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GEOLOGIAN TUTKIMUSKESKUS
Pohjois-Suomen Yksikkö
Rovaniemi
18.8.2014 73/2014
3D geological modelling of the Sotkavaara
intrusion, Rovaniemi, Northern Finland
Irmeli Huovinen, Ilkka Lahti, Tuomo Törmänen
GEOLOGIAN TUTKIMUSKESKUS
18.8.2014
GEOLOGICAL SURVEY OF FINLAND DOCUMENTATION PAGE
Date / Rec. no.
Authors
Huovinen, Irmeli, Lahti, Ilkka, Törmänen, Tuomo
Type of report
Archieve report
Commissioned by
Geological Survey of Finland
Title of report
3D geological modelling of the Sotkavaara intrusion, Peräpohja schist belt
Abstract
The aim of the geological 3D modelling of the Sotkavaara was to visualize the data in 3D and make subsurface interpretation
and interpolation of geology and shape of the intrusion and the PGE reef type occurrence. 3D modeling of the Sotkavaara
intrusion has been part of the project 2141006 “Lapland mineral systems and exploration models”. The 3D modelling is based
on both geological and geophysical data, including drill hole data and geophysical ground measurements including magnetic,
gravity, IP, VLF-R, Sampo and AMT measurements.
Sotkavaara intrusion is composed of Sotkavaara pyroxenite which has a maximum thickness of 350 m. It is surrounded by up
to 100 m thick gabbro-amphibolite zone. The shape of the intrusion is bowl-like. Intrusion was intruded into the Peräpohja
schist belt sedimentary rocks. Intrusion hosts PGE- reef type occurrence which is located near the contact of pyroxenite and
gabbro.
Keywords
3D modelling, pyroxenite intrusion, geophysics, PGE
Geographical area
Finland, Laplanc provinence, Rovaniemi, Sotkavaara
Map sheet
T434 1F
Other information
Report serial
Archive code
73/2014
Total pages
Language
Price
Confidentiality
Public
Unit and section
PSY/VA501
Project code
2141006
Signature/name
Irmeli Huovinen
Signature/name
Ilkka Lahti Tuomo Törmänen
GEOLOGIAN TUTKIMUSKESKUS
18.8.2014
GEOLOGIAN TUTKIMUSKESKUS KUVAILULEHTI
Päivämäärä / Dnro
Tekijät
Huovinen, Irmeli, Lahti, Ilkka, Törmänen, Tuomo
Raportin laji
Arkistoraportti
Toimeksiantaja
Geologian tutkimuskeskus
Raportin nimi
3D geological modelling of the Sotkavaara intrusion, Peräpohja schist belt
Tiivistelmä
Sotkavaaran pyrokseniittisen intruusion 3D- mallinnus on tehty hankkeen 2141006 puitteissa. 3D- mallinnuksen tavoitteena
oli intruusion geologian, muodon ja koon visualisointi sekä kairaamalla paikannetun PGE-esiintymän jatkuvuuden arviointi.
Mallinnus perustuu hankkeiden 2551006 ja 2551011 tuottamaan geologisiin ja geofysikaalisiin aineistoihin, joihin lukeutuu
magneettinen maanpintamittaus, painovoimamittaus, sähkömagneettiset VLF-, VLF-R, SAMPO ja AMT mittaukset sekä 10
kairareikää. Magneettisesta aineistosta on tehty magneettinen inversiomalli ja painovoima-aineistosta karkea 3D-tulkinta.
Sotkavaaran intruusion keskiosa koostuu pyrokseniitista joka on paksuimmillaan 350 m jota ympäröi maksimissaan 100 m
paksu gabro-amfiboliittikehä, joka erottuu magneettisessa aineistossa korkeamman suskeptibiliteetin ansiosta. Intruusion
keskiosassa esiintyy kapeita peridotiittisia juonia/kerroksia. PGE-reef-tyyppinen mineralisaatio esiintyy lähellä pyrokseniitin
ja gabron kontaktia. Intruusiota ympäröi peräpohjan liuskevyöhykkeen sedimenttikivet.
Asiasanat (kohde, menetelmät jne.)
