FURTHER INTERPRETATTON of the Gu-Fe-Ni SULFTDE MINERALIZATION in the DULUTH COMPLEX, NORTHEASTERN MINNESOTA JILL DILL PASTERIS

Canadian Mineralogist Yol.22, pp. 39-53(1984)

FURTHERINTERPRETATTON OF THE Gu-Fe-Ni SULFTDE MINERALIZATIONIN THE DULUTHCOMPLEX, NORTHEASTERNMINNESOTA

JILL DILL PASTERIS Department of Earth and Planetary sciences, McDonnell center for the space sciences, Ilashington University, Box j169, St. Loui.s, Missouri 6SnA, U.S.e.

ABSTRACT Par contre, la pr6sencede trofite et de graphite, d6pos6 en phasevapeur, et d'ilmdnite sansmagndtite, dansla suite Approximately lQ sample5mineralized wilh Cu, Fe and Minnamax, indiqueraientun milieu plus r6ducteurque celui Ni sulfidesfrom the South Kawishiwi (Inco deposit)and du glte Inco. Les observationssuivantes montrent qu'une Partridge River (Minnamax deposit) intrusive complexes phase fluide mobile r6active fut active au stade tardif du werestudied by meansof the optical microscopeand the magmatisme:graphite d6pos6par phasevapeur dans les electron microprobe. Cathodoluminescence,microther- fractures de silicates non alt6r6s, inclusions fluides secon- mometry and liaser-Ramanspectroscopy were used on some daires chlorif&res dans l'olivine, remplacementd'agrdgats samples.In the two suites,both dominatedby troctolite, grossiersde pyrrhotine par de la cubanite massive,et pr6- the sulfides are mineralogically similar (predominantly pyr- cipit6s de sulfure de fer et nickel i grain fin le long de frac- rhotite, pentlandite, chalcopyrite, cubanite) and also textures dansl'olivine serpentinis6e.La grandevariation de turally similar. In the Inco suite, however, both magnetite composition de la pentlandite indiquerait aussi un r6dqui- and ilmenite are present,whereas troilite is rare. In con- libre post-magmatiquepartiel. Un bain sulfur6immiscible trast, the occurrence of troilite and vapor-deposited pr6coce(d rapport Fe:Cu 6lev6)et un fluide magmatique graphite, as also the presen.sel ilmenite without magnetite, ultdrieur enrichi en cuivre sont responsablesde la min6ra- in the suite from Minnamax, suggeststhat the environment lisation dansles troctolites:le liquide sulfur6, par pr6cipi there was more reducing than at Inco. The following obser- tation primaire et le fluide cuprifbre, surtout par rempla- vations show that a mobile, reactivefluid phasewas ac- cement. Le mouvement et la composition du fluide durent tive during late-stagemagmatism: vapor-deposited graphite avoir 6t6 gouvern6spar un procepsusintercumulus normal in fractures in unaltered silicates, Cl-bearing secondary dansle complexeintrusif, hypothbsequi du moins concorde fluid-inclusions in olivine, replacementof coarsepyrrhotite avecles donn€essur isotopesdu soufre(Ripley l98l) pour segregationsby massivecubanite, and precipitatesof fine- le glte de Dunka Road, situd dansle pluton de Partridge grainediron-nickel sulfide along fracturesin serpentiniz- River. ed olivine. Widely variable compositionsof pentlanditealso suggestpartial postmagmaticre-equilibration. Both an early Traduit par la Rddaction) immiscible sulfide melt (with a high Fe:Cu ratio) and a later Cu-enrichedmagmatic fluid producedmineralization in the Mots-clds:sulfures de Cu, Fe et Ni, complexede Duluth, troctolites, the sulfide melt by primary precipitation, the pluton de Partridge River, gite Minnamax, pluton de Cu-bearingfluid mainly by replacement.The movemenr South Kawishiwi, glte Inco, Minnesota. and composition of the fluid were controlled by normal intercumulusprocesses in the complex.This hypothesisis in accordwith the sulfur-isotopedata of Ripley (1981)for INtnorucrroN the Dunka Road depositin the Partridge River pluton. Sample locqtion Keywords:Cu, Fe and Ni sulfides,Duluth complex,Par- tridge River pluton, Minnamax deposit, South Approximately 20 specimen$of mineralized drill- Kawishiwi pluton, Inco deposit, Minnesota. core and hand samples were studied from the Inco and Minnamax mines sites in the Duluth Complex SOMMAIRE (Fig. l). The Inco material (provided by M. Boucher) consists predominantly of troctolites and augite- On a 6tudi6au microscopeet A la microsondeenviron bearing troctolites from the South Kawishiwi in- 20 6chantillons min6ralisdsen sulfures de Cu, Fe et Ni des trusive body, near the site ofthe Inco shaft (Cooper complexesintrusifs de South Kawishiwi (gite Inco) et par- et al. 1978t Fig. I ofthis paper). The exact localities tridge River Glte Minnamax). Certains dchantillons furenr of the individual samples are not 6tudi6s par cathodoluminescence,microthermom€trie et documented. spectroscopieRaman au laser.Dans les deux suites. domi- The Minnamax samples, representing a section of n6espar la troctolite, les sulfures(surtout: pyrrhotine, pent- the adjacent Partridge River troctolite body, are landite, chalcopyrite, cubanite) sont min6ralogiquementet macroscopically similar to the Inco suite. These texturellement semblables.Dans la suite Inco, la magndtire documented drill-core samples are from hole 146 in et I'ilmdnite sont presentes,alors que la troilite y est rare. the southern end ofthe Tiger Boy deposit, and repre- 39

