ARTICLE

3D modelling of hydrothermal alteration associatedwith VHMS deposits in the Kristineberg area, Skelleftedistrict, northern Sweden

Riia M. Chmielowski & Nils Jansson &

Mac Fjellerad Persson & Pia Fagerström

Received: 1 July 2014 /Accepted: 22 December 2014# Springer-Verlag Berlin Heidelberg 2015

Abstract This contribution presents a 3D assessment ofmetamorphosed and deformed, hydrothermally altered volca-nic rocks, hosting the massive sulphide deposits of theKristineberg area in the 1.9 Ga Skellefte mining district innorthern Sweden, using six calculated alteration parameters:the Ishikawa alteration index, the chlorite–carbonate–pyriteindex and calculated net mass changes in MgO, SiO2, Na2Oand Ba. The results, which are also available as film clips inthe Supplementary data, confirm inferences from geologicalmapping; namely that the sericite- and chlorite-rich alterationzones have complex and cross-cutting geometries and thatmost of these zones are semi-regional in extent and rangecontinuously from surface to over a kilometre deep. The majorknown massive sulphide deposits occur proximal to zonescharacterised by coincidence of high values for the alterationindex and chlorite–carbonate–pyrite index and large MgOgains, which corresponds to zones rich in magnesian silicates.These zones are interpreted as the original chlorite-rich,

proximal parts the alteration systems, and form anomalies ex-tending up to 400 m away from the sulphide lenses. In addition,the stratigraphically highest VHMS are hosted by rocks rich intremolite, talc, chlorite and dolomite with lesser clinozoisite,which have high chlorite–carbonate–pyrite index and low–medi-um alteration index values, reflecting a greater importance ofsome chlorite-carbonate alteration at this stratigraphic level.Vectoring towards massive sulphide deposits in this area can beimproved by combining the AI and CCPI indexes with calculat-ed mass changes for key mobile elements. Of the ones modelledin this study, MgO and SiO2 appear to be the most useful.

Keywords 3Dmodel .Lithogeochemistry .VHMSdeposits .

Mass change . Kristineberg . Skellefte district

Introduction

One of the key features of volcanic-hosted massive sulphide(VHMS) deposits is the occurrence of hydrothermally alteredzones in the stratigraphic footwall of the deposits. In somecases, these alteration zones can extend into the stratigraphichangingwall to envelop the deposit. In both cases, the alter-ation zones are typically mineralogically and chemicallyzoned and also volumetrically more extensive than the orelenses themselves (Sangster 1972; Kranidiotis and MacLean1987; Galley 1993; Ohmoto 1996; Hannington et al. 1998;Large et al. 2001b; Goodfellow et al. 2003). Documentingthe geometry, zonation patterns and intensities of such alterationzones is an important step in exploring for newVHMS depositsand also helps to constrain genetic models for the deposits.

Many studies have examined the geochemistry and mineralzonation of hydrothermally altered zones related to VHMSdeposits (e.g. Large et al. 2001b; Barrett et al. 2005; Zuo

Editorial handling: B. Lehmann

Electronic supplementary material The online version of this article(doi:10.1007/s00126-014-0572-x) contains supplementary material,which is available to authorized users.

R. M. Chmielowski (*)Department of Civil, Environmental and Natural ResourcesEngineering, Division of Geosciences, Luleå University ofTechnology, 971 87 Luleå, Swedene-mail: riia.chmielowski@ltu.se

N. Jansson :M. F. Persson : P. FagerströmBoliden Mines, Exploration Department, Kontorsvägen 1,Prospekteringen 936 81, Boliden

Present Address:N. JanssonDepartment of Civil, Environmental and Natural ResourcesEngineering, Division of Geosciences, Luleå University ofTechnology, 971 87 Luleå, Sweden

Miner DepositaDOI 10.1007/s00126-014-0572-x

et al. 2009; Lotfolah Hamedani et al. 2012; Li et al. 2013).However, only in recent years has it become possible to con-tour geochemical data in three dimensions (3D) using tech-niques such as polyharmonic radial basis functions (Carr et al.2001) to interpolate smooth surfaces from scattered datapoints. These advances in computer technology provide animproved understanding of the 3D geometry and chemicalzonation of hydrothermally altered zones, including the masschanges, which in turn help to define the 3D pathways ofmineralizing hydrothermal fluids.

A regional 3Dmodel of the structure and tectonic setting ofthe long-producing Skellefte mining district in northernSweden has recently been developed (e.g. Bauer et al. 2011;Skyttä et al. 2013). However, alteration zones in this area haveto date only been studied in 2D and on the deposit scale (e.g.Schlatter et al. 2003, 2006; Barrett et al. 2005; Schlatter 2007).

In this contribution, we present and interpret the 3Ddistribution of selected alteration parameters in theKristineberg area of the western Skellefte district withthe objective of assisting exploration in the Kristinebergarea. When expanded to other deposits, this approachshould improve our understanding of alteration in theSkellefte district in general.

Regional geology

The Palaeoproterozoic Skellefte district in the Fennoscandianshield of northern Sweden (Fig. 1) has a century-long historyof mining and exploration. It is one of the most importantmining regions in Europe, containing more than 85 pyriticZn–Cu–Au–Ag massive sulphide deposits (Allen et al.1996; Kathol et al. 2005). One of the most important areasin the district is the about 120-km2 Kristineberg area (Fig. 1;Årebäck et al. 2005), which hosts the currently minedKristineberg VHMS deposit (in this paper, the term Bregionalscale^ refers to the entire Skellefte district, whilst Bsemi-regional^ refers to the Kristineberg area as shown inFig. 1a). The Kristineberg mine has produced ∼27.8 Mt, frommore than ten lenses since 1941, at an average grade of 3.74%Zn, 1.04 % Cu, 0.24 % Pb, 1.4 g/t Au and 39 g/t Ag (Janssonet al. 2013). The Kristineberg area also includes several small-er past-producing mines including Rävlidmyran (7.18 Mt,mined 1950–1991), Rävliden (1.56 Mt, mined 1941–1988),Kimheden (0.13 Mt, mined 1967–1969 and 1974–1975) andHornträskviken (0.64 Mt, mined 1981–1991) (Fig. 1a andJansson et al. 2013). The entire Kristineberg area is still ofconsiderable interest for mineral exploration.

