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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...
The intrusion is a flat, oval-shaped brachy-
anticline structure, 26 km x 29 km in size (see
geological map, Appendix 3). The interior is
made up of footwall rocks of the intrusion –
Archaean granitoid gneisses, overlying late
Archaean to early Proterozoic (“Lapponian”)
supracrustal rocks, pre-Koitelainen gabbroic
intrusions and ultramafic dykes. The Archaean
gneisses form two domes. The smaller one, im-
mediately west of the peridotitic basal cumu-
lates of the Koitelainen intrusion, consists of
tonalites, trondhjemites and granodiorites (Pel-
tonen, 1986), yielding a U-Pb zircon age of
3100 Ma (Kröner et al., 1981); however, Sm-
Nd isotopic ratios suggest a prior crustal resi-
dence of about 250–500 Ma (Jahn et al., 1984).
The few outcrops of the much larger easterly
Kiviaapa dome (U-Pb zircon ages 2765–2821
Ma, Mutanen, 1989b) are located near the base
contact of the Koitelainen intrusion. The east-
ern contact of the Kiviaapa dome gneisses with
the Koitelainen intrusion has been intersected
by two DDHs. These rocks were thermally af-
fected by the intrusion.
A deep gravity low in the core of the Kivi-
aapa dome, with distict gradient boundaries
against surrounding gneisses, suggest the pres-
ence of a granite mass with a great depth ex-
tent here.
The pre-Koitelainen supracrustal rocks
around the Archaean gneisses beneath the in-
trusion include regionally metamorphosed sub-
arkoses, various acid and intermediate volcani-
clastic rocks, volcanic breccias (U-Pb zircon
age 2526±42 Ma, Peltonen, 1986; Pihlaja &
Manninen, 1988), diamictites, tuffs and
tuffites, and basaltic lavas and tuffs. The most
common, but rarely exposed (as a result of
grussification), rock type is a biotite-plagiocla-
se mica gneiss that contains various amounts
of felsic volcanic material.
The Koitelainen intrusion is overlain by a
thick succession of metamorphosed volcanic
and sedimentary rocks. In the roof, within
reach of the thermal effects of the intrusion,
high-aluminous schists (for composition, see
anal. 1, Table 3) and komatiitic volcanic rocks
(Mutanen, 1976a), accompanied by smaller
amounts of basaltic effusives and tuffs and
subarkosic quartzites, predominate. Higher up,
there must be a change from the pre-Koitelai-
nen to post-Koitelainen, pre-Keivitsa supracrus-
tal formations. The boundary has not yet been
located; tentatively I would place it just above
the level of calcareous rocks, micaceous arkos-
ic rocks and low-Ti basalts between the
Koitelainen and Keivitsa intrusions (see Ap-
pendix 4).
Emplaced among the volcanic-sedimentary
rocks are several small mafic intrusions and
extensive komatiitic layered sills. Some of
these are older than, and probably unrelated to,
the Koitelainen intrusion; these include
“komatiite gabbros” and ultramafic intrusions
of the komatiite kin, and a differentiated,
“evolved” mafic sill found by diamond drilling
immediately beneath the Koitelainen intrusion,
at the eastern boundary of the Kiviaapa dome.
I conceive the two layered komatiitic sills
between the Koitelainen and Keivitsa intru-
sions (see geological map, Appendix 4) as
roughly consanguineous with the former.
These sills occur all around the western perim-
eter of the Koitelainen intrusion. When close
to the upper contact of the Koitelainen intru-
sion, the lower part of the lowermost sill regu-
larly contains spots of augite (now mostly al-
tered to amphibole), enclosing clusters of oliv-
ine crystals (for composition, see anal. 2, Ta-
ble 3). I interpret the spots as having formed
through the thermal effect of the Koitelainen
intrusion.
THE KOITELAINEN INTRUSION
General geology