U-Pb zircon-titanite-apatite age constraints on basin development and basin inversion in the Kiruna mining district, Sweden

• Published in: Precambrian research Vol. 372; p. 106613
• Main Authors: Andersson, Joel B.H., Logan, Leslie, Martinsson, Olof, Chew, David, Kooijman, Ellen, Kielman-Schmitt, Melanie, Kampmann, Tobias C., Bauer, Tobias E.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.05.2022
• Subjects: Basin development; Basin inversion; Centre - Centre for Advanced Mining & Metallurgy (CAMM); Centrumbildning - Centrum för avancerad gruvteknik och metallurgi (CAMM); Den föränderliga jorden; Geochronology; IOA; Kiruna; Malmgeologi; Ore Geology; The changing Earth; Geochronology; Basin inversion; IOA; Kiruna; Basin development
• ISSN: 0301-9268; 1872-7433; 1872-7433
• DOI: 10.1016/j.precamres.2022.106613

•We provide geochronological constraints on the tectonothermal evolution of the Kiruna mining district in northern Sweden.•Basin development at 1887 ± 3 Ma.•Syn-volcanic faulting at 1889 ± 26 Ma coeval with iron ore emplacement and associated hydrothermal alteration.•Burial and upper greenschist facies conditions and subsequent cooling due to tectonic exhumation at 1805 ± 26 Ma.•Basin inversion at 1812 ± 3 Ma during a protracted tectonothermal event that may have lasted up to c. 20 m.y. The basin inversion coincides with the regional D2 event in northern Norrbotten. To constrain the tectonothermal evolution of the type locality for iron oxide-apatite deposits, we have obtained U-Pb zircon, titanite, and apatite age data for the Kiruna mining district in northernmost Sweden. The results indicate that the host basin initiated in an overall extensional regime as indicated by the deposition of alluvial conglomerates and greywackes. A volcanic intercalation in a conglomerate unit approximately 1 km northwest of the Luossavaara iron oxide-apatite deposit yields a U-Pb zircon concordia age of 1887 ± 3 Ma representing the timing of the earliest Orosirian volcanism in the central Kiruna mining district coinciding with the onset of basin development. In-situ analysis of titanite sitting on hydrothermally altered fracture planes within a cataclastic fault damage zone (c. 270 m from the fault core system associated with the Luossavaara iron oxide-apatite deposit) yields complex U-Pb data. Applying a strict discordance filter yields seven analyses (<1.5 % relative discordance which included six common Pb-corrected analyses and one analysis where no significant common Pb was detected) that yield a 207Pb/206Pb weighted average age of 1889 ± 26 Ma that we interpret as the most probable minimum age of fault initiation. The overlap in age of the host volcanic rock and the fault implies that during basin development, contemporaneous volcanism and syn-volcanic faults may have played an important role during iron ore emplacement. A U-Pb lower intercept age of apatite from the Nukutus iron oxide-apatite deposit provides a first indication that the area was subsequently buried and metamorphosed under upper greenschist-facies conditions and later tectonically exhumed and cooled below the apatite closure temperature at 1805 ± 26 Ma. Further temporal constraints on exhumation and basin inversion is given by syn-tectonic titanite along a brittle-ductile reverse shear zone to the east of the study area that shows limited presence of significant common Pb and is part of a sodic-calcic + Fe + Cl hydrothermal alteration assemblage. Titanite grains that show a well-developed zonation yield a U-Pb concordia age of 1812 ± 3 Ma, which we interpret as the onset of basin inversion. Titanite grains from the same sample that show less pronounced zonation yield a U-Pb concordia age of 1802 ± 8 Ma, tentatively indicating that the tectonothermal activity lasted up to c. 20 m.y.