Geology and ultramafic rocks of the Paleoproterozoic Pulju ...

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INTEGRATED TECHNOLOGIES FOR MINERAL EXPLORATION
PILOT PROJECT FOR NICKEL ORE DEPOSITS
Brite-EuRam BE-1117 GeoNickel
Task 1.2 Mineralogy and modelling of Ni sulfide deposits in
komatiitic/picritic extrusives
Technical Report 6.5
geology and ultramafic rocks
of the paleoproterozoic
pulju greenstone belt,
western lapland
Heikki Papunen
Turku University
Department of Geology
1998

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Contents
6.5 Geology and ultramafic rocks of the Paleoproterozoic Pulju Greenstone Belt,
Western Lapland
6.5.1 Introduction
6.5.2 General Geology of the Pulju Belt
6.5.2.1 The supracrustal belt and surrounding granitoids
6.5.2.2 The Pulju Belt as a part of the succession of the Central
Lapland greenstone belt
6.5.2.3 Structure and metamorphism of the Pulju Belt
6.5.2.4 Stratigraphy and rock sequence
6.5.2.5 Rock types of the Mertavaara Formation
6.5.2.6 Ultramafic rocks
6.5.2.6.1 General division
6.5.2.6.2 Non-fractionated komatiitic flows
6.5.2.6.3 Fractionated komatiitic flows - komatiitic cumulates
6.5.2.7 Mineralogy of ultramafics
6.5.2.7.1 Olivine
6.5.2.7.2 Amphiboles
6.5.2.7.3 Chlorite
6.5.2.7.4 Phlogopite
6.5.2.7.5 Pyroxenes
6.5.3 Geochemistry of the Mertavaara Group rocks
6.5.3.1 Methods and samples
6.5.3.2 Felsic and mafic rocks
6.5.3.3 Non-fractionated ultramafic flows
6.5.3.4 Fractionated komatiititic flows - komatiitic cumulates
6.5.3.4.1 Main elements
6.5.3.4.2 Trace elements
6.5.3.4.3 Factor analysis
6.5.3.5 Geochemical classification of komatiites
6.5.3.5.1 Non-fractionated komatiitic flows (chlorite-amphibole rocks)
6.5.3.5.2 Komatiitic cumulates
6.5.3.6 Alteration of the cumulates
6.5.4 Sulphides
6.5.5 Summary
Tables
References

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Figure 6.5.1 Generalized geological map of northern Fennoscandia; the Pulju area is deliated
6.5 Geology and ultramafic rocks of the Paleoproterozoic
Pulju Greenstone Belt, Western Lapland
6.5.1 Introduction
The Pulju greenstone belt, in the west of Kittilä municipality, forms the western edge of the
central Lapland greenstone complex in Finnish Lapland (Fig. 6.5.1). With abundant ultramafic
rocks and associated sulfidic metasediments, the belt trends NNE from the Ounasjoki
river to Pulju village, where it is intersected by a circular body of the Pulju granite. Beyond
the Pulju granite, it continues northwards as the Kietsimä-Peltotunturi belt and further into
Norway, where its extensions are the Anarjokka (Gammon 1977) and Karasjok Greenstones
(Barnes and Often 1990).
The ultramafics of the belt have been known since the pioneering work of Mikkola (E.
Mikkola 1941). Suomen Malmi Oy undertook fieldwork in the area in the early 1960s to

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check high-altitude aeromagnetic anomalies measured by the Geological Survey of Finland
(GSF) (Söderholm 1966). At the beginning of the 1970s, a reconnaissance geochemical
survey of stream sediments conducted by Outokumpu Oy revealed a number of nickel,
chromium and base metal anomalies, and the follow-up geological fieldwork pinpointed
some outcrops of sulfide-bearing ultramafics. The area was one of the targets studied by the
Department of Geology, University of Turku, in its investigations of the ultramafics of
Lapland in 1974-1979 (Papunen et al. 1977, 1980, Heino 1977, Idman 1980). At the same
time, some shallow diamond drill holes were made in the sulfide-bearing ultramafics of the
Siettelöjoki area. After low-altitude airborne geophysical surveys in the mid-1970s,
Outokumpu Oy continued prospecting in the area, with detailed geological mapping,
geochemical till sampling, ground geophysics and diamond drilling (Lahtinen 1979, Inkinen
et al 1984).
The bedrock crops out selectively in some of the hilly areas, most of the area being
covered with bogs and Quaternary glacial deposits. Geological maps are therefore based on
geophysics, geochemical till sampling and drilling data.
To reinterpret the existing geological and geochemical data and the new information
gleaned from geophysical low-altitude surveys conducted by the GSF in 1990, the Department
of Geology of the University of Turku reinvestigated the area in 1990 (Meriläinen
1993), the focus this time being on the origin of the ultramafics and their potential for sulfide
nickel deposits. In this respect, the experience gained in Western Australia of the physical
volcanology and geochemistry of komatiites (Gole et al. 1987, Barnes et al. 1988, Hill
et al. 1990) is relevant and has been applied in interpretations presented here.
6.5.2 General Geology of the Pulju Belt
6.5.2.1 The supracrustal belt and surrounding granitoids
The Pulju Belt lies west of the Kittilä Greenstone area proper (for the geology of Central
Lapland, see Lehtonen et al. 1998). The belt comprises a sequence of supracrustal rocks,
starting with continental lithofacies with quartzites and biotite gneisses, and passing upwards
into pelitic metasediments interlayered with ultramafic cumulates and lavas, and
further into mafic volcanics and metasediments probably of a marine environment. In the
west, the belt is bordered by the Hetta granite, which intersects the Pulju Belt as numerous
veins. The Hetta granite is considered to be part of a reworked Archaean granitoid crust that
was remobilized during the Svecokarelian orogeny, about 1950 Ma ago (Lehtonen et al.
1985). The Pulju granitoid body intersects the supracrustal belt as a small isolated cupola
of the Hetta granitoid. At Kiimatievat, in the northern part of the Pulju Belt, Hakanen (1977)
studied a small molybdenite occurrence associated with calc-silicate rocks at the contacts
of conformable pegmatite veins. It is not sure whether the molybdenum pegmatite is related
to the Hetta synorogenic granitoid or to the anorogenic granitoids of Tepasto (1760 Ma)
located southeast of the belt, but the appearance of the veins along the orogenic deforma-

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tion structures suggests an association with the synorogenic Hetta granite.
The northern extension of the Pulju Belt, here called the Kietsimä-Peltotunturi belt,
is not known as well as the Pulju Belt proper. Reconnaissance mapping of the ultramafics
of the Kietsimä-Peltotunturi belt (Papunen et al. 1977) indicates, however, that the rock
types of both are similar although carbonation is more extensive and the grade of metamorphism
somewhat higher in the Peltotunturi ultramafics (Julku 1982, Papunen et al. 1977).
According to Henriksen (1983), Barnes and Often (1990) and Davidsen (1994), the
Karasjok Greenstone Belt, which is in the possible extension of the Peltotunturi-Kietsimä
Belt, comprises metamorphosed and deformed equivalents ranging upwards from continental
clastic sediments to shallow marine sediments and subaerial to shallow marine mafic
volcanics, komatiitic and tholeiitic volcanics, and ending with a formation composed of
mafic to intermediate volcanics and clastic sediments.
6.5.2.2 The Pulju Belt as a part of the succession of the Central
Lapland greenstone belt
The general geology and stratigraphy of the Central Lapland area have been reviewed by
Lehtonen et al. (1985, 1992, 1998), Silvennoinen (1985), Räsänen et al. (1989) and
Manninen (1991). Recent understanding of the stratigraphy of the supracrustal sequence of
the Central Lapland Greenstone Belt (CLGB, Fig. 1) as presented by Lehtonen et al. (1998)
divides the sequence into seven Groups. The two lowermost Groups, Salla and Onkamo,
are characterized by intermediate to felsic metavolcanics and tholeiitic to komatiitic
metavolcanics, respectively. They are considered to have been deposited c. 2.50-2.44 Ga
ago. The overlying Sodankylä (> 2.21 Ga) Group is characterized by the Virttiövaara quartzite
Formation and the albitized metasediments and mafic metavolcanics of Honkavaara.
Upwards, the Savukoski Group is divided into the Matarakoski phyllite-black schist-dolomite
Formation (> 2.13 Ga), the Linkupalo tholeiitic volcanite Formation (> 2.05 Ga), the
Sotkaselkä picritic metavolcanite Formation and the overlying Sattasvaara komatiitic
metavolcanite Formation (> 2.05 Ga). The Savukoski Group is overlain by the Kittilä Group
(c. 2.012 Ga), which is composed of the Fe-tholeiitic Kautoselkä, BIF-characterized
Porkonen and Mg-tholeiitic Vesmajärvi formations, and the Pyhäjärvi Formation, which is
characterized by mica schists and greywackes. The Lainio Group overlies the Kittilä Group
and contains three formations: Tuulijoki (> 1.883 Ga), with lamprophyric metavolcanics;
Latvajärvi (1.883 Ga), with intermediate to felsic metavolcanics; and Ylläs (< 1.883 Ga),
with quartzites, mica schists and conglomerates. The youngest sedimentary unit, the Kumpu
Group, is composed of quartzites of the Levi Formation.
The layered mafic igneous complexes in central Lapland, Koitelainen, Akanvaara etc.
(2.44 Ga), are thus coeval with the Onkamo Group metavolcanics, and the Kevitsa mafic
igneous complex (2.05 Ga) intersects the Sattasvaara-type komatiites.
The Matarakoski Formation of the Savukoski Group consists principally of graphitic
and sulfidic sediments interlayered with dolomites and calc-silicate rocks. The komatiites
of the Sattasvaara Formation overlie the Matarakoski sedimentary unit and frequently display
pillows and pyroclastic breccia structures (Saverikko 1983, 1985). Krill et al. (1985)

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Figure 6.5.2 Genralized geological map and cross sections of the Pulju belt

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obtained an age of 2.085 Ga for the pyroclastic komatiites of Finnmark, Norway, which
have been correlated with the komatiites of the Sattasvaara Formation in central Lapland
(Manninen 1991). The lower part of the Kittilä Group consists of Fe-tholeiitic and Mgtholeiitic
mafic volcanics separated by an extensive chemosedimentary unit, the Porkonen
Iron Formation. In geochemistry, the volcanics display, in general, pristine mantle characteristics
and are less contaminated than the volcanics of the Savukoski and Sodankylä
Groups.
According to Lehtonen et al. (1998), the surface between the Onkamo and Sodankylä
Groups is weathered, indicating a hiatus, and the contact between the Savukoski and Kittilä
Groups is tectonic, with lenses of ultramafic cumulates interpreted by Hanski (1997) to be
a part of the ancient obducted oceanic crust. The sequence of supracrustal rocks in the
Sodankylä, Savukoski and Kittilä Groups suggests an intracontinental basin evolution that
terminated with the formation of oceanic crust in the Kittilä Group. The depth of the depositional
basin varied in time and place. Magmatic activity was associated with periods of
regional hydrothermal processes, implying long-lived thermal activity in the mantle.
The relationship of the Pulju Belt with the Central Lapland Greenstone Belt (CLGB)
has been a subject of controversy. Meriläinen (1976) reported a U-Pb zircon age of 2720
Ma for an albite diabase intersecting the Karasjok quartzite in Norway, in the northern
extension of the Pulju Belt. This date and the greenstone belt association of the Pulju Belt
rocks led Ga‡l et al. (1987) to consider the belt Archaean in age. However, the recent description
of the stratigraphy of the CLGB by Lehtonen et al. (1998) correlates very well with
that of the Pulju Belt, which can thus be considered the western extension of the CLGB.
6.5.2.3 Structure and metamorphism of the Pulju Belt
The Pulju Belt occupies an open V-shaped area on the geological map (Fig. 6.5.2). The lowaltitude
aeromagnetic maps produced by the GSF in 1991 give us a reliable basis on which
to interpret the extent and structures of the Belt. The comprehensive study of the deformation
structures of the Sirkka-Pahtavuoma area of the Kittilä greenstone belt by Koistinen
and Virransalo (1985) provided the background for Rastas and Kilpeläinen (1991), who
compiled a dynamic model of deformation phases in the Pulju Belt by investigating the
deformation structures in the Mertavaara key area on outcrop and micro scales. The following
deformation analysis and notes on metamorphism are based on their observations.
The rocks of the Pulju Belt are not favourable for deformation analysis, because the
frequently outcropping mafic and ultramafic metavolcanics do not display refolding and
overprinting deformation features as well as do pelitic rocks. The metapelites lying between
the volcanics are not appropriate either, because, being rich in sulfides and graphite, they
intensify all possible deformations displaying only the latest features. The best rock types
for deformation analysis among those cropping out in the area are arenitic-argillic metasediments,
and the Mertavaara area is the only place where they are exposed, and even there
only sparsely.
Primary sedimentary layering is visible in the metasediments as regular compositional
banding (Fig. 6.5.3). The various deformation phases can be followed down to the oldest
one, which deforms the primary layering.

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Figure 6.5.3 Folded primary layering in a quartzite outcrop at Mertavaara
The first deformation phase, D 1
, is well represented by the strong foliation, S 1
, parallel
or subparallel to the layering. S 1
is also the strongest foliation in the non-layered rock
types of the Mertavaara area, and thus provides a basis for correlations of the other deformation
phases. In the metasediments, the S 1
foliation is manifested as thorough parallelism
of all sheetlike minerals. S 1
is also prominent in the ultramafic komatiitic flows. The weak
foliation locally observed in massive serpentinites parallels the main foliation, S 1
, of the
amphibole-chlorite rocks nearby, and the rare foliation in serpentinites has been interpreted
as S 1
. Shear zones and 1-20 cm wide mylonites subparallel to the layering characterize D 1
as does small-scale intrafolial isoclinal folding, F 1
.
The main folding phase at Mertavaara was F 2
, a tight folding that deforms both S 0
and
S 1
. The fold axes of F 2
trend northeast and plunge southwest at a small angle. The axial
plane dips gently to the west, and an intense crenulation cleavage, S 2
, parallels the axis of
F 2
. The shallow dipping or subhorizontal D 2
structures are only visible in vertical sections
of the outcrops. The gentle dip of F 2
folding has been considered a primary D 2
feature and
not a consequence of younger deformation phases, thus indicating overthrusting from the
northwest.
Two deformation phases younger than D 2
, that is, D 3
and D 4
, have been encountered.
Both exist as open folds deforming D 1
and D 2
structures. For example, an east-west-trending
crenulation cleavage in the mica-rich layers of the Mertavaara arkose quartzites deforms the
S 2
cleavage. We can interpret the interference figure of the magnetic anomaly pattern by
assuming that the more common east-west-trending folding is F 3
and that the north-southtrending
folding is the youngest, F 4
. The axial planes of open folding are subvertical.
The tectonic interpretation of the area in Fig. 6.5.2 is based on the interpretation of
magnetic maps as an interference figure of D 1-4
.

