Chapter 5 The Pechenga Ni-Cu Sulfide Deposits ...€¦ · sulfide-bearing crustal rocks (e.g.,...

145

IntroductionAS SUMMARIZED by Naldrett (2004, p. 613), generation of ex-ploitable magmatic Ni-Cu sulfide deposits is a consequenceof a long chain of geologic processes and dependent on sev-eral essential factors: (1) in the mantle source region, the de-gree of partial melting should be large enough for magma tobecome undersaturated in sulfide and consequently acquire asufficiently high concentration of chalcophile elements, (2)the magma should remain undersaturated in sulfide during itsascent to crustal levels where, (3) an immiscible sulfide liquidcan segregate from the magma due to changes in intensiveparameters or magma composition, (4) separated sulfide liq-uid should react with a sufficiently large amount of magma tocollect economic amounts of base and precious metals, and(5) in order to constitute a commercially viable deposit, thesulfides should be mechanically concentrated in restrictedplaces by a suitable hydrodynamic magma flow regime orgravitational forces. There is ample evidence that in manycases the cause of sulfide immiscibility has been the additionof external sulfur to magma through its contamination withsulfide-bearing crustal rocks (e.g., Lesher and Groves, 1986;Ripley et al., 2002; Keays and Lightfoot, 2010).

The importance of magmatic plumbing systems, relevant tofactors (4) and (5), has become evident, for example, in thestudy of the Noril’sk Ni-Cu deposits. These deposits are related

to open-system magma chambers where upgrading of sulfidicore took place by reaction with passing magma (e.g., Li et al.,2009). In contrast, Ni-Cu deposits of Voisey’s Bay andJinchuan were generated by accumulation of magmatic sul-fides in a narrow feeder conduit between two gabbroic intru-sions or an ultramafic feeder below a funnel-shaped differen-tiated intrusion, respectively (Naldrett et al., 2009; Song etal., 2009). It appears that rich economic deposits rarely formwithout strong hydrodynamic forces leading to mechanicalenrichment of sulfides (Naldrett and Campbell, 1982; Riceand Moore, 2001; Naldrett, 2004). As a result of open-systembehavior of magmatic bodies, metal tenors of sulfides canvary widely within a Ni-Cu deposit or between different de-posits, and the sulfide/silicate ratio within a body can be ap-preciably higher than expected on the basis of sulfur solubil-ity in silicate magma. Also, the immediate country rocks maynot have necessarily been the main source of external sulfurfor the ore-bearing intrusions, even though they may be richin sulfides, and isotopic disequilibrium within a magmaticbody may be recognized.

Economically important Paleoproterozoic Ni-Cu sulfidedeposits at Pechenga, northwestern Russia, were discoveredin the 1920s and have been exploited without interruptionsince the 1940s. They are associated with the lower parts ofdifferentiated, sill-like ultramafic-mafic bodies emplaced intoa sedimentary environment. From the beginning of the re-search history, there has been a consensus that the depositsrepresent magmatic ores generated by immiscible sulfidemelt exsolved from a silicate magma (e.g., Gorbunov, 1968).In the first comprehensive account on the ores, Väyrynen

Chapter 5

The Pechenga Ni-Cu Sulfide Deposits, Northwestern Russia: A Review with New Constraints from the Feeder Dikes

EERO J. HANSKI,1,† ZHEN-YU LUO,1,* HARRY ODURO,2 RICHARD J. WALKER2

1 Department of Geosciences, P.O. Box 3000, 90014 University of Oulu, Finland2 Department of Geology, University of Maryland, College Park, Maryland 20742

AbstractThe Paleoproterozoic, synvolcanic Ni-Cu sulfide deposits at Pechenga are hosted by conformable, sill-like

ferropicritic differentiated intrusions, injected into carbonaceous and sulfidic graywackes and shales of the Pro-ductive Formation. Due to the presence of abundant sulfides in the country rocks, assimilation of S-rich sedi-mentary material has commonly been attributed as the most significant factor that triggered sulfide immisci-bility and led to the formation of the Pechenga ores. Several Ni-Cu sulfide prospects are associated with aferropicritic dike system that transects the thick pillow lava succession of the Kolosjoki Volcanic Formation un-derlying the mentioned sedimentary unit, showing that the magma was saturated in sulfide prior to reachingthe stratigraphic level where pyritic black shales occur. Our new rhenium-osmium isotope data from the Pah-tajärvi prospect (γOs in the range of +52 to +69) reveal that a significant component of radiogenic Os was pre-sent in the magma. This together with new S isotope data is compatible with the Pahtajärvi ultramafic dike act-ing as a feeder conduit to ore-producing magma chambers in the upper part of the Productive Formation. Ourresults and other evidence, indicating potential nonradiogenic osmium of seawater in the basin where sedi-ments of the Productive Formation were deposited, requires that the current model invoking country rocks asthe main source of sulfur and radiogenic osmium in the Ni-Cu deposits needs to be reevaluated. Exogenic sul-fur from Archean supracrustal rocks is not supported by the absence of mass-independent fractionation of sul-fur isotopes in the Pahtajärvi sulfides.

† Corresponding author: email, [email protected]*Present address: Key Laboratory of Isotope Geochronology and Geo-

chemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sci-ences, 510640 Guangzhou, China.

© 2011 Society of Economic GeologistsReviews in Economic Geology, v. 17, p. 145–162

(1938) proposed that segregation of sulfide melt occurred atsome depth and its injection took place independently afterthat of the silicate magma. Gorbunov and his coworkersmaintained that the intrusion of ore-bearing massifs tookplace during folding related to the Svecofennian orogeny(Gorbunov, 1968; Gorbunov et al., 1985). It is now well es-tablished that the Pechenga Ni-Cu ores are synvolcanic andgenetically related to the emplacement of primitive ferropi-critic magmas into the thickest sedimentary formation in thePechenga belt (Hanski, 1992). Besides the settling of sulfidesby gravitational forces, evidence for enrichment of the sulfideliquid by hydrodynamic forces has been observed (cf. Gor-bunov et al., 1989).

The source of sulfur in the Pechenga ores has remainedenigmatic. On the basis of sulfur isotope data, Grinenko et al.(1967) concluded that most sulfur is of a mantle origin andonly in some cases in the Eastern ore camp could assimilationof country-rock sulfur have happened. Grinenko and Smolkin(1991) regarded the Archean basement as a source for sulfurfor the economic intrusions, having a near meteoritic S iso-tope composition. Later sulfur derived from pyritic blackshales has played a fundamental role in many ore generationmodels at Pechenga (Melezhik et al., 1994; Walker et al.,1997; Green and Melezhik, 1999; Brügmann et al., 2000;Barnes et al., 2001; Naldrett, 2004). This development hasbeen due to the acquisition of more sulfur isotope data from

black shale country rocks (Melezhik et al., 1994, 1998), andOs isotope data for Ni-Cu ores (Walker et al., 1997). Althoughthere is also physical evidence of contamination with sedi-mentary material in the form of country-rock xenoliths insome intrusions (Smol’kin, 1977), the extent to which sulfur-rich country-rock sediments contributed to the ore formationrequires further investigation.

In this paper, we present a review of the geologic and geo-chemical characteristics of the Pechenga Ni-Cu deposits andprovide new evidence from a mineralized feeder dike system,indicating that at least some portion of ferropicritic magmawas already sulfur saturated before intruding to the strati-graphic levels where the ore-bearing differentiated ultramafic-mafic sills and associated sulfur-rich sedimentary rocks are lo-cated. We will also show that this magma had similar isotopecharacteristics to those of economic Ni-bearing intrusions.

Geologic BackgroundThe Pechenga greenstone belt represents the northwestern

part of the Paleoproterozoic Pechenga-Imandra and/or Varzugabelt running in a northwest-southeast direction through theKola Peninsula, northwestern Russia (Fig. 1), with minor ex-tensions into northern Finland and Norway (Melezhik andSturt, 1994). The supracrustal rocks of the Pechenga green-stone belt are assigned to the older North Pechenga Groupand the younger South Pechenga Group, which are separated

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20 km

N

S

Ferropicritic intrusiveand volcanic rocks

Pilgujärvi SedimentaryFormation (Productive Fm)

V

V

Kolosjoki Volcanic Fm

Pilgujärvi Volcanic Fm

Eastern ore fieldWestern ore field

Pilgujärvi intrusion

Kaula

Kammikvi

Sampling site

Sampling site

Kotselvaara

Ortoaivi

3 km

Finland

Russia

Barents SeaPECHENGA

IMANDRA-VARZUGA

Norway

68 O

30O

Southern Pechenga Group

Kolosjoki Sed.