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and fine-grainedgraphite was done by F. Adar (In- struments SA) and F. Purcell (Spex Corp.). The author performed later analyseswith an Instruments SA RamanorU-l 000. Quantitativewavelength-dispersion analysis(WDS) was carried out on selected ..6 automated "so)Iir!"'..$_..n' sulfides,oxides and silicateswith a fully ..a. t\i ... - trr JEOL 733 electron microprobe at Washington University. The standards are natural minerals and synthetic materials. Correction proceduresfor the analysesare based on Bence& Albee (1968) and Albee & Ray (1970).The operatingconditions were 15 kV acceleratingvoltage with currents of 50 nA for oxide, 30 nA for feldsparand 30 nA for sulfide phases.The sulfide dataarethe primary concernof this paper. l:;cli'.'".0* Previous studies Two previous detailed studies addressedthe Yi."'"' mineralogy,textures, relative abundances and com- .oap"ff positions of sulfide phasesin troctolitic sequences at the Inco site and in a drill hole south of Babbitt [Weiblen& Morey (196) and Boucher(1975), respec- tivelyl. Other reports describe the more general geologicalrelationships of the sulfide depositsto the from Coopgr, potential 6t a|.1978 Duluth Complexas well astheir economic (e.g., Bonnichsen 1972, I 974, Tyson & Chang I 984). More recently, oxygen-and sulfur-isotopeanalyses have been periormed on the Dunka Road deposit (Ripley& Rao 1980'Ripley 1981)and rubidium and FIG. l. Generalizedgeologic map of a portion of the strontium-isotopeanalyses on a Minnamax drill core Duluth Complex,showing sampling areas and other (Grant & Molling 1981),b.oth in the PartridgeRiver sulfidedeposits. 1 Inco site,2 Minnamaxsite, 3 U.S. studies, together with the (map troctolite. The above Steelsite, 4 WaterHen intrusivecomplex after & NaldretN(1977) Cooperet al. 1978and Ripley1981). sulfur-isotopework of Mainwaring on the Cu-Ni mineralizationin the Water-Hen in' trusive complexto the south, suggestthat reaction of melt and country rocks may havestrongly affected sentdepth levelsof 173to 586metres (samples pro- the sulfide precipitation in the Duluth Complex; they vided by S. Watowich, AMAX). The mine site is concludedthat much of the sulfur may have been pres(:ntlyowned by Kennecott,but the name Min- derived from the intruded sediments.The chemistry nam€x is retained here because AMAX was the and pfunicaltransport aspects of the mechanismremain operator during this program of drilling and sam- to be explained.It thus appears,from petrographicand pling. The silicaterock-types are predominantly troc- strontium-isotopeanalysis (Grant & Molling 1981), tolitic, but a hornfels zone that may represent that vast amountsof the underlying carbonaceous, xenoliths of underlying metasedimentary and sulfide-bearing Virginia Formation were not metavolcanicrocks also was s€urpled.The Cu-Ni physically assimilated by the Duluth magmas. sulfide mineralization consists of both the However, H2O, CO2 and other volatiles may have disseminated and massivg (Local Boy) type been selectivelyassimilated by the melt (c/ Weiblen (watowich 1978). & Morey 1976,Hollister 1980,Ripley 1981). The immediateaims of the presentstudy are (l) Techniques of analysis to compare and contrast sulfide relationshipsob- servedin the two depositschosen (Iable l) with those This overview involved a study by optical previously reported and (2) to re-interpret thesedata microscopyof about 2.0doubly polishedthin sections in light of more recentinformation on the composi- jn transmitted and reflected light. Microther- tion of late magmatic fluids and on intercrrmulus mometry, Raman spectroscopyand cathodolumines- processes.Of particular coniern are the following cencewere performed on severalsamples. The in- points: the timing of sulfide immiscibility in the itial Raman-microprobeanalysis of fluid inclusions magmatic sequence, the factors causing sulfide

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Flne spinel Coarse Graphlte Coarse Coarse & llm. lamellae Sample Po Tr Cp Cb graphlte in veinlets ilmenite rngnetite in sllicates

ARAA A in plag & Cpx

MINNAI'IAX ASAA A in p'lag & Cpx

Key'lJte' to abbrevlations: A abundant, S some, R rare, N not observed, po pyrrhotlte, Tr trol- Cp chalcopyrite, Cb cubanite.

precipitation,the numberof episodesof mineraliza- similar in the Inco and Minnamax troctolitic samples; tion and the possibility of subsequentremobilization the samples,therefore, will be consideredtogether. of sulfides. As documentedby previous investigators (e.g., Foose 1982,Tyson & Chang 1984),the in- Sulfide mineralogy ond textural types dividual troctolite bodiesat Duluth representmulti- ple injections of melt and locally strong interaction Samplesfrom the Minnamax Tiger Boy deposit with wallrock. With the smallnumber of samplesin representmassive mineralization of the Local Boy the presentstudy, only broad-scaledifferences be- type (-586 m depth), disseminatedsulfides in the tween the suitescan be recognized. basalzone (-490 m depth), and the "cloud zone" The following abbreviationsare usedin the text mineralization ( - 335m depth) that typically lies 200 and figure captions:Cp chalcopyrite,Cb cubanite, or more metres above the basal sulfide zone Pn penflandite, Bn bornite, Tr troilite, Po pyrrhotite, (Watowich 1978).Values of the Cu-Ni ratio reported mPo monoclinic pyrrhotite, hPo hexagonal pyr- in AMAX assaysrange from2,25 in an isolatedzone rhotite, sssolid solution, WDS wavelength-dispersion of hornfels to 5.01 in the upper basal zone. The spectroscopy, and EDS energy-dispersionspec- valuesare quite erratic, but averageabout 4.2, which troscopyon the electronmicroprobe. In the sample is distinctly higher than the averageCu-Ni ratio in numbersfor tlte Minnamax drill-core specimens.i.e., the Inco depositsto the north (SpruceRoad -2.7, 146-XXXX, the secondpart of the number refers Maturi -3.3) and of the U.S. Steeldeposir (-3.3) to the depth (in feet). immediatelyto the south (S. Watowich,pers. comm. 1982). OPAQUEMwnnaT_Sn-IcATE RELATIoNSHIPS The dominant sulfide minerals in the Inco and Minnamax samplesare pyrrhotite (in somecases, two Textural relationships of the sulfides are very compositions),chalcopyrite, cubanite and pentland-

TABLE2. REPRESENTATIVEBULK COMPOSIIION OF SULFIOES

Sample* Troillte(3) Pyrfhotlte Chalcopy- Cubanlte(7) Bornlte(3) Nl-Mack.lna-fleazlewood- Cp?(3) Bn?(3) (7) rlte(s) wtte(6) Jte(z)

Composlt.ionsin weight percent f9 63.83(.23)60.64(.19) 30.47(.zs) 41,49( ,37) l l .75 58.66(.74) 2.07(.o1) 2s.?7(.24\13.6s(.57) Ni 0.0(0) o,2t(.tolo.ozl.o2l 0,0(0) 0.05 4.97(.16) 69.51(1,i5)'r.34(.r3) 0.16(.28)0.13(.te) cu 0.c6(.10) 0.20(.20\s+.stl.ssl 22.88(,37) 62.20 o.++1.00) 36.01(.33) 57.e5(r.7e) co 0.0( 0 ) 0.01(.01)o.o (.orl 0.0(0) 0.02 0.02(.38) 0.01(.01) 0.03(.02)0.03(,04) s 36.57(.13)38.4.1(.51) sa.sri.+zi 35.24(.44) 26.44 35.r2(.81)28.54(1.63) 34.30(.40) 27.31(1.01) Totai 100.46(.42)99.47(.401 gs.st(.ssi 99,61(.42)100.46 99.21(.86\10i.47(.36) 99.77(,eo)99.07(.55) Composit'ionsin atomlc percent 't0 Fe 50 48 25 34 -- 46.8 22412 Ni ?o 56 Lu 25 ::to 49 .01 Co _t S5052b0 ?2 1: 50 41 48.8 41 50 43 Formula MS MgSro MzSz M:S: Ms.zsSq Mr,osS Mz.eeSz Mr.gaSz Ms,srS,, FeS FeeSls CuFeS2 CggoFezonS: Cr,"zeFgseSqF9e6Ni.0BS Nl273Fe10Cu05S2 Crr"ozFe.seS,Cu€ 9FeL12S4 "1919!lll9:u: after sample namelndicate numberof anatyses that were averaged. Parenthesesafter wl.% analyses showone standard dev.latiorr. Trollite: 146-]961(AMAX)' darker reglon of Po-Tr intergrowth. Pyrhotite: DG4(INco),gralns lnclude large lntercumulus- DG3-_(INco).,srains lnciude coarsl cp rameilae in cu,'itirnseirn iiii.ii"i-cp-pJ.-" !"ffiil13;.l:l^l::ll',1:-91:,''ft;lfilloliTiih,n[3]"i'i,i3Jio;'"!?,1'Tilli:"'1,!:;"50;f3;,'l;ii;i3; !!]lggnvrite: small-sraln,in ,,cp,,-pn-pn, il"llyii"U.'iiliilE;""3iill'i;ult3;'li,9o,r,- ' cp-Bn-Hz.cp?:,,i46-569 (AMAx),.sratns-tnglude 6i;tsii ;ne i;;nisnea isoiiiea sraini. Bn?: 146-1176,Cp-,,8n,, and dp-"Bn"-pn;,iompoiitions slm.iiai to thosein 146-569.Electron-mtcroprobe data.

