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377

The Canadian Mineralogist

Vol. 40, pp. 377-394 (2002)

THE PLATINUM-GROUP MINERALS IN THE UPPER SECTION

OF THE KEIVITSANSARVI NiCuPGE DEPOSIT, NORTHERN FINLAND

FERNANDOGERVILLA

Instituto Andaluz de Ciencias de la Tierra(Universidad de Granada CSIC) andDepartamento de Mineraloga y Petrologa, Facultad de Ciencias, E18002 Granada, Spain

KARIKOJONEN

Geological Survey of Finland, Betonimiehenkuja 4, FIN02150 Espoo, Finland

ABSTRACT

The Keivitsansarvi deposit, in northern Finland, is a low-grade dissemination of NiCu sulfides containing 1.326.6 g/t PGE.It occurs in the northeastern part of the 2.05 Ga Keivitsa intrusion and is hosted by olivine wehrlite and olivine websterite,metamorphosed at greenschist-facies conditions. The sulfide-mineralized area shows variable bulk S, Ni, Co, Cu, PGE, Au, As,Sb, Se, Te and Bi contents. S and Au tend to decrease irregularly from bottom to top of the deposit, whereas Ni, Ni/Co, PGE, As,Sb, Se, Te and Bi tend to increase. Thus, the upper section of the deposit has low S (

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378 THECANADIANMINERALOGIST

INTRODUCTION

Many early Proterozoic layered intrusions in centralFinnish Lapland and in Russian Karelia contain miner-alized zones enriched in the platinum-group elements(PGE; Alapieti et al.1990). In some deposits, PGE en-richment is associated with sulfide precipitation duringmixing processes between highly differentiated silicatemelts and new pulses of primitive maficultramafic melt(e.g.,Halkoaho et al.1990a, Huhtelin et al.1990). Inother deposits, such enrichment was produced by theprecipitation of PGE from fluids or fluid-enriched re-sidual melts that were transported upward through thepile of cumulates (e.g.,Halkohaho et al. 1990b, Iljina1994, Barkov et al.1995a, 2001). A third possibility hasbeen proposed by Mutanen et al.(1988) and Mutanen

(1997), who argued that transport of PGE in theKoitelainen and KeivitsaSatovaara complexes takesplace in the form of melt-soluble Cl complexes and thatconcentration of PGE occurred by the breakdown ofsuch complexes due to the crystallization of primary,cumulus or postcumulus chlorapatite, Cl-rich horn-blende and Cl-rich biotite. Nevertheless, the mineral-ogy and texture of the PGE-rich upper section of theKeivitsansarvi NiCu deposit, in the Keivitsa intrusion,involve a more complex interpretation, at least combin-ing the effects of magmatic crystallization, serpentiniza-tion, hydrothermal alteration and metamorphism.

In this paper, we present the results of a detailedmineralogical and chemical study of selected samplesfrom the upper section of the Keivitsansarvi NiCuPGE deposit. This study is mainly focused on the miner-alogy and mineral chemistry of the PGE and associatedsulfide and arsenide assemblages, as well as on thewhole-rock distribution of noble metals. Our results leadto an understanding of the distribution of Pt and Pd inthe ores, and suggest the existence of discrete, discon-tinuous PGE-rich horizons (similar to reef-type depos-its) at various depths in the upper section of the deposit.

GEOLOGICALCONTEXT

The Keivitsansarvi NiCuPGE deposit is locatedin the northeastern part of the Keivitsa layered intrusionin central Finnish Lapland (Fig. 1). The Keivitsa intru-sion occupies a surface area of approximately 16 km2

and consists of a lower ultramafic unit (> 2 km thick)overlain by a gabbro unit with a maximum thicknessexceeding 500 meters that grades upward into a grano-phyre unit of variable thickness (a few tens of meters)(Mutanen 1997). The ultramafic unit is composed ofpyroxeneolivine cumulates rich in intercumulus pla-gioclase, containing discontinuous layers and masses ofpyroxenite and gabbro. These ultramafic rocks also con-tain abundant xenoliths of komatiite-related volcanicrocks toward the upper part of the unit, where the

Keivitsansarvi deposit is located, and pelitic xenolithstoward the base of the intrusion. The gabbro unit is madeup of pyroxene gabbro, ferrogabbro and magnetite gab-bro containing discontinuous layers of olivine pyroxen-ite near the top of the unit, overlying a stratigraphic levelrich in pelitic hornfels and minor xenoliths of komatiite.Magnetite gabbro grades upward into a granophyremainly composed of sodic plagioclase, quartz and sec-ondary hornblende.

The same sequence of cumulate rocks can be ob-served in the Satovaara intrusion, which is located 23km to the east of the Keivitsa complex (Fig. 1). Mutanen(1997) considered the two bodies as blocks of a singleintrusion, the KeivitsaSatovaara Complex, separatedby a NE-trending fault zone.

