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Geological Survey of Finland, Bulletin 395
Tapani Mutanen
ordinary pelites, as such, did not contribute any-
thing of value to the ores; quite the contrary, it
diluted the magma with Ni-poor material.
However, the relatively high Au in sulphides,
when compared with other Ni-Cu(-PGE) de-
posits (Fig. 76), may be explained by the addi-
tion of Au from assimilated rocks. In fact, in
the southern part of the intrusion, several sul-
phide-bearing hornfels intersections have high
Au values (270 ppb/0.7 m, 1510 ppb/1.4 m,
15–88 ppb/9.3 m). Thus, it may not be mere
coincidence that false ore sulphides in this
same area are noted for their relatively high
Au and high Au/Pd.
Olivine xenocrysts from disintegrated
komatiites were residue phases (see above).
Insoluble as they were, they were not chemi-
cally inert, but reacted and exchanged compo-
nents with the magma they arrived in. The
dunite-serpentinite, with 0.2 – 0.4% Ni, is the
rock richest in Ni in the complex, the Keivit-
sansarvi deposit included. On the other hand,
the Ni content of the parent magma was only
ca 100 ppm (Table 6) and, so, with the frac-
tionation of olivine and pyroxenes, Ni was
doomed to decline in the residual liquid. With
no evidence of added primitive magma, the
only possibility that remains is that a great part
of the Ni in the Keivitsansarvi deposit was
supplied by Ni-rich komatiite debris.
The melting of pelites left a residue of sul-
phide liquid, most of it not soluble in the hy-
brid magma, which was charged with cumulus
crystals. Mixing and equilibration with the
main magma and komatiitic olivine xenocrysts
produced a beautiful mixing line between pe-
litic sulphides and komatiitic dunites (see Figs
62 and 65). The main magma was undersatu-
rated in sulphide and acquired little of the add-
ed sulphur. It evidently reached terminal sul-
phide saturation only at the level of the upper-
most ferrogabbros.
Note (Fig. 52) that depletion of S did not oc-
cur in the roof (if anything, there is an in-
crease in S towards the magma). Thus, magma
did not acquire S by selective diffusion or by
trickling of anatectic sulphide melt down to
magma (cf. Thompson & Naldrett, 1984).
Partial melting of black schists left a residue
of graphite. Under the total pressure conditions
of the intrusion, part of the graphite oxidized
to CO
2
and part dissolved in melt as carbon
(see Mathez & Delaney, 1981). That graphite
can crystallize from magmas in layered intru-
sions has been realized since the early 1980s
(e.g., Elliott et al., 1981; Ulmer, 1983).
At Keivitsa, graphite crystallized locally
from carbon-saturated magma as euhedral
crystals, and the local residual liquid remained
saturated with graphite (Mutanen, 1989b). In-
tercumulus graphite is particularly common in
the Ni-PGE ore type.
Assimilation of carbonaceous sediments
caused reduction of magma. The low ferric/
ferrous ratio is reflected in the suppression of
magnetite crystallization, in the relatively low
Mg/Fe of the equilibrated komatiitic olivine
and in the early appearance of primary pigeo-
nitic pyroxene and fayalitic olivine.
The presence of intercumulus graphite indi-
cates that the graphite buffer had not been con-
sumed but had survived to subsolidus tempera-
tures. The sulphide paragenesis, with troilite,
talnakhite and very rare secondary magnetite,
reflects reducing subsolidus conditions.
Graphite and graphite-rich xenoliths, having
a density lower that that of the magma, were
able to float and stack at the roof. The trajecto-
ries of any crystalline phases, whether cumu-
lus or residual, would have crossed this reduc-
ing float layer. The graphite gabbro may have
crystallized from this kind of carbon-saturated,
graphite-enriched melt. The absence of prima-
ry magnetite and the presence of inverted pi-
geonite in this rock are testimonies of the low
oxidation state of Fe. In olivine pyroxenite im-
mediately below the graphite gabbro, graphite
occurs as roundish, radial-textured aggregates.
As in the contaminated KOI-type intrusions,
olivine and plagioclase did not crystallize to-
gether cotectically. In the late stage of the evo-
lution of the gabbroic main magma plagiocla-