Correlation between distribution and shape of VMS deposits and regional deformation patterns, Skellefte district, northern Sweden

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ARTICLE

Correlation between distribution and shape of VMS depositsand regional deformation patterns, Skellefte district,northern Sweden

Tobias E. Bauer & Pietari Skyttä & Tobias Hermansson &

Rodney L. Allen & Pär Weihed

Received: 13 February 2013 /Accepted: 9 December 2013 /Published online: 4 January 2014# Springer-Verlag Berlin Heidelberg 2014

Abstract The Skellefte district in northern Sweden is host toabundant volcanogenic massive sulphide (VMS) depositscomprising pyritic, massive, semi-massive and disseminatedZn–Cu–Au ± Pb ores surrounded by disseminated pyrite andwith or without stockwork mineralisation. The VMS depositsare associated with Palaeoproterozoic upper crustal extension(D1) that resulted in the development of normal faults andrelated transfer faults. The VMS ores formed as sub-seafloorreplacement in both felsic volcaniclastic and sedimentaryrocks and partly as exhalative deposits within the uppermostpart of the volcanic stratigraphy. Subsequently, the district wassubjected to deformation (D2) during crustal shortening.Comparing the distribution of VMS deposits with the regionalfault pattern reveals a close spatial relationship of VMS de-posits to the faults that formed during crustal extension (D1)utilising the syn-extensional faults as fluid conduits.Analysing the shape and orientation of VMS ore bodies showshow their deformation pattern mimics those of the hostingstructures and results from the overprinting D2 deformation.Furthermore, regional structural transitions are imitated in thedeformation patterns of the ore bodies. Plotting the aspectratios of VMS ore bodies and the comparison with unde-formed equivalents in the Hokuroko district, Japan allow anestimation of apparent strain and show correlation with the D2

deformation intensity of the certain structural domains. A

comparison of the size of VMS deposits with their locationshows that the smallest deposits are not related to known high-strain zones and the largest deposits are associated withregional-scale high-strain zones. The comparison of distribu-tion and size with the pattern of high-strain zones provides animportant tool for regional-scale mineral exploration in theSkellefte district, whereas the analysis of ore body shape andorientation can aid near-mine exploration activities.

Keywords Palaeoproterozoic . Skellefte district . VMSdeposits . Ore body shape . Deformation

Introduction

Volcanogenic massive sulphide (VMS) deposits are typicallystratiform and partially strata-bound bodies of sulphide min-erals precipitated from hydrothermal fluids at or immediatelybelow the seafloor (Sangster and Scott 1976; Franklin et al.1981). Franklin et al. (1981) showed that distinct structuresare controlling both volcanism and hydrothermal activity inactive VMS systems. The Palaeoproterozoic Skellefte districtof Sweden is host to 79 VMS ore bodies. Numerous publica-tions deal with the geochemistry and origin of the VMS oresin the Skellefte district (Broman 1987; Weihed et al. 1992,2002a, 2005; Hannington et al. 2003; Wagner et al. 2004;Årebäck et al. 2005; Barrett et al. 2005; Weihed 2010) andtheir volcanic setting (Allen et al. 1996; Doyle and Allen2003; Montelius et al. 2007; Schlatter 2007). Recent structuralgeological and geochronological investigations have signifi-cantly improved our understanding of 3D geometry and of thestructural evolution inside different domains in the Skelleftedistrict (Skyttä et al. 2010, 2012; Bauer et al. 2011, 2013). Inparticular, Allen et al. (1996) and Bauer et al. (2011) suggestedthat early normal D1 faults and associated transfer faultsformed a complex geometry of syn-extensional sedimentary

Editorial handling: F. Tornos

Electronic supplementary material The online version of this article(doi:10.1007/s00126-013-0503-2) contains supplementary material,which is available to authorized users.

T. E. Bauer (*) : P. Skyttä :R. L. Allen : P. WeihedDivision of Geosciences and Environmental Engineering, LuleåUniversity of Technology, 971 87 Luleå, Swedene-mail: [email protected]

P. Skyttä : T. Hermansson : R. L. AllenThe Boliden Group, 936 81 Boliden, Sweden

Miner Deposita (2014) 49:555–573DOI 10.1007/s00126-013-0503-2

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basins and hence influenced both stratigraphy and subsequentdeformation patterns. Moreover, Bauer et al. (2013) indicatedthat the syn-extensional faults could have acted as pathwaysfor hydrothermal fluids. Despite the large number of publica-tions dealing with VMS deposits and structural geology, littlehas been reported about the structural control on the distribu-tion and shape of VMS ore deposits in general (e.g.Blenkinsop 2004) and the shape of VMS deposits in theSkellefte district in particular (Gavelin 1939; Grip,unpublished company report 1970; Grip and Frietsch 1973).Local studies on the mineralogy, geochemistry and origin ofVMS deposits in the Skellefte district have been conducted(e.g. Weihed et al. 2002a; Montelius 2005; Årebäck et al.2005; Schlatter 2007; Skyttä et al. 2013), but no regionalstudy summarising the setting of VMS deposits and the influ-ence of structures is available. The main objective of thispaper is to unravel the relationship of ore formation anddeformation events in the Skellefte district by showing thelocation, shape and orientation of VMS ore bodies andhighlighting their relation to present fault systems.We providea model on the structural control of syn-extensional faultingand VMS ore formation and subsequent transposition duringcrustal shortening. Based on these results, an interpretation ofpossible locations and shapes of undiscovered mineralisationsis made. This provides a tool for prospectivity mapping inboth field and near-mine exploration in the Skellefte districtand in comparable VMS districts throughout the world. Tomeet the above objectives, information about the 3D orienta-tion, shape and size of 79 VMS ore bodies located throughoutthe Skellefte district were gathered and 21 deposits weremodelled in three dimensions. Furthermore, the location, sizeand orientation of the deposits were visualised on structuralmaps showing the spatial relationships to major and minorfault systems. The VMS deposits occurring within the differ-ent structural domains, as specified by Skyttä et al. (2012),were integrated with the host rock structures within eachdomain. In particular, the orientation, shape and aspect ratioswere compared to both local and regional deformation inten-sity and deformation patterns of the surrounding host rocks.

Geological background

The Skellefte district is loosely defined by the occurrence ofrocks belonging to the volcanic arc series of the SkellefteGroup. The district consists of 1.9–1.8 Ga supracrustal andassociated intrusive rocks that were deformed and metamor-phosed during the same time interval in connection with theSvecokarelian orogeny (Lundström et al. 1997; Mellqvistet al. 1999; Kathol and Weihed 2005). To the north theSkellefte district is bordered by sub-aerial volcanic rocks ofthe Arvidsjaur Group comprising rhyolite to basalt. North ofthe Skellefte and Arvidsjaur area Palaeoproterozoic and

reworked Archaean rocks form a part of the Norrbottencraton. The Bothnian Basin metasedimentary rocks occursouth and east of the study area, whereas the Skelleftedistrict represents a transitional zone between those twomajor tectonic units (Fig. 1). The Archaean–Proterozoicboundary has been defined by a shift in Nd signature(Lundqvist et al. 1996; Wikström et al. 1996; Mellqvistet al. 1999) that coincides with a south-dipping seismicreflector interpreted as reflecting NE-verging thrust tec-tonics (BABEL Working Group 1990).

