Hydrothermal alteration and lithogeochemical marker units at the Svärdsjö Zn-Pb-Cu deposit, Bergslagen, Sweden, and their implications for exploration

Sammanfattning: In exploration, a lithogeochemical approach can be used to aid the characterisation of rocks surrounding metamorphosed and hydrothermally altered deposits. Accurate description of the geological setting of deposits is crucial for understanding the ore forming processes and identifying targets for exploration. The Svärdsjö Zn-Pb-Cu deposit is located in the heavily mineralised and metamorphosed Bergslagen ore province of south-central Sweden. The deposit and surrounding minor occurrences were actively mined for over 500 years, producing more than 1 Mt of Zn-Pb-Cu-Ag massive sulphide ore. The combination of strongly metamorphosed and hydrothermally altered rocks in Svärdsjö makes geological interpretation challenging. Therefore, an approach combining lithogeochemical and petrographic methods is used in this study. The characterisation of the rocks and hydrothermal alteration surrounding the deposit allowed for an interpretation of ore formation and its implications for further exploration in the Svärdsjö area. The results verified that the Svärdsjö mineralisations are hosted by 2–15 m thick dolomitic marble units, commonly altered to skarn. Surrounding the deposit are subvolcanic intrusions and volcanoclastic rocks of mainly dacitic composition. The combined approach also helped identifying a strong to intense hydrothermal chlorite-sericite alteration enveloping the mineralised marble units and resulted in large mass gains of Fe and Mg whereas Na was depleted. Multiple episodes of alteration and metamorphism are evident from cross-cutting relationships with less altered dykes and overprint by metamorphic minerals such as cordierite and anthophyllite. An ore formation model involving sub-seafloor volcanic-associated replacement is suggested for the Svärdsjö deposit based on (i) the presence of a zoned hydrothermal alteration system within a volcanoclastic rock sequence and (ii) the irregular stratabound sulphide lenses hosted by thin marble units in the centre of the alteration system. Additionally, it is inferred that the stratabound nature of the deposit is caused by the neutralisation of a hot acidic fluid, resulting in precipitation of the sulphides within the marble. Finally, two geochemically distinct lithological units have been identified adjacent to the mineralised zones, providing new, larger exploration targets in the area. Mass change calculations reveal that Fe and Mg enrichment and Na depletion are useful vectors towards mineralisation, with detectable changes extending for up to 100 m from the mineralised lenses. These findings showcase the usefulness of the incorporation and careful interpretation of lithogeochemical data when exploring for metamorphosed hydrothermal ore deposits in mineralised provinces of the Fennoscandian Shield or elsewhere in the world.