Pilgujärvi Sedimentary Fm

Pilgujärvi Volcanic Fm

Kuetsjärvi Sed.and Volc. Fms

and Volc. Fms

Ahmalahti Sed.and Volc. Fms

FIG. 1. Geologic map of the Pechenga area showing the sampling sites of feeder dikes (modified after Brügmann et al.,2000).

by a major tectonic discontinuity. The North Pechenga Groupforms a southeast-southwest−dipping (30°–60°), asymmetricmonocline with a length of 70 km and a maximum width of ca.30 km. The sequence has a maximum thickness of more than10 km and represents a temporal geologic evolution of morethan 400 m.y. It is characterized by the predominance of vol-canic rocks in the strata and the cyclical repetition of volcanicand sedimentary rocks. The lower part of the North PechengaGroup is dominated by cratonic sediments, including con-glomerates, sandstone, and dolomites, and subaerially eruptedmafic to intermediate lavas and minor rhyolites. The upperpart was deposited under deeper water conditions and con-tains the thickest sedimentary unit of the North PechengaGroup, the Pilgujärvi Sedimentary Formation, which is alsoknown as the Productive Formation, due to the presence ofintrusions hosting the ca. 1980 Ma Pechenga Ni-Cu deposits.The Productive Formation reaches a thickness of ca. 1 kmand is mainly composed of graywackes and shales rich in sul-fides and carbonaceous matter. It is underlain and overlain bythick piles of submarine, massive or pillowed mafic lavas ofthe Kolosjoki and Pilgujärvi Volcanic Formations, respec-tively (see Fig. 1). The former is younger than ca. 2060 Ma(Melezhik et al., 2007), while the age of the latter is ca. 1980Ma (Hanski et al., 1990).

The Productive Formation has been divided into threelithostratigraphic units, Members A to C (Akhmedov andKrupenik, 1990; Melezhik et al., 1998). The lowermost Mem-ber A is composed of Corg- and S-bearing sandstones, silt-stones, and subordinate polymictic conglomerates and Mem-ber B contains highly carbonaceous and sulfidic graywackewith Bouma cycle rhythmites. Member C (Lammas Member)is represented by ferropicritic tuffs and tuffites, which areconcentrated in two eruptive centers. The only notable sedi-mentary interlayer within the Kolosjoki and Pilgujärvi Vol-canic Formations is represented by the 50- to 150-m-thickBlack Shale Member in the middle part of the Kolosjoki Vol-canic Formation (Predovsky et al., 1974).

The Pechenga Ni-Cu sulfide deposits are located in thelower part of concordant or subconcordant, differentiated,mafic-ultramafic bodies, often referred to as gabbro-wehrliteintrusions (Hanski, 1992). These magmatic bodies were em-placed as concordant, sill-like injections into the sediments ofthe Productive Formation. Some differentiated ore-bearingbodies show glass-bearing textures in their upper part, indi-cating rapid cooling (Hanski, 1992), and could be thick lavaflows (Green and Melezhik, 1999). The total amount of thegabbro-wehrlitic intrusions has been estimated to be ca. 25percent of the volume of the Productive Formation(Smol’kin, 1977). The thickness of the intrusions generallyranges between 5 to 250 m and along strike they can be fol-lowed from 100 m to 6.5 km (Zak et al., 1982). The largestmagmatic body, the Pilgujärvi layered intrusion, attains athickness of ca. 600 m (Smol’kin, 1977). Intrusions hostingsignificant amounts of sulfides are restricted to the centralpart of the Pechenga belt, where the Productive Formation isthickest. They occur in a ca. 20-km-long, arcuate zone, whichhas been divided into the Eastern and Western ore camps(Fig. 1). As indicated by Figure 1, the intrusions tend to be lo-calized in the upper part of the Productive Formation in theWestern ore camp and in the central or lower part in the

Eastern ore camp. Current mining activities are concentratedin a huge open pit related to the Pilgujärvi intrusion in theEastern ore camp, whereas the deposits associated withsmaller intrusions (Kaula, Kotselvaara, Kammikivi) in theWestern ore camp have already been mined out over the longperiod of exploitation.

The differentiated intrusions are composed of an ultramafic,wehrlitic lower part and a gabbroic upper part, separated byan intermediate zone of clinopyroxenites, and record a crystal-lization order of chrome spinel, olivine, clinopyroxene, mag-netite, and plagioclase. Orthopyroxene is characteristically ab-sent as a cumulus phase. The Pilgujärvi intrusion mentionedabove is exceptional in showing magnetite earlier, togetherwith olivine, as a liquidus phase. A hydrous nature of theparental magma is demonstrated by orthomagmatic kaersutiticamphibole and titanian phlogopite occurring as intercumulusphases in ultramafic cumulates (Hanski, 1992; Fiorentini etal., 2008). Postmagmatic alteration has been variable in ultra-mafic rocks, resulting in a suite of rocks ranging from relativelyfresh olivine-pyroxene rocks to serpentinites and talc-carbon-ate rocks, though textural preservation is generally good.

Most of the Ni-Cu ore of the area occurs as disseminated ornet-textured sulfides in ultramafic rocks at the base of the in-trusions. Also, massive and breccia ores are found at the baseof the intrusions with the breccia ores having been formed atthe contact zones that have been tectonically active after theformation of the massive ores. In some areas, economic-gradeenrichment of epigenetic sulfides has taken place by post-magmatic mobilization of sulfidic material into sedimentarycountry rocks along fault zones running at the basal contactsof the intrusions (Gorbunov et al., 1985). The mineralogy ofthe ores is rather simple; major ore minerals are pyrrhotite,pentlandite and chalcopyrite with the mobilized ores beingricher in chalcopyrite than the nonmobilized ores. Less abun-dant and accessory phases include pyrite, magnetite, violarite,sphalerite, cubanite, mackinawite, and vallerite (Gorbunov etal., 1985). In terms of Ni/(Ni + Cu), the western and easternorebodies are very similar: based on the data presented byZak et al. (1982), the average Ni/(Ni + Cu) ratio of 618 syn-genetic ore samples from the Western ore camp (Kotselvaara,Kammikivi, Semiletka) is 0.67, while the ratio of 715 similarore samples from the Pilgujärvi intrusion (east) is 0.68.

The parental magma to the ore-bearing intrusions was a hy-drous, Fe-rich primitive magma called ferropicrite (Hanski andSmol’kin, 1995). It is uncertain whether all the ore-bearingbodies are intrusive in nature as ferropicritic magma also pro-duced extrusive rocks in the area, including layered, spinifex-textured lava flows and thick agglomeratic accumulations. Ap-proximately 5 vol percent of the volcanic rocks of the PilgujärviVolcanic Formation are ferropicritic and coeval with the intru-sive equivalents in the underlying sedimentary formation.

The Northern Soukerjoki, Pahtajärvi, and Kolosjoki Ore Prospects

The intrusions hosting the northern Soukerjoki and Pahta-järvi ore showings are found in the boundary zone betweenthe Eastern and Western ore camps (see Figs. 1, 2) and havebeen briefly described by Gorbunov et al. (1985, 1999). Thenorthern Soukerjoki ultramafic intrusion is located at the contact between the pillowed mafic volcanic rocks of the

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Kolosjoki Volcanic Formation and overlying phyllitic meta -sediments of the Pilgujärvi Sedimentary Formation (Fig. 2).On the surface, the body has a funnel shape and a length of450 m. It has a thickness of less than 50 m and dips, firstdeeply and then more gently, to the southwest, plunging to adepth of 400 m. The sulfide mineralization forms a 10- to 15-m-thick, weak dissemination in the lower contact zone of theintrusion with local lenses of richer dissemination. Close tothe tectonic zone running at the eastern and northeasterncontact of the magmatic body, ore-bearing rocks are shearedand have undergone talc alteration. In the eastern part of theore zone, some massive lenses are met with Ni contents of upto 6 wt percent.

The Pahtajärvi ore prospect is associated with a sheetlike,northwest-trending ultramafic body cutting diagonally a pileof pillow lavas in the upper part of the Kolosjoki Volcanic For-mation (Fig. 2). It has a thickness of a few tens of meters anddips 75° to 80° toward the southwest with a maximum depthof 300 m. At its southeastern end, the dikelike body appearsto join the northern Soukerjoki magma chamber, though theactual contact is disturbed by faulting (Fig. 2). The intrusionis composed of altered wehrlites and olivine pyroxenites and,close to the contacts, amphibole rocks after pyroxenites. Sul-fides in the form of weak and locally higher grade dissemina-tion occur throughout the body apart from the contact zonepyroxenites. According to Gorbunov et al. (1999), the em-placement of the Pahtajärvi dike was controlled by a north-west-trending fault (Pahtajärvi fault) that can be traced for adistance of 5 km.

The Pahtajärvi fault has a northwest direction in the Pahta-järvi area, but farther to the northeast, it is gradually curvedto an east-west direction being almost parallel to the strike ofthe lavas of the Kolosjoki Volcanic Formation (see Naldrett,2004, fig. 5.8). Besides the Pahtajärvi ultramafic body, the

fault also contains other lens- or dike-shaped intrusions whenfollowed into deeper parts of the Kolosjoki Volcanic Forma-tion. It is this part of the fault where the Kolosjoki oreprospect is located, having a distance of almost 3 km to thelower contact of the Productive Formation on the presenterosional surface (Gorbunov et al., 1999). It is important tonote that the prospect is located stratigraphically beneath theBlack Schist Member, which occurs in the middle part of thethe Kolosjoki Volcanic Formation. The ore-bearing intrusionhas pinch-and-swell structures, a length of ca. 700 m, and athickness between 3 and 25 m. The rock types vary betweenwehrlites and gabbros. Ore material is unevenly distributed inaltered wehrlites (amphibole-bearing serpentinites) in thecentral part of the intrusion and occurs as disseminated andveined-disseminated sulfides, rarely as massive sulfides.

