869

Thz C anadian M he raln g i s tVol.36, pp. 869-886 (1998)

COMPOSITIONAL VAR|ATIONS lN Cu-N|-PGE SULFIDES OF THE DUNKA ROADDEPosrr'DULUrHcl#'h',t#[tBTlHl+':'l[t"9#SoFcoMBrNED

ROBERT D. TTffiNU,ULII AIP SARAH-JANE BARNES

Ddpanement des Sciences Appliquies, Universit| du Qudbec d Chicoutimi, Chicoatimi, Qudbec G7H 2BI

ABSTRAST

The Dunka Road deposit is one of rcn occlurences of Cu-Ni sulfides bearing platinlm-group elements (PGE) on thenorthwestern margin of the Duluth Complex, in Minnesota. Mineralization has been linked to contamination of the hostFoctolitic magma through assimilation of argillaceous rocks from the Virginia Formation. On the basis of texture andcomposition, the sulfide mineralization is divided into five types: 1) norite-hosted disseminated sulfides, 2) troctolite-hosteddisseminated sulfides, 3) PGE-rich disseminated sulfide horizons,4) pyrrhotite-rich massive sulfides, and 5) chalcopyrite-richdisseminated sulfides. The norite-hosted sulfides exhibit featues suggestive of the magma's substantial s6ntamin4ti6a, sssfoashighproportionsofpyrrhotiteandarsenideminerals,andhighmeanvaluesof S/Se(9,700)and6sS (ll.2Voo).T\eyareatsogenerally metal-poor, implying that the sulfides interacted with a relatively low vohme of silicate melt (i.e., low R factor). Thetroctolite-hosted sulfides formed at moderate degrees of contamination, as indicated by their intermediate mean values ofS/Se (4,600) and 6sS (7.\Voo).T\e PGE-rich sulfide horizons show little sign of contamination, and have mantle-like meanvalues of S/Se (2,600) and 6345 (2.1%o). Their very high PGE contents suggest that they formed at elevated R factors. Thepyrrhotite-rich massive sulfides and associated chalcopyrite-rich disseminated sulfides have relatively high mean values ofS/Se (8,000) and 6345 QO.z%o), indicative of significant contamination. The former are interpreted to represent a cumulateof monosulfide solid-solution (mss), whereas the chalcopyrite-rich sulfides represent the fractionated sulfide liquid. A generalincrease in the degree of contamination is observed toward the base of the intrusion, associated with a decrease in R factor andmetal concentration of the sulfides. This likely results from the introduction of partial melt from the metasedimentary country-rocks,which was cooler than the mafic magma and led to the early crystallization of the sulfide liquid.

Keywords: sulfide deposits, assimilation, sulfur isoiopes, S/Se ratios, R factor, nzss fracrionation, rroctoliteo platinum-groupelements, nickel, copper, arsenides, Dunka Road deposit, Duluth Complex, Minnesota.