The VHMS deposits in the Kristineberg area are hosted bythe Skellefte Group, a thick succession of Palaeoproterozoic(1.9 Ga) greenschist to amphibolite facies, felsic to intermedi-ate, submarine metavolcanic rocks (Fig. 1; Allen et al. 1996;Årebäck et al. 2005). A sample from the Kristineberg

hanging-wall rhyolite has been dated at 1883±6 Ma bySkyttä et al. (2011). The Skellefte Group is overlain bythe dominantly metasedimentary Vargfors Group, whichconsists of fine-grained turbiditic metagraywackes, graphit-ic phyllites, metaconglomerates and mafic-intermediatemetavolcanic rocks and intrusions (Allen et al. 1996;Årebäck et al. 2005).

Massive sulphide deposits occur in this area in two differentstratigraphic settings. One group occurs near the top of theSkellefte Group, close to, or at, the contact with the overlyingVargfors Group (i.e. the Rävliden, Rävlidmyran, and Hornträskorebodies), while the other group occurs lower in stratigraphywithin the Skellefte Group (i.e. the various Kristinebergorebodies, and the Kimheden ore body). The orebodies fromeach of these stratigraphic settings have distinct characteristicsin terms of size, composition, host lithologies, and alterationminerals present (Allen et al. 1996; Årebäck et al. 2005).

In general, the deposits occurring in the lower stratigraphiclevels (e.g. Kristineberg) are dominated by sphalerite, pyriteand chalcopyrite with lesser galena and Ag-rich sulphosaltsand contain local zones of Au enrichment (Årebäck et al.2005). They are hosted by strongly altered rhyolitic rocks,which are now schistose and rich in chlorite, sericite andquartz, with metamorphic porphyroblasts of cordierite andandalusite. Zones of massive pyrite occur in various parts ofthe Kristineberg deposit and also at the Kimheden deposit.

In contrast, deposits in the upper stratigraphic levelsof the Skellefte Group are dominated by sphalerite andpyrrhotite with lesser pyrite, chalcopyrite, arsenopyriteand Ag-rich sulphosalts; they are hosted by rocks richin tremolite, talc, chlorite and dolomite with lesserclinozoisite, and are commonly underlain by chalcopy-rite–pyrrhotite-dominated stringer zones which are dis-cordant relative to stratigraphy. The stringers are developedin intensely chloritized and/or silicified felsic volcanic rocksthat are flanked by quartz-sericite schists.

To the east of the mine, and underlying the SkellefteGroup, is the syn-volcanic calc-alkaline Viterliden intru-sive complex (VIC), dated by Skyttä et al. (2011) at1892±3 and 1889±3 Ma. This complex has intrudedthe volcanic succession that forms the footwall of theKristineberg deposit (Galley and Bailes 1999). Thesupracrustal package is intruded towards the south (Fig. 1)by the Revsund granitoid of late- to post-Svecokarelian age(1.80–1.78 Ga; Skyttä et al. 2012).

Methods

Whole-rock lithogeochemistry

This project made use of 3070 lithogeochemical analysesfrom 212 drill holes and 91 outcrop locations to create various

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Fig. 1 a General geology of the Skellefte district. Intrusions: ViViterliden; Ga Gallejaur; Rg Rengård; Ka Karsträsk; GI, GII, GIII, GIVJörn-type intrusions, phases I–IV (geology after Bergman Weihed 2001;Kathol and Weihed 2005). b A General geology of the Kristineberg area,

Skellefte district, Sweden (modified after Bauer et al. 2014). The namesof the fault blocks (A–G) used in this paper are given in circles. c Locationof the Skellefte District within Scandinavia

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3D models of the alteration. Most of these analyses (2624samples) were derived from earlier studies [includingHannington et al. 2003 (33 samples); Barrett et al. 2005(249 samples)] and legacy data from the database of BolidenMines. However, a total of 446 samples, from 29 drill holes,were collected specifically for this project. All of thelithogeochemical analyses used for this project weremade between 1994 and 2013. The sampled drill holesrange from distal to proximal relative to the knownsulphide deposits.

The new sample set was analysed byAcme Laboratories Ltd.in Canada. Major and selected trace elements were analysed byinductively coupled plasma (ICP)–emission spectroscopy afterlithium borate fusion and nitric acid digestion. Rare earth ele-ments (REEs) and refractory trace elements were analysed byICP–mass spectrometry. Total carbon and sulphur weredetermined by the Leco method. Analytical qualitywas monitored by submitting unidentified reference ma-terials. The same laboratory and procedures were usedfor most of the legacy data (prior to 2001 the laboratorywas called XRAY). However, during the period 1995–2001, 537 samples were analysed at XRAL in Toronto,using X-ray fluorescence methods, with glass beads formajor elements and some trace elements (Ba, Co, Cr,Ni, V, Cu and Zn), and pressed pellets for Zr, Y, Nb,Ga, Mo, Pb, Rb and Sr.

3D data treatment

In this paper, alteration in the Kristineberg area is assessedusing six parameters: two alteration indices and calculatedmass changes in four selected mobile elements. Results foreach parameter are shown as 3D shapes that outline the sizeof the main alteration zones as well as alteration gradients. Thealteration indices used are the Ishikawa alteration indexAI = [100(K2O +MgO)] / (K2O +MgO +Na2O +CaO)(Ishikawa et al. 1976) and the chlorite–carbonate–pyriteindex CCPI=[100(MgO+FeO)]/(MgO+FeO+Na2O+K2O)(Large et al. 2001a).

The Aranz LeapfrogGeo software (version 1.3.1) wasused to generate the 3D surfaces and shapes of thealteration zones, using built-in interpolation algorithms.Interpolation settings (Table 1) were chosen to take intoaccount the direction of the fold hinges of the area,which are undulatory but generally plunge about 20 °to the west. The boundaries for the isosurfaces for theAI and CCPI models were set to correspond to theranges given by Large et al. (2001a). The mass-changemodels were divided first into net gain and net lossgroups (where the isosurface boundary is zero); the re-maining isosurfaces boundaries were set to yield ap-proximately the same number of samples within eachisosurface (Table 2).