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ite, all in widely varying proportions. Quantitative electron microprobe). Weiblen & Morey (1976) electron-microprobeanalysis Clable 2) confirmed the believed that some veinlets consist solely of presenceof cubanite and chalcopyrite, but not of the chalcopyrite and cubanite. Owing to the small grain- related lower-temperatureCu-Fe sulfidestalnakhite, sizeof the sulfide veinlets,point counting was not mooihoekite and haycockite. Lesser amounts of possible.However, it appearsthat veinletsconnect- mackinawite,bornite and an unknown Cu-rich Cu- ing larger sulfide bodieshave the samemineralogy Fe-sulfide also were identified optically and by as the bodies,whereas isolated narrow veinletswithin electron-microprobeanalysis. silicate grains may be Cu-enriched. Weiblen's & Morey's (1976)sulfide-silicate tex- Two other types of veinletsare restrictedto serpen- tural classification is applicableto both the Inco and tinized olivine grains: secondarysulfide and oxide AMAX samples:1) interstitial sulfides' bearingan in- stringers.These, commonly, are only a few micro- tercumulusrelationship to the plagioclasenetwork, metresin width, but locally are very abundant(Fig. 2) sulfide inclusionsin plagioclaseand clinopyroxene, 3). The sulfide veinletsconsist of a mottled white' 3) fine sulfide veinletstraversing grains of serpen- polygranular, strongly anisotropicNi-mackinawite tinized olivine as well as of unalteredsilicates, and (Table 2). Other thin veinletsconsist of magnetite 4) sulfide-silicateand sulfide-oxide intergrowths. (optical and microprobe confirmation), which is Interstitial sulfides are the dominant textural type physically separate from the sulfide stringers, in both the Inco and AMAX samples(Fig. 2). They although both opaquephases commonly occur in the consist of massivepolygonal blebs of pyrrhotite, samesilicate grain. pentlandite, chalcopyrite a cubanite, which are There is a wide variety of intergrowths(sulfide- locally accompaniedby a halo of irregular, finely silicate,sulfide-oxide, sulfide-sulfide), whosepro- dispersedsulfides. The texturesare intercumulate. per interpretation may help to definemore precise- Consideredin the categoryof included sulfidesare ly the timing and mechanismsof sulfide movement abundant, dominantly Cu-rich, elongate sulfide in the troctolites. It should be kept in mind, however, bodiesparallel to the (010)planes (i.e., albite twin- that small-scale textural anomalies and re- lamellae)in the intercumulusplagioclase grains and equilibration can lead to confusion. Although in adcumulus plagioclase overgrowths. Olivine sulfidescommonly lie directly adjacentto or within grains,in contrast,rarely havesulfide inclusions(as grains of a mafic silicate (clinopyroxene,biotite, noted also by Weiblen & Morey 1976). more rarely hornblende) and oxide (magnetite, il- Severaltypes of narrow veinletsof opaquephases menite), much of the sulfide is in contact with arepresent in the Inco and AMAX silicates.The widest plagioclase only (cl, Ripley 1981, Dunka Road veinlets (10-20 pm on average)cross-cut silicate deposit). grains and appearto connectthe large (up to a few As emphasizedby Boucher(1975), there are abun- millimetres)bodies of interstitial sulfide. The wider dant cases of replacement textures :rmong the veinlets contain optically identifiable pyrrhotite, pentlandite, chalcopyriteand cubanite (confirmed by

Ftc. 2. Interstitial grain of sulfide between plagioclase grains.Sulfide is predominantlymassive cubanitewith Frc. 3. Iron sulfide veinletsin serpentinizedolivine. The imall grains of chalcopyrite (not distinguishable in sulfideis strongly anisotropic,and in somecases is Ni- photoexaph)around the edge.Reflected light. Scalebar mackinawite. OL olivine, S serpentine.Reflected light, 200 um. Minnamax drill-core 1216,500 m depth. dry objective. Scalebar I mm. Inco sampleDG-8.

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aal4.1 ;.4i*

Frc. 4. Mediumgxey host is ilmenite,ILM. Light grey, Frc. 6. Pyrrhotite (mediumgrey) with pentlanditeinclu- largeinclusion is titanomagnetite(MT) with lamellae sions(small, bright white), replaced by massivecubanite of exsolvedspinel (black) and oxidized-exsolved (ight grey).No chalcopyriteobserved. In someareas, lamellaeof ilmenite(difficult to see).Within titanomag- pentlanditeforms inclusions in cubanite.Reflected light, netiteis whiteinclusion of chalcopyriteand pyrrhotite oil immersion.Scale bar 50 pm. Inco sampleDG-7. (s).Reflected light, oil immersion.Scale bar 50 l,lm. Inco sampleDG-8. of ilmenite and oxidized - exsolved titanomagnel- ite, the sulfide body is next to the magnetite (Fig. 4). There are many examplesof sulfides replacing sulfides.These are found in both the Inco and AMAX magnetitein magnetite-ilmeniteintergrowths @g. sarnples.Polyphase sulfide bodiescommonly man- 5). tle and appearto replacebiotite and Fe-Ti oxides. Many examplesin the Inco and Minnamax speci- Cu-Fe sulfides, in somecases, fill cleavageinterfaces mens also confirm Boucher's (1975) and Bon- in biotite $ains. Only rarely are sulfide bodiesen- nichsen's(1972) interpretations that Cu-Fe sulfides closedin Fe-Ti oxides.However, in almost everycase commonly replaced earlier pyrrhotite. Irregular in which the enclosingoxide is a coarseintergrowth islandsof pynhotite occur in cubaniteor chalcopyrite

Frc.7. Major phase(medium grey) is troilite, wirh lighter lamellaeof pyrrhotite (48 at.9o Fe). Round inclusions Ftc. 5. Remnant of lamellar intergrowth of magnetite and are lamellar intergrowths of chalcopyrite (white) and ilmenite (i.e., oxidized-exsolvedtitanomagnetite). Cu- cubanite (ieht grey). Appears to have been coexisting Fe sulfide (white) has selectivelyreplaced magrretite and pyrrhotite solid-solution and blebs of Cu-Fe-sulfide liq- left ilmenite (dark erey lamellae). Reflected light, oil uid (iix). Reflectedlight, oil immersion. Scalebar 50 immersion. Scalebar 8 pm. Inco sampleDG-2; pm. Minnamax drill-core 1216,558 m depth.