The parental magma of the KeivitsaSatovaara com-plex intruded 2.05 Ga ago (Huhma et al.1995) into avolcanosedimentary sequence slightly above anotherolder (2.44 Ga) and larger (~750 km2) igneous body,the Koitelainen intrusion (Mutanen 1997). Thevolcanosedimentary sequence mainly consists ofkomatiite-related flows and tuffs intercalated within athick sequence of pelitic sedimentary rocks, including

Pt dans la plupart des cas. Toutefois, les roches montrent un enrichissement global en Pd par rapport au Pt. Cette anomaliedcoule du fait que la plupart du Pd logerait en solution solide dans la structure de la gersdorffite, la nickeline, la maucherite etla pentlandite. Les assemblages de minraux et leurs textures dans la partie suprieure du gisement de Keivitsansarvi sont lersultat cumulatif de la serpentinisation, de laltration hydrothermale et du mtamorphisme dun minerai pr-existant dissemin, faible teneur en NiCu, form par la cristallisation intercumulus dune faible fraction de liquide sulfur immiscible. Laserpentinisation a caus un enrichissement en Ni des sulfures et a conserv les teneurs originales en EGP. Plus tard, lors dunmtamorphisme aux conditions du facis schistes verts, les EGP et une partie de larsenic, ainsi que dautres semi-mtaux, furentlessivs de la zone minralise par une venue de fluides hydrothermaux, probablement transports sous forme de complexeschlorurs, et prcipits le long dhorizons devenus enrichis en NiCuEGP, comme cest le cas vers la partie suprieure dugisement. La recristallisation mtamorphique a aussi caus une dissolution partielle et une redistribution des aggrgats de sulfureset darsniures, contribuant ainsi un enrichissement accru du minerai sulfur en nickel.

(Traduit par la Rdaction)

Mots-cls: minerai de nickelcuivre, minraux du groupe du platine, bismuthotellurures palladifres, intrusion de Keivitsa,Finlande.

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PGMINTHEKEIVITSANSARVIDEPOSIT, FINLAND 379

black shales (Fig. 1). These sedimentary rocks containvariable but locally important amounts of graphite andsulfides (especially abundant where the Keivitsa com-plex was emplaced), and were transformed to hornfelsesalong the contact with the intrusion. Nevertheless, allthese rocks (including those of the Keivitsa intrusion)were later metamorphosed (and hydrothermally altered)during a regional metamorphic event that reachedgreenschist-facies conditions (Tyrvinen 1983).

THEKEIVITSANSARVIDEPOSIT

The host rocks

The Keivitsansarvi deposit occurs in the upper partof the ultramafic unit and is hosted by variably meta-morphosed olivine websterite and olivine wehrlite(Fig. 1). According to Mutanen (1997), the unmetamor-phosed rocks consist of cumulus augite, olivine,orthopyroxene and magnetite, with intercumulus plagio-

clase, phlogopite, hornblende, chlorapatite, monazite,graphite, ilmenite and sulfides. Olivine commonlyshows a pseudomorphic transformation to lizardite, andplagioclase grains generally are altered to fine aggre-gates of phyllosilicates. Where metamorphosed, thepyroxenes are partially replaced by amphiboles, olivineand specially the early-formed lizardite are replaced byantigorite, which develops interpenetrating plates thatcut and overprint earlier pseudomorphic textures, andplagioclase is transformed into chlorite, epidote, calciteand minor scapolite. Minor talc, chlorite and secondaryphlogopite also are present in the metamorphic assem-

blage.The primary modal composition of these rocks is

relatively constant along the ultramafic sequence exceptfor a slight, irregular increase in the clinopyroxene:orthopyroxene ratio upward. Similarly, the compositionof the major minerals exhibit a crude trend, with increas-ing Di in clinopyroxene and En in orthopyroxene, An inplagioclase, and Fo and especially Ni in olivine from

ROVANIEMI

HELSINKI

Keivitsa

Koitelainen

Satovaara

1 2 3 4 5 6 7 8 9 10 11

5km

FINLAND

KEIVITSA

SATOVAARA7511

7513

3498 3500

FIG. 1. Simplified geological map of the KeivitsaSatovaara intrusion, redrawn after Mutanen (1997). The following symbolsare shown on the map: 1: CuNiPGE ore, 2: pelitic and black schist hornfels, 3: komatiite-related ultramafic rocks, 4: olivinewerhlite and olivine websterite, 5: gabbro, 6: granophyre, 7: mica schists and other pelitic rocks, 8: black schists, 9: arkosicquartzite, 10: chloriteamphibole rocks, 11: fault.

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the lower ores upward. Representative whole-rock com-positions of selected samples from the drill cores stud-ied are listed in Table 1.

These mineralogical and chemical variations occuralong with an increasing abundance of komatiite xeno-liths (in some cases containing pelitic intercalations) inthe host ultramafic rocks toward the top of the ore de-posit. Toward the roof of the intrusion, the abundanceand size of the xenoliths increase; these vary from mm-size single crystals of olivine to bodies one hundredmeters across (Mutanen 1997). These arguments, to-gether with other trace-element (e.g.,REE) and isotopedata (S, SmNd and ReOs: Huhma et al.1996, Hanskiet al.1996), were used by Mutanen (1997) to develop afractionation model based on the contamination of theparental melt of the KeivitsaSatovaara Complex by theassimilation of pelites and black shales at the base ofthe magma chamber and by its chemical re-equilibra-tion with komatiite xenoliths (plus small amounts ofpelites) from the roof of the intrusion.

The sulfide ores

The Keivitsansarvi NiCuPGE deposit is a low-grade dissemination of sulfides, mainly characterized bysignificant variations in Ni content (from 87 ppm to~5%) and in the Ni:Co ratio (from 1 to 80), associatedwith variations in the content of semimetals (As, Sb, Se,

Te and Bi) and of noble metals (platinum-group ele-ments and gold). From bottom to top, sulfide mineral-ization shows increasing amounts of Ni, semimetals andplatinum-group elements.