The geochemical interpolations have been bounded andconstrained using faults and major lithological contacts fromthe 3D structural model of the Skellefte district proposed bySkyttä (2012) and Skyttä et al. (2012). The upper and westernboundaries of the modelled zone conform to the surface topog-raphy (max elevation=610 m above sea level) and to thewestward-dipping contact between the Skellefte Group and theVargfors Group (Fig. 1a). The base of the zone wastaken as the contact between the Skellefte Group andthe underlying VIC (Fig. 1a) or a maximum depth of2000 m below sea level (the deepest sample is fromapproximately 1400 m below sea level). The northernboundary is defined by a Bdomain boundary^ fault (labelledBD1^ on Fig. 1a and c), which separates the Skellefte Groupfrom the Vargfors Group at the surface, while the southernboundary is defined by the contact with the Revsund granitoid(Fig. 1a). Due to the presence of major faults (Skyttä et al.2013), the geochemical modelling has been carried out sepa-rately in each of seven individual fault blocks, labelled A–G inFig. 1a.

Mass change calculations

The techniques used for calculating mass change in altered sam-ples with respect to least altered precursor lithologies follows

Table 1 Settings used for the 3D interpolations of the alteration parameters examined in this study. AI=Ishikawa alteration index, CCPI in chlorite–carbonate–pyrite alteration index, ΔMgO to ΔBa are calculated mass changes

Model Global trend Ellipsoid ratios Interpolant

Dip Azimuth Pitch Max. Int. Min Interpolant Drift Sill Accuracy Range Nugget

AI 70 176 160 4 4 1 Linear Constant 500 0.500 3000 0

CCPI 70 176 160 4 4 1 Linear Constant 300 0.500 3000 0

ΔMgO 70 176 160 4 4 1 Linear Constant 20 0.300 4000 0

ΔNa2O 70 176 160 4 4 1 Linear Constant 2.0 0.050 3000 0

ΔSiO2 70 176 160 4 4 1 Linear Constant 500 0.200 4000 0

ΔBa 70 176 160 4 4 1 Linear Constant 800,000 10.000 4000 0

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those of MacLean and Barrett (1993) and Barrett et al. (2005).As a first step, samples were grouped by rock type (mafic tofelsic) based on their immobile-element ratios (Table 3, Fig. 2).Samples were then further divided into tholeiitic, transitional,and calc-alkaline affinities based on their Zr/Y ratios (Fig. 2d).For each main rock type, a least-altered sample was selected asan approximation of the precursor composition on the basis ofhaving a near-normal major element composition a low LOIvalue and minimal secondary minerals (Table 4). The least-altered samples define reasonable fractionation trends inplots of various oxides against Zr (Fig. 2). The assumptionwas made that all altered samples were derived from aseries of about ten homogeneous precursor compositions.A mass-change correction factor was then calculated foreach altered sample by dividing the Zr (ppm) value forthe least-altered sample by that of the altered sample (foreach specific rock type). The mass-change correctionfactor was then applied to each oxide and element inthe altered sample to obtain Breconstituted^ values.Finally, the mass change in each oxide or element isobtained by subtracting the Breconstituted^ values fromthe precursor values.

Results

Lithogeochemistry and alteration assemblages

Geochemical classification based on immobile elements

Due to hydrothermal alteration and metamorphism in theKristineberg area, it is not possible to identify the precursorsof the volcanic rocks based on their mineralogy and major-element composition. Precursor rock groups can, however, beidentified using ratios of immobile elements (particularlythose involving Al2O3, TiO2 and Zr). Using this approach,Barrett et al. (2005) recognised several rock groups, includingthree rhyolite types (A, B and X), a dacite, an andesite and amafic group. In the present study, which includes a largersample set and covers a larger area, we have added four furtherrock subtypes (Fig. 2, Table 3).

Alteration indices

When dealing with a very large legacy data set collect-ed by a variety of geologists over several decades, 3Dvisualisation of AI and CCPI variability serves to high-light volumes of anomalous chemistry that can beinterpreted in the context of typical alteration zonationpatterns around VHMS deposits (Large et al. 2001a). Asa first step, before the determination of precursor com-position mass change calculations (see below), we haveused the alteration box plot (Large et al. 2001a) toT

able2

Calculatedinterpolationvalues

fortheboundaries

betweeneach

interpolated

shell(values

norm

alfont)andthenumberof

samples

with

values

falling

with

intheinterpolated

shells(valuesin

italics)

Interpolated

values

formasschanges

inMgO

,Na2O,S

iO2andBa

Darkred

Red

Pink

Purple

Light

blue

Med

blue

Darkblue

Black

Total

ΔBa

490

−630

492

−290

497

0347

200

337

435

336

765

342

2841

ΔNa 2O

688

−4.61

691

−4.13

680

−3.15

688

0116

2863

ΔMgO

468

−0.95

474

0474

0.95

488

1.9

462

3.5

479

2845

ΔSiO

2379

−14.5

392

−7.5

372

−3.5

391

0336

2.9

334

7.5

332

16.0

327

2863

Interpolated

values

forthecalculated

AIandCCPIalteratio

nindexes

Grey

Blue

Purple

Red

Darkred

AI

9325

619

65333

80453

901572

3070

CCPI

527

50502

60681

70573

80787

3070

The

names

ofthecoloursused

torepresenteachshellare

show

nabovethelistin

gs

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display general alteration effects and to group samplesbased on their altered composition. In Fig. 3a, whichshows all rock samples from the Kristineberg area, mostsamples show high to very high AI values (see alsoTable 2). Samples with both high AI and high CCPItend to be concentrated within 400 m of the knownore deposits (Fig. 3b). Samples with only one high in-dex are found both near the deposits and distally tothem (usually within 1.6 km) (Fig. 3b). Samples withCCPI >80 but AI <25 occur in only two areas: (1) theupper part of the Skellefte Group near the Vargfors con-tact (at elevations between ∼400 m above and ∼100 m belowsea level) both between the Rävliden and Rävlidmyran oredeposits and extending up to 2.7 km to the south of theRävlidmyran deposit; and (2) about 200 m to the north-west of the L-lens of the Kristineberg ore deposit, at adepth of ∼380 m below sea level.