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tions betweenthe Inco and Minnamax suites.There are somemineralogical distinctions, however, as indicated in Table I and discussedin a later section. Almost all the samplesfrom both localitiescontain cubanite and chalcopyrite (or phasesof similar composition). In somecases, the chalcopyrite(Cp) and cubanite (Cb) occur as lamellar intergrowths with each other. In a few samples,lamellar Cp-Cb occurs in round bodiesenclosed in coarsegrains ofpyr- rhotite (Fig. 7). The lamellar intergrowth probably resulted from the subsolidus exsolution of a higher-temperature Cpo phase. In some cases, however, chalcopyrite and cubanite form distinctly separateregions in a sulfide body, or cubaniteoc- curs almost to the exclusionof chalcopyritein a sample. The former ca$emay representtextural re- Frc.8. Irregularbleb of pyrrhotitesurrounded by rim of equilibration of exsolvedbodies, but the latter case very finely crystsllinegraphite (arrows). Enclosed in requiresanother explanation (seediscussion section). serpentinevein. Reflected light, oil immersion.Scale In those samplesin which there is a strong associa- bar 20 pm. Minnamaxdrill-hole 146, 173m depth. tion of sulfideswith plagioclase,massive cubanite appearsdominant over chalcopyrite. In contrast, pyrrhotite seemsto be the dominant phasein sulfide (or both), and residualpentlandite bodies are isolated segregationsassociated with biotite. in the two phases@ig. O. Chalcopyrite and cubanite Bornite is one of the minor sulfide phases.It oc- can both be seenenclosing pentlandite grains and fill- curs only in disseminated, fine-grained sulfide ing abundant fractures in the pentlandite (cl, bodies, not in the larger interstitial segregations.Bor- Boucher 1975).In one sample,however, chalcopyrite nite usually comprisesthe lessercomponent of Cp- fills fractures in a pentlandite grain that is fully Bn and, morerarely, of Cp-Pn-Bn intergrowths(see enclosedin cubanite,indicating that cubanitelocal- Table 2). ly may postdateand replacechalcopyrite. Sulfide-sulfide textures differ among the in- Other opoque phases dividual samples,but there are no obvious distinc- In the Inco and Minnamax samPles, titanomagnetite, ilmenite and graphite occur. .E Graphite occursin two forms, which togetherusually comprisemuch lessthan lVo of individual samples. Thereare isolatedblades (> 100pm) and clustersof bladesbetween and within silicategrains (Hollister 1980),which may be xenolithic fragmentsfrom the wall rock. In addition, extremely fine-grained :. l::si ': : graphite(on the order of a few micrometres)occurs $",x along some silicate grain-boundaries,in fine fractures in silicates,and as thin rims around sulfide segregations(Fig. 8). The latter may be a magmatic precipitate. Ilmenite and titanomagnetite characteristicallyoc- cur (Fig. 4) as large (up to several millimetres), anhedralintercumulus bodies, as smallerround inclusions in plagioclaseand mafic phases,and as oriented rods (-3 x L2 pm) in plagioclaseand clinopyroxene(Fig. 9). The coarseroxide grainscom- monly are associatedwith intercumuluspyroxene and grains (A) plagioclase(Pl); Frc. 9. Adjacent of augite and biotite and, in many cases,with sulfides. Ilmenite mutual grain-boundary. In each case, it arrow shows grains are unzoned and have no exsolution bodies . appears that a cumulus core contains oxide (mostly magnetite) lamellae (-L), whereas adcumulus over- (confirmed by EDS analysis).The magnetiteaverages growths (-C) do not. SeeTable 4 for plagioclasecom- about 4-8 w1.90TiO2 and commonly has abundant positions. Transmitted light. Scalebar 500 pm. Inco exsolution lamellae of spinel (sensu stricto), sampleDG-8. ulv6spinel and ilmenite (Fig. a). In the Inco samples,

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TABLE3. REPRESENTATIVEBULKCOMPOSITIONS OFPENTLANDITE

Sample* A (7) B (2) D (2) E (6) F (4) c (l)

Compositlonsin weight percent

Fe ?3.32 (.28) 40. oc 30.80 (.3s) 32.29 (.18) JI.JO r?q 33.15 (,54 J3.dv Ni 42,72 (.41) 37.60 (.18) 35.12 (.41) ?? 2n | 1?\ 34.61 (.40 32.78 (.54 29.88 na7 ( (.17 ( 0 0,25 Cu (.14) 'I0.64 .50) 0.02 (.04) 'Io.04 (.05) 0,12 0.0 't Co 0.97 (.04) .21 i nr\ 0.99 (.12) .63 ( .01) ( .03 1,32 1.1? .'l0 ?t 7a (.18) a nol I to\ ?t ro I tl\ (.44 32.5r ( .34 s J5.a+ '100.48 vJ.ru \..r/ '100.49 IOf,al I00.36 (.84) 99.33 (.r3) (.46) r00.64 (.25) 100,77 (.77 99,76 (,32 ComposI ti ons in atomicpercent (nonnalized to Fe+Ni+S. 100)

Fe ta 2? 25 ao 26 ?7 30 'A Ni JJ 30 27 26 27 23 ) 48 48 48 48 47 47 47 iParenthesesafter samplenumber lndicate numberof analysesthat were averaged. Parenthesesafter wt.%analyses show one standarddevlation. A: 146-1176(AI'1AX) small Pn-Cpcompound graln. B: 146-469(AMX) Cp-Bn-Pn compound graJn. C: 146-569(AtrlAX) Pn matrix of Pnvrith fine lamellae.D: DG4(INco) large Pn lnclusionin Po. E: 146-1443(Al''mx) large, isolated Pn. F: 0G4(Al'iAX) large Pn 1n large Cphosted by Po. G: DG7(iNCo) Pn-Tr small compound graln. Electron-mlcioprobedata.