Gorbunov et al. (1985) regarded the northern Soukerjokiand Pahtajärvi ore showings as an offset-type mineralizationbut did not give any explanation on what they meant by thisclassification. Smol’kin and Borisova (1995) proposed that theinjection of magma into the Pahtajärvi synsedimentary faultwas part of the first of the four stages of ferropicritic magma-tism at Pechenga and the Pahtajärvi body possibly representsa feeder conduit for some ferropicritic lavas. Based on the ev-idence shown below, we conclude that the Pahtajärvi dike sys-tem is comagmatic with ore-bearing intrusions in the upperpart of the Productive Formation.

Samples and MethodsThe sampling sites are shown in Figure 1. In order to

compare the isotopic compositions of sulfide minerals fromthe Pahtajärvi feeder dike and the differentiated ore-bear-ing intrusions and, hence, assess the potential role of themagma-sediment interaction during ore formation, we sepa-rated sulfide fractions from five Pahtajärvi samples, using

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A

C

B

D

Disseminated ore in serpentinite

Serpentinized peridotite

Pyroxenite

Mafic volcanic rock

Tuffogenic sedimentary rocks

Fault Overburden

Kolosjoki Volcanic Formation

C

A

D

B

NorthernSoukerjoki

Pahtajärvi

100 m

N

S

PilgujärviSedimentary Formation

FIG. 2. Map showing the northern Soukerjoki and Pahtajärvi ore prospects (after Gorbunov et al., 1985).

standard heavy liquid methods, and measured their osmiumand sulfur isotope ratios. We also sampled the medium-grained central part of a ca. 5-m-thick ultramafic dike occur-ring among tholeiitic pillow lavas ca. 2 km east of the Pahta-järvi dike and having an approximate stratigraphic distance of500 m to the upper contact of the Kolosjoki Volcanic Forma-tion (Fig. 1). The rock is now mainly composed of secondarytremolite, serpentine and talc after original olivine andclinopyroxene, and fine-grained groundmass with sulfide dis-semination. The groundmass also contains needlelike kaersu-tite crystals, which are typical of ferropicritic rocks. In orderto evaluate the state of sulfur saturation of the feedingmagma, we determined the S and major oxide content of asample from the dike by X-ray fluorescence analysis (XRF). Achromite concentrate was obtained from the lower, ultramaficpart of a ca. 20-m-thick layered lava flow occurring in thelower part of the Pilgujärvi Volcanic Formation in the Kotsel-vaara area. The Re-Os isotope composition of the concentratewas determined to supplement earlier chromite analyses(Walker et al., 1997) from barren ferropicritic bodies.

Osmium isotope analyses were performed at the Universityof Maryland. Samples were digested in sealed borosilicateCarius tubes using a 1/2 mix of 12M HCl and 16M HNO3 at260°C for >72 h, with isotopically enriched multielementspikes. Osmium was purified by carbon tetrachloride solventextraction and microdistillation (Cohen and Waters, 1996;Birck et al., 1997), prior to measurement as OsO–

3 ions, usinga Sector 54 thermal ionization mass spectrometer. Externalprecision of 187Os/188Os, determined via individual measure-ments of 20 to 50 pg Os standards (UMCP Johnson andMatthey) during the analytical campaign, was better than ±2per mil (187Os/188Os = 0.11364 ± 6; n = 15; 2σ). After osmiumseparation, rhenium was recovered and purified from resid-ual solutions by an anion exchange separation technique.Rhenium analyses were accomplished using an Nu Plasmamulticollector ICP-MS. Rhenium and Os blanks averaged 1.0and 0.7 pg, respectively, and were inconsequential for thesemeasurements. For more details, the reader is referred toDay et al. (2010).

Sulfur isotopes were determined at the University of Mary-land using the following procedure. Approximately 0.5 to 1.0g of ground sample was sequentially treated with differentacid solutions to convert the various components of sulfurpresent in the sample into Ag2S. Acid volatile sulfur consist-ing of free sulfide and metal sulfides was extracted by distilla-tion with 5N HCl. Pyritic sulfur was reduced with chromiumacid distillation, after Canfield et al. (1986). Organic boundedsulfur was reduced by Raney nickel hydrodesulfurization(Granatelli, 1959; Oduro et al., 2011) followed by sulfatecomponent in the sample, which was reduced into H2S gas byboiling with a 25-ml solution mixture consisting of 320 ml HI,524 ml HCl, and 156 ml H2PO4 (Forrest and Newman, 1977).In all distillation-reduction reactions, evolved H2S gas wasquantitatively trapped into a silver nitrate solution by precip-itating as Ag2S.

Samples of Ag2S were reacted in Ni bombs with ten-foldexcess F2 gas at 320°C for approximately 8–12 hours. Prod-ucts of SF6 were cryogenically purified from F2 (at −196°C)and HF through distillation (at −115°C) to condense traces ofHF contaminants, before transferring it into an injection loop

of a gas chromatograph (GC). Final purification of SF6 byGC-TCD (thermal conductivity detection) was accomplishedusing a composite column made up of two sections, a 1/8-in-diam, 6-ft-long packed column containing a type 5A molecu-lar sieve and a 1/8-in-diam, 12-ft-long Hayesp-Q™ column. Acarrier flow of He set at 20 ml min−1 was utilized with a GCtemperature of 50°C to elute SF6 peaks between 12 and 18min. The SF6 gas eluting from the column was captured bydiverting it together with He carrier gas into a glass spiral trapchilled at liquid nitrogen temperature (−196°C), where theHe gas was slowly pumped off from the trap. Sulfur isotopemeasurements of purified SF6 were performed on a FinniganMAT 253 mass spectrometer. Analytical uncertainties in the Sisotope measurements (derived from IAEA-S1, IAEA-S2,and IAEA-S3 reference materials), estimated from long-termreproducibility of Ag2S fluorinations, are 0.02, 0.008, and 0.20(1σ) for δ34S, Δ33S, and Δ36S, respectively.

ResultsTable 1 provides XRF analysis data for the 5-m-thick ultra-

mafic dike together with average analyses of other ultramaficrocks from the Pahtajärvi dike system and intrusions from theProductive Formation. On a volatile-free basis, the narrowdike contains (in wt %) 22.8 MgO, 15.1 FeOtotal, 5.65 Al2O3,and 1.71 TiO2, and 1,750 ppm Ni. The high MgO suggests thepresence of some cumulative olivine component as it hasbeen estimated that the ferropicritic magma had ca. 15 wtpercent of MgO (Hanski, 1992). The Al2O3/TiO2 ratio of thedike is low (3.3), which is diagnostic for ferropicritic magmasand ultramafic cumulates crystallized from such magmas(Table 1). The sulfur content is relatively high, ca. 1.0 wt per-cent, indicating an S-saturated nature of the magma in thedike.

Analytical results for five sulfide separates from the Pahta-järvi dike are presented in Table 2. They have high concen-trations of Re and Os, ranging from 39.4 to 73.8 ppb and 13.3to 35 ppb, respectively, and relatively low 187Re/188Os ratiosbetween 9.62 and 14.7. The samples show radiogenic osmiumisotope compositions with 187Os/188Os in the range of 0.1786and 0.1914, corresponding to γOs(1980 Ma) values between+58.5 and +69.5. These results are compatible with earlierisotopic data obtained for Ni-Cu ore samples from intrusionswithin the Productive Formation (Walker et al., 1997). Com-pared to the sulfides, the chromite concentrate from the lay-ered lava flow has lower concentrations of Re and Os (1.88and 2.80 ppb). It also shows lower 187Re/188Os (3.27) and anonradiogenic Os isotope composition with γOs(1980 Ma) of2.9. This is again in agreement with earlier results for barrenferropicritic lava flows containing chromites with relativelylow initial 187Os/188Os (Walker et al., 1997).

The same sulfide samples analyzed for Re-Os isotope sys-tematics were also analyzed for multiple sulfur isotopes(Table 3). The δ34S values vary between 0.3 and 2.8 per mil.Separated bulk sulfides, pyritic sulfur, organic sulfur, and sul-fate record, respectively, the following δ34S values (‰): 0.7 to1.5, 2.2 to 2.8, 1.8 to 2.8, and 0.3 to 0.7. All measured isotopiccompositions fall in the range of orebodies in the Western orecamp (−1 to +2‰) but are lower than the typical values ofsulfides from the Eastern ore camp (+3.5 to +5‰; Grinenkoet al., 1967). The measured Δ33S values (between −0.06 and


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TABLE 2. Rhenium and Osmium Concentrations (ppb) and Os Isotope Compositional Data for Sulfide Separates from the Pahtajärvi Feeder Dike and a Chromitite Concentrate from a Ferropicritic Layered Flow from the Kotselvaara Area

Sample no. Mineral Os Re 187Re/188Os 187Os/188Os 187Os/188Os(1980) γOs(1980)

5-EJH-07 Sulfide separate 25.53 63.32 12.70 0.6096 0.1836 61.76-EJH-07 Sulfide separate 21.89 62.38 14.71 0.6766 0.1832 61.4Duplicate 28.74 71.80 12.81 0.6211 0.1914 68.67a-EJH-07 Sulfide separate 13.35 39.36 15.25 0.6914 0.1799 58.5Duplicate 21.36 55.13 13.26 0.6344 0.1898 67.17b-EJH-07 Sulfide separate 35.93 68.28 9.622 0.4958 0.1731 52.5Duplicate 29.92 73.82 12.65 0.6166 0.1925 69.514-JTK-06 Chromite concentrate 2.803 1.877 3.268 0.2264 0.1168 2.9

