199
Geological Survey of Finland, Bulletin 395
Geology and ore petrology of the Akanvaara and Koitelainen mafic layered intrusions and the Keivitsa-Satovaara...
et al., 1989; Brügmann et al., 1990; Cowden et
al., 1990; Halkoaho et al., 1990; Saini-Eidukat
et al., 1990; Bird et al., 1991; Eales et al.,
1993; Scoon & Teigler, 1994; Izoitko &
Petrov, 1995; Reeves & Keays, 1995).
Sulphide Ni-Cu ores, epitomes of mantle
magmas and mantle sulphides, are low in PGE
in general, sometimes surprisingly so, as in the
Bruvann ore in Norway (Boyd et al., 1987;
Barnes, 1987). Whether sulphides are rich or
poor in PGE, sulphide liquid is always found
either as the cause of enrichment or culprit of
impoverishment of the PGE (e.g., Barnes,
1987).
In some cases PGE seem to be chalcophobic
more than chalcophile: Stone et al. (1991) de-
scribe a case where PGE-Au hike coincides
with a drop of Ni, Cu and S. As in the Keivitsa
intrusion, massive sulphides are sometimes de-
pleted of PGE (Dillon-Leitch et al., 1986). The
PGE often behave independently even in the
presence of sulphide liquid (Mutanen, 1989b;
Legendre & Augé, 1992).
It is too seldom noticed that PGE are, above
all, siderophile elements (for rare exceptions,
see Hiemstra, 1979; Tredoux et al., 1995). In
general, it seems that too much is made of the
chalcophile character of the PGE (as in the
statement “Equilibration with sulfide is the
only known mechanism that can effectively re-
move PGE from terrestrial magmas”; Naldrett
& Duke, 1980).
A spatial connection of Cl minerals with
PGE is commonly noted. We suggested earlier
(Mutanen et al., 1987, 1988) that halogens ac-
quired from sediments formed melt soluble
complexes with PGE. These were able to enter
the sulphide liquid only after breakdown of the
complexes. Thus, the formation and break-
down of PGE complexes would govern the
seemingly arbitrary, even irrational strati-
graphic distribution of PGE, with little respect
for sulphides. Others have also pleaded for the
melt-soluble halogen complexes of PGE (Mill-
er et al., 1988; Gorbachev et al., 1994b). Ring-
wood (1955) already accounted the enrichment
of Cr and V in the upper part of the Skaergaard
intrusion for melt-soluble complexes. The ex-
istence of haloid-metal autocomplexes in sili-
cate liquids was suggested by Anfilogov and
co-workers (1984).
The breakdown of PGE complexes could
lead to the liberation of PGE and formation of
metallic and other PGE compounds. In fact, di-
rect crystallization of PGM from magmas is
often observed or indicated (Distler & Laputi-
na, 1981; Augé, 1986; Rosenblum et al., 1986;
Distler et al., 1986; Lee & Tredoux, 1986; Lee
& Parry, 1988; Nixon et al., 1990; Barkov et
al., 1991; Legendre & Augé, 1992; Peck et al.,
1992; Scoon & Teigler, 1994). The independ-
ent PGM particles would be available for oth-
er, non-solvent collectors.
That PGM grains are so often located at the
sulphide/silicate grain boundaries makes one
wonder whether the PGE-sulphide bind is me-
chanical rather than chemical. It is tempting to
think that the sulphide liquid droplets acted as
phase boundary collectors for tiny PGM parti-
cles that had already crystallized from silicate
liquid (Mutanen, 1992; see and cf. Hiemstra,
1979). The particles should be real, stable min-
erals, not atomic-scale clusters collected by
sulphide droplets (Tredoux et al., 1995). The
process would thus be analogous to the old-
time oil flotation.
Our experimental tests showed that the idea
is feasible: skeletal crystals of a Cu-Pt alloy
were found on sulphide beads (Fig. 88; see
Mutanen et al., 1996; Mutanen, 1995). Further
experiments should confirm whether the PGM
(alloy particles in the experiment) nucleated
on sulphide droplets (heterogeneous nuclea-
tion) or were mopped along by sinking sul-
phide droplets. In either case, the experiment
indicated that the PGM alloy was a true liquid-
us phase. Remarkable was the absence of Pd
phases on sulphide beads, suggesting that Pd
behaved more like a chalcophile, while Pt was
siderophile. The experiments on the distribu-
tion of Pt and Pd between silicate liquid, sul-
phide liquid and liquid metal, by Marakushev