coarseilmenite and coarsemagnetite grains common- Pn-Tr, 2) moderate-Nipentlandite for Pn-Po-Cp ly are directly adjacent (Pasteris1983, in prep.). associations,stringers in silicate, coarsePn in Po, isolated Pn in silicates, Pn remnants in Cb, 3) CouposrrroN oF SULFIDEMINERALS moderate-to high-Ni pentlanditein Cp-Bn-Pn and Pn-Cp associations,and 4) high-Ni pentlandite Approximately 200electron-microprobe analyses where extremelyfine lamellaeof Cp cross-cutthe (Tables2, 3) were performed on 8 doubly polished pentlandite. It therefore appearsthat the most Ni- thin sections(3 from Inco, 5 from Minnamax). The rich pentlandite occurs in the most Cu-rich compositionsof cubaniteand chalcopyriteaxe ne€fly assemblages.In comparison, Weiblen & Morey stoichiometric and do not vary systematicallybe- (1976)reported 26-28 atomic 9o Ni in all of their tween the samples.The only "pyrrhotite" phases samples of pentlandite, placing them in the present are Feeslo @exagonal)and troilite, with moderate-Nigroup. hexagonalpyrrhotite occurring in both suites.As rec- Thin sections 146-569 and 146-1176 from ognizedoptically, abundant coarsetroilite occurson- Minnamax have the most Ni-rich compositionsof ly in the Minnamax samples,u$ually with lamellar pentlandite studied, although the samesections also intergrowths of hexagonalpyrrhotite. Somesmall in- contain typical, moderate-Ni grains. Sample tercumuluscompound grains of troilite and other 146-1176also has the most oxidized Fe-Ti oxide sulfideswere identified by WDS in the Inco suite. assemblagethat was analyzed from the Minnamax Another phasewith little compositionalvariation suite @asteris1983, in prep.). In at leasttwo cases, €rmongthe samplesis Ni-mackinawite (Table 2). It the high-Ni pentlandite contains very fine lamellae appearsto be a secondaryphase, as is typical for of chalcopyrite (Fig. 11). In one example, the mackinawite. This sulfide ocqus as nrurow stringers pentlandite also is hosted by chalcopyrite, but in the in serpentinizedolivine, as large feather-likeinclu- other case,the pentlandite is isolatedbetween silicate sionsin chalcopyriteand cubanite,and as isolated grains. rough-texturedblebs that may be associatedwith pentlanditeor cubanite.The phaseis slightly metal- Minor minerals: presence and absence rich comparedto troilite and containsapproximately 5 wt.9o Ni. Severalminerals of low abundancewere identified The compositionally most variable phase is by electron-microprobeanalysis. Small subhedral pentlandite(Table 3). Even within a singlethin sec- grains (not the typical "exsolution stars") of Fe- tion, the,Fecontent of the pentlanditevaries up to rich sphalerite (approximately 12 wt.tlo Fe) were 9 fi.90. flowever, no significantzoning was found found in a large chalcopyrite grain in Inco sample within individual, grains. Figure l0 is a plot of DG4 (see also Boucher 1975). In AMAX sample representativecompositional gloups of pentlandite 146-1787,a 150-pmround inclusionof approximate (approximately70 individual compositions)and their composition (Ni,Cu,Co)3As2was distinguishedin coexisting Fe sulfides. Generalizations are difficult coarsecubanite by EDS analysis(Fig. l2d). The grain to make. However, the following broad composi- containsapproximately 50 wt.9o Ni, 4 9o Cu, and tional patterns were observed among the 3 9o Co, As apparentlybeing the only other compo- mineralogicalassociations: 1) low-Ni pentlanditein nent. The arsenideis whiter and hasa higher reflec-

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/\ 'l{l Fo

S /" \

40 o

/ Fe -)

Frc. 10. Representativecompositions of sulfides,normalized to Fe+Ni+S = 100at.Vo, showingcoexisting pentlandite and Fe sulfides.See Table 2 for descriptionsof pyrrhotite and Ni-mackinawite samples,Table 3 for pentlandite descriptions.

Frc. 1l. Back-scatteredelectron imagesof very fine lamellae (dark grey) of chalcopyrite in pentlandite. a. Chalcopyrite- pentlanditeintergrowth on edgeof small grain of chalcopyrite.Minnamax samplelzt6-1176. b. Pentlanditegrain isolatedin plagioclase.Minnam* sample146-569. See Table 3 and Figure l0 for pentlanditecompositions. Images photographed at 15 kV exciting voltage; scale bar l0 pm.

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tance than pentlandite. In comparisonto its cubanite sulfidesresembling tarnished chalcopyrite. The above host, the arsenideappears almost metallic white and phaseswere not detected,nor was Hall's & Weiblen's has approximatelythe samehardness as cubanite. (1967) phase X @e-rich chalcopyrite). However, In crossedpolarizers, the phaseshows a fine-grained several grains of a metal-poor, Cu-rich mosaic structure and is weakly anisotropic (cl, "chalcopyrite" (Cp? in Table 2) and a metal-poor, oregonite,as in Antun et al. 196A. The one identi- Cu-poorbornite @n?in Table2),locally intergrown, fied grain of heazlewooditecontains minor Fe and were documented in AMAX samples 146-569 and Cu, but no Co (Table 2). 146-1176.These two samplesare also distinguished The presenceof Ni-mackinawite (seealso Weiblen by the abundanceof bornite and the presenceof Ni- & Morey 1976)was discussedabove. Owing to the rich pentlandite filled with minute lamellae of wide range of optical properties, in spite of the chalcopyrite.The samplesmay be exihibiting par- limited range of compositions determined, this tial exsolution (Cp-Bn, Pn-Cp) during re-equilibra- "phase" probably representsseveral stages of low- tion. temperaturebreakdown products in addition to true Three other mineralogicalfeatures were noted that mackinawite (c/ Antun et ql. 1966).An occasional differ from the resultsand interpretationsof Weiblen companionof mackinawitein Cu-Ni-Fe sulfide oc- & Morey (1976) for their samplesfrom the South currences,the sheetsilicate - sulfide valleriile (see Kawishiwi intrusive complex("Inco"). In the pres- Antun e/ al. 1966) was not identified. However, ent study, only troilite and hexagonalpyrrhotite were several l0-15-pm-wide lamellae with the optical identified by electron-microprobeanalysis (in agree- featuresof mackinawiteor valleriite were observed ment with Boucher 1975), not hexagonal and in coarse,octahedrally fractured pentlanditein a large monoclinic pyrrhotite. Weiblen & Morey (1970 grain of pyrrhotite. EDS analysis showed these reported that they found no monomineralic veinletsprobably to contain Mg-serpentine. pentlanditeassemblages, whereas they do appearin Following the work of Hardyman (1969) and the presentsuite as both small and coarsegrains. In Weiblen & Morey (1976),sulfides of the so-called addition, the cubanite grains in the presentstudy are chalcopyrite group (talnakhite, mooihoekite, consistently nonst'oichiometric. Values of the haycockite) were sought among the disseminated metal:sulfur ratio are 3:3 (Table 2), but the grains

Ftc. 12. Energy-dispersionspectra at 15 kV; counting time (in seconds)recorded at far left baseof spectrum;full ver- tical scale1024 counts. Samplenumber listed under eachspectrum. a. Olivine grain host to multiple pits that are probably openedfluid inclusions.b, c. Analysesof the pits show Ca and Cl in both cases,as well as K and S in one case.The pits may be remnantsof brine-filled fluid inclusions.d. Analysis of a 200-pmelliptical inclusion in a large grain of cubanite. It is a Ni arsenidewith a few wt.9o Co and minor Cu.

