Gold deposits in northern Finland - Arkisto.gsf.fi

No clear examples of subduction-related
magmatism between 2.70 and 2.05 Ga have
been found in Fennoscandia. The 2.02 Ga, felsic
sub-volcanic and possibly volcanic rocks in
Finnish Lapland (Kittilä in Fig. 1) occur in association
with oceanic island arc-type rocks and
are the oldest candidates for Palaeoproterozoic
subduction-related rocks (Hanski & Huhma
2005). Associated continental within-plate volcanic
rocks are possibly related to the continued
break-up of the craton. Bimodal alkaline–
tholeiitic magmatism in central Lapland (Hanski
et al. 2005) and rift-related magmatism in
Kola (Pechenga) show that rift magmatism continued
until 1.98 Ga. The Jormua–Outokumpu
ophiolites (Fig. 1), tectonically intercalated with
deep-water turbidites, are unique examples of
Archaean subcontinental lithospheric mantle
with a thin veneer of oceanic crust formed at
1.95 Ga along the western edge of the present
Karelian province (Peltonen 2005).
The main Palaeoproterozoic orogenic evolution
of Fennoscandia can be divided into the
Lapland–Kola orogen (1.94–1.86 Ga, Daly
et al. 2006) and the composite Svecofennian
orogen (1.92–1.79 Ga, Lahtinen et al. 2005,
2008). The latter is divided into the Lapland–
Savo, Fennian, Svecobaltic and Nordic orogens.
Whereas the Lapland–Kola orogen shows only
limited formation of new crust, the composite
Sveco fennian orogen produced a large volume
of Palaeoproterozoic crust in the Svecofennian
province. It should be noted that igneous rocks
of 1.9–1.8 Ga age in northern Finland show
a large contribution of older LREE-enriched
litho sphere (Huhma et al. 2011) verifying the
occurrence of Archaean crust below the Palaeoproterozoic
cover in the Karelian province.
Palaeoproterozoic 2.45–1.88 Ga
supracrustal sequences
Palaeoproterozoic supracrustal sequences overlie
much of the northern part of the Fennoscandia.
In Finland, the main sequences are the Central
Lapland Greenstone Belt, Peräpohja and Kuusamo
schist belts and the Lapland granulite belt
(Figs. 1, 3–6). Figure 4 summarises the stratigraphy
of the Central Lapland Greenstone Belt
(CLGB), which is the dominant supracrustal sequence
of the region. This stratigraphy has been
applied also to the Peräpohja and Kuusamo belts
with variable success. Figures 5 and 6 show the
major stratigraphic units for the Peräpohja belt.
The lowermost units of the greenstones, the
Vuojärvi and Salla groups, lie unconformably
on the Archaean TTG rocks. The rocks of the
Salla Group are clearly of early Palaeoproterozoic
age. However, the age of the Vuojärvi Group is
uncertain and can even be of the latest Neoarchaean.
The Vuojärvi and Salla groups are followed
by sedimentary and volcanic units which
precede the c. 2.2 Ga igneous event and comprise
the Kuusamo and Sodankylä Groups in
the CLGB and in the Kuusamo schist belt. The
Sodan kylä Group also hosts many, but not all,
of the known Palaeoproterozoic syngenetic sulphide
occurrences in the CLGB.
The mafic to ultra mafic volcanic and shallow
marine sedimentary units of the Savukoski
Group were deposited during 2.2–2.01 Ga in
the CLGB, and similar units were also formed
in the Kuusamo and Peräpohja belts (Lehtonen
et al. 1998, Rastas et al. 2001). In the latter belt,
rocks aged from the Petäjäskoski Formation to
Väystäjä Formation (Fig. 5) seem to be contemporaneous
with the Savukoski Group (Kyläkoski
et al. 2012a). Age data from the Palaeoproterozoic
greenstones (e.g. Perttunen & Vaasjoki
2001, Rastas et al. 2001, Väänänen & Lehtonen
2001, Kyläkoski et al. 2012a) suggest a major
magmatic and rifting event at c. 2.1 Ga with the
final break up taking place at 2.06 Ga. Extensive
occurrences of 2.13 and 2.05 Ga dolerites
also support these dates. Thick piles of mantlederived
volcanic rocks, including komatiitic and
picritic high-temperature melts, are restricted to
the Kittilä–Karasjok–Kauto keino–Kiruna area
and are suggested to represent plume-generated
volcanism (Martinsson 1997). The rifting of the
Archaean craton along a north-west-trending
12 Pasi Eilu & Tero Niiranen (ed.)