3D mallinnus, pyrokseniitti intruusio, geofysiikka, PGE
Maantieteellinen alue (maa, lääni, kunta, kylä, esiintymä)
Suomi, Lapin lääni, Rovaniemi, Sotkavaara
Karttalehdet
T434 1F
Muut tiedot
Arkistosarjan nimi
Arkistotunnus
73/2014
Kokonaissivumäärä
Kieli
Englanti
Hinta
Julkisuus
Julkinen
Yksikkö ja vastuualue
PSY/VA501
Hanketunnus
Allekirjoitus/nimen selvennys
Irmeli Huovinen
Allekirjoitus/nimen selvennys
Ilkka Lahti Tuomo Törmänen
GEOLOGIAN TUTKIMUSKESKUS
18.8.2014
Contents
1 INTRODUCTION 1
2 GEOLOGY 2
3 DATA AND METHODS 3
4 3D MODEL 6
5 CONCLUSIONS AND REMARKS 10
6 REFERENCES 11
GEOLOGIAN TUTKIMUSKESKUS 1
18.8.2014
1 INTRODUCTION
The Sotkavaara pyroxenitic intrusion is located in the Rovaniemi municipality about 25 km E-SE of the
city of Rovaniemi in northern Finland, within the UTM base map sheet T434 (Fig. 1). The Sotkavaara
intrusion was intruded into the sedimentary rocks of the Pöyliövaara Formation which consist mostly of
mica schists and gneisses and minor black schists. The Pöyliövaara Formation rocks are surrounded by
older Oikaraisenvaara Formation quartzites and amphibolites. The intrusion hosts a reef type PGE occur-
rence and thin Ni-bearing massive sulphide veins. (Törmänen et al., 2014)
Geological research of the Sotkavaara started in 2007 by outcrop sampling, ground geophysical surveys
and continued later by diamond drilling in 2009 and 2012. Total of 10 drill holes (M361409R398,
M361409R399 and T4342012R5-R12), for 2624 m were drilled. (Lauri et al., 2013, Törmänen et al.,
2011, Törmänen et al., 2014).
The aim of the geological 3D modelling of the Sotkavaara intrusion was to visualize the data in 3D and
carry out subsurface interpretation and interpolation of geology and the shape of the intrusion and the
PGE occurrence.
Figure. 1. Location of the Sotkavaara study area marked as a red square.
GEOLOGIAN TUTKIMUSKESKUS 2
18.8.2014
The study of the intrusion was carried out in co-operation with Geological Survey of Finland (GTK) pro-
jects 2551006 “Ore potential of mafic and ultramafic rocks of Northern Finland” and 2551011 “Mineral
potential estimation of the Central Lapland area, Finland, 2011-2012”. 3D modeling of the Sotkavaara
intrusion has been part of the project 2141006 “Lapland mineral systems and exploration models”.
2 GEOLOGY
The Sotkavaara intrusion is located in the Paleoproterozoic (2.4-1.9 Ga) Peräpohja Belt in northern
Finland in the municipality of Rovaniemi. The age of the intrusion is unknown, but age data for the
Pöyliövaara Formation which hosts the intrusion (Hanski et al., 2005) indicate an age younger than 1.98
Ga. At the current bedrock surface the intrusion extends is 1.5 x 2.5 km and based on the drill hole data
the pyroxenitic part of is up to 350 m thickand the the lowermost gabbro-amfibolite zone is up to 100 m
thick. The intrusion was intruded into the schists and gneisses of the Pöyliövaara Formation. The
Pöyliövaara Formation rocks overlie the older Oikaraisenvaara Formation quartzites and amphibolites
(Fig. 2). (Törmänen et al., 2014).
Figure. 2. Bedrock map of the Sotkavaara. (DigiKP by Geological survey of Finland)
GEOLOGIAN TUTKIMUSKESKUS 3
18.8.2014
Sotkavaara pyroxenite is predominatly composed of clinopyroxene and amphibole. The susceptibility of
the pyroxenites varies somewhat, probably due to subtle variations in the amount of magnetite. There are
1-5 m thick serpentinite altered dunite-peridotite layers or dykes in the central part of the intrusion. The
outer zone of the intrusion is composed of amphibolites and two types of gabbros: low-TiO2 and high-
TiO2 gabbros. Gabbroic dykes are also present in the pyroxenitic part. The amphibolites are associated
with gabbro. The gabbros and amphibolites have higher susceptibility compared to the pyroxenite and the
surrounding sedimentary rocks. Although there is clear chemical variation between the two types of gab-
bro units, no geophysical signature was discovered to separate them. The relationship between the amphi-
bolites and the Sotkavaara intrusion rocks is unclear. The amphibolites probably represent inclusions of
the surrounding supracrustal units, rather than being part of the intrusion. The drill hole M361409R398
was drilled trough the intrusion and shows that the gabbro-amphibolite zone is followed by quarz-feldspar
gneis, schist and amphibolite. Locally the upper part of the intrusion contains xenoliths and amphibolite
and gabbro pegmatite veins/dykes are abundant throughout the intrusion.
