Kiruna and Malmberget, the rocks of the Kiirunavaara group host IOA deposits. The ages of these rocks are known from U-Pb secondary ion mass spectrometry (SIMS) ages of 1880 ± 3 Ma from footwall and hanging-wall rocks, respectively, of the Kiruna deposit (Westhues et al., 2016) and 1885 ± 6 and 1881 ± 6 Ma at Malmberget (Sarlus et al., 2020). Locally, unconformably overlying both the Kiirunavaara and Porphyrite groups and particularly well developed in the west of Norrbotten, are sedimentary rocks of the Younger Svecofennian Supracrustals, also termed the Upper Sediment group (Bergman et al., 2001a). Where well-exposed, these occur as thick sequences of metamorphosed sandstone, arkose, conglomerate, and quartzite and are interpreted to have been deposited as continental or near-shore deposits (Witschard and Zachrisson, 1995a, b; Bergman et al., 2001a) in a series of graben-like basins (Witschard, 1984). Recently, Andersson et al. (2021) proposed an intracontinental back-arc basin depositional setting for both the Porphyrite and Kiirunavaara groups as well as the Hauki quartzite in the central Kiruna area. Comagmatic plutons of monzodiorite, diorite, and gabbro, belonging to the Haparanda suite, and granite, monzonite, quartz monzonite, quartz-monzodiorite, and ultramafic layered complexes of the Perthite Monzonite suite intrude the Porphyrite and Kiirunavaara groups and are related to a phase of continental arc magmatism that occurred from ca. 1.89 to 1.86 Ga (Bergman et al., 2001a; Martinsson, 2004; Bergman, 2018).

A phase of medium-grade metamorphism (M1) at uppergreenschist to middle-amphibolite facies conditions occurred in the Norrbotten area ca. 1.88 to 1.86 Ga and is interpreted to have resulted from accretionary process affecting the Norrbotten craton that resulted in deformation and basin inversion of the Rhyacian-Orosirian supracrustal and intrusive sequence (Bergman et al., 2001a; Weihed et al., 2002; Lahtinen et al., 2009; Bauer et al., 2011; Skytta et al., 2012; Andersson et al., 2020, 2021). Following this deformational event, a suite of dominantly granitic plutonic rocks, minor gabbro, quartz monzonite, monzonite, quartz monzodiorite, and monzodiorite were intruded ca. 1.81 to 1.77 Ga (Bergman et al., 2001a; Sarlus et al., 2018, 2020) throughout Norrbotten as large batholiths (Bergman and Weihed, 2020). These are referred to as the Edefors and Lina suites, previously termed the Granite-Pegmatite Association (Bergman and Weihed, 2020). Skiöld (1982) proposed that melting and assimilation of the older Paleoproterozoic intrusives and supracrustal rocks with minor input of juvenile melts resulted in the emplacement of the Lina suite as S-type granites at midcrustal levels (Öhlander et al., 1987). Intrusion and emplacement of the Lina granite was coincident with a phase of deformation and metamorphism (M2) at low to intermediate pressures confined to major deformation zones and margins of contemporaneous batholiths (Bergman and Weihed, 2020; Sarlus et al., 2018, 2020). Intrusions of granite-syenite-gabbro occur locally in Norrbotten and form part of the Transscandinavian igneous belt rocks, which are more common and voluminous in south and central Sweden (Bergman et al., 2001a) and interpreted to have been intruded above an eastward-dipping, N-S–orientated subduction zone ca. 1.81 to 1.66 Ga (Åhäll and Larson, 2000; Weihed et al., 2002).

Within Norrbotten, a clear temporal discontinuity exists between the formation of the recognized IOA and IOCG deposits. Major IOA deposits formed early, with the Kiruna deposit dated to between 1888 and 1874 Ma (Cliff et al., 1990; Romer et al., 1994; Westhues et al., 2016), the Mertainen deposit to 1880 ± 3 Ma (Martinsson et al, 2016), and a similar age inferred for the Malmberget deposit (Bauer et al., 2018). Two phases of IOCG formation have been recognized in Norrbotten. The oldest occurred during an interval from ca. 1880 to 1860 Ma (e.g., Rakkurijärvi and Pikkujärvi in the Kiruna area) with a younger phase at ca. 1800 to 1750 Ma (e.g., Nautanen, Aitik overprinting event, Särkivaara, Tjårrojåkka, Ferrum and Gruvberget) (Billström and Martinsson, 2000; Wanhainen et al., 2005; Edfelt, 2007; Smith et al., 2007, 2009). Limited IOA-style mineralization is also recognized during this younger event with formation of the iron-rich components of the Tjårrojåkka deposit (Edfelt, 2007).

Geology of the Gällivare-Malmberget area

The rocks of the Gällivare-Malmberget area are composed of volcanic, volcaniclastic, and sedimentary units attributed to the Porphyrite and Kiirunavaara groups, surrounded by younger batholithic intrusions (Fig. 1). In the east of the area, Porphyrite group rocks dominate as metamorphosed basaltic to andesitic volcanic and volcaniclastic rocks and associated sedimentary rocks (Fig. 1). Lynch et al. (2018) report a magmatic U-Pb SIMS age of 1878 ± 7 Ma for an andesitic unit from within the Nautanen deformation zone. In the western portions of the area, intermediate to felsic volcanic and volcaniclastic rocks of the Kiirunavaara group form the bulk of the exposed sequence (Sarlus et al., 2020). The nature of the contact between the two groups within the Gällivare-Malmberget area is poorly understood. However, the mapped sequence along the western margin of the Nautanen deformation zone indicates rocks of these groups may be interbedded and are locally overlain by arkoses and quartzites of the Upper Sediment group (Fig. 1).

The Gällivare-Malmberget area contains abundant plutonic rocks. A quartz monzodiorite stock in the structural footwall of the Aitik deposit has been dated to 1887 ± 8 Ma by U-Pb TIMS geochronology and is part of the Haparanda suite (Wanhainen et al., 2006). At the Liikavaara deposit (Fig. 1), a granodiorite in the footwall has a similar age of 1887 ± 22 Ma (Warlo et al., 2020). In the southwest portion of the area, the Dundret layered gabbroic intrusion has been dated to 1880 ± 3 Ma by U-Pb SIMS geochronology (Sarlus et al., 2018). The Vassavaara gabbro intrusions, located between Dundret and Aitik, have been dated to 1798 ± 4 Ma (Sarlus et al., 2018). The Gällivare-Malmberget inlier is rimmed by voluminous granitic intrusions with associated pegmatites of the Lina Suite, which have been interpreted to have been emplaced between ca. 1.81 and 1.77 Ga (Bergman et al., 2001a; Sarlus et al., 2019).

Metamorphic and deformation events: In the Gällivare area, a 1.88 to 1.86 Ga metamorphic event (M1) resulted in upper greenschist facies conditions in the east and middle amphibolite facies conditions within the Nautanen deformation zone and rocks to the west (Bergman et al., 2001a). Peak metamorphic conditions were coincident with the initial phase of crustal shortening (D1; Bauer et al., 2018). This metamorphic event resulted in generation of a variably intense, planar penetrative foliation (S1) in rocks of the Porphyrite group and the >1.87 Ga intrusive rocks in the east of the inlier (Lynch et al., 2015). The S1 foliation is northwest to north-northwest striking and moderately to steeply southwest to south-southwest dipping and is often parallel to primary layering and therefore difficult to distinguish (S0/S1; Lynch et al., 2015; Bauer et al., 2018, 2022). In the west of the inlier, a strong penetrative foliation (S0/S1) is present at Malmberget and is generally aligned east-west to northeast-southwest and dips moderately to the south and southeast (Bauer et al., 2018).

