Gold deposits in northern Finland - Arkisto.gsf.fi

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line from Ladoga to Lofoten was accompanied by the formation of north-west- and north-easttrending rift basins (Saverikko 1990) and injection of 2.1 Ga dyke swarms parallel to these (Vuollo 1994). Rifting culminated in mafic and ultramafic volcanism and the formation of oceanic crust at c. 1.97 Ga. This is indicated by the extensive komatiitic and basaltic lavas of the Kittilä Group of the CLGB (Figs. 3 and 4). The 1.97 Ga stage also included deposition of shallow to deep marine sediments, the latter indicating the most extensive rifting within Fennoscandia. Fragments of oceanic crust were subsequently emplaced back onto the Karelian craton in Finland, as indicated by the Nuttio ophiolites in central Finnish Lapland and the Jormua and Outokumpu ophiolites farther south (Kontinen 1987, Sorjonen-Ward et al. 1997, Lehtonen et al. 1998). In northern Finland, pelitic rocks in the Lapland Granulite Belt were deposited after 1.94 Ga (Tuisku & Huhma 2006). In central Lapland, the Kittilä Group greenstones are overlain by intermediate to felsic volcanic and terrestial sedimentary rocks. Regionally, they exhibit considerable variation in lithological composition due to partly rapid changes from volcanic to sedimentary dominated facies. Within the CLGB, these rocks are mainly represented by the Kumpu Group (Lehtonen et al. 1998) and by the Paakkola Group in the Peräpohja area (Perttunen & Vaasjoki 2001). The molasse-like conglomerates and quartzites comprising the Kumpu Group were deposited in deltaic and fluvial fan environments after 1913 Ma and before c. 1880 Ma (Rastas et al. 2001). Palaeoproterozoic intrusive magmatism Early rifting and layered intrusions The 2.50–2.44 Ga rifting event, possibly related to a major mantle plume, is indicated by the intrusion of numerous layered mafic igneous complexes and associated silicic intrusions in northern Fennoscandia (Alapieti et al. 1990, Lauri et al. 2012a). Most of the intrusions are located along the margin of the Archaean domains, either at the boundary against the Proterozoic supracrustal sequence, totally enclosed by the Archaean granitoids, or enclosed by a Protero zoic supracrustal sequence. Most of the intrusions define the west-trending Tornio– Näränkävaara belt of layered intrusions (Iljina & Hanski 2005). The rest of the intrusions are in north-western Russia, central Finnish Lapland and north-western Finland. Mafic dykes Mafic dykes are locally abundant in northern Finland, and show a variable strike, degree of alteration and metamorphic recrystallisation which, with age dating, indicate multiple igneous episodes. Albite diabase (a term commonly used in Finland and Sweden for any albitised dolerite) is a characteristic type of intrusion that forms up to 200 m thick sills. They commonly have a coarse-grained central part dominated by albitic plagioclase and constitute laterally extensive, highly magnetic units. In northern Finland, albite diabases, both sills and dykes, form age groups of 2.2, 2.13, 2.05 and 2.0 Ga (Vuollo 1994, Lehtonen et al. 1998, Perttunen & Vaasjoki 2001, Rastas et al. 2001, Figs. 4 and 5). These ages also reflect extrusive magmatism in the region. The dykes vary in width from less than 1 m to one kilometre. Nearly all show internal differentiation and igneous textures but consist of metamorphic and altered mineral assemblages (carbonate, sericite, epidote, biotite and scapolite). In areas with greenschist facies regional metamorphism they are commonly surrounded by albitised and carbonated country rocks (Eilu 1994). Granitoids A major part of the bedrock in northern Finland is composed of various types of granitoid rocks. A conspicuous feature in the bedrock map is the Central Lapland Granitoid Complex (CLGC), which lies south of the Central Lapland Green 18 Pasi Eilu & Tero Niiranen (ed.)