It is also possible to express the composition of the chem-ically altered samples in terms of the hydrothermal alterationminerals that were likely present prior to regional metamor-phism, namely chlorite, sericite, carbonate and pyrite, asshown in Fig. 5a for five main mineral groups (cf. Largeet al. 2001a). Samples with CCPI >80 and/or AI >80, areinferred to have originally comprised zones of strong carbon-ate, chlorite–carbonate, chlorite–pyrite, sericite–chlorite andsericite alteration. These minerals in part were subsequentlyconverted to metamorphic phases such as cordierite, andalu-site, phlogopite and tremolite. Geologic mapping in this areaindicates that the locations of the various alteration zones havecross-cutting relationships with the stratigraphy of the area(e.g. Schlatter et al. 2003)

About half of the strongly altered samples in theBmineralogy^ box plot are dominated by sericite–chlorite.Although many of these samples are located adjacent tothe known orebodies, some occur distally to them(Fig. 4b). Within the sericite–chlorite group, sampleswith AI >90 are notably more abundant than those with

AI 80–90 (834 vs 227 samples, respectively). However,the latter group shows a stronger spatial correlation withthe known orebodies.

About one quarter of the strongly altered samples in theBmineralogy^ box plot are dominated by chlorite–pyrite.These samples are generally located close to the knownorebodies, with only a few of them occurring in other areas(Fig. 4c). The chlorite–pyrite samples with AI=80–90 show astronger correlation with known ore deposits than do those withAI >90.

Samples with inferred strong carbonate or chlorite–car-bonate alteration tend to be located in the area east ofKristineberg and near the contact with the VargforsGroup; they are fairly proximal to the stratigraphicallyhigher deposits at Rävliden and Rävlidmyran, where zonesrich in dolomite and/or tremolite are locally associatedwith the orebodies (Årebäck et al. 2005) (Fig. 4d, e). Incontrast, zones dominated by sericite occur at thestratigraphically deeper level of the Kristineberg mine,but are rather distal to the orebodies (Fig. 4f).

Mass changes

Calculated mass-change results for the main mobile el-ements are summarised as a series of histograms inFig. 5. The results (mass gains and mass losses) foreach element tend to be normally distributed, with thepeaks around zero. However, in some cases, the distri-bution is asymmetrical; this is related to differences ininitial sample composition and how much of a givenelement can be lost from a sample. However, all rocktypes show a consistent and substantial loss in Na2O,and consistent gains in K2O. The peaks for both MgOand FeO show a slight positive shift. As discussed be-low, samples with high MgO gains show a spatial rela-tion to known ore deposits.

Table 3 Rock types

The ranges of composition for theimmobile-element ratios used toclassify the various rock types andthe number of samples withineach type

No. of samples Zr/Al2O3 Al2O3/TiO2 Zr/TiO2 Zr/Y

Lowest Highest Lowest Highest Lowest Highest Lowest Highest

Rhyolite X 170 11.7 16.6 45.1 56.7 496.8 746.2 3.2 25.0

Rhyolite A 317 15.8 23.6 34.0 52.2 689.1 989.5 3.5 32.5

Rhyolite B 831 15.4 20.3 30.7 41.7 580.4 719.4 4.5 33.2

Rhyolite C 384 14.0 18.8 27.8 39.8 465.9 604.1 2.8 32.9

Dacite A 335 11.7 17.2 24.0 38.6 383.5 509.0 4.4 39.0

Dacite B 478 8.1 15.9 20.6 35.0 256.4 408.8 3.9 41.2

Andesite A 204 7.3 11.1 19.3 34.3 166.0 271.0 3.0 60.9

Andesite B 169 4.8 7.5 17.3 32.6 126.2 214.5 2.9 56.2

Mafic A 41 6.6 7.0 13.6 30.1 67.2 128.0 3.1 6.4

Mafic B 18 2.0 3.5 12.9 20.9 36.7 72.6 1.2 4.4

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OlA

32

%

Zr ppm

f

y = -0.033x + 19.878R² = 0.635

2

4

6

8

10

12

14

16

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24

00 100 20050 150

OiT

2%

Zr ppm

e

y = -0.563ln(x) + 3.321R² = 0.790

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

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Y

Zr/TiO2

Rhy X

DaciteA

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0

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Calc-alkaline

Tholeittic

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Al O /TiO2 3 2

c

0 5 10 15 20 25 30 35 40 45 50 55 600

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Colour

Mafic B

Mafic A

Andesite B

Andesite A

Dacite B

Dacite A

Rhyolite X

Rhyolite C

Rhyolite B

Rhyolite A

Fig. 2 Rock-type classifications based on immobile-element ratios, cal-culated using ppm values for Zr and wt% values for TiO2 and Al2O3. a Zr/Al2O3 vs Al2O3/TiO2. b Zr/Al2O3 vs Zr/TiO2. c Zr/TiO2 vsAl2O3/TiO2. dZr/Yvs Zr/TiO2. e TiO2 vs Zr. fAl2O3 vs Zr. The regression lines for eachrock type are ideal alteration lines along which all altered samples wouldlie if they were derived from a series of compositionally distinct,

homogenous precursors. The heavy dashed curves and equations in eand f are the estimated igneous trends based on curve fitting of least-altered samples (using Excel). Samples plotting towards the origin haveexperienced net mass addition, whereas those plotting in the oppositedirection away from the trends have experienced net mass loss

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Table 4 Chemical composition of least-altered samples of each rock type and affinity within the Kristineberg area, northern Sweden