Regional geology

The lowest stratigraphic unit in the Skellefte district is repre-sented by metasedimentary rocks of the Bothnian Supergroupconsisting mainly of volcanogenic turbiditic greywackes andargillites and other conglomerates with intercalations of felsicandmafic volcanic rocks withmainly rhyodacitic composition(Kathol and Weihed 2005; Skyttä et al. 2012). The BothnianSupergroup forms the inferred basement to the 1.89–1.88 Ga,mainly felsic volcanic and volcaniclastic Skellefte Group(Figs. 2 and 3; Allen et al. 1996; Billstrom and Weihed1996; Montelius 2005; Skyttä et al. 2011). The ore-bearingSkellefte Group comprises mainly subaqueous lava domes,porphyritic cryptodomes, lavas and volcaniclastic rocks withlargely rhyolitic and minor basaltic, andesitic and daciticcomposition. Intercalations of grey to black mudstone,volcaniclastic siltstone, sandstone, breccia–conglomerate andcarbonate-rich volcaniclastic rocks are common (Allen et al.1996; Kathol and Weihed 2005; Montelius et al. 2007). Thestratigraphic thickness of Skellefte Group volcanic rocks isapproximately 3 km in the northern part of the district (Allenet al. 1996). The dominantly sedimentary Vargfors Group(1.88–1.87 Ga) overlies the Skellefte Group and comprisescarbonate-rich mudstones and conglomerates overlain byturbiditic mudstones, sandstones and monomict conglomer-ates sourcing from the Skellefte Group. The monomict unitsare unconformably overlain by polymict conglomerates andsandstones (Bauer et al. 2013). Exposure of contact relation-ships is generally poor, but detailed studies in the Vargforssyncline show varying contact relationships ranging fromprimary conformable and unconformable contacts to tectoniccontacts (Allen et al. 1996; Bauer et al. 2011, 2013).Carbonate-rich mudstone and conglomerate occur predomi-nantly at the Skellefte Group–Vargfors Group contact (Baueret al. 2011). Metasedimentary rocks immediately south of thecentral Skellefte district (Fig. 1) are regarded as VargforsGroup rocks due to their similar character and lithologies(Kathol and Weihed 2005). Their transition to BothnianSupergroup metasedimentary rocks to the south of the districtwas drawn in an arbitrary manner (Kathol and Weihed 2005).Locally occurring Vargfors Group mafic volcanic andvolcaniclastic rocks are commonly spatially associated with

556 Miner Deposita (2014) 49:555–573

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both the SSW–NNE and ESE–WNW striking high-strainzones in the district (Bauer et al. 2013).

The oldest intrusive rocks in the Skellefte district are rep-resented by 1.89–1.88 Ga granitoids and intermediate or basicrocks, including the oldest phase (GI) of the so-called Jörnintrusive complex (Wilson et al. 1987; Gonzales Roldan 2010;Bejgarn et al. 2012) and the Viterliden intrusion (Skyttä et al.2011) both tonalitic to granodioritic in composition. Theserocks are suggested to be co-magmatic with the volcanic rocksin the Skellefte Group. Younger phases of the Jörn intrusivecomplex (GII to GIV; granitic to tonalitic) as well as theintrusive rocks in the so-called Perthite-Monzonite suite with

mainly syenitic to monzonitic composition formed between1.88 and 1.86 Ga (see data compilation in Bejgarn et al. 2012)and post-date the volcanic rocks in the Skellefte Group. Thedistrict is bordered by 1.82–1.78 Ga late Svecokarelian intru-sive rocks with mainly granitic to syenitic composition(Kathol and Weihed 2005).

Structural geology

The Skellefte district is characterised by complex fault pat-terns ofWNW–ESE-striking and associated NNE–SSW-strik-ing faults that are inferred to have syn-extensional origin

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Supracrustal rocks

VMS mine, in operation Lode gold mine, in operation Major shear zones Major high-strain zone separating deep and shallow crustal domain Location of VMS ore body

Late to post-orogenic intrusive rocks ~1.82-1.78 Ga Granite - syenitoide, Revsund type Gabbro - diorite Granite, Skellefte-Härnö suite

Perthite-Monzonite suite intrusive rocks ~1.88-1.86 Ga Metasyenite - Metamonzonite Metagabbro - Metadiorite Jörn GIV metagranite Jörn GIII metagranite Jörn GII metagranodiorite- metatonalite

Early-orogenic intrusive rocks~1.89-1.88 Ga Jörn GI metagranodiorite- metatonalite

Unclassified, ~1.90-1.86 Ga Metagranite - Metatonalite Metagabbro - Metadiorite

Other

Map source: Geological Survey of Sweden (SGU)

Coordinates in the Swedish national grid (RT90)

200 km

Phanerozoic coverCaledonian orogenic beltSveconorwegian orogenSvecokarelian and Lappland-Kola orogens

Archaean rocks partly affacted by the Svecokarelian and Lapland-Kola orogenies

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Fig. 1 Inset map: Generalised geology of the Fennoscandian Shield.Geological domains: BB Bothnian basin, NC Norrbotten craton, CSDcentral Skellefte district (Fig. 2), KB Kristineberg area (Fig. 3). Thedashed line represents the boundary between rocks with Proterozoicand rocks with Archean Nd signatures (Mellqvist et al. 1999). Geology

drawn after Koistinen et al. (2001). Main map: Geological map of theSkellefte district. GI, GII, GIII, GIV: Jörn intrusive complex, phases I–IV;GaGallejaur, KaKarsträsk, RgRengård, Si Sikträsk, ViViterliden. Mod-ified after Bergman Weihed (2001), Kathol et al. (2005) and Skyttä et al.(2012)

Miner Deposita (2014) 49:555–573 557

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558 Miner Deposita (2014) 49:555–573

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(Fig. 2; Allen et al. 1996). Bauer et al. (2011) show theVargfors syncline and how these fault patterns of normal andtransfer faults developed during crustal extension, as evi-denced from kinematic indicators and facies variations withineach of the fault-bounded blocks of the sub-basin. These faultpatterns are most distinct in the central Skellefte district butmay be recognised throughout the district (Skyttä et al. 2012).Skyttä et al. (2012) suggest that the fault pattern either resultsfrom regional NE–SW transpression or NW–SE transpression,forming a pull-apart basin that favours the emplacement of

early orogenic intrusive rocks and the deposition of SkellefteGroup volcanic rocks. A major ESE–WNW-striking shearzone with inferred syn-extensional (D1) origin (Fig. 1;Dehghannejad et al. 2012) separates two crustal domains withcharacteristic deformational signatures (Skyttä et al. 2012).The earliest tectonic deformation (D1) at 1.89–1.87 Ga(Lundström et al. 1997, 1999; Lundström and Antal 2000;Rutland et al. 2001a, b) is constrained to deeper crustal levelssouth of the crustal detachment. The D1 deformation in thedeeper crustal level is suggested to be synchronous with the

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inclined with dip and lineationvertical

inclined with dip and lineationverticalunspecified

Fault traceMajor high-strain zone separating deep and shallow crustal domain

VMS deposits

inclined with dip and lineationverticalunspecified

3020

3020

3020

Intrusive rocks

Late to post-orogenic intrusive rocks~1.82-1.78 Ga

Vargfors Group, ~1.88-1.87 Ga

VMS deposit, > 1 Mt

VMS deposit, 0.1 - 1 Mt

Perthite-Monzonite suite intrusive rocks~1.88-1.86 Ga

Early-orogenic intrusive rocks~1.89-1.88 Ga

Unclassified intrusive rocks~1.90-1.86 Ga

Supracrustal rocks

Structures

Granite, Revsund-typeGabbro - diorites, Revsund-typeGranite, Skellefte-Härnö suite

Metagabbro - MetadioritesMetasyenite - Metamonzonite

Jörn GI metagranodiorite - metatonalite

Metagabbro - metadiorite

Gallejaur meta-andesite - metabasalt

Meta-argillite - metaconglomerate

Skellefte Group, ~1.89-1.88 Ga

Metarhyolite - metadaciteMetabasalt - Meta-andesite

Bothnian Supergroup, ~1.96-1.86 Ga

Metagreywacke - meta-argillite

Unclassified

Metabasalt - meta-andesite

VMS deposit, < 0.1 Mt

20

45

50

46

72

7971

75

Trace of D2 antiform and synform, plunge direction indicated

Mineral lineation

12

27

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Profile

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A

A’

Inferred ore horizon

Fig. 3 Geological map of the Kristineberg area showing the location, size and orientation of VMS deposits. Modified after Allen et al. (1996), Katholet al. (2005) and Skyttä et al. (2013). See Table 1 for numbering and names of the deposits

Miner Deposita (2014) 49:555–573 559

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crustal extension at higher crustal levels in the northern do-main, which seems to be unaffected by D1 deformation (Skyttäet al. 2012).

