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 stratigra- phy, petrography with focus on microstructures, X-ray computed tomography imaging of sulfide-structure rela- tionships, 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 struc- tures, 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 © 2021 Gold Open Access: This paper is published under the terms of the CC-BY license. 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 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 2 ANDERSSON ET AL. 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