HoleID Mafic B Andesite A Mafic A Dacite A Dacite B Rhy A Rhy X

HOO15 KRC2627 KRC3540 KRC2627 KRC2541 R2E1774 Outcrop13

Depth (m) 28.7 729.9 83.1 256.4 644.1 373.9 Surface

SiO2 (wt%) 49.41 64.13 51.09 68.77 66.89 70.89 72.34

Al2O3 (wt%) 16.85 15.42 15.72 12.72 14.74 14.15 14.28

FeO (wt%) 11.71 5.43 11.34 2.43 4.86 2.92 2.53

MgO (wt%) 6.16 4.07 5.95 3.09 2.11 0.62 0.65

CaO (wt%) 4.48 1.74 7.21 2.74 0.94 1.55 0.72

Na2O (wt%) 3.20 4.81 3.57 3.67 4.71 4.52 5.59

K2O (wt%) 1.73 0.69 1.50 1.05 1.04 3.43 2.36

MnO (wt%) 0.15 0.07 0.17 0.02 0.01 0.06 0.04

TiO2 (wt%) 1.302 0.523 0.678 0.363 0.45 0.33 0.322

Cr2O3 (wt%) 0.01 0.002 0.011 0.009 0.01 0.019 0.013

P2O5 (wt%) 0.223 0.234 0.166 0.079 0.23 0.075 0.06

As (ppm) 0.025 0.7 0.8 0 0 2.7 260.2

Ba (ppm) 184 315 365 349 701 816 895

Y (ppm) 20.2 28 14.2 25 22.8 44 40.6

Zn (ppm) 151 26.4 50 7 9 121 96

Zr (ppm) 52 118.1 78.3 157.6 137.2 264.7 208.2

LOI 3.2 2.1 1 4.7 3.4 1.1 0.7

Sum 99.75 99.84 99.73 99.9 99.99 99.84 99.84

Zr/Y 2.6 4.5 5.5 6.3 6.0 6.0 5.1

Affinity Tholeiitic Tholeiitic Transitional Transitional Transitional Transitional Transitional

AI 50.7 42.1 40.9 39.2 35.8 40.0 32.3

CCPI 78.4 63.3 77.3 53.9 54.8 30.8 28.6

HoleID Andesite B Dacite A Dacite B Rhy A Rhy B Rhy C Rhy X

R2F654 KRC1994 KRC3518 KRC2781 KRC1033 KRC3114 KRC2627

Depth (m) 585.9 700.6 41.9 648.7 1005.2 315.6 804.7

SiO2 (wt%) 57.19 71.18 66.16 76.73 74.21 74.99 72.79

Al2O3 (wt%) 16.18 13.88 15.31 12.86 12.54 12.84 13.06

FeO (wt%) 6.02 4.13 3.78 1.53 2.34 2.16 3.67

MgO (wt%) 4.84 1.78 4.44 0.9 1.33 0.94 1.33

CaO (wt%) 7.64 2.48 0.42 0.79 0.29 1.09 2.09

Na2O (wt%) 4.96 4.45 3.21 4.67 5.03 4.28 4.67

K2O (wt%) 0.28 0.65 1.68 1.07 0.93 1.30 0.41

MnO (wt%) 0.13 0.03 0.15 0.07 0.02 0.03 0.02

TiO2 (wt%) 0.591 0.426 0.516 0.338 0.326 0.383 0.324

Cr2O3 (wt%) 0.012 0.001 0 0.007 0 0 0.002

P2O5 (wt%) 0.157 0.14 0.125 0.021 0.049 0.079 0.064

As (ppm) 189.6 – 1 2.1 1.1 3.7 0.6

Ba (ppm) 158 328 1440 517 1206 376 116

Y (ppm) 21 18.3 17.9 22.2 19.8 21.3 14.9

Zn (ppm) 16.1 27 191 40 24 107 5

Zr (ppm) 114.0 176.2 175.4 248 210.3 198.8 185.2

LOI 1.1 0.55 3.4 1.8 7.7 1.5 4.8

Sum 99.76 100.2 99.67 100.2 99.89 99.85 99.99

Zr/Y 7.1 9.6 9.8 11.2 10.6 9.3 12.4

Affinity Calc-alkaline Calc-alkaline Calc-alkaline Calc-alkaline Calc-alkaline Calc-alkaline Calc-alkaline

AI 28.9 26.0 62.8 26.5 29.8 29.4 20.5

CCPI 67.5 53.7 62.7 29.7 38.1 35.7 49.6

See Fig. 2 for the compositional range of each rock type. Immobile-element ratios for the main rock types at the Kristineberg mine itself are given inBarrett et al. (2005)

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Some samples show large losses in both Na2O and CaO.About 60 % of these samples fall plot in the chlorite–pyritefield of Fig. 4, with nearly 10 % falling in the sericite field.Most of these samples show gains in MgO, FeO and K2O,which suggests that both chloritisation and sericitisation haveoccurred. However, those with the largest gains in MgO+FeOshow a loss in K2O, implying that very strong chloritisationcan also remove K from the rocks. Conversely, in the mostsericitised samples (highest K gains), MgO+FeO have beenlost from the rock.

3D model interpolations

Based on 3D contouring of the Kristineberg data, interpo-lated values have been calculated for six alteration param-eters: CCPI, AI and mass changes in SiO2, MgO, Na2Oand Ba (Table 5). Results are shown in Figs. 6 to 8 onthree lines of section, both subparallel and normal to thestrike of the orebodies. Each of the section lines in thesethree figures intersects one or more orebodies. The objec-tive of these plots is to better understand the spatial

Colour

Size

AI < 25

CCPI > 80

AI 25 - 65

CCPI 70 - 80

AI 65 - 80

CCPI 60 - 70

AI 80 - 90

CCPI 50 - 60

AI > 90

CCPI < 50

Alteration Box Plot

AI

Least-Altered Rhyolite

Least-Altered Dacite

Least-Altered Andesite-Basalt

IPC

C

a

10050

50

10

10

00

20

20

30

30

40

40

60

60

70

70

80

80

90

90

100

b

Kristineberg A-Lens

Kristineberg L-Lens

Kimheden

Rävlidmyran

Rävliden

Plunge +19Azimuth 022

0 500 1000

Fig. 3 a Alteration box plot. AI variations are shown by symbol colour,and CCPI variations shown by symbol size. Five Bbins^ were used foreach variable. b 3D plot of alteration indices for samples in the

Kristineberg region. This region includes the Rävlidmyran, Rävliden,Kimheden and Kristineberg orebodies (shown in black)

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Colour

chl-carb

carb

Ser-chl (AI >90)

Ser-chl

sericite (AI > 90)

sericit e

Chl-Py (AI > 90)

Chl-Py

Size

AI 80-98 and/or CCPI > 80

AI > 90

AI & CCPI < 80

sericite-chlorite altered samples

Plunge +22Azimuth 032

0 1000

1500500

b

AI

Fig. 4 Alteration box plot as in Fig. 3a, showing fields of strongestalteration indicated by large circles and likely primary alterationminerals (indicated by different colours). b–f 3D plots showinglocations of strongly altered samples dominated by sericite-chlorite (b),chlorite-pyrite (c), carbonate (d), chlorite-carbonate (e) and sericite (f).