Sol aarns

Le gisement de Dunka Road fait partie d'un groupe de dix gisements de sulfures de Cu-Ni enrichis en 6l6ments du groupedu platine @GP) regroup6s le long de la bordure nord-ouest du complexe de Duluth, au Minnesota. Ia min6ralisation serait li6ed une contamination du magma-hdte [octolitique due d une assimilation de roches argileuses de la formation de Vuginia.D'aprds leur texture et leur composition, les sulfures sont divises en cinq types: 1) sulfures diss6min6s dans la norite commeh6te, 2) sulfures dis5Smin$s dans Ia troctolite comme h6te, 3) horizons de sulfures diss6min6s enrichis en EGR 4) sulfuresmassifs enrichis en pynhotite, et 5) sulfures diss6min6s enrichis en chalcopyrite. I.es caract€ristiques des sulfures dans la norite,par exemple les proportions 6lev6es de pynhotite et de minf,14gx ars6nifOres, et des valeurs moyennes 6lev6es de S/Se (9 700)et 6uS (lI.2Voo), font penser que le magma a subi une importante contamination. De plus, ces sulfures sont g6n6ralementappauvris en m6taux, ce qui implique qu'ils ont r6agi avec un faible volume de magma silicat6 (i.e., fuble facteur R). Lessulfures dans la troctolite se sont form6s I des niveaux moyens de s6atamin4fi6n, tel qu'indiqu6 par leurs valeurs moyennesinterm6diaires de S/Se (4 600) et 6345 (7.8%o). Les horizons de sulfures enrichis en EGP montrent trds peu de signes decontamination, et ont des valeurs moyennes de S/Se (2 600) et EaS (2.lVoo) quli rappellent celles du manteau. Leur trbs forteteneur en EGP t€moigne de facteurs R 6lev6s. Les sulfures massifs enrichis en pyrrhotite, et les sulfures diss6min6s enrichis enchalcopyrite avec lesquels ils sont associds, ont des valeurs moyennes relativement 6lev6es de S/Se (8 000) etE3:S (L0.2%o),indications d'une imponanF contamination. Les sulfures massifs rdsulteraient d'une accumulation d'une solution solide demonosulfure (nss), alors que les sulfures enrichis en chalcopyrite representent un liquide sulfur6 fractionn6. Une augmentationgln€rale du degr6 de contamination est observ6e vers la base de I'intrusion, associ6e d une diminution du facteur R et de laconcen8ation en m6taux des sulfures. Ceci rdsulte possiblement de I'introduction d'un produit de fusion partielle ddriv6 des rochesmdtasddimentaires encaissantes, moins chaud que le magma mafique, qui aurait ainsi men6 I la cristallisation h0tive du liquide sulfur6,

Mots-clds: gisement de sulfures, assimilation, isotopes de soufre, rapports S/Se, facteur R, fractionnement de rass, troctolite,complexe de Duluth,6l6ments du groupe du platine, nickel, cuivre, arsdniures, gisement de Dunka Road, complexe deDuluth, Minnesota.

I Cunent address: MinistEre des Ressources naturelles du Qudbec, Qu6bec, QC CIH 6Rl.E- mail addre ss: sgq@ mrn.gouv.qc.ca

870 THE CANADIAN MINERALOGIST

hnnorucrroN

Most studies of the Duluth Complex have focussedon the Cu-Ni-PGE sulfide deposits that occur alongthe base of troctolitic intrusions (Th6riault et al. 1997,and references therein). Country-rock assimilation iswidely recognized as having played a significant role inthe genesis of the mineralization. Evidence of assimilationis based on stable isotopes (Mainwaring & Naldrett1977, Ripley 1981, 1986, Rao & Ripley 1983, Ripley& Al-Jassar 1987, Ripley et al. 1993, Lee & Ripley1995), Se/S ratios @ipley 1990a Th6riault et el. 1997),major- and trace-element geochemistry fllson &Chang 1984, Ripley &Alawi 1988, Severson & Hauck1990) and silicate mineralogy (Foose & Weiblen 1986,Geerts 1991).

Although the categories of sulfide occurrences inthe Duluth Complex have previously been defined (e.9.,Foose & Weiblen 1986, Martineau 1989), no systematicdescription and classification of the different typesof mineralization forming a deposit have ever beenattempted. This situation may explain the general lackof agreement conceming the mechanisms of concenfrationinvolved in ore formation. In particular, models involvingboth magmatic and hydrothermal processes have beenproposed to explain the localization of platinum-groupelements (PGE) in the Duluth Complex.

In this paper, we describe the mode of occurrence offive types of sulfide mineralization observed near tlebase of the Duluth Complex in the Dunka Road area.We show that compositional variation among the typesof mineralization results from the sequential actionof three processes, namely counfiy-rock assimilation,interaction between the sulfide liquid and the silicatemelt (R factor), and fractional crystallization of thesulfide liquid.