In the samples of the upper section of the Keivit-sansarvi deposit selected for this study, the sulfides areinvariably interstitial to silicates, unless they were re-mobilized by late metamorphic fluids. In the latter case,sulfides (and arsenides) occur close to their primary lo-

cation, filling fractures, included in, and following thecleavage planes of amphiboles, chlorite and antigorite,and exhibiting irregular, serrated contacts with thesemetamorphic silicates. They also develop a fine dusting(submicrometric scale) throughout the serpentinizedolivine (replaced by lizardite). The mineral assemblagevaries slightly from one drill core to the next. In thesamples from drill core R695, the main sulfides are pent-landite, pyrite and chalcopyrite; monoclinic pyrrhotiteand millerite occur occasionally, and gersdorffite,bornite and mackinawite are rare. Pyrite displays tex-tural patterns varying from variably fractured, euhedralto subhedral crystals included in pentlandite or chal-

copyrite to extremely fine intergrowths with pentland-ite and, to a lesser extent, chalcopyrite (Figs. 2AD). Itis possible to observe all intermediate stages between

FIG . 2. Reflected-light microscopy images of sulfide,arsenide and sulfarsenide intergrowth textures from theupper section of Keivitsansarvi deposit. A. Broken andpartly dissolved pyrite (py) in pentlandite (pn) matrix,sample R713/22.40 m, width of the image 250 m. B.Pyrite intergrown with pentlandite and chalcopyrite (cpy)within pentlandite, sample R713/22.40 m, width of the

image 550 m. C. Graphic intergrowth consisting of thinbands of pyrite and chalcopyrite within pentlandite, sampleR695/81.15 m, width of the image 125 m. D. Graphicintergrowth of pyrite and chalcopyrite at the margins of apentlandite crystal, sample R695/81.15 m, width of theimage 250m. E. Aggregate of pentlandite, and gersdorffite(gd) surrounding niccolite (nic), sample R713/29.95 m,width of the image 250 m. F. Interstitial pentlandite andchalcopyrite with niccolite and gersdorffite, sample R713/29.97 m, width of the image 250 m.

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A B

C D

E F

pn nic

gd

nic

gd

cpy

pnpn

gd

gd

nic

pn

pn

py

py

cpy

cpy

py

cpy

pn

pn

py

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the textural relationships described; in these cases, py-rite seems to be replaced by pentlandite and chalcopy-rite. Fine intergrowths of pentlandite and chalcopyrite

occur occasionally along the contact between these twominerals. Chalcopyrite also occur filling late fracturesin silicates. Monoclinic pyrrhotite is associated withchalcopyrite, pyrite, and granular pentlandite, and usu-ally exhibits abundant flame-shaped exsolution-inducedblebs of pentlandite. Millerite is associated with pent-landite and is the main constituent of the dusting of sul-fides in the pseudomorphic lizardite. Gersdorffite formseuhedral to subhedral crystals included in pentlanditeand chalcopyrite. Mackinawite replaces pentlandite, andbornite invariably occurs in association with chalcopy-rite. The samples from drill core R713 show almost thesame mineralogy and texture as those from R695,although they are rich in nickeline, maucherite and,especially, gersdorffite (Figs. 2EF). Nickeline andmaucherite occur separately but are associated withchalcopyrite and pentlandite (or pentlanditepyriteintergrowths), or both, and are invariably surrounded bygersdorffite. The latter is locally replaced along outergrain-boundaries by a second generation of maucheriteand, in rare cases, it exhibits minute inclusions of pyr-rhotite and pentlandite at the replacement front.Gersdorffite also forms isolated euhedral to subhedralcrystals included in or, more commonly, at the grainboundaries of pentlandite.

Electron-microprobe analyses of the main sulfidesshow that Ni content of pentlandite is very variable,from 33 to 42 wt.% Ni (2632.5 at.%); Ni:Fe ratios fallbetween 1.0 and 1.55 (Table 2). This chemical variabil-ity is closely related to the mineral assemblage. In thepentlanditepyrite assemblage (containing small

amounts of chalcopyrite), the Ni content of pentlanditevaries from 30 to 32.5 at.%, whereas in the assemblagepentlandite pyrite monoclinic pyrrhotite chalcopy-rite, pentlandite contains between 26 and 27 at.% Ni.These compositional ranges indicate that the pentland-ite from the upper section of the Keivitsansarvi depositis similar to that equilibrated at relatively low tempera-ture in association with pyrite, pyrrhotite or millerite inthe Marbridge deposit, Quebec (Graterol & Naldrett1971). In fact, the composition reported here for pent-landite in the pentlanditepyrite assemblage overlapsthat of pentlandite in the divariant field of pentlanditepyrite in the condensed system FeNiS at 230C(Misra & Fleet 1973). The coexisting pyrite contains0.171.97 at.% Ni and 0.182.37 at.% Co, and the pyr-

rhotite contains 0.090.77 at.% Ni (Table 2). The Ni andCo contents of pyrite are higher than those of pyriteassociated with pentlandite, pyrrhotite and millerite inthe Marbridge deposit (Graterol & Naldrett 1971) and,according to the results of Misra & Fleet (1973), couldbe explained by assuming temperatures of equilibrationabove 140C for the assemblage pentlandite pyrite chalcopyrite monoclinic pyrrhotite described here.The composition of the pyrrhotite suggests that Ni dif-

fusion stopped at very low temperatures, in agreementwith phase relations at 140C (Misra & Fleet 1973).