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Figure 6.5.4 P-T diagram depicting the metamorphic peak conditions of the Pulju area (Rastas and
Kilpeläinen 1991)
The rock types of the area are not suitable for sophisticated interpretation of the metamorphic
grade. Garnet is a common metamorphic mineral in metapelites, but when alone
it does not tell us much about the metamorphic conditions. Qualitatively, the well preserved
primary sedimentary features and the lack of migmatization indicate that high temperatures
were not attained during regional metamorphism.
Andalusite porphyroblasts that formed in a reaction between staurolite, muscovite and
quartz have been recognized in the mica gneiss interlayers of arkosic quartzite at Mertavaara.
Parallel to them are minute oxide mineral exsolutions. As the temperature rose,
fibrolitic sillimanite and biotite crystallized, and andalusite turned partly into sillimanite,
but muscovite and quartz were still stable phases. The recrystallization of Al-silicates and
biotite, which was syn-D 1
deformational, started in the stability field of andalusite, and
terminated in the sillimanite field. In the mica gneisses of Hotinsaajo, southwest of Mertavaara,
chlorite decomposed into cordierite and staurolite, which then reacted to form biotite
and fibrolite parallel to S 1
; finally, garnet crystals were formed from biotite and fibrolite.
Zoned Ca-rich plagioclase that cystallized during D 1
contains local remnants of staurolite
in its cores. However, abundant small plagioclase inclusions in the garnet porphyroblasts
indicate that garnet crystallized after plagioclase. The last metamorphic feature is the
growth of biotite flakes almost perpendicular to the biotite of S 1
foliation. A related late
feature is the growth of chlorite in the cracks of plagioclase and garnet.
The amphibole-chlorite rocks of Mertavaara commonly contain long metamorphic
blades of olivine porphyroblasts parallel to S 1
. D 2
deformed the olivine porphyroblasts.
The mineral assemblages and reactions in pelitic rocks imply that metamorphism was
of the intermediate grade (Winkler 1976) and syn-D 1
. Decomposition of staurolite, the
andalusite-sillimanite reaction and the muscovite-quartz stability field define the peak
metamorphic PT conditions to ca. 3.5 kb and 600-650°C (Fig. 6.5.4).
The study of the structural geology of the Pahtavuoma area, Kittilä, by Koistinen and
Virransalo (1985) deals with the metavolcanics and metsediments of a typical CLGB area
ca. 40 km south of Pulju, but the authors also briefly describe the Pulju Belt. They consider
that the deformation history of both areas can be correlated despite the higher metamorphic

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grade at Pulju. They also suggest that both areas belong to the same tectonic megablock and
cannot find any structural features indicating a difference in their deformation ages. However,
Rastas and Kilpeläinen (1991) demonstrated that S 1
foliation predominates in the Pulju
Belt and S 2
in the Sirkka-Pahtavuoma area. They pointed out that the difference might be
subjective, as one emphasizes a feature more dominant than the other, or that the difference
really does exist and that the Pulju Belt was metamorphosed earlier as regards deformation
phases.
In their structural study of the CLGB, Ward et al. (1989) considered that the belt underwent
early north-verging deformation opposite in polarity to the granulite deformation
that thrust the North Lapland granulite belt at least 125 km in a southeast direction (Marker
1988). As the S 1
of the Pulju Belt is subparallel to the primary bedding, the first deformation
phase was overthrusting in character, but the direction can no longer be determined.
The structures of D 2
are flat-lying and the shallow dip of the axial plane of the S 2
foliation
is to the northwest, indicating a southeast-vergent overthrust. Neither of these fit the northeast
- or southwest-trending thrust patterns that deformed the CLGB or the granulite arch.
However, the southwestern margin of the Pulju Belt can be interpreted as a tectonic thrust
zone dipping to the southwest, and thus it is consistent with the regional clockwise torque
caused by granulite deformation as proposed by Ward et al. (1989).
A deep seismic POLAR traverse (Gaál et al. 1989) across the CLGB indicates that the
belt is rather shallow, only 5-7 km deep, lying on an Archaean granitoid basement. The Pulju
Belt was not included in the seismic profile and its vertical extent is not known. The
syntectonic Hetta granitoid is a wide, oval body trending roughly north-south, and on the
basis of the gently dipping structures and numerous intersecting granitoid veins, the Pulju
Belt is here interpreted as a shallow sheet lying in a flat position at the southern end of the
granitoid batholith. Geophysics indicates that the northwestern margin is the thinnest part
of the schist belt, which peters out as scattered remnants in the intruding granite. The shallow
dip of fold planes and the gently plunging axes are responsible for the interference
structure depicted on the geological map (Fig. 6.5.2).
In conclusion, the Pulju volcanic-sedimentary sequence underwent a complex structural
evolution combined with metamorphic recrystallization. Hydrothermal alteration occurred
in several stages, starting with volcanic-hydrothermal and diagenetic alterations
before major structural deformation, and culminating in syn-metamorphic alteration during
granitization of the large Hetta batholith. Polyphase deformation, recrystallization and
alteration processes provide the framework for the evolution of the sequence.
6.5.2.4 Stratigraphy and rock sequence
The complex polyphase deformation and poor outcropping of the area render detailed study
of the stratigraphic succession difficult. Although reliable younging directions are visible
only in the outcrops of metasediments at Mertavaara, the order of rock sequences gives us
an idea of the stratigraphy, which can be supplemented by geophysical surveys and the
geochemistry of basal till samples in non-outcropping areas. A stratigraphic scheme initially
presented by Inkinen et al. (1984) is here revised in the light of outcrop mapping and geophysical
data (Fig. 6.5.5). Lithostratigraphically, the supracrustal rocks of the Pulju Belt are
correlated with the Sodankylä, Savukoski and Kittilä Groups of the CLGB. The stratigraphy
of the Pulju Belt is divided into the Sietkuoja, Mertavaara and Vittaselkä Formations,
and the stratigraphic observations, classification and correlation are briefly described.

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Figure 6.5.5 Stratigraphic column of the Pulju belt; reviewed from the scheme presented by Inkinen et al.
(1984).
The basal contact of the Pulju Belt does not crop out, but Inkinen et al. (1984) described
felsic gneiss outcrops to the east of the Belt that, owing to their migmatitic appearance,
were considered part of the basement complex. The gneisses of the Belt are seldom
migmatized.
Due to exploration activities the description focuses on four areas, viz. Mertavaara,
Hotinvaara, Lutsokuru and Siettelöjoki, with the emphasis on the stratigraphic position of
the ultramafic members.
The Sietkuoja Formation consists of scattered occurrences of mafic metavolcanics in
the lower part of a transgressive continental sedimentary sequence mainly composed of
arkosic quartzites. Locally green due to the presence of fuchsite, these quartzites grade
upwards into biotite-banded quartzites and terminate with biotite and biotite-hornblende

12
gneisses, which indicate incipient volcanism in the succession. The sedimentary rock sequence
of the Sietkuoja Formation corresponds to that of the Virttiövaara Formation of the
Sodankylä Group (Lehtonen 1998). Owing to poor outcropping, the mafic volcanics in the
lower portion of the Sietkuoja Formation cannot be correlated with the volcanics of the
Sodankylä or Onkamo Groups.
The overlying sequences are composed of felsic arenitic, argillic and chemical sediments
and variable mafic and ultramafic volcanics. Sedimentary rocks commonly occupy
the lower part and volcanics the upper part of the sequence, but due to low-angle layering
and polyphase tectonics the order cannot be verified everywhere, and hence the sedimentary-volcanic
sequence was stratigraphically united as the Mertavaara Formation. Drill cores
and geophysical maps were used to combine details of the sulfidic and graphitic sedimentary
strata, but the large areas devoid of outcrops leave room for speculation.
In the Mertavaara area proper (Fig. 6.5.6), the stratigraphic younging direction is northwards.
Here the strata of the Mertavaara Formation begin with felsic metavolcanics
interbedded with a relatively thick chert sequence. The large body of serpentinite marked
in the central part of Fig. 6.5.6 is underlain by and interbedded with metasediments and
cherts, the presence of which is easily seen in electromagnetic surveys due to the abundance
of sulfides and graphite. The serpentinite is overlain by mafic metavolcanics and banded
amphibolites, which are also interbedded (or interfolded) with ultramafics. The extensive
area of chlorite-amphibole rocks northeast of the serpentinite body has been interpreted as
an overlay of flat-lying, relatively thin strata, which, owing to a gently dipping fold axis,
F4a, occupy a large area on the ground surface. Drilling intersected a small section of gabbro
and hornblendite, evidently representing a cumulate phase of mafic volcanics.
The tectonically complex Hotinvaara area (Fig. 6.5.7) exhibits an interference structure
of gently dipping strata folded by northeast-trending F 2
and east-west-trending open F 3
folds. In the west, the F 4
structure refolded the section in a NNE-SSE direction, and the
northwest-trending “branch” of the ultramafics in the western part of the area is an interference
structure produced by a late thrust folding, F 4a
, interpreted as the reflection of a
northeast-verging deformation (Ward et al. 1988). On the map the ultramafic metacumulatesulfidic
sediment sequence is visible as a folded, single stratigraphic unit over the whole
area. Sulfidic metasediments are more common in the upper contact zone of ultramafic
metacumulates, where calc-silicate rocks and sulfidic-graphitic cherts frequently alternate
with ultramafics. The uppermost amphibolite (high-Mg metabasalt) of the Mertavaara
Formation crops out in a few places and has been met with in drill cores to the north of this
eastern branch.
At Lutsokuru, F 4a
is subparallel to F 2
, and the stratigraphic younging direction is from
west to east. The sequence begins with amphibolites overlain by two belts of serpentinites.
These are interbedded with graphitic-sulfidic chemosediments that pass upwards into felsic
metasediments. As a separate layer higher up in the strata, a sequence of amphibole-chlorite
rocks is overlain by amphibolites and a sequence of argillic metasediments interbedded
with graphitic phyllites.
At Siettelöjoki, an ultramafic stratum was intersected by four shallow drill holes
(Papunen et al. 1979). The stratigraphic younging direction of the section is to the east.
Felsic gneisses form the lowermost stratum in the drilled section and also overlie ultramafics,
but in the east they are interbedded with an amphibolite. To the east of the section,
there are outcrops of ultramafics that differ in petrography from serpentinites and meta-

13
Figure 6.5.6 Detailed geological map of the Mertavaara area; a schematic cross section depicts the
stratigraphic scheme of the area (the map according to Meriläinen, 1993, and Lahtinen, 1979).
peridotites, being metacumulates of tholeiitic magma and constituting a feeder channel for
related volcanics. Owing to the considerable abundance of pyroxenes and magnetite, they
have high density and magnetic susceptibility (Fig. 6.5.8); the high magnetic anomalies
combined with high gravity anomalies in the belt have been attributed to similar cumulates.

14
Figure 6.5.7 Geological map of the Hotinvaara area; compiled on the basis of drill core data, geophysical
maps and geological observations presented by Lahtinen (1979).
Figure 6.5.8 Density-suskeptibility diagram of rocks of the Pulju belt.
The internal stratigraphy of the Mertavaara Formation thus varies from one study area
to another. The mutual correlation of ultramafic flows (chlorite-amphibole rocks) and ultramafic
metacumulates is a controversial subject. Previous authors (Inkinen et al. 1984)
considered all the ultramafic rocks as a single stratigraphic unit, but here the flows and
cumulates are regarded as separate layers, with the lenses of komatiitic metacumulates on

15
a lower stratigraphic level than the ultramafic flows. The ultramafic metacumulates are
always interbedded with sulfidic and graphitic chemosediments, especially in their upper
contact zones, whereas the ultramafic flows are associated with mafic metavolcanics. The
Mertavaara Formation begins with either felsic metasediments/metavolcanics (Mertavaara,
Hotinvaara, Siettelöjoki) or a layer of mafic metavolcanics (Lutsokuru), and terminates with
banded amphibolites or, more generally, high-Mg metabasalts. The sulfidic sediments of the
Mertavaara Formation display similarities with the Matarakoski Formation of the CLGB
Savukoski Group. The mafic metavolcanics of the Pulju area mainly overlie sulfidic
metasediments and can be correlated with the Linkupalo Formation of the CLGB. However,
because they are commonly also related to ultramafic flows or, locally, to felsic volcanics,
they should more correctly be correlated with the Sattasvaara Formation of the CLGB.
Ultramafic flows display many similarities with the type localities at Sattasvaara, in the
CLGB, and in the Karasjok Greenstone Belt, Norway (Barnes and Often 1990), but the
ultramafic cumulates are uncommon in the CLGB. The picritic ultramafics of the CLGB
have not been encountered in the Pulju Belt. Felsic metavolcanic and sedimentary interlayers
are abundant throughout the Mertavaara Formation, a variability that is characteristic
of volcanic sequences.
The uppermost Vittaselkä Formation has been met with in only a few outcrops and drill
holes, but it is seen on geophysical maps as a uniform area characterized by strong electromagnetic
and weak magnetic anomalies. The Formation has been interpreted as consisting
of pelitic metasediments interlayered with graphitic gneisses. The geophysical anomaly
pattern indicates a relatively thin, flat-lying stratum. The existence of this sequence calls
for supplementary studies.
6.5.2.5 Rock types of the Mertavaara Formation
Sections of the ultramafic members of the Mertavaara Formation were drilled at Siettelöjoki,
Mertavaara, Hotinvaara, Aihkiselkä, Lutsokuru, Hotinsaajo and Holtinvaara, where the
outcrops locally also permit mapping in the field. However, geophysical maps imply that
these rocks exist throughout the Pulju area together with those of the Mertavaara Formation.
Diamond drill cores from Siettelöjoki, Mertavaara, Hotinvaara and Lutsokuru were
reinvestigated for this study, and the descriptions of the Hotinvaara and Mertavaara Formations
are based on them.
Felsic gneisses, composed mainly of quartz, feldspars, muscovite and biotite in variable
proportions, are abundant constituents of the Mertavaara Formation. The composition
ranges from leucocratic quartz-feldspar rock of felsic metavolcanic origin to biotite-rich,
banded gneisses with abundant detrital material.
Amphibolites have been met with at different stratigraphic levels of the Mertavaara
Formation. The uppermost thick amphibolite member is banded, but interlayers of more
homogeneous or amygdaloidal modifications are locally present in cherts and felsic gneisses
in the middle of the Formation. Plagioclase (andesine) and green hornblende are the main
minerals, but cummingtonite, diopside, biotite and epidote are also common in places. The
banding is due to variations in plagioclase and hornblende, but diopside and other calc-silicate
bands also occur. Quartz-bearing felsic fragments up to 1 cm across and carbonateepidote
amygdales are rare remnants of volcaniclastic structures. Uralitized relics of

16
clinopyroxene phenocrysts are frequent in hornblende-rich layers. Amphibolites were primarily
mafic metavolcanics, either homogeneous lavas or layered tuffs and tuffites.
In a few drill cores from Mertavaara and Hotinvaara, homogeneous hornblende gabbros
and hornblendites exist together with amphibolites. Hornblendites frequently contain
pseudomorphs after clino- and orthopyroxene, indicating pyroxenitic origin. They have been
interpreted as fractionated portions of thick mafic lava flows or lava lakes, where ponded
lava was fractionated under tranquil, near-surface conditions. The gabbros and hornblendites
are often totally recrystallized and the primary textures obliterated, but the ultramafic,
probably tholeiitic, metacumulate of Siettelöjoki is characterized by well preserved
primary cumulus textures of uralitized pyroxenes. Abundant disseminated magnetite and
ilmenite were formed during metamorphic alteration, and hence the magnetic susceptibility
of the rock is high .
Calc-silicate rocks, cherty quartzites and sulfide-graphite schists exist together as a
“chert member”. Cherts with abundant sulfides and graphite are spatially associated and
frequently interlayered with the ultramafic cumulates. In the lower part of the Mertavaara
Formation at Mertavaara, drilling intersected a considerable section of cherts, but folding
of a flat-lying stratum evidently enhanced the thickness. The layers are often only a few
meters thick. Because of persistent sulfide and graphite contents, they are revealed by their
magnetic and electromagnetic anomalies, even in areas covered by Quaternary overburden.
The layered calc-silicate rocks are composed of coarse-grained diopside, tremolite,
plagioclase (An40-45) and carbonates and locally of quartz, chlorite, microcline, garnet
(grossularite) and clinozoisite. In some localities, there is pure albite instead of oligoclase.
Sericite and clinozoisite are secondary hydrous minerals after plagioclase. At the northwestern
margin of the Belt, calc-silicate rocks associated with granite pegmatite veins locally
contain abundant molybdenite as specks and scattered flakes. An occurrence was studied
at Kiimatievat but the mineral deposit was too scattered to warrant further prospecting
(Hakanen 1977). Porphyroblasts of potassium feldspar are common constituents of calcsilicate
rocks in the vicinity of pegmatites. Grossularite occurs locally as folded bands,
indicating the primary layered structure of the calc-silicate rocks.
The thin black-and-white-layered cherty quartzites are composed of fine-grained
quartz and variable amounts of graphite and sulfides. With an increase in the contents of
calc-silicates and biotite, phlogopite and chlorite, they grade into calc-silicate rocks and
sulfide-graphite schists with banded intermediate varieties. Small-scale disharmonious
folding is typical. Oxide facies banded iron formations are rare, and sulfides prevail, even
in the iron-rich varieties. Some layers of sulfidic cherts in the lower part of the Mertavaara
Formation at Mertavaara and Lutsokuru contain above-average concentrations of zinc.
6.5.2.6 Ultramafic rocks
6.5.2.6.1 General division
The ultramafic rocks were tentatively divided in the field into chlorite-amphibole rocks and
serpentinites-metaperidotites. They are always associated with the sedimentary and volcanic
members of the Mertavaara Formation and commonly interbedded with sedimentary and
volcanic layers, indicating that they are essential members of the stratigraphic succession
and were thus originally ultramafic volcanics.