A second phase of high-temperature, low-pressure metamorphism and deformation (M2/D2) in the area due to E-W–directed crustal shortening occurred at 1.80 to 1.78 Ga (Sarlus et al., 2020; Bauer et al., 2022). In the volcanosedimentary rocks to the east of the Nautanen deformation zone, a discordant, NNE-striking and subvertically dipping crenulation cleavage (S2), orthogonal to bedding planes was developed (Lynch et al., 2015). Similar S2 foliation is absent at Malmberget; however, broad-scale folding of the sequence hosting the Malmberget orebodies is interpreted to have developed at this time (Bauer et al., 2018). Similar large-scale folds are present directly east of the Nautanen deformation zone (Lynch et al., 2015; Bauer et al., 2022).

The deformation zone, which hosts the Nautanen North deposit (Fig. 1), is a regional-scale, NNW-SSE–striking, 1- to 2.5-km-wide composite shear zone (Lynch et al., 2015) that extends more than 150 km. The deformation zone contains several subvertical to moderately W-dipping, NNW-SSE–trending, first-order shear zones with oblique reverse kinematic and related north-northwest–south-southeast second-order shear zones (Bauer et al., 2022). Shear zones dominantly strike north-northwest and show mainly W-block-up reverse kinematics and oblique dextral components (Lynch et al., 2015).

Brittle deformational structures are common throughout the Gällivare-Malmberget area and occur as N- to NNE- and NNW-striking, moderate to subvertical dipping faults with variable or undetermined offset. They displace D1 and D2 deformational features and are attributed to a late phase of extension tectonics (Lynch et al., 2015; Bauer et al., 2018).

Mineralization events: Several major mineral deposits are present in proximity to Nautanen North. The Malmberget IOA deposit (Fig. 1), hosted within felsic to andesitic rocks of the Kiirunavaara group, consists of a number of iron oxide orebodies along approximately 5 km of strike length (Bauer et al., 2018). Magnetite and hematite make up massive, breccia, and layered bodies with accessory apatite, amphibole, pyroxene, and biotite with minor pyrite, chalcopyrite, bornite, and molybdenite (Lund, 2013). The orebodies display a strong penetrative foliation (S1), indicating mineralization occurred prior to or at the onset of D1 at ca. 1.88 to 1.87 Ga. Iron oxide mineralization was associated with widespread sodic (Na) alteration (albitization; Bauer et al., 2018). A second compressional event at ca. 1.8 Ga (D2) resulted in open asymmetric synformal folding and boundinage of the host rocks and IOA orebodies. In some high-strain zones, this event included growth of amphibole + quartz + magnetite + hematite + biotite + sulfides and K-feldspar + magnetite + apatite + biotite + hematite (Bauer et al., 2018). This event has been interpreted by Bauer et al. (2018) to relate to regional-scale IOCG alteration at ca. 1.8 to 1.75 Ga that overprints portions of the Aitik deposit (Wanhainen et al., 2006).

The Aitik Cu-Au-Ag-Mo deposit (Fig. 1) has been interpreted by Wanhainen (2005) to be related to emplacement at 1887 ± 8 Ma of a quartz monzodiorite intrusion in the footwall of the deposit. This intrusion is cut by porphyry Cu-style chalcopyrite-bearing quartz veins with associated biotite and K-feldspar alteration that have been dated to 1876 ± 6 Ma by Re-Os methods (Wanhainen et al., 2005, 2006). Later growth of biotite, sericite, epidote, K-feldspar, amphibole, garnet, scapolite, tourmaline, and apatite was coincident with potential remobilization of sulfides that formed higher-grade Cu zones (Wanhainen and Martinsson, 2003; Wanhainen et al., 2005). These higher-grade zones occur along NNW-SSE–orientated ductile structures and fold hinges of D2 age and have been related to a ca. 1.8 to 1.75 Ga IOCG event (Wanhainen et al., 2006; Drejing-Carroll et al., 2015; Bauer et al., 2022).

The Liikavaara Cu-(W-Au) deposit (Fig. 1) contains chalcopyrite, pyrrhotite, and pyrite with minor sphalerite, galena, scheelite, and molybdenite within quartz ± tourmaline-calcite veins and calcite veins that crosscut biotite-amphibole schists and gneiss of the Porphyrite group (Warlo et al., 2020). Alteration was dominated by biotitization of primary and early amphibole. Biotite also occurs in selvages to quartz and calcite veins and may be accompanied by tourmaline and epidote. Locally, sericite and epidote are observed to partly replace secondary biotite and feldspar, and a late assemblage of chlorite and K-feldspar is common throughout the deposit (Warlo et al., 2020). The age of the mineralization event has not been constrained; however, Warlo et al. (2020) interpreted the deposit to have formed coincident with the emplacement of a granodiorite intrusion at 1887 ± 22 Ma in the footwall to the deposit.

Small-scale mining took place within the Nautanen deformation zone in the vicinity of the Nautanen North deposit, with the longest and most extensive period occurring from 1902 to 1907. The mines exploited chalcopyrite-pyrite-magnetite veins, breccias, and disseminations associated with strong biotite, K-feldspar, garnet, sericite, tourmaline, amphibole, and scapolite alteration (Geijer, 1918; Danielson, 1985; McGimpsey, 2010). McGimpsey (2010) determined the protoliths of the mineralized schists and gneisses within the mined areas to be basaltic andesitic to trachy andesitic volcanic and volcaniclastic rocks of the Porphyrite group. Exploration drilling conducted by the company SGAB within this historic mining area culminated in 1985 with the definition of two mineralized zones, a zone of higher grades containing 0.63 Mt at 2.36% Cu, 1.3 g/t Au, and 11g/t Ag, and a lower-grade zone containing 2.3 Mt at 0.34% Cu and 0.3 g/t Au (Danielson, 1985).

This study is based on geologic logging of 21,039 m of diamond drill core from 30 representative drill holes. Broader geologic information collected during exploration and resource definition at Nautanen North and its surroundings was compiled from the Boliden Mines exploration database. This information described rock types, alteration minerals and intensity of alteration, structures, and occurrence of metallic minerals and included 750 oriented core measurements collected by Boliden Exploration personnel utilizing goniometers, kenometers, and wraparound protractors. Fifty-five structural measurements for this study were taken from 450 m of oriented diamond drill core in the northern portion of the deposit. They were collected on drill core as direct measurements of dip, dip direction, and plunge via a compass-clinometer with core orientated on a rock launcher rig with dip and dip direction set from corresponding downhole deviation surveys. Where oriented core was unavailable, alpha angles measured utilizing a protractor were used conservatively and with caution to guide cross section interpretations.

Lithogeochemical data was compiled and interpreted from the Boliden Mines Exploration database, which included 1,365 samples from Nautanen North. Sampling for lithogeochemistry was conducted in accordance with Boliden Mines Exploration guidelines with collection of a half drill core (NQ2 and BQ core diameter) sample with an approximate length of 25 cm. Post-2017 sampling included the determination of the magnetic susceptibility and density of the samples. Samples were prepared at ALS Piteå, Sweden, and analyzed at ALS Vancouver, Canada, and Loughrea, Ireland. Samples were dried and crushed until 70% passed a 2-mm screen, then split via riffle splitter, and a sample split of up to 250 g was pulverized until 85% of the sample passed through a 75-µm screen. Major oxides were analyzed via lithium borate fusion and then digested and measured via inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Loss on ignition (LOI) was determined using a thermal decomposition furnace. Lithophile elements were quantified by digesting lithium borate fused discs with the resultant solution analyzed via inductively coupled plasma-mass spectrometry (ICP-MS). Total S and C were analyzed by infrared (IR) spectroscopy. Base metals were analyzed by four-acid digestion with ICP-AES, while volatile elements were analyzed via aqua regia digestion with ICP-MS; Au was analyzed by fire assay with an ICP-AES finish. Lithogeochemical data from Nautanen North were plotted on the IOCG alteration (AIOCG) diagram of Montreuil et al. (2013) as molar barcodes utilizing the approach of Blein et al. (2022).