The main deformation event (D2) at 1.87–1.86 Ga caused astrong strain partitioning in the heterogeneous post-D1 do-mains. The southern domain is characterised by non-coaxialhigh-strain deformation, accompanied by lateral stretchingand gently plunging mineral lineations. In the northern do-main, D2 deformation resulted in inversion of syn-extensionalfaults (Bauer et al. 2011), upright folding (1.87 Ga, Skyttäet al. 2013) and vertical stretching with steep to sub-verticalmineral lineations. Bauer et al. (2011) showed that re-activation of early syn-extensional faults in the Vargfors syn-cline was favoured by carbonate-rich lithologies at the bottomof the sedimentary sub-basin. Skyttä et al. (2012) suggest twoalternative scenarios for D2 deformation: (a) one event around1.87 Ga with approximately SSE–NNW bulk compressionand strong partitioning between the southern domain and thenorthern domain. The latter was dominated by a coaxialSSW–NNE-shortening component. Alternatively, (b) twoseparate events, a SW–NE transpressional event at 1.87 Gain the northern domain and a SSE–NNW transpressionalevent in the southern domain, are suggested (Skyttä et al.2012). Due to the close relation of the two events in thealternative model by Skyttä et al. (2012), they are both includ-ed into the main compressional D2 deformation.

The latest major deformation event (D3) at 1.82–1.80 Ga(Weihed et al. 2002b) is inferred to have resulted from E–Wcrustal shortening causing reactivation of the N–S-strikinghigh-strain zones with reverse kinematics (Bergman Weihedet al. 1996; Bauer et al. 2011; Skyttä et al. 2012).

Peak metamorphism (M1) in the eastern and southern partsof the district is synchronous with the oldest deformationevent (D1) pre-dating the c. 1.88 Ga intrusions (Lundströmet al. 1997, 1999; Skyttä et al. 2012). Highest metamorphicconditions with partial melting are observed in the SE part ofthe district decreasing towards the north. The metamorphicpeak in the western and central parts (M2) post-dates theupright folding (Årebäck et al. 2005; Skyttä et al.2012). In the Kristineberg area, sub-solidus PT conditions(∼3 kbar, ∼600 °C) were observed (Lundström et al. 1997,1999; Kathol and Weihed 2005).

Geological setting of VMS deposits

Information about the location, status (mine, closed mine,deposit or mineralisation), size, orientation, proportions, ton-nage and metal grade of 79 VMS ore bodies throughout theSkellefte district are shown in Table 1. The VMS deposits inthe Skellefte district are hosted within the uppermost part ofSkellefte Group stratigraphy (Allen et al. 1996). The majorityof VMS-hosting volcaniclastic rocks are interpreted to havedeposited rapidly and therefore the slowly forming VMS ores

are suggested to have formed as sub-seafloor replacement(Allen et al. 1996). Some VMS ores are hosted in facies withlow sedimentation rates and are therefore interpreted asexhalative deposits. The sulphide ores comprise massive,semi-massive and disseminated ores surrounded by dissemi-nated pyrite and with or without stringer vein networkmineralisation. The majority of deposits contain abundantpyrite and Zn–Cu–Au ± Pb. The main ore minerals are pyrite,sphalerite, chalcopyrite, arsenopyrite and galena (Allen et al.1996). The dominating hydrothermal alteration ischaracterised by Q-ser-py hydrothermal assemblage andshows more intensity in the stratigraphic footwalls of thedeposits. Allen et al. (1996), Weihed et al. (2002a) andBauer et al. (2013) inferred that the ore-forming hydrothermalfluids utilised the syn-extensional faults (D1) as fluid conduitsand that the ores precipitated in the vicinity of these faults.Consequently, it was suggested that subsequent fault-inversion (D2) transposed the VMS ore bodies into theirpresent-day geometry (Årebäck et al. 2005; Coller, unpub-lished company report 2008; Coller, personal communication2011; Bauer et al. 2013). This is in line with the studies byTalbot (1988), Bergman Weihed et al. (1996) and Årebäcket al. (2005) showing localised shearing and stronger defor-mation within the VMS ores than in the hosting volcanicrocks. This deformation is visible as internal folding withinthe massive sulphide ores as reported for the Rävlidmyran(deposit number 62 in Table 1; Grip and Frietsch 1973),Näsliden (Dep.) 49 and 50; Grip and Frietsch 1973; Rickardand Svenson 1983), Långdal (Dep. 36; Grip and Frietsch1973; Talbot 1988; Weihed et al. 2002a) and many otherdeposits (c.f. Grip and Frietsch 1973 and references therein).A varying degree of re-mobilisation of during deformationhas been inferred from specific deposits. In the Kristinebergdeposit (Dep. 35), remobilisation is evident from accumula-tion of sulphides in F2-fold hinges and recrystallisation due toD2 shear-induced remobilisation (Årebäck et al. 2005). The J-and K-lenses of the Kristineberg ore are suggested to resultfrom D2 remobilisation (Jolley, unpublished company report2001; Årebäck et al. 2005). Remobilisation of sulphides in theLångdal deposit has been described as polymodal with bothsolid-state and liquid-state type (Weihed et al. 2002a).Furthermore, Weihed et al. (2002a) suggest that gold has beenremobilised from the sulphides into shear zones during orslightly after peak metamorphism (M2).

Many of the VMS deposits are located within second-orderstructures, such as splays of major N–S-trending shear zones(Fig. 2), as reported for the Boliden (Dep. 20; Grip,unpublished company report 1970; Grip and Frietsch 1973;Bergman Weihed 2001) and Näsliden deposits (Dep. 49 and50; Grip and Frietsch 1973; Rickard and Svenson 1983), orsplays of reactivated faults, such as the Sjömalmen (Dep. 67;Grip 1951; Grip and Frietsch 1973) and Svansele deposits (c.f.Grip and Frietsch 1973).

560 Miner Deposita (2014) 49:555–573

Page 7

Tab

le1

Listo

fVMSdepositsin

theSk

elleftedistrictwith

locatio

n,tonnage,shape(X–Y–Z),planar

andlin

earorientationof

orebody

(dip

direction,dip,trendandplunge)andmetalgrades

Nam

eNr.

Status

Dipdir

Dip

Trend

Plunge

X(m

)Y(m

)Z(m

)To

nnage(1000t)

Au(ppm

)

Åkulla_W

1Abandoned

mine

270

80315

7620

155

200

980

0.7

Åkulla_E

2Prospect

6085

585

60200

2.1

Ålid

en3

Mineralisation

4570

1Åsen_WC

4Prospect

300

10–20

700

1Åsen_W

5Prospect

Åsen_EC

6Abandoned

mine

210

70300

10–20

3450

450.7

Åsen_E

7Prospect

Bastuheden_C

8Prospect

260

87100

910.2

Bastuheden_N

9Prospect

260

86200

681.7

Bastuheden_S

10Prospect

225

87200

290

0.7

Bastuheden_E

11Prospect

242

860.5

100

Björklid

en12

Mineralisation

Bjurfors_C

13Prospect

200

6025

100

600

0.25

Bjurfors_W

14Prospect

190

5030

185

Bjurfors_E

15Abandoned

mine

190

60266

2330

185

65200

0.5

Bjurliden

16Prospect

180

45257

2510

120

180

720

0.3

Bjurträsk

17Prospect

190

60253

387

140

150

377

0.25

Bjurträsk_S

18Prospect

27.9

Bjurvattnet

19Prospect

Boliden

20Abandoned

mine

176

8096

4540

590

430

8280

15.5

Boliden_S

21Prospect

Brattm

yrhögen

22Mineralisation

Brattm

yrhögen_N

23Prospect

Brännan

24Prospect

7565

Elvaberget

25Prospect

225

65177

5520

300

110

930

0.5

Fjällboheden

26Prospect

200

Granlunda

27Abandoned

mine

110

80192

4012

20350

125

Holmtjärn

28Abandoned

mine

023

52023

5235

100

440

500

7.4

Hornträskviken_C

29Abandoned

mine

315

88226

3025

45418

880

0.6

Hornträskviken_E

30Prospect

600

Högkulla_E

31Abandoned

mine

180

65239

50113

0.6

Kankberg

32Activemine

230

55216

5415

200

366

1800

2.6

Kedträsk

33Abandoned

mine

210

58251

5015

330

356

3900

0.5

Kim

heden

34Abandoned

mine

145

85229

5018

130

450

1000

0.4

Kristineberg

35Activemine

190

60245

3730

800

3300

23049

1Långdal

36Abandoned

mine

9079

3571

25600

400

4330

1.9

Långsele

37Abandoned

mine

135

6587

5540

315

680

12000

0.9

Långträskåsen

38Prospect

Långviken

39Prospect

7.5

75105

0.5

Lillhomsberget

40Mineralisation

Lom

viken

41Prospect

50.38

Maurliden_N

42Prospect

230

70868

0.7

Maurliden_W

43Activemine

230

88230

8846

300

200

6900

0.9

Maurliden_E

44Activemine

3510

140

200

501265

0.8

Mörkliden

45Mineralisation

335

0.3

Miner Deposita (2014) 49:555–573 561

Page 8

Tab

le1

(contin

ued)

Nam

eNr.