Three drill holes T4342012R6, T4342012R12 and M361409R398 intersected low-sulfide reef type PGE
zone. Drillhole M361409R399 intersected 25 cm thick massive sulfide vein containing 2.1% Ni, 0.47 %
Cu, 0.26% Co and trace PGEs (79 ppm Pd). The lower contact of the PGE mineralization is sharply and
the upper contact is gradual. Total thickness of the PGE is unknown due to the lack of assays from the
uppermost part of the reef. The best intersection in drill hole M361409R6 contains 6m @ 0.99 ppm
Au+2PGE and with additional 7 m @ 0.51 ppm Au+ 2PGE. The pyroxenites have low whole-rock S con-
tents whereas the gabbros contain locally sparse disseminated sulfides (M361409R399: 31.5m @ 0.1%
Cu) (Törmänen et al., 2014).
3 DATA AND METHODS
The data used for the 3D modeling consists of both geological and geophysical data. The geological map
of the Sotkavaara area (Geological Survey of Finland, 2010) is based mainly on geophysical interpreta-
tion, although there are a few outcrops in the Sotkavaara area. The first two drill holes (M361409R398
and M361409 R399) were drilled in 2009 and after the discovery of the PGE-potential of the intrusion
exploration continued by sparse diamond drilling program in 2012 (drill holes T4342012R5-R11)(Fig.3).
Petrophysics were measured from three drill holes M361409R398, M361409R399 and T4342012R12
and consisted of susceptibility, remanence and density measurements.Borehole DC –resistivity (mise-a-
la-masse) measurements were carried out of two drill holes M361409 R399 and T4342012R12 in 2013.
In addition to GTK’s standard airborne geophysical measurements ground measurements have also been
done in the Sotkavaara intrusion area including magnetic, gravity, IP, VLF-R, Sampo and AMT meas-
urements. Two gravity lines were measured in 2008 and a first rough 3D model of the intrusion gravity
data was made based on the gravity data. Ground magnetic measurements were carried out in 2008. The
site and line spacing of measurements were 10 m and 100 m, respectively. 32 N-S oriented lines with the
GEOLOGIAN TUTKIMUSKESKUS 4
18.8.2014
Figure 3. Location of the drill holes, on top base map as a background, below the bedrock map as a back-
ground (DigiKp by Geological survey of Finland)
GEOLOGIAN TUTKIMUSKESKUS 5
18.8.2014
length of 2.2 km were measured using proton magnetometers (Fig. 4.) Short time variations of the Earth’s
magnetic field were removed by using a magnetic base station that was established near the survey area.
Resulting magnetic maps show several high intensity anomalies with peak amplitudes less than 7000 nT.
Drillings have shown that the anomalies are due to magnetite in gabbros and pyroxenites and no large
sulphide bodies have yet been found by the drillings.
Figure 4. Location of the ground magnetic survey lines and gravity lines. Magnetic map as a background.
The densely spaced magnetic dataset enables the use of geophysical modeling and inversion techniques in
order to obtain information on magnetic anomaly sources and 3D geometry of the intrusion. 3D inver-
sions were performed using the UBC-GIF inversion software of Li and Oldenburg (1996). Prior to inver-
sions the magnetic data was preprocessed and regional trends were removed using various trend removal
techniques. Inversions were done both as unconstrained that leads to very smooth models and also as con-
strained inversions, which yields more discrete sharp boundary solution. In the constrained inversion the
susceptibility is either fixed or enabled to vary within assigned bounds. Constraining is highly recom-
mended but it requires a priori information like drillings, geological mapping or information from other
GEOLOGIAN TUTKIMUSKESKUS 6
18.8.2014
geophysical measurements. In the case of Sotkavaara such additional information has been mainly ac-
quired from drillings.
Homogenous half-space was used as a starting model in all 3D inversions. Used 3D mesh consisted of
cells having fixed horizontal dimension of 25m x 25m and varying vertical dimension, which limits the
total number of cells and consequently decrease the duration of inversion. Total size of 3D mesh was ap-
proximately 400 000 and it took 3-4 hours to get final results. The aim of inversion is to find such theo-
retical model that produces measured magnetic anomalies of the target area. In general all inversions were
successful as very low misfit values were obtained. Modelled susceptibilities are < 0.7 SI units. Inversion
results show that magnetized bodies have dip towards to the central part of the intrusion therefore sug-
gesting bowl-shape form for the intrusion.
