8 Introduction Pär Weihed, Olof Mart<strong>in</strong>sson Luleå University of Technology, Luleå, Sweden Pasi Eilu Geological Survey of F<strong>in</strong>l<strong>and</strong>, Espoo, F<strong>in</strong>l<strong>and</strong> The Fennosc<strong>and</strong>ian Shield forms the north-westernmost part of the East European craton <strong>and</strong> constitutes large parts of F<strong>in</strong>l<strong>and</strong>, NW Russia, Norway, <strong>and</strong> Sweden (Fig. 1). The oldest rocks yet found <strong>in</strong> the shield have been dated at 3.5 Ga (Huhma et al. 2004) <strong>and</strong> major orogenies took place <strong>in</strong> the Archaean <strong>and</strong> Palaeoproterozoic. Younger Meso- <strong>and</strong> Neoproterozoic crustal growth took place ma<strong>in</strong>ly <strong>in</strong> the western part, but apart from the anorthositic Ti-deposits <strong>in</strong> SW Norway, no major ore deposits are related to rocks of this age. The western part of the shield was reworked dur<strong>in</strong>g the Caledonian Orogeny. Economic m<strong>in</strong>eral deposits are largely restricted to the Palaeoproterozoic parts of the shield. Although Ni–PGE, Mo, BIF, <strong>and</strong> orogenic <strong>gold</strong> de- Geological <strong>and</strong> tectonic evolution of the northern part of the Fennosc<strong>and</strong>ian shield Stefan Bergman Geological Survey of Sweden, Uppsala, Sweden Pär Weihed, Olof Mart<strong>in</strong>sson Luleå University of Technology, Luleå, Sweden Pasi Eilu Geological Survey of F<strong>in</strong>l<strong>and</strong>, Espoo, F<strong>in</strong>l<strong>and</strong> Markku Ilj<strong>in</strong>a Geological Survey of F<strong>in</strong>l<strong>and</strong>, Rovaniemi, F<strong>in</strong>l<strong>and</strong> Regional geology The oldest preserved cont<strong>in</strong>ental crust <strong>in</strong> the Fennosc<strong>and</strong>ian Shield was generated dur<strong>in</strong>g the Saamian Orogeny at 3.1–2.9 Ga (Fig. 1) <strong>and</strong> is dom<strong>in</strong>ated by gneissic tonalite, trondhjemite <strong>and</strong> granodiorite. Rift- <strong>and</strong> volcanic arc-related greenstones, subduction-generated calc-alkal<strong>in</strong>e volcanic rocks <strong>and</strong> tonalitic-trondhjemitic igneous rocks were formed dur<strong>in</strong>g the Lopian Orogeny at 2.9–2.6 Ga. Only a few Archaean economic to subeconomic posits, <strong>and</strong> some very m<strong>in</strong>or VMS deposits occur <strong>in</strong> the Archaean, virtually all economic examples of these deposit types are related to Palaeoproterozoic magmatism, deformation <strong>and</strong> fluid flow. Besides these major deposit types, the Palaeoproterozoic part of the shield is also known for its Fe-oxide deposits, <strong>in</strong>clud<strong>in</strong>g the famous Kiruna-type Fe-apatite deposits. Large-tonnage low-grade Cu–Au deposits (e.g., Aitik), are associated with <strong>in</strong>trusive rocks <strong>in</strong> the northern part of the Fennosc<strong>and</strong>ian Shield. These deposits have been described as porphyry style deposits or as hybrid with features that also warrant classification as iron oxide–copper–<strong>gold</strong> (IOCG) deposits (Weihed 2001, Wanha<strong>in</strong>en et al. 2005). Dur<strong>in</strong>g this field trip to northern F<strong>in</strong>l<strong>and</strong>, we will emphasize <strong>gold</strong> deposit characteristics, their diversity, <strong>and</strong> speculate on temporal <strong>and</strong> spatial relationship between different deposits. The deposits are discussed <strong>in</strong> terms of their tectonic sett<strong>in</strong>g <strong>and</strong> relationship to the overall geodynamic evolution of the shield. Also considered are deposit-scale structural features <strong>and</strong> their relevance for the underst<strong>and</strong><strong>in</strong>g of the ore genesis. m<strong>in</strong>eral deposits have been found <strong>in</strong> the shield, <strong>in</strong>clud<strong>in</strong>g orogenic <strong>gold</strong>, BIF <strong>and</strong> Mo occurrences, <strong>and</strong> ultramafic-to mafic-hosted Ni-Cu (Weihed et al. 2005, Fennosc<strong>and</strong>ian Ore Deposit Database 2010). Dur<strong>in</strong>g the Palaeoproterozoic, Sumi-Sariolian (2.5–2.3 Ga) clastic sediments, <strong>in</strong>tercalated with volcanic rocks vary<strong>in</strong>g <strong>in</strong> composition from komatiitic <strong>and</strong> tholeiitic to calc-alkal<strong>in</strong>e <strong>and</strong> <strong>in</strong>termediate to felsic, were deposited on the deformed <strong>and</strong> metamorphosed Archaean basement dur<strong>in</strong>g extensional events. Layered <strong>in</strong>trusions, most of them with Cr, Ni, Ti, V <strong>and</strong>/or PGE occurrences, represent a major magmatic <strong>in</strong>put at 2.45–2.39 Ga (Amel<strong>in</strong> et al. 1995, Mutanen 1997, Alapieti & Laht<strong>in</strong>en 2002). Periods of arenitic sedimentation preceded <strong>and</strong> followed extensive komatiitic <strong>and</strong> basaltic volcanic stages at about 2.2, 2.13, 2.05 <strong>and</strong> 2.0 Ga <strong>in</strong> the northeastern part of the Fennosc<strong>and</strong>ian Shield dur<strong>in</strong>g extensional events (Mutanen 1997, Lehtonen et al. 1998, Rastas et al. 2001). Associated with the subaquatic extrusive <strong>and</strong> volcaniclastic units, there are carbonate rocks, graphite schist, iron formation <strong>and</strong> stratiform sulphide occurrences across the region.
Fig. 1. Simplified geological map of the Fennosc<strong>and</strong>ian Shield with major tectono-stratigraphic units discussed <strong>in</strong> text. Map based on Koist<strong>in</strong>en et al. (2001), tectonic <strong>in</strong>terpretation after Laht<strong>in</strong>en et al. (2005). LGB = Lapl<strong>and</strong> Greenstone Belt, CLGC = Central Lapl<strong>and</strong> Granitoid Complex, BMB = Belomorian Mobile Belt, CKC = Central Karelian Complex, IC = Iisalmi Complex, PC = Pudasjärvi Complex, TKS = Tipasjärvi–Kuhmo–Suomussalmi greenstone complex. Shaded area, BMS = Bothnian Megashear. 9
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