Status

Dipdir

Dip

Trend

Plunge

X(m

)Y(m

)Z(m

)To

nnage

(1000

t)Au(ppm

)

Mörkliden

346

Mineralisation

820.6

Mörkliden

447

Prospect

115

0.1

Näset

48Mineralisation

250

Näslid

en49

Abandoned

mine

255

60–80

260

7020

200

600

5012

1.3

Näslid

en_N

50Abandoned

mine

300–100

900

0.9

Norrliden_N

51Prospect

200

65200

6540

180

140

2952

0.4

Norrliden_S

52Prospect

Nyborg

53Prospect

296

0.2

Österbäcken

54Prospect

240

20–40

0.3

Petik

näs_N

55Abandoned

mine

1300

5.6

Petik

näs_S

56Abandoned

mine

5090

140

6030

250

400

6500

2.3

Rakkejaur

57Abandoned

mine

240

88319

8065

450

320

20000

1Renström_K

yrkväg

58Prospect

2.8

Renström_W

59Activemine

7275–90

7275–90

20300

940

9261

2.8

Renström_E

60Abandoned

mine

1590

105

7010

100

235

Rävlid

en61

Abandoned

mine

195

60269

2625

100

860

1936

0.6

Rävlid

myran

62Abandoned

mine

390

278

25–30

40460

1500

6823

0.8

Rundklumpen

63Prospect

Rutselheden

64Mineralisation

Rågangen

65Prospect

0.7

Salm

onLindersMalm

66Abandoned

mine

Sjöm

almen_Inner

67Abandoned

mine

4070

329

4528

100

200

2300

0.8

Sjöm

almen_O

uter

68Prospect

1490

100

2.9

Skäggträskberget

69Prospect

190

50–70

Skoberget

70Mineralisation

Snåtterm

yran

71Prospect

190

4020

180

170

100

Stenbrånet

72Mineralisation

8530

8530

0.8

200

260

Svansele

73Prospect

220

40220

402900

0.2

Svansele_sel

74Prospect

205

302–3

300

Svansele_väg

75Prospect

196

30196

30360

0.9

Tistelm

yran_E

76Mineralisation

Udden

77Abandoned

mine

210

80278

6530

430

300

5946

0.7

Vikborg

78Prospect

Vindelgransele

79Prospect

141

Nam

eAg(ppm

)Cu(%

)Zn(%

)Pb

(%)

Structuraldomain

Eastin

gNorthing

Figure

Onlineresource

Reference

Åkulla_W

121

Boliden

1710320

7207700

21

6,12,13,15

Åkulla_E

141.7

Boliden

1710650

7208150

22

6,12

,13,15

Ålid

en24

0.1

Maurliden

1678879

7223025

212,13,15

Åsen_WC

641.74

1.84

0.46

Holmträsk

1692850

7209800

212,13,15

562 Miner Deposita (2014) 49:555–573

Page 9

Tab

le1

(contin

ued)

Nam

eAg(ppm

)Cu(%

)Zn(%

)Pb

(%)

Structuraldomain

Eastin

gNorthing

Figure

Onlineresource

Reference

Åsen_W

Holmträsk

1691600

7210420

212,13,15

Åsen_EC

341.22

0.84

0.2

Holmträsk

1693950

7209150

23

6,12

,13,15

Åsen_E

Holmträsk

1694850

7208500

212,13,15

Bastuheden_C

151.39

0.11

Boliden

1710700

7207700

212,13,15

Bastuheden_N

860.07

4.6

1.06

Boliden

1711250

7208700

212,13,15

Bastuheden_S

60.96

0.08

Boliden

1710300

7207200

212,13,15

Bastuheden_E

Boliden

1712050

7208700

212,13,15

Björklid

enKristineberg

1636120

7214550

315

Bjurfors_C

183.5

0.2

Maurliden

1679250

7216950

210,12,13,15

Bjurfors_W

Maurliden

1678570

7217200

210,12,13,15

Bjurfors_E

2.6

Maurliden

1679560

7216800

26,10,12,13,15

Bjurliden

460.1

3.5

0.35

Snåtterm

yran

1683100

7215900

27,10,12,13,15

Bjurträsk

70.3

6.6

Snåtterm

yran

1684930

7215680

28,10,12,13,15

Bjurträsk_S

0.03

1.69

Snåtterm

yran

1684840

7215590

29,13,15

Bjurvattnet

Boliden

1711600

7203000

215

Boliden

501.43

0.92

0.27

Boliden

1716500

7204250

24

5,6,12,13,15

Boliden_S

Boliden

1716600

7204100

212,13,15

Brattm

yrhögen

Kristineberg

1645250

7226900

112,13,15

Brattm

yrhögen_N

Kristineberg

1646700

7225000

112,13,15

Brännan

Boliden

1709000

7205550

212,13,15

Elvaberget

220.46

1.69

0.18

Maurliden

1667300

7219600

212,13,15

Fjällboheden

30.5

0.2

Boliden

1713020

7217580

113,15

Granlunda

150.76

1.3

0.9

Kristineberg

1621730

7225330

32,12,13,15

Holmtjärn

920.4

40.4

Maurliden

1676820

7227920

25

6,12,13,15

Hornträskviken_C

651.08

4.6

0.5

Kristineberg

1627910

7223220

36

6,12,13,15

Hornträskviken_E

12.3

Kristineberg

1628250

7223460

36,12,13,15

Högkulla_E

110

0.18

12.6

0.65

Maurliden

1673984

7217709

210,12,13,15

Kankberg

521.4

1.8

0.3

Boliden

1711000

7209450

26,12,13,15

Kedträsk

240.4

2.9

0.2

Snåtterm

yran

1692770

7211540

27

6,12,13,15

Kim

heden

61.17

0.7

Kristineberg

1631200

7223400

38

6,12,13,15

Kristineberg

351

3.7

0.5

Kristineberg

1629950

7220600

39

4,6,12,13,14,15,20,21

Långdal

149

0.1

5.8

1.7

Boliden

1710070

7198570

210

6,12,13,15,22

Långsele

250.6

3.9

0.3

Boliden

1712050

7201150

211

6,12,13,15

Långträskåsen

Kristineberg

1620900

7226290

315

Långviken

191.5

0.68

0.11

Maurliden

1669050

7221730

212,13,15

Miner Deposita (2014) 49:555–573 563

Page 10

Tab

le1

(contin

ued)

Nam

eAg(ppm

)Cu(%

)Zn(%

)Pb

(%)