Also shown are the Kristineberg, Kimheden, Rävliden and Rävlidmyranorebodies (black), and the trace of the contact between the SkellefteGroup and the Vargfors Group at an elevation of +425 (approximatelythe topographic surface). Offsets of this contact are due to faults (seeFig. 1)

Miner Deposita

relation between alteration zones and the locations of oredeposits. The alteration results are also shown as videoclips in the Supplementary data files (one for each alter-ation parameter).

Discussion

Relationship to ore deposits

Very high CCPI values are associated with most of theKristineberg ore lenses. CCPI values are generally somewhatlower adjacent to orebodies from the higher stratigraphic set-ting (Rävliden, Rävlidmyran, Hornträsk), as well as theKristineberg L-lens and Kimheden orebodies, although theseareas do contain some samples with very high CCPI values.

The known ore deposits are mainly located within areas ofhigh AI values (and decreasing beyond about 400 m). In gen-eral, the size of the deposit is roughly proportional to the sizeof the high-AI shell. For example, the high-AI zones aroundthe two biggest VHMS deposits in the area (Kristineberg andRävlidmyran) are much more extensive than that around thesmaller Hornträsk deposit (the high AI shells for Kristinebergand Rävlidmyran are >2 km wide and 5 km long, while theone associated with Hornträsk is <2 km long and 500mwide).

AI values around and particularly directly above thestratigraphically higher deposits (Rävliden, Rävlidmyran andHornträsk) are commonly not much elevated despite the pres-ence of significantly alteration as inferred from drillcore log-ging. The Rävliden deposit, for example, is located partly inAI 65–80 shell and partly in the AI 25–65 shells. The com-monly lower AI values around orebodies in thestratigraphically higher levels of the Skellefte groupare probably due to a higher proportion of carbonatesand metamorphically derived tremolite relative to thedeeper Kristineberg level. The carbonate-bearing rockshave generally experienced less Na depletion and Kaddition, which, together with dolomite addition, leadsto lower AI values.

Large Na losses are found near most of the orebodies, usu-ally to one side of them. The larger zones of Na loss arelocated at deeper stratigraphic levels (Kristineberg andKimheden orebodies), reflecting extensive plagioclase-destructive hydrothermal alteration.

Large MgO gains occur near all of the orebodies, usuallyconcentrated towards one side of the orebody; these areinterpreted as chlorite-dominated zones. The largest zone ofMgO gain is located at depth near the northwest end of theKristineberg orebodies and extends up to 2.5 km from theseorebodies. Smaller zones of high MgO gain are present on the

sericite altered samples

f

Plunge +22Azimuth 032

0 1000

1500500

Plunge +22Azimuth 032

0 1000

1500500Plunge +22Azimuth 032

0 1000

1500500

Plunge +22Azimuth 032

0 1000

1500500

dc

chlorite-carbonate altered samples

chlorite-pyrite altered samples

e

carbonate altered samples

Fig. 4 (continued)

Miner Deposita

north side of the Rävliden and Rävlidmyran orebodies and onthe south west sides of the Hornträsk and Kimheden orebodies.

SiO2 mass changes show an interesting distribution:The largest gains occur near one end of each orebody,while SiO2 loss commonlyoccurs near the other end.Silica gain is mainly represented by the addition of freequartz. Silica loss typically occurs in zones of strongchloritisation and/or sericitisation.

The largest Ba gains occur near the Kimheden deposit, withmoderate gains near the Kristineberg L-lens. Ba shows a fullspectrum of gains to losses around all other orebodies.

These findings are compatible with those of Barrettet al. (2005), who showed that most of the volcanicrocks in the mine area are strongly depleted in Na, withvariable gains in Mg and Fe, and large gains to largelosses in Si.

Fig. 5 Histograms showing the number of samples gaining or losingspecific amounts of each element with respect to the least-altered sampleof the same rock type.Mass changes are given as absolute amounts (eitherwt% or ppm). Only a few elements have been consistently added (K2O)or removed (CaO, Sr, Na2O) across all rock types. The histograms were

calculated using the smallest number of bins necessary to bring the countwithin each bin to under 600. For those elements that have only a fewsamples in the highest and/or lowest bins, the X-axis has been shortenedin order to focus on the main cluster of samples

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Footprint of the Kristineberg hydrothermal alteration system

Although the Kristineberg hydrothermal alteration system issemi-regional in extent, only a small portion of this area con-tains known massive sulphide ore deposits. Many VHMS de-posits are associated with hydrothermal upflow zones, whichin turn follow synvolcanic faults that were active at the time ofmineralisation (e.g. Galley et al. 2007). Although it is difficultto recognise syn-volcanic faults where regional metamor-phism and post-ore deformation are strong, it has been sug-gested that many of the known faults in the central Skelleftedistrict are re-activated normal faults associated with tectonicinversion (Allen et al. 1996; Bergman Weihed 2001; Baueret al. 2011), and similar models have been proposed for theKristineberg area (Skyttä et al. 2013; Bauer et al. 2014).