TABLE 3. Sulfur Isotope Values of Sulfide (AVS), Pyritic Sulfur (CRS), Organic Bound Sulfur, and Sulfate in Samples from the Pahtajärvi Feeder Dike Together with Analytical Results of IAEA Standards

Sample no. Sulfur species δ33S δ34S δ36S Δ33S Δ36S

5-EJH-07 AVS 0.35 0.70 1.11 –0.014 –0.23Pyritic sulfur 1.42 2.80 5.32 –0.025 –0.01Organic sulfur 1.00 2.07 4.33 –0.064 0.39Sulfate 0.22 0.41 0.98 0.005 0.20

6-EJH-07 AVS 0.65 1.30 2.39 –0.022 –0.07Pyritic sulfur 1.19 2.36 4.43 –0.027 –0.06Organic sulfur 1.11 2.27 4.34 –0.055 0.03Sulfate 0.37 0.70 1.36 0.007 0.03

7a-EJH-07 AVS 0.74 1.49 2.82 –0.029 –0.01Pyritic sulfur 1.10 2.19 4.14 –0.026 –0.02Organic sulfur 0.88 1.78 3.52 –0.033 0.14Sulfate 0.14 0.31 0.44 –0.018 –0.16

7b-EJH-07 AVS 0.74 1.49 2.76 –0.028 –0.07Pyritic sulfur 1.24 2.44 4.65 –0.022 0.00Organic sulfur 1.40 2.77 5.39 –0.029 0.11Sulfate 0.26 0.55 1.05 –0.021 0.01

IAEA standards IAEA-S1 11.34 22.06 42.11 0.043 –0.22IAEA-S2 –0.08 –0.33 –1.51 0.093 –0.89IAEA-S3 –16.82 –32.53 –62.08 0.069 –1.17

Notes: δ values (‰) are reported relative to Vienna Cañon Diablo Troilite (V-CDT); the magnitude of mass-independent fractionation of 33S or36S is shownas Δ33S or Δ36S values defined as the deviation of a measured δ33S or δ36S value from the δ33S or δ36S value expected from the mass-dependent fractiona-tion relationships (see Farquhar et al., 2000)

TABLE 1. Chemical Composition of Narrow Ultramafic Dike Cutting Pillow Lavas of the Kolosjoki Volcanic Formation (1) Compared with Average Compositions of Ultramafic Rock Types from the Pahtajärvi Feeder Dike (2–3) and

Intrusions Within the Productive Formation (5–6) (1 = this study, 2–6 from Smolkin and Borisova, 1995)

1 2 3 4 5 6

n 1 3 9 23 61 27SiO2 (wt %) 46.44 42.06 45.58 44.74 40.27 46.91TiO2 1.71 1.71 2.16 2.06 0.93 1.91Al2O3 5.65 5.67 7.33 7.14 3.25 5.94Fe2O3 2.82 7.30 4.19 3.46 9.52 3.25FeO 12.57 12.70 11.96 13.52 9.35 11.41MnO 0.14 0.24 0.22 0.22 0.23 0.20MgO 22.84 23.88 15.37 18.50 31.88 14.98CaO 6.13 3.97 11.84 8.88 2.94 13.67Na2O 0.08 0.19 0.55 0.27 0.13 0.65K2O 0.02 0.08 0.14 0.09 0.15 0.28P2O5 0.18 0.20 0.22 0.19 0.13 0.19S 1.06 1.19 0.13 0.55 0.42 0.36(LOI) (4.89) (8.10) (4.94) (5.97) (7.34) (3.39)Al2O3/TiO2 3.3 3.3 3.4 3.5 3.5 3.1Cr (ppm) 1420 3170 1690 1920 3910 1040Ni 1750 2730 640 1210 2790 570Co 210 100 130 230 300V 280 220 320 210 220 310Cu 180 1860 320 360 730 320

Notes: 1 = ultramafic dike, 2 = peridotite, 3= pyroxenite, 4 = pyroxenitic lower chill, 5 = peridotite, 6 = olivine pyroxenite; n = number of samples

+0.01) do not indicate any mass-independent fractionation ofsulfur (MIF-S; cf. Farquhar et al., 2010) isotopes in the Pah-tajärvi feeder dike.

DiscussionIn the following discussion, we combine our new results

with previously obtained geochemical and isotopic data fromPechenga and evaluate how they fit with the common ore for-mation model invoking interaction of ferropicritic magmawith sulfur-rich country-rock sediments. We commence withgeochemical information, considering first sulfur, selenium,and the semimetals As and Te, and then PGE and lithophileelements, and finally move to radiogenic and stable isotopeconstraints.

Geochemical constraints

Hanski (1992) discussed the S-Se-As-Te relationships in thePechenga Ni-Cu ores and country-rock metasediments, butthe actual analytical results were not tabulated and are there-fore included in this paper as supplementary data (see foot-note, p. xxx). Figure 3a displays S and Se contents of differentNi-Cu ore types, black shale and ore mineral separates. Sele-nium contents (0.01–11 ppm) of black shale samples are lowcompared to the abundances in most ore samples, in whichSe commonly reaches levels of tens of ppm. The pyrite andpyrrhotite separates from black shale analyzed by Hanski(1992) contain only ca. 4.0 and 0.3 ppm Se, respectively. Thepyrite separates from concretions analyzed by Barnes et al.(2001) have higher Se levels (up to 53 ppm). Even thoughmost of the Ni-Cu ore samples have relatively high Se, theystill plot on the left side of the line indicating the primitivemantle S/Se value. This could be interpreted as evidence foraddition of sedimentary sulfur to magma and consequent in-crease in S/Se of the contaminated magma. For comparison,Figure 3 also contains analyses of ore samples from the Paleo -proterozoic komatiite-related Thompson Ni deposit, Canada,which has been shown to comprise a substantial external sulfur component from sedimentary sources (Ekstrand et al.,1989). These samples plot on a trend having higher S/Se than

most of the Pechenga samples. This is also true for manyother komatiite-associated deposits, though it must bepointed out that S/Se varies very much in these deposits, withaverage values for each deposit falling mostly between 3,000and 6,000 (Lesher and Barnes, 2009). Instead of crustal con-tamination, a small amount of sulfide segregation can also ex-plain the relatively high S/Se ratios in the Pechenga ore sam-ples (cf. Barnes et al., 2009), which is compatible with the lowPGE levels in the ores (see below).

Barnes et al. (2001) drew attention to the high As contents(>100 ppm) which they had measured for several Ni-Cu oresamples from Pechenga (Fig. 3b) and suggested that As wasderived from country-rock sediments. Pyritic black shales canindeed have high As concentrations (e.g., Meyer and Robb,1996; Yu et al., 2009) and thus, upon assimilation, can poten-tially contribute to high As in magmatic ores. Microprobeanalyses have given As abundances up to 1.8 wt percent forPechenga sedimentary pyrite (Abzalov et al., 1997). The com-positions of the pyrite (up to 720 ppm) and pyrrhotite sepa-rates mentioned above suggest that, for the most part, arsenicin black shale resides in the lattice of pyrite and the As/S ratioin whole-rock black shale samples (e.g., Abzalov et al., 1995)seems to be controlled by the corresponding ratio in pyrite.The interpretation of the behavior of As in Ni-Cu ores is notstraightforward as its content in ore samples is highly variable.In addition to high As samples, there are many ore samples,including massive ones, which have As contents of less than10 ppm (Fig. 3b). There is no correlation between the As/Sratios and δ34S values among the Ni-Cu ore samples (consid-ering the Western and Eastern ore camps separately: r = 0.02,n = 42; r = −0.33, n = 19). Positive correlations that were ob-served between As and Se (see Fig. 3b) or As and S amongthe Thompson Ni deposit samples are not observed for thePechenga ore samples. Instead, there is a wide scatter of dataand many samples have low As/Se similar to that of primitivemantle, or even lower. Abzalov et al. (1997) carried out a de-tailed study of the arsenic mineralogy in the Pechenga Ni-Cuores and their country rocks and observed that disseminatedores that still have their primary magmatic textures preserved


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Se (ppm)

A

s (p

pm

)

PMS

PMS

pyrite

pyr

rhot

ite

a b

0 50 100 150 2000

10

20

30

40

50

Se (ppm)

S

(Wt-

%)

primitiv

e

man

tle

primitive mantle

0 40 80 120 160 2000

200

400

600

800

Thompson Ni ore

Black shale (BS)Pyrrhotite from BSPyrite from BS

Disseminated ore (DO)Sulfide sep. from DOMassive oreBreccia ore

FIG. 3. Samples of different Ni-Cu ore types and black shale and their mineral separates plotted on (a) S vs. Se and (b)As vs. Se diagrams. Pechenga data from Hanski (1992) and Barnes et al. (2001) and comparative data on the Thompson Nideposit (Canada) from Naldrett (2004). PMS denotes primitive mantle sulfide of Hattori et al. (2002). Primitive mantle S/Seand As/Se ratios after McDonough and Sun (1995).

commonly do not contain arsenic minerals, whereas these arefound in massive, breccia and stringer ores. They concludedthat upon metamorphic recrystallization of authigenic As-bearing pyrite to As-free pyrrhotite in sediments, arsenic wasmobilized and formed various hydrothermal mineralization inthe area. This may partly explain the wide scatter and highlevels of As in massive and breccias ores (Fig. 3b) that occurat or close to the intrusion-footwall contacts.