Structuraldomain

Eastin

gNorthing

Figure

Onlineresource

Reference

Lillhomsberget

Kristineberg

1617750

7223000

315

Lom

viken

12.7

0.8

0.7

0.06

Maurliden

1668940

7222420

212,13,15

Maurliden_N

270.14

1.06

0.16

Maurliden

1676603

7221656

215

Maurliden_W

490.2

3.4

04Maurliden

1674690

7222554

212

6,12,13,15,17

Maurliden_E

161.3

0.7

Maurliden

1676350

7221409

213

6,12,13,15,17

Mörkliden

690.18

3.56

0.34

Kristineberg

1625170

7218060

312,15

Mörkliden

35

0.88

0.1

Kristineberg

1627380

7218400

312,15

Mörkliden

425

0.17

4.9

0.4

Kristineberg

1627840

7218540

312,15

Näset

Snåtterm

yran

1685640

7216970

28,15

Näsliden

351.1

30.3

DNSZ

1654950

7225900

214

6,12,13,15,18

Näsliden_N

280.6

1.8

0.2

DNSZ

1655060

7226350

212,13,15,18

Norrliden_N

440.8

2.5

0.2

Maurliden

1678351

7218173

215

10,12,13,15

Norrliden_S

Maurliden

1677978

7217910

215

Nyborg

410.53

4.8

0.5

Kristineberg

1636325

7227025

315

Österbäcken

Maurliden

1680986

7218036

210,12,13,15

Petik

näs_N

103

1.3

5.6

0.9

Holmträsk

1700580

7210720

215,19

Petik

näs_S

108

1.1

4.8

0.9

Holmträsk

1700300

7210150

216

6,15,19

Rakkejaur

500.3

2.4

0.2

DNSZ

1657250

7230700

217

6,12,13,15

Renström_K

yrkväg

155

0.8

61.7

Holmträsk

1702940

7209620

213,15

Renström_W

155

0.8

6.5

1.5

Holmträsk

1702900

7209120

218

6,12,13,15

Renström_E

Holmträsk

1703440

7208940

213,15

Rävlid

en67

1.9

3.14

0.1

Kristineberg

1625750

7220500

319

6,12,13,15

Rävlid

myran

511

3.9

0.6

Kristineberg

1625350

7221900

320

6,12,13,15

Rundklumpen

170.08

1.6

0.1

Maurliden

1668940

7223220

212,13,15

Rutselheden

Snåtterm

yran

1682215

7216090

28,15

Rågangen

181.4

0.4

0.1

Maurliden

1668410

7222370

212,13,15

Salm

onLindersMalm

Kristineberg

1626850

7226260

32,15

Sjöm

almen_Inner

470.5

2.2

0.5

Maurliden

1667400

7223480

211,12,13,15

Sjöm

almen_O

uter

502.1

3.65

0.39

Maurliden

1666850

7223750

211,12,13,15

Skäggträskberget

Maurliden

1673137

7217707

210,12,13,15

Skoberget

Boliden

1711800

7202800

213,15

Snåtterm

yran

150.3

6.6

0.17

Snåtterm

yran

1686929

7215414

210,12,13,15,16

Stenbrånet

Holmträsk

1696180

7193350

21,3,15

Svansele

40.28

0.32

0.6

Snåtterm

yran

1689420

7214180

212,13,15

Svansele_sel

Snåtterm

yran

1690450

7214230

212,13,15

564 Miner Deposita (2014) 49:555–573

Page 11

Methodology

The location, size and orientation of the ore deposits werevisualised on structural maps over both the central Skelleftedistrict (Fig. 2) and the Kristineberg area (Fig. 3). Due to theavailability of sufficient data, 21 deposits were modelled inthree dimensions. Modelling was performed in the gOcadsoftware platform (Paradigm), utilising existing mine modelsprovided by Boliden Mines for the Kristineberg (Dep. 35),Maurliden W (Dep. 43), Rävliden (Dep. 61), Rävlidmyran(Dep. 62) and RenströmW (Dep. 59) deposits. The remainingdeposits were modelled from available digital and paper levelplans, cross sections and drill hole information (Fig. 4). Theshape of complex ore bodies was simplified to a certain degreeto aid visualisation, but without modifying the general char-acter of the deposit (Fig. 4). Dip, dip direction, trend andplunge of the ore bodies as well as their dimensions weredetermined from the 3D models (Fig. 5). The extent of thebodies is given by X representing the thickness of the body(perpendicular to the strike), Yindicating the width of the bodyand (along strike) and Z the length of the ore body (parallel todip; Table 1 and Fig. 4b). For the majority of the remainingdeposits, these values were sourced from the literature(Gavelin 1939; Grip, unpublished company report 1970;Grip and Frietsch 1973; Claesson et al. 1980, 1981;Claesson and Johansson 1981; Lindberg and Andersson1981; Rickard and Svenson 1983; Bergman Weihed et al.1996; Årebäck H 1998; Årebäck H and Sandström 2001;Weihed et al. 2002a; Hannington et al. 2003; Årebäck et al.2005; Kathol and Weihed 2005; Åkerman 2007; Monteliuset al. 2007; Schlatter 2007; Skyttä et al. 2009; Skyttä et al.2010; Skyttä et al. 2013). The aspect ratios of the VMS orebodies were visualised by plotting their x/y and y/z ratios in amodified Flinn diagram where x represents the largest, y themedium and z the smallest extent of the body (Fig. 6; Flinn1962; Ramsey and Huber 1983). As an undeformed analogue,the younger Kuroko deposits in Japan show how the VMSdeposits originally formed as lensoidal bodies with unknowndimensions (Ishihara et al. 1974; Ohmoto and Skinner 1983).Therefore, finite strain cannot be determined. As for theSkellefte district, the Maurliden E deposit (Dep. 44) was usedas a proxy for primary shape, assuming that it represents theleast deformed VMS deposit in the district due to its aspectratios comparable to undeformed Kuroko deposits and theabundant undeformed primary textures, such as banded mas-sive sulphides and stringer zones, and the stratiform and sub-horizontal character of the ore body (Fig. 5a and OnlineResource 13). Furthermore, the average and bulk of aspectratios from VMS ore bodies in the Hokuroko district, Japan(c.f. data compilation in Tanimura et al. 1983) were plottedagainst VMS ore bodies in the Skellefte district (Fig. 6). Withthis approach, apparent strain combinedwith the ore orientationmay be used to evaluate the intensity of tectonic transposition.T

able1

(contin

ued)

Nam

eAg(ppm

)Cu(%

)Zn(%

)Pb

(%)

Structuraldomain

Eastin

gNorthing

Figure

Onlineresource

Reference

Svansele_väg

70.25

3.14

0.04

Snåtterm

yran

1690050

7214100

212,13,15

Tistelm

yran_E

Maurliden

1673750

7225270

215

Udden

360.4

4.3

0.3

Snåtterm

yran

1690450

7212150

221

6,12,13,15

Vikborg

Holmträsk

1701600

7201900

215

Vindelgransele

145

0.23

16.6

3.7

Kristineberg

1617228

7223780

312,13,15

References:1Åkerm

an(2007),2

Årebäck

(1998),3

Årebäck

andSandström

(2001),4

Årebäck

etal.(2005),5Bergm

anWeihedetal.(1996),6Boliden

(unpublishedcompany

report),7Claessonetal.

(1980),8

ClaessonandJohansson(1981),9

Claessonetal.(1981),10

Gavelin(1939),11Grip(1951),12Grip(unpublishedcompany

report1970),13

GripandFrietsch(1973),14Hanningtonetal.(2003),

15Katholand

Weihed(2005),16LindbergandAndersson

(1981),17Monteliu

setal.(2007),18

Rickard

andSv

enson(1983),19Schlatter(2007),20

Skyttäetal.(2009),21

Skyttä

etal.(2010),22

Weihed

etal.(2002a)

Miner Deposita (2014) 49:555–573 565

Page 12

Description and structural setting of VMS deposits

Location

Plotting the VMS deposits on the geological map of theSkellefte district reveals a close spatial relationship to faults(Figs. 1, 2 and 3). In the central parts of the district, theSjömalmen (Deps. 67 and 68), Rundklumpen (Dep. 63) andNorrliden (Deps. 51 and 52) deposits are associated with adistrict-scale inverted normal fault which was observed in thefield and in reflection seismic profiles (Dehghannejad et al.2012). Furthermore, the Bjurfors (Deps. 13, 14 and 15),Rutselheden (Dep. 64), Bjurliden (Dep. 16), Bjurträsk(Deps. 17 and 18) and Snåttermyran (Dep. 71) deposits, andprobably also the Svansele (Deps. 73, 74 and 75) deposits, areassociated with a fault splay related to the previously namedfault and interpreted from magnetic anomaly maps (Bauer2010). Moreover, the Rundklumpen (Dep. 63), Bjurfors W(Dep. 14), Bjurliden (Dep. 16) and Bjurträsk (Deps. 17 and18) deposits are located in the vicinity of the intersection ofnormal and transfer faults which were interpreted from mag-netic anomaly maps and partly observed in the field (Bauer

2010). Also the Udden (Dep. 77) and Kedträsk (Dep. 33)deposits are associated with district-scale faults.