Geol-ocrcAr. Snrrnqc

The Duluth Complex consists of an arcuate mass ofmafic intrusions of Middle Proterozoic age (1100 Ma)associated with the Keweenawan flood basalt provinceof the Midconfinent Rift of North America (VanSchmus & Hirze I 985). The complex exlends for moretb^\ 225 km along the northwestern margin of LakeSuperior, from Duluth, Minnesotao to the Canadianborder (Fig. 1). It was emplaced as a succession ofdistinct mafic intrusions along an unconformitybetween volcanic rocks of the North Shore VolcanicGroup and older Proterozoic (1800-2300 Ma: Hemminget al. 1995) basement rocks of the Animikie Group. Ingeneral, rocks forming the Duluth Complex consist ofan upper anorthositic series underlain by a more mafictroctolitic series (Taylor 1964, Weiblen & Morey1980). Recent U-Pb ages (1098.6 + 0.5 to 1099.3 + 0.3Ma) obtained by Paces & Miller (1993) suggest thatboth intrusive series are essentiallv coeval.

Frc. l. Geological map showing the emplacement of theDuluth Complex and associated sulfide mineralizationwithin the Keweenawan flood basalt province of centralNorth America (modified after Naldrett 1989). PRI:Partridge River intrusion, SKI: South Kawishiwi inrusion.Black circles represent Cu-Ni-PGE sulfide deposits(l: Water Hen; 2: Wyman Creek; 3: Wetlegs; 4: DunkaRoad; 5: Babbitt; 6: Serpeatine; 7: Dunka Pit; 8: BirchLake; 9: Maturi; 10: Spruce Road); 82: drill hole from theMesabi deep-drilling project.

Significant Cu-Ni-PGE sulfide mineralization hasbeen observed in tle basal portions of two of thetroctolitic intrusionso namely the Parridge River andSouth Kawishiwi intrusions (Severson & Hauck 1990,Severson 1994). These are the host to more than 6billion tonnes of mineralization grading 0.66Vo Cu wtd0.2VaNi (Listerud & Meineke 1977, Ripley 1990b).The sulfide-bearing troctolitic intrusions occur alongthe northwestern margin of the complex, where theyintrude Proterozoic metasedimentary rocks of tleVirginia and Biwabik formations (Fig. 1). The latter isa cherry and slaty banded iron-formation (Morey1.992), whereas the overlying Virginia Formationconsists mainly of argillite, siltstone, and greywacke,The argillaceous horizons commonly contain finelydisseminated pyrite, especially in proximity to theBiwabik Formation (I-ucente & Morey 1983).

Axarvrcer Mergoos

Rock samples for this work were taken from drillcore provided by the Minnesota Department of Natural

Cu-Ni-PGE SULFIDES, DUNKA ROAD DEPOSIT 871

leucotroc{olite, troctolite, olivinegabbro, nodte (unditferentiated)

dunite, m6latroctolite

oxide-rich pyroxenito

Resources, Hibbing, Minnesota. Mineralized surmplesof the Dunka Road deposit were collected from drillholes #26107,26117 and 26143 (Fie.2, Table 1),whereas argillite samples of the Virginia Formationwere taken from hole #B2 (Fig. 1, Table l), which islocated approximately 40 km to the southwest of theDunka Road deposit and well beyond the metamorphicaureole of the complex.

A total of 28 whole-rock samples were alalyzed forS, Ni and Cu by X-ray fluorescence spectrometry atAnalabs - Caleb Brett in England. Concentrations ofthe trace elements Co, As, Sb, Se and Au were deter-mined by instrumental neutron-activation analysis(INAA) at the Universit6 du Qu6bec d Chicoutimiusing the method of B6dard & Barnes (1990). Sampleswith a low level of Se were further analyzed for thiselement by atomic absorption at Chemex Laboratoriesin Vancouver. Concentrations of the PGE and Re weredetermined by INAA in Chicoutimi, after preconcen-

Viroinia Formation(adillite, homf els xenoliths)

Biwabik Formation

sulfide occunences (a = PGE-rich)

trating the metals into a Ni-sulfide bead following themethod of Robert et aI. (197 l).

The ratio of sulfur isotopes was determined on 24whole-rock samples by mass spectrometry at theOttawa-Carleton Geoscience Centre in Ottawa. Anadditional four measurements were obtained frommineral separates at Indiana University.

