87
Geological Survey of Finland, Bulletin 395
Geology and ore petrology of the Akanvaara and Koitelainen mafic layered intrusions and the Keivitsa-Satovaara...
ed in the vertical distribution of secondary
minerals: in graded layers biotite is most abun-
dant in the upper part, hornblende in the mid-
dle and plagioclase in the lower part. In some
cases the lowermost 10 cm consists mainly of
poikilitic plagioclase and chromite.
Of the primary minerals cumulus pyroxene
either crystallized or, more probably, settled,
with chromite (Fig. 25a). Big (up to 1 cm)
euhedral cumulus crystals of plagioclase are
common at Jänessaari, southwestern Koite-
lainen (see Mutanen, 1989b, p. 44 and 49).
Large crystals of fluorapatite, partly of cumu-
lus status, occur in places; fluorapatite oiko-
crysts are seen to enclose chromite crystals. Il-
menite, too, sometimes occurs as cumulus crys-
tals. Tourmaline (3.1% Cr
2
O
3
by electron micro-
probe) is always present in small amounts; it may
be a primary postcumulus mineral.
The intercumulus consists mostly of second-
ary silicates and other secondary minerals: bi-
otite (1.4% Cr
2
O
3
by electron microprobe),
colourless amphibole, greenish amphibole,
bright green amphibole (1.1–2.1% Cr
2
O
3
by
electron microprobe), titanite, rutile, two kinds
of chlorite, epidote-clinozoisite, porphyroblas-
tic scapolite, carbonate, hydrobiotite-vermicu-
lite, zeolites, and the present paragenesis of
sulphides (two generations of pyrite, chalcopy-
rite, pyrrhotite, pentlandite, covellite, miller-
ite, violarite, galena and marcasite). In some
strongly carbonatized places the margins of
chromite crystals have been altered to a ferro-
magnetic spinel (Cr-magnetite ?).
Of the PGM, moncheite (Fig. 25k) and a Rh-
S phase were found first, but ruarsite (RuAsS)
is most common; it occurs as tiny (<10 mu)
euhedra in both chromite and matrix. Laurite
and sperrylite have also been encountered.
When the UC layer was found and prelimi-
narily studied, it soon became apparent that the
chromite has a unique and quite unexpected
composition (anal. 1, Table 4). The high Ti is
due to exsolved ilmenite rods. The Mg is very
low, and even the modest MgO values in anal-
yses are mainly due to secondary biotite inclu-
sions. Narrow beam analyses give MgO 0.0%
and TiO
2
0.2–0.4% for “pure” chromite. All
chromites analysed are very poor in Mg and
rich in Fe. The UC chromite is conspicuously
rich in V.
The chromite as a whole, even in different
parts of the intrusion and certainly re-equili-
brated under different postcumulus conditions,
is surprisingly homogeneous in composition.
However, a strong vertical variation in V has
been observed. In the case studied in detail, the
V in chromite was high in the upper part
(range 0.5–1.34% V) and lower part (range
1.34–1.46%) of the UC layer, but only 0.30–
0.50% V in the middle.
The calculated normative oxide (spinel + il-
menite) compositions of the chromite are (in
wt%): chromite 56–71%, ilmenite 0.7–15%,
hercynite 12–18%, magnetite 7–12%, frankli-
nite 0.6–1.8%, jacobsite 1–4% and coulsonite
1.0–3.2%. Evidently the original high-T spinel
was above the solvus of the continuous ulvite-
chromite solid solution system (see Arculus,
1974; Arculus & Osborn, 1975). The “pure”,
low-Ti matrix chromite represents a very low-
T composition on the solvus, with the oxida-
tion-exsolved ilmenite. Understandably, be-
cause of the necessary combination of “primi-
tive” (Cr) and “evolved” (Ti, Fe, V, Zn) com-
ponents, there were not many opportunities for
the natural chromite ores to experiment with
the system. However, the series is seen in as-
sociation with accessory spinels in both lavas
and intrusive rocks (Evans & Moore, 1968;
Gunn et al., 1970; Evans & Wright, 1972;
Thompson, 1973; Zolotukhin et al., 1975; Ne-
radovskii & Smolkin, 1977; Rozova et al.,
1979; Genkin et al., 1979; Muraveva et al.,
1979; Eales, 1979; Eales & Snowden, 1979; El
Goresy & Woerman, 1977; Smolkin & Pakho-
movskii, 1985; Neradovskii, 1985; see also:
Cameron & Glover, 1973).
According to experimental data given by
Maurel & Maurel (1983, 1984), the liquid in
equilibrium with the UC chromite contained
Al
2
O
3
11–12% and MgO << 0.5%.