Strongly deformed metasedimentary rocks associated withthe ESE–WNW-striking shear zone that is suggested to sepa-rate domains with coaxial deformation at higher crustal levelsand non-coaxial deformation at deeper crustal levels (Skyttäet al. 2012) host the Åsen deposits (Deps. 4, 5, 6 and 7). TheLångdal (Dep. 36), Långsele (Dep. 37) and Boliden (Deps. 20and 21) deposits in the western part of the district are related tothe inferred continuation of this major shear zone (Skyttä et al.2012). Splays of the suggested continuation of this domainboundary in the Kristineberg area (Skyttä et al. 2012) host theBrattmyrhögen (Deps. 23 and 22; Fig. 1) deposits. TheGranlunda (Dep. 27), Kristineberg (Dep. 35), Mörkliden(Deps. 45, 46 and 47), Rävliden (Dep. 61), Rävlidmyran(Dep. 62) and Hornträskviken (Deps. 29 and 30) deposits inthe Kristineberg area are also located on high-strain zones.Furthermore, local-scale inverted normal faults have beenreported in relation to the Maurliden W deposit (Dep. 43;Montelius et al. 2007), the Petiknäs deposits (Deps. 55 and56; Schlatter 2007) and the Renström deposits (Deps. 58, 59and 60).

50 m

c

50 m

d

50 m

a

50 m

b

strikedip

linear trend

Y

X

Z

Fig. 4 Modelling of the Petiknäs S deposit (deposit Nr. 56 in Table 1): adigitalising of georeferenced paper level plans, bmodelled surface of theore body. Modelling of the Boliden deposit (Dep. 20): c digitalising and

simplification of georeferenced paper level plans, d 3D model of theBoliden ore body with marking of proportions and strike and dipvisualisation

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Deposits that show no relation to inverted normal faults butto transfer faults are the Åliden (Dep. 3), Holmtjärn (Dep. 28),Långviken (Dep. 39), Lomviken (Dep. 41), Maurliden N(Dep. 42), Maurliden E (Dep. 44), Rågangen (Dep. 65) andVikborg (Dep. 78) deposits. Deposits associated withregional-scale N–S-trending high-strain zones are theNäsliden (Deps. 49 and 50) and Rakkejaur (Dep. 57) depositslocated within the Deppis-Näsliden shear zone and the Åkulla(Deps. 1 and 2), Bastuheden (Deps. 8, 9, 10 and 11), Brännan

(Dep. 24), Fjällboheden (Dep. 26; Fig. 1) and Kankberg (Dep.32) deposits within the Vidsel-Röjnoret shear system.

No obvious association to known high-strain zones orfaults was noticed for the Björkliden (Dep. 12), Kimheden(Dep. 34), Långträskviken (Dep. 38), Lillholmsberget (Dep.40), Salmon Linders Malm (Dep. 66) and Vindelgransele(Dep. 79) deposits in the Kristineberg area and theBjurvattnet (Dep. 19), Elvaberget (Dep. 25), Högkulla E(Dep. 31), Näset (Dep. 48), Österbacken (Dep. 54),

N

0

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Maurliden EMaurliden W

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50 m

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B-lens

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Main ore body

B-lenses

Einarsson ore

Einarsson W ore

J-lens

K-zinc

M-zone

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D-lens

C-lens

50 mNorth

North

North

a b

c d

e fFig. 5 Screenshots of 3D models of ore bodies in the Skellefte district: aModel of the Maurliden synform with the steeply dipping Maurliden W(Dep. Nr. 43 in Table 1) deposit, the sub-horizontal Maurliden E (Dep.44) deposit and 3D visualisation of faults; bmodel of the down-dip linear

Kedträsk (Dep. 33) deposit; cmodel of the planar Åsen EC deposit (Dep.6); d model of the cigar-shaped Hornträskviken deposit (Dep. 29); emodel of the Kristineberg deposit (Dep. 35); fmodel of the cigar-shapedHolmtjärn deposit (Dep. 28)

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Skäggträskberget (Dep. 69), Skoberget (Dep. 70) andStenbrånet (Dep. 72) deposits in the central Skellefte district.

Ore body orientation

The dip direction and dip of the mainly planar and lensoidalore bodies are summarised in Table 1 and visualised in Figs. 2and 3. Furthermore, the trend and plunge of the greatestlongest axes of the ore bodies are also displayed.Additionally, the 3D orientation of the ore bodies can be seenin Fig. 5 and the Online Resources 1 to 21. Based on varyingstructural styles, Skyttä et al. (2012) defined specific structuraldomains within the Skellefte district. The sections belowpresent firstly structural overviews, which are followed bydescriptions of the ore body orientations in each domain.

Maurliden domain The Maurliden domain can be separatedinto four units: the Vargfors and Maurliden synclines to thenorth, the Finnliden antiform in the centre and an extensivearea with Vargfors Group metasedimentary rocks to the south(Fig. 2). The distinct fault pattern of NNW–SSE-strikingreverse faults and associated NE–SW-striking faults is verypronounced in this domain and has been interpreted in theVargfors syncline as WNW–ESE-striking inverted normalfaults and associated NNE–SSW-striking reactivated transferfaults (Bauer et al. 2011). The NNW–SSE-striking reverse

faults show great lateral extent and can be traced through largeparts of the central Skellefte district (Skyttä et al. 2012).Especially the larger scale reverse faults show mainly listricgeometries, with predominantly southerly dips, thereforeinterpreted as inverted normal faults (Dehghannejad et al.2012). The NE–SW-striking transfer faults are of more localcharacter and dominate the northern part of this domain(Skyttä et al. 2012). They divide the Vargfors and Maurlidensynclines into distinct blocks with differing strain intensities(Bauer et al. 2011). This is particularly noticeable for thetransfer fault separating the low-strain block hosting theMaurliden E deposit (Dep. 44) from the high-strain blockhosting the Maurliden W deposit (Dep. 43). Moreover, thesame low-strain–high-strain pattern can be traced withinblocks in the Vargfors syncline north of the Maurliden de-posits. The dominant folds are upright and mainly open withsub-horizontal, undulating fold axes. Within high-strain do-mains, folding is more isoclinal and fold axis is locally trans-posed into steep orientation (e.g. in the vicinity of theMaurliden W deposit). Towards the south, fold axes havesteeper plunges. In the vicinity of high-strain zones, strataare transposed into steep to partly overturned attitudes(Bauer et al. 2011; Skyttä et al. 2012). Mineral lineation issteep to sub-vertical and the main foliation is sub-parallel tothe axial surfaces of the folds. The metamorphic grade is lowand increases slightly towards the south (Skyttä et al. 2012).

log

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)

log (y/z) 15.0 25.1

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16

Holmträsk domain

Snåttermyran domain

Maurliden domain

Boliden domain

Deppis-Näsliden shear zone

Kristineberg Northern Antiform

Kristineberg Southern Antiform

Avarage VMS deposit in the Hokuroko Basin, Japan

80 % of VMS deposits in the Hokuroko Basin, Japan15

25

6732

33

43

51

56

60

62

7277

35

5949

61

571

220

29

36

37

28

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71

27

oblate

prolate

Fig. 6 Modified Flinn diagram showing the aspect ratios of VMS orebodies in the Skellefte district and the average and bulk aspect ratios ofVMS ore bodies in the Hokuroko district, Japan. Data source for

Hokuroko deposits: Table 2 in Tanimura et al. (1983). Diagram modifiedafter Flinn (1962) and Ramsey and Huber (1983)

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Deposits in the northern half of this domain dip mainlytowards the northeast and southwest, with dip angles varyingfrom vertical to sub-horizontal and the long axes plungingperpendicular to the dip direction. Changes in orientation canbe observed in the Maurliden deposits. Whereas theMaurliden W deposit (e.g. Maurliden W, Dep. 43; Fig. 5aand Online Resource 12) is oriented sub-vertically, parallel toa fault, the Maurliden E deposit (Dep. 44; Fig. 5a and OnlineResource 13) dips gently towards the northeast, and theMaurliden N deposit (Dep. 42) dips steeply towards thesouthwest, concordant with the fold limbs of the Maurlidensyncline (Fig. 5a). The deposits in the southern half of thedomain dip 60° to 70° towards northeast or south to south-west, and the long axes of the ore bodies plunge 45° to 55°towards northwest or south to southwest (Fig. 2). Only theBjurfors deposits (Deps. 13 and 15) are gently W plunging,hence representing a transition to the nearby Snåttermyrandomain.