Low As in disseminated ores is evident from Figure 4a con-taining a sample set from the Western ore camp. Figure 4aalso illustrates the strikingly different Te/As ratios in the mag-matic ores and associated sediments. Tellurium data are cur-rently available only for disseminated ore types, in which amaximum of 13.6 ppm has been measured. In black shales,the highest Te concentration of 0.41 ppm has been obtainedfor a pyrite separate, while whole-rock samples have yieldedTe contents in the range of 0.01 to 0.29 ppm (Hanski, 1992),which are very low compared to the world average value of2.0 ± 0.3 ppm determined for black shales by Ketris and Yu-dovich (2009). As demonstrated by Figure 4b, Te and Pt havea very good mutual correlation in the ore samples, which alsoapplies to other PGE, suggesting that the occurrence of PGEis largely controlled by tellurides in disseminated ores andthese elements have remained more or less immobile duringpostmagmatic processes.

The Pechenga Ni-Cu sulfide ores have relatively low totalcontents of PGE compared to Ni and Cu, an observation thatis compatible with segregation of a small amount of sulfide atdepth prior to the final emplacement of the magma (Brüg-mann et al., 2000; Barnes et al. 2001). Barnes et al. (2001)placed Pechenga ore compositions on a Ni/Pd versus Cu/Irdiagram in which the samples plotted away from the commonkomatiite-tholeiite field, indicating depletion of noble metalscompared to base metals in a similar fashion as midoceanridge basalts (MORBs) in the same diagram. Figure 5 com-pares the total PGE and Ni + Cu contents in the Pechengaore samples (comprising all main ore types) to those in sam-ples from the Jinchuan and Voisey’s Bay ore-bearing intru-sions, as well as various komatiite-hosted ore deposits. Most

of the Pechenga samples plot in an area lying between thetrends defined by the komatiite-related deposits and Voisey’sBay ores. The depletion in PGE in the low-grade ores fromJinchuan is similar to that at Pechenga, but the high-gradeores at Jinchuan are clearly more depleted in PGE. Brüg-mann et al. (2000) found a positive correlation between eachPGE in ferropicritic lava flows from the Pilgujärvi VolcanicFormation, which is not normally observed in sulfide-under-saturated ultramafic magmas where (Ir group PGE) IPGEbehave generally compatibly and (Pd group PGE) PPGE in-compatibly (cf. Brügmann et al., 1987; Puchtel et al., 2007).They concluded that, in addition to the ore-bearing intru-sions, the barren ferropicritic lava flows were also sulphur saturated during their differentiation and segregated a smallamount of sulfide liquid. If external sulfur was involved in

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0 100 200 300 400 500 600 700 8000 0

10 10

20 20

As (ppm)

Te

(pp

m)

Te (ppm)

PMS

pyritepyrrhotite

Disseminated Ni-Cu ore

Pyrite from BSPyrrhotite from BSBlack shale (BS)

a b

prim

itive

man

tle

0 1000500 1500 2000

Pt (ppb)

FIG. 4. (a). Te and As contents of disseminated Ni-Cu ores from the Western ore camp sills (Kammikivi, Ortoaivi, Kot-selvaara), black shales, and pyrite and pyrrhotite separates from black shales. Data from Hanski (1992). PMS denotes prim-itive mantle sulfide of Hattori et al. (2002). Primitive mantle Te/As ratio taken form McDonough and Sun (1995). (b). Te vs.Pt diagram for disseminated Ni-Cu ores from the same sills as in (a). Data from Appendix 1 and Brügmann et al. (2000).

0.6 1 10 20.06

0.1

1

7

ΣP

GE

(pp

m)

Ni+Cu (wt%)

Komatiitic ore

Pechenga

Jinchuan

Voisey’ s Bay

FIG. 5. Total PGE and Ni + Cu contents in sulfide ores from Pechenga,Voisey’s Bay, Jinchuan, and komatiite-related deposits. Analytical data takenfrom Abzalov and Both (1997), Brügmann et al. (2000), Barnes et al. (2001),Naldrett (2004), and Song et al. (2009).

these processes, it is not seen in the Os isotope compositionof the flows in a similar fashion, as in the ore-bearing intru-sions (see below).

In order to assess the effects of contamination on thelithophile element systematics, compositions of shale samplesare normalized to an average ferropicrite with 15 wt percentMgO in Figure 6. The most distinct differences in the olivine-incompatible, immobile element ratios between black shalesand ferropicritic magma are the higher U/Th, Th/Nb ratiosand, due to low P in black shales, the higher La/P, Zr/P, andYb/P ratios in black shales compared to those in ferropicriticmagma. Our data coupled with the literature survey showedthat in ferropicritic rocks measured Th/U varies between 2.4and 5.0 and averages 4.1, while in the sedimentary rock theratio rarely exceeds 2.0 with an average value being 1.4. Themeasured average Th/Nb ratios in ferropicritic rocks andpelitic sediments are 0.1 and 0.3, respectively. Thus, it seemsthat, due to the mentioned differences in certain trace ele-ment ratios, trace element geochemistry could be used to de-cipher the potential effects of country-rock assimilation to thecontaminated magma composition. However, the precision ofthe analyses and alteration of the rocks leading to mobility ofelements (particularly of U) barely allow barren and ore-bear-ing ferropicritic bodies to be distinguished using theirlithophile trace element ratios. Barnes et al. (2001) also con-sidered several incompatible element ratios (U/Th, K2O/Sm,Lu/Sc, Yb/Sm) and could not find definitive evidence forcrustal contamination.

Isotopic constraints

Previous studies have shown that uranogenic Pb isotopescannot be used to discriminate between ore-bearing and bar-ren ferropicritic bodies (Hanski, 1992). Instead, thorogenicisotopes have indicated some potential. Pushkarev et al.(1988) and Hanski (1992) reported Pb isotope analyses ofpelitic sediments and their iron sulfides from Pechenga. Con-sistent with the low Th/U ratios measured for black shales,

whole-rock sediment samples and separated minerals havelow 208Pb/206Pb ratios and form a distinct, low-angle trend ina 208Pb/204Pb versus 206Pb/204Pb plot, apart from the trend offerropicritic rocks (Fig. 7). Hence, assimilation of sedimen-tary material by magma might be seen in a shift of rock com-positions toward lower 208Pb/204Pb at given 206Pb/204Pb. Thishas indeed been observed for the samples from the Pilgujärviintrusion, which plot on a line with a lower coefficient thanthe trend of barren ferropicritic lava samples (Fig. 7). The dif-ference in the Pb isotope composition is most distinct in themost radiogenic samples, but as shown by the two lines inFigure 7, projecting to the field of sulfide ores, resolution ofbarren and ore-bearing systems becomes more difficult in thecase of less radiogenic samples.

So far only a few Nd isotope measurements have been per-formed on sedimentary rocks from the Productive Forma-tion, but the available data indicate differences depending onthe rock type. A gritstone, containing clastic detritus fromArchean basement, has yielded an εNd (1980 Ma) value of−5.1, while sulfide-poor and sulfide-rich shales show higherεNd(1980 Ma) values of −3.0 and +0.1, respectively (Hanskiand Huhma, unpub. data). The close to chondritic Nd isotopecharacter of sulfide-rich sediments, together with their rela-tively low Nd concentration compared to that of the ferropi-critic magma (Fig. 6) render the assimilation of sulfide-bear-ing sedimentary material ineffective in changing the Ndisotope composition of a contaminated magma. This is con-sistent with the observation that the ore-bearing intrusionsand barren ferropicritic lava flows do not differ in their initialNd isotope compositions. Consistent with this, Hanski (1992)obtained an initial εNd value of 1.4 ± 0.4 from an isochron forferropicritic volcanic rocks, while individual samples fromore-bearings intrusions (Pilgujärvi, Kammikivi) commonlygave similar initial εNd values in the range of 1.3 to 2.6 (Han-ski, 1992; Walker et al., 1997).

As carbonaceous shales can have high Re and Os concen-trations relative to mafic to ultramafic magmas (1–400 ppb vs.


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1

10

20

U Th Nb Ta La Ce P Nd Zr Sm Eu Ti Y Yb V Sc A l

Se

dim

en

t /

Fe

rro

pic

rite

FIG. 6. Compositions of black shales from the Productive Formation nor-malized to an average ferropicrite. Analyses of the sedimentary samplestaken from Hanski (1992) and Barnes et al. (2001), shown as gray and blacksymbols, respectively.

14 18 21 25 29 33 36 4030

33

36

39

42

46

49

52

55

58

206Pb/204Pb

208 P

b/20

4 Pb

PilgujFerropicriteNi-Cu ore

ärvi intrusion

Black shale (BS)Pyrrhotite from BSPyrite from BS

FIG. 7. Thorogenic and uranogenic Pb isotope ratios for Ni-Cu sulfideores, ferropicritic rocks, and pelitic sediments. Analytical data taken fromPushkarev et al. (1988), Hanski (1992), Smol’kin (1992), and Abzalov et al.(1995).