Metal assay data was obtained from both exploration and resource drill hole samples. Sample preparation was conducted at ALS Piteå and Malå, Sweden, along the same guidelines outlined for lithogeochemistry samples. Analysis was predominantly conducted at ALS Loughrea, Ireland. Copper, Ag and Mo were analyzed via aqua regia digestion with an ICP-AES finish, Au was analyzed by fire assay with an ICP-AES finish, total S was analyzed by IR spectroscopy, and magnetic susceptibility was analyzed by a Satmagan-135 saturation magnetization analyzer. The data sets were compiled and interpreted from the Boliden Mines Exploration database to produce grade contour cross sections and level plans.

The combined geologic and geochemical data set was utilized to produce a series of geologic cross sections through the deposit, from which two level plans were produced. Cross sections were produced at 1:2,000 scale, as hand-drawn interpretations with the aid of a light table. Level plans were produced as hand-drawn interpretations from these cross sections, at the same scale. Digitization of cross sections and level plans was undertaken in Adobe Illustrator. Distribution and percentages of alteration minerals together with lithogeochemical results were plotted on the cross sections and level plans, utilizing a Dell Canvas digital screen and the Autodesk Sketchbook software to produce a digital set of alteration cross sections and level plans, which were finalized in Adobe Illustrator.

A total of 55 samples were collected from drill core, representing type examples of host rocks as well as altered and mineralized rock types. Forty-one polished thin sections were produced from this sample set. Alteration and mineralization paragenesis were determined via examination of hand specimens and transmitted- and reflected-light petrographic analysis of polished thin sections utilizing a Leitz Wetzlar Orthoplan Trinocular microscope.

In the discussion of hydrothermal alteration and associated mineral assemblages, + is utilized to denote minerals commonly occurring together, – is utilized to denote minerals occurring within vein assemblages, and ± is utilized to denote minerals occasionally occurring together.

The host rocks of the Nautanen North deposit comprise amphibole-biotite and biotite schists and gneisses containing bands of sericite-garnet schists and gneisses (Fig. 2). Although the sequence underwent significant hydrothermal alteration, remnant primary volcanic textures are sometimes observed.

Rocks on the eastern side of the deposit are dominated by a 200- to 300-m-wide package of intercalated amphibole-biotite and biotite gneisses (Fig. 2). Amphibole-biotite gneiss rocks are medium grained and dark in color (Fig. 3A), whereas biotite gneiss rocks are somewhat lighter in color (Fig. 3B). Both contain variable amounts of biotite, amphibole, quartz, plagioclase, epidote, and pyroxene, and localized zones contain garnet porphyroblasts. The amount of amphibole within the rock was used to name rock types; rocks with greater than 10 vol % amphibole were designated as amphibole-biotite gneiss. Both amphibole-biotite and biotite gneiss can in rare instances be observed to show a clastic-like texture. Biotite gneiss transitions eastward to banded biotite gneiss (Fig. 2), which displays distinct light gray- and black-colored compositional gneissic bands (Fig. 3C). Banded biotite gneiss is prominent in the southern portion of the deposit, where it can form up to a 40-m-wide zone. In northern portions of the deposit, banded biotite gneiss is observed as layers less than 1 m in thickness throughout the amphibole-biotite and biotite gneiss package. Banding within the banded biotite gneiss varies from 0.2 to 4 cm in width. The light gray bands are composed of quartz and feldspar, with darker bands composed of coarse- to very coarse grained biotite with minor quartz, garnet porphyroblasts, and coarse-grained magnetite. The distinctive texture of the unit makes it an important marker horizon.

Banded biotite and biotite gneisses transition sharply westward to sericite-garnet gneiss or schist (Fig. 2), which is the dominant host rock to high-grade mineralized zones at Nautanen North. The sericite-garnet gneiss package varies in width from 1 to 120 m (Fig. 2), strikes south-southeast, and has a subvertical to steep west-southwest dip. The sericite-garnet gneiss is fine to medium grained, light gray to light pink in color, and displays a moderate to strong foliation or, locally, a schistose texture; in rare instances, a deformed porphyritic texture is preserved. The unit is composed of sericite, garnet, quartz, plagioclase, microcline, biotite, and chlorite. Garnet porphyroblasts are abundant and range from sub-mm to over 10 cm in diameter. The sparse diamond drilling to the west of the sericite-garnet gneiss indicates repetition of the biotite and amphibole-biotite gneiss sequence.

At depth, particularly in northern portions of the deposit, a weakly to moderately foliated granitic rock is observed (Fig. 2). It is dominantly pink in color (Fig. 3E) with the pink color becoming less apparent with distance from the mineralized zone. It is coarse grained, with megacrysts of zoned perthitic plagioclase and potassium feldspar in a groundmass of microcline, plagioclase, and quartz with minor biotite and trace pyrite and molybdenite. Mineralogically, the rock classifies as a monzogranite. The contact between the granite and surrounding rock is most often sharp and commonly contains narrow zones (<0.5 m) of pegmatite; shearing in both the granite and the country rock adjacent to the contact is commonplace. The granite is generally unmineralized. However, where the granite has intruded rocks containing low-grade (>0.2 wt % Cu) Cu sulfides, it may contain disseminations or a micromesh veinlet network of chalcopyrite over narrow (1–2 m) intervals. Coarse-grained pegmatites are common at Nautanen North and crosscut both the gneisses and the granite; they are interpreted to be comagmatic with the latter. Where pegmatites intruded mineralized rocks, sulfides occur along the pegmatite margins.

Lithogeochemistry of the Nautanen North rocks

Lithogeochemical samples from amphibole-biotite, biotite, and banded biotite gneisses have SiO2 versus TiO2, Sc versus Zr, Sc versus Ti, and Sc versus Nb values (Fig. 4A-D) and immobile element ratios (Zr/Ti vs. Nb/Y) suggesting derivation from dominantly basaltic to alkali basaltic composition volcanic rocks (Fig. 4E). Lithogeochemical results from the amphibole-biotite, biotite, and banded biotite gneisses do not display tight clusters on immobile element plots, indicating an incoherent protolith. Samples from sericite-garnet gneiss show SiO2 versus TiO2 and Zr versus Sc values that are indicative of a more intermediate source (Fig. 4A-C). The tightly clustered immobile element ratios of the sericite-garnet gneiss indicate a coherent andesitic basalt to trachyandesite protolith is likely (Fig. 4E). Chondrite-normalized rare earth element (REE) plots (McDonough and Sun, 1995) indicate that the Nautanen North granite is strongly depleted in Eu but enriched in both heavy and light REEs (Fig. 4F). On Ta versus Yb plots (Fig. 4G), samples of the granite plot primarily in the “within plate” granite field (Pearce et al., 1984) and within the granite and alkali granite fields on R1-R2 plot for granitoids (Fig. 4H; de la Roche et al., 1980).