Snåttermyran domain The Snåttermyran domain ischaracterised by distinct, laterally continuous WNW–ESE-striking and south-dipping reverse faults with partly strike-slip components and abundant NNE–SSW-striking faults.Dehghannejad et al. (2012) and Skyttä et al. (2012) inferredthese faults as inverted normal faults and associated transferfaults. In the eastern parts, isoclinal and recumbent folds oflocal extent were reported by Skyttä et al. (2012). These areoverprinted by open, NE-verging folds, with SE-plunging foldaxes, formed during the main compressional event. In thevicinity of the major high-strain zone in the southern part ofthis domain, bedding parallel foliation was overprinted byupright crenulations. Mineral lineation in the central part ofthe Snåttermyran domain generally plunges gently towardsWSW (Skyttä et al. 2012). The metamorphic grade is low(greenschist facies) but increases to upper amphibolite faciestowards the south (Skyttä et al. 2012).

The planar VMS ore bodies in the Snåttermyran domaindip 30–80° towards the south and southeast, with moderatedip angles dominating (Fig. 2). The long axes of the ore bodiesplunge typically gently to moderately (25–40°) towards west.Some ore bodies plunge more steeply with up to 65° towardswest (e.g. Kedträsk, Dep. 33; Fig. 5b and Online Resource 7).Only the long axis of the Svansele deposit (Dep. 73) plungesdown dip.

Holmträsk domain The Holmträsk domain is dominated bythree parallel early orogenic events (Figs. 1 and 2). In contrastto the previously described domains, the configuration offaults in this domain, especially north of the Rengård intrusion(Fig. 1), is more complex in nature. The northernmost intru-sion (the Rengård intrusion) has an unfoliated low-strain coreand is bounded by a high-strain zone along its northernmargin(Skyttä et al. 2012). In contrast, the foliation in the Kartsträsk

and Sikträsk intrusions is penetrative. Metamorphic gradeincreases from greenschist facies in the north to upper am-phibolite facies in the south.Mineral lineations are steep in thenorthern parts of the domain and gently dipping to sub-horizontal in the southern parts, corresponding to a domainat shallower crustal level and deeper crustal level, respectively(Skyttä et al. 2012).

The northern part of the Holmträsk domain is dominated bythe sub-vertically oriented Petiknäs (Deps. 55 and 56;Fig. 4a, b and Online Resource 16) and Renström (Deps. 58,59 and 60; Online Resource 18) deposits (Allen et al. 1996;Schlatter 2007). The long axes of the ore bodies plungetypically 60–70° and parallel to the dip direction. In contrast,the central ore bodies of the Åsen deposit (Deps. 4 and 6;Fig. 5c and Online Resource 3) dip steeply towards south withsub-horizontal westward plunges. In the southernmost partof the domain, the Stenbrånet deposit (Dep. 72) forms agently E-dipping plane with the long axes parallel to thedip direction.

Boliden domain For the Boliden domain, Skyttä et al. (2012)suggested the continuation of a major high-strain zone, iden-tified in the Maurliden, i.e. Snåttermyran and Holmträsk do-mains, along the contact between Skellefte Group and theBothnian Supergroup in the eastern parts of the domain(Figs. 1 and 2).

Deposits in the northwestern part of the Boliden domainare either sub-vertically oriented, as the Åkulla E (Dep. 2;Online Resource 2) and Bastuheden (Deps. 8, 9, 10 and 11)deposits, or steeply inclined towards west and southwest asthe Åkulla W (Dep. 1; Online Resource 1) and Kankberg(Dep. 32) deposits. The long axes of the ore bodies plungesteeply towards west and southwest. The Brännan deposit(Dep. 24), south of the latter, dips 65° towards the east. Thesoutheastern part of the domain has S- to SE-steep dips andmoderate to steep NE to SE plunges that characterise theLångdal (Dep. 36; Online Resource 10), Långsele (Dep. 37;Online Resource 11) and Boliden (Deps. 20 and 21; Fig. 4c, dand Online Resource 4) deposits.

Deppis-Näsliden shear zone The steeply W-dipping Deppis-Näsliden shear zone shows down-dip lineations and reverseswest-side-up shear sense, resulting from E–W crustal short-ening (Bergman Weihed 2001). It was interpreted as a large-scale syn-extensional fault and suggested to control the open-ing of a pull-apart basin during the formation of SkellefteGroup volcanic rocks (Skyttä et al. 2009, 2012).

The Rakkejaur deposit (Dep. 57; Online Resource 17), inthe Deppis-Näsliden shear zone, is sub-vertical and steeply Nplunging. This is in contrast with the Näsliden deposits (Deps.49 and 50; Online Resource 14), which show steep dipstowards the southwest with approximately down-dip orienta-tion of the long axes of the ore bodies.

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Kristineberg area The Kristineberg area is characterised bytwo W- to SW-plunging antiforms separated by a shear zone(Fig. 3), which divides the area into a domain with dominantlycoaxial deformation to the north and a transpressional domainto the south (Skyttä et al. 2013). Skyttä et al. (2013) suggestthis geometry to be a result of the formation of splays at thewestern termination of the crustal detachment. Strainpartitioning in the southern antiform resulted in a characteris-tic “flat-steep-flat” geometry with a non-cylindrical, W-plunging fold hinge. The characteristic fault pattern as identi-fied in the central Skellefte district is absent, but the pro-nounced E–W-striking shear zones were suggested to repre-sent inverted normal faults (Skyttä et al. 2013).

Ore bodies in the southern antiform in the Kristineberg areaare generally oriented parallel to the major high-strain zones.The Hornträskviken C (Dep. 29; Fig. 5d and Online Resource6), Kimheden (Dep. 34; Online Resource 8) and Rävlidmyran(Dep. 62; Online Resource 20) deposits, located on the north-ern limb of the southern antiform, are sub-horizontal, whereasthe Kristineberg (Dep. 35; Fig. 5e and Online Resource 9) andRävliden (Dep. 61; Online Resource 19) deposits dip 60°towards south. The long axes of all deposits plunge gently-to-moderately towards the west and southwest, subparallel tothe fold axis. However, the orientation of the Kristinebergdeposit (Dep. 35) can only be determined approximately,since geometries significantly vary with depth as an effect ofthe flat-steep-flat geometry in the Kristineberg area (Skyttäet al. 2013). The Granlunda deposit (Dep. 27) in the northernantiform dips 80° towards the east and plunges moderatelytowards the south.

Aspect ratios of the ore bodies

Since the ore bodies’ primary shapes were variably lensoidalwith unknown dimensions, only a tentative comparison ofapparent strain is possible (c.f. “Methodology” section above).Bearing this in mind, varying apparent strain intensities can beobserved by comparing the 3D models of the ore bodies.Some of the deposits show planar geometries such as theÅsen EC (Dep. 6; Fig. 5c and Online Resource 3) andKedträsk (Dep. 33; Fig. 5b and Online Resource 7) deposits,whereas others are dominantly cigar shaped such as theHornträskviken C (Dep. 29; Fig. 5d and Online Resource 6),Holmtjärn (Dep. 28; Fig. 5f and Online Resource 5) andRävliden (Dep. 61; Online Resource 19) deposits. Plottingthe aspect ratios of the deposits in a modified Flinn diagram(Fig. 6) reveals that deposits from the Snåttermyran domainshow a relatively more oblate deformation than deposits fromthe Boliden and Holmträsk domains. Furthermore, ore bodieswithin the Holmträsk, Boliden and Snåttermyran domainsshow higher apparent strain relative to the ore bodies withinthe Maurliden domain. Ore body shapes in the Maurlidendomain range from relatively oblate to slightly prolate. Ore

bodies in the Kristineberg area range from oblate to prolateshapes with relatively high apparent strain. The Stenbrånetmineralisation (Dep. 72) south of the Skellefte district ischaracterised by oblate deformation and conspicuously highapparent strain.