0.5–1.3 ppb for Re and 0.1–8 ppb vs. 0.001–2.2 ppb for Os;e.g., Ravizza and Turekian, 1989; Shirey and Walker, 1998)and, due to their high Re/Os, tend to develop radiogenic Osisotope compositions with time, the Re-Os isotope system canserve as a powerful tool in detecting the effects of black shaleassimilation in sulfide-bearing igneous rocks. Figure 8 sum-marizes the initial Os isotope values and Os concentrationsthat have been measured for minerals and whole-rock sam-ples from Pechenga. It is obvious that all ore-bearing bodieshave quite radiogenic initial Os isotope composition with ini-tial γOs values varying between 43 to 240. These include sul-fide separates from the Pahtajärvi feeder dike having γOs inthe range of 53 to 69 (Table 1). In terms of the Os isotopecomposition, the Pahtajärvi sulfides are similar to some dis-seminated ore samples from the Kammakivi sill in the West-ern ore camp. Massive and breccia ore samples from the Pil-gujärvi intrusion in the Eastern ore camp are considerablymore isotopically homogeneous and slightly less radiogenicwith γOs values between 43 and 50 (Walker et al., 1997). Thereis some heterogeneity in the osmium isotope compositions inthe Western ore camp, also within an individual intrusivebody, as exemplified by a chromite separate from the Kam-mikivi sill (Fig. 8); this has a lower γOs value (20) than the un-derlying sulfide-bearing rocks occurring in the basal part ofthe same sill.

Figure 8 also shows analytical data for mineral separatesfrom ferropicritic layered flows from in the Pilgujärvi Vol-canic Formation, without association of any known significantenrichment in Ni-Cu sulfides. In the study of Walker et al.(1997), chromites and accessory sulfides gave γOs values of ca.6, which was considered the original Os isotope compositionof uncontaminated ferropicritic magma, inherited from themantle source and compatible with the ocean-island basalt(OIB)-type character of the magma. The chromite concen-trate of the present study yielded a lower γOs value of 2.9, rep-resenting the most unradiogenic osmium isotope composition

measured for Pechenga ferropicritic rocks. The osmium iso-tope studies have, thus, revealed broadly two kinds of fer-ropicritic magmas containing slightly radiogenic (γOs ca. 6 orless) and radiogenic Os values (commonly γOs >40), respec-tively, with the latter representing sulfide-bearing systems,and importantly also the feeder dikes within the KolosjokiVolcanic Formation. The available geologic and isotopic datafrom the feeder dikes demonstrate that at least some portionsof the ore-producing magma were sulfide saturated and con-tained radiogenic Os already prior to injection into the sulfur-rich sediments of the Productive Formation.

The above conclusion raises the question of whether assim-ilation of sedimentary material played any role in the ore for-mation at Pechenga. For solving this problem, it is essential todetermine the initial Os isotope composition of the potentialS-rich contaminants among the country rocks. This task is noteasy for many reasons:

1. Black shale samples are thought to record the isotopiccomposition of contemporaneous seawater from which thesediments were deposited but, as shown by the seawatercurve established for the latest 200 m.y. (Fig. 9), the isotopiccomposition of seawater can fluctuate irregularly within alarge range through time, depending on the erosional trans-port of Os from continental weathering and its hydrother-mal input from juvenile oceanic crust (e.g., Peucker-Ehren-brink and Ravizza, 2000). The potential change in 187Os/188Os of seawater over the time period of deposition ofslowly accumulating sediments requires that for construc-tion of a reliable isochron, only closely spaced samplesshould be considered.

2. Sediments can contain a detrital silicate componentmasking the Os isotope signature of the hydrogenous compo-nent and causing variability in the initial Os isotope ratio.

3. Permeable sedimentary system may remain isotopicallyopen for variable periods of time (Hannah et al., 2004).

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0.05 0.1 1 10 100 2001

10

100

1000

2000

Os (ppb)

Kotselvaara

Ortoaivi

Kammikivi

PilgujärviPilgujärvi

Chromite

Sulfide+ilmenite

Disseminated ore

Massive and breccia ore

Olivine

Pyrrhotite from black shale

Pyrite from black shale

Pahtajärvifeeder dike

Layered lava flows

γOs

Kotselvaara

x

x

FIG. 8. Osmium isotope data on whole-rock samples and mineral separates from four ore-bearing intrusions (Kotselvaara,Ortoaivi, Kammikivi, Pilgujärvi), Pahtajärvi feeder dike, three layered lava flows, and black shale. Data from Walker et al.(1997) and this study. Near-chondritic initial Os isotope compositions obtained by Hannah et al (2006) for a suite of blackshale samples are not shown, because Os concentration data were not published.

4. Because of their high 187Re/188Os ratio, the present-dayisotopic compositions of ancient black shale samples are com-monly highly radiogenic and require significant correctionsfor in situ decay of 187Re to 187Os, decreasing the precision ofthe calculated initial 187Os/188Os ratios.

5. Postdepositional mobilization, particularly of rhenium,can disturb the primary isotopic systematics (e.g., Siebert etal., 2005; Walker and Nisbet, 2005; Kendall et al., 2009b;Williams et al., 2010). In spite of these complications, Re-Osisochrons have recently been obtained for many sample suitescovering a wide-age spectrum (see references in the Fig. 9caption).

Published Os isotope data on the sedimentary rocks fromPechenga are still scarce. Walker et al. (1997) reported resultsof six analyses, which did not give consistent isotopic compo-sitions as three samples yielded negative initial γOs values,likely due to postdepositional gain of Re, and three othersamples gave values between 294 and 1,130 (Fig. 8). In theirmixing modeling, Walker et al. (1997) used γOs values of 300and 600 for the sedimentary end member with 1 ppb Os andcame to the conclusion that the radiogenic Os isotope com-positions of the ore-bearing systems can be explained by 4 to10 percent addition of crustal osmium. Utilizing sulfide andorganic matter fractions, Hannah et al. (2006) produced anisochron for the Pechenga black shales with an age of 2004 ±

9 Ma and an initial 187Os/188Os ratio indicating close to chon-dritic isotope evolution (γOs 18 ± 18), thus, differing signifi-cantly from the earlier assumption on the existence of radi-ogenic osmium in the Pechenga pelitic metasediments. Thisresult suggests that the effects of contamination with blackshale material would be very limited on Os isotopes in fer-ropicritic magma.

Figure 9 shows initial osmium isotope compositions ofblack shales and other carbonaceous sedimentary rocks vary-ing in age from Archean to Cretaceous, together with Os iso-tope evolution of seawater from Jurassic to present. All ages,initial ratios and their errors in this figure are based onisochron data. There is a general increase in initial 187Os/188Osfrom nonradiogenic values in Archean and Paleoproterozoicblack shales toward highly radiogenic values at the end of Pre-cambrian and then a decrease to lower but variable ratios dur-ing the Phanerozoic eon. The three oldest nonradiogenic car-bonaceous shales come from the Transvaal Supergroup,South Africa (ca. 2316 Ma; Hannah et al., 2004), the Hamer-sley Basin, Western Australia (ca. 2500 Ma; Anbar et al.,2007) and the southwestern Superior province, Canada (ca.2695 Ma; Yang et al., 2009). The position of Pechenga in thediagram is based on data from Hannah et al. (2006). In addi-tion to these, Os isotope data on kerogen from carbonaceousand sulfidic shales of the ca. 1850 Ma Virginia Formation (Rip-ley et al., 2001) and on Ludicovian pyrobitumen (shungite)


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0.2

0.4

0.6

0.8

1.2

1.4

1.6

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800

Chondritic evolution γOs=0

Averagepresent-dayseawater Os

1.0

γOs=500

γOs=1000

Age (Ma)

GREAT

OXIDA

TION E

VENT

Pechenga

187 O

s/18

8 Os

FIG. 9. Isochron-based initial Os isotope ratios for black shales and other carbonaceous sediments through time with datataken from Cohen et al. (1999), Singh et al. (1999), Mao et al. (2002), Li et al. (2003), Selby and Creaser (2003, 2005), Han-nah et al. (2004, 2006), Kendall et al. (2004, 2006, 2009a-c), Anbar et al. (2007), Jiang et al. (2007), Selby (2007), Turgeon etal. (2007), Azmy et al. (2008), Selby et al. (2009), Xu et al. (2009), Yang et al. (2009), Finlay et al. (2010), and Rooney et al.(2010, 2011),. Dashed curve showing the Os isotope evolution of seawater from Jurassic to present after Peucker-Ehrenbrinkand Ravizza (2000).

from the ca. 2050 Ma Onega basin, Russian Karelia (Hannahet al., 2010), indicate low initial 187Os/188Os ratios. The non-radiogenic composition in Archean and Paleoproterozoicblack shales is explained by the predominance in ancient seasof hydrothermally derived osmium over radiogenic osmiumproduced by oxidative weathering of continental crust (Han-nah et al., 2004; Anbar et al., 2007). The trend shown in Fig-ure 9 makes it plausible that seawater in the basin where theblack shales of the Productive Formation were deposited hada nonradiogenic Os isotope composition. However, there isother evidence, not shown in Figure 9, indicating that Pale-oproterozoic seawater could have had a highly radiogenic os-mium isotope composition. Shirey and Barnes (1994) re-ported Os isotope analyses of shales associated with the 1.9Ga komatiitic Ni-Cu sulfide-bearing sills in the Cape Smithfold belt, Canada, having an analogous geologic setting tothat of the Pechenga ore-bearing intrusions. The Os isotopecomposition of these rocks is variable but highly radiogenicwith the initial 187Os/188Os ratios falling in the range of 0.99to 6.82, which are equivalent to γOs(1888 Ma) values of 680to 5300.

