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

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.

Energy Drive for the Kiruna Mining District Mineral System(s): Insights from U-Pb Zircon Geochronology

“…Approximately three to four successive mid-to late-Paleoproterozoic deformation events (described below) are recorded in the Norrbotten lithotectonic unit and have been constrained by ages of magmatic rocks, hydrothermal alteration, and structural characterization (cf. [2] and references therein; [5,[18][19][20][21]49,50]).…”

“…An added element of complexity to the mineral systems approach occurs in regions with long and complicated tectonic histories, such as the Norrbotten province, which has experienced several phases of extension and deformation resulting in compound structural patterns and overprinting magmatic and metamorphic events (cf. [2,5,[18][19][20][21][22]). Furthermore, the landscape has been shaped by Quaternary glaciation leaving extensive till coverage and poor outcrop exposure.…”

“…However, increasing structural and geochronology data link IOCG-style mineralization to a late phase of the tectonic evolution (ca. 1.82-1.78 Ga; [5,[18][19][20][21]27]). These data raise an important question about the timing of introduction and/or remobilization of Cu ± Au when assessing the genetic relationship between IOA and IOCG systems in Norrbotten.…”

“…[31] and reference therein, [33]). In the Kiruna mining district, structural evidence suggests the area was at a higher crustal level and too shallow to develop ductile fabrics during the early Svecokarelian [21]. Moreover, metamorphic fingerprints from the early (M 1 ) phase are either absent, masked, or unidentified (cf.…”

“…Moreover, metamorphic fingerprints from the early (M 1 ) phase are either absent, masked, or unidentified (cf. [21]). Consequently, this study focuses on the absolute timing of magmatic intrusions as a more robust way to elucidate the timing of energy driving processes.…”