Isotopic identification of paleo rift zones within the Sveconorwegian Province; implications for nickel sulphide mineralisations in the SW Fennoscandian Shield

The southwestern Fennoscandian Shield hosts significant orthomagmatic nickel sulphide deposits linked to pre-Sveconorwegian crustal evolution. This study integrates new U-Pb zircon geochronology, Lu-Hf isotopic data, and whole-rock geochemistry from the Kongsberg Unit of the Sveconorwegian Province to investigate its magmatic and metallogenetic evolution. The results reveal distinct magmatic episodes across the three complexes of the Kongsberg Unit, highlighting notable temporal and isotopic heterogeneity. Magmatism in the Eastern Kongsberg Complex is confined between 1610 ± 11 and 1524 ± 11 Ma, coeval with igneous activity during the Gothian orogeny and consistent with an arc-related tectonic setting. Meanwhile, the Western Kongsberg Complex records younger, more juvenile magmatism between 1528 ± 9 and 1153 ± 3 Ma. In contrast, the intervening Modum Complex represents extensional basin development with sedimentation < 1475 Ma and juvenile mafic magmatism between 1305 ± 8 Ma and 1185 ± 12 Ma. Nickel sulphide mineralisation, including the Ertelien (1584 ± 19 Ma) and Langedal (1546 ± 2 to 1524 ± 11 Ma) deposits in the Eastern Kongsberg Complex, predominantly correlates with Gothian magmatism, whereas deposits in the Western Kongsberg Complex, such as Grågalten (1164 ± 13 Ma), are linked to later magmatic events. A pronounced westward-younging trend in zircon U-Pb crystallisation ages and Hf model ages supports a dynamic lithospheric evolution driven by long-lived crustal extension and subsequent Sveconorwegian orogenic assembly. Additionally, widespread platinum group element (PGE) depletion across the province suggests a pyroxenitic mantle source, rather than repeated sulphide extraction during magmatic differentiation. These findings emphasise the critical role of pre-Sveconorwegian lithospheric architecture and mantle source characteristics in controlling nickel sulphide mineralisation, offering valuable insights for future exploration strategies in the region.