Also noteworthy are the measured Os concentrations ofblack shales and their sulfide fractions at Pechenga. The bulkOs concentrations can be relatively high, varying between 0.3to 2.4 ppb, but pyrite-rich bands or pyrite separates haveyielded very low Os contents of <0.1 ppb (Walker et al., 1997).Pyrrhotite can have a slightly higher concentration. Never-theless, it seems that Os is more concentrated in the Corg-richfraction than in the sulfide fraction in these sediments. This isconsistent with the data obtained by Ripley et al. (2001) onpartitioning of Os in the 1.85 Ga Virginia Formation argillite, inwhich Os enrichment factors for kerogen relative to whole rockrange from 4 to 210. It follows that melting or devolatization of

pyrite from country-rock black shales is ineffective in chang-ing the Os isotope composition of the interacting magma.

The first sulfur isotope study on the Pechenga rocks re-ported by Grinenko et al. (1967) revealed that there is a cleardifference in the S isotope composition between ores fromthe Eastern and Western ore camps. They also showed thatthe S isotope composition is independent of the Ni-Cu oretype. These general features have been confirmed by later in-vestigations (Abzalov and Both, 1997; Barnes et al., 2001) andare illustrated in Figure 10, which presents a compilation ofsulfur isotope compositions of Ni-Cu ores from the Westernand Eastern ore camps, divided into four types (disseminated,massive, breccias, and vein). The orebodies from the west(Kaula, Kotselvaara, Kammikivi, Semiletka) display a coher-ent, symmetric distribution with a distinct maximum at δ34Svalues of 0 to 0.5 per mil and 97 percent of the samples fallingbetween −2.0 and +2.0 per mil, that is within the range whichis thought to be typical for uncontaminated, mantle-derivedmafic magmas (0 ± 2‰; Ripley, 1999). The isotopic resultsfrom the Pahtajärvi dike, shown in the same diagram, overlapthe data from the Western ore camp. This is also the case forsulfides from barren ferropicritic flows in the Pilgujärvi Vol-canic Formation (Grinenko and Smol’kin, 1991; Melezhik etal., 1994). The Ni-Cu ore samples from the Eastern ore camp(Pilgujärvi and Kierdzhipori intrusions) form a single groupbut, compared to the Western ore camp analyses, they aredisplaced to higher δ34S values, having a maximum at δ34Svalue of 4.0 to 4.5 per mil and most samples in the range of3.0 to 5.5 per mil (Fig. 10). There is no overlap between thedistribution patterns of the western and eastern intrusionswith the boundary being at the δ34S value of 2.5 per mil.

Several studies have been carried out to characterize the sul-fur isotope composition of sedimentary rocks in the Productive

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15

20

25

30

35

40

14 16 18 20 22 24

Kaula,Kotselvaara,Kammikiviand Semiletkaintrusions(west)

Sδ-6-8

Feeder dike sulfide

Disseminated oreMassive oreBreccia oreVein ore

Sulfides from sediments

Pilgujärvi andKierdzhiporiintrusions(east)

Pah

tajä

rvi

34

FIG. 10. Histogram comparing the Pahtajärvi sulfur isotope measurements (black) with literature data on different oretypes and sedimentary sulfides. The dashed line presents the boundary between the sulfur isotope compositions of ore sam-ples from the western and eastern intrusions. Literature data sources: Grinenko et al. (1967), Pushkarev et al. (1988), Han-ski (1992), Melezhik et al. (1998), Barnes et al. (2001).

Formation (Grinenko et al., 1967; Akhmedov and Krupenik,1990; Abzalov and Both, 1997; Barnes et al., 2001), providingisotope data which can be compared with those from the sul-fide-bearing intrusions (Fig. 10). In the most comprehensiveanalytical campaign, Melezhik et al. (1998) determined the Sisotope composition of sulfides from different stratigraphiclevels of the Productive Formation and from samples inwhich iron sulfides have different modes of occurrence de-pending on the postdepositional diagenetic or cataclasticprocesses they have undergone. Figure 11 shows S isotopedata from Melezhik et al. (1998), arranged according to theapproximate stratigraphic position of the samples in a com-posite stratigraphic column starting from the middle part ofMember B. Melezhik et al. (1998) classified iron sulfides inblack shales into six categories varying from synsedimentaryand early diagenetic (class 1) to late-stage diagenetic and cata-genetic (class 6). As indicated in Figure 11, the sulfides in thelower parts of the stratigraphic section have undergone low tomoderate diagenetic transformation and display a bimodaldistribution of δ34S values, with abundant measurements be-tween 2 and 4 per mil and between −4 and 0 per mil. Theupper parts of the section, in turn, were affected by more ad-vanced diagenesis and have a wide spread of δ34S values,though mostly contain relatively heavy sulfur with δ34S valuesin the range of 6 to 9 per mil.

Figure 10 combines the sulfur isotope data from the ore-bearing intrusions and sedimentary country rocks. The near-chondritic δ34S values of the western intrusions have beenregarded as a sign of a mantle derivation of sulfur (Grinenkoet al., 1967; Abzalov et al., 1995) or its origin from Archeanbasement (Grinenko and Smol’kin, 1991), but it is also com-patible with assimilation of poorly consolidated synsedimen-tary or early diagenetic sulfides with δ34S values close tozero (Melezhik et al., 1994). However, as shown in Figure 11,most of the low δ34S values have been determined for sedi-ments from the lower part of the section, whereas the west-ern intrusions with chondritic δ34S were emplaced in theupper part of the Productive Formation. Considering theother δ34S maximum observed in the eastern intrusions (Fig.10), their relatively heavy S isotope signature has been unan-imously attributed to incorporation by magma of heavy sulfurfrom country-rock sediments (e.g., Grinenko et al., 1967;Hanski, 1992; Abzalov et al., 1995; Melezhik et al., 1994).Melezhik et al. (1994) suggested that the eastern intrusionswere emplaced into consolidated sediments and thereforetheir relatively high δ34S values reflect the average S isotopecomposition of all Productive Formation diagenetic stages.From Figure 11 it can be noted that, in the lower part of thesection, the measured δ34S values of sedimentary sulfides sel-dom reach the level found in the eastern intrusions. Thereexist numerous determinations of δ34S values of around 8 forblack shale samples, but these have been predominantly ob-tained from the upper part (Member C) of the ProductiveFormation (Fig. 11). Thus the presently available S isotopedata indicate a stratigraphic decoupling of the S isotope com-positions between the ore-bearing intrusions and sulfidic sed-iments. It follows that the sulfur isotope ratios measured forimmediate country rocks may not be representative indicatorsof the source of sulfur involved in the genesis of the Ni-Cumineralization.


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-5 0 5 10 15 20

-5 0 5 10 15 20

-5 0 5 10 15 20

-5 0 5 10 15 20

-5 0 5 10 15 20

-5 0 5 10 15 20

-5 0 5 10 15 20

-5 0 5 10 15 20

-5 0 5 10 15 20

100

300

200

400

500

600

m 0

Ultramafic tuff

Ultramafic tuffite

Greywacke and rhythmite

Gritstone and conglomerate

Massive sulfide layer

Diag. sulfide concretion

Multizoned pyrite concretion

Gritstone and conglomerate

Diag. calcite concretion

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ber

CM

emb

er B

Sδ34

6

6

7

5

5

4

3

3

2

1

Ad

van

cin

gd

iag

enes

is 6

7

5

5

43

3

21

FIG. 11. Sulfur isotope data for sedimentary sulfides arranged in a com-posite stratigraphic column through Members B and C of the ProductiveFormation. Data from Melezhik et al. (1998): 1 = synsedimentary to early di-agenetic pyrite, 2 = early diagenetic pyrrhotite-pyrite concretions, 3 = earlydiagenetic pyrite layers and lenses, 4 = middle diagenetic pyrrhotite micro-concretions, 5 = late diagenetic sulfide concretions, 6 = late-stage diagenetic-catagenetic sulfide concretions. 7 = data on unclassified pyrite or pyrrhotitefrom Hanski (1992) and Barnes et al. (2001).

As shown in Figure 2, the northern Soukerjoki intrusion isemplaced at the contact between the Productive Formationand underlying volcanic rocks and could have interacted withthe sedimentary rocks. One option is that the magma becamesulfur saturated by this interaction and the sulfide liquid set-tled deeper into the feeder dike. This process is regarded asunlikely, particularly in the case of the Kolosjoki prospect,which is located stratigraphically several hundred metersbelow the above-mentioned contact.

If the major part of the sulfur present in ores was not of amantle origin or from the Productive Formation, its source re-mains problematic, as there are no abundant sulfur-rich sedi-mentary lithologic units in the underlying Paleoproterozoicrock sequence. The only sedimentary unit that contains a sig-nificant amount of sulfides is the 50- to 150-m-thick BlackShale Member in the middle part of the Kolosjoki VolcanicFormation. It is uncertain to what extent this pile of blackshale acted as a source of sulfur for ascending ferropicriticmagma as individual intrusive lenses within the Pahtajärvi dikesystem are mineralized, not only above, but also beneath theBlack Shale Member, among the latter being the magmaticbody hosting the Kolosjoki Ni-Cu prospect. When consideringpotential Paleoproterozoic sources, it should be pointed outthat a major thrust zone (the Luchlompolo fault; Kozlovsky,1987; Gorbatsevich et al., 2010) exists between the KolosjokiVolcanic Formation and underlying sedimentary rocks, whichwas previously thought to represent a seismic boundary be-tween the Proterozoic and Archean complexes. Hence an un-known amount of potential sulfur sources in pre-KolosjokiVolcanic Formation supracrustal rocks is now missing. Fur-thermore, our sulfur isotope analyses did not reveal any

mass-independent sulfur isotope fractionation in the feederdike sulfides, which could have been used to confirm anArchean sedimentary source for sulfur (cf. Bekker et al., 2009).