The earliest foliation observed at Nautanen North is the S0/S1 foliation, which is a planar composite fabric commonly parallel or tightly oblique to primary compositional banding. This foliation is best preserved in the sericite-garnet gneiss to the west of the main mineralized zone, where it dips on average 38° east-southeast; however, shallow W-dipping S0/S1 fabrics are also observed. The S2 foliation is the dominant fabric at Nautanen North and has an average dip of 77° east-northeast. Ductile deformation zones mark both the western and eastern margins of the high-grade mineralized zone in the deposit and are common throughout the deposit (Fig. 2). Foliation in these S2 zones displays average dips of 71° east. The relationship between the S0/S1 and S2 fabrics is best observed along the western boundary of the high-grade mineralized zone, where the S0/S1 fabric can be observed wrapping into and being overprinted by the S2 foliation (Fig. 5A-C).

Mesoscale parasitic folding of primary compositional banding, S0/S1 foliation, and zones of foliation-parallel disseminated magnetite and/or sulfides are present in the core of the Nautanen North system (Fig. 5B-C). This central zone of folding tapers upwards, effectively forming an antiformal hinge zone which is bound by S2 shears and zones of intense ductile deformation.

In the northeast portion of the deposit, a NW-SE–trending, NE-dipping (70°–75°) zone up to 150 m wide of intense brittle deformation and faulting is present. This zone is intensely brecciated and veined by epidote with hematite and K-feldspar staining. Limited drilling in the hanging wall of this fault zone has encountered hydrothermally altered and mineralized rock with identical characteristics to Nautanen North, and the fault is interpreted to have downthrown to the east with an unknown amount of displacement.

The alteration assemblages outlined here represent those present within the Nautanen North deposit (Table 1). Alteration assemblages have been designated utilizing Corriveau et al.’s (2022a) alteration facies scheme for IOAA (IOA-IOCG) systems. Transitional mineral assemblages occur between each of the proposed alteration facies within the deposit and are described where relevant. A summary of the interpreted paragenetic sequence observed at Nautanen North, in reference to the main phase of Cu mineralization, is provided in Figure 6. Appendix 1 outlines how these alteration mineral facies at Nautanen North are interpreted to relate to those in other mineral deposits within the region.

Sodic alteration and sodic-calcic-iron

The Na facies consists of fine-grained albite and coarse-grained to porphyroblast-like scapolite (Fig. 7A), which have replaced primary plagioclase and can occur with accessory apatite. The Na facies occurs regionally and throughout the Nautanen deformation zone. However, within proximity to Nautanen North, albite and scapolite were largely replaced by subsequent alteration mineral assemblages. The Na assemblage is best preserved within biotite and amphibole-biotite gneisses east of the main mineralized zone and only locally preserved west of the mineralized zone in rocks with original mafic compositions. At the boundary between zones of Na and high-temperature calcic-iron (HT Ca-Fe) facies alteration, veins containing HT Ca-Fe mineral assemblages discontinuously display selvages of albite-scapolite and may represent the evolution from pure Na to HT Ca-Fe facies conditions and are interpreted as a transitional sodic-calcic-iron (Na-Ca-Fe) alteration facies (Fig. 7B).

High-temperature calcic-iron

The HT Ca-Fe facies consists of a mineral assemblage of amphibole (hornblende, actinolite) + magnetite ± garnet (spessartine-almandine; Waara, 2015) with accessory epidote ± pyroxene ± apatite. This facies occurs throughout the Nautanen deformation zone and on the periphery of the deposit as complete wall rock replacements, as replacement bands (Fig. 7C), and in veins. In replacement zones, coarse-grained amphibole replaces primary magmatic amphibole and early hydrothermal biotite and amphibole, while patchy epidote replaces primary and earlier-formed hydrothermal feldspar, and garnet occurs locally as syntectonic porphyroblasts from 1 mm to 10 cm in diameter. Veins containing a mineral assemblage of amphibole (hornblende, actinolite)-magnetite ± chalcopyrite ± pyrite ± molybdenite ± apatite ± titanite ± garnet are common in this facies. Individual veins average 1 to 2 cm in width, although wider examples up to 1 m have been observed, and extensive vein stockwork zones are common. Local folding and shearing of veins within D2 shear zones are observed.

High-temperature calcic-potassic-iron and potassic-iron

A transitional high-temperature calcic-potassic-iron (HT Ca-K-Fe) facies occurs along the boundary between HT Ca-Fe and high-temperature potassium-iron (HT K-Fe) facies-altered rock and is coincident with S2 shear zones (Fig. 8A-B). The HT Ca-K-Fe facies contains a mineral assemblage of amphibole + biotite + magnetite + K-feldspar ± garnet (Fig. 7D), whereas the HT K-Fe facies consists of an assemblage of biotite + magnetite + K-feldspar ± sericite (Fig. 7E). The HT K-Fe facies occurs in the biotite, banded biotite, and amphibole-biotite gneisses. Biotite in this facies is coarse grained and replaced magmatic and hydrothermal amphibole in addition to earlier-formed biotite and epidote. Medium- to coarse-grained K-feldspar pervasively replaced primary and earlier hydrothermal feldspars in HT K-Fe facies rocks. Intense K-feldspar alteration led to textural destruction of the rock and the development of breccia zones commonly containing abundant tourmaline.

Low-temperature potassic-iron

A low-temperature potassic-iron (LT K-Fe) facies is present as a sericite-dominated assemblage with K-feldspar + garnet ± biotite and accessory tourmaline ± chlorite ± epidote ± calcite ± hematite ± apatite and rare allanite (Fig. 7G). This facies dominates within sericite-garnet gneisses but can occur within banded biotite and biotite gneiss zones. Sericite occurs as very fine to fine-grained, shreddy replacement of igneous feldspars and hydrothermal Na-feldspars and as coarse-grained sericite within S2 shear zones that crosscut the sericite-garnet gneisses. Garnet occurs as coarse-grained porphyroblasts which are occasionally skeletal and contain abundant inclusions of quartz, magnetite, tourmaline, and occasionally sulfides, with pressure shadows on garnets commonly containing the same minerals as those forming inclusions. Tourmaline occurs as disseminated grains and veins within the LT K-Fe facies. Where tourmaline occurs with garnet, it is present as inclusions in rims of the garnet porphyroblasts but absent from garnet cores. The transition from HT K-Fe to LT K-Fe facies is often sharp and associated with a marked increase in overall sulfide content within the LT K-Fe–altered rocks, defining the high-grade Cu mineralization at the Nautanen North deposit.

Late calcic-potassium-iron

A late calcic-potassium-iron (Ca-K-Fe) facies consisting of an epidote + hematite + K-feldspar mineral assemblage (Fig. 7H) occurs as wall rock replacement, localized patches and bands, and in veins randomly throughout the deposit, rarely forming easily mappable zones. Areas of Ca-K-Fe facies wall rock replacement and veining are generally spatially coincident with brittle deformation zones, with a prominent Ca-K-Fe facies zone spatially associated with the wide brittle fault zone, east of the deposit. Calcic-potassic-iron–altered zones appear to overprint all other mineral assemblages other than quartz-tourmaline veins.

Quartz-tourmaline veins

Veins containing coarse-grained quartz and tourmaline crosscut all alteration assemblages at Nautanen North. These veins (Fig. 7I) vary in width from subcentimeter to over 1 m. Wall rock adjacent to these veins can be extensively replaced by tourmaline and, to a lesser extent, by K-feldspar. The veins may contain sulfides where they cut previously mineralized rock.