The composition of gersdorffite also indicates that

its final re-equilibration took place at a relatively lowtemperature. This mineral shows a wide compositionalrange, from (Ni0.54Co0.35Fe0.10)0.99As0.99S1.0to the Co-and Fe-free Ni end-member, with a Co:Fe ratio varyingfrom 0.33 to 3.69, and with nearly constant metal to non-metal ratio. From Klemms (1965) experimental resultsin the system NiAsSCoAsSFeAsS, these composi-tions correspond to gersdorffite grains crystallized or re-equilibrated on cooling below 500C. The analyzednickeline and maucherite contain minor amounts of Fe(0.280.87 and 0.11.67 at.%, respectively) and lessthan 0.1 at.% Co.

THEPLATINUM-GROUPMINERALS

A total of 120 grains of platinum-group minerals(PGM) were found in fifteen selected polished sectionsfrom the two drill cores studied. Of these grains, 54%occur as isolated PGM grains included in silicates (cu-mulus pyroxenes in some cases, but mainly hydrous sili-cates: amphibole, antigorite and chlorite), 39% areassociated with sulfides (attached to grain boundaries),

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and only 6% of the grains occur included in sulfides.Their grain size is very small, ranging from a sub-roundish crystal of 35 m across to grains less than

1 m. Most of the identified PGM (80%) correspond to

minerals in the system PtPdNiTeBi, includingmoncheite, merenskyite, michenerite, Pt- and Pd-richmelonite, and minerals of the kotulskite sobolevskite

(sudburyite) solid-solution series. Sperrylite grains areabundant as well, representing 14% of the total PGMfound. Other PGM encountered are cooperite, braggite,PtFe alloys and mertieite II or stibiopalladinite. Somegrains of native gold also were found.

PGM of the system PtPdNiTeBi

Moncheite[(Pt, Pd, Ni) (Te, Bi)2] is the most abun-dant mineral of this group, representing 55% of the tel-luridebismuthide grains found. It occurs as relativelylarge grains (from 35 to 5 m) included in silicates, atsilicatesulfide grain boundaries or, less commonly, atpentlanditemillerite grain boundaries or associatedwith chalcopyrite filling late fractures in silicates. Itschemical composition shows an extensive substitutionof Te for Bi and of Pt for Pd (Table 3, Figs. 3, 4). Bicontent varies from 7.16 to 22.24 at.%, and Pd substitu-tion for Pt extends from the composition of Pd-freemoncheite to that of platinian merenskyite (Figs. 3, 4),in agreement with the existence of complete solid-solu-tion between merenskyite and moncheite. Minoramounts of Ni (

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cal composition of the moncheite plots within the com-positional range reported in the literature (Fig. 4).

Merenskyite [(Pd, Pt, Ni) (Te, Bi)2] and melonite

[(Ni, Pd, Pt) (Te, Bi)2] are less abundant (13% and 9%of the total telluridebismuthide grains, respectively),and the grains are smaller (

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in three different small regions of the melonite compo-sitional field within the system NiTe2PdTe2PtTe2(Fig. 3, Table 5), corresponding to the formulae: (Ni0.67

Pd0.23Pt0.06Fe0.04)1.0(Te1.93Bi0.05Sb0.02)2.0, (Ni0.39Pd0.32Pt0.22Fe0.04Cu0.01)0.98(Te1.54Bi0.43)1.97 and (Ni0.48Pd0.46Fe0.07Pt0.01)1.02(Te1.73Bi0.21Sb0.02As0.02S0.01)1.99.

Only four grains of michenerite(PdBiTe) have beenfound. Two of them occur within hydrous silicate, andthe two others are associated with chalcopyrite, filling anarrow fracture in hydrous silicate. Their grain size var-ies from 9 5 m to 2 1 m; they present irregular,curved grain-boundaries. Electron-microprobe analysisof the largest grain gives a Ni-free, metal-rich composi-tion, corresponding to the formula: (Pd0.97Fe0.05Pt0.02Os0.01)1.05(Bi0.89Sb0.04As0.02)0.95Te1.01.

Minerals of the kotulskite sobolevskite (sud-buryite) solid-solution series occur as elongate, smallirregular grains (the largest measures 12 7 m) in-cluded in hydrous silicates or at silicatesulfide grainboundaries. They commonly are fractured and cut bylate laths of chlorite or antigorite. As shown in Figures5 and 6 and Table 5, the analyzed grains extend in com-position from bismuthian kotulskite [(Pd0.93Fe0.02)0.95(Te0.81Bi0.21Sb0.01)1.03] to antimonian sobolevskite[(Pd0.97Fe0.03Ni0.01Pt0.01Os0.01)1.03(Bi0.62Sb0.29Te0.05S0.01)0.97]. This compositional trend provides furtherevidence of the existence of continuous solid-solutionbetween kotulskite and sobolevskite, as shown byEvstigneeva et al.(1976), but with a substitution of Tefor Bi + Sb, confirming their assumption of the prob-

able existence of phases intermediate among kotulskite,sobolevskite and sudburyite. Pd substitution for Pt andNi is negligible (Table 5).