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are commonly Fe-rich. This may reflect the in- somewhatricher in Fe; others are very enrichedin completereaction of Cu-Fe sulfidespecies with pre- Ni (Fig. 10). The cobalt contentof all the pentland- existing Fe sulfides. ite samplesis about l-2 wt,a/0.The most Fe-rich gtrains,which coexist with Fe sulfides, texturally ap- STcNIF'TceNcsoF SULFIDETEXTURES pearto be primary. Their compositionsQ3-2A at.tlo AND MINERALOGY Ni) plot betweenthe compositionalranges reported by Misra & Fleet (1973)for the natural assemblages As recognizedby Bonnichsen (1972), Boucher Tr-Pn (lower Ni) and Tr-hPo-Pn (higher Ni). (1975)and Weiblen & Morey (1976),almost all of The compositionof the high-Ni pentlanditefrom the sulfide separationin the troctolites occurred con- compound grains of Cp-(Bn)-Pn and the high-Ni siderablybelow the liquidus, presumablyas an im- pentlandite (30-33 at.qo Ni) with fine Cp lamellae misciblemelt. The sulfidesare interstitial and vein- correspondsto that usually found in more sulfur- like; those that occur as inclusions in plagioclaseare rich assemblages,such as those containing pyrite, in adcumulusovergrowths or in intercumulusgrains. and more Ni-rich assemblages,such as those with The texturesdo not support a cause-and-effectrela- millerite, heazlewooditeand godlevskite(Misra & tionship betweenthe precipitation of sulfides and of Fleet 1973).No pyrite wasidentified at Duluth, and Fe-Ti oxides.Thus, there is little evidencethat sulfide only one grain of Fe-bearingheazlewoodite (Table segregationwas induced by a suddendecrease in iron 2) was found near a Ni-rich Pn. content of the melt as oxidesbegan to precipitate. These textural and compositional data on the Primary magnetite is rarely found in the sulfide sulfidessuggest that at leasttwo episodesof sulfide bodies, suggestingthat the imr-nisciblesulfide melt mineralizationoccurred, and that the productsnow did not have an oxygen content as high as that observedrepresent (partial) re-equilibration below reported in other igneousenvironments (cl, Skinner 200oC.The early mineralizationwas dominatedby & Peck 1969). a homogeneous,low-sulfur Fe-(Ni-)S phase(rzss) Severalinferences follow from the specificcom- and intermediatesolid-solution (iss of the Cu-Fe-S positions of sulfidesand assemblagesfound in the system).Subsequent re-equilibration eventually pro- two troctolites. A low-sulfur bulk composition for duced the coexistingintergrowths of Tr-hPo and the parent melts is indicated by l) the lack of Cp-Cb (Fig. 7). According to acceptedinterpreta- monoclinic pynhotite (a common FeSphase in many tions of the phaseassemblages (e,g,, Crue 1973), other deposits)in preferenceto hexagonalpyrrhotite pentlanditeqhould have formed from the Fe-(Ni-)S t troilite in both suites,2) the lack of sulfur-rich phase at about 600oC. However, large coarse phasessuch as pyrite, and 3) the presenceof the grains of Pn with or without Po and small, inter- metal-rich phaseheazlewoodite (in one sqction from cumulus Pn grains (isolated, with chalcopyrite alone 146-569;see Table 2). The apparentinconsistency or, rarely, with Po) are abundant. This raisesthe betweenlow fugacity of sulfur and abundant sulfide questions:1) could Pn be a liquidus phase?(unlike- minerals is explained by the self-buffering capacity ty); 2) wasthere extensive, late-stage silicate-sulfide of sulfide-rich rocks (cl, Eckstrand 1975). recrystallizationat Duluth? (probably); and 3) did The compositionsand assemblagesof the sulfides some pre-existing compositions of pentlandite also show a lack of equilibration and suggestmore become enriched in Ni during subsequent than one episode of sulfide precipitation or re- re-equilibration? (possibly). The Ni-rich phase equilibration. For instance,the observedassemblages heazlewoodite(stable below -556"C) typically is Cp-Tr and Cp-hPo are metastablewilh respectto regardedas a product of the reduction of pentland- Cb-Tr and Cb-hPo (Craig & Scott 1974).This may ite during serpentinization(Craig & Scott 1974). account for the reaction relationship observedin However,the Fe (-2 wt.9o)and Cu (- 1.3wt.9o) both troctolite suitesbetween Fe sulfides and Cu- contentdof the one analyzedgrain of headewoodite Fe sulfides. suggest a possible higher-temperature,primary Previous studies of the system Fe-Ni-S have origin. shown that in natural and artificially produced An episodeof later mineralization or, at least' later assemblages,the composition of the pentlandite re-equilibration affected large portions of the grains is closely correlated with the specific sulfides. This is most clearly observed in the mineralogyof the coexistingsulfides (e.9., Graterol widespread replacement of Fe sulfides by cubanite & Naldrett 1971,Harris & Nickel 1972,Misra & Fleet and chalcopyrite. Replacement could explain the 1973, Crug I 973).In the two suitesunder investiga- common occurrenceof small grains of pentlandite tion, thereis no consistentcompositional distinction in massivecubanite (Fig. 6) and the abundanceof betweenpentlandite coexisting with hexagonal pyr- massivecubanite in the absenceof chalcopyrite. The rhotite alone and that coexistingwith troilite (Fig. high content of Ni in somesmall, intercumulusPn l0). Most samplesof pentlandite are in the range grains coexistingwith chalcopyrite(some of the most 25-27 at.t/oNi and 25-27 at.VoFe. Somegrains are Ni-rich Pn compositionsshown in Fig. l0) could be

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TABLE4. REPRESENTATIVEBULK COMPOSITIONS OF SITICATE MINERALS

Sampler A B CD EF GH IJ KL Si02 ilt.% 53.16 47.75 53.90 52.83 an ?4 ?( 02 36.05 36.69 36.98 37.12 35.61 36.49 KzO 0.30 0.07 0,32 0.29 0.15 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Ti02 0.02 0.0 0.11 0.13 0.02 0.0 0.0 0.0 0.19 0.07 0,0 0.0 Afz0 g 27.94 3l .57 27,53 27.92 29,81 0.0 0.0 0.0 0.0 0.0 0,0 0.0 lvlso 0,02 0.0 0.03 0.02 0.01 28.57 27.83 32.q3 a, 1l ?2 00 29.C3 30.27 Cao 13.02 17.25 12.69 13.02 IJ.!J U.U? 0.05 0.01 0.01 0.03 0.02 0.03 Na20 3.82 1.73 4.27 3.97 2.84 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Mn0 0.52 nE7 n?o 0.44 0.42 .0.53 0,51 FeO 0.18 0.80 0.21 0.27 0.27 35.81 36.68 30.6? 31,39 30.2.l 34,30 33.93 Ni0 0.14 o.il 0.18 0.20 0.21 0.21 9.23 0.0 0.0 Cr203 0.0 0.0 0:; 0:; 0.0 ',l0l.010.0 0.02 0.01 'I0l.350.03 0.06 '100. Total 98.46 99,17 99.06 98.45 98.99 l0t.31 100.33 I01.II 10 l0l .46 An (Fo) 64 g4 61 63 75 59 58 65 65 66 60 6l Ab (Fa) 34 I 5 37 35 zs 4t 42 35 35 34 40 39 or2lzzo