17
The division of ultramafic rocks into chlorite-amphibole rocks and serpentinitemetaperidotites
calls for comment. Because the ultramafics satisfy the definition of komatiites
(Arndt and Nisbett 1982), as will be discussed in detail in the section on geochemistry,
the komatiite classification (Hill et al. 1990) can be applied, even though the primary
textures have been almost totally obliterated by metamorphism and deformation. The amphibole-chlorite-rocks
are fairly homogeneous and foliated, or then they are layered, fragmentary
volcanic breccias or locally pillowed. They do not, however, show the compositional
A and B layering of typical fractionated Archaean komatiitic flows. In mineral composition
and geochemistry, they bear a very close resemblance to the komatiites of Sattasvaara,
Central Lapland (Lehtonen et al. 1998) and Karasjok (Barnes and Often 1990) and
are here considered as weakly fractionated, liquid-rich, aphyric non-cumulate extrusives,
either thin flows or volcaniclastics. For simplicity, they are here called (non-fractionated)
komatiitic flows (NFKF).
The chemical composition of the serpentinites and metaperidotites corresponds to that
of the komatiitic cumulates, either olivine adcumulates or mesocumulates. However, they
often contain interlayers and irregular masses of chlorite-amphibole rocks, which complicate
the classification. The serpentinite-associated chlorite amphibole rocks have a
geochemical signature of their own. It differs from that of the NFKF, and therefore the exact
division can only be made on the basis of geochemistry. Reasons for the differences between
the chlorite-amphibole rock types will be discussed at greater length after the description
of geochemical features. If the mineralogical data on metamorphosed komatiitic flows (Hill
et al. 1990) are applied in the classification, the chlorite-amphibole rocks associated with
the serpentinites can tentatively be considered as the A layers and the serpentinites as the
B layers of fractionated komatiitic flows. They are thus the result of near-surface fractionation
of komatiititic melt and are here called fractionated komatiitic flows (FKF). The classification
and characteristics of the different types of chlorite-amphibole rocks and komatiites
will be discussed in the following.
6.5.2.6.2 Non-fractionated komatiitic flows
The non-fractionated komatiitic flows (NFKF) constitute layers ranging in width from a
metre to several tens of metres. Thick sequences of NFKF were intersected by drilling at
Mertavaara and in the eastern ultramafic belt of Lutsokuru. They occur as independent layers
in many localities without a direct connection with metaperidotites; at Lutsokuru, for
example, the eastern belt contains only NFKF and the western belt FKF. At Mertavaara, the
NFKF display layering and breccia structures that can be interpreted as primary volcanic
structures - tuffs and volcaniclastics; a breccia even has fragments of ultramafic cumulate
in the NFKF (Fig. 6.5.9). At Holtinvaara, tuff layering, volcanic breccias and pillow structures
are clearly visible. From the northern extension of the Pulju Belt, the Karasjok Greenstone
Belt, Norway, Barnes and Often (1990) described compositionally similar rocks in a
similar geological environment with well-preserved volcanic primary textures such as pillow
lavas, volcanic breccias, tuff layers and agglomerates.
The NFKF are foliated, green on the weathered surface and commonly spotted with
brown olivine porphyroblasts ranging in size from 0.5 to 1 cm. Foliation is a pervasive
feature, and micro-scale folding is often present, too.
The olivine content varies, resulting in weak compositional layering, but primary structures
may also be totally obliterated. A search for primary textures in outcrops, drill cores

18
Figure 6.5.9 Breccia structures of chlorite-amphibole rocks (komatiitic flows); note the ultramafic
cumulate xenoliths. Mertavaara
and thin sections revealed only some questionable linear arrays of magnetite grains that
could be due to serpentinized elongated olivines; other indications of primary skeletal crystals
are lacking. The rocks are totally recrystallized but in all probability the primary ultramafic
extrusives were aphyric, composed of tiny mafic crystallites in partly glassy matrix
that were subsequently hydrated to Mg-Fe amphiboles and sheet silicates. The present
mineral composition, olivine, tremolite, chlorite, magnetite and ilmenite, is the outcome of
metamorphic recrystallization under amphibolite facies conditions.
Magnesian tremolite and chlorite (Mg-clinochlore) exist in slightly variable proportions,
and with a grain size ranging from 0.1 mm locally up to 1 mm. They form a finegrained
matrix for large metamorphic olivine porphyroblasts, which are often roundish but
in places also elongated, without definite crystal forms. Large olivine grains are only marginally,
but small grains totally, retrograde, being serpentinized into intensely coloured green
or brown lizardite. They contain numerous inclusions of ilmenite, magnetite, chlorite and
tremolite. The olivine is richer in iron than it is in the FKF, and the average Fo content is
as low as 65%, much lower than that in the FKF. The Fo-poor olivine was formed by a
metamorphic dehydration reaction from hydrous precursors. Peltonen (1990) proposed that
the metamorphic, low-Fo olivine in the Vammala metamorphosed ultramafic flows crystallized
from chlorite as a result of the reaction:
chlorite —> olivine + spinel + H 2
O.
This reaction cannot, as such, be applied here, because green spinel is absent from or
rare in the assemblage. Carbonates exist in variable amounts as thin veins or accumulations
of grains that could be remnants of primary amygdales.
The oxide minerals are ilmenite and magnetite - the ilmenite as separate grains and the
magnetite locally as thin sheets between the chlorite flakes. The lines and bands of finegrained
magnetite in association with olivine porphyroblasts may imply the presence of
grains of the primary elongated spinifex type, but their precursors can no longer be recognized.
Pyrrhotite-dominated sulfides exist as dissemination but in considerably smaller
amounts than in the FKF.

19
6.5.2.6.3 Fractionated komatiitic flows - komatiitic cumulates
The fractionated komatiitic flows (FKF) contain abundant olivine that is retrograde and
altered to colourless or green lizardite. The grade of serpentinization varies from total
serpentinization to fresh olivine-bearing rocks.
A representative section of the FKF is in drill core Hov-7 from Hotinvaara in Fig
6.5.10. As the stratigraphic younging direction is not self-evident in the sections, the features
indicating the stratigraphic top were examined carefully. The interlayers of sulfidic
Figure 6.5.10 Drill core data of the drill core Hov-7, which was drilled through an ultramafic cumulate
lrns; for location of the drilling site see Fig. 6.5.7; Hotinvaara

20
sediments locally display graded bedding, but in some places the layers are intensely folded
and the facing of the sediment does not warrant far-reaching conclusions. Thin layers of
serpentinite and amphibole-chlorite rocks interbedded with sulfidic metasediments and
calc-silicate rocks exist in the shallow portion (upper part) of drill core Hov-7 and have been
interpreted as representing primary thin flows of komatiites. The occurrence of fractionated
ultramafic flows with metasedimentary interlayers often observed at the top of thick cumulate
bodies has been used as a stratigraphic guide. Also, the gradual change from amphibole-chlorite-serpentine
rocks to pure serpentinites in thin ultramafic flows can be attributed
to the variation in primary komatiitic A and B zones; the stratigraphic younging direction
is then consistent with the other observations. The existence of thin interlayer sediments
suggests that the cumulates were probably not sills, but accumulated from an ultramafic lava
flow and that the mainly chemical sedimentary material precipitated between flows. An
indication of a younging direction in the drilled section could be that the interlayer sediments
lie on top of a chlorite-amphibole rock (A zone) and that a serpentinite (B zone)
overlies the interlayer sediment. Downwards in drill core Hov-7, the rocks grade into a thick
body of komatiitic cumulate that, in the deep portion, passes into pure, fresh dunite.
Dunite is a light-green olivine rock without serpentine or sulfide minerals. It is characterized
by blades of olivine crystals, up to 5 cm long, enclosing small olivine grains. The
longitudinal parting of the blades imparts a special luster to both the mineral and the rock.
The difference between FKF-related amphibole-chlorite rocks and the NFKF is not
always clear in drill core logging. Two distinctive features of the FKF are, however, the
general lack of foliation and the coarser grain size of silicate minerals. Moreover, in texture
the FKF amphibole-chlorite rocks display replacement of olivine by tremolite and of
chlorite by phlogopite. The final division, however, has to be made on the basis of the
chemical composition of the rocks and minerals.
The main variation in the mineral composition of the FKF is due to the presence of
amphibole (tremolite/cummingtonite), a magnesian sheet silicate (chlorite/phlogopite) and
olivine/serpentine. At Lutsokuru, enstatite and diopside are the main constituents. The rocks
are a homogeneous grey or green when dominated by serpentine and olivine but, with an
increase in amphibole and chlorite contents, they become heterogeneous, with light-green
amphibole/chlorite patches in green serpentine matrix, passing eventually into spotty
serpentinite, with dark green serpentine spots in light-green amphibole/chlorite matrix. The
grade of serpentinization is not related to the amounts of amphibole and chlorite in the rock.
In metamorphosed fractionated komatiite flows of the Archaean succession (Western
Australia as an example), the variation in chlorite/tremolite and olivine is largely a primary
compositional feature caused by variation in melt and cumulus olivine in the A and B layers
of the flow (Hill et al. 1990). Pure dunites represent primary olivine adcumulates, and,
with increasing tremolite and chlorite, the precursor primary rock grades into either pyroxene-bearing
cumulates, meso- and orthocumulates or spinifex-textured lavas with a considerable
portion of trapped primary melt. In some parts of the Pulju Belt, however, the lightcoloured
amphibole-chlorite mass replaces the dunitic matrix, forming an irregular network
in the rock. In this case, the rock underwent metasomatic alteration during metamorphism
and the fluids introduced elements that altered the primary olivine rock into more silicic
chlorite and amphibole-bearing variants. The alteration will be discussed in a separate section.

Carbonation of ultramafics is not common. Talc- and magnesite-bearing zones are
intensely tectonized and of scattered occurrence, mainly in the Mertavaara area. Secondary
thin retrograde carbonate veining is fairly common but is not then associated with talc
crystallization.
The textures of the FKF are totally metamorphic. A 5-cm-long section in drill core
Hov-32 (164 m) displays relict spinifex textures with long, thin blades of olivine (sheeted
spinifex), but generally the minerals are totally recrystallized.
The olivine grains vary in size from fine-grained to 10-cm prisms, but their metamorphic
origin is revealed by the habitus of the crystals and the trace element composition.
Some large olivine grains are porphyroblastic, having small olivine grains as inclusions. The
crystal forms are well developed, especially against sulfides, but amphibole and chlorite
exhibit their own crystal forms against olivine. Serpentine is mainly lizardite, but antigorite
has locally grown as larger flakes in lizardite matrix or as bundles of flakes at the margins
of olivine.
Single euhedral grains of amphibole and chlorite are enclosed in olivine/serpentine in
dunitic rocks or olivine-rich peridotites, but when their abundances increase they form a
coarse-grained mass replacing serpentinized or fresh olivine grains. Primary magmatic textures
have not been preserved. Owing to the metamorphic growth of olivine, the scattered
amphibole and chlorite grains exist both in interstices and as inclusions in olivine. Amphibole
is mainly colourless tremolite; cummingtonite is rare.
Enstatite is locally the main constituent in the Lutsokuru metaperidotites. It is coarsegrained,
clear, without exsolution laminae and often serpentinized to a varying degree.
Enstatite is not present in the Hotinvaara and Mertavaara samples, but diopside was met
with in a few samples from Lutsokuru.
Phlogopite is common in dunites and metaperidotites. In thin section, it is very pale
brown, often even colourless. It exists together with chlorite or, locally, as interlayers or
interlocking grains with phlogopite cores and chlorite margins. The intergrowths do not
display the alteration textures or transitional forms typical of the alteration of biotite into
chlorite but seem, instead, to be a result of coexisting recrystallization. The chlorite to
phlogopite ratio varies from phlogopite-free chloritic metaperidotites to a pure phlogopite
rock that constitutes a 0.5-m-wide zone at the margin of the ultramafic body at Hotinvaara.
The accessory minerals are chromite, magnetite and sulfides. Locally, chromite grains
form small aggregates, probably indicating primary interstitial chromite accumulation. In
recrystallized rocks, the aggregates are now frequently enclosed in olivine. A metamorphic
chromite exists in some metacumulates. Irregular in form, translucent and homogeneous,
it is without magnetite rims and was formed by metamorphic decomposition of chromiferous
chlorite. Metamorphic chromite will be described in the context of the Kellojärvi
chromites (Appendix 6.7). Magnetite exists as small, independent grains or together with
serpentine as a secondary dust. It occurs as interlayers between chlorite flakes or as an
oxidation product of sulfides. Magnetite accumulations with interstitial forms are due to the
total oxidation of sulfides.
21