Kotulskite Sudburyite

Sobolevskite

PdTe PdSb

PdBi

FIG. 6. Composition of kotulskite sobolevskite (sud-buryite) solid-solution series in at.% from the upper part ofthe Keivitsansarvi deposit plotted in the PdBiPdTePdSbdiagram. Symbols: dots: samples from drill core R695,and crosses: samples from drill core R713.

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Sperrylite(PtAs2)

Sperrylite is, together with moncheite, the most

abundant PGM in the upper section of the KeivitsansarviNiCuPGE deposit. It occurs as rounded or irregulargrains included in silicates, either isolated or associatedwith pentlandite. Only one grain was found included inpentlandite. Its grain size varies from 20 11 m to 1 1.5 m. Electron-microprobe analyses of the differ-ent grains (Table 6) reveal a composition with minoramounts of Fe (0.52.52 at.%), Ni (

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proportions of Fe (1.391.86 at.%) could be the resultof contamination from the enclosing silicate.

TRACEPLATINUM-GROUPELEMENTSINSULFIDES,ARSENIDESANDSULFARSENIDES

The results of the analyses of trace Ir, Rh, Pt and Pdin sulfides, arsenides and sulfarsenides from the uppersection of the Keivitsansarvi NiCuPGE deposit arelisted in Table 7. This table does not include Ir becauseit has not been detected above the minimum detection-limit (MDL) in any run. These results show that the maincarriers of PGE (especially Pd) are gersdorffite and, toa lesser extent, nickeline and maucherite, in agreement

with the previous data of Gervilla et al. (1997, 1998)and the experimental results of Gervilla et al. (1994).Traces of Rh have been detected (up to 454 ppm) inthree analyses performed on three different grains ofgersdorffite, suggesting that this mineral can dissolvesignificant amounts of Rh. Traces of Pt have also beenfound in several analyses performed on different miner-als. It has been detected in one analysis of pentlanditepyrite micro-intergrowths (42 ppm), in three analysesof chalcopyrite (3454 ppm), in one analysis ofnickeline (36 ppm), and in two analyses of gersdorffite(31, 277 ppm). These results show that although minoramounts of Pt can be dissolved in sulfides, arsenidesand sulfarsenides, this element is dominantly present inthe form of discrete PGM (mainly moncheite and sper-

rylite). In contrast, Pd minerals are comparatively lessabundant than Pt minerals, and Pd tends to occur in thestructure of the main ore minerals. As cited above, themain carrier of this PGE is gersdorffite, which usuallydissolves several hundred ppm Pd (up to 868 ppm).Similarly, Pd has been detected in all analyses per-formed on nickeline and maucherite, which contain,respectively, 47342 ppm and 25241 ppm Pd. Discretegrains of pentlandite and pentlandite intergrown with

pyrite also carry significant amounts of Pd (

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dritic values), with variable levels of Os. In arsenide-rich samples, Ir and Ru contents increase up to 0.2 timeschondritic values (Fig. 7). These data show a close cor-relation between arsenides and the more refractory PGE.Although we have not observed any PGM containingthese elements in the samples studied, the presence ofcomposite sulfarsenide crystals containing a core ofirarsite surrounded by hollingworthite and cobaltite inheavy concentrates from other drill cores of the Keivitsaintrusion and in disseminated sulfide ores of theSatovaara intrusion (Mutanen 1997) suggests that thesesulfarsenides could be the carriers of Ir, Rh and, to alesser extent, Os and Ru in the Keivitsansarvi deposit.

PGE values in the upper section of the Keivitsansarvi

deposit (Table 8) are, on the whole, ten times higherthat those reported by Mutanen (1997) for NiPGE ores.Furthermore, the reported average concentrations of Pt(1734 ppb in drill core R695 and 2377 ppb in R713)and Pd (2428 ppb in drill core R695 and 1472 ppb inR713) approach those of some reef-type PGE depositslike the Sompujrvi (sulfide-type) reef and the AlaPenikka (normal) reef in the Penikat layered intrusion(Halkoaho et al.1990a, b), and the Merensky Reef in

the Bushveld Complex (Barnes et al.1985), which con-tain, respectively, 2667 ppb, 1965 ppb and 3740 ppb Pt,and 2700 ppb, 7125 ppb and 1530 ppb Pd, on average.These data suggest that the section studied represents aPGE-enriched horizon. There may be significant eco-nomic potential to the Keivitsansarvi deposit.

DISCUSSION

Mineralogical and textural relations of the Keivit-sansarvi PGM show evidence that, although some ofthese phases could have crystallized at a magmatic tem-perature (e.g., those that remain included in cumulussilicates), most of them formed or re-equilibrated at

moderately low-temperature conditions. The fact thatmost of these grains occur included in hydrous silicatesor at the contacts between sulfides and silicates indi-cates that late magmatic, hydrothermal or metamorphicprocesses had to play an important role in the genesis ofthese PGM.

Moncheite is stable up to 1150C (Elliot 1965); how-ever, the compositional trend shown by the grains weanalyzed (Figs. 3, 4), with extensive substitution of Te

0.00

0.01

0.10

1.00

10.00

100.00

Os Ir Ru RhPt PdAu

R695

0.00

0.01

0.10

1.00

10.00

100.00

Os Ir Ru RhPt Pd Au

R713

1

11

PGEsamp

les

/PGEc

hon

dri

tes

FIG. 7. Chondrite-normalized PGE diagrams of samples from drill cores R695 and R713, representative of the upper part of theKeivitsansarvi deposit. Patterns marked with dots represent samples containing arsenides and sulfarsenides.

