Hydrothermal alteration, lithogeochemical marker units and vectors towards mineralisation at the Svärdsjö Zn-Pb-Cu deposit, Bergslagen, Sweden

ABSTRACT

The Svärdsjö Zn-Pb-Cu deposit is situated in the heavily mineralised Bergslagen lithotectonic unit of the Fennoscandian shield, south-central Sweden. Intense hydrothermal alteration followed by a strong overprint by deformation and metamorphism during the Svecokarelian orogeny complicate interpretation of the local geology. Integration of whole-rock lithogeochemical and petrographic methods has allowed the mainly dacitic volcanic host succession and effects of ore-related hydrothermal alteration to be characterised. Mineralisation is hosted by 2–15 m thick, commonly skarn-altered dolomitic marble interbeds. Zones of strong–intense hydrothermal chlorite-sericite alteration envelop the marble units, recording mass gains of Fe and Mg, as well as Na depletion. Minerals such as cordierite, anthophyllite and sillimanite formed in these rocks during regional metamorphism. Mineralisation via sub-seafloor replacement is suggested for the deposit based on alteration zoning and the irregular, stratabound, marble-hosted style of sulphide lenses. It is inferred that mineralisation formed via neutralisation of hot, acidic metalliferous fluids. Geochemically and lithologically distinct units adjacent to the mineralised zones can serve as marker units to aid further exploration in the area. Mass change calculations reveal that Fe and Mg enrichment, as well as Na depletion exhibit detectable changes extending up to 100 m from the mineralised lenses, providing exploration vectors.

KEYWORDS:

• Lithogeochemistry
• Bergslagen
• alteration
• massive sulphides
• Skarn
• VMS

Acknowledgements

This paper is based on the first author’s MSc thesis at Luleå University of Technology. Boliden Mineral AB provided the financial support for the study and is thanked for allowing the results to be published. We are grateful to Eemeli Rantala at Boliden Mineral AB for providing valuable expertise about the study area, as well as aid and instructions in the logging and sampling of drill cores. The technical employees at the Boliden Mineral AB in Garpenberg core shed are acknowledged for assisting with sample preparation. Stefan Andersson (Geological Survey of Sweden) and Nikola Denisova (Memorial University of Newfoundland) are thanked for detailed and constructive reviews. Magnus Ripa is thanked for editorial handling and additional reviews which helped improve this contribution.

Disclosure statement

No potential conflict of interest was reported by the author(s).