GBoLocY on rrn Duu<e Roar Deposn

119 prrnka Road Cu-Ni-PGE deposit occurs withinthe basal part of the Partridge River intrusion, wherethe host mafic rocks are in contact with sulfide-bearingargillites of the underlying Vrginia Formation (Fig. 1;.As shown from a drill-hole-interpreted cross-section(Fig. 2), the igneous stratigraphy has been subdividedinto correlatable cyclic units (Severson & Hauck 1990,Th6riault et al. 1997). Each cyclic unit consists of athin ultramafic laver of melatroctolite to dunite overlaia

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TTIE CANADIAN MINERALOGIST

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cu-Ni-PGE SULFIDES. DUNKA RoAD DEPOSIT

Flc. 3. A. Country-rock xenoliths hosted in heterogeneous norite and olivine gabbro near the base of the Dunka Pit deposit. Notetle presence of granitic selvages along the margins of the xenoliths. B. Bedded plnrhotite unit of the Virginia Formationshowing thin laminations of pyrrhotite within hornfelsed argillite. Sample taken adjacent to the Dunka Pit deposit (DC-70,Tables 1, 2). Scale in centimeters for both photographs.

873

by a much thicker sequence dominaa{y consisting ofleucotroctolite and olivine gabbro, with minor n'octoliteand norite.

In contrast to rocks of the upper cyclic units, whichare generally well layered and contain very sparsesulfide, rocks of the basal part of the intrusion (lower250 m) are mineralized and very heterogeneous interms of texture and composition. Variations in grainsize and modal minslalogy are considerable throughoutthis zone, and leucotroctolite, olivine gabbro and noritecommonly occur together on a local scale.

Hornfels xenoliths derived from the underlyingVirginia Formation comprise up to 25Vo of the volumeof the basal sequence (Frg. 3A). Norite typically occursin proximity of country-rock xenoliths or directly at thebase of the intrusion. The norite was interpreted ashaving crystallized from a hybrid magma, producedthrough mixing between the parental basic magma anda granitic partial melt derived from the surroundingargillite (Th6riault et al. 1997). Evidence of partialmelting is indicated by the occurrence of graniticselvages along the margin of country-rock xenoliths(Fig. 3A). Furthermore, on the basis of trace-elementgeochemistry (Tlson & Chang 1984) and sulfurisotopes (Ripley 1981, Ripley & Alawi 1988), heatfrom the intrusion is interpreted to have led to the earlytransfer of a sulfur-bearing hydrous fluid from thefoorwal rocks into the basic magma. Hence, the highlyheterogeneous and mineralized character of the basalrocks hosting the Dunka Road deposit appears to berelated to a process of magma contamination throughmixing with derivatives from the underlying VirginiaFormation. An imporlant contribution of sulfur seemsto have come from what has been termed the beddedpyrrhotite unit (Severson & Hauck 1990), a 10- tol5-m-thick argillite horizon located approximately

25 m above the base of the Virginia Formation andcontaining 10-2OVo banded pyrrhotite (Fig. 3B). Asshown from the drill-hole-interpreted cross-section ofthe deposit (Fie.2), it is more than likely that this unithas been completely assimilated by the intrudingmagma on tJle basis of the reduced thickness (10-20 m)of the Virginia Formation in the Dunka Road area.

NATLTRE oF THE MINERALIZAnoN

AT TIIE DITNKA ROAI DNPOSTT

11" punka Road Cu-Ni-PGE sulfide deposit hasestimated resources of 1450 million tonnes grading0.397 wt.Vo Cu, 0.094 wt.% Ni, 445 ppb Pd, 118 ppbPto and 6l ppb Au (Wright Engineers Ltd. 1991).

The bulk of the mineralization (>95 vol.%) consistsof disseminated sulfideso which comprise no more than5 vol.Vo of the rock. The disseminated sulfides are typi-cally interstitial to the silicates, and generally consistof pyrrhotite, chalcopyrite, pentlandite, 4nd sgfunnife,with minor quantities of mackinawite, sphalerite, andbornite. Small amounts of the arsenides maucherite,niccolite, and cobaltite-gersdoffite, as well as theplatinum-group mineral @GM) froodite @dBir), wereobserved locally with the sulfide minerals. Less than5 vol,Vo of the mineralization consists of zones ofmassive sulfide dominated by pyrrhotite, with lesserpentlandite, chalcopyrite, cubanite, mackinawiteomaucherite, and niccolite.

