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

Abstract: 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 relationships. In contrast, the timing of an overprinting D2 crustal shortening is directly constrained by U-Pb geochronology in titanite indicating an age of approx. 1.8 Ga during an event tentatively interpreted to have lasted up to 20 m.y. This time span is coeval with the exhumation of the Kiruna mining district as recorded by an U‑Pb reset age in apatite in association to an IOA deposit. The D2 deformation is characterized by reactivation of older structures and responsible for juxtaposition of blocks from different crustal levels and tectonic exhumation into upper crustal domains. Transposition of fabrics and ore bodies into re-activated listric faults during basin inversion explains sub-parallel relationships between the ore-proximal structures, bedding, and stratiform/stratabound orebodies.Sodic-calcic + Fe ± Cl alteration is widespread and generally sits in early structural positions and interpreted as pre‑ to syn‑D1. However, U-Pb titanite results indicate that sodic-calcic alteration was developed also during the younger D2 event and shows that the alteration style is temporally and spatially widely distributed. Commonly, the alteration styles associated with D2 deformation are potassic in character and associated to Fe- and Cu-sulphide minerals. These potassic alteration assemblages sit in structurally late positions, often brittle in character. Sulphides were remobilized into D2-structures and the entrapment style is mainly controlled by rock competency.A least two additional overprinting deformation phases are identified (D3 and D4). Clockwise rotation of the overall crustal shortening direction resulted in a gentle refolding of the inverted basin and influences the shape of some IOA deposits in the Kiruna mining district. Dominant joint structures at the Malmberget IOA deposit are indicated as relatively early features and their development is controlled by pre-existing foliation and crosscut by hydrothermally altered structures, that may be coeval with hydraulic fracturing in the Kiruna mining district that crosscut all other fabrics.