The area south of the Kristineberg A- and B-lenses hasbeen interpreted as the stratigraphic hangingwall by Årebäcket al. (2005). The alteration at surface is, in fact, less intense tothe south of the A-lens (Fig. 8h). However, at depth (−290 to

−900 m), the alteration is generally more intense to the south.If the Kristineberg host succession is stratigraphically coher-ent, the alteration pattern to the south of the A-lens wouldimply that hydrothermal fluids penetrated hangingwall strataafter deposition of the A-lens orebody. However, it is alsoconceivable that the apparent alteration Breversal^ at depth isthe result of repetition of an interval of altered footwall rocksby late folding or thrusting.

Although AI and CCPI values are high near all of theorebodies, they are highest near the Kristineberg andKimheden deposits and the volcanic rocks near theKristineberg and Kimheden orebodies, which are locateddeepest in the original volcanic sequence. The high AI andCCPI values result from larger orebodies showing larger de-pletions of Na2O andmore consistent gains inMgO than at theother orebodies. These mass changes (as well as silica loss)point towards more extensive zones of chloritisation aroundthe deeper orebodies. The Kristineberg L-lens is an exceptionto these orebody size vs alteration patterns, as samples

2 3

A’Ag

Interpolated CCPI

A’A

d

0004361+

)X(tsa

E+16

3200

0

+16

3000

000 082 61+

A’

2

Ae

+16

3400

0)

X(tsaE

000236 1++

1630

000

+16

2800

0

A’

A

b

A’Ah

> 90

65 - 80

26 - 65

< 25

80 - 90

> 80

60 - 70

50 - 60

< 25

70 - 80 < -0.95

--

--

--

--

--

--MgOAI CCPI

0 - 0.95

0.95 to 1.9

1.9 to 3.5

> 3.5

-0.95 to 0

niagssa

Mssol

ssaM

< -4.16

-4.13 to -3.15

-3.15 to 0

> 0

-4.16 to -4.13

Na O2

< -14.5

SiO2

-7.5 to -3.5

-3.5 to 0

0 to 2.9

2.9 to 7.5

7.5 to 16

>16

-14.5 to -7.5 < -630

Ba

0 to 200

200 to 435

435 to 765

>765

-630 to -290

-290 to 0

A’

A

a

A’+5000

Elev (Z)

-500-1000-1500

-2000

A

f

A+5000

Elev (Z)

-500-1000-1500

-2000

cA’

InterpolatedAI

block D

blockE blockFblockG

block C

0004361+

)X(tsa

E+16

3200

0

+16

3000

00008261+

Alterationparamatercolour codes forFigures 6-8

Fig. 6 Interpolated 3D values of alteration parameters on section A–A′through the Kristineberg area. a Location of section A–A′ on geologicalmap. Note that this section is subparallel to the regional fold axes. b Planview showing interpolated surface values of alteration index (AI). c–hInterpolated values along section A–A′ for AI, CCPI, ΔSiO2, ΔMgO,ΔNa2O and ΔBa, respectively. The Δ symbol indicates calculated mass

change. The colour scheme and numerical ranges used to depict thedifferent degrees of alteration are shown to the left of the index map.MgO, Na2O and SiO2 are in wt%, Ba is in ppm. The Kristinebergorebodies are shown in yellow on all sections; the main fault blocks areidentified on the first section

Miner Deposita

+5000

Elev (Z)

-500-1000-1500

-2000

e

B’B

2 3

+5000

Elev (Z)

-500-1000-1500

-2000

f

B’B

2

+5000

Elev (Z)

-500-1000-1500

-2000

g

B’B

h

B’

Ba

block D

bloc

k E

bloc

k F block G

block C

+5000

Elev (Z)

-500-1000-1500

-2000

c

Interpolated AI

B’B

+72

1800

0

+72

2000

0

+72

2200

0

+72

2400

0 )Y( htr o

N

Interpolated CCPI

+5000

Elev (Z)

-500-1000-1500

-2000

d

B’B+

7218

000

+72

2000

0

+72

2200

0

+72

2400

0 )Y( htr o

N

+7218000

+7220000

+7222000

North (Y)+7224000

+16

2400

0

+16

3000

0

+16

2600

0

+16

2800

0

B’

Bb

Interpolated ∆Ba

+5000

Elev (Z)

-500-1000-1500

-2000

B’B

Fig. 7 Interpolated 3D values of alteration parameters on section B–B′across the Kristineberg area. a Location of section B–B′ on geologicalmap. b Plan view showing interpolated surface values of alteration index(AI). c–h Interpolated values along section B–B′ for AI, CCPI, ΔMgO,

ΔNa2O, ΔSiO2 and ΔBa, respectively. The Δ symbol indicates calculatedmass change. The main fault blocks are identified on the first section; theKristineberg A-lens orebody is shown in yellow on all sections. Colourcodes for the alteration parameters are given in Fig. 6

Miner Deposita

collected from its host rocks tend to have lower CCPI valuesthan those at the other orebodies, which may be explained bythe fact that a number of these show strong losses in MgO (asmuch as −4.35). Note that a few (∼15%) of the samples in thisarea with strong losses in MgO also show a strong enrichmentin CaO (up to 8.4), while others (∼10 %) show little to noenrichment in any of the elements.

While it is fairly typical in the Kristineberg area for both theAI and CCPI values to be high if the other is, there are samplespresent that have a very different alteration pattern—the com-bination of high (>80) CCPI values with very low (<25) AIvalues. This occurs near the top of the Skellefte Group, whereit extends several kilometres southwards from the Rävlidenand Rävlidmyran orebodies below the contact with theVargfors Group (to depths of only 130 m below sea level).This high-CCPI/low AI pattern also occurs at deeperlevels, to the north side of the L-lens (at elevations of300–900 m below sea level). Large et al (2001a)discussed two alteration trends that result in high CCPIand low AI values, chlorite–carbonate and sericite–carbon-ate alteration. They indicate that the former is developedin the footwall adjacent to massive sulphide lenses, whilethe latter is developed in the hanging wall to massivesulphide deposits, or located along a favourable strati-graphic unit.