22
6.5.2.7 Mineralogy of ultramafics
6.5.2.7.1 Olivine
Outokumpu Exploration did a great number of olivine microprobe analyses as part of the
prospecting program, but only Fe and Ni were determined. The analyses are plotted in the
diagram in Fig. 6.5.11. In the present study, one set of mineral analyses was made with a
Cambridge S-200 SEM-EDS at Turku University. The analyses reported in Tables 6.5.1-
6.5.5 were made with a Camebax WDS microprobe of CSIRO, Western Australia.
The olivines of the NFKF are low in Mg, containing only 59.5-61.3 mol.% Fo (Table
6.5.1). The olivines of the cumulates are richer in Fo, the contents ranging from 88.6 mol.%
to 92.0 mol.% at Mertavaara, 90.5 mol.% to 92.53 mol.% at Hotinvaara and up to 92.6-92.9
mol.% at Lutsokuru. The values correspond well with the Mg values of the host rocks. The
contents of Cr and Ca are below the detection limits. The Mn content varies from 0 wt.%
to 1.01 wt.% MnO, and the correlation with the MnO content of the rock is obvious, as the
NFKF contain 0.23 wt.% MnO in the rock and 0.87-1.01 wt.% MnO in olivine; the corresponding
figures for serpentinites are 0.16 wt.% in the rock and only 0.11-0.18 wt.% in
olivine. This good positive correlation is contradictory to the negative rock Mn/olivine Mn
correlation described by Peltonen (1990) for the Svecofennian metamorphic olivines. The
nickel content is lowest in the olivines of Lutsokuru, with only 0.0-0.07 wt.% NiO; it is
somewhat higher in the Mertavaara and Hotinvaara serpentinites, and highest of all in the
Fo-poor olivines of the Mertavaara NFKF, with 0.25-0.29 wt.% NiO.
6.5.2.7.2 Amphiboles
Anthophyllite, magnesio-cummingtonite and tremolite exist in ultramafic cumulates (Table
6.5.2). Magnesio-cummingtonite displays a slightly lower mg’ value (0.868-0.873) than do
the coexisting tremolites (0.951-0.968) [Note: mg number or mg’ is the atomic ratio Mg/
(Mg+Fe)]. The calcic amphibole of the NFKF is actinolite, with mg’ values ranging from
0.853 to 0.878, reflecting the lower mg number of the host rocks. The calcic amphibole of
the FKF is tremolite with low contents of Na, mainly substituting for Ca in the M(4) site
of the lattice. Most of the Al is in the tetrahedral site; the small amount in the octahedral
site corresponds to the amount of Na in the M(4) site (Table 6.5.2).
The chemical composition of calcic amphibole has been used as a pressure indicator
in the metamorphic assemblages in which Na is buffered by the presence of albite, Al by
chlorite and f O2
by magnetite (Brown 1977, Bird et al. 1984). Because the ultramafic rocks
have low concentrations of Na, the geobarometer gives only minimum pressures, but apparent
maximum values of 2.6-2.8 kb were calculated for the FKF and NFKF of Mertavaara,
3.2-3.6 kb for the Hotinvaara FKF and 3.3 kb for the Lutsokuru FKF (Fig. 6.5.12).
Consistent with the amphibole geobarometer, a higher metamorphic grade was derived for
the Lutsokuru area from the mineral assemblage containing metamorphic diopside and
enstatite instead of calcic amphiboles.
For comparison, some hornblendes of the amphibolites of Mertavaara were analysed.
They have lower Ni and Cr contents and a higher Ti content than the tremolites of ultramafic
rocks (Table 6.5.2).

23
Figure 6.5.11 The diagram depicts the variation of Ni and Fo contents of olivines from the Pulju area; also
analysed olivines from some other ultramafic rocks of Lapland are presented for comparison. The
numbers refer to target areas presented in Papunen et al. (1977).
Figure 6.5.12 Na(M4) vs. Al(IV) plot for the Pulju amphiboles; isobars from Brown (1977).

24
6.5.2.7.3 Chlorite
Chlorite is clinochlore with a high Cr content (Table 6.5.3). Alkalis are negligible and Ti
is on a slightly lower level than in coexisting tremolite. Chlorite exhibits a considerable
variation in substitution, and FeMg -1
, Al 2
Mg -1
, Si -1
, CrAl -1
and FeAl -1
are the most significant
substitutions. In the samples analysed, FeMg -1
parallels the variation in Mg# in the
rock. Moreover, the Tschermak type of substitution, Al 2
Mg -1
Si -1
, of Al has a positive correlation
with FeMg -1
substitution (see Fig. 12). Cr is high in the chlorites of the FKF, up to
3.2 wt.%, and lower in the NFKF and amphibolites, reflecting the chromium content of the
host rock.
6.5.2.7.4 Phlogopite
Chlorite is a common secondary alteration product of biotite, but at Hotinvaara and
Lutsokuru phlogopite and chlorite exist together in apparent equilibrium; evidently the
concentration and availability of potassium were crucial for the crystallization of phlogopite
instead of chlorite. Phlogopite also includes more silica and slightly less aluminium,
iron and magnesium than does chlorite.
The phlogopites analysed are close to the Mg end member of the annite-phlogopite
series (Table 6.5.3). Al is totally in a tetrahedral position. Na is below detection limits, and
only in a few cases is the (XII) site wholly occupied by K atoms. The Ti content is low, but
is, on average, three times higher than in coexisting chlorite. In contrast, chromium prefers
chlorite and is, on average, four times higher in chlorite than in coexisting phlogopite.
The absorption colour of phlogopite varies somewhat, from colourless to pale yellowish
brown, but no clear correlation with a specific element was detected. The highest absorption
is parallel to the (0001) crystallographic direction.
6.5.2.7.5 Pyroxenes
Orho- and clinopyroxenes were analysed from the ultramafics of Lutsokuru drill core
LK-3 (Table 6.5.4). The clinopyroxene is almost pure diopside and the orthopyroxene contains
6.5-9.6 mol.% ferrosilite component. The pyroxenes are metamorphic in origin.
6.5.3 Geochemistry of the Mertavaara Group rocks
6.5.3.1 Methods and samples
A set of 156 samples from drill cores and outcrops was analyzed for major and trace elements
with combined XRF, ICP-MS and INAA methods at the XR Analytical Laboratories,
Ontario, Canada. Also at our disposal were the assays of Ni, Cu, Co, Zn and S made on drill
cores by Outokumpu Oy for prospecting purposes.
Average compositions of rock types are shown in Tables 6.5.5-6.5.6.
The analyses were made on outcrop samples from the Mertavaara area, three drill cores
(Hov-7, Hov-30 and Hov-32) from Hotinvaara and two drill cores (Lk-2 and Lk-3) from
Lutsokuru. For the study of areal variation, the analyses of different areas were treated separately.
As the main emphasis here is on ultramafic rocks, the geochemistry of felsic and
mafic rocks is discussed only briefly.

25
6.5.3.2 Felsic and mafic rocks
Quartz-feldspar rocks and schists were analysed from Mertavaara. The analysed samples
interpreted as felsic volcanics in origin were selected on the basis of their fine-grained and
locally porphyritic textures. However, the low CaO concentrations and the high values of
the alumina index, from 0.9 to 1.24, imply a sedimentary rather than an igneous origin. In
the Na 2
O+K 2
O vs. SiO 2
diagram the rocks plot in the field of rhyolites; in the Zr/Ti vs Nb/
Y diagram (Winchester and Floyd 1976) they plot in the field of rhyodacites and trachyandesites.
In the Ti/100-Zr-Y *
3 diagram (Pearce and Cann 1973), however, the rocks plot
outside the calc-alkaline field, towards the Zr apex, indicating a sedimentary heavy mineral
accumulation in rock composition. The mantle-normalized spidergrams (Fig. 6.5.16 B)
are characterized by enrichment and flat distribution of LILE and LREE elements and relatively
low concentrations of Nb and Sr relative to other LILE, and also by low P, Ti, Sc and
V relative to Hf, Zr and HREE. The trace element patterns are similar to those of the CLGB
andesitic volcanics of Möykkelmä described by Räsänen et al. (1989), who considered the
andesites contaminated final members of volcanic fractionation sequence. The rocks can
thus be regarded as reworked felsic volcanics with a small to moderate detrital component.
Amphibolites represent mafic volcanics, lavas, tuffs and tuffites.
Banded amphibolites, which constitute the uppermost unit of the Mertavaara Formation,
are tuffitic and volcaniclastics in character and contain a minor amount of sedimentary
material; however, with low TiO2 (0.77 wt.% compared with 1.41 wt.% in the tholeiitic
amphibolites) and high Cr (650 ppm) and Ni (329 ppm) they display a distinct, more
primitive, geochemical characteristic of the volcanic component. Like the other primitive
signatures, the Mg# [Mg# = atomic MgO/(MgO+0.9*FeOtot)] is high (0.62) compared
with that of the tholeiitic amphibolites (0.47). The banded amphibolites are considered to
be variants of high-Mg basalts.
The mantle and MORB-normalized spidergrams (Fig. 6.5.16 B) of the massive and
lava-textured amphibolites display a relatively flat pattern of HFSE. In the mantle-normalized
diagram, Hf, Zr and HREE are high compared with Ti, LREE, Sr, P and Sc. As a result
of the high Zr and Nb, the interelement ratios, such as Nb/La and Ti/Zr, are low, evidently
indicating crustal contamination. The Cr and Ni concentrations are at about the level
of MORB. The low Mg-high Ti amphibolites have been considered tholeiitic basalts in
origin.
The high Mg metabasalts differ from their tholeiitic counterparts in their higher LILE
concentrations (Table 6.5.5), evidently due to their tuffitic character and the sedimentary
material involved. The high peak of U is a consequence of synsedimentary/volcanic precipitation
and is comparable to that in cherts and calc-silicate rocks, which, as deduced from
their high graphitic carbon and sulfide concentrations, evidently precipitated under low Eh
conditions. Low values of Y and Sc compared with mantle-normalized V are characteristic.
In magmatic fractionation, Sc is concentrated in clinopyroxene, because a high cpx/melt
distribution coefficient and a negative anomaly suggest early fractionation of pyroxene from
the melt. However, a negative Sc anomaly is characteristic of all the ultramafic rocks in the
area.
Biotite-hornblende gneisses deposited during incipient volcanism constitute a mixture
of residual sedimentary and mafic volcanic material. Characteristic features are low P and
Sr values, a large variation in incompatible elements and high Cr and Ni concentrations. The
spidergrams reveal clear similarities between cherts and biotite-hornblende gneisses.

26
Figure 6.5.13 Ternary plots depicting the variation of chemical compositions of felsic, mafic and
ultramafic volcanic rocks of Mertavaara
6.5.3.3 Non-fractionated ultramafic flows
The non-fractionated komatiitic flows (NFKF) were analysed at Mertavaara and Lutsokuru
only (Table 6.5.6). The MgO contents of the rocks, recalculated to volatile free 100 % oxides,
range from 18.68 wt.% to 26.48 wt.%, with 23.48 wt.% as the average. The average
nickel content is 959 ppm, and the average Mg# 0.78, which is much lower than the corresponding
value for ultramafic cumulates (FKF), but higher than the Fo percentage (64)
of metamorphic olivines of the NFKF (Fig. 6.5.21). Mg# = 0.84 was calculated from the
molecular proportion diagram of Mg/Ti vs. Fe/Ti although the scatter of points does not suggest
a good correlation. A compositional gap in MgO contents exists in the variation diagrams
(Fig. 6.5.14) for the FKF and NFKF analysed.
In an ultramafic discrimination diagram based on Australian komatiites and mafic
metavolcanics (Rock 1991), the high TiO 2
concentrations of the NFKF transfer the points
outside the field of komatiititic flows although Cr and Ni values are well in the preferred
limits. However, if the average MgO value calculated from several flows refers to the primary
melt composition, the rocks fit well the komatiite definition of Arndt and Nisbet
(1982).
Nesbitt et al. (1979) classified komatiites as Al-depleted and Al-undepleted types. In
that classification, the chlorite-amphibole rocks are geochemically atypical (Fig. 6.5.14).
The concentrations of Al are close to the level of Al-undepleted komatiites, and the Al 2
O 3
/
TiO 2
ratio (8.83) is also consistent with them, whereas the concentrations of Sc and Yb are

27
Figure 6.5.14 Diagrams depicting the incompatible element vs. MgO in the Mertavaara and Hotinvaara
metacumulates and ultramafic lavas. Trend lines of Al-depleted and undepleted komatiites from Barnes
and Often (1990)
at the level of Al-depleted komatiites. The Ca to Al ratios are close to 1 and refer to Alundepleted
komatiites (Fig. 6.5.13). This could, of course, be due to metamorphic depletion
of Ca, which is a mobile element in alteration.
The relatively small number of NFKF samples analysed does not, however, warrant
far-reaching conclusions about fractionation trends. The molecular Mg/Fe diagrams are too
scattered to be indicative of fractionated minerals and their compositions (Fig. 6.5.21). The
irregular variation in composition is due either to the primary random mixtures of pyroclastic
material with variable amounts of melt and cumulus crystals or to the rocks having been
altered in metamorphism.
The chemical compositions of the NFKF and FKF are depicted in variation diagrams
and spidergrams (Figs 6.5.14-6.5.16 A). The spidergrams of mantle-normalized elements
(Fig. 6.5.16 A) display a wide scatter and relatively high values of low-charge large-ion
lithophile (LIL) elements (Cs, Rb, Ba, K and Nb) whereas the concentrations of incompatible
high-field strength elements (HFSE) are more constant and their average inter-element
ratios not far from the corresponding mantle values. Of the elements analysed, the light rareearth
elements (LREE) La and Ce, together with Sr and Sc, display somewhat lower mantlenormalized
values than do P, Hf, Zr, Ti and the heavy REE (HREE) Lu and Yb. In this respect,
the rocks fit the classification of Al-depleted komatiites. In the MORB-normalized
plot the high concentrations of Cr and Ni are striking whereas K and Sr are well below the
MORB values.
The NFKF share many geochemical similarities with the Karasjok komatiites de-

28
scribed by Barnes and Often (1990), who drew attention to the differences between the
concentrations of the incompatible Ti-associated elements (TAE) Ti, Zr and Sm and the
HREE-associated elements (HAE) V, Sc, Ca and Al (see Fig. 6.5.14). The Pulju NFKF have
TAE/HAE ratios of only about 1.5 to 2, thus resembling the northern Karasjok komatiites
(pillow lavas and volcaniclastics). The Sattasvaara komatiites (Lehtonen et al. 1998) do not
differ from the Pulju NFKF.
As the variation diagrams were too complicated to define the mutual correlations of
the elements, factor analyses (obliquely rotated Varimax) were conducted on the samples
analysed, and the factors were geologically interpreted on the basis of the element loadings
(Table 6.5.7). The factors with a high loading of only main elements were considered with
caution owing to the possibility of self-correlation between the constant sum percentage
values. In all calculations, the number of samples exceeded that of variables.
Magmatic crystal fractionation can well explain the correlations between elements, e.g.
Ti, Zr, Sc, Si, K, Y and La, that are not compatible in early crystallizing mafic silicates. The
effect of crystal fractionation manifests itself as high loadings of the elements on a “residual
melt factor”. On the other hand, Ti has no correlation with V in the NFKF. Sulfur correlates
with Cu, but very weakly with Ni and Co and not at all with Zn and Pb. Au and Bi have a
positive mutual correlation as do Ni, Co and As. The results will be discussed in greater
detail in the context of the corresponding analyses of the FKF.
6.5.3.4 Fractionated komatiititic flows - komatiitic cumulates
6.5.3.4.1 Main elements
Komatiitic cumulates were analysed from Mertavaara, Hotinvaara and Lutsokuru. The results
for each of the areas were treated separately (Table 6.5.6).
Pure serpentinites and dunites were considered to represent primary olivine adcumulates,
and their geochemistry closely indicates primary olivine composition. The primary
composition of cumulate olivine can be calculated from a variation diagram of MgO vs an
olivine-incompatible element such as Ti, V, Ca or Al. On that basis, at Mertavaara the most
magnesian primary olivine contained 47 wt.% MgO, corresponding to 86.9 mol.% Fo (see
Hill et al. 1990).
In fractionated ultramafic and mafic rocks, the fractionated minerals can be identified
with Pearce element ratio diagrams, in which the main component of the mineral is normalized
with an element conserved in the melt fraction during crystallization (Pearce 1968,
Russell and Nicholls 1988, Stanley and Russell 1989). Accordingly, the molecular Fe/Mg
ratio of an early crystallizing single mineral can be calculated from a diagram in which the
Fe and Mg of the rock are related to an incompatible element.
For the most magnesian Mertavaara samples, Si/V vs FM/V (FM = Fe+Mg) and Si/
Ti vs FM/Ti diagrams show that olivine was the main fractionated mineral; a molecular
proportion diagram, Mg/Ti vs Fe/Ti, gives a value of Fo 90,6 mol.% and an Mg/V vs Fe/
V diagram Fo 91.4 mol.% (Fig. 6.5.21). The difference from the above value is due to fractionation
of the Mertavaara serpentinites, as implied by metamorphic alteration and the
introduction of metamorphic tremolite into the rock.
Most of the Mertavaara samples contain small amounts of nickeliferous sulfides. Even

