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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...
Keivitsa intrusion, selective melting of layers
of low-melting-point compositions above the
intrusion resulted in progressive cave-in of the
roof. Dehydration contraction of the floor
rocks ripped gashes, to be filled by sulphide
liquid of the Cu-rich “offset” deposits (e.g.,
Godlevskii, 1968). All these processes resulted
in effective demolition of the envelope of the
intrusion, continuous comminution and, ulti-
mately, melting and mechanical disintegration
of xenolithic material.
The magma swallowed and digested signifi-
cant quantities of country rocks. Pelitic rocks,
now pyroxene-plagioclase hornfels, are found
as large xenoliths and as half-digested (rotten)
xenoliths. Komatiitic rocks occur as solid lay-
ered, banded and massive ultramafic xenoliths
(Fig. 48) or as mechanically disintegrated rot-
ten xenoliths. Composite komatiitic-pelitic xe-
noliths are also present.
Fresh komatiite xenoliths (komatiite horn-
felses) are composed of olivine, chromite,
clinopyroxene and orthopyroxene, the mineral
proportions depending on the original chemis-
try of the rocks (Table 7). The original chro-
mite is sometimes recrystallized into chrom-
magnetite, suggesting that inherited chromites
served as seed crystals of cumulus chrom-
magnetite.
Komatiite xenoliths are often overlain by
contaminated material rich in pyroxene and
sulphides, presumably from the melted sedi-
mentary part of the original composite xeno-
liths. It is often seen that the cumulus mush
has intruded into the cracks of komatiite xeno-
liths. Olivine pyroxenites beneath many xeno-
liths contain abundant primary brown horn-
blende, which often has euhedral cores; euhe-
dral hornblende crystals occur inside orthopy-
roxene oikocrysts.
The komatiite xenoliths contain fine-grained
disseminated sulphides corresponding in bulk
composition to the interstitial sulphides in the
surrounding cumulate matrix. The xenoliths in
barren olivine pyroxenites do not contain sul-
phides. Thus, it seems that sulphides infiltrated
(as liquid) into the xenoliths from the sur-
rounding magma.
On account of graphite dissociation and ac-
companying disintegration of xenoliths high-
carbonaceous xenoliths are rare, but small
graphitic xenoliths (ca 5–10 cm across), how-
ever, do occur – telltale signs of incorporated
graphite-rich black schist material. Graphite-
rich pelitic hornfels xenoliths are associated
with false ore sulphides ca 2 km west of the
Keivitsansarvi deposit (see map, Appendix 4).
Graphite is found in many intrusions and in
many forms, often in clots suggesting half-di-
gested xenoliths. Graphite is closely associated
with PGE in Merensky Reef and Stillwater J-
M reef PGE deposits, the Platreef deposit in
the Bushveld Complex (Buchanan & Rouse,
1984) and it occurs in ultramafic pegmatoids
below the Stillwater J-M reef (Volborth &
Housley, 1984). Sedimentary origin of graph-
ite is often indicated by field relationships and
by the carbon isotope composition (Lieben-
berg, 1970; Buchanan & Rouse, 1984; Touys-
inthiphoexay et al., 1984; Fletcher, 1988). A
primary igneous origin is sometimes suggested
(e.g., Kornprobst et al., 1987). In the Keivitsa
intrusion, graphite crystallized as beautiful
euhedral cumulus crystals (Mutanen, 1989b)
from a magma that had digested carbonaceous
sediments.
Gaseous or solid carbonaceous contaminants
will be dissolved in magma as carbon, or de-
gassed, until equilibrium between C, CO and
CO
2
is attained (Grinenko, 1967). This de-
pends on the magma composition and, above
all, on total pressure. At a P(tot) of only 400
bar (ca 1.5 km depth) graphite is stable in ba-
saltic magmas without metallic iron, i.e., at ox-
ygen pressures above the IW buffer line
(Goodrich & Bird, 1985). The association of
carbon and chlorine is established already in
magmas (Hoefs, 1965); thus, their close asso-
ciation in Stillwater (Volborth et al., 1986)
may well be a feature that reflects the compo-
sition of pore liquid (see also Slobodskoi,
1981).