This study led to the identification of five rypes ofrullids mineralization within the Dunka Road deposit.These are: 1) norite-hosted disseminated sulfides,2)troctolite-hosted disseminated sulfi des, 3) PGE-richdisseminated sulfide horizons, 4) pyrrhotite-richmassive sulfides, and 5) chalcopyrite-rich disseminatedsulfides surrounding lenses of massive sulfide.

874

Norite -hosted dis s eminated suffide s

The norite-hosted disseminated sulfides aretypically found adjacent to country-rock xenoliths orwithin approximately 30 m of the basal contact ofthe intrusion. The mineralization consists mainlyof pyrrhotite and ilmenite, with lesser chalcopyrite,cubanite, and pentlandite (Fig. 4A). Maucherite,niccolite and cobaltite-gersdorffite are also commonlyobservedo occurring as isolated grains enclosed withinsulfides. This relationship suggests that the arsenidesare primary magmatic phases crystallized from thesulfide liquid.

Tro c t o li t e - ho s t e d di s s e minat e d sulfi de s

The troctolite-hosted disseminated sulfi de m i nsrali-zation occurs throughout the lower 250 m of theintrusion, and forms more than half of the deposit.The sulfides are mainly in leucotroctolite, disributedinterstitially among the silicate minerals. They consistmainly of pyrrhotite, pentlandite, chalcopyrite, andcubanite, with ftace amounts of mackinawite and bornirc(Figs. 48, C). The ratio of pyrrhotite to Cu- and Ni-richsulfide phases is lower here tlan in the norite-hostedsulfides. Furthermore, only minor amounts ofmaucherite were observed.

PGE-rich disseminated sulfide horizons

TWo horizons of disseminated sulfides distinctlyenriched in the PGE, l0 m thick on average, weresampled from the upper portion of the mineralizedzone. They occur 100 to 200 m above the basal contact(Fig. 2), and are hosted in leucotroctolite. The horizonstypically underlie layers of melatroctolite to dunite.They were first identified by Geerts (1991), who relatedtheir origin to a magmatic process rather than tosecondary enrichment by a hydrothermal fluid.

The PGE-rich disseminated sulfide horizons gentainup to 2.8 ppm Pt + Pd (Table 1), and consist 6ainly ofinterstitial grains of chalcopyrite, cubanite, pentlandite,and lesser pyrrhotite (Fig. D). Unlike the norite-

and troctolite-hosted sulfide mineralization, arsenideminerals were not observed in the PGE-rich horizons.

Pynhotite -rich m.as s iv e suffide s

The pyrrhotite-rich massive sulfides occur as lensesin the basal sequence ando less commonly as pods ofmassive sulfide bordering country-rock xenoliths. Themassive sulfide pods (l-3 cm wide) are typicallyhosted in norite, and consist of 9G-957o pyrrhotite, withminor chalcopyrite and pentlandite. They also containmaucherite and niccolite (Figs. 4E, F), and this isreflected in the elevated As content of sample DC-75(Table 1). The massive sulfide pods and their noritichost are likely derived from in situ partial meltingwithin the adjacent, sulfide-bearing country-rockxenoliths.

Several narrow lenses of massive sulfide reachingup to 5 cm in width occur in the basal mineralizedrocks. They are usually at a sharp angle to the base ofthe intrusion, and are thus interpreted to originate asinjections of sulfide liquid through a filter-pressingmechanism. The lenses contain 85-90Vo pyrrhotite,S-lOVo pentlandite, and lesser chalcopyrite, cubaniteand mackinawite. They are typically zoned, from acentral core of massive pyrrhotite with minor flame-likepentlandite, to a narrow im (l-2 mm) of subhedralpentlandite and marginal intergrowths of chalcopyrite,cubanite and minor mackinawite (Fig. 4G).