29
Figure 6.5.15 SiO 2
and CaO vs. MgO diagrams depicting the compositions the ultramafic flows and
Mertavaara and Hotinvaara metacumulates. Cmpositions of tremolite, olivine and chlorite are marked for
comparison; symbols as in Fig. 6.5.14.
so, the nickel concentration of primary olivine can be calculated from the MgO vs. Ni diagrams
of the samples with a low S content (Barnes et al. 1988), giving a value of 1700 ppm
Ni. The same value can also be obtained from a S vs. Ni diagram as the Ni concentration
corresponding to 0 wt.% S.
The Hotinvaara data give 48.5 wt.% MgO for primary olivine, which corresponds to
88.7 mol.% Fo. A value of 91.4 mol.% Fo can be calculated from the molecular proportion
diagram Fe/Ti vs. Mg/Ti. A Ni vs. S diagram indicates 1320 ppm Ni in a rock with 0 wt.%
S, whereas the Ni vs. MgO diagram gives a somewhat higher value, 2066 ppm, for the primary
Ni content in olivine.
The corresponding values for the Lutsokuru samples are 1500 ppm Ni in olivine,
which, according to the Fe/V vs. Mg/V mol.prop. diagram, contains 93.4 mol.% Fo. The
Ti related mol.prop. diagram is too scattered to give a reliable correlation. The MgO concentration
of primary olivine, as approximated from MgO vs. Al 2
O 3
, CaO and incompatible
element variation diagrams, was 51wt.%, corresponding to 92.2 mol.% Fo.
Because Al 2
O 3
and TiO 2
are not compatible in the early cumulus minerals, olivine in
particular, it is presumed that the whole-rock Al 2
O 3
/TiO 2
ratio of cumulates reflects the
composition of melt. In ultramafics, the average ratio varies from 14.6 to 43.1, with a considerable
scatter. All the values are well above those of amphibole-chlorite rocks (Fig.
6.5.14) and militates against a genetic link between these two rock types. The role of sec-

30
Figure 6.5.16.A Rock / primitive mantle (Thompson) spidergrams of fractionated (serpentinites) and nonfractionated
(chlorite-amphibole rocks) komatiitic flows from Mertavaara, Hotinvaara and Lutsokuru
ondary alteration processes in the composition, and especially the mobility of Al, will be
discussed in the section dealing with alteration.
In most of the variation diagrams, the scatter of the analytical values is wider than the
variation caused by analytical inaccuracy and does not follow the rules of magmatic fractionation.
In fractionated ultramafics, the incompatible elements, such as Ti and Al, are
accommodated mainly in the intercumulus melt portion if olivine is the sole crystallizing
phase. Thus, the contents of Ti and Al should increase parallel to those of the other elements
conserved in the sequence: olivine adcumulate-mesocumulate-orthocumulate. A good linear
correlation between these elements and decreasing MgO is evident, for example, in the well-

31
Figure 6.5.16.B Rock/primitive mantle spidergrams of metasediments and metavolcanics from
Mertavaara.
preserved thick sequences of komatiitic cumulates of the Walter Williams formation, Western
Australia (see e.g. Hill et al et al. 1990). Because the opx/melt distribution coefficient
of Ti is slightly above 1, some enrichment of Ti is possible in the cumulus crystals if opx
is a liquidus phase. However, the Si/Mg molecular proportion diagram does not support the
existence of pyroxene in these samples.
In the sequences of ultramafic samples analysed, some TiO 2
and Al 2
O 3
values in the
TiO 2
vs. MgO and Al 2
O 3
vs. MgO diagrams are extremely low, and the existence of two
populations of samples can be deduced from the distribution of these elements (see Fig.
6.5.14).

32
Figure 6.5.17 Rock/MORB and rock/mantle spidergrams
of Mertavaara non-fractionated flows
Figure 6.5.18 Rock/MORB spidergrams of the
ultramafic flows of the Lapland greenstone belt
(Hanski and Smolkin 1989, Räsänen et al. (1989)
To explain the mutual variations in SiO 2
, CaO and MgO concentrations, the corresponding
variation diagrams are presented in Fig. 6.5.15. Clearly a mixture of olivine and
an ultramafic melt cannot explain a major part of the SiO 2
-MgO variation with a composition
corresponding to that of the NFKF. This would be the case if the ultramafics were
mixtures of fractionated magmatic cumulus olivine and intercumulus melt. The SiO 2
-MgO
variation could be explained if opx and olivine were both cumulus minerals. However, the
distribution of Sc contradicts the existence of magmatic opx because Sc has a distribution
coefficient of >1 opx/melt, and there is no correlation between the calculated opx content
and the content of Sc. If the rocks are a cumulus-intercumulus mixture of olivine crystals
and melt only, the melt was richer in SiO 2
and/or MgO than were, say, the NFKF. This
possibility will be discussed in a later section. In fact, however, the mutual variation in
olivine and tremolite ± chlorite could well explain the observed scatter because the samples
plot along the olivine-tremolite mixing line.
The CaO-MgO variation diagram displays a similar wide scatter of concentrations. The
mixing of intercumulus mafic melt having Ca/Mg ratios of komatiitic flows with cumulus
olivine could explain much of the variation, assuming post-crystallization resetting of Ca.
Even then, the tremolite-olivine ± chlorite control lines explain the compositional variations;
magmatic crystal fractionation is hardly the basic petrological reason for this mineralogical
variation.

33
Figure 6.5.19 Rock/MORB spidergrams of metacumulates and metasediments in drill core Hov-7. Note
the effects of amphibole (Ca-Si metasomatism) and phlogopite (K-metasomatism) for trace elements of
serpentinites in the lower diagram.
6.5.3.4.2 Trace elements
The variation in the trace element concentrations of komatiitic cumulates is presented in
Table 6.5.6 and depicted as primitive mantle and MORB normalized spidergrams in Fig
6.5.16 A and 6.5.19. All diagrams show a wide scatter of concentrations similar to that already
noted in the main elements.
The concentrations of many of the lithophile trace elements were below the detection
limits and thus the inter-element ratios in Table 6.5.6 were calculated only from significant
values above detection limits.
The relatively high concentration of TiO 2
in the komatiitic flows is not reflected in the
cumulates, which have very low contents of TiO 2
(Fig. 6.5.18). For example, a chondritenormalized
Ti/Zr (N)
value, which is 1.32 in amphibole-chlorite rocks, is only 0.30-0.64 in
the serpentinites except in some samples richer in Ti from the eastern Hotinvaara area, for
which an anomalously high Ti/Zr (N)
value, 1.59, was calculated (Table 6.5.6). A similar
difference between amphibole-chlorite rocks and metacumulates is shown by Ti/Y (N)
ratios.
Concentrations of Zr vary (Fig. 6.5.14) and have no correlation with MgO, whereas high
Sm values correlate with low MgO values.
A feature common to all the MORB spidergrams is the high values of Rb, which at
Lutsokuru and in drill core Hov-7 are associated with high K and Ba. In contrast, Hov-30
to 32 and Mertavaara display very low K values. The samples from Mertavaara show a peak
of Nb values. All cumulates display high values of Cr and Ni as do all the other rock types
of the area except tholeiitic amphibolites.

34
Figure 6.5.20 A Element variation in drill core Hov-7; for rock types see Fig. 6.5.10
Figure 6.5.20 B Rock type and element variation in the drill core LK-3, Lutsokuru. Note the good
correlation and anomalously high TiO 2
and V concentrations in banded amphibolites and calc-silicate
rocks. Tremolite-bearing ultramafics locally display high values of Ni/S

Figure 6.5.21 Pearce element ratios (Pearce 1968) of the Mertavaara ultramafic lavas (chlorite-amphibole
rocks - Δ), serpentinites (ultramafic cumulates - +) and mafic low-Ti metavolcanics (o); calculated mg#
for different rock sequences are indicated. The ratio 1:2 in Si/Ti vs. FM/Ti diagram indicates that olivine
has been the only fractionated mineral in the cumulates
35

36
Figures 6.5.20 A and B depict the geochemistry and the variation in composition of
the wall rocks and komatiititic cumulates for typical drill profiles of Hotinvaara (Hov-7)
and Lutsokuru (LK-3) described above. The MgO of the cumulates increases slightly in a
stratigraphic downward direction, attaining a pure dunitic composition in the lower part of
Hov-7. The elements combined with the interstitial melt fraction therefore decrease, indicating
the primary meso- to adcumulate nature of the cumulate. Only at a depth of 140-150
m do TiO 2
, Al 2
O 3
and the incompatible elements attain higher values; an interesting peak
of sulfur and sulfide-bound elements is noted at the same level. Combination of these two
groups of elements indicates the existence of primary intercumulus melt that was enriched
in magmatic sulfides, too. The composition and characteristics of sulfides are discussed in
a separate section.
The mutual ratios of olivine-incompatible elements vary along the profiles. The Al 2
O 3
/
TiO 2
ratio, for instance, displays a slight zoning in the komatiitic cumulates of Lutsokuru
that is due either to variation in the composition of primary cumulus minerals or to secondary
alteration and mobilization of either Al or Ti. Al has a slightly higher crystal/melt distribution
coefficient for pyroxenes than has Ti, and the Al 2
O 3
/TiO 2
ratio may thus increase
in the pyroxene-bearing cumulates. However, the main elements do not indicate the existence
of pyroxene cumulates. Secondary mobilization of Al is discussed in the section on
alteration.
Potassium displays a wide variation in the profiles, the peak values being in the phlogopite
rocks. They evidently represent intensely tectonized shear zones of ultramafics,
where phlogopite crystallized instead of chlorite because of high activity of K in the fluid
phase in the permeable fracture zone. The presence of phlogopite in the cumulates is reflected
in the increased values of K in the profiles, indicating that hydrous K-bearing fluids
percolated throughout the cumulate bodies.
6.5.3.4.3 Factor analysis
To explain the wide scatter and mutual correlations of the main and trace elements,
correlation coefficients were calculated and a Varimax-rotated factor analysis was conducted
as described for the NFKF. Logarithms of the percentages were used to avoid overweighting
of the scattered high contents of the trace elements. Table 6.5.7 lists the main factors for
NFKF and FKF separately for each of the three study areas.
The most prominent factor (#1) in all areas is characterized by a high positive loading
of SiO 2
, variable weak positive loadings of Al 2
O 3
, K 2
O, CaO, Na 2
O, La and Sm, and
strong negative loadings of MgO, FeO, Cr and LOI. This factor is due to the elemental
variation in magmatic crystal fractionation: the concentrations of Mg and Fe and Cr decrease
with increasing Si of the cumulates. If pyroxene-bearing cumulates exist, the increase
in silica correlates with the other pyroxene-associated elements, mainly Ca and alkalies.
The positive loadings of Ti, Al, Zr, P, V, Sc at Mertavaara and Hotinvaara on a common
factor (#2) were explained by the intercumulus melt factor. As the intercumulus melt
accommodates the elements not incorporated in the accumulated crystal phases, the factor
gets high scores in the samples, e.g. orthocumulates and, less prominently, mesocumulates,
with high melt proportions. This factor does not occur in the NFKF, in which the fractionation
factor accommodates Ti and Zr. Note that, at Lutsokuru, the factor characterized by
TiO 2
and Al 2
O 3
also has high loadings of FeO, S, Cu, Ni, Co, Pb and Pd, typical chalcophile

37
elements. This indicates that sulfides accumulated together with intercumulus melt, and the
concentration of sulfides correlates with that of the residual silicate melt, a finding held to
be strong evidence of magmatic sulfide accumulation.
Table 6.5.7 Results of factor analysis (Varimax, orthogonal rotation; due to brevity only the principal loadings
and their types have been indicated)
Factor
Principal loadings
Mertavaara NFKF
1) Residual melt factor: -MgO, -FeO,- Cr, +SiO 2
, +Na 2
O, +K 2
O, +CaO, +Y, +Ti, +Zr, +Sc, +La
2) Sulfide factor: +S, +Cu, +Zr, +Zn, -Cl, -Sr, -Pb
3) Gold: +Bi, +Au
4) Ni-As factor: +Ni, +Co, -As
Mertavaara serpentinites (komatiitic cumulates):
1) tremolite alteration factor: -MgO, -FeO, -LOI, +SiO 2
, +K 2
O, +CaO, +Pb, +La, +Sm, +Au
2) residual melt factor: +TiO 2
, +Al 2
O 3
, +Zr, +Sc, (+V)
3) sulfide/Mo factor: +S, +Mo, +Zn, +Cu, + SiO 2
, -Cr
4) sulfide/Na factor: +Na 2
O, +S, +Cu, +SiO 2
,
5) +Cr, +Zr, +Pd, +Bi (+V, +Sm)
6) Ni-As factor: +Ni, +Co, +As
7) +Ba, +Cl, +Br
Hotinvaara serpentinites (komatiitic cumulates):
1) tremolite alteration factor: -MgO, -FeO, +SiO 2
, +CaO, +Na 2
O, +Sr, +La, +Ce
2) residual melt factor: +TiO 2
, +Al 2
O 3
, +P 2
O 5
, +V, +Sc, +CO 2
3) potassic alteration: +K 2
O, +Ba, +Br
4) carbonation factor: +CO 2
, +Pb (-Au)
5) sulfide factor: +S, +Ni, +Cu, +Pd, +Co
Lutsokuru serpentinites (komatiitic cumulates):
1) residual melt + sulfide factor: +TiO 2
, +FeO, + S, +P, +Al 2
O 3
,+La,+ Ce, + V, -Cr, +Cu, +Ni,+Co, +Pb, +Pd
2) tremolite alteration factor: +SiO 2
, -MgO, -MnO, +CaO, +Na 2
O
3) serpentinization: +MgO, +Cl, +Br
4) potassic alteration: +K 2
O, +Al 2
O 3
, +Ba, +V, +Sc
Sulfides behave in interesting and individual ways in different target areas. The
Mertavaara serpentinites do not display a correlation between sulfur and Ni. Instead, two
distinct sulfide factors exist: one with loadings of Na 2
O, S, Cu and SiO 2
, and the other with
loadings of Mo, Zn, Cu and SiO 2
. As an individual nickel-arsenide factor, Ni correlates with
Co and As but not with S. The Mertavaara sulfide factors are interpreted as the late hydrothermal
introduction of sulfur into a fluid containing Cu and Si together with Na or Mo and
Zn. A definite sulfide factor with high positive loadings of S, Ni, Cu, Pd and Co was established
in the Hotinvaara samples, and in the Lutsokuru cumulates the sulfides correlate with
the abundance of residual melt. In both cases, some of the sulfides, at least, deposited in the
magmatic stage. The origin of sulfides will be discussed in a separate section.

