Gold deposits in northern Finland - Arkisto.gsf.fi

already reached in the northern part. The last
phase of granitic magmatism comprises tourmaline-
and beryl-bearing pegmatites that occur
as dykes and small intrusions at the southern
parts of the CLGC. The age of the pegmatites is
uncertain. However, they sharply cross-cut the
1.77 Ga, Th-rich granites and are thus younger
(J.P. Ranta, unpublished).
Palaeoproterozoic deformation
and metamorphism
The Palaeoproterozoic rocks in the northern part
of the Fennoscandian Shield have undergone
several phases of deformation and metamorphism.
Metamorphic grades vary from lowergreenschist
in the CLGB to granulite facies in
the Lapland Granulite Belt.
A sequence of ductile deformation events in
the CLGB is reported in Hölttä et al. (2007)
and Patison (2007) and references therein. The
main deformation features relate to thrusting
events at the margins of the CLGB. North- to
north-east-directed thrusting in the southern
margin was driven by the Svecofennian orogenic
events and south-west-directed thrusting
of the Lapland Granulite Belt in the north by
collision of the Kola and Karelian cratons. It is
currently unclear if these events were contemporaneous
or successive. Clear overprinting deformation
features of these thrusting events are
lacking, therefore they are generally referred to
as the D1–2 stage (e.g. Patison 2007). The earliest
detected foliation (S1) is bedding-parallel
and can locally be seen in F2 fold hinges and as
inclusion trails in andalusite, garnet and staurolite
porphyroblasts. The main deformation features
consist of flat-lying to gently-dipping S2
foliation and recumbent or reclined F3 folding.
The D1–2 structures are overprinted by sets of
D3 structures consisting of F3 folding and late
shear zones. The orientation of F3 folds is highly
variable with east and north striking axial
traces dominating. The dips vary from horizontal
through moderately dipping to vertical. The
ductile deformation features are overprinted by
brittle faulting related to the latest deformation
stage D4.
The north- to north-east-directed thrusting
during the D1–2 stage resulted in the development
of two major thrust zones: the Sirkka
and Venejoki thrust zones (Fig. 3). The Sirkka
thrust zone is a rheological boundary between
the Savu koski Group volcano-sedimentary sequence
in the south and the Kittilä Group in
the north, and it hosts numerous gold deposits
and occurrences. It consists of a series of closelyspaced,
south-dipping thrusts, vertical to subvertical
shear zone segments and, based on seismic
data, it dips at about 40 degrees to the south
reaching a depth of at least 9 km (Patison et al.
2006). The D3 stage resulted in the development
of a set of north to north-east striking strike-slip
shear zones which intersect and, in some places,
displace the early thrust zones. A number of
gold occurrences are spatially correlated to these
structures and the largest known gold deposit,
Suurikuusikko, is hosted by one of such structures.
There are also clear indications of reactivation
of the early thrust structures during D3.
Abrupt changes in metamorphic grade are associated
with D3 shear zones suggesting that they
were active after the peak of the metamorphism.
Direct age data on the deformation and
metamorphism is limited. The south-west directed
thrusting of the Lapland Granulite belt
took place in 1.91–1.88 Ga (Tuisku & Huhma
2006) and can be considered as the maximum
age for the D1–2 stage. The Haparanda suite
magmatism during 1.89–1.86 Ga is considered
to be contemporaneous with D1–2 suggesting
that the minimum age for the D1–2 is around
1.86 Ga. A minimum age for the D3 deformation
is given by post-collisional 1.77 Ga Nattanen-type
granites. This is also the maximum age
for D4. The maximum age of D3 is unknown,
but is very likely around 1.86 Ga. The regional
metamorphic peak was reached during the
D1–2 stage.
The metamorphic grade is mainly of low to
intermediate-pressure type, from lower green­
20 Pasi Eilu & Tero Niiranen (ed.)