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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 petrographic description of the ultrama-
fic rocks presented earlier applies in general to
the host rocks of the ores, too, and so is not re-
peated. Here, only ore minerals and their tex-
tures are described.
In most of the ore types sulphides crystal-
lized from the sulphide liquid and equilibrated
to low temperatures. However, the present ore
parageneses, which are characterized by the
presence of troilite, talnakhite (Fig. 80) and
graphite and the scarcity of primary and sec-
ondary magnetite, were predetermined by re-
ducing magmatic conditions. Secondary mag-
netite, so common in sulphides in traditional
Ni sulphide ores, is scantu in Keivitsa ores.
Rare findings of big sulphide beads (Figs
78a-d) allow a striking window into the secrets
of phase separation in the Keivitsa intrusion. It
is readily seen that olivine, clinopyroxene and
plagioclase crystallized in equilibrium with
two liquids, each with an identical composi-
tion (at a given time) in both liquids. Of
course, because of the low solubility of sili-
cates in sulphide liquid, silicates embedded in
sulphide grew by diffusion through, not from,
sulphide liquid. Postcumulus growth and reac-
tions obscured these relationships in the sur-
rounding silicate cumulate, but in sulphide
beads they are clear – literally – as a mirror:
the silicates crystallized in the order clinopy-
roxene -> olivine -> plagioclase (Fig. 78c-d).
The decreasing ease of nucleation (Wager,
1959) is seen in the increase of grain sizes
from olivine to clinopyroxene to plagioclase,
which is also the order from simpler to more
complicated compositions and lattices.
The sulphide droplets are additional evi-
dence of settling of glomerophyric crystal
packets. It can be seen that the surface of the
sulphide droplet acted as a phase boundary
collector for clinopyroxenes and olivines. This
could also explain the mantled orthopyroxene
euhedra (Fig. 77): sulphide liquid wetted the
surfaces of orthopyroxene euhedra in magma
suspension (phase boundary collector princi-
ple; Mutanen, 1992), prohibiting their growth.
The beads offer a clue to the cumulus sili-
cate assemblages: clinopyroxene was the first
primary silicate phase, then joined the olivine
cotectic and both later joined the plagioclase
cotectic. The nucleation of plagioclase was
difficult in silicate liquid. Because of its
oikocrystic growth in many cumulates plagi-
oclase is judged too hastily as a postcumulus
phase. The ghosts of euhedral plagioclase in
postcumulus plagioclase in pyroxene cumu-
lates of the Koitelainen and Akanvaara intru-
sions, the euhedral plagioclase included in or-
thopyroxene in Keivitsa intrusion (see above)
and the tiny euhedral plagioclases in intercu-
mulus spaces in the Porttivaara intrusion
(Mäkelä, 1975) all suggest that plagioclase
was often a cumulus mineral that failed to nu-
cleate properly.
In false and regular ores the sulphide grain
aggregates are mostly interstitial to silicates.
These primary aggregates are joined by very
thin “chicken wire” sulphide veins with a tex-
ture similar in appearance to nerve cells (Fig.
79). These mu-thin sulphide veins represent
the fractal “ends” of the sulphide vein system.
Secondary magnetite, associated with serpenti-
nized olivine, also forms networks, but the sul-
phide veins do not follow them.
The sulphide/silicate grain boundaries are
plain and smooth in unaltered rocks but irregu-
lar and serrated in metaperidotites, and an au-
reole of fine sulphide dust surrounds the pri-
mary sulphide grain aggregates.
The main sulphides are troilite, hexagonal
pyrrhotite, chalcopyrite, pentlandite, talna-
khite and cubanite; pyrite and monoclinic pyr-
rhotite are rare, and occur only in association
with retrograde alteration, as in carbonate and
albite-carbonate veins. Mackinawite and
sphalerite occur regularly; molybdenite, ga-
Mineralogy of ore types