3D geological modelling of the Sotkavaara intrusion ...tupa.gtk.fi/raportti/arkisto/73_2014.pdf3D...

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


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


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


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.


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)


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


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)


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


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.


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.


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.


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.


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


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.

3D geological modelling of the Sotkavaara intrusion ...tupa.gtk.fi/raportti/arkisto/73_2014.pdf3D...

Documents

Transcript of 3D geological modelling of the Sotkavaara intrusion ...tupa.gtk.fi/raportti/arkisto/73_2014.pdf3D...