Chapter 4 Paleoproterozoic (2.0–1.8 Ga) syn-orogenic sedimentation, magmatism and mineralization in the Bothnia–Skellefteå lithotectonic unit, Svecokarelian orogen

Abstract

The Bothnia–Skellefteå lithotectonic unit is dominated by turbiditic wacke and argillite (Bothnian basin), deposited at 1.96 (or older)–1.86 Ga, metamorphosed generally under high-grade conditions and intruded by successive plutonic suites at 1.95–1.93, 1.90–1.88, 1.87–1.85 and 1.81–1.76 Ga. In the northern part, low-grade and low-strain, 1.90–1.86 Ga predominantly magmatic rocks (the Skellefte–Arvidsjaur magmatic province) are enclosed by the basinal components. Subduction-related processes in intra-arc basin and magmatic arc settings, respectively, are inferred. Changes in the metamorphic grade and the relative timing of deformation and structural style across the magmatic province are linked to major shear zones trending roughly north–south and, close to the southern margin, WNW–ESE. Zones trending WNW–ESE and ENE–WSW dominate southwards. Slip along the north–south zones in an extensional setting initiated synchronously with magmatic activity at 1.90–1.88 Ga. Tectonic inversion steered by accretion to a craton to the east, involving crustal shortening, ductile strain and crustal melting, occurred at 1.88–1.85 Ga. Deformation along shear zones under lower-grade conditions continued at c. 1.8 Ga. Felsic volcanic rocks (1.90–1.88 Ga) host exhalative and replacement-type volcanogenic massive sulphide deposits (the metallogenic Skellefte district). Other deposits include orogenic Au, particularly along the ‘gold line’ SW of this district, porphyry Cu–Au–Mo, and magmatic Ni–Cu along the ‘nickel line’ SE of the ‘gold line’.

Ore Remobilization History of the Metamorphosed Rävliden North Volcanogenic Massive Sulfide Deposit, Skellefte District, Sweden

“…The Skellefte district belongs to the Bothnia-Skellefteå lithotectonic unit of the 2.0 to 1.8 Ga Svecokarelian orogen (Fig. 1) in the Fennoscandian shield (Skyttä et al, 2020;Stephens, 2020). The extent of the district is loosely defined by the distribution of Paleoproterozoic metavolcanic and siliciclastic metasedimentary rocks belonging to the Bothnian, Skellefte, Vargfors, and Arvidsjaur groups (Fig.…”

“…The extent of the district is loosely defined by the distribution of Paleoproterozoic metavolcanic and siliciclastic metasedimentary rocks belonging to the Bothnian, Skellefte, Vargfors, and Arvidsjaur groups (Fig. 2), which have undergone regional metamorphism from greenschist to amphibolite facies grade (Skyttä et al, 2020). Metasedimentary rocks of the Bothnian supergroup have been interpreted to stratigraphically underlie the Skellefte group, at least in some parts of the district (Skyttä et al, 2020).…”

“…2), which have undergone regional metamorphism from greenschist to amphibolite facies grade (Skyttä et al, 2020). Metasedimentary rocks of the Bothnian supergroup have been interpreted to stratigraphically underlie the Skellefte group, at least in some parts of the district (Skyttä et al, 2020). The Vidsel-Röjnoret shear zone limits the surface distribution of the Vargfors and Skellefte groups to the east (Fig.…”

“…Rävliden North and the other deposits in the Kristineberg area are hosted by a system of west-plunging anticlines (Årebäck et al, 2005). The deposits and their host rocks have been subjected to greenschist to lower amphibolite facies metamorphism and polyphase deformation at c. 1.9 to 1.8 Ga during the Svecokarelian orogeny (Skyttä et al, 2020). This has resulted in considerable change in the geometry of ore lenses, including remobilization of primary mineralization (Årebäck et al, 2005;Skyttä et al, 2013).…”

Large-scale, flat-lying mafic intrusions in the Baltican crust and their influence on basement deformation during the Caledonian orogeny

“…The Jämtland volcanic porphyries may display structural heterogeneities between the succession of lava flows and the batholiths of the Transscandinavian Igneous Belt, and the Svecofennian domain may consist of amalgamated, flat-lying lenses of granite with both sub-horizontal contacts and magmatic foliations (e.g., Bartley et al, 2012;Cruden and Weinberg, 2018, and references therein). However, even if this characteristic batholith structure could exist (e.g., Mattsson and Elming, 2001a, for the easternmost part of the Rätan granite), it is very unlikely that the intrusive contacts and foliations extend over distances as large as 100 km, as suggested by the complex nature of the foliation in the Svecofennian basement (Skyttä et al, 2020). The presence of older, large-scale, flat-lying shear zones controlling sill emplacement is also unlikely considering the prominent vertical Storsjön-Edsbyn Deformation Zone and Hassela Shear Zone in the basement of central Sweden.…”