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for Bi and of Pt for Pd, suggests a lower temperature offormation. Although no experimental data are availablein the systems PtPdTe and PtBiTe, this suggestion

is in agreement with the low thermal stability of PdTe2(752C: Kim et al.1990) and -PdBi2(420C: Shunk1969). Merenskyite thus had to crystallize well belowmagmatic temperatures. Although it could be arguedthat the high Pt content of the merenskyite we analyzedshould increase its thermal stability (752 C: Kim et al.1990), Hoffman & MacLean (1976) showed that thestability of this mineral strongly decreases with the sub-stitution of Te for Bi. The kotulskite end-member of thekotulskite sobolevskite (sudburyite) solid-solutionseries decomposes at 746C (Kim et al.1990) but, prob-ably, this limit of thermal stability diminishes with thesubstitution of Te for Bi, as occurs in merenskyite.Michenerite is stable only below 500C (Hoffman &MacLean 1976).

According to the experimental results in the systemPtSPdSNiS (Verryn & Merkle 2002), the analyzedgrain of cooperite crystallized around 950C, and itscomposition does not reflect equilibrium with the onlygrain of braggite found. The chemical composition ofthe latter plots outside the compositional fields estab-lished by Verryn & Merkle (2002) between 1200and700C. According to these experimental results, thesolubility of Ni in braggite tends to increase as tempera-ture decreases, which suggests that our analyses couldcorrespond to a mineral equilibrated below 700C. Thereported composition of braggite is similar to that fromthe Lukkulaisvaara layered intrusion, which crystallizedbelow 600C (Barkov et al.1995b). Nevertheless, analternative interpretation of PdPt(Ni) sulfides whosecompositions plot inside the miscibility gap between

braggite and cooperite established experimentally(Verryn & Merkle 2002) is that they are metastablephases resulting from the loss of Pd during hydrother-mal alteration (R.K.W. Merkle, pers. commun., 2001).The preferential leaching of Pd could be explained bythe lower stability of PdS, compared to PtS, in aqueoussolutions under different thermodynamic conditions(Gammons et al.1992, Wood et al.1992).The Pd-im-poverished altered grains should preserve their original(braggite) structure (e.g.,Barkov et al.1995b).

The texture of the sulfide assemblages described canbe interpreted on the basis of phase relations in the sys-tem FeNiS at 725C (Karup-Mller & Makovicky1995), 650C (Kullerud et al.1969), 400C (Craig etal.1968) and 230C (Misra & Fleet 1973). These ex-

perimental results predict that Ni-rich pyrite formedslightly above 725C in equilibrium with monosulfidesolid-solution (mss) and Fe-rich vaesite (NiS2) or withmssalone for Fe-richer compositions. On cooling, Fe-rich vaesite became unstable for the average bulk-com-position of pentlanditepyrite assemblages (~5459at.% S, ~2930% Fe and ~1012% Ni; estimated fromchalcopyrite-free composite aggregates of coexistingpyrite and pentlandite) and upon reaching 400C, re-

acted with the coexisting mssto form a mssricher in Niin equilibrium with Ni-rich pyrite. As temperature con-tinued to drop, Ni-rich mssdecomposes, forming pyrite

and pentlandite at 230C. This reaction sequence explainsthe abundance of fine pyritepentlandite intergrowthsin the polished sections studied. Slight variations in thebulk composition of the original, interstitial sulfide meltgave rise to different proportions of pyrite and pentlan-dite showing the diverse textures described. In addition,the presence of chalcopyrite in many interstitial aggre-gates of sulfides indicates that this mineral could be inequilibrium with pyrite and intermediate solid-solution(iss) when it becomes stable at 557C (Cabri 1973). Oncooling, this ternary assemblage changes to chalcopy-rite pyrite pyrrhotite. This conversion could explainthe presence of euhedral crystals of pyrite in chalcopy-rite and of pyritechalcopyrite intergrowths. However,the common absence of monoclinic pyrrhotite in theseassemblages, as well as the presence of monoclinic pyr-rhotite associated to intergrown pentlandite and pyrite,suggest that other factors (e.g., hydrothermal alteration,metamorphism) modified the expected low-temperaturephase relations, giving rise to non-equilibrium assem-blages.

The above-described evolutionary picture of the dis-seminated NiCu mineralization, formed by inter-cumulus crystallization of a small fraction of immisciblesulfide melt, is somewhat different in arsenide-richsamples from drill core R713. Experimental results onthe partitioning of platinum-group elements in the sys-tem FeNiCuS (containing minor amounts of As, Teand Bi: Fleet et al.1993), as well as the textural rela-tions of nickeline and maucherite with sulfide aggre-gates, suggest that these minerals could have formed

later during the crystal fractionation of the sulfide melt,together with chalcopyrite and iss. On cooling, nickelineand maucherite reacted with the coexisting sulfides toform gersdorffite coronae around Ni arsenides or dis-crete crystals where the amount of arsenides was verysmall. The compositional trend shown by gersdorffiteindicates that it could start to crystallize around 500C(for the Co-richest compositions, Klemm 1965), but re-equilibrated on cooling by the substitution of Co for Ni,under moderately high anion (S and As) fugacities (Hemet al.2001).