38
At both Hotinvaara and Lutsokuru, K 2
O exists in a distinct factor together with Ba and
at Lutsokuru with Al 2
O 3
. This is evidently due to the existence of phlogopite in these target
areas and can be interpreted as a postmagmatic hydrothermal influx of potassic fluids.
Even though all the samples have high contents of Rb, this element does not seem to correlate
with K.
The observed correlation between MgO and LOI is attributed to retrograde serpentinization
of olivine. On the other hand, a positive correlation between Br, Cl and MgO, also
noted as a distinct factor, indicates that the serpentinizing fluids introduced halogens as well
as B, which also displays a positive correlation with MgO and LOI. In this respect, the
serpentinizing fluids have qualitative characteristics of marine or connate water.
6.5.3.5 Geochemical classification of komatiites
6.5.3.5.1 Non-fractionated komatiitic flows (chlorite-amphibole rocks)
On the basis of the field studies, the layered or massive amphibole-chlorite rocks were
above considered as an effusive phase of ultramafic komatiitic magma. Except in the
Holtinvaara area, primary structures seldom warrant resolution of the volcanic characteristics
of the rocks, and compositionally similar rocks elsewhere in Lapland, for example,
at Sattasvaara (Lehtonen et al. 1992) and Karasjok (Barnes and Often 1990), frequently
display volcaniclastic, breccia and pillow lava features. These komatiites are here called
non-fractionated komatiite flows (NFKF).
The major element geochemistry of the NFKF, as depicted in the MgO-Al 2
O 3
-CaO
diagram (Fig. 6.5.13), is comparable, among others, to that of the thin komatiitic flows in
the upper stratigraphic level of the Forrestania greenstone belts of Western Australia (see
e.g. Perring et al. 1995). The Forrestania thin komatiitic flows were metamorphosed in
middle to high amphibolite facies and have mineral compositions similar to those of the
NFKF of the Pulju Belt; even the texture is often foliated, in contrast to the massive texture
of the thicker, fractionated ultramafic flows. The thin Forrestania flows occur together
with ultramafic cumulates or occupy belts of their own, commonly on higher stratigraphic
levels of the succession. A lateral and upstrata change from the komatiitic metacumulates
to non-fractionated komatiitic flows is also evident in the Pulju Belt, where they exist as
individual layers at Mertavaara and Lutsokuru (see Fig. 6.5.6).
The strong foliation characteristic of the NFKF contrasts with the massive texture of
the komatiitic metacumulates. This difference is typical of the rock associations elsewhere,
too, and reflects the differences in their competence in ductile deformation. Chlorite- or talcrich
ultramafics are incompetent compared with massive olivine-rich metacumulates, which
display ductile deformation only in their marginal parts or along thin deformation zones,
where the rock has altered into chlorite or talc schist, probably at a late stage of deformation
when olivine had already been altered by retrograde serpentinization. This characteristic
difference in tectonic competence has resulted in misleading interpretations, with
massive ultramafic metacumulates being understood to represent an intrusive phase younger
than the foliated komatiitic flow sequence, for example, in the early interpretations of the
origin of the “intrusive metaperidotites/dunites” and komatiitic flows in the Wiluna greenstone
belt, Western Australia (Naldrett and Turner 1977), or in komatiitic cumulates and

39
flows of the Kuhmo greenstone belt, Finland (Hanski 1986, Piquet 1982).
Deformation and the lack of primary textures make it difficult to assess the primary
melt composition of the NFKF. A weighted average composition calculated for different
extrusive types might give a rough estimate of the liquid, assuming that no intratelluric
phenocrysts were involved in the extrusion. Although the number of samples analysed (11)
does not fulfil statistical criteria, the calculated average value is here used as a model of a
melt, which thus contained 23.5 wt.% MgO. This value is not far from that for ultramafic
tuffs of the Karasjok komatiites (Barnes and Often 1990) and fits well the definition of
komatiites (Arndt and Nesbitt 1982). For further consideration, note that a komatiite melt
with 23.5 wt.% MgO is in equilibrium with Fo 92
olivine (Hill et al. 1990).
The concentrations of Ti, Al and some trace elements are not as in typical komatiites,
however, although the compatible elements, Cr and Ni, are consistent with their concentration
limits. Barnes and Often (1990) classified the similar Ti-enriched Karasjok komatiites
as Al-depleted types. They considered that the ultramafic volcanics resulted from a
relatively small amount of decompression melting of a rising mantle plume in a rifting
environment when garnet was the residual phase, implying partial melting of the mantle at
pressures greater than 40 kb. Arndt and Lesher (1992) concluded that the low-Al 2
O 3
/TiO 2
and HREE-depleted Barberton-type komatiites are formed by a low degree of mantle melting
at pressures greater than 15 GPa (420 km) in the presence of garnet or majorite. The low
values for Sc/Ti and Sc/V in the Mertavaara ultramafic flow indicate partial melting, with
garnet as a residual phase, because the garnet/melt partition coefficient of Sc is considerably
higher than that of Ti and V.
Despite the higher-than-average komatiitic TiO 2
and trace elements associated with Ti,
the rocks have other characteristics of Al-depleted komatiites, too. Some of the picritic
volcanics of the Fennoscandian Shield (Hanski 1992, Hanski and Smolkin 1987) are extremely
rich in Ti, with values as high as 2.5 wt.% TiO 2
in iron-rich ferropicrites. The NFKF
of this study differ from picrites in their high MgO and the komatiite-type characteristics
of the compatible elements. The Onkamo Group komatiites reported from the Möykkelmä
area, Central Lapland (Räisänen et al. 1989, Lehtonen et al. 1998), display lower TiO 2
values
at the same MgO content as the corresponding rocks of the Pulju Belt and have other
characteristics of Al-undepleted komatiites as well. The CLGB komatiites of Sattasvaara,
in contrast, show higher TiO 2
contents than typical Al-undepleted (Munro type) komatiites
and in their trace element contents closely resemble the Pulju NFKF (Lehtonen et al. 1998).
Lehtonen et al. (1998) also report the existence of LREE-depleted high-Ti ultramafic
metavolcanics in the CLGB, which they called picrites. The ultramafics are compared in
Fig. 6.5.18.
6.5.3.5.2 Komatiitic cumulates
Barnes and Often (1990) described massive types of Karasjok komatiites, which, due to
their preserved primary textures, were considered cumulates of the flows. Their MgO ranges
up to 38.4 wt.% (volatile free), a value that is, however, far below those of the rocks considered
to be metacumulates of the Pulju Belt. The layers of massive metacumulates typical
of the Pulju area seem to be lacking in Karasjok. At Pulju, they are probably more widespread
and abundant than the chlorite amphibole-rocks and are composed of variable
amounts of retrograde serpentinized or fresh olivine, chlorite, tremolite, enstatite and

40
chromite. The most magnesian members are pure olivine rocks. Representing primary olivine
adcumulates, such rocks have locally recrystallized to coarse-grained dunites. The
primary olivine mesocumulates and orthocumulates in the Mertavaara and Hotinvaara areas
contain chlorite and tremolite and, at Lutsokuru, also enstatite. The textures are totally
recrystallized, obscure remnants of primary spinifex textures having been noted in only a
few localities. The serpentinites and associated chlorite-amphibole rocks form layers of
variable thickness, with the most magnesian members commonly occupying the cores of
thick layers, as shown by drill core Hov-7 (Figs. 6.5.10 and 6.5.20 A). Metasedimentary
interlayers are, however, more common above the ultramafics than at the lower margin, and
a slight but regular decrease in MgO upwards in the ultramafic cumulate sequence is evident,
for example, in the well analysed sections of Hotinvaara and Mertavaara. The graphite-
and sulfide-bearing felsic schists are metasedimentary interlayers between metacumulates
and show that the massive metacumulates and interlayer sediments were formed
lit-par-lit, and that the cumulates and sediments deposited alternately. The cumulates are
therefore hardly sills; merely basal parts of flows in which the olivine accumulating from
overflowing lavas forms layers of adcumulates and mesocumulates. The rare existence of
preserved spinifex textures of tremolite and chlorite-bearing rocks in the upper part of the
cumulate sequence suggests a ponded lava flow origin for this sequence. However, the
possibility of cumulate sills must be looked into more closely in follow-up studies of all the
drilled sections. The upward decrease in MgO is due either to fractionation of the melt or
to a change from olivine adcumulate to mesocumulate and further to orthocumulate and
finally to a rapidly crystallized melt phase of a ponded flow. The almost constant Mg number
of the sequence favours the latter option.
6.5.3.6 Alteration of the cumulates
It was noted above that the structures and textures imply secondary enhancement of tremolite
and chlorite. The geochemical indications of alteration will be discussed below.
The regular geochemical variation caused by the trapped intercumulus liquid/cumulus
mineral relationship is visible in the variation diagrams of unmodified komatiites as a
linear increase in melt-associated elements with decreasing MgO content, and a linear increase
in olivine-compatible elements (Ni) with increasing MgO. Fractionation of the melt
and crystallization of the less magnesian cumulus minerals after olivine in a closed magmatic
system, as in a ponded lava lake, with or without pulses of fresh melt, operates in the
same way. In Western Australia, such relationships are typical of the cumulates of the large
Walter Williams Formation (Hill et al. 1990, Donaldson 1982).
In the present samples, the minor and trace element compositions of the cumulates
vary, but not as regularly as they would if the amount of trapped intercumulus liquid and/
or fractionation alone were responsible for the geochemical variation. The variation diagrams
of MgO vs. Al 2
O 3
or TiO 2
or other incompatible elements display a wide scatter, and
the existence of more than one population of cases can be deduced. As the MgO content
(anhydrous) ranges from 27 wt.% to 49 wt.%, the Al 2
O 3
values are in general below 2 wt.%,
and the TiO 2
values, even in samples with only 27 wt.% MgO, below 0.2 wt.%. These values
and trends are far below the corresponding Al 2
O 3
and TiO 2
concentrations in unaltered
komatiite ortho- and mesocumulates with similar MgO values. However, in the factor analysis
the effect of variation in the intercumulus liquid is seen as an Al-Ti (Ca) factor, because

Al and Ti are the most abundant elements not incorporated in olivine.
In a MgO-SiO 2
diagram (Fig. 6.5.15), the points scatter along the olivine-orthopyroxene
control line, indicating that the rocks may originally have been mixtures of these
minerals. As pointed out above, the variation in Sc does not support the existence of pyroxene.
The observed variation in SiO 2
-MgO cannot even be attributed to the mixing of
intercumulus liquid with olivine because the variation trend in Fig. 18 implies a liquid
anomalously rich in SiO 2
and poor in Al and Ti.
The chemical compositions of the analysed chlorite and tremolite coexisting with olivine
are depicted in the Al-Mg, Ti-Mg and Si-Mg variation diagrams (Figs. 6.5.14 and
6.5.15), which indicate that the mixtures of olivine, tremolite and chlorite well explain the
compositions of the rocks analysed. This is in contrast to the magmatic trends, but such
compositions can be reached through the alteration of cumulate under metamorphic p-T
conditions of the chlorite-olivine-tremolite stability field if excess SiO 2
is available in the
metamorphic fluid phase. As the alteration affected mainly primary olivine ad- and mesocumulates,
the concentrations of the trace elements not incorporated in the fluid phase remained
at a low level in altered rocks. There is, however, considerable variation in the elements
in metamorphic fluids. Ti is mainly incorporated in tremolite and phlogopite. The
low concentrations of TiO 2
in tremolite explain the low-TiO 2
population of the whole rock
analyses and are in accordance with the tremolite-olivine control in the TiO 2
-MgO diagram
(Fig. 6.5.15).
Although Zr is considered an immobile element in hydrothermal alteration, the irregular
variation in Zr in ultramafics is here more likely a result of secondary readjustment. This
idea is supported by a shear-associated phlogopite rock in drill hole Hov-7 at a depth of
142.60 m; at 27 ppm Zr, the content is more than ten times that in the adjoining ultramafics.
In thin section, the great number of minute zircon inclusions in phlogopite are indicated by
multiphase pleochroic haloes. This phlogopite rock has been interpreted as a hydrothermally
altered sheared ultramafic. High contents of Rb, Cs, and K characterize the phlogopitebearing
rocks. The behaviour of Al is also erratic and, for example, the ratio Al 2
O 3
/TiO 2
in
ultramafics varies within wide limits. A zonal pattern of Al 2
O 3
/TiO 2
can, however, be verified
in the ultramafic cumulates of Lutsokuru (Fig. 6.5.20 B). Al is commonly an immobile
element because in hydrothermal alteration it passes directly from the primary phases
into secondary layer-lattice minerals without marked readjustment with the altering fluids.
Marshall and Mancini (1994), however, observed a mass gain of Al 2
O 3
, K 2
O, BaO, Rb 2
O
and SiO 2
in the alteration zones of peridotite around granitic dykes of the Vammala ultramafic
body. The exchange reactions took place under metamorphic conditions of midamphibolite
facies, at about 600°C and 4.5 kb along a local D 3
fracture system. Although
these alteration zones are quite thin compared with the more regional alteration zones of
the Hotinvaara and Lutsokuru ultramafics, the gain of Al in the process is evident. With a
more regional hydrothermal influx of granitic fluids the process can advance further and
produce wide, regional alteration fronts in which primary olivine has been altered to chlorite
with a hydrothermal gain of Al. Tremolite has been noted to replace olivine as a result of
alteration by silica and Ca-bearing fluids. A similar alteration is distinct in the metamorphosed
adcumulates of Pulju.
Hence, here, the Al 2
O 3
/TiO 2
ratio is not necessarily indicative of the primary melt
composition as it is in less altered ultramafic sequences.
41

