Platinum-Group Minerals in the Early Proterozoic Kuusijärvi ...

Platinum-Group Minerals in the Early Proterozoic Kuusijärvi Marginal Series,
Koillismaa Layered Igneous Complex, Northeastern Finland
K. Kojonen
Geological Survey of Finland, FIN-02150 Espoo Finland
M. Iljina,
Geological Survey of Finland, FIN 96101 Rovaniemi Finland
ABSTRACT: The Kuusijärvi layered intrusion of 2.44 Ga age, northeastern Finland, hosts a 15-33 m thick
platinum-group mineral(PGM)-Ni-Cu sulfide bearing gabbro layer in the Marginal Series, which is
mineralized by disseminated chalcopyrite, pyrrhotite, pentlandite and pyrite and platinum-group minerals
merenskyite (62 % of the PGM), michenerite (1.3 %), kotulskite (5 %) , Pd-rich bismuthian melonite (25.3%),
sperrylite (6.6.%) and platinum group element (PGE)-rich cobaltite. The PGE- Cu-Ni sulfide bearing gabbro
has Cu/(Cu+Ni) 0.53 and Pt/(Pt+Pd) 0.29. The PGM have grain sizes of

4 THE KUUSIJÄRVI DRILL CORE PROFILE
Figure 2. The Koillismaa Layered Igneous Complex (KLIC),
Northern Finland, redrawn after Alapieti & Piirainen (1984).
KLIC has three structural units, the eastern Näränkävaara
intrusion and the Western Intrusion, together with an
unexposed feeder intrusion, which forms the strong positive
gravity anomaly between the two exposed bodies. The Western
Intrusion has several smaller blocks, which have the names:
Pirinvaara, Syöte, Porttivaara, Pyhitys, Kuusijärvi, Lipeävaara,
Kaukua and Murtolampi. The Western Intrusion is capped with
a up to 1 km thick granophyre. Albite-quartz rock, probably of
metasomatic origin, occurs at the base of the intrusion in
contact to the footwall granite gneisses (Ilijina et al. this
volume).
2 SAMPLING
Samples were taken from the diamond drill cores
358 and 359 (Figure 3). Samling was done in core
lengths of 20-50 cm, 48 samples from drill core 358,
from depth 65.70 to 88.08 m, and 12 samles from
drill core 359, from depth 179.06 to 183.60 m.
The igneous stratigraphy of Kuusijärvi Block is
composed of two zones (Figure 3): The Marginal
Series (20-50 m thick) forms the basal part of the
intrusion and is followed by the Layered Series of
50-150 m thickness. The lower third of the Marginal
Series is characterized by gabbroic cumulates
followed by pyroxenitic cumulates which in turn are
overlain by olivine rich cumulate peridotites. The
Layered Series of the Kuusijärvi Block is almost
exclusively composed of gabbroic cumulates, e.g.,
olivine gabbronorites. Diabase veins occur both in
the Marginal Series and in the Layered Series. In the
lower contact zone of the Kuusijärvi Block there is a
ca 10 m thick layer of albite-quartz rocks, which is
considered to have formed by metasomatic
processes.
The Cu-Ni-PGE-Au enriched sulphides are
disseminated in the Marginal Series in thickness of
15-33 m. The Kuusijärvi disseminated sulphide
containing gabbro has an average Cu/(Cu+Ni) 0.53,
with an average of 0.28 wt% Cu and 0.24 wt% Ni.
The average Pt/(Pt+Pd) is 0.29. The average values
of various PGE and Au are shown in Table 1.
Maximum PGE+Au values are 3211 ppb and 3587
ppb in the drill holes R358 and R 359, respectively.
3 ANALYTICAL METHODS
A polished section was prepared from each sample
from drillholes 358 and 359 and studied with
polarising microscope. Subsequently the sections
were analysed with a Cameca SX50 electron
microprobe and automatic searches for PGM were
made (Johanson & Kojonen 1995). Manual
searching of heavy mineral concentrates of two drill
core samples were also done using a variable
vacuum scanning electron microscope, Jeol JSM-
5900LV. The PGM’s found were analysed with
energy dispersive spectrometer (EDS) and with
wavelength dispersive spectrometers (WDS) using
15 kV electron gun voltage and 20 nA beam current
if the grains were large enough for quantitative
analyses. Metals and natural minerals were used as
standards. The rest of the drill core samples were
crushed and milled and analysed for major elements
and a suite of trace elements by XRF and with MS-
ICP for PGE and Au using fire assay with Ni sulfide
collection.
Figure 3. Cross section of the drill holes 358 and 359
modified after Iljina (1999), Kuusijärvi block, Koillismaa
Layered Igneous Complex