i indicates that the element (tNC0), (A) (B) plagioclase grain. was not analyzed for. A, B: D03 core and clear'146-569 edge of C, D, E: DG3,core (c), core near oxide lamellae (D) and edge (E) of plaqioclase. F: (AMX) larqe qrain of olivine. G: 146-569small grain of ollvine. H: DG3targe, subhedral graln'of oiivine. I: DG4(INco) olivine'with-triJl of fluld inclusions. ,J: DG4olivine wlth flne Fe-Ti oxide larnellae. K: DG7(INCo) large grain of ollvlne. L: DG7small graln of olivine. Each colum represents the results of one electron-m'icroprobeanalysis.

due to partial re-equilibration at this time. These Electron-microprobe analysisshows a consistentin- distinctive compositionsalso might be due to the late- creasein Ca with accompanyingdecrease in Na and, stageinflux of more Ni- and Cu-rich sulfides.The to a lesserextent, K from the core of plagioclase presenc€of massivecubanite and troilite constrains grains to their sulfide-enclosingrim. The core-rim the temperatureof final reaction to below about differencesreach 4 wt.9o CaO (Table 4). 200'C and about l40oc, respectively. The same compositionalpopulations of plagio- Other mineralogical evidence suggestsJhat this claseare defined by the coresof grains filled with later episodewas a distinct eventof mineralization lamellae (spinel, magnetite, ilmenite) and their clear, involving copper enrichment. The disseminated often irregular, nonlamellar rims (Fig. 8). sulfides in adcumulus plagioclasegrains are Cu-rich Cathodoluminescencemicroscopy showed the same comparedto the commonly more m€rssiveinterstitial scaleof compositional distinctions betweenapparent sulfides(Boucher 1975, Weiblen & Morey 1976;this early cumulates (with oxide lamellae) and later ad- study). Cu-Fe sulfidesselectively replace exsolved cumulates (without lamellae). Fe-Ti oxides (Fig. 5) that have equilibration The silicates,therefore, appear to have undergone temperaturesfrom about 700oCto 900.C (Pasteris somehiatus during their growth. In particular, they 1983). were undergoingadcumulus growth or dissolution The apparentCu-enrichment trend, in part, may and reprecipitation during late-s1age sulfide reflect the nonnal fractionation of a mafic melt as mineralization.At this time, the Fe and Ti contents it acquiresan increasingCu:Ni ratio. Experiments of the melt had decreasedsufficiently to preventthe in the systemCu-Fe-Ni-S (e,g,, Craig & Kullerud previously widespreaddevelopment of oxide lamellae 1969) indicate that as temperaturedecreases, the in the grains of plagioclaseand augite (Fig. 8). sulfide liquid coexistingwith solid pyrrhotite will becomeincreasingly Cu-rich. Thus, simple fractiona- CoNrnor-s oN TrrETnrarNc AND LocALrzATroN oF tion may account for the confinementof the high- SULFIDE PRECIPITATIoN (Cu:Fe) phasebornite to late.stagesulfide dissem- inations. There is ongoing discussionabout how sulfide However, it is clear that the silicates (and segregationwas initiated at Duluth, from wherewere presumably also the sulfides) at Duluth have the sulfur and metalsderived, and how can the par- undergonemore than simple fractionation (Foose ticular localization of and compositional variations 1982, Tyson & Chang 1984). Weiblen & Morey in the sulfide mineralization be explained(e.9., Bon- (1976)reported that the plagroclaserims that enclose nichsen1972, Boucher 1975, Weiblen & Morey 1976, Cu-rich sulfides are more anofihite-rich than the Mainwaring & Naldrett 1977, Ripley l98l). The cores of the grains; similarly, in ferromagnesian sulfur-isotopework of Mainwaring & Naldrett (197) grains with marginal sulfide inclusions,the silicate and Ripley (1981)gives strong backing to Mainwar- rims are more Mg-rich than the cores. Further ing's & Naldrett's (1974) original suggestionthat analysiswas done in the presentstudy to evaluate much of the sulfur was derivedfrom the sediments the pervasivenessand significance of this effect. intruded by the Duluth Complex. An inherent part