Alteration lithogeochemistry

Lithogeochemical data from Nautanen North was plotted on the AIOCG diagram of Montreuil et al. (2013) as molar barcodes utilizing the approach of Blein et al. (2022). Gneissic rocks display an apparent trend from least-altered and weakly Ca-Fe–altered through Ca-K-Fe and K-Fe alteration zones (Fig. 9A). The majority of samples from within the least-altered zones display molar proportions dominated by Na, suggesting early Na metasomatism was later overprinted by Ca and K alteration. Granitic rocks display an apparent trend from least-altered to transitional Na-Ca-Fe and K alteration zones, reflecting both Na and K metasomatism.

Samples with Cu values above 1,000 ppm display lithogeochemical compositions indicating they underwent K-Fe and/or Ca-K-Fe alteration (Fig. 9B). Samples with Cu values below 25 ppm are below crustal average abundances expected for Cu (Hu and Gao, 2008), suggesting they have been depleted in Cu. These samples plot dominantly as a tight vertical cluster within the Ca-Fe/weakly altered to Ca-K-Fe fields. The insert map provided within Figure 9B indicates that these depleted samples (<25 ppm) correspond largely to the peripheral wall rocks of the deposit, namely amphibole-biotite gneiss and HT Ca-Fe–altered zones. Cobalt values increase in line with Ca and Fe (Fig. 9C). Samples with Co values above 29 ppm are found to the east of the main mineralized zone, where biotite and amphibole-biotite gneisses and HT Ca-Fe, HT Ca-K-Fe, and HT K-Fe facies alteration dominates. Rocks throughout the Nautanen North deposit are strongly elevated in Ba, averaging 2,275 ppm (Fig. 9D). Highly elevated Ba values (>6,000 ppm) occur in samples that underwent intense K alteration within the LT K-Fe–altered and high-grade Cu zones at the core of the deposit. V shows a similar trend to Co on the AIOCG plot (Fig. 9E), although V-rich samples (>200 ppm) cluster east of the high-grade Cu mineralized zone within HT Ca-Fe, HT Ca-K-Fe, and HT K-Fe facies-altered biotite and amphibole-biotite gneisses.

Mineralization at Nautanen North forms a series of high-grade Cu lenses (>0.9 wt % Cu) with strike lengths between 500 and 700 m and known vertical extents of over 400 m. These high-grade lenses occur within an envelope of lower-grade mineralized rock (>0.1 wt % Cu). High-grade zones are largely confined to the eastern edge of the sericite-garnet gneiss, although in the southern portion of the deposit, zones of higher grade can extend 15 to 20 m into the adjacent banded biotite gneisses. Localized zones of higher-grade Cu occur both to the east and west of the main mineralized zone. Mineralized zones contain several different styles of mineralized rock which vary laterally across the deposit (Fig. 10) and are interpreted to have occurred in several distinct paragenetic phases. Photomicrographs of different styles of mineralized rocks are provided in Appendix 2.

Magnetite and sulfide impregnation along foliation (low-grade)

Magnetite and sulfide minerals within lower-grade zones, particularly in the western portion of the deposit, occur as disseminated grains along shear bands and compositional layering parallel to S0/S1 foliation (Fig. 11A) and within mesh stockworks (Fig. 11B-C). Disseminated magnetite becomes more abundant away from the core of the system within Ca-K-Fe and Ca-Fe facies-altered rock. Magnetite occurs as 1- to 5-mm diameter, anhedral to euhedral grains, commonly forming between 5 and 20 vol % of the host rock, and can show alteration on rims to hematite. Sulfides present in lower-grade zones consist of pyrite, pyrrhotite, and chalcopyrite in decreasing order of abundance. Sulfide grains range from 1 to 5 mm in diameter and generally display anhedral shapes. Pyrite and pyrrhotite appear to have replaced earlier-formed magnetite and, in higher-grade zones, chalcopyrite replaced magnetite, pyrrhotite, and pyrite.

Sulfides in brittle veins, vein stockworks, and shears

Brittle veins and vein stockworks containing sulfides occur as micromesh networks and veinlets that cut the S0/S1 fabric and as foliation-parallel shear veins within the low-grade envelope surrounding the high-grade zones. Individual veins vary in width from <1 mm to over 5 cm, with thicker veins in higher-grade mineralized zones. Large veins can transition to mineralized breccia zones. Brittle veins are commonly branched (Fig. 12A), suggesting that they may form stockworks on a larger scale than can be observed in drill core. Brittle vein contacts are generally sharp but can display irregular margins (Fig. 12B). Variability in vein geometries and the preponderance of veins within F2 fold hinges suggest they formed soon after the transition from ductile to brittle conditions.

Chalcopyrite is the most common vein-filling sulfide and often occurs without significant accessory minerals. However, magnetite, pyrite (Fig. 12C), pyrrhotite, tourmaline, biotite, sericite, anhydrite (Fig. 12D), fluorite, and/or molybdenite are observed in sulfide veins locally. Magnetite, pyrite, and pyrrhotite display textural evidence of replacement or overgrowth by chalcopyrite in LT K-Fe facies rocks (Fig. 12A, D). Iron sulfides decrease in abundance with depth in the deposit, whereas anhydrite appears to increase with depth (Fig. 12E).

Sulfide matrix breccias (high-grade)

The core of the Nautanen North deposit is composed of sulfide matrix breccias, colloquially termed “pebble breccias.” These are fault-vein to cataclasite-like zones up to 2 m in width, but more commonly 0.2 to 0.5 m, that display a durchbewegung texture (Craig and Vaughan, 1994; Vokes, 1969) with pebble-like clasts, varying in shape from subangular to rounded. Clast size ranges from sand to pebble size, and clasts are composed of brecciated sulfides (Fig. 13A), mineralized (Fig. 13A) and unmineralized wall rock, pegmatite and tourmaline (Fig. 13B), quartz veins (Fig. 13A, C), and garnet (Fig. 13D). The matrix of the breccias is composed of rock flour, sulfides, magnetite, K-feldspar, tourmaline, and, with increasing depth, anhydrite and fluorite. Cement to breccias is predominantly sulfides, but tourmaline and K-feldspar are also observed. Clasts of K-feldspar and LT K-Fe–altered rock show alteration on their rims to very fine clay minerals, indicating that both physical and chemical abrasion occurred during breccia formation.

Amphibole veins

Amphibole veins within HT Ca-Fe and HT Ca-K-Fe facies-altered rock can contain variable amounts of chalcopyrite, pyrite (Fig. 14A), magnetite (Fig. 14B), molybdenite (Fig. 14C), and, in rare instances, bornite. Magnetite in the veins is commonly replaced by sulfides. The veins are often 1 to 5 cm wide, but meter-scale examples are noted. Both folded and unfolded examples are present and can locally form stockwork zones.

Structural controls on mineralization

Ductile shear zones control the location and geometry of the mineralized lenses at Nautanen North. The eastern margin of the high-grade Cu zone is an NNE-striking, subvertical to E-dipping D2 shear zone. The intersection of this shear zone with an NNE-trending, subvertically W-dipping D2 shear zone forms the western edge of the high-grade mineralized zone and effectively bounds the deposit in the near-surface. An inflection of the orientation of these controlling shears, which may represent an E-dipping relay between two subvertical shear zones, is observed between 0 and 300 m above sea level in the southern portion of the deposit and can be tracked along the entire strike length of the southern mineralized lens (Fig. 15). This inflection has a shallow northerly and southerly dip producing an antiformal culmination in the center of the southern lens, where Cu grades are the highest and the high-grade mineralized zone is at its widest. Steepening of the shear zones resulted in narrowing of the high-grade mineralized zone (Fig. 15). At 0 m above sea level (Fig. 16), two NNW-striking broader zones of mineralized rock are present, which are bounded on their western margin by shear zones dipping on average 80° to east-northeast. These lenticule-like zones are linked by a narrow, N-S–orientated mineralized zone at a point where the controlling shears are vertically dipping. The zones merge upwards toward 100 m above sea level, producing a single high-grade (>1 wt % Cu) mineralized zone which strikes north-northwest. The shear zone controlling this lens dips 70° north-northeast.

