22
Geological Survey of Finland, Bulletin 395
Tapani Mutanen
granophyre caps have been preserved (Pert-
tunen, 1991).
The most completely analysed of the best
preserved intrusions is Akanvaara, which
shows (Fig. 5) that the bulk composition of the
intrusion corresponds to a fairly normal tholei-
ite, with ca 52% SiO
2
, 6–8% MgO, 0.7% TiO
2
,
and low Cr and Ni. Thus, the magma was not
particularly “primitive”. It was rather similar
in bulk composition to the Bushveld layered
series (see von Gruenewaldt, 1979), and Bush-
veld magma at the Merensky Reef fractionat-
ing stage (Cawthorn, 1995, 1996).
From what we know of the fractionation of
the Koitelainen and Akanvaara magmas (low
H
2
O) and the bulk intrusion compositions
(MgO, Cr and SiO
2
), it is clear that the mag-
mas were not boninitic (see Crawford et al.,
1989) as proposed by Alapieti (1996). The un-
tainted chill compositions have a Cr content
appropriate to tholeiitic basalts, low REE and a
flat REE CN pattern (see Alapieti, 1982).
SHMB-type volcanism (see Sun et al., 1989)
preceded the Akanvaara intrusion, but the in-
trusion itself is not of the SHMB type.
The parent magmas of the 2440–2490 Ma
intrusions were low in Ti and P, and the Fe
3+
/
Fe
2+
ratio was low. Thus, ilmenite never crys-
tallized as a “legal” cumulus mineral, cumulus
apatite appeared only in the uppermost ferrodi-
orites, and cumulus magnetite (original ti-
tanomagnetite) entered late (at Koitelainen at
about 94 PCS level).
The magmas were also heavily undersaturat-
ed in sulphur (sulphide liquid), and hence,
aside from transient sulphide saturation due to
contamination, terminal saturation of sulphide
liquid (high Fe and Cu/Ni) occurred at or
above the magnetite-in phase contact. In this
respect the magmas were quite similar to that
of other tholeiitic intrusions, e.g., the Bush-
veld Complex (Liebenberg, 1970) and Kigla-
pait intrusion (Morse, 1969a, 1979b).
The mafic layered intrusions in northern
Finland and their counterparts in Russian
Karelia and the Kola Peninsula host a variety
of ore deposits. With their numerous known
prospects of PGE (Razin, 1974; Mäkelä, 1975;
Isohanni, 1976; Piirainen et al., 1977; Mu-
tanen, 1979, 1989a,b, 1996; Vuorelainen et al.,
1982; Piispanen & Tarkian, 1984; Lavrov &
Pekki, 1987; Mutanen et al., 1987, 1988;
Krivenko et al., 1989; Lazarenkov et al., 1989,
1991; Yakovlev et al., 1991; Barkov et al.,
1991, 1994; Pchelintseva & Koptev-Dvorni-
kov, 1993; Golubev & Filippov, 1995; Mitro-
fanov et al., 1995a,b; Nikitichev et al., 1995;
Trofimov, 1995, Trofimov et al., 1995), of Ti
and V (e.g., Proskuryakov, 1967; Juopperi, 1977,
Mutanen, 1989b) and of chromite (Veltheim,
1962; Kujanpää, 1964; Kozlov et al., 1975; Bar-
kanov, 1976; Mutanen, 1979, 1981, 1989a,b,
1996; Dokuchaeva et al., 1982a, b; Lavrov &
Trofimov, 1986; Lavrov & Pekki, 1987; Alapieti
et al., 1989; Smirnova et al., 1996), they are jus-
tifiable targets for the present exploration pro-
grammes in the Shield. Taken together, the
chromitite layers of Koitelainen, Akanvaara,
Monchegorsk, Imandra and Burakovka already
constitute the largest known undeveloped re-
source of chromite in the world.
A belt of layered intrusions runs in a general
southeasterly direction through Finnish Lap-
land and on into Russia (Fig. 1). The name La-
pland Intrusion Belt (LIB) is used here for the
belt. The LIB contains intrusions, sills and
dykes with ages from 2490 Ma to 2000 Ma and
younger. The intrusions of the 2440 Ma age
group occur in the axial part, and throughout
the length, of the LIB, from Tsohkkoaivi-
Kaamajoki in the northwest to Kivakka,
Tsipringa and Lukkulaisvaara (the Olanga
group) in Russia. Small intrusions and dykes
of the general magma type of 2440 Ma intru-
sions are met with southeast of Olanga (see
Fig. 1); the belt may extend still farther south-
eastwards, to Burakovka in the Lake Onega re-
gion, Russia.
Relics of a pre-Koitelainen intrusion were
intersected below the Koitelainen intrusion by
two diamond drill holes, and its rocks occur as
xenoliths near the base of the Koitelainen in-