Chapter 6 Paleoproterozoic (1.9–1.8 Ga) syn-orogenic magmatism, sedimentation and mineralization in the Bergslagen lithotectonic unit, Svecokarelian orogen

Abstract

Felsic volcanic rocks (c. 1.91–1.89 Ga) and interlayered limestone, hosting Zn–Pb–Ag ± Cu ± Au ± Fe sulphide and Fe oxide deposits, characterize the Bergslagen lithotectonic unit, Svecokarelian orogen, south-central Sweden. Three sulphide mines are currently in operation. Siliciclastic sedimentary rocks stratigraphically envelop this volcanic succession and all the rocks are intruded by a dominant calc-alkaline, c. 1.91–1.87 Ga plutonic suite. Fabric development associated with folding and localized shear deformation followed at c. 1.87–1.86 Ga (D1) and was succeeded by strongly partitioned strain (D2). Dextral transpression along steeply dipping, WNW–ESE or NW–SE shear zones prevailed in the northern and southern domains, whereas major folding with east to northeasterly axial surface traces and shearing along limbs occurred in the central domain. Open folding (D3) subsequently affected the western areas. Polyphase metamorphism under low-pressure and variable temperature conditions included anatexis at c. 1.86 Ga (M1) and 1.84–1.80 Ga (M2). More alkali–calcic magmatic activity, combined with the emplacement of anatectic granite and pegmatite, overlapped and succeeded the M1 and M2 migmatization events at c. 1.87–1.83 Ga and c. 1.82–1.75 Ga, respectively. The younger granites are genetically linked in part to W skarn deposits and host Mo sulphide mineralization. Switching between retreating and advancing subduction systems during three separate tectonic cycles along a convergent, active continental plate margin is inferred.

Origin of Palaeoproterozoic, sub-seafloor Zn-Pb-Ag skarn deposits, Sala area, Bergslagen, Sweden

“…1), and in detail especially to the marble interbeds. This reflects the role of metamorphosed limestone interbeds in the volcanic succession as hosts for the principal types of iron oxide and sulfide deposits in the BLU, which are calcic and magnesian iron skarn deposits and SVALS-type deposits, respectively (Stephens and Jansson 2020). It also reflects the presence of stratiform, syngenetic deposits, including banded iron formations and stratiform Zn-Pb-Ag sulfide deposits, which occur in stratigraphic intervals dominated by metamorphosed volcaniclastic siltstone and limestone (Allen et al 1996).…”

“…2) as well as the outer contact of the distal, more cryptic halo of Mn enrichment (Fig. 11A) indicates that alteration and associated mineralization formed prior to D 1 deformation at c. 1.87 Ga (Stephens and Jansson 2020). This explains why mineralization at Sala mine plunges sub-parallel to F 1 fold axes, and is folded by F 2 (Fig.…”

“…Some of the GDG felsic plutons are interpreted to have been coeval and co-magmatic with the volcanic succession they intrude into, i.e. to be subvolcanic (Allen et al 1996;Jansson and Allen 2011;Stephens and Jansson 2020). Facies associations in the 1.91-1.89 Ga volcanic succession are indicative of a predominantly shallow marine depositional environment, and during pauses in volcanism, stromatolitic limestone interbeds formed (Allen et al 1996).…”

“…1). Notable deposits include the 28.1 Mt Falun deposit, the 6.7 Mt Stollberg ore field, and the currently mined > 144 Mt Garpenberg deposit (Stephens and Jansson 2020), one of the largest zinc mines in the world. A long history of polyphase ductile deformation and medium-grade regional metamorphism complicate ore genetic studies and classification.…”

Zinkgruvanite, Ba<sub>4</sub>Mn<sup>2+</sup><sub>4</sub>Fe<sup>3+</sup><sub>2</sub>(Si<sub>2</sub>O<sub>7</sub>)<sub>2</sub>(SO<sub>4</sub>)<sub>2</sub>O<sub>2</sub>(OH)<sub>2</sub&a

“…1.89 Ga and overprinted by polyphase metamorphism and ductile deformation at ca. 1.87-1.86 and 1.84-1.80 Ga during the Svecokarelian orogeny (Stephens and Jansson, 2020). Peak metamorphic conditions are constrained to 750 ± 50 • C and 0.4-0.6 GPa in the area (Andersson et al, 1992;Gunn, 2002;Stephens et al, 2009).…”