42
In the factor analysis, K and Ba have high loadings on a common factor, evidently
indicating potassic alteration of the rocks. Potassic alteration was not observed at Mertavaara,
but in the Hotinvaara serpentinites K 2
O is associated with Ba and Br, and at Lutsokuru
Al 2
O 3
has a high loading on the factor together with K 2
O, Ba, V and Sc. The metamorphic
formation of phlogopite explains the gain of all these elements.
As pointed out above, the geochemical variation diagrams cannot be interpreted on the
basis of crystal fractionation alone. The main variation in the chemical composition can be
explained by the gain of Si, Al and Ca in the olivine parent rock during the metamorphic
crystallization of tremolite and chlorite. This requires the prevailing fluid phase during
metamorphic recrystallization to have had high activity of these elements. In the light of the
data so far available, the origin of the fluids can only be assumed, but a probable source is
the granitization process that syntectonically reworked the surrounding Hetta granite into
its present composition and shape. Note that some ultramafic samples from Lutsokuru display
W values well above the detection limit, and that some analyses from Mertavaara show
considerable contents of Mo (these elements are not included in Table 6.5.6 because most
of the analyses were below the detection limits). These granite-bound elements may also
originate from postorogenic granitoids of the Tepasto type (Haapala et al. 1987), which
display high contents of Mo, Nb and Rb, typical lithophile trace elements of the altered
komatiites at Mertavaara, too. Because the Tepasto granite was emplaced about 1.77 Ga ago,
it is considerably younger than the postulated deformation age of the Pulju Belt. On the
other hand, isotope geochemistry (Patchett et al. 1981, Kouvo et al. 1983) indicates that
granites of the Nattanen-Tepasto type are remelted Archaean crust in origin. Their trace
element characteristics can thus be deduced from Archaean granitoids, which were the precursors
of the synorogenic Hetta granite, too (see Fig. 6.5.23).
Some of the anions, e.g. CO 3
2-
and S 2- , evidently originate from the sedimentary sequence,
into which they were captured from decomposing carbonates and sulfides by percolating
metamorphic fluids.
The first phase of alteration was probably contemporaneous with the extrusion of
volcanics in a marine environment and their subsequent hydration by ocean-floor hydrothermal
alteration. In this respect, the alteration may, to a certain extent, be comparable to
the early alteration described by Eilu (1994) from the central Lapland Lapponium. There
is, however, no direct evidence of early hydration and alteration, because metamorphic
recrystallization in the Pulju area obliterated all the igneous and early hydrothermal textures.
The partial oxidation of sulfides and deposition of magnetite may have occurred during this
period. The major phase of alteration was associated with regional metamorphism and tectonics,
and especially the coeval remobilization of old Archaean granites was the main
generator of the percolating fluids that carried silica and other altering elements to the
ultramafics. This phase of alteration occurred close to the peak of regional metamorphism,
and hence the secondary mineral association has characteristics of middle to high amphibolite
facies, including the minerals En-Di-Tre-Kum-Ol, Chlo-Phlog-Antig in varying proportions.
Retrograde serpentinization altered the metamorphic olivine to lizardite and deposited
secondary magnetite and secondary sulfides (valleriite and mackinawite).
The alteration has slightly different characteristics in different target areas. Potassic
alteration is typical of Lutsokuru and Hotinvaara, but is very weak at Mertavaara. Silicification
and the associated introduction of Ca and Al () to form secondary amphiboles and

43
chlorite at the expense of olivine affected the whole area. Carbonation is not a pervasive
alteration, and talc is a common mineral only at Mertavaara. The retrograde serpentinization
is associated with the accumulation of Br, Cl, and, to a lesser extent, B. This is reflected in
a factor with high loadings of MgO, LOI, Br, Cl and B. The retrograde serpentinizing fluid
was probably related to marine water.
6.5.4 Sulfides
The NFKF contain sulfides only when in contact with sedimentary sulfide layers, but disseminated
sulfides are common throughout the komatiitic cumulates. Pyrrhotite and pentlandite
are the main minerals. Mackinawite and valleriite have been met with only in a few
serpentinite samples from drill cores Hov-30 to 32. Chalcopyrite is rare. The pyrrhotite/
pentlandite ratio varies in the sulfide grains, which are often interstitial to silicates. In
serpentinites and in the rocks formed from olivine adcumulates, the sulfides exist as roundish
anhedral grains interstitial to the euhedral olivines. Chlorite and amphiboles also exhibit
their crystal forms against sulfides, and blades of antigorite locally intersect the sulfide
grains. The sulfides are oxidized to varying degrees. The sulfides of the amphibole-rich
rocks are fresh, but in serpentinized rocks magnetite replaces first pyrrhotite and finally even
pentlandite as a result of retrograde oxidation.
In the serpentinites, which contain veins and patches of amphibole-chlorite material
replacing olivine, the sulfides envelop light-green amphibole-chlorite patches and occur as
dissemination in dark-green serpentinized olivine. In sulfide-rich samples, sulfides also
exist together with amphiboles, as irregular grains pierced by amphibole needles.
The textures indicate that the sulfides have undergone metamorphic recrystallization
of the rocks, and, due to the low energy needed to form euhedral crystals, the recrystallizing
silicates intersect the “soft” sulfides. Retrograde serpentinization of olivine was accompanied
by deposition of secondary magnetite, which locally replaces the sulfide grains.
However, early pre-metamorphic oxidation of sulfides has also taken place, because some
of the rocks with fresh metamorphic olivine contain oxidized sulfide grains as well. This
oxidation is thought to be associated with the early postvolcanic serpentinization of olivine
The variation in the pentlandite/pyrrhotite ratio is shown in the geochemistry, and a
Ni vs. S diagram of Mertavaara displays no correlation between sulfur and nickel (Fig.
6.5.22). The highest Ni values are about 2000 ppm, or roughly the same as the nickel content
of calculated primary igneous olivine. Thus the sulfide-rich samples are rich in pyrrhotite,
and pentlandite is more abundant only in weak dissemination. This feature is also evident
in the factor analysis of the Mertavaara samples (Table 6.5.7), in which two different
sulfide factors appear, but Ni has no loading on either of them. Instead, Ni, Co and As form
a factor of their own, called the nickel arsenide factor, which shows up even if the content
of As is low and nickel arsenides are only rarely met with in polished sections. The correlations
between these elements are better than that between nickel and sulfides.
The case of Mertavaara evidently indicates a secondary influx of sulfur during metamorphic
alteration and a reaction with the host rock to form sulfides. However, the sulfides
forming under metamorphic conditions have extracted nickel only from the close environment
of the grains, and hence the nickel content is on the level of igneous olivine, which

44
Figure 6.5.22 Ni vs.S and PGE vs.Ni plots of the Mertavaara and Hotinvaara rocks
was the main primary carrier of nickel in the barren host-rock. The sulfides are more abundant
at the margins of tremolite-chlorite patches and veins, and sulfur has no correlation
with nickel. The weakest sulfide dissemination may contain a considerable amount of nickel
when calculated to 100 % sulfides, but because these sulfides were not enriched or mobilized,
nickel did not concentrate to form high-grade ores. Accordingly, the metamorphic
olivine of sulfide-bearing samples displays low contents of nickel (Fig. 6.5.22).
The formation of secondary sulfides is thus evidently associated with metamorphic
alteration processes. The aqueous fluids originating from the underlying remobilized granitic
crust contained ions of Si, Al, K and Ca and migrated through the rock sequence under
metamorphic conditions. From the sulfidic sediments they captured sulfur liberated by
the high-temperature pyrite to pyrrhotite metamorphic reaction and probably removed as
polysulfide complexes to the ultramafic rocks in which the silica of carrying fluids reacted
with serpentine or virgin olivine to form amphiboles and chlorite. The reactions changed
the fluid Eh-pH conditions and, as a result, the polysulfide complexes decomposed, and
sulfide ions were liberated to react with the host rock to form sulfide minerals. The sulfur
mobilization and reactions took place in hydrothermal systems. Interelement correlations
demonstrate that Cu and some of the Fe were removed from the sediments by hydrothermal
fluids together with sulfur, but that Ni and Co were captured from the silicates at sites

45
Figure 6.5.23 Ni vs. Fe contents of olivines in the Pulju ultramafics and Ni in olivine vs. sulfur content of
the rock
where polysulfides decomposed and sulfides precipitated. The analyses from Mertavaara
show that nickel was immobile in the metamorphic deposition of sulfides.
The Lutsokuru and Hotinvaara samples differ, however, from those from Mertavaara
(Fig. 6.5.22). The Hotinvaara analyses show a special nickel sulfide factor with high loadings
of S, Ni, Cu, Co and Pd, and at Lutsokuru the sulfide factor has positive loadings of
S, Ni, Co, Pd, Pb, Fe, TiO 2
, Al 2
O 3
, and the trace elements La and Ce.
Although there are good positive correlations between Ni and S in the scattergrams of
Lutsokuru and Hotinvaara (Fig. 6.5.22), the individual samples differ in their Ni/S ratios.
The samples with high contents of sulfide nickel display a Ni/S ratio of 0.23 at Lutsokuru,
and of up to 0.40 at Hotinvaara, but only a few samples with relatively high nickel contents
have Ni/S ratios between 0.15 and 0.11, which is a common ratio at Mertavaara. Most of
the samples analysed plot in the range of 0.2 wt.% to 0.3 wt.% Ni. The contents of nickel
are well above the values for igneous primary olivine owing to the concentration of nickel.
The correlations of Co, Pd and Cu with S are all positive and high.
Examples from Hotinvaara (Hov-30 and 32) show that the Ni/S ratio varies in different
mineralized flows (Fig. 6.5.22). A lava flow intersected by two drill holes contains heavy
dissemination of sulfides and the Ni/S ratio is 0.3, corresponding to about 9 wt.% Ni in the
100 % sulfides. Massive sulfides have been encountered at the base of another mineralized
flow, about 25 m below the upper mineralized flow, and the sulfides assay approximately
4 wt.% Ni. Variable contents of nickel are due to the difference in the sulfide-silicate melt
ratio, the R-factor.
The most probable mechanism for the high Ni values of the Hotinvaara and Lutsokuru
sulfides is the accumulation of nickel sulfides at the magmatic stage. However, a late hydrothermal
influx of sulfur and the formation of metamorphic sulfides have locally decreased
the initially high Ni/S values of the magmatic sulfides.

46
The good correlation of sulfides with Al, Ti and incompatible trace elements can be
explained by the accumulation of magmatic sulfides together with the intercumulus silicate
liquid in meso- and orthocumulates, which contained more trapped liquid than did the
adcumulates. Cu, Pt and Pd are incompatible elements in the crystallization of olivine and
will be enriched in the evolved late silicate melt. Their use as indicators of magmatic sulfides
is questionable, however, because of their tendency to metamorphic mobilization.
Metamorphic mobilization (for definition, see Marshall and Gilligan 1993) of disseminated
sulfides and their accumulation in high-strain zones have been postulated as a mechanism
for the formation of the massive sulfides in Kambalda (Groves et al. 1979) or the
brecciated sulfides in metamorphosed Svecofennian Ni-Cu deposits (see Papunen &
Gorbunov 1985). Here, however, the tectonic upgrading of sulfides is disputable because
the samples with high-grade Ni sulfides are not tectonized and the sulfide textures are interstitial
disseminations as in other sulfide types.
During the oxidation of sulfides, magnetite first replaces pyrrhotite in pyrrhotite-pentlandite
grains, the secondary oxidation thus upgrading the sulfides. This kind of mechanism
is common in weak sulfide disseminations in serpentinites. In the sulfides of Lutsokuru and
Hotinvaara, the high-grade Ni-rich varieties contain both pyrrhotite and pentlandite, without
a marked magnetite content, and oxidation cannot be a major factor in the formation of
high-grade sulfides. We therefore conclude that some of the sulfides inherit their high
nickel, cobalt, platinum and palladium tenors from immiscible igneous sulfide melt trapped
in olivine ortho- and mesocumulates. The presence of igneous sulfide melt in komatiitic
cumulates makes the rock sequence a highly potential host rock for nickel ores.
Sulfide saturation of the silicate melt should leave its mark on the geochemistry of the
crystallizing igneous rocks and their minerals. The silicate melt will be depleted in elements
such as Ni, PGE, Cu and Co, which have a high sulfide/silicate melt distribution coefficient,
if a sufficient amount of sulfide melt is segregated. As a result, the nickel content of the
crystallizing igneous olivine will be low, a fact that has often been used as a prime indicator
of sulfide saturation. The calculated nickel content of igneous olivine at Mertavaara is
about 1700 ppm and at Hotinvaara and Lutsokuru somewhat higher, close to 2000 ppm, but
in all of them it is considerably lower than that of olivines crystallized from sulfide-unsaturated
high-Mg komatiitic melts (Smith and Erlank 1982, Arndt 1986). To account for the
Ni depletion of olivine, the parental liquid must have lost some nickel by fractional extraction
of a sulfide melt in an early phase of emplacement (see e.g. Barnes et al. 1988). On the
basis of the Fo content of olivine and using the Fe/Mg distribution coefficients of komatiitic
melts, we find that the Mg content of the melt was ca. 18 wt.% at Mertavaara and up to 25
wt.% at Lutsokuru (Roeder and Emslie 1970, Hill et al. 1990). On the other hand, in chlorite-amphibole
rocks with 25 wt.% MgO, which were considered to represent a melt-rich
komatiite phase, the nickel content ranges from 600 to 1200 ppm, with 950 ppm as an average.
Even if the chlorite-amphibole rocks have a component of accumulated olivine these
values are within the broad range of Ni and MgO concentrations of spinifex-textured unsaturated
komatiitic rocks (Arndt 1986, Arndt and Lesher 1992). A silicate melt in equilibrium
with olivine Fo 92
and 2000 ppm Ni should contain 200 ppm Ni if the K d
Nisulph melt/silicate
melt
is 10). This value is far lower than the lowest Ni concentration of chlorite amphibole
rocks. The Mertavaara komatiitic flows (chlorite-amphibole rocks) thus probably represent
a sulfide unsaturated pulse of magma different from that of the komatiitic cumulates.

47
6.5.5 Summary
The Pulju supracrustal belt is a part of the Paleoproterozoic greenstone succession extending
from Karasjok to N. Sweden, Central Lapland and Salla. The main stratigraphic units of the
CLGB have been observed in the Pulju belt, although the subhorizontal layering, polyphase
folding and scarce outcropping of the Pulju belt complicate the correlation and interpretations.
The supracrustal sequence of Pulju is not so thick as in the type sections of Sattasvaara
and Kittilä areas probably indicating that Pulju located at the margin of the rift basin. The
Pulju area is characterized by abundant ultramafic cumulates that locate stratigraphically
in the sulfide- and graphite-bearing sedimentary unit, corresponding the Matarakoski Formation
of CLGB. Ultramafic cumulates are rare elsewhere in the Matarakoski Formation.
The cumulates seem to be in a lower stratigraphic position in than the ultramafic flows. The
flows correspond Sattasvaara-type komatiites, they are not fractionated and display frequently
breccia, pillow, tuffitic and agglomeratic structures. The Sattasvaara-type komatiites
probably extruded subaereally or in a shallow water and the eruption could be related to a
vigorous explosion (Saverikko 1983, 1985). It is an open question if the komatiitic cumulates
and the flows are comagmatic because the melt-rich parts of the fractionated flows are
not easy to recognize due to metasomatic elemental change of ultramafic rocks.
Figure 6.5.23 Schematic representation of the alteration process of the Pulju belt: Intrusion of granitoids
caused fluid activity along zones of tectonic weakness. Primarily the granitic fluids carried Sr, Pb, La,
Sm, Rb, Ba Br and Mo, and the fluids may incorporate additional Ca, Si, Al, K and Na and possibly also
CO 2
and S from sedimentary rocks. The fluids reacted in amphibolite facies metamorphic conditions with
ultramafic rocks to form tremolite and chlorite along the tectonic zones, and serpentine and metamorphic
sulfide dissemination in olivine cumulates. Fluids finally crystallized as molybdenite-bearing granitic
pegmatites.