Chalc opy rite- rich dis s eminated salfides

Chalcopyrite-rich disseminated sulfi des are typicallyfound in association with the pyrrhotite-rich lenses ofmassive sulfide. The disseminated sulfides occur assmall interstitial grains to the silicate minerals orlocally as very tlin veinlets, and consist of.50-75Vochalcopyrite, lS-3OVo pyrrhotite, 5-l 07o cubanite andl-3%o mackinawite, with minor sphalerite and bornite.Small subhedral grains of froodite surround pentlanditewithin a coarse-grained lamellar intergrowth ofchalcopyrite and cubanite (Fig. aH). The frooditeappears to have nucleated on the pentlandite prior to theformation of the Cu-rich sulfide phases, which favors a

Ftc. 4. A. Ttpical norile-hosted disseminated sulfide mineralization showing the close association of pyrrhotite (Po) and ilmenite(Ilm), with minor chalcopyrite (Cp) and cubanite (Cb). Field of view is 1.25 mm in width. B. Troctolite-hosted disseminatedsulfide mineralization consisting of pynhotite (Po), pentlandite (Pn), chalcopyrite (Cp) and cubanite (Cb). Field of view is1.25 mm in width. C. Troctolite-hosted mineralization illustrating the interstitial nanue of the sulfides relative to fte silicateminerals. Field of view is 1.25 mm in width. D. Interstifial sulfides from a PGE-rich horizon, consisting of pentlandite (Pn),cubanite (Cb) and chalcopyrite (Cp). Note the absence of pyrrhotite. Field of view is 1.25 mm in width. E. Grain ofmaucherite (Ma) with minor chalcopyrite (Cp) within a pynhotite-rich massive sulfide pod at the margin of a country-rockxenolith. Field of view is 1.25 mm in width. F. Subhedral crystals of niccolite (Nc) within the above pyrrhotite-rich massivesulfide pod. Pentlandite (Pn) and chalcopyrite (Cp) complete the sulfide assemblage. Field of view is 1.25 mm in width. G.Zoned massive sulfide lens consisting of a pynhotite-rich core, an inner rim of pentlandite (Pn) and a marginal intergrowthof chalcopyrite (Cp) and cubanite (Cb). Field of view is 5 mm in width. H. Subhedral crystals of froodite (Fr) at the margiaofpenflandite (Pn), within gu-ri.h disseminated sulfides composed ofintergrown chalcopyrite (Cp) and cubanite (Cb). Fieldof view is 0.3 mm in width.

Cu-Ni-PGE SULFIDES. DIJNKA ROAD DEPOSm

876 TTIE CANADIAN MINERALOGIST

magmatic origin for the PGM, as opposed to its crystal-lization from a late hydrothermal fluid.

Sumun Isoropns

Sulfur isotope studies carried out on several Cu-Nisulfide deposits of the Duluth Complex indicate thatmost of the sulfur has been derived from argillaceousrocks of the underlying Virginia Formation(Mainwaring & Naldreu 1977, Ripley 1981, 1986,Ripley &Al-Jassar 1987). Ripley (1981) obtained 6sSvalues ranging from 0.2 to l5.3Voo from mineralizedintrusive rocks of the Dunka Road deposit, valuesthat cover a range similar to those values he measuredin the Virginia Formation.

Virginia Formatinn

Sulfur isotope measurements were obtained fromsix core samples of unmetamorphosed pyritiferousargillite taken approximately 20 km from the intrusivecontact (drill hole#BZ, Fig. l). The samples are consid-ered representative of the unit, having been collectedbetween 3 and 345 m above the basal contact with theBiwabik Formation. Values of 6sS range from 4.5 to8.6Voo (Table l, Fig. 5), and are similar to the value of7.4Voo determined by Ripley (1981) on material fromthe same drill hole, and to the range ofresults of sixanalyses reported by Ripley & Al-Jassar (1987) (5.6 to8.8%o) from core samples located 10 km from themargin of the Complex.