Conclusions

3D modelling of the AI and CCPI alteration indices in com-binationwith calculatedmass changes for key elements allowsthe 3D shapes of stronger versus weaker alteration zones to beoutlined and thus can identify the more proximal portions ofhydrothermal systems. Ultimately, it might be possible to rec-ognise Bhot^ zones of fluid upflow that point towards seafloordischarge locations where massive sulphides might haveformed. In the Kristineberg area, the deposits are associatedwith zones of high MgO gains, and zones of either high gainsor losses in SiO2, regardless of stratigraphic level. The largest

deposits on both the lower and upper stratigraphic levels—Kristineberg and Rävlidmyran, respectively—are associatedwith the zones of highest MgO and SiO2 gain. Based on theinferred-mineralogy box plot (Fig. 4a), the original alterationzones around the orebodies were dominated by either chlo-rite–pyrite or carbonate–chlorite, with the latter assemblagemore common in the upper stratigraphic levels of theSkellefte Group (Fig. 4b).

Although the AI and CCPI indices are useful for mappingout the extent and geometries of semi-regional alterationzones, especially if the precursor lithologies have a relativelyuniform composition, definition of the more prospectivezones is best achieved by also factoring in calculated masschanges for the mobile elements. At deeper levels within theSkellefte Group, the best exploration targets should show acombination of high AI, high CCPI, gains in MgO, substantialgains or losses in SiO2 and near complete losses in Na2O. Athigher levels, less emphasis should be placed on AI and CCPIvalues, and more on calculated mass changes for MgO, whichprovide a better vector to mineralisation.

The present study shows that alteration features previouslydocumented in the Kristineberg mine (Barrett et al. 2005),such as Mg addition in chloritised zones, form parts of muchlarger alteration zones that are semi-regional in extent, andcompositionally zoned. Within these large volumes of hydro-thermally altered rocks, the stratigraphically lower depositsare generally accompanied by zones of coincident high AIand high CCPI, whereas the stratigraphically higher depositscommonly display lower AI. These differences reflect a great-er abundance of hydrothermally derived dolomitic carbonatesand metamorphically derived tremolite skarns at the higherlevel of the Rävliden–Rävlidmyran deposits, as opposed tothe originally more phyllosilicate-dominated alteration assem-blages (now including metamorphically derived cordierite andandalusite) associated with the deeper-level Kristineberg orelenses. Vectoring towards massive sulphide deposits in thearea can be improved by combining the AI and CCPI indexeswith calculated mass changes for key mobile elements. Of

Table 5 General features of alteration parameters in the vicinity of orebodies in the Kristineberg region

Kristineberg A, E,EW, J and K lenses

Kristineberg A lens Kristineberg L lens Kimheden Rävliden Hornträsk Rävlidmyran

Section A–A′ B–B′ C–C′ (Not available) (Not available) (Not available) (Not available)

AI High High High High Med Med High

CCPI High High Med Med Med–high Med Med–high

ΔMgO Greatest gain Greatest gain Greatest loss Med gain Greatest gain Transition fromloss to gain

Greatest gain

ΔNa2O Greatest loss Greatest loss Greatest loss Greatest loss Med loss Low loss Greatest loss

ΔSiO2 Full spectrum Full spectrum Full spectrum Greatest gain Greatest gain Full spectrum Greatest gain

ΔBa Full spectrum Full spectrum Moderate gain Greatest gain Full spectrum Full spectrum Full spectrum

AI=Ishikawa alteration index, CCPI in chlorite–carbonate–pyrite alteration index, ΔMgO to ΔBa are calculated mass changes. See Figs. 6, 7 and 8 forlocations of sections A–A′, B–B′ and C–C′. Additional alteration details are shown in the 3D video clips provided as an appendix to this paper

Miner Deposita

00032 27+

)Y(

htroN+

7221

00000 0912 7

++72

1700

0

+5000

Elev (Z)

-500-1000-1500

-2000

c

Interpolated AI

bloc

k D

bloc

k Ebl

ock

F

block G

blockC

block A

blockB

C’C

Interpolated CCPI

C C’+5000

Elev (Z)

-500-1000-1500

-2000

d

+5000

Elev (Z)

-500-1000-1500

-2000

e

’CC C

Interpolated 2 3

C’+5000

Elev (Z)

-500-1000-1500

-2000

f

C C’

2

+5000

Elev (Z)

-500-1000-1500

-2000

g

C C’+5000

Elev (Z)

-500-1000-1500

-2000

h

0003227+

)Y(

htroN+

7221

0000009127

++72

1700

0

C’

C

a

C’

C

b

North (Y)+7223000

+7221000

+7219000

+7217000

0004261+ +

1630

0000006261

+ +16

2800

0

Fig. 8 Interpolated 3D values of alteration parameters on section C–C′across the Kristineberg area. a Location of section C–C′ on geologicalmap. b Plan view showing interpolated surface values of alteration index(AI). c–h Interpolated values along section C–C′ for AI, CCPI, ΔMgO,

ΔNa2O, ΔSiO2 and ΔBa, respectively. The Δ symbol indicates calculatedmass change. The main fault blocks are identified on the first section; theKristineberg L-lens orebody is shown in yellow in all sections. Colourcodes for the alteration parameters are given in Fig. 6

Miner Deposita

these, MgO and SiO2 appear most useful. MgO is ubiquitous-ly elevated proximal to the known massive sulphide depositsat both stratigraphic levels. SiO2 displays large mass gains andlarge mass losses near the known deposits, The large amountsof Mg added to the alteration zones in the Kristineberg areaindicates that relatively normal seawater was commonlyentrained into the volcanic sequence near the orebodies, whilethe deposition of silica (and carbonate) points towards rapidcooling of fluids at or near the seafloor.

Acknowledgements The project was undertaken by Luleå University ofTechnology with the support of Boliden Mines. We would like to expressour appreciation to PärWeihed, and Rodney Allen, who were instrumentalin getting this collaboration underway. We thank the many geologists atBoliden who have, over the years, collected the drill core samples andlithogeochemical analyses, which formed an essential part of this study.We also thank the personnel at the Boliden core facility for retrieving fromstorage many historic drill holes for the new lithogeochemical sampling.Tim Barrett and Thomas Monecke provided thoughtful and comprehen-sive reviews, which considerably improved the quality of this manuscript.Additional thanks are due to Tim Barrett for his willingness to send helpfulreplies to queries sent him during all stages of this research project.

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