The geometry of the mineralized zones highlights the interrelationship of the strike and dip of the bounding shears with the development of broad relay-like zones that appear to have controlled the extent of high-grade mineralization. These relationships are highlighted by grade contour maps of the deposit (Fig. 16). The lower-grade (0.1–0.5 wt % Cu) zone contains disseminated sulfides, widely spaced mineralized shear veins, and sulfide-bearing brittle veins. In the eastern portion of the lower-grade envelope to the deposit, these mineralized shear veins are orientated approximately north-south and dip on average 83° to the east. In the lower-grade envelope to the west of the deposit, similar mineralized shear zones are orientated north-northwest and dip on average 82° to the west. In areas where the high-grade core of the deposit is narrow or absent, mineralized shears and microveins are orientated north-south to north-northwest–south-southeast and dip on average 86° to the east, effectively pinching and closing off the system.

Breccias within the core of the deposit host the highest Cu grades. Controls on breccia development are best observed in the north of the deposit from –200 to 0 m elevation (Fig. 17). Here, the high-grade zone contains a series of NW-trending, moderately to steeply (60°–90°) SW-dipping breccia bodies that appear to be interconnected by NW-trending, steeply (80°–90°) SW-dipping mineralized shears. In drill core, such shears are observed to fold the S0/S1 fabric and contain foliation-parallel grains of sulfides. Deformation produced a vertical, relay-like, breccia-dominated mineralized zone between the subvertically dipping bounding shear zones. The folded S0/S1 fabric within this zone retains an overall westerly dip in contrast to the easterly dip in rocks to the east of the mineralized zone. The relationships suggest folding was the result of reverse movement along the easterly bounding shear zone. Similar controls on the location of breccias and more intensely mineralized zones are observed throughout the deposit.

Host rocks to the Nautanen North deposit

Results from this study show that the host rocks to the Nautanen North deposit have immobile element concentrations consistent with mafic to intermediate igneous protoliths (Fig. 4A-E). Although intense hydrothermal alteration combined with deformation largely destroyed primary igneous textures, there are rare instances of preserved volcaniclastic and, less commonly, porphyritic textures consistent with flows or subvolcanic intrusions. The spread of lithogeochemical data from amphibole-biotite, biotite, and banded biotite gneiss is consistent with a mafic volcaniclastic protolith. In contrast, the tightly clustered lithogeochemical data from the sericite-garnet gneisses suggest derivation from intermediate composition-coherent volcanic flows or subvolcanic intrusions. At the historical Nautanen mine, 3 km south of Nautanen North, McGimpsey (2010) has shown that the rocks have similar immobile element ratios to the sericite-garnet gneisses at Nautanen North, classifying the rocks as a sequence of shoshonitic andesites and the host of the Nautanen mine mineralization. Based on outcrop observations, Lynch et al. (2015) suggested the rocks of the Nautanen deformation zone were derived from andesitic to dacitic tuffs with some agglomerates. To the east of the deformation zone, they identified additional basaltic components and interpret those rocks as part of the same sequence as observed within the deformation zone based on both field observations and immobile element geochemistry (Lynch et al., 2015). Monro (1988) reported comparable intermediate lithogeochemical values for the host rock gneisses to the Aitik deposit. The apparent abundance of basaltic protoliths at Nautanen North is probably real, and the presence of these iron-rich rocks may be of importance in localization of the deposit.

The age of the host rocks at Nautanen North has not been published to date. However, Lynch et al. (2018) produced an age of 1878 ± 7 Ma via U-Pb SIMS for an andesitic rock from the Nautanen deformation zone some 8 km along strike to the south of the deposit. This date conforms to the age of the Porphyrite group rocks regionally and is currently considered the approximate age for the host rock sequence at Nautanen North.

The weakly to moderately foliated Nautanen North granite is mineralogically classified as a monzogranite. Its texture and composition are typical of many intrusions making up the Lina granite suite. The classification of monzogranite is typical of the Lina suite granites, and the textural observations made at Nautanen North of weak to moderate foliation and a strong association with pegmatites is also a common textural feature and association of the suite (Bergman et al., 2001a). Immobile element data indicate the Nautanen North granite should be classified as an alkali granite or granite; available geochemical data suggest it is a within-plate granitoid (WPG). However, the lithochemistry of the granite at Nautanen North differs from most other Lina granite suite intrusions (Öhlander et al., 1987; Bergman et al., 2001a) with a higher degree of REE enrichment, which could be attributed to assimilation of already metasomatized rocks or in situ hydrothermal alteration.

Hydrothermal alteration, structural controls, and mineralization at Nautanen North compared to other IOA-IOCG deposits in Norrbotten

Hydrothermal alteration facies at Nautanen North consist of Na, Na-Ca-Fe, HT Ca-Fe, Ca-K-Fe, K-Fe, and LT K-Fe, with later Ca-K-Fe and very late quartz-tourmaline veining. Rocks within the deposit, and within the broader Nautanen deformation zone, exhibit a degree of complexity that negates a simple streamlined evolution of the system. Transitional mineral associations are commonly observed, and overprinting relationships can be complex. This is best illustrated in the case of HT Ca-K-Fe facies-altered rock present at the boundary between the HT K-Fe and Ca-Fe facies. While the HT Ca-K-Fe facies is interpreted to represent a transitional assemblage between the HT K-Fe and Ca-Fe facies, the possibility remains that it may alternatively represent HT K-Fe alteration overprinting a Ca-Fe facies assemblage within the core of the system. Furthermore, the position of the bulk of Nautanen North samples within the least-altered field of the AIOCG highlights the complex overprinting nature of hydrothermal alteration at the deposit. Molar barcode plots of samples within the least-altered field indicate that the bulk of analyzed samples have molar proportions dominated by Na, suggesting that the sequence of rocks was initially altered by Na fluids before overprinting by Ca-Fe, Ca-K-Fe, and K-Fe, producing an apparent least-altered chemical signature in what are very strongly altered rocks.

Although constraining the precise ages of the alteration and mineralization events at Nautanen North is the focus of ongoing research, the relative timing of the events can be inferred based on crosscutting relationships and comparison with regional events and other mineral deposits. Na alteration assemblages are poorly preserved at Nautanen North. However, an Na alteration signature is evidenced by the lithogeochemical data (Fig. 9A). In other IOA-IOCG deposits within Norrbotten, Na facies alteration mineral assemblages are better preserved. For instance, at Tjårrojåkka, some 50 km to the west of Kiruna, albite occurs early in the paragenetic sequence (Edfelt et al., 2005; Table 2), and at the Rakkurijärvi Cu-(Au) IOCG prospect, close to Kiruna, albite and scapolite alteration of the host rocks is observed, with albite also present in breccias and veins (Smith et al., 2007). Na-Ca-Fe facies alteration is commonly observed to overprint Na and Ca-Fe–altered rocks at Nautanen North. This dominantly vein-filling assemblage of amphibole-magnetite-albite-(scapolite) and its accessory minerals (apatite, titanite, pyroxene, garnet) is similar to the main alteration mineral assemblage of albite + magnetite + amphibole + pyroxene + apatite + hematite in close proximity to the massive magnetite orebodies at Malmberget (Table 2; Bauer et al., 2018). A younger amphibole-dominated mineral assemblage consisting of coarse-grained amphibole + quartz + magnetite + hematite + biotite + chalcopyrite has also been reported from Malmberget. This mineral assemblage appears to have been associated spatially with pegmatic granites (Bauer et al., 2018) and may correlate with HT Ca-K-Fe alteration at Nautanen North. Similar associations are observed at Tjårrojåkka (Table 2), where hornblende is present in the wall rock and in veins, whereas actinolite occurs within veins throughout the deposit; both hornblende and actinolite may be associated with K-feldspar (Edfelt et al., 2005).

