8: Precambrian geodynamics and ore formation: The Fennoscandian Shield

Abstract

15Compared with present-day global plate tectonics, Archaean and Palaeoproterozoic plate tectonics may have involved faster 16 moving, hotter plates that accumulated less sediment and contained a thinner section of lithospheric mantle. This scenario also 17 fits with the complex geodynamic evolution of the Fennoscandian Shield from 2.06 to 1.78 Ga when rapid accretion of island 18 arcs and several microcontinent-continent collisions in a complex array of orogens was manifested in short-lived but intense 19 orogenies involving voluminous magmatism. With a few exceptions, all major ore deposits formed in specific tectonic settings 20 between 2.06 and 1.78 Ga and thus a strong geodynamic control on ore deposit formation is suggested. 21All orogenic gold deposits formed syn-to post-peak metamorphism and their timing reflects the orogenic younging of the 22 shield towards the SW and west. Most orogenic gold deposits formed during periods of crustal shortening with peaks at 2.72 to 23 2.67, 1.90 to 1.86 and 1.85 to 1.79 Ga. 24The ca. 2.5 to 2.4 Ga Ni-Cu F PGE deposits formed both as part of layered igneous complexes and associated with mafic 25 volcanism, in basins formed during rifting of the Archaean craton at ca. 2.5 to 2.4 Ga. Svecokarelian ca. 1.89 to 1.88 Ga Ni-Cu 26 deposits are related to mafic-ultramafic rocks intruded along linear belts at the accretionary margins of microcratons. 27All major VMS deposits in the Fennoscandian Shield formed between 1.97 and 1.88 Ga, in extensional settings, prior to 28 basin inversion and accretion. The oldest bCyprus-typeQ deposits were obducted onto the Archaean continent during the onset of 29 convergence. The Pyhäsalmi VMS deposits formed at 1.93 to 1.91 Ga in primitive, bimodal arc complexes during extension of 30 the arc. In contrast, the Skellefte VMS deposits are 20 to 30 million years younger and formed in a strongly extensional intra-arc 31 region that developed on continental or mature arc crust. Deposits in the Bergslagen-Uusimaa belt are similar in age to the 32 Skellefte deposits and formed in a microcraton that collided with the Karelian craton at ca. 1.88 to 1.87 Ga. The Bergslagen- ARTICLE IN PRESS33 Uusimaa belt is interpreted as an intra-continental, or continental margin back-arc, extensional region developed on older 34 continental crust. 35Iron oxide-copper-gold (IOCG) deposits are diverse in style. At least the oldest mineralizing stages, at ca. 1.88 Ga, are 36 coeval with calc-alkaline to monzonitic magmatism and coeval and possibly cogenetic subaerial volcanism more akin to 37 continental arcs or to magmatic arcs inboard of the active subduction zone. Younger mineralization of similar style took place 38 when S-type magmatism occurred at ca. 1.80 to 1.77 Ga during cratonization distal to the active N-S-trending subduction zone 39 in the west. Possibly, interaction of magmatic fluids with evaporitic sequences in older rift sequences was important for ore 40 formation. 41Finally, the large volumes of anorthositic magmas that characterize the S...

U-Th-Pb zircon geochronology on igneous rocks in the Toija and Salittu Formations, Orijärvi area, southwestern Finland: Constraints on the age of volcanism and metamorphism

“…Allen et al, 1996;Kumpulai nen et al, 1996) but in Finland, volcanic arc, back-arc rifting and rifting of older continental margin settings have been proposed (e.g. Lindroos & Ehlers, 1994;Kähkönen, 2005;Weihed et al, 2005). A combination of these settings was proposed by and Väisänen & Westerlund (2007).…”

Reconnaissance-scale conceptual fuzzy-logic prospectivity modelling for iron oxide copper – gold deposits in the northern Fennoscandian Shield, Finland

“…The northern region of the Fennoscandian Shield is recognised as an economically important metallogenic province dominated by generally small Fe oxide and Au -Cu deposits, some with IOCG affinities (Niiranen et al 2002;Edfelt et al 2005;Niiranen 2005;Weihed et al 2005). These deposits mainly occur within or close to (5100 km) the margin of the Archean craton (Figure 1), above Archean continental crust and subcontinental lithospheric mantle, and are hosted by juvenile Paleoproterozoic volcano-sedimentary sequences formed during rifting of the Archean craton (Weihed et al 2005).…”

“…These deposits mainly occur within or close to (5100 km) the margin of the Archean craton (Figure 1), above Archean continental crust and subcontinental lithospheric mantle, and are hosted by juvenile Paleoproterozoic volcano-sedimentary sequences formed during rifting of the Archean craton (Weihed et al 2005). As summarised by Weihed et al (2005), geochronological studies indicate that Paleoproterozoic hydrothermal mineralisation was multiphase, probably occurring at least over 160 million years from 1.9 to 1.76 Ga.…”

“…These deposits occur in Archean to Proterozoic crust along the rifted southwest margin of the Archean Karelian craton where they are temporally associated with calc-alkalic to alkalic magmatism. This rifted margin contains other ores and volatile-rich magmas, including: (i) a large number of epigenetic occurrences with an unusual metal association (variable combinations of Au, Co, Cu, F, Fe, LREE, Nb, Ni, Pd, P, Th, U, and Zr) (Weihed et al 2005); (ii) the Outokumpu Cu -Co -Zn -Ni -Au massive-sulfide deposits (Gaá l et al 1975;Kontinen et al 2005); (iii) the Siilinjä rvi carbonatite-hosted apatite deposit (Puustinen & Kauppinen 1989); (iv) lamprophyres (O'Brien et al 2005); and (v) kimberlites (Lehtonen 2005;O'Brien et al 2005). The craton margin has been recognised as one of the most significant metallogenic belts in the Fennoscandian Shield, resulting in decades of debate over possible genetic connections between the ore types located there (Hietanen 1975;Gaá l & Gorbatschev 1987;Peltonen 2005).…”

“…Boundaries of the major geologic units from Lahtinen et al (2005). Filled circles represent deposits and showings with IOCG affinities (Eilu 1999;Niiranen et al 2002;Edfelt et al 2005;Weihed et al 2005). Filled diamonds define the location of kimberlites (Lehtonen 2005).…”