Gold deposits in northern Finland - Arkisto.gsf.fi

Introduction
Pasi Eilu & Tero Niiranen, Geological Survey of Finland
The Fennoscandian shield forms the northwestern
part of the East European craton, and
comprises most of Finland, Sweden, Karelia,
the Kola Peninsula and, with the Caledonides,
most of Norway (Fig. 1). The oldest rocks have
been dated at 3.6 Ga (Huhma et al. 2004), but
most of the shield comprises Neoarchaean to
Palaeoproterozoic rocks (Lahtinen et al. 2008).
Meso- to Neoproterozoic crust dominates in the
south-western part of the shield, and the Caledonides
form a major part of the west of the region.
Economic mineral deposits occur in rocks of
nearly all ages, with major peaks of formation
during the Palaeoproterozoic and Caledonian
orogenies.
Northern Finland comprises rocks of c. 3.1
to 1.8 Ga. Metallogenic belts cover large parts
of the area (Fig. 2). Despite extensive Neoarchaean
rocks in the area, no economic mineralisation
predating 2.45 Ga has so far been
discovered. The detected Archaean BIF, epigenetic
Au and komatiitic Ni occurrences are
rather small (FODD 2013). Major mineralisation
stages in northern Finland relate to 2.45 Ga
layered intrusions (PGE ± Ni-Cu, Cr, V-Ti-Fe),
2.1–1.9 Ga (?) supracrustal sequences (komatiitic
Ni, stratabound Cu-Zn, BIF, possible Au),
c. 2.06 Ga mafic to ultramafic intrusions (Ni-
Cu ± PGE), and the 1.94–1.79 Ga Lapland–
Kola and Svecofennian orogenies (Au ± Cu ±
Co, IOCG).
Presently, mining takes place in the 2.45 Ga
and 2.06 Ga intrusions (Cr and Ni-Cu-PGE,
respectively) and the Palaeoproterozoic supracrustal
rocks mineralised during the Lapland–
Kola to Svecofennian orogenies (Au). This field
trip targets on the latter: orogenic gold and of
possibly other types of epigenetic gold deposits
(Suurikuusikko, Saattopora, Rompas), but we
will also see a gold deposit that may be of syngenetic
type (Pahtavaara). Despite mining, a lot of
genetic uncertainties still relate to these deposits,
matters that we expect to be widely discussed
during this field trip.
Geological and tectonic evolution of the northern part
of the Fennoscandian shield
Pasi Eilu, Tero Niiranen & Laura Lauri, Geological Survey of Finland
The bedrock of northern Finland comprises
Mesoarchaean to Palaeoproterozoic rocks. Their
geological evolution can only be described in
the context of the entire Fennoscandian shield.
Below, we describe this evolution in three parts,
the Archaean, the Proterozoic predating the
Svecofennian orogeny and the Svecofennian.
This description is essentially based on Lahtinen
(2012) unless otherwise is indicated.
The Archaean crust, either exposed or concealed
under Palaeoproterozoic cover rocks
and granitoids, occurs in the east and north of
Fenno scandia. Four major Archaean provinces
have been outlined (Fig. 1, modified after Hölttä
et al. 2008). The western and eastern parts of the
Karelian province comprise Mesoarchaean, 2.8–
3.0 Ga rocks, but rocks older than 3.0 Ga occur
locally. The central part of the Karelian province
is mainly Neoarchaean, having plutonic and volcanic
rocks that are 2.75–2.70 Ga in age. The
volcanic belts formed in various geo dynamic settings,
including oceanic plateau, island arc, back
arc, intra-arc and intra-continental rift. Neoarchaean
accretion at c. 2.83–2.75 Ga and sub­
8 Pasi Eilu & Tero Niiranen (ed.)