One sample of metamorphosed argillite from thebedded pynhotite unit (DC-70; Table 1, Fig. 5) yieldeda much higher value, l5.8Voa, and falls within the rangeof 6g5 values determined by Severson (1994) (1.4.0to 2I.L%o). Sulfur isotope measurements of the sameunit were also obtained by Zanko et al. (1994) ftom al7-m-thick xenolith occurring along the base of theSerpentine deposit. The 6sS values show a systematicincrease from 6.3 to 29.lVoa (mean of 19.4Voo) towardthe top of the xenolith (theh Fig. 10). This increase isassociated with an upward increase in the grade ofmetamorphism; reported mineral assemblages andtextures are indicative of complete dehydration andlocalized partial melting near the top, whereas relictchlorite and muscovite found at the base indicate thatdehydration reactions did not go to completion. Similarvariations in 6sS values with proximity to the intrusivecontact have also been reported from the MuskoxIntrusion (Sasaki 1969) and the Pechenga ore field inRussia (Melezhk et al. 1994). Hence the strongpositive correlation observed between 6sS values ofthe argillaceous rocks and grade of meramorphismmakes it improbable that the isotopic variability in theVirginia Formation is related to a primary process ofbacterial fractionation, as was suggestedby Zanko etat. (1994).

0 4 a 1 2 1 6 2 0 2 4 2 4

634S (%. )

Frc. 5. Histogram showing the distribution of sulfur isotopicvalues for the five types of sulfide mineralization formingthe Dunka Road deposit, as well as the bedded pyrrhotiteunit and unmetamorphosed argillite of the VirginiaFormation. Black boxes: this study; white boxes: Geerts(1994); Srey boxes: Ripley (1981), Ripley & Al-Jassar(1987), Zanko et al. (1994); striped boxes: Severson(1994),Za*o et al. (1994).

o 4co=croTL

Massive sulfides (M)

Cu-Ni-PGE SULFIDES. DUNKA ROAD DEPOSff 877

As proposed by Theng (1990) for an open system,we argue that the significantly higher 6sS valuesobserved in metamorphosed argillaceous rocks near thecontact originate from the breakdown of pyrite intopyrrhotite under hydrous conditions, with release of a[ght-sulfur-enriched vapor into the magma prior to theonset of crystallization. Such a process has previouslybeen demonstrated experimentally by Kqiwara et al.(1981) through the thermal decomposition of pyriteinto pyrrhotite at 600"C. They observed a significantdepletion in 6sS, by up to l2Voo,in the early fraction ofsulfur released, with the pyrrhotite residue having a6sS value abott4.SVoobtgfier than the starting material.Hence the early release of a light-sulfur-enriched vaporphase into the magma most likely led to a significantincrease in the 6sS ofthe residual pyrrhotite-bearingmetasedimentary rocks, the later subsequentlydischarging important quantities of isotopically heaviersulfur near the base of the intrusion during their partialassimilation.

Norite-hosted, troctolite-hosted and PGE-richdisseminated suffides

Sulfur isotope measurements obtained from thesethree types of disseminated sulfide mineralizationshow a general increase in 6aS toward the base of theintrusion and in proximity to country-rock xenoliths(1.e., where magma contamination is likely to be mostimportant). Hence the average 6sS value of the norite-hosted sulfides reaches ll.2%o, whereas that of thehoctolite-hosted and PGE-rich sulfides is 7 .8 and 2.lVoo,respectively, with only a slight overlap between thegroups ofvalues (Table I, Fig. 5). A similal variationwith respect to distance from the basal contact wasnoted by Ripley (1981) based on 128 sultur isotopeanalyses from the mineralized intrusive rocks. Of these,14 samples from the uppermost PGE-rich horizon (the"Red Horizon" of Geerts 1994) yielded an average 6ySvahte of 3.6Voo (Fig. 5), which is comparable to that ofthe present study.

Pyrrhotite -rich mas sive and chalc opyrite - richdisseminated sulfides

Sulfur isotope measurements obtained from thepyrrhotite-rich and chalcopyrite-rich sulfi des yieldedrelatively high 6sS values, ranging from 8.4 to 16.0%oClable l), which suggest that a large proportion ofthesulfur is sedimentary in origin. The very high valueobtained for the massive sulfide pod (sample DC-:|5,16.0%0) is almost identical to that measured forthe bedded pyrrhotite unit. This is not surprising,as DC-75 vyas sampled directly adjacent to a country-rock xenolith. As for the pyrrhotite-rich lenses andassociated chalcopyrite-rich disseminated sulfides, fhevery similar values obtained within each pair ofsamples imply that little to no isotopic fractionation

occurred during their formation. A similal conclusionwas reached by Ripley & Al-Jassar (1987) on the basisof 6vS values of mineral separates from adjacentpyrrhotite and cubanite within the Babbitt deposit.

