Mutanen, Tapani., 1997. Geology and ore petrology of the Akanvaara and
Koitelainen mafic layered intrusions and the Keivitsa-Satovaara layered com-
plex, northern Finland. Geological Survey of Finland, Bulletin 395, – pages,
88 figures, 11 tables and 5 appended maps.
The Akanvaara and Koitelainen intrusions, ca 2440 Ma old, represent a
group of cratonic mafic layered intrusions (aged from 2500 to 2440 Ma) of
the Fennoscandian (Baltic) Shield. These intrusions, which show all the stand-
ard types of igneous layering, host deposits of chromite, vanadium, titanium,
platinum-group elements (PGE) and gold.
The cumulate stratigraphy in Akanvaara and Koitelainen reflects both
normal tholeiitic fractionation and superimposed intermittent selective and
wholesale contamination by anatectic salic melt (now granophyre) from the
floor and the roof, and by refractory residual phases after anatexis of the roof
rocks. Cumulus phases separated mainly in a hybrid melt between the mafic
main magma and the overlying buoyant acid melt. The contaminants are found
in cumulates as fossil melt and magma inclusions in olivine, chromite, or-
thopyroxene and plagioclase, and as “granitic” intercumulus material. The
composition of the intercumulus material in the most heavily contaminated
cumulates deviates strongly from what one would expect of the mafic main
magma. Unusual minerals (loveringite, chlorapatite. zircon, zirconolite, bad-
deleyite, thorite, allanite, galena, perrierite, a Ti-Th-REE-P silicate) are com-
mon in the intercumulus of the ultramafic cumulates; ilmenite, chlorapatite
and zirconolite occur as daughter and occluded minerals in melt and magma
inclusions in olivines and orthopyroxene. Chlorapatite, ilmenite and loverin-
gite are the most common Doppelgänger phases (Mutanen, 1992), of which
only the apatite phase (as a fluorapatite) appears as a late cumulus mineral.
The melting occurred both early and late in the crystallization history,
corresponding to the water-saturated and dry melting of the granitic constit-
uents of the surrounding crustal rocks. There is no evidence for, nor need of,
multiple magma pulses; the reversals can readily be explained by contami-
nants transmitted by two-phase convection. Contamination of the magma by
crustal material shifted the liquidus phase boundaries, resulting in excessive
separation first of olivine and, subsequently of orthopyroxene. Refractory
aluminous phases after partial melting of pelitic roof rocks were responsible
for the anorthositic cumulates.
Chromitite layers occur from near the base of the intrusions to the 86 PCS
level. Almost all the chromitites have elevated concentrations of PGE. The
gangue is generally rich in biotite-phlogopite. All chromites are very low in
MgO (from 0.0% to 1.2%). These features imply that chromite crystallized and
was equilibrated in a melt poor in magnesium and rich in potassium. It is
suggested that the chromium of the uppermost chromitites (UC layers) was
exotic: it was liberated from melted high-aluminous schists averaging 500
ppm Cr. Exotic chromium may well have contributed to the genesis of some
other anorthosite-associated chromitites, too (e.g., Fiskenaesset, Sittampundi,
Soutpansberg, Bushveld, Stillwater).
In the Akanvaara and Koitelainen intrusions the PGE show little respect for
sulphides, and are typically associated with oxides (chromite, ilmenomagnet-
ite, ilmenite). Evidently the PGM phases crystallized from the silicate liquid
direct.