Exploring the depths: A journey into the world's largest underground iron mine
In April 2023, eight members of the SGA Baltic Chapter embarked on a two-day visit to the Kiruna mine in Norrbotten, northern Sweden (Fig. 1). The mine in Kiruna is operated by the state-owned Swedish mining company LKAB and is the largest underground iron mine in the world. The Kiirunavaara apatite iron ore (IOA) deposit, along with a few other iron mines in the area, serves as the primary source of iron produced in Europe today. The company plays a crucial role in the European Union's iron ore production, accounting for c. 90 percent of the total output (2021).
The northern Norrbotten ore province has a long history of industrial mining dating back to the 17th century. The ore province is dominated by iron and copper production, with the additional extraction of silver and gold. In addition to the conventional iron and base metal resources, the region exhibits promising potential for rare earth elements that could possibly contribute to the transition towards a fossil-free society (Lindberg 2023) .
Fig 1: Generalized geology of northern Norrbotten ore province, covering the Kiruna area. KNDZ = Kiruna-Naimakka deformation zone, NDZ = Nautanen deformation zone, PSZ = Pajala shear zone (Modified after Andersson et al., 2021).
The apatite iron ore in Kiirunavaara is distinguished by the presence of apatite together with high-grade iron mineralisation, mostly magnetite. Moreover, geochemically, the iron ore demonstrates generally a low Ti content combined with a high content of V (Bergman et al., 2001). The genesis of the Kiruna-type of IOA deposits has been a topic of ongoing research and debate for decades. Two main hypotheses have been proposed to explain the origin of the ores. One well-established hypothesis suggests a magmatic origin, where iron oxide magmas intruded the host rocks (e.g. Frietsch 1978). In contrast, an alternative hypothesis leans towards a hydrothermal explanation, suggesting that the ores were deposited epigenetically by hydrothermal fluids (e.g. Westhues et al. 2017). The Kiruna-type deposit has been discussed to have a potential genetic relation to IOCG deposits due to the presence of Cu and Au (Martinsson et al. 2016). Additionally, during the 1970s, a pseudo-sedimentary-exhalative origin was postulated (e.g. Parák 1975). Because of this famous geological dispute, the first event of the field trip was an evening debate among the participants, representing three different suggested ore genetic models: magmatic, hydrothermal and sedimentary-exhalative.
Early the following day, we entered the mine and began our descent to a depth of 540 metres from the top of the Kiirunavaara mountain where the visitor centre is located (Fig. 2). Following an introduction on the mine's history and modern processing methods, a presentation was held by geologist Sergio F. Castro Reino covering the regional geology, host lithologies and deposit related alterations. The stratigraphic column in the area is believed to have formed in an intracontinental back-arc basin. Within the geological setting, the deposit is found within a succession of porphyritic volcanic and volcaniclastic rocks.
Fig. 2: Group picture together with the LKAB staff taken at the visitor centre.
The magnetite-apatite deposit itself consists of a four kilometre-long and approximately 90 meters wide, E-dipping (60°–70°) ore body. The emplacement timing of apatite iron ore in Kiruna is constrained by reported crystallization ages at; 1888 ± 6 Ma (U-Pb, titanite, Romer et al. 1994), 1878 ± 4 Ma (U-Pb, titanite, Martinsson et al. 2016) and 1877 ± 4 and 1874 ± 7 Ma (U-Pb, zircon, Westhues et al. 2016). The magnetite mineralisation is situated at the contact between a trachyandesitic footwall and a rhyodacitic to rhyolitic hanging wall. Both the footwall and the hanging wall are brecciated by the magnetite ore (Bergman et al. 2001). Based on structural analysis and age dating, it has been determined that the hydrothermal mineral assemblages in Kiruna can be attributed to a minimum of four distinct tectonic events that occurred at different times (Andersson et al. 2021, 2022).
After the coffee break, another presentation was held by Sergio that focused on the structural styles in the ore body. In 2020, the mine was affected by a 4.3 magnitude local-scale seismic event, resulting in significant damage to infrastructure. Since then, there has been a greater focus on detailed structural mapping. After the final presentation we continued our journey deeper into the mine and visited the underground core logging facilities there, we had the opportunity to look at drill core of the different lithologies and alteration styles typical for the Kiruna deposit. The participants learned that the ore types are defined by their Fe and P content and are classified into five categories ranging from high-Fe and low-P to lower-Fe and high-P. The low-phosphorus ore type is massive, fine-grained, and homogeneous with a Fe content of around 68-69% and a low sulphur content. The higher phosphorus ore types contain bands of apatite and often have a higher silica content with a Fe content of around 59–61% and a P content of approximately 1.5%. We also got the opportunity to get a short introduction to the CAD program used by the drift geologists for mapping lithologies. Just before we left the mine, Sergio philosophically concluded that geology is the most artistic form of science. Like an artist, a geologist must be observant of colours and textures and be able to visualize features in 3D.
After returning to ground level, we made a final stop at the near mine exploration departments of Kiruna and Svappavaara (open pit mine c. 40 kilometres southeast of Kiruna). We were introduced to how a Minalyzer has been successfully implemented in the workflow of core logging. This device uses X-ray fluorescence to scan drill core , causing the minerals to emit characteristic X-ray signatures. The data obtained is valuable for geological interpretation and resource assessment. It is a useful tool to understand the lithology, mineralogy, and geochemical characteristics of the subsurface rocks. Before returning back to Luleå, the participants examined several drill cores and a collection of minerals at the logging facility.
The participants of the trip would like to thank all the staff at LKAB for allowing us to visit their mine and logging facilities.
References
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(2021) LKAB Annual and Sustainability Report in Brief 2021. LKAB in cooperation with Rippler Communications, Luleå, Sweden