S/Se Verues

The ratio Se/S has been used effectively to determinetJre source of sulfur in magmatic Ni{u sulfidedeposits (e.9., Paktunc 1989, Eckstrand et aL 1989,Ripley 1990a, Th6riault et al.1997). Its usetulness isparticularly sigpificant where country-rock assimilationseems to have played a role in the genesis of minerali-zation, as ssdimentarj rocks are generally depleted inselenium relative to mantle-derived rocks (Eckstrand &Hulbert 1987).

Norite-hosted, troctolite-hosted and PGE-richdisseminated sulfides

Our data show that whole-rock S/Se values decreasesystematically from the norite-hosted sulfides (mean of9,700) to the fioctolite-hosted sulfides (mean of 4,600),and reach a minimum in the PGE-rich sulfide horizons(mean of 2,600) (Table 1). In light of the elevatedS/Se values of the argillite samples from the VrginiaFormation, which range from 15,500 to 54,000, weattribute this regular decrease in S/Se to varyingdegrees of its assimi-lation by the mafic magma Similar'S/Se values (mean of 23,300) were also obtained inthe district by Eckstrand & Cogulu (1986) fromargillaceous rocks of the equivalent Rove Formation inproximity to the Ni-Cu sulfide deposit in the CrystalLake Gabbro.

Pyrrhotite-rich mns sive and chalcopyrite - richdisseminated suffides

The pyrrhotite-rich massive sulfides have significantlyhigher S/Se values than the chalcopyrite-rich dissemi-nated sulfides (Table l). The high value obtained forthe massive sulfide pod (sample DC--75:23,800) fallswithin the range for the argillaceous rocks; this is inagreement with its very high 6vS value and points to asignificant input of metasediment-derived sulfur.

DrscussloN

C o untry - ro c k as s imilat io n

A number of lines of evidence presented abovestrongly support the assumption that country-rockassimilation was a key process in the genesis of theDunka Road Cu-Ni-PGE deposit. With respect to fieldobservations, the abundance of hornfels xenolithstypically rimmed by granitic materialo along withthe common presence of norite near the base of themineralized intrusion, are indicative of significant

o Nodt+h@bd 6ulid6a Tddh+hGbd sulfid4@ PGE-rich 6ulfid4. lv@ivo oullld@o Cp-rich oulfld@o Unmetam. a€lllltoI Baddedpyrrhotleunn

t r o

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d P o r' \ o E l . vt g ld

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a

878 THE CANADIAN MINERALOGIST

Q roooo

a significant proportion of the sulfur is sedimentary inorigin, being most likely derived in large part from thebedded pyrrhotite unit of the Virginia Formation. Thewide range in 6vS values of the mineralized intrusiverocks is consistent with a simple mixing modelbetween "igneous" sulfur contained in the magma(6vS = 0%o) and externally derived sedimentary sulfurrepresented by the bedded pynhotite unit (6345 =I5.8%o). The assumed 6sS value for magmatic sulfur issupported by recent isotopic measuremens obtained byLee & Ripley (1995) for 12 unmineralized samples oftroctolite from the South Kawishiwi intrusion, whichyielded values ranging from -3.4 to +l.2%oa (mean of+O.lVoo). Assuming the two end members to be valid,1 1 0 5 0

6 34S (7- )

FIc. 6. Whole-rock S/Se ratio versus 6:+5 value for the fivetypes of sulfide mineralization, bedded pyrrhotite unit andunmetamorphosed argillite of the Virginia Formation.Range in S/Se ratio and 6sS value of the mantle takenfrom Ecksffand & Hulbert (1987) and Hrlbert et aI.(1988), respectively.

o Noril+hoslod sulffdesa Troctolite-