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of sucha processis the large-scaletransport and mix- (e.g.., Ferry l98l) and carbon species(e.9., Frost ing of the sulfur in the melt. 1979)probably weremobilized from the wall rocks The developmentand evolution of a high-tempera- as volatile componentsand dissolvedin the melt ture fluid phaseseem critical to the explanationof (e.g., Burnham 1979,Ripley l98l) without large- the Duluth sulfides. Ripley (1981)has discussedsome scalephysical assimilation of xenoliths(cf. Grant & of the thermodynamicsof producing fluids that con- Molling l98l). tain H2O, H2S, CO2 and CHo. Physical evidencefor The pervasiveinfluence of thesevolatiles on the the passageof fluids over a rangein temperatureis flor) andflS) of the local igneousenvironment ap- found in the abundanceof fluid inclusions (mostly parently is reflectedin the mineralogyof individual empty now) along healedfractures in the silicatesand deposits. In the samples from the Inco deposit, apatite, by the presence of stringers of troilite is rare, but both coarsemagnetite and ilmenite microcrystallinegraphite in the silicates,and by the coexist,and the magnetitedemonstrates subsolidus occurrenceof irregular veinlets of low-temperature oxidation-exsolution.In contrast,samples from the Ni-bearing mackinawitein serpentinizedolivine (Fig. Minnamax drill-core contain abundant troilite [re- 3). The texture of the fine-graindd,fracture-filling quiring a lower sulfur:metal ratio than the other pyr- graphite suggeststhat it was vapor-deposited(cl, rhotite phases,thus a lower/(O)1, coarseilmenite Frost 1979).More information on the temperature (but no magnetite),and veinletsof vapor-deposited and/(O) of the fluid is requiredto determinefrom graphite in association with the sulfides. A what vapor speciesthe carbon precipitated (cl, temperaturey'(O) analysis of the Fe-Ti oxides in Pasterisl98l). The narrow, irregularveinlets of iron both troctolites (Pasteris1983, in prep.) showsthe sulfidesin altered olivine probably were produced ilmenite-magnetite assemblagesto approximate the when iron from the orieinal olivine wasexcluded dur- quartz - fayalite - magnetite buffer, and the Minn- ing serpentinization,in the presenceof mobile, sul- amax ilmenite to lie at somewhatlower fugacities of fur-bearing fluids (c/. Eckstrand 1975, Groves & oxygen. Keays 1979). The closeassociation of presumablymagmatic car- CoNct usIoNs bon and sulfides has been noted in other igneous suites(e.9., Pasteris1981, Pedersen 1981, Elliott el The processof intercumulus infiltration in layered al. 1982),It may reflectfiO) control on the timing intrusive complexes,as describedby Irvine (1980), of sulfide segregationbecause the presence of may provide the key to understandingthe evolution graphite in the assemblageconstrains the lO) of the Duluth Complex. In part, the model allows (e.g., Ohmoto & Kerrick 1977,Frost 1979,Ripley for intercumulus liquid from an earlier, more 1981).Buchanan & Nolan (1979)experimentally pro- primitive melt, from the baseof a fractionating se- duced immiscible sulfide liquids in sulfide-saturated quence,to percolateupward through the cumulates melts at different fugacities of oxygen. They of a later, more fractionated sequencein a layered demonstratedthat a much higher value of /(S) is complex. It providesa meansby which the follow- necessaryto nucleatean immisciblesulfide phasein ing effectscan be brought about: l) the mineralogy a highy'(O) than in a low-flO) melt (for instance, and compositionin the upper portion of an igneous overthe logflO) range-8.50 to -11.50at 1200"C). sequencecan be affected by the chemicalparameters However, they also demonstratedthat for a given establishedearly in the crystallization history (at the temperature and /(S) in a sulfur-undersaturated baseof a sequence),2) mineral compositionsin the liquid, the reducedmelt could dissolvemuch more sequencecan be given a more primitive signature sulfur than the oxidizedmelt before a sulfide liquid than otherwisewould occur, and 3) a more gradual was segregated(c/ Haughton et al, 1974).In addi- evolution (as distinguishedfrom episodicchange) oc- tion, this reducedmelt would be very sensitiveto curs in the intercumulus liquid as it partly re- slight increasesin/(S) andflO), which could lead equilibrates with successivebatches of lessprimitive to the nucleation of an immiscible liquid. solids and intercumulusliquid. This mechanismis The particular T-f(O) conditions in the Duluth especiallysignificant for the Duluth troctolites, which magmas(as controlled by graphite and coexisting have a multiinjection history and in which the phases)are consideredto have causedsulfide satura- sulfides clearly are intercumulus products. The tion to be delayeduntil late in the magmatic se- developmentof a high-temperaturefluid is suggested quence.Sulfide saturation postdatedthe precipita- to have beenpart of the overall intercumuluspro- tion of most of the olivine, and coincidedwith the cessat Duluth; in part, the fluid may account for precipitation of biotite and other intercumulus the metasomatizingpotential of the infiltrating in- phases. The Ni content of the olivine grains tercumulusliquid. (-0.18-0.22 wt.7o NiO for Fo66-5r)is only slightly In summary, sulfide mineralization in the Duluth higher than expected for coexisting sulfide and troctolitesmay well reflect a two-stageprocess. Ini- silicatemelts (c/ Fleet& MacRae1983). Both sulfur tially, volatiles (e.g., CO2, H2O, H2S, CHa) were

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dissolvedin the silicatemelt at the time when an early the effect of this evolving fluid on/(Or),/(S) and immiscible sulfide liquid segregated.The sulfide had sulfur content and capacity of the melt. At some a high Fe:Cu ratio and subsequentlycrystallized point in its fractionation, cooling and assimilation, primarily massive,interstitial pynhotite (with some the melt again becamesulfide-saturated. The ir- exsolutionproducts upon seeling).Later in the crys- regularitiesin the sulfur-isotopedata (Ripley 1981) tallization sequence,the volatiles separatedfrom the may reflect the fact that someof the analyzedsulfides silicatemelt as a distinct phase,and containedcon- are direct precipitatesof a sulfide liquid, whereas siderabledissolved copper and sulfur. This fluid was others have undergonereaction with a later, sulfide- capable of compositionally altering pre-existing bearing fluid. phases. Thus, the three dominant textures of The presentstudy is basedon a small number of chalcopyriteand cubanitemay representa continuum samples,so that only broad-basedinterpretations are from sulfide liquid immiscibility to high-temperature possible.However, it is significantthat thereare ob- reaction of a fluid and a sulfide phase: l) rounded vious distinctionsin the opaquemineralogy between bodies of lamellar Cp-Cb in Po (original coexistence the two adjacentbodies of troctolire. This is impor- of rssand PoJ, 2) irregular, small islands of pyr- tant for future evaluationof thesedeposits for both rhotite a pentlanditein a cubanite + chalcopyrite their Cu-Ni and Ti contents.Future studiesshould host (partial replacement),and 3) homogeneouslarge addresschanges in the sulfide and oxide mineralogy bodies of cubanite (completed reaction FeS + with depth in severaldrill-cores. CuFeS2- CuFe2S3).An active, late-stageCu- and S-bearing fluid would account for the selective replacementby Cu-Fe sulfide of magnetite regions ACKNOWLEDGEMENTS in oxidized and exsolved magnetite-ilmenite intergrowths(Fig. 5), for cubanitebodies that appear The author thanks the following, who provided pseudormorphicafter biotite and titanomagnetite, samples, helpful discussionsand permission to and for Cu-Fe sulfides replacing interstitial pyr- publish the data they supplied:S.N, Watowich and rhotite in troctolite and xenolith assemblages(Fig. 2). J.B. Malcolm of AMAX (Minnamax samples)and This fluid phase is consideredto have evolved M.L. Boucher of the U.S. Bureau of Mines, Min- vrith, but to havebeen more mobile than, the inter- neapolis (Inco samples).A.J, Naldrett, P.W. cumulusliquid. As distinct from the typical model Weiblen, T. O'Reilly and T.A. Abrajano are thanked of a hydrothermal fluid, this magmatic fluid con- for earlierdiscussions. Three anonymousreviewers tained relatively little water and was of higher gave helpful commentson an earlier draft of the temperature. In the presentstudy, preliminary fluid- paper. However, the author acceptsfull responsibili- inclusion analysisby laserRaman spectroscopy(cl, ty for all opinionsexpressed. This work was funded Rosasco& Roedder 1979)revealed small amounts in part by NSF grant EAR-8025255;the support of of CO, in some fluid inclusions. Microthermometry the ResearchCorporation is also acknowledged. was unsuccessfulin detectingphase changes upon cooling down to -196C; the inclusionsmay now be empty. However, the energy-dispersionelectron- REFERENcES microprobeanalysis of someopened, 10-20 pm pits in olivine grains in polished thin section(Fies. l2a-c) ArnBsA.L. & Rav, L. 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Rosasco,c.J. & RoBnorn,E. (1979):Application of a ne'w Raman microprobe spectrometerto Received June 5, 1982, revised manuscript accepted Oc- nondestructiveanalysis of sulfateand otherions in tober 17, 1983.

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