4 3D MODEL
The geological modeling software used included Gocad® by Paradigm and Gocad® Mining Suite by
Mira. Geophysical data was processed by geophysical modeling programs and the results were translated
for Gocad format. Direct observation data used for modeling is mainly drill hole data, although there are a
few outcrop observations included. Indirect data used for modeling is geophysical data as discussed in
previous chapter. Drill hole logs contain information about rock types, geochemical and geophysical data.
The dimensions of the model are 2700m x 1500 m x 975 m.
Topography determines the upper surface of the 3D model. Topography is based on digital elevation
model (dem) made by Land Survey of Finland. Topography represents the quaternary deposits covering
the bedrock.
Contacts between lithological units are modeled as surfaces in Gocad software. Main lithological units are
defined based on the drill hole data. Rock type classification has been simplified. Originally there were 37
rock names, but after the classification 11 were used for modeling and only 3 are present in the 3D model.
Amphibolite and gabbro pegmatite veins/dykes are not included in the model. All sedimentary rocks out-
side the intrusion are included in one lithologic unit (Pöyliövaara Formation). Lithological units in 3D
model are intrusive rocks: Sotkavaara pyroxenite, gabbro (this unit includes also amphibolites outside the
Sotkavaara pyroxenite) and sedimentary rocks of the Pöyliövaara formation. (Fig. 5)
Drill hole data shows that pyroxenite comprise the bulk of the intrusion. The maximum thickness of the
pyroxenite according to drill hole data is 350 m. The contact between the pyroxenite and surrounding
gabbro is intersected by several drill holes. Thin serpentinite interlayers or dykes, which represent meta-
morhpsed peridotites and dunites occur in the middle of the pyroxenite unit. Individual serpentinite lay-
ers/dykes cannot be unequivocally connected between drill holes, but they are mostly concentrated in the
middle (and upper) parts of the Sotkavaara pyroxenite unit.
Based on drill hole and geophysical data the outer zone of the intrusion is composed of gabbros and am-
phibolites. The magnetic anomalies are caused by magnetite related to the intrusive rocks and by mag-
netic pyrrhotite related to the Pöyliövaara Formation black schists.
GEOLOGIAN TUTKIMUSKESKUS 7
18.8.2014
Figure 5. Sotkavaara lithology in 3D: Pöyliövaara formation as blue, gabbro as light brown and Sot-
kavaara pyroxenite as dark brown. First picture: ground magnetic map of the Sotkavaara.
The contact between lowermost gabbro unit and pyroxenitic unit was intersected by several drill holes.
Despite the geochemical differences within the gabbroic rocks, the two types of gabbros are modelled as
single lithological unit. Gabbros also occur within the pyroxenites as irregular layers or dykes. These thin
and discontinuous gabbro layers are not included in 3D model except in the SW border of the intrusion,
where the a gabbro unit can be outlined based on the magnetic data and drill hole data (Fig 5.).
The contact between intrusion and surrounding sedimentary rocks is outlined based on the magnetic in-
version, gravity data and drill hole data. The contact between the Sotkavaara pyroxenite and the gabbro is
intersected by several drill holes, but the contact between surrounding sedimentary rocks and the intrusion
is located by one drill hole M361409R398. Drill hole data indicate that the contact between the intrusion
and Pöyliövaara formation rocks is located approximately 100 m below the contact of the Sotkavaara py-
roxenite and the gabbro. The magnetic inversion result indicates that the magnetized bodies have a dip
toward the central part of the intrusion. The density of the intrusion rocks is higher in comparison to the
surrounding sedimentary rocks. Based on this density data and the gravity measurements a 3D gravity
forward model was used to in outlining the contact between the intrusion and surrounding supracrustal
units.
GEOLOGIAN TUTKIMUSKESKUS 8
18.8.2014
Figure 6. Cross section of the 3D-model and the data used for modeling. On top a plan view of the geo-
logical model of the intrusion. In cross sections location of the drill hole M361409R398, PGE-reef as a
red peak.