SummaryFigure 12 displays a schematic stratigraphic cross section

from the upper part of the Northern Pechenga Group, illus-trating the mode of emplacement of ferropicritic magmapulses into different environments. Within a thick pile of sul-fide-bearing black shales and graywackes, synvolcanic, sill-like ultramafic-mafic bodies hosting Ni-Cu sulfide deposits inthe lower parts occur at two main stratigraphic levels. In theupper part of the sediment unit, ferropicritic magma eruptedviolently and formed thick accumulations of pyroclastic de-posits. Some portion of magma ascended later through theoverlying pillow basalt pile to sea floor where it crystallized aslayered and spinifex-textured lava flows. Also shown in Figure12 is a magma chamber at the contact between the KolosjokiVolcanic Formation and overlying sediments, which was fedby a magma conduit for sulfide-laden ferropicritic magmathat traversed through a pillow lava succession. In terms of Osand S isotopes, the Pahtajärvi prospect in the feeder dike issimilar to ore-bearing intrusions of the Western ore camp inthe upper part of the Productive Formation, suggesting thatsulfide-laden ferropicritic magma could have reached thisstratigraphic level from the feeder dike. The Productive For-mation sediments seem to have acted as a buoyancy trapthrough which dense sulfide-laden magma was incapable tointrude but started to be injected along horizontally beddedsedimentary strata and potentially received more sulfur by in-teracting with country-rock iron sulfides. Instead, magma that

158 HANSKI ET AL.

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Kolasjoki Volcanic Fm.

Ferropicritic tuffsPilgujärvi

SedimentaryFm.

(Productive Fm)

Pilgujärvi Volcanic Fm.

W E

Lava flowsγOs 3~6

γOs ~65, ~250

Feeder dike

34

3434

34

γOs 52~59δ S +1 to +2

δ S -1 to +2

γOs 43~50; δ S +3 to +5

δ S 7~9

δ S -3 to 5

Black Shale Mb.

34

FIG. 12. A schematic presentation on the occurrence of ferropicritic igneous rocks at different stratigraphic levels of theupper part of the northern Pechenga Group. Also shown are isotopic data on ferropicritic rocks and black shales.

was undersaturated or barely saturated with sulfide during itsascent managed to pass through the Productive Formationand formed barren lava flows higher up in the stratigraphy.

Convincing physical evidence exists for preemplacementsulfide saturation of at least some portion of the magma thatgenerated the Pechenga Ni-Cu ores. This raises the questionof the role of contamination with sulfidic country-rock mate-rial. However, because of the high lithophile element contentof the ferropicritic magma, the extent country-rock assimila-tion is not easily assessed using trace element geochemistry orNd isotopes. Support for sulfidic black shale assimilationcomes from Se/S ratios of ores and sulfur isotope composi-tions of the eastern intrusions. The most robust evidence forcrustal contamination is provided by Os isotopes, which showa distinctly more radiogenic signature for ore-bearing intru-sions, compared to barren layered lava flows (Walker et al.1997). Our new rhenium-osmium isotope measurements ofsulfide separates from the Pahtajärvi dike (γOs in the range of52−69) reveal that a significant component of radiogenic Oswas present in the magma already prior to its entering theoverlying magma chambers within sedimentary rocks. On theother hand, recent isotopic evidence indicates the presence ofnonradiogenic osmium in ancient seas (Anbar et al., 2007), in-cluding the Pechenga depositional basin (Hannah et al.,2006). Although marine black shales in general develop veryradiogenic Os isotope compositions over time, the negligibleage difference between the ferropicritic magmatism and sed-imentation at Pechenga prevented this to happen before theemplacement of the ore-producing intrusions. This impliesthat the significance of the assimilation of sedimentary coun-try-rock material during ore formation needs to be reevalu-ated and more work is needed to confirm the initial Os iso-tope signature of the sedimentary rocks.

AcknowledgmentsThe authors are thankful to Victor Melezhik for valuable dis-

cussions and Sarah-Jane Barnes and an anonymous reviewerfor their constructive reviews. Financial support by the Acad-emy of Finland (grant 116845) is gratefully acknowledged.This is a contribution (paper no. 2) to the ICDP FAR-DEEP.

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APPENDIX 1

S Ni Co Cu As Te Se Pt PdSample no. Area Rock type (wt %) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppb) (ppb)

28 Kaula Breccia ore 30.9 56900 1230 38000 5.78 37.930 Kaula Breccia ore 26.2 56000 1130 8600 3.99 29.9116-2 Pilgujärvi Breccia ore 8.8 1530 394 212 0.14 17.9236-1 Pilgujärvi Breccia ore 11.4 29500 715 6380 2.76 18.065 Pilgujärvi Disseminated ore 14.9 33500 957 5671 1.53 11.130029 Kotselvaara Disseminated ore 3.9 5614 186 2524 0.41 1.730029/dupl Kotselvaara Disseminated ore 3.7 5528 184 2510 0.43 1.830037 Kotselvaara Disseminated ore 1.4 4479 180 1205 0.46 1.130051 Kotselvaara Disseminated ore 3.1 7474 285 3338 0.74 1.8200202 Kotselvaara Disseminated ore 7.4 22107 654 6136 2.33 9.5200225 Kotselvaara Disseminated ore 1.6 4020 229 2057 0.35 1.037-3 Pilgujärvi Disseminated ore 10.1 33500 574 13500 2.80 16.0Kotsel 1a Kotselvaara Disseminated ore 12.1 40542 678 27521 7.00 17.3Kotsel 1b Kotselvaara Disseminated ore 11.9 40613 680 28405 7.40 17.4Kotsel 1 Kotselvaara Disseminated ore 12.1 40542 678 27521 5 7.00 17.3 900 1020Kotsel 1/dupl Kotselvaara Disseminated ore 11.9 40613 680 28405 7.40 17.4 760 1020Kotsel 2 Kotselvaara Disseminated ore 12.3 28043 656 37106 5 5.60 17.4 500 740Ortoaivi 1 Ortoaivi Disseminated ore 15.1 61000 1080 19600 10 13.60 41.6 1460 1930Ortoaivi 2 Ortoaivi Disseminated ore 12.5 22673 623 15432 5 2.40 14.3 160 340Pet1-40.60 Kammikivi Disseminated ore 11.9 29555 585 3609 5.17 4.95 11.2 301 150Pet1-41.55 Kammikivi Disseminated ore 10.9 35848 679 11649 12.5 4.22 10.0 373 467Pet1-42.75 Kammikivi Disseminated ore 12.4 43707 759 15061 23.5 5.66 13.1 525 719Pet1-44.00 Kammikivi Disseminated ore 13.5 48855 853 20350 28.8 9.09 18.2 678 1104Pet1-45.05 Kammikivi Disseminated ore 16.4 36557 706 42567 9.85 2.64 13.0 380 396Pet1-46.65 Kammikivi Disseminated ore 7.0 10516 252 57862 1.61 3.23 7.726 Kaula Massive ore 34.0 73900 1590 40200 2.56 35.426/dupl Kaula Massive ore 28.1 76839 1682 40615 3.40 36.0S-2904-5 Kierdzipori Massive ore 34.1 21800 549 9781 5.10 23.529-EJH-92 Kotselvaara Phyllite 1.7 138 24 85 3 0.06 0.62-EJH-92 Kotselvaara Phyllite 4.0 680 95 155 88 0.19 0.630-EJH-92 Kotselvaara Phyllite 3.4 73 30 117 3 0.04 0.633-EJH-92 Kotselvaara Phyllite 2.0 85 26 60 29 0.29 0.342-EJH-92 Kotselvaara Phyllite 1.6 370 58 111 3 0.10 0.69-EJH-92 Kotselvaara Phyllite 3.7 223 47 126 39 0.11 1.2Kotsel 3 Kotselvaara Phyllite 20.2 257 30 395 320 0.08 0.0Kotsel 4 Kotselvaara Phyllite 20.2 267 33 323 427 0.09 0.0Kotsel 5 Kotselvaara Phyllite 3.6 145 40 157 11 0.01 0.1Kotsel 6 Kotselvaara Phyllite 3.3 137 39 136 3 0.01 0.1Kotsel 7 Kotselvaara Phyllite 7.4 209 84 200 30 0.02 0.2Pet1-47.50 Kammikivi Phyllite 0.5 111 35 1740 32 0.11 1.0Pet1-48.40 Kammikivi Phyllite 0.9 100 32 734 32 0.14 1.2Kv-12A Kotselvaara Pyrite from phyllite 49.9 456 437 707 0.40 0.3Kv-12B Kotselvaara Pyrite from phyllite 48.4 407 351 717 0.41 0.3Kv-13A Kotselvaara Pyrrhotite from phyllite 36.5 983 232 390 72 0.02 3.8Kv-13B Kotselvaara Pyrrhotite from phyllite 36.5 877 213 361 65 0.04 4.9

Notes: Selenium, Te, Pt, and Pd contents of ore samples were determined by graphite furnace atomic absorption spectrometry (GFAAS) and Ni, Cu, Coand As contents by inductively coupled plasma-atomic emission spectrometry (ICP-AES) at the Geological Survey of Finland (Rovaniemi)

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