Although most textural relations can be ascribed tothis magma-related magma-related mineralization uponcooling, the low thermal stability of the bismutho-tellurides associated to hydrous silicates, the composi-

tion of PdPt(Ni) sulfides and the presence ofnon-equilibrium assemblages in the sulfides clearly in-dicate the activity of fluids. Our present knowledge doesnot allow us to establish unequivocally the nature ofsuch fluids nor the timing of a possible hydrothermalevent related to low-density fluids separated from thecrystallizing intercumulus melt (such as those describedby Boudreau & McCallum 1992). It is evident, how-ever, that the rocks of the upper section of the

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Keivitsansarvi deposit suffered partial serpentinizationfollowed by metamorphism under greenschist-faciesconditions, associated to extensive hydrothermal alter-

ation (Tyrvinen 1983, P. Hltt, pers. commun.).Serpentinization caused Ni release from olivine, sincelizardite does not contain much of this element (Barnes& Hill 2000) and, consequently, Ni was collected bythe coexisting sulfides, which favored the stabilizationof Ni-rich assemblages containing pentlandite and mil-lerite (Eckstrand 1975, Barnes & Hill 2000). In arsenide-bearing assemblages, serpentinization could causepartial loss of As, stabilizing maucherite-bearing (Ni-rich) assemblages. Hydrothermal alteration associatedwith metamorphism could remobilize PGE, probably inthe form of chloride complexes (the host rocks of theKeivitsansarvi deposit are very rich in Cl; Mutanen1997), and precipitate them in discrete horizons. PGE-rich ores do not constitute a single, continuous horizon(as should be expected from magmatic crystallization),but form discontinuous bands at variable depths. Three-dimensional modeling of PGE distribution and prelimi-nary analytical data on other drill cores (with Pt and Pdreaching up to 2640 and 1160 ppb, respectively) pro-vide evidence for the presence of several PGE-enrichedhorizons in the upper section of the Keivitsansarvi de-posit. The hydrothermal fluids carried some arsenic (andother semimetals), as the existence of a second genera-tion of maucherite replacing gersdorffite along the outerboundary of grains shows. Consequently, metamor-phism of the host maficultramafic rocks partly dis-solved the sulfide (and arsenide) aggregates, as well asthe PGM, but mostly redistributed them. That is whythese minerals generally occur intersected by late am-phibole, antigorite and chlorite, along cleavage planes

of the hydrous silicates, and fill late fractures and veins.During this process, disseminated sulfides recrystallizedand re-equilibrated with olivine, resulting in furtherenrichment of Ni in the sulfides (e.g., Barnes & Hill2000, Mancini & Papunen 2000).

During the magmatic crystallization of the sulfideore, most of the Pt was concentrated in discrete miner-als, whereas Pd behaved according to its crystal-chemi-cal affinity during the fractional crystallization of thesulfide melt (Makovicky et al.1986, Gervilla et al.1994). Thus, Pd entered the structure of moncheite athigh temperature and formed other Pd tellurides, ars-enides and antimonides on cooling. Nevertheless, amajor part of Pd partitioned, in arsenide-free samples,into the mssand was accommodated in the pentlandite

structure as it crystallized. In arsenide-bearing samples,Pd was fractionated with As to the residual Cu-rich sul-fide melt, and was collected by the crystallizingnickeline and maucherite, and later, entered the struc-ture of gersdorffite. The stabilization of both pentland-ite and maucherite during serpentinization, preserved(and could even increase) the Pd content of these min-erals (Makovicky et al.1986, Gervilla et al.1994). Themetamorphism associated with hydrothermal alteration

of the ores resulted in the remobilization of PGM fol-lowed by their re-equilibration and precipitation ingeochemically favorable horizons, and in the redistri-

bution of Pd minerals combined with intercumulus hy-drous silicates and arsenides, especially high-Pdgersdorffite.

ACKNOWLEDGEMENTS

The authors dedicate this paper to Louis J. Cabri onthe occasion of his official retirement. It is clear thatalthough retired, he will continue to be a leader in theinvestigation of PGE-enriched assemblages in ores. Inthis respect, Dr. Cabri tested the electron-microprobe-derived trace-PGE results on sulfides and arsenides bycomparing them with PIXE data; a collaborative paperis now in preparation. The authors thank the GeologicalSurvey of Finland for permission to publish this paper,and the Junta de Andalucia (Research Group No. 4028)for partial financial support. Mr. Bo Johanson kindlyassisted in the electron-probe micro-analyses. Construc-tive comments of H. OBrien, T.A.A. Halkoaho, R.K.W.Merkle, A.Y. Barkov and R.F. Martin improved thiscontribution significantly. This paper is a contributionto IGCP Project No 427.

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The samples used for this study were selected fromtwo different drill cores (R695 and R713) with an aimto collect representative material from the upper sectionof the Keivitsansarvi NiCuPGEAu deposit where,according to Mutanen (1997), platinum-group elements(PGE) are concentrated. The samples were taken inlengths of 2050 cm from the drill cores.

Electron-probe micro-analysis

The mineralogy and texture of the PGM, sulfides aswell as their relations with the associated silicates, wereinvestigated in polished sections and polished thin sec-tions by both reflected- and transmitted-light micros-copy, systematic scanning electron microscopy (SEM)in back-scattered electron image mode (BEI) and en-ergy-dispersion spectroscopy (EDS) analyses for rapididentification followed by quantitative wavelength-dis-persion spectrum (WDS) electron-microprobe analyses.

