Tracing ore-forming processes in the REE-Fe-polymetallic mineral system of the REE-line, Bergslagen, Sweden: Insights from LA-ICP-MS/MS Re-Os geochronology of molybdenite

The REE-Fe-polymetallic mineral system along the REE-line in Bergslagen, south-central Sweden, encompasses a range of magnetite-rich iron oxide deposits with variable REE and polymetallic enrichments (e.g., Cu, Co, Bi, Mo, Au), including the notable Bastnäs-type REE deposits. As part of the EU-funded Exploration Information Systems project, in-situ Re-Os LA-ICP-MS/MS geochronology of molybdenite has been conducted across various mineralisations along the REE-line. This research aims to better delineate the timing of key ore-forming processes, link them to regional geological events, and assess the implications for mineral system modelling.

Mineralogical and textural evidence indicates a prolonged evolution of mineralisation within this system, involving multiple stages of REE mineral and Fe-Cu-Mo-Bi-(Co) sulphide formation. The sulphide-rich assemblages often occur as fracture fillings, veins, or bands with, and occasionally as inclusions within, allanite-group minerals. These assemblages are hosted within various mineralisation types, including recrystallised cerite-(CeCa) and bastnäsite-(Ce) ores, hydrothermally altered and metamorphosed volcanic rocks, carbonate rocks with serpentine-dominated pseudomorphs (“ophicalcite”), and amphibole ± pyroxene or andradite-dominated magnetite skarns.

The new Re-Os geochronology, combined with previously published data, reveals two primary age domains: ∼1.91–1.88 Ga and ∼1.87–1.83 Ga. The earlier domain aligns with primary mineralisation, formed through hydrothermal replacement of carbonate interlayers in a volcano-sedimentary succession during shallow-marine, sub-seafloor, and back-arc volcanic activity during the Svecokarelian orogeny. The younger domain is synchronous with ∼1.87–1.84 Ga magmatism in parts of Bergslagen and the peak of regional metamorphism in the studied area. The combined textural and age data are best explained by the remobilisation of different ore-forming components (e.g., REE, Fe, Cu, Mo, Bi) during regional metamorphism and deformation.

The findings offer new insights into mappable proxies for prospectivity mapping within the REE-Fe-polymetallic mineral system, particularly for processes related to pathways and sinks. They further underscore the protracted nature of mineralisation and highlight the significance of deformation- and metamorphism-related structures and features as additional exploration targets, particularly for identifying mineralisation types that diverge from the classical carbonate-replacement styles of Bastnäs-type deposits.

We acknowledge funding from the European Union’s Horizon Europe research and innovation programme for the project Exploration Information Systems under grant agreement No. 101057357.