48
During metamorphism the ultramafic cumulates were altered by granitic fluids that
transported elements from the sedimentary layers and changed the compositions of ultramafics.
Tremolite, chlorite, phlogopite and ferrous sulfide minerals are the results of metamorphic
reactions. Pervasive weak sulfide dissemination in ultramafic cumulates is the
result of metamorphic reactions, but in addition to metamorphic sulfides also Ni-rich magmatic
sulfides exist in the cumulates. The magmatic sulfides are either disseminated or
minor massive layers that exist at the base of ultramafic cumulate. Only Ni and PGE assays
discriminate the magmatic and metamorphic sulfides. The metamorphic alteration
reactions occurred in reduced conditions and as the result also metamorphic chromite was
deposited in the decomposition reaction of chromiferous chlorite.
If the komatiitic flows and cumulates are comagmatic, the cumulates have lost considerable
amounts of nickel because the komatiite magma contains 900 ppm Ni but the first
crystallizing olivine of the cumulates only 2000 ppm, that indicates far lower content of Ni
in the parental magma. The differences in Ni contents could serve as a guide for further
exploration and research for magmatic nickel deposits
Tables

55
References
Arndt, N.T. (1986) Differentiation of komatiite flows. Journ. Petrology 27, 279-301.
Arndt, N.T. and Nisbet, E.G. (1982) Komatiites: London, George Allen & Unwin, 526 p.
Arndt, N.T. & Lesher, C.M. (1992) Fractionation of REEs by olivine and the origin of Kambalda komatiites,
Western Australia. Geochemica Cosmochem. Acta 56, 4191-4204.
Barnes, S.J., Hill, R.E.T. & Gole, M.J. (1988) The Agnew nickel deposit, Western Australia, II Sulfide
geochemistry, with emphasis on platinum group elements. Econ. Geol. 83, 537-550.
Barnes, S.J., Hill, R.E.T. and Gole, M.J. (1988) The Perseverance ultramafic complex, Western Autralia:
the product of a komatiite lava river. J.Petrol. 29: 305-331.
Barnes, S.-J., and Often, M. (1990) Ti-rich komatiites from northern Norway. Contrib. Mineral. Petrol.
105: 42-54.
Bird, D.K., Schiffman, P. Elders, W.A., Williams, A.E. & McDowell, S.D. (1984) Calc-silicate mineralization
in active geothermal systems. Econ. Geology 79, 671-695.
Brown, E.H. (1977) The crossite content of Ca-amphibole as a guide to presure of metamorphism. J.
Petrol. 18, 53-72.
Eilu, P. (1994) Hydrothermal alteration in volcano-sedimentary rocks in the Central Lapland greenstone
belt, Finland. Geol. Survey Finland Bull. 374, 145 p.
Davidsen, B. (1994) Stratigrafi petrologi og geokjemi med vekt pΠkomatiittiske bergarter innen den
nordligste del av Karasjok gr¿nnsteinsbelte, Brennelv, Finnmark. Unpublished M.Sc. Thesis,
University of Tromsö, Department of Biology and Geology. 380 p.
Donaldson, M.J. (1982) Mount Clifford-Marshall Pool are In: Groves, D.I. & Lesher, C.M. (eds.)
Regional geology and nickel deposits of the Norseman-Wiluna Belt, Western Australia. University
of Western Australia Extension Service, Publication 7, C53-C67.
Gaál, G., Mikkola, A. & Söderholm, B. (1987) An outline of the Precambrian evolution of the Baltic
Shield. Precambrian Res. 6, 199-215
Gaál, G. Berthelsen, A., Gorbatchev, R., Kesola, R., Lehtonen, M.I., Marker, M. & Raase, P. (1989)
Structure and composition of the Precambrian crust along the POLAR Profile in the northern Baltic
Shield. Tectonophysics 162, 1-25.
Gammon, J.B. (1977) Nikkelletning i Anarjokka. Bergverkens Landssammenslutnings Industrigruppe
(BVLI). Referat fra Nikkelm¿te, 63-77
Gole, M., Barnes, S.J. & Hill, R.E.T. (1987) The role of fluids in metamorphism of komatiites, Agnew
nickel deposit, Western Australia. Contrib. Mineral. Petrol. 96, 151-162.
Haapala, I, Front, K., Rantala, E. & Vaarama, M. (1987) Perology of Nattanen-type granite complexes,
northern Finland. Precambrian Research. 35, 225-240.
Hakanen, P. (1977) Kittilän Puljun liuskejakson karsivyöhyke. Unpublished M.Sc. Thesis, Turku University,
Department of Geology. 80 p.
Hanski, E. (1986) The Gabbro-Wehrlite Association in the Eastern Part of the Baltic Shield. In: Friedrich,
G.H., Genkin, A.D., Naldrett, A.J., Ridge, J.D., Sillitoe, R.H., and Vokes, F.M. (editors) Geology
and Metallogeny of Copper Deposits. Proceedings of the Copper Symposium 27th International
Geological Congress Moscow, 1984. Special Publication No. 4 of the Society for Geology Applied
to Mineral Deposits. Springer-Verlag, Berlin Heidelberg: 151-170.
Hanski,E.J. (1992) Petrology of the Pechenga ferropicrites and co-genetic Ni-bearing gabbro-wehrlite
intrusions, Kola Peninsula, Russia. Geol. Survey Finland Bull. 367, 192 p.
Hanski, E.J. & SmolkinV.F. (1989) Pechenga ferropicrites and other early Proterozoic picrites in the
eastern part of the Baltic Shield. Precambrian Research 45, 63-82.
Hanski, E. (1997) The Nuttio serpentinite belt, Central Lapland: An example of Paleoproterozoic
ophiolitic mantle rocks in Finland. Ofioliti 22, 35-46
Heino, T. (1977) Puljun liuskejakson ja Peltotunturin alueen ultramafiittien keskinäistä vertailua. Unpubli-

56
shed M.Sc. Thesis, Turku University, Department of Geology, 89 p.
Henriksen, H. (1983) Komatiitic chlorite-amphibole rocks and mafic metavolcanics from the Karasjok
greenstone belt, Finnmark, northern Norway: a preliminary report. Norges geologiske unders¿kelse
Nr. 382, Bulletin 71, 17-43
Hill, R.E.T., Barnes, S.J., Gole, M.J. and Dowling, S.E. (1990) The physical volcanology of komatiites in
the Norseman - Wiluna belt , Western Australia. Geol. Soc. Australia, Western Australia Division,
Excursion Guide 1. 100 p.
Idman, H. (1980) Lapin ultramafiittien oksidifaasista. Unpublished M.Sc. Thesis, Turku University,
Department of Geology, 147 p
Inkinen, O., Ilvonen, E. & Pelkonen, R. (1984) Puljun liuskejakson ja Hotinvaaran tutkimukset 1982-
1984. Unpublished report, Outokumpu Oy Exploration 54 p.
Julku, T. (1982) Länsi-Inarin liuskejakson sagvandiiteista. Unpublished M.Sc. Thesis, Turku University,
Department of Geology. 83 p.
Krill, A.G., Bergh, S., Lindahl, I., Mearns, E.W., Often, M., Olerud, S., Oelsen, O., Sandstad, J.S.,
Siedlecka, A. & Solli, A. (1985) Rb-Sr, U-Pb and Sm-Nd isotopic dates from Precambrian rocks of
Finnmark. Norges Geologiske Unders¿kelse, Bulletin 403, 37-54
Koistinen, T & Virransalo, P. (1985) Kittilän Pahtavuoma-Sirkka-alueen rakennegeologiasta ja malminetsinnästä.
Unpublished report, Outokumpu Oy Exploration, Lapin Malmi, 33 p.
Kouvo, O., Huhma, H. & Sakko, M. (1983) Isotopic evidence for old crustal involvement in the genesis
of two granites from North Finland. Terra Cognita 3, p 135.
Lahtinen, J. (1979) Kittilän Mertavaaran Ni- ja Zn-tutkimukset. Pallasjärven lentoalue 1, 2742. Unpublished
report, Outokumpu Oy Exploration 14 p.
Lehtonen, M.I., Manninen, T., Rastas P., Väänänen, J., Roos, S.I. & Pelkonen, R. (1985) Explanation to
the geological map of central Lapland. Geological Survey of Finalnd, Report of Investigation 71, 35
p.
Lehtonen, M.I., Manninen, T., Rastas P. & Räsänen J. (1992) On the early Proterozoic metavolcanic rocks
in Finnish central Lapland. In: Balagansky, V.V & Mitrofanov, F.P. (eds.) Correlation of Precambrian
formations of the Kola-Karelia region and Finland. Apatity, Russian Academy of Sciences, 65-
85.
Lehtonen, M.I., Airo, M.-L., Eilu, P., Hanski, E., Korteialinen, V., Lanne, E., Manninen, T., Rastas, P.,
Räsänen, J. & Virransalo, P. (1998) Kittilän vihreäkivialueen geologia, Lapin vulkaniittiprojektin
raportti. Geolical Survey of Finland, Report of investigation 140. 1-144.
Manninen, T. (1991) Sallan alueen vulkaniitit (Volcanic rocks in the Salla area, northeastern Finland).
Geological Survey of Finland, Report of Investigation 104, 97 p.
Marker, M. (1988) Early Proterozoic (c. 2000-1900 Ma) crustal structure of the northeastern Baltic
Shield: tectonic division and tectonogenesis. Norges Geologiske Unders¿kelse Bull. 403, 55-74.
Marshall, B. & Gilligan, L.B. (1993) Remobilization, syn-tectonic processes and massive sulfide deposits.
Ore Geology Reviews 8, 39-64.
Marshall, B. & Mancini, F. (1994) Major- and minor-element mobilization, with implications for Ni-Cu-
Fe sulfide remobilization, during retrograde metasomatism at the Vammala Mine, southwest Finland.
Chemical Geology 116, 203-227.
Meriläinen, K. (1976) The granulite complex and adjacent rocks in Lapland, northern Finland. Geol.
Survey Finland, Bulletin 281, 129 p.
Meriläinen, M. (1993) Kittilän Mertavaaran kivien petrografiasta ja geokemiasta. Unpublished M.Sc.
Thesis, Turku University, Department of Geology, 82 p.
Mikkola, E. (1941) Kivilajikartan selitys, Lehdet B7-C7-D7, Muonio-Sodankylä-Tuntsajoki. Geologinen
tutkimuslaitos, Espoo.
Naldrett, A.J. & Turner, A.R. (1977) The geology and petrogenesis of a greenstone belt and related nickel
sulfide mineralization at Yakabindie, Western Australia. Precambrian Research 5, 43-103.
Nesbitt, R.W., Sun, S.-S. and Purvis, A.C. (1979) Komatiites: geochemistry and genesis. Can. Miner. 17:
165-186.

57
Papunen, H., Ilvonen, E., Idman, H., Pihlaja, P., Neuvonen, K.J. & Talvitie, J. (1977) Lapin ultramafiiteista
(Ultramafics of Lapland). Geologinen tutkimuslaitos, Report of Investigation 23, 87 p
Papunen, H., Pihlaja, P. & Idman H. (1980) Komatiittiprojektin 1977-1979 datamateriaali. Unpublished
report, University of Turku
Papunen, H. & Gorbunov, G.I. (eds.) (1985) Nickel-copper deposits of the Baltic Shield and Scandinavian
Caledonides. Geol. Surv. Finland Bull. 333, 394 p.
Patchett, P.J., Kouvo, O., Hedge, C.E. & Tatsumoto, M. (1981) Evolution of continental crust and mantle
heterogeneity: evidence from Hf isotopes. Contrib. Mineral. Petrol. 78, 279-297.
Pearce, J.A. & Cann, J.R. (1973) Tectonic setting of basic volcanic rocks determined using trace element
analyses. Earth Planet. Sci Letters 19, 290-300
Pearce, T.H. (1968) A contribution to the Theory of Variation Diagrams. Contrib. Mineral. Petrol. 19, 142-
157.
Peltonen, P. (1990) Metamorphic olivine in picritic metavolcanics from southern Finland. Bull. Geol.
Soc. Finland 62, 99-114.
Perring, C.S., Barnes, S.J. & Hill, R.E.T. (1995) The physical volcanology of Archaean komatiite sequences
from Forrestania, Southern Cross Province, Western Australia. Lithos 34: 189-207.
Piquet, D. (1982) M•canismes de recrystallisation m•tamorphique dans les ultrabasites: exemple des
roches vertes Arch•ennes de Finlande Orientale (ceinture de Suomussalmi-Kuhmo) Th•se 3e
cycle. Rennes. 246 p.
Räsänen, J., Hanski, E. & Lehtonen, M. (1989) Komatiites, low-Ti basalts and andesites in the Möykkelmä
area, Central Finnish Lapland. Geological Survey of Finland, Report of Investigation 88, 41 p.
Rastas, J. & Kilpeläinen, T. (1991) Tektonis-metamorfiset tutkimukset Mertavaarassa, Puljun liuskejaksolla.
Turku University, Publication of the Department of Geology and Mineralogy 25, 19 p.
Rock, N.M.S. (1991) MacSuite, An integrated suite of Macintosh geoscientfic programs. Rockware Inc.,
Colorado, USA, 54 p.
Roeder, P.L. & Emslie, R.F. (1970) Olivine-liquid equilibria. Contrib. Mineral. Petrol. 29, 275-289.
Russell, J.K. & Nicholls, J. (1988) Analysis of petrogenetic hypotheses with Pearce element ratios.
Contrib. Mineral. Petrol. 99, 25-35.
Saverikko, M. (1983) The Kummitsoiva komatiite complex and its satellites in northern Finland. Geol.
Soc. Finland, Bull. 55, 111-139.
Saverikko, M. (1985) The pyroclastic komatiite complex at Sattasvaara in northern Finland. Bull. Geol.
Soc. Finland 57, 55-87.
Silvennoinen, A. (1985) On the Proterozoic stratigraphy of Nothern Finland. In: Laajoki, K. & Paakkola,
J. eds. Proterozoic exogenic processes and related metallogeny. Proceedings of the symposium held
in Oulu, Finland, August 15-16, 1983. Geological Survey of Finland Bulletin 331, 107-116.
Smith, H.S. & Erlank, A.J. (1982) Geochemistry and petrogenesis of komatiites from the Barberton
greenstone belt, South Africa. In: Arndt, N.T. & Nisbet, E.G. (eds.) Komatiites. George Allen &
Unwin, London. 347-398.
Söderholm, B. (1966) Om berggrunden i Mertavaara och omgivningar i Kittilä kommun. Unpublished
M.Sc Thesis, University of Helsinki, Department of Geology and Mineralogy
Stanley, C.R & Russell, J.K. (1989) Petrologic hypothesis testing with Pearce element ratio diagrams:
derivation of diagram axes. Contrib. Mineral. Petrol. 103, 78-89.
Ward, P., Härkönen, I. & Pankka, H.S. (1989) Structural studies in the Lapland greenstone belt, northern
Finland, and their application to gold mineralization. In Autio, S. ed: Geological Survey of Finland,
Current Research 1988. Geol. Survey Finland, Spec. Paper 10, 71-78
Winchester, J.A. & Floyd, P.A. (1976) Geochemical magma type discrimination: Application to altered
and and metamorphosed basic igneous rocks. Earth. Planetary Sci Lett. 28, 459-469.
Winkler, H.G.F. (1976) Petrogenesis of metamorphic rocks. Springer-Verlag, New York. 334 p.