Two phases of magnetite mineralization have been recognized at Malmberget. An early phase of coarse-grained, disseminated, and massive magnetite formed the orebodies, which preserve the S1 foliation and contain apatite, albite, amphibole, and hematite. A less significant second phase of magnetite veins and irregular replacements of wall rock is also present. Magnetite of this phase generally cuts S1 foliation, is associated with K-feldspar alteration, and locally contains sulfides (Bauer et al., 2018). Two phases of magnetite mineralization are also recognized at Nautanen North. The early phase resulted in precipitation of disseminated magnetite and apatite parallel to the S0/S1 fabric. This style of magnetite is best preserved within Na, Na-Ca-Fe, and HT Ca-Fe and K-Fe alteration facies. A second phase of magnetite mineralization occurred in association with the Na-Ca-Fe and HT Ca-Fe alteration events. This magnetite is present in amphibole veins, which transgress S0/S1 fabrics and locally transgress S2 fabrics. The veins are folded by F2, suggesting they formed immediately prior to and progressed into the main phase of D2 deformation. Massive bodies of magnetite, like those at Malmberget, have not been observed at Nautanen North, but the early phase of magnetite + apatite mineralization may represent a distal signature of the 1.88 Ga event that formed the Malmberget system.

High-temperature potassic-iron alteration at Nautanen North is common to the central portions of the system as well as the main mineralized portions of the Aitik deposit, and both deposits’ alteration mineral assemblages contain K-feldspar + quartz + biotite + muscovite (Table 2; Monro, 1988). At Tjårrojåkka, pervasive and vein K-feldspar and biotite + scapolite alteration is reported to have occurred with the main phase of Cu mineralization (Edfelt et al., 2005). At Rakkurijärvi, an HT K-Fe alteration phase produced a mineral assemblage of K-feldspar and biotite in wall rocks and a biotite-dominated assemblage within breccia zones (Table 2). This phase of alteration predated the main phase of sulfide mineralization (Smith et al., 2007), similar to the sequence at Nautanen North. At Malmberget, extensive and pervasive K-feldspar + magnetite + apatite + biotite + hematite alteration is observed (Bauer et al., 2018), comparable to the HT K-Fe assemblage observed at Nautanen North.

LT K-Fe facies-altered rocks are the main host to high-grade Cu mineralization at Nautanen North, and a similar mineral assemblage of muscovite + pyrite and chlorite + sericite + magnetite ± epidote has been reported for the western portions of the mineralized zone at Aitik (Monro, 1988). However, the bulk of economic grades at the Aitik deposit occur within adjacent biotite + K-feldspar (HT K-Fe)-altered rocks. At Rakkurijärvi, an LT K-Fe facies of epidote + sericite + chlorite + hematite + quartz + sulfides, similar to that at Nautanen North, has been reported (Smith et al., 2007). This mineral assemblage occupies veins and breccia matrix as well as altering wall rocks. Unlike at Nautanen North, where high-grade mineralized breccias appear to have formed at the same time or after the LT K-Fe facies alteration event, breccias at Rakkurijärvi are interpreted to have formed prior to the LT K-Fe alteration event (Smith et al., 2007).

The formation of the Nautanen North deposit was associated with deformation within the Nautanen deformation zone during the D2 event. Individual shear zones within the deformation zone controlled the location of mineralization. High-grade breccia zones appear to have formed in response to reverse movement along these structures, either during D2 deformation or subsequent reactivation. The Aitik deposit is also hosted within the Nautanen deformation zone, and D2-age structures at the deposit have been shown to control the location of high-grade Cu mineralized zones and IOCG-style alteration assemblages (Drejing-Carroll et al., 2015; Bauer et al., 2022). The Tjårrojåkka deposit is situated at the conjunction of two shear zones (Edfelt, et al., 2006), forming part of a larger NW-SE–trending zone of deformation termed the Fjällåsen shear zone, with hydrothermal activity within the shear zone interpreted to have occurred during D2 deformation (Andersson et al., 2020). At Rakkurijärvi, Cu sulfide-bearing breccias are thought to have formed along ENE-trending shear zones, though these are likely to be of D1 age (Smith et al., 2007).

Mineralization at Nautanen North resulted in precipitation of disseminated magnetite and sulfides (chalcopyrite, pyrite, and pyrrhotite with lesser molybdenite and bornite) as well as magnetite and sulfides in veins and within the matrix of breccias. A similar sulfide mineral assemblage is present in the Aitik deposit (Monro, 1988; Wanhainen et al., 2006) as disseminated grains and in veins; mineralized breccias are absent. At Tjårrojåkka, massive magnetite formed by replacement of host rocks as well as in veins while chalcopyrite, bornite, pyrite, and molybdenite together with magnetite occur as disseminations and veinlets (Edfelt et al., 2005). Breccias form the main host for Cu-Au mineralization at Rakkurijärvi. In addition to chalcopyrite, pyrite, and rare molybdenite, the breccia matrix at Rakkurijärvi contains albite, actinolite, magnetite, quartz, epidote, scapolite, calcite, and chlorite (Smith et al., 2007). The silicate mineral assemblages observed within the mineralized breccias at Rakkurijärvi display a dominant Na-(Ca-Fe) alteration facies assemblage that was subsequently affected by LT K-Fe alteration. In contrast, the breccias at Nautanen North contain only HT K-Fe– and LT K-Fe–altered clasts and older sulfide clasts with matrix and cement dominated by sulfides and tourmaline with lesser K-feldspar, suggesting that the breccia at Nautanen North formed late in the deposit’s history.

Within the zone of HT K-Fe alteration in the center of the Nautanen North deposit, amphibole in veins appears to have been replaced by biotite, whereas magnetite in the veins was replaced by Fe sulfides and chalcopyrite. The relationship suggests that amphibole-magnetite veining likely preceded or was synchronous with the main phase of sulfide mineralization. The paucity of magnetite in the high-grade core of the deposit probably resulted from enhanced magnetite replacement by sulfides.

The Nautanen North granite shows petrographic and geochemical evidence of K and Na alteration. However, in drill core, the granite can be seen to intrude mineralized rock, and associated pegmatites crosscut the mineralized and hydrothermally altered rock. This suggests granite emplacement occurred late in the development of the Nautanen North system, but when the hydrothermal system was still active, or that the granite and surrounding rocks experienced an additional hydrothermal activity as a result of emplacement. The ca. 1.77 Ga age proposed for intrusion of the Lina granite suite regionally (Bergman et al., 2001a; Sarlus et al., 2019) may therefore constrain the age of the bulk of the Nautanen North mineralization.