The electron-microprobe analyses were done at theGeological Survey of Finland using a Cameca SX50instrument (Johanson & Kojonen 1995). The platinum-

group minerals (PGM) were located automatically in thepolished sections using the TURBOSCAN rare-phasesearch program (Walker 1990), which is based on thebrightness of the back-scattered electron (BSE) signalfrom heavy phases exceeding 1 m in size. Normally,six round 2.5-cm polished sections were scanned over-night; the following morning, the PGM were relocatedaccording to the coordinates listed by the program, ana-lyzed by EDS and, where large enough, by WDS forquantitative analysis. Each PGM grain was photo-graphed with the PGT Imix image-analysis program fordocumentation and grain-size analysis. WDS analysesused an accelerating voltage of 15 kV and probe currentof 20 nA for PGM and 20 kV and 45 nA for sulfides.The analytical lines and crystals used for the platinum-

group elements were: PtL on LiF, OsM on TAP,PdL, IrM, RuLand RhLon PET. The selection ofanalytical X-ray lines, background-measurement posi-tions and analyzing crystals was planned to eliminatethe inter-element peak overlaps of different PGE. Stan-dards for PGE were metallic Pt, Pd, Os, Ir, Rh and Ru.Other standards were: HgS for Hg, pentlandite for Feand Ni , pyrite for S, chalcopyrite for Cu, cobaltite forCo and As, Bi2Se3for Bi and Se, metallic Ag and Sb2Te3

for Sb and Te. The Cameca software applies the PAPdata correction method (Pouchou & Pichoir 1984).

Although it is generally considered that the electronmicroprobe cannot be used for trace-element analysis,detection limits of 520 ppm are routinely achieved(Cousens et al. 1997, Gervilla et al. 1997, 1998,Robinson et al.1998). Even sub-ppm detection limitshave been achieved for favorable elements (e.g.,Si, Ti,Cr) using higher currents and longer counting times thannormally used in major-element analysis (Robinson etal.1998). Trace analyses of Ni in silicates, Au in pyriteand arsenopyrite and PGE in sulfides, arsenides andsulfarsenides have recently been done at the GeologicalSurvey of Finland using the Australian CSIROTraceanalysis program specially designed for the CamecaSX50 (Johanson & Kojonen 1995, 1996, Kojonen et al.1998, Weiser et al.1998, Kojonen & Johanson 1999a,b). The conditions of analysis in trace analysis of PGEwere: accelerating voltage 35 kV, probe current 500 nA,measuring time 600 s (300 s for peak, 150 s for back-ground on both sides of the peak); metallic standardsPt, Pd, Ru, Ir; analysis lines PtL, PdL, RuL, IrM;

analysis crystals: LiF for Pt, TAP for Ir, PET for Pd,Rh. The statistical standard deviations () calculated bythe CSIROTrace program for these analyses were 914 ppm, corresponding to minimum detection limits(2, 95% confidence) of 18 to 28 ppm. In this study, thepure PGE metals were used as standards and measuredwith the same accelerating voltage as the sample, butwith a smaller sample current (ca. 10 nA) to avoid de-tector deadtime problems. The detection limits calcu-lated from the pure element peak/backgroundmeasurements according to the Ziebold equation(Ziebold 1967) correspond roughly to twice the stan-dard deviation of the trace-element measurements by theCSIROTrace program. The calculated limits of detec-tion are: 3034 ppm Pt, 2428 ppm Ru, 2022 ppm Ir

and 1820 ppm Pd.For trace-element analysis using an electron micro-probe, it is necessary to study the background carefullyaround the trace-element X-ray lines of interest to findout the shape of the background and possible overlapsof minor peaks from other elements present in thesample. This may be done using the SpectrumScanprogram of the CSIROTrace program package usingpredefined step-lengths and dwell-times. In some cases,

APPENDIX: SAMPLESANDANALYTICALMETHODS

WOOD, S.A., MOUNTAIN, B.W. & PAN, PUJING (1992): Theaqueous geochemistry of platinum, palladium and gold:recent experimental constraints and a re-evaluation oftheoretical predictions. Can. Mineral.30, 955-982.

ZIEBOLD, T.O. (1967): Precision and sensitivity in electronmicroprobe analysis.Anal. Chem.39, 858-861.

Received September 28, 2000, revised manuscript accepted

December 17, 2001.

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the background shape may be concave, convex or in-clined. The background positions were determined fromthe target minerals using the above mentioned proce-

dure and measured on both sides of the lines (PtL,PdL, RhLand IrM). The results of the scans wereplotted in xy diagrams to determine the backgroundoffsets on both sides of the peak.

The final minimum limit of detection depends onnumber of counts on peak and background, the peak-to-background ratio, and the X-ray excitation efficiency,which depend on the characteristics of the diffractioncrystals. The detection limit is strongly dependent onthe accelerating voltage used, especially with the LIFcrystal (Robinson & Graham 1992). For TAP and PET

crystals, the improvement of the detection limit is notso large with increasing accelerating voltage.

Platinum-group-element and whole-rock analysis

The drill-core samples were crushed and ground forthe inductively coupled plasma mass spectrometry(ICPMS) and X-ray-fluorescence (XRF) analysis(Juvonen et al.1994). For the ICPMS analyses, thePGE and Au were preconcentrated by nickel sulfide fireassay before analysis. The XRF analyses for the majorelements were done on powder pellets using a funda-mental parameter correction program developed in theReseach Laboratory of the Rautaruukki Company,Raahe, Finland, by Mr. Ilmari Alavainio.

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