Discussion

The comparison of the distribution of ore deposits with resultsfrom recent geological studies in the Skellefte district showsthe close spatial relationship of VMS ore deposits and high-strain zones, apart from some exceptions. Furthermore, aremarkable amount of deposits are located at, or close to,intersections of faults. This is in agreement with the presentedidea that the majority of faults in the Skellefte district formedduring crustal extension (Allen et al. 1996; Bauer et al. 2011;Dehghannejad et al. 2012; Skyttä et al. 2012) and that VMSdeposits formed during a phase of crustal extension, utilisingsyn-extensional faults as fluid conduits (Allen et al. 1996;Bauer et al. 2013). However, numerous deposits are hostedwithin volcaniclastic and sedimentary rocks in the vicinity ofthe faults but not exactly within the fault zone. A possiblescenario is that hydrothermal fluids flow along faults and thesulphides precipitate below the sea floor in water-saturated,porous volcaniclastic sediments or precipitate as exhalativesulphides on the sea floor in the vicinity of the fault.

Analysing the orientation and shape of the ore bodies showshow ore body deformation reflects the deformation pattern ofthe structures in the host rocks. For example, the steeply S-dipping ore bodies in the Snåttermyran domain are parallel tothe main foliation, whereas the gentle westerly plunge of theore bodies resembles the mineral lineation in the area.Furthermore, the aspect ratios of the VMS ore bodies showan apparent correlation with the deformation intensity of cer-tain structural domains. Relatively low strain in the Maurlidendomain indicates a relatively high crustal level during defor-mation, whereas higher strain in the Holmträsk and Bolidendomains and the Kristineberg area indicates an increasingcrustal depth at the time of crustal shortening. However, cau-tion has to be exercised with interpretation of the aspect ratiosas the primary shape can only be inferred from less deformeddeposits (e.g. Maurliden E, Dep. 44, Online Resource 13) orfrom younger, less deformed analogues (e.g. Hokuroko dis-trict; Tanimura et al. 1983). The original shape prior to defor-mation remains unknown, hence analysing the aspect ratioscan only give tentative values. For example, deposits with alarge primary extent (e.g. the Kristineberg deposit; Dep. 35)might show higher apparent strain after the deformation than itactually experienced. Nevertheless, bearing this in mind, theaspect ratios of ore bodies, especially for deposits with ton-nages of the same order of magnitude, can act as a tentativecomparison of apparent strain within ore bodies in the district.

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The conspicuously high apparent strain, combined with thegently dipping orientation of the Stenbrånet mineralisation(Dep. 72), results from high-strain non-coaxial deformationat deeper crustal levels (Skyttä et al. 2012). Local strainpartitioning and variations in strain intensity within the struc-tural domains, as reported by Bauer et al. (2011) and Skyttäet al. (2012), are also reflected in the orientation of ore bodies.This can be observed in the Maurliden synform with the sub-vertically oriented Maurliden W deposit, located in the tightlyfolded western part of the synform, and the sub-horizontal togently dipping Maurliden E deposit, on the southern limb ofthe openly folded eastern part of the synform. The NW–SE-striking transfer fault, which separates the high-strainMaurliden W block and the low-strain Maurliden E block,can be traced towards northeast into the Vargfors syncline,where a similar pattern of strain partitioning can be observed(c.f. Bauer et al. 2011). This shows the importance of transferfaults for strain partitioning during crustal shortening in theSkellefte district.

Although re-mobilisation is regarded as a minor feature inthe Kristineberg deposit (Årebäck et al. 2005), indications forre-mobilisation could have been masked by peak metamor-phism post-dating folding and fault inversion (c.f. Skyttä et al.2012) consequently varying with the degree of metamor-phism. Nevertheless, Skyttä et al. (2013) show that the stron-gest deformation in the area of the Kristineberg deposit islocalised in the ore, hence requiring a certain degree of re-mobilisation. Similar patterns of strain partitioning, withhigher strains within the ore bodies relative to that in thesurrounding host rocks, are reported by Talbot (1988) andBergman Weihed et al. (1996). In the Långdal deposit, asignificant amount of re-mobilisation characterises the oreand influences for instance the occurrence of gold in thesystem (Weihed et al. 2002a). This suggests that re-mobilisation of ore minerals due to strain partitioning playedan important role. The intensity of re-mobilisation might besteered by the degree of fluid flow along the nearby fault and,consequently, varies with the size of the latter.

A comparison of the size of VMS deposits with theirlocation shows that the smallest deposits, such as the Näset(Dep. 48), Österbacken (Dep. 54) and Högkulla (Dep.31)deposits, are not associated with known faults, whereas thelarge tonnage deposits, such as Boliden (Dep. 20), RenströmW (Dep. 59) and Rakkejaur (Dep. 57), are associated withdistrict-scale faults. Both regional-scale high-strain zones, theDeppis-Näsliden shear zone (DNSZ) and the Vidsel-Röjnoretshear system host large deposits in terms of tonnage, whereasthe faults controlling the Renström (Deps. 58–60) andPetiknäs (Deps. 55 and 56) deposits could be connectedto the Vidsel-Röjnoret Shear System. The coupling be-tween size of the hosting high-strain zone and associatedVMS deposit could be explained either by (a) favouredfluid flow along large structures during ore formation or

(b) favoured enrichment of ore minerals and associatedre-mobilisation of massive sulphides along large structuresduring shearing. The reason that some deposits, such as theNäset (Dep. 48), Österbacken (Dep. 54) and Högkulla(Dep. 31) deposits, do not show a correlation with knownfaults might be that the controlling faults are so far notdetected or that the hydrothermal fluids utilised other path-ways for circulation (e.g. high permeability in volcaniclasticrocks). Anyhow, the sizes of deposits that cannot be correlat-ed with known faults indicate decreased fluid flow during oreformation compared with the larger deposits. Irrespective ofthe timing of enrichment, the correlation of ore deposit andhigh-strain zone size provides a valuable tool for mineralexploration in the Skellefte district. By constraining apredefined distance to regional shear zones, coupled withthe comparison of distribution and size with the pattern ofhigh-strain zones, areas with increased potential for large-scale deposits can be defined and, hence, provides an impor-tant tool for regional-scale mineral exploration in theSkellefte district. Additionally, the analysis of ore body shapeand orientation can aid near-mine exploration activities, andespecially the interpretation of geophysical data, bypredicting the expected shape and location of VMS orebodies.

Conclusion

The distribution of VMS deposits in the Skellefte districtshows a strong correlation with the regional fault patternresulting from upper crustal extension (D1). The 3D orienta-tion, shape and size of the ore bodies reflect the deformationpattern of the hosting structures and, for this reason, it isinferred that the present-day geometries mainly result fromD2 crustal shortening. The aspect ratios of VMS ore bodiesand the comparison with undeformed equivalents in theHokuroko district, Japan allow an estimation of apparentstrain and show correlation with the deformation intensity ofthe certain structural domains. A comparison of the size ofVMS deposits with their location shows a coupling betweensize of the hosting high-strain zone and size of the associatedVMS deposit, thus providing an important tool for mineralexploration in the Skellefte district or comparable VMS dis-tricts in the world.

Acknowledgments We appreciate the constructive comments by jour-nal reviewers Dr. Joseph Zulu and one anonymous reviewer. Dr. DaveColler is acknowledged for contributions, ideas and discussions. Geolo-gists from the Boliden Group, especially Rolf Jonsson, are thanked forcontributions and support with data. Dr. Michael Stephens from theGeological Survey of Sweden is thanked for valuable comments on themanuscript. This work is part of the VINNOVA 4D modelling of theSkellefte district project funded by VINNOVA and the Boliden Groupand the ProMine project partially funded by the European Commissionunder the Seventh Framework Programme.

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