Table 1. Average PGE and Au values of drill core samples
from Kuusijärvi Marginal Series.
6 RESULTS OF PLATINUM GROUP
MINERALS ANALYSES
ddh Pt Pd Rh Ru Ir Os Au
358 179 439 18 17 8 6 218
359 264 645 22 29 11 5 81
Total of 150 PGM grains located by SEM and
EPMA were analysed and fotographed for image
analysis. Most of the grains found occur within
silicates as discrete grains. To lesser extent, the
PGM are intergrown on the grain borders of sulfides.
The grain sizes measured are below 40 µm, and
most of the grains were 5-10 µm in diameter. The
major part of the PGM found belongs to the system
(Pd+Ni)-Bi-Te including minerals merenskyite (62
%), michenerite (1.3 %), kotulskite (5 %) and Pdrich
melonite (25.3 %). Other PGM found were
sperrylite (6 %) and PGE-rich cobaltite which was
observed within sulfides (see Table 2).
Table 2. Average electron microprobe analyses of the
Kuusijärvi PGM, Koillismaa Layered Igneous Complex
Figure 5. Chondrite normalised PGE + Au of the analysed
samples from drill holes 358 and 359, Kuusijärvi Marginal
Series, Koillismaa Layered Igneous Complex.
5 ORE MINERALS
The disseminated sulfide ore minerals consist of
chalcopyrite, pentlandite, pyrrhotite and pyrite.
Accessory sulfide minerals include covellite,
violarite, marcasite, mackinawite, sphalerite and
argentian pentlandite. The oxide ore minerals are
magnetite, ilmenite and chromite. The PGM
previously found are merenskyite, froodite,
michenerite and sperrylite (Alapieti 1982, Alapieti &
Piirainen 1982). Other accessory ore minerals
identified previously are hessite, electrum, native
gold and copper. Piispanen and Tarkian (1984)
report essentially the same minerals from
Römetölväs, Syöte block, adding cobaltite and
palladian bismuthian melonite to the list. Lahtinen
(1985) reports additional Cu-rich sulfides covelline
and bornite.
wt% 1 2 3 4 5 6
Pd 19.73 23.99 40.86 8.86 0.69 1.18
Pt 0.83 0.88 0.24 0.17 53.36 1.49
Ru 0.02 0.02 0.05 0.02 0.02 2.42
Rh 0.01 0.01 0.00 0.02 0.49 2.29
Os 0.21 0.52 0.29 0.16 0.18 1.04
Ir 0.00 0.05 0.04 0.00 0.00 1.68
Ni 4.73 0.04 0.04 11.95 0.69 9.99
Co 0.18 0.01 0.02 0.02 0.11 14.06
Cu n.d. n.d. n.d. n.d. n.d. 0.22
Fe 0.01 0.69 0.24 0.15 0.45 5.61
As n.d. n.d. n.d. n.d. 43.51 42.43
S n.d. n.d. n.d. n.d. 0.32 16.65
Se n.d. n.d. n.d. n.d. 0.19 n.d.
Bi 12.59 40.77 15.27 7.43 n.d. 0.01
Sb 0.39 n.d. 0.96 0.45 n.d. 0.01
Te 60.58 32.52 42.02 70.07 n.d. 0.01
Total 99.58 99.50 100.0 99.28 100.0 99.09
N 50 6 15 31 10 18
N=number of analyses n.d.=not determined
1. Merenskyite, 2. Michenerite, 3. Kotulskite,, 4. Pd-rich
melonite, 5. Sperrylite 6. Cobaltite
Hessite is often found intergrown with the PGEtellurides,
altaite more seldom. Native gold, silver
and electrum occur as discrete grians inside silicates
or as inclusions in the Pd-Ni-Bi tellurides (Table 3).

Table 3. Average electron microprobe results of Au-Ag
minerals from the Kuusijärvi Marginal Series, Koillismaa
Layred Igneous Complex
wt% 1 2 3 4
Au 1.09 72.97 93.69 0.21
Ag 61.67 25.66 4.35 100.18
Sb 0.15 0.01 0.01 0.00
Bi 0.07 0.53 0.59 0.01
Te 36.31 0.05 0.09 0.15
Total 99.30 99.22 98.63 100.55
N 4 20 6 1
N=number of analyses
Hessite, 2. Electrum, 3. Native gold, 4. Native silver
7 DISCUSSION AND CONCLUSIONS
The chondrite normalised PGE+Au patterns of the
Kuusijärvi Marginal Series (Figure 5) show smooth
positive slopes with enrichment in Pd and Au. They
resemble the patterns of Platreef, South Africa and
Sudbury, Canada ( Naldrett & Cabri 1976, Cabri &
Naldrett 1984). The higher grade PGE-deposits of
Penikat and Portimo show Ru depletion (Lahtinen et
al. 1989) which is not visible in the Kuusijärvi PGE
distribution pattern. Most of the PGM discovered in
the samples of drill cores 358 and 359 in the
Kuusijärvi section are Pd-Ni-Bi tellurides, which are
often intergrown with hessite, sperrylite and native
gold. It is noteworthy that approximately 80 % of the
PGM occur within silicates and only 20 % at the
grain borders of sulfides. The grain sizes are < 40
µm , majority < 15 µm. The mode of occurrance of
the PGM in the samples of the Kuusijärvi Marginal
Series suggests their reletively late crystallization,
later than the sulfides and about the same time as the
silver tellurides, native gold and electrum and with
the hydrosilicates, possibly as a result from volatilerich
residual liquids, as suggested by Piispanen &
Tarkian (1984). On the other hand, their formation
may have been caused by the regional
metamorphism, which has altered the original
magmatic mineral assemblage of the layered
intrusion. During retrogressive metamorphism the
PGM may have been crystallising in the
intergranular spaces with the hydrosilicates and on
the surfaces of the sulfide grains.
ACKNOWLEDGEMENTS
The friendly assistance of Mr Bo Johanson and Ms Marja
Lehtonen with the electron microprobe and SEM analyses is
gratefully acknowledged. Ms. Pirkko Kurki is thanked for
drawing the geological maps. North Atlantic Natural
Resources AB is thanked for permission to publish this paper.
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