Nautanen North within an IOCG framework

The Nautanen North deposit comprises Cu-Au–mineralized rocks in which sulfide mineralization largely postdated precipitation of magnetite. The overall magnetite content at Nautanen North varies from >5 vol % in the core of the system and up to 20 vol % on its peripheries, within the range expected of IOCG deposits (>15 vol %; Williams et al., 2005; Skirrow, 2022). Copper at Nautanen North is associated with enrichment of Ag, Mo, Co, Ba, and V and, to a lesser extent, U, LREEs, and F.

Nautanen North displays intense hydrothermal alteration, which transitioned from Na to Na-Ca-Fe to HT Ca-Fe to Ca-K-Fe to K-Fe to LT K-Fe facies. Sodic alteration occurred early within the deposit’s history at a regional scale. Sodic facies mineral assemblages were largely replaced by subsequent alteration mineral assemblages in the core of the Nautanen North system and were preserved distally, a common feature of IOCG deposits (Williams et al., 2005; Corriveau et al., 2022b; Skirrow, 2022). Nautanen North lithogeochemical data display a trend from least-altered to Ca-K-Fe– and K-Fe–altered rocks with molar barcodes (Montreuil et al., 2013), indicating alteration overprinted previously strongly Na-altered rocks, and few examples of true least-altered rocks remain. These alteration assemblages also conform to the mineral assemblages proposed by other authors (e.g., calcic, potassic; Hitzman et al., 1992; Skirrow, 2022).

Copper mineralization at Nautanen North was temporally and spatially related to LT K-Fe facies alteration and, to a lesser extent, HT K-Fe alteration. Chalcopyrite is the dominant Cu sulfide with only very minor bornite present. The majority of chalcopyrite at Nautanen North is present in syndeformational veins and shear veins, with a volumetrically smaller but higher-grade component hosted in breccias. These styles of mineralized rock are common to the IOCG deposit group (Hitzman et al., 1992; Sillitoe, 2003; Williams et al., 2005; Groves et al., 2010; Skirrow, 2022). The syndeformational mineralization styles exhibited at Nautanen North suggest the deposit formed in a midcrustal setting at or near the brittle-ductile transition during D2. The Nautanen North granite is inferred to postdate the bulk of mineralization at the deposit and is not regarded as genetically linked to the introduction of sulfides at the deposit. Though there is no clear link to a single intrusive body, the deposit appears to have formed during a period of igneous activity (ca. 1.80–1.77 Ga) within the Gällivare area (Sarlus et al., 2019, 2020).

Quartz veining is uncommon at Nautanen North. Most quartz veins observed cut mineralized rock. These veins commonly contain tourmaline and may contain Cu and Fe sulfides where they cut previously mineralized rock. The lack of syn-sulfide quartz veins conforms to the criteria of IOCG deposits proposed by Williams et al. (2005), Groves et al. (2010), and Skirrow (2022).

The paragenetic sequence and mineral assemblages of hydrothermal alteration at Nautanen North are similar to those of several deposits in the Carajás district of Brazil (Table 3), particularly Sossego (Lindenmayer and Teixeria, 1999; Monterio et al., 2008). The ubiquitous occurrence of tourmaline at the Salobo deposit in the Carajás district is similar to that observed at Nautanen North (Lindenmayer, 1990). Breccias are a common feature of many IOCG systems; however, at Nautanen North, they form a volumetrically small component of the deposit. The syndeformational and locally brittle textures present at Nautanen North suggest its hydrothermal system developed at a crustal position that allowed cyclic transition between brittle and ductile conditions, though ductile deformation appears to have dominated. In this sense, the deposit shares similar structural and textural features with the Sequerinho deposit in Carajás, Brazil (Monteiro et al., 2008), and the Eloise deposit in Cloncurry, Australia (Baker and Laing, 1998).

The Nautanen North deposit meets all the criteria to be classified as an IOCG (magnetite group) deposit as proposed by Williams et al. (2005) and Williams (2022), matches the criteria to be classified as a sensu stricto IOCG deposit (Groves et al., 2010), and conforms to a classification as an orogenic-type IOCG deposit as proposed by Skirrow (2022).

The Nautanen North Cu-Au-(Ag-Mo) deposit contains hydrothermal alteration mineral assemblages that paragenetically evolved from early Na and Na-Ca-Fe through to HT Ca-Fe and HT Ca-K-Fe to later HT K-Fe and LT K-Fe facies. Magnetite mineralization at Nautanen North is coincident with Na, Na-Ca-Fe, Ca-Fe, and Ca-K-Fe assemblages and appears to have occurred in two phases, the younger of which is interpreted to have transitioned into the main phase of sulfide mineralization, which was coincident with HT K-Fe and LT K-Fe alteration. Sulfides commonly replaced magnetite and occur as veins, shears, breccias, and to a lesser extent, disseminated grains. Chalcopyrite is the dominant Cu sulfide. High-grade breccias within the deposit contain clast components which indicate they formed syn- to post-LT K-Fe alteration within narrow zones that developed as vertical relays between bounding shear zones at the core of the deposit with a reverse sense of shear. The HT Ca-Fe, HT Ca-K-Fe, HT K-Fe, and LT K-Fe facies alteration as well as the majority of sulfide mineralization appears to have been synchronous with the development of the D2-aged host shear zone (Nautanen deformation zone). The Lina granite in the footwall of the deposit with its associated pegmatites crosscut mineralized and altered rock and may constrain the formation of the deposit, but additional dating is required. Nautanen North may have a shared metasomatic history with the nearby Malmberget IOA deposit and displays similarities with other IOCG-type deposits in Norrbotten. The paragenetic sequence of alteration/mineralization as well as the mineralogy and textures of Nautanen North suggest it belongs to the IOCG class of deposits. Based on the current resource estimates (21 Mt at 1.46% Cu, 0.13 g/t Au, and 1.55 g/t Ag), Nautanen North, which remains open to depth and along strike, represents the largest IOCG deposit yet discovered in Fennoscandia.

This research work was funded by Boliden Mines AB and Science Foundation Ireland (SFI) research grant number 16/RP/3849. Boliden Mines are kindly thanked for permission to publish. Since the Nautanen North deposit was discovered in 2012, many Boliden Exploration personnel have contributed to advancing the understanding of this complex system. The input and discussions with the entire team over the years have contributed to the interpretations presented here. The authors wish to thank Oakley Tuner, Nils Edblom, Janne Kaukolinna, Jonna Tirroniemi, Niko Laaksonlaita, Josef Bruun Nielsen, Christian Stenvall, Brendon Dean, Jakob Fahlgren, Paulina Nordfeldt, and Sean Johnson for countless discussions on the deposit and the research. Scott Halley is thanked for assistance with compilation, interpretation, insights, and discussions on the regional and deposit lithogeochemistry data set. Edward P. Lynch, James Kidder, Christian Stenvall, and Tobias Hermansson provided feedback on an early version of the manuscript, which improved it significantly, and we kindly acknowledge them. Constructive feedback and comments from Louise Corriveau, Irene del Real, and one anonymous reviewer greatly improved the manuscript. We thank Louise Corriveau and Olivier Blein for the help provided with geochemistry and the workflow for creating and interpreting molar barcode plots.

David Drejing-Carroll is an exploration geologist with Boliden Mines and a Ph.D. candidate at University College Dublin/iCRAG studying IOCG deposits in northern Sweden. In 2012, he was the project geologist responsible for the discovery of the Nautanen North deposit and since then has contributed to its exploration and development. Prior to this, he was part of the exploration team taking the world-class Gounkoto gold deposit, Mali, from discovery into production for Randgold Resources. In addition, he has worked on exploration and mining projects throughout Europe. He holds an undergraduate degree from the University of Edinburgh and an M.Sc. from Camborne School of Mines.

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