Structural evolution of the Palaeoproterozoic Sala stratabound Zn-Pb-Ag carbonate-replacement deposit, Bergslagen, Sweden

Abstract

A structural investigation of the Sala Zn–Pb–Ag deposit in the Bergslagen mining district of southern Sweden shows that it is associated with two tectonic structures: the N–NW-trending Storgruveskölen shear zone (SSZ), which is parallel to the strike of the mined ore bodies, and the F₁ Sala syncline with a fold hinge plunging c. 35° towards NNW, which is parallel to the plunge of the entire mineralised system. The Sala syncline was refolded by F₂ folds, leading to flattening and local reversals in the plunge of F₁ folds and the ore bodies. Field evidence suggests that the SSZ represents both a phase of D₃ reverse dip-slip shearing and a later (D₄) phase of dextral strike-slip reactivation. However, a high concentration of pre- to syn-D₁ skarn- and sulphide-bearing vein networks and breccias adjacent to the SSZ, which are gradational into the mined massive sulphide ore bodies, suggest that stages in the formation of the SSZ predated D₃ and D₄. It may consequently constitute a reactivated pre- to syn-D₁ structure. The distribution of breccia and hydrothermal alteration together with the highly discordant nature of the deposit are consistent with a pre- to syn-D₁ timing of ore formation, involving of cross-stratal fluid flow along the proto-SSZ and subordinate fluid flow parallel to volcanic interbeds in the host carbonate rock. Three δ³⁴S determinations on sphalerite (2.1–2.4‰) and galena (1.2‰), respectively, are consistent with a magmatic source for ore sulphur, as has been suggested for many other sulphide deposits in Bergslagen.

Keywords:

• Sala
• Bergslagen
• mineralisation
• ore
• stromatolites
• structural geology

Acknowledgements

The manuscript is modified after the authors MSc thesis, originally written as a project in Earth Science for Uppsala University, under the supervision of Prof. Hemin Koyi and Prof. Christopher Talbot. The choice of subject was done partly on behalf of the personal interest of the author and partly after a suggestion from Tumi Resources Ltd. Thin-sections were funded by Tumi Resources Ltd. Discussions with Prof. Rodney Allen (Boliden) and Dr Magnus Ripa (Geological Survey of Sweden) have stimulated the continued development of the early thoughts and ideas into their present form. Dr Kjell Billström (Swedish Museum of Natural History) performed the S isotope analyses on two samples provided by the author during an analytical session in Salamanca, Spain. Reviews by Stefan Luth (SGU) and Sabina Strmić Palinkaš (UIT) helped improve this contribution. Adam McElroy (Boliden) is thanked for providing a language check. This research would never have been possible without the helpfulness of Sala Silvergruva AB, who runs the current tourism operations.