Paleoproterozoic deformation in the Kiruna‑Gällivare area in northern Norrbotten, Sweden : Setting, character, age, and control of iron oxide-apatite deposits

• Main Author: Andersson, Joel
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Luleå tekniska universitet, Geovetenskap och miljöteknik 2021
• Subjects: Kiruna; Gällivare; structural geology; Paleoproterozoic; deformation; Geology; Geologi; Malmberget; Norrbotten
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-87741

This thesis covers the structural evolution of the Kiruna‑Gällivare area in the northern Norrbotten ore province, Sweden. The study area hosts several economically significant iron oxide-apatite (IOA) deposits and includes the type locality for this ore type. Despite the abundant work on the genesis of IOA-systems, their structural setting and control is poorly constrained. This highlights the need for multi-scale structural studies that can help to unravel structural controls on the genesis and overprinting deformation histories. Four IOA-hosting key study areas were under investigation covering multi-scale structural controls from regional to deposit scale. Extensive geological mapping focused on structures, stratigraphy, and hydrothermal alteration, combined with multi-scale structural analysis and U-Pb geochronology was conducted. Results are synthesized in a time-constrained tectonothermal model for IOA deposits and host rocks of the Kiruna‑Gällivare area. The results indicate that the IOA deposits in Norrbotten formed in an overall extensional regime coeval with basin development in a backarc setting. The onset of basin development is indicated by a U-Pb age in zircon from a volcanic intercalation in a stratigraphically basal alluvial conglomerate of the ore-bearing sequence. A titanite age indicates that an ore-proximal cataclastic fault has syn-volcanic origin and formed coeval with basin development and ore formation. A similar origin is postulated for ore-proximal biotite-bearing structures at the Malmberget IOA deposit. In comparison with Kiruna, the Malmberget area experienced higher metamorphic conditions and records a more complex deformation history. Following backarc extension, subsequent crustal shortening resulted in basin inversion and re-activation of structures. Crustal scale, reverse shear zones developed in favourable lithologies and inferred pre-existing structures during D1. The timing of D1 crustal shortening coincides with peak metamorphism (M1) and is bracketed by crosscutting ...