45
Geological Survey of Finland, Bulletin 395
Geology and ore petrology of the Akanvaara and Koitelainen mafic layered intrusions and the Keivitsa-Satovaara...
rare; they do occur, however, in the very low-
ermost chromite-rich layer 9 m above the top
of the lower chilled margin. The layer, which
occurs in cumulate gabbros, is 0.51 m thick
and consists of alternating chromite-rich and
pyroxene-rich layers. Quartz, plagioclase and
biotite are abundant in the intercumulus, which
also contains loveringite(?), zircon and fluora-
patite. Near the base of this layer is an autolith
of fine-grained microgabbro.
The compositions of chromite from the
chromitite layers are listed in Table 1. There
are no systematic upward trends in the LC lay-
ers, although there are clear differences (e.g.,
in Al, Mn, Fe, Cr and Mg) between the layers,
intra-layer variation (Mg, Fe, Al, Cr) and even
slight but distinct variation between grains in
one sample. There is no correlation between Fe
and Mn, Fe and V, but a positive one between
Cr and V. There is a general, but not tight,
negative correlation between Al and Fe, indi-
cating Al
3+
/Fe
3+
substitution. In LLC layers the
substitution is between Cr
3+
and Fe
3+
. The com-
position of the chromite from a pyroxene-chr-
omite cumulate between two chromitite layers
differs from that of the chromitites (Table 1).
Upwards from the LC to the ULC there is a
slight decrease in Mg and an increase in Zn
and Mn, but chromites with similar Mn values
occur in the lowermost LC layers. The UC is
distinct from the ULC and LC. Its chromite has
an even lower Mg and higher Ti, V and Zn
content; Mn is high, being about the same as in
ULC chromite.
Post-cumulus re-equilibration was probably
not very effective. Analyses of LC chromite on
one sample (DDH322/258.27 m) in which
chromite is enclosed by three different poikil-
itic matrix minerals – sulphide, plagioclase
and carbonate – showed that the compositional
differences between these chromites are negli-
gible. Chromite in plagioclase is slightly high-
er in Al, whereas chromite in sulphide is high-
er in Fe and Ti and lower in Mg, Cr and Al.
Other evidence for the lack of significant
post-cumulus changes come from the UC lay-
er, where the mg# in bulk silicates is even low-
er than that in underlying and overlying sili-
cate cumulates (Fig. 17). This indicates that
the low Mg in UC chromite is an original fea-
ture and was not markedly lowered by postcu-
mulus (magmatic plus metamorphic) re-equili-
bration.
As the D
Mg
chr/liq
in all imaginable magma com-
positions is > 1 (see Maurel & Maurel, 1982b,
1984), the low Mg in chromite implies that the
chromites of all chromitite layers crystallized in
an environment very poor in Mg.
The UC layer (see Appendix 2) is similar to
the UC layer of the Koitelainen intrusion. It
was found in an outcrop in 1991 and was later
traced (1995–1996) by diamond drilling in the
southern part of the intrusion. Between the
eastern and western fault borders it is continu-
ous over a strike length of 8 km. From east to
west the relative stratigraphic elevation of UC
succession decreases several hundred metres
in relation to the base of the magnetite gabbro,
whereas the combined thickness of the MZ and
UZ remains approximately constant.
Figure 10 shows the general stratigraphy of
the UC succession, and details of UC DDH in-
tersections. The homogeneous MZ gabbro has
a sharp boundary with the overlying UZ. This
begins with a 0.5–4.6-m-thick unit of alternat-
ing layers of mottled anorthosite, gabbro-
anorthosite, anorthosite gabbro and medium-
grained non-cumulate gabbro and microgab-
bro. The layers are irregular and do not always
persist laterally. Below the UC the mottled
anorthosite is from 0.2 to 1.2 m thick, but in
five DDH intersections it is lacking altogether.
The irregularity of the sub-UC unit is ascribed,
as at Koitelainen, to strong magmatic erosion.
Closely associated with the UC are micro-
gabbro, medium-grained non-cumulate gabbro,
pyroxenite, pyroxene pegmatoid and pyrox-
ene-plagioclase pegmatoid, all of which con-
tain chromite as dissemination, schlieren and
thin layers. The UC is overlain by layers of
mottled anorthosite and other plagioclase-rich
cumulates, gabbro and layers/autoliths of non-