stone Belt (CLGB). The CLGC is geologically not as well known as the surrounding supracrustal belts, but recent data suggest that the complex hosts several granitoid suites that range in age from >2.1 Ga to 1.76 Ga (Lauri et al. 2012b). The classification of granitoid suites used in Sweden (see Bergman et al. 2007) thus seems to be only partly applicable to the CLGC. Isotope geochemical data (Huhma 1986, Ahtonen et al. 2007) suggest that the CLGC has a strong Archaean source component. However, no Archaean rocks have so far been found within the complex. The oldest, >2.1 Ga granitoids are situated at the north-eastern margin of the complex, adjacent to the CLGB, forming a 150 km long line of intrusions with approximately similar ages (Lauri et al. 2012b). The central parts of the CLGC contain deformed granitoids with ages of 1.85 Ga (Ahtonen et al. 2007). Even older, c. 1.99 Ga, heavily deformed porphyritic granites are found within the supracrustal Peräpohja belt in the south (J.P. Ranta, unpublished). In the Lapland Granulite Belt, arc-related rocks of the norite-enderbite series intruded the supracrustal sequence at 1920–1905 Ma (Tuisku & Huhma 2006). A major suite of c. 1.88–1.86 Ga calc-alkaline granitoids is known in both Sweden and western Finnish Lapland. The name Haparanda Suite was originally assigned to intrusions in south-eastern Norrbotten, Sweden (Ödman et al. 1949). Later, it was extended to comprise petrographically similar rocks in northern Norrbotten and Finland (Ödman 1957, Hiltunen 1982). These intrusions are medium- to coarse-grained, even-grained, moderately to intensely deformed grey tonalites and granodiorites, which are associated with gabbros, diorites and rare true granites (Ödman 1957). Compositional variations are not prominent in individual intrusions, and the variation from intermediate to felsic types can mainly be seen between separate intrusive bodies. The geochemical signature of the Haparanda suite is typical for ‘volcanic arc granitoids’ with low Rb, Y and Nb contents (Mellqvist et al. 2003). They define a calc alkaline trend and are metaluminous to slightly peraluminous. An age range of 1.89–1.86 Ga has been defined for the suite in the western parts of northern Finland (Huhma 1986, Perttunen & Vaasjoki 2001, Rastas et al. 2001, Väänänen & Lehtonen 2001, Figs. 4 and 5). The compositional range and the chemical characteristics of the Haparanda Suite are very similar to that of the arc-related volcanic rocks in northern Sweden. Thus, the Haparanda Suite intrusions are regarded as comagmatic with extrusive phases of the early Svecofennian arc magmatism. This is supported by their calcalkaline character (Bergman et al. 2001) and also by the contemporaneity with subduction in the shield (Lahtinen et al. 2005). The north-western and south-eastern corners of the CLGC host crustally derived, leucocratic microcline granites with ages of 1.81 Ga (Väänänen & Lehtonen 2001, Ahtonen et al. 2007, Lauri et al. 2012b). These are abundant especially in the south-eastern part of the complex, where they seem to be limited to the extent of the Archaean Pudasjärvi complex underneath the Palaeoproterozoic cover. In the central part of the CLGC a suite of mantle-derived, appinitic rocks, ranging in composition from quartz diorites to syenites, has an age of c. 1796 Ma (Väänänen 2004, Ahtonen et al. 2007). The main phase of the CLGC seems to be constrained between 1.79–1.76 Ga (Ahtonen et al. 2007, Lauri et al. 2012b). Abundant granites intruded the older rock types at this stage. The granites vary from deformed, migmatitic varieties to crosscutting, undeformed types. The southern margin of the CLGC seems to be mainly composed of 1.77 Ga, pink, even-grained granite that has an exceptionally high Th/U ratio (J.P. Ranta, unpublished). Despite the similar age these granites are different from the Nattanen-type granites in the northern part of the CLGB that occur as postcollisional, discordant stocks (Heilimo et al. 2009). It seems that the tectonic regime was still mainly compressional in the southern part of the CLGC whereas the post-tectonic stage was Excursion Guidebook FIN1 19
Page 1 and 2: 12th Biennial SGA Meeting 12-15 Aug
Page 3: Excursion Guidebook FIN1 Gold depos
Page 7 and 8: Contents Programme ................
Page 9 and 10: Saattopora Levi Suurikuusikko (Kitt
Page 11 and 12: Timanide orogen 500 km Murmansk Lap
Page 13 and 14: Metallogenic Areas and Mineral Depo
Page 15 and 16: 24°0'0"E 26°0'0"E 68°0'0"N Suuri
Page 17 and 18: 24°0'0"E 25°0'0"E 26°0'0"E N Rom
Page 19: Table 2. Selected, potentially sign
Page 23 and 24: schist to upper amphibolite facies.
Page 25 and 26: of Ag, Au, As, CO 2 , K, Rb, S, Sb
Page 27 and 28: 3472000 3476000 3480000 3484000 750
Page 29 and 30: 30% decrease in net volume. Amphibo
Page 31 and 32: 7525300 A’ 3391900 3390500 A 200
Page 33 and 34: eralisation took place at 300-350
Page 35 and 36: comm. 2002). Ore-grade rock was fou
Page 37 and 38: Figure 18. Loading ore. Underground
Page 39 and 40: from progressive alteration of mafi
Page 41 and 42: least 25 km (Fig. 15). The dip of t
Page 43 and 44: Table 4. Alteration minerals presen
Page 45 and 46: tober 15, 2012 (Fig. 28). The prese
Page 47 and 48: 3 395 000 mE 3 400 000 mE 3 405 000
Page 49 and 50: ciated with the gold and uraninite.
Page 51 and 52: Table 6. North Rompas drilling inte
Page 53 and 54: Grönholm, P., 1999: The mesotherma
Page 55 and 56: J.S., Siedlecka, A. & Solli, A., 19
Page 57: Sorjonen-Ward, P., Ojala, V.J. & Ai

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