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Geological Survey of Finland, Bulletin 395
Geology and ore petrology of the Akanvaara and Koitelainen mafic layered intrusions and the Keivitsa-Satovaara...
vated Cl in metasediments and serpentinite,
with common occurrence of scapolite porphy-
roblasts in eruptive and metasedimentary
rocks.
The increase in sulphides and graphite in
pelites coincides with the appearance of acid
volcanic dropstones (Fig. 40c). Evidently,
from then on acid volcanism contributed mate-
rial to the pelitic sediments.
Emplaced in the lower part of this succes-
sion are two thick differentiated komatiitic
sills composed of thick olivine cumulates
(+chromite) overlain by pyroxene cumulates
and gabbro. At the top of the upper sill is a
thin seam of granophyre, with dendritic horn-
blende (the “spider rock” of Jorma Isomaa).
The peridotitic main part of the lowermost sill
contains augite oikocrysts enclosing olivine
crystals. These “spotted komatiites”, which I
formerly considered lavas, hug the roof con-
tact of the Koitelainen intrusion round its
western half. The komatiitic peridotites of the
sills are lower in MgO than the dunitic to peri-
dotitic komatiites which occur as xenoliths in
the Keivitsa intrusion.
About 20 m below the base of the Keivitsa
intrusion a 10 m-thick peridotite sill has been
intersected by two diamond drill holes. Xeno-
liths of similar rocks occur in the deeper parts
of the intrusion.
Evidently, there was a long time break in the
supracrustal succession between the Koitelai-
nen and Keivitsa intrusions. I tentatively con-
sider the lower sediments and komatiitic sills
older than the Koitelainen intrusion (2435
Ma); from the arkosic quartzites upwards the
rocks are younger than Koitelainen but some-
what older than Keivitsa (2057 Ma).
Recumbent folds in hornfelsed pelite have
been intersected by diamond drill holes at
Keivitsa. The thermometamorphism postdates
the folding. There is other evidence of pre-
Keivitsa folding. The folds may have been
formed by slumping of unconsolidated sedi-
ments due to crustal instability that was associ-
ated with volcanism and in anticipation of the
Keivitsa event; also, like the tight folds below
the basal contact of the Palisades Sill (Walker,
1969), they may have been formed in connec-
tion with, and by, the emplacement of the
Keivitsa magma.
Several bodies of peculiar albite-rich brec-
cias occur in a wide area west and northwest of
the intrusion. The Puilettilampi hydrothermal
breccia pipe west of Keivitsa (see Appendix 4)
contains abundant carbonate, magnetite and
pyrite. In the same area there are stocks of to-
nalite(-trondhjemite) which often have wide
albitized aureoles and albitite contact breccias.
The tonalites and breccias are massive, non-fo-
liated rocks which have undergone regional
metamorphism. The stocks and breccia bodies
cut through high-aluminous schists, presuma-
bly of pre-Koitelainen age, but not the Keivitsa
intrusion and the pelites immediately below
the intrusion. Closely associated with the Pui-
lettilampi hydrothermal pipe are porphyritic
keratophyres (Fig. 40a) and gabbros. I inter-
pret the breccia bodies and tonalite stocks as
coeval with – and related to – the pre-Keivitsa
acid volcanism. They may represent roots of
volcanoes and volcanic hydrothermal vents.
Younger dyke rocks will be described and
discussed later.
The grade of regional metamorphism
reached the amphibolite facies, and embraced
the rocks of the KSC and dyke rocks. Meta-
morphism obliterated almost all the magmatic
minerals of small igneous bodies as well as the
original hornfels minerals around the KSC and
the komatiitic sills. Thermometamorphic min-
erals are locally preserved inside the Keivitsa
intrusion. Large portions of the ultramafic
rocks in Keivitsansarvi have undergone sur-
prisingly little jointing and are mineralogically
fresh. There are also unaltered parts in the gab-
bro cumulates, but the granophyres and upper-
most gabbros are wholly altered.
During the subsequent folding the intrusions
and their hardened hornfels aureoles behaved
as competent blocks, and the deformation in-
side the intrusions was largely elastic. Fold