GEOLOGIAN TUTKIMUSKESKUS 9
18.8.2014
PGE- reef is intersected by three drill holes (T4342012R6, T4342012R12 and M361409R398). It is lo-
cated above the contact between the gabbro and pyroxenite and below the serpentinite interlayers. (Fig. 7)
The lower contact of the mineralization is sharp, but the location of the upper contact is undefined due to
the lack of assay data. The PGE concentration decrease upwards. The PGE enriched zone was not de-
tected in fourth drill hole (M361409R399). The PGE-reef is, in most cases, associated with the mottled
textured, altered, plagioclase-bearing pyroxenite, however, the mottled pyroxenite is partly unmineralized
as indicated e.g. by drill hole data from T4342012R7. The mottled pyroxenite is located within the lower
part of the pyroxenite, near the contact between the gabbro and the pyroxenite, probably throughout the
intrusion as it has been intersected in shallow drill holes drilled to the margins of the intrusion. The low-
ermost mottled pyroxenite has been modeled as a continuous surface (Fig 8).
Figure 7. Ground magnetic map of the Sotkavaara intrusion with pyroxenite contact. The PGE reef (red
peaks) is located above the contact between pyroxenite and gabbro.
GEOLOGIAN TUTKIMUSKESKUS 10
18.8.2014
Figure 8. From above on the left: the voxet model of the Sotkavaara, cross section the magnetic inversion
model of the Sotkavaara, cross sections of Sotkavaara voxet and the location of the PGE- reef as gray sur-
face and the mottled textured pyroxenite as blue surface.
5 CONCLUSIONS AND REMARKS
The 3D data visualisation and modelling of the Sotkavaara intrusion has given new information about the
geology, the shape of the intrusion and the location of the reef-type PGE occurrence. New inversion
methods were also tested. Magnetic inversions were done using both as unconstrained and constrained
methods.
The geological 3D model was done based on both geological (drill hole) and geophysical data. The strong
positive magnetic anomalies detected in the area are caused by magnetite related to gabbros, amphibolites
and pyroxenites. The magnetic inversion data shows that the high magnetic bodies dip towards the centre
of the intrusion and the shape of the intrusion is bowl-like. The contact between gabbros and pyroxenites
has been intersected by several drill holes. One drill hole was drilled completely trough the intrusion and
gives the maximum thickness of 350 m for Sotkavaara pyroxenite and 100 m for the gabbro-amphibolite
GEOLOGIAN TUTKIMUSKESKUS 11
18.8.2014
outer zone. 3D model indicates that the intrusion has zoned structure including magnetite bearing gabbro
as an outer zone and a pyroxenitic core.
The PGE-reef has been intersected by three drill holes. Based on the drill hole data it is located above the
contact between the gabbro and pyroxenite. Because the sulphide-poor nature of the PGE-reef, it can be
detected only via geochemical methods. More assay data is needed to solve the continuity of the PGE-reef
and its relationship to the alteration of pyroxenites.
Due to limited outcrop data we emphasize the need of oriented drill cores and structural measurements
from them should there be further modelling of the Sotkavaara intrusion.
6 REFERENCES
Bedrock of Finland - DigiKP. Digital map database [Electronic resource]. Espoo: Geological Survey
of Finland [referred 14.8.2014]. Version 1.0.
Hanski, E., Huhma, H., Perttunen, V. 2005. SIMS U-Pb, Sm-Nd isotope and geochemical study of an
arkosite-amphibolite suite, Peräpohja Schi st Belt: evidence for ca. 1.98 A-type magmatism in
northern Finland. Geological Survey of Finland, Bulletin 77:5 29.
Lauri, S., L., Konnunaho, J., Lepistö, S., Peltoniemi-Taivalkoski, A., Salmirinne, H., Sandgren, E.,
Sarala, P., Törmänen, T. 2013. Keski-Lapin Mineraalipotentiaalin arviointi 2011-2012. Geologian
tutkimuskeskus, arkistoraportti 104/2013. English summary. 112 p.
Li, Y. and Oldenburg, D. W. 1996. 3-D inversion of magnetic data. Geophysics, 61, 394-408.
Törmänen, T., Heikura, P., Konnunaho, J., Salmirinne, H. 2011. Hanke 251006 Pohjois-Suomen
mafis-ultramafisten magmakivien malmipotentiaali 2009-2010 loppuraportti. Geologian
tutkimuskeskus, hankeraportti 55/2011. English summary. 31 p., 7 appendix.
Törmänen, T., Huovinen, I. & Konnunaho J. 2014. New type of low-sulphide PGE-reef of the
Sotkavaara pyroxenite intrusion, Rovaniemi, northern Finland. In: Lauri, L., S., Heilimo, E.,
Leväniemi, H., Tuusjärvi, M., Lahtinen, R & Hälttä, P. (eds.) 2014. Current Research: 2nd GTK
Mineral Potential Workshop, Kuopio, Finland, May 2014. Geological Survey of Finland, Report of
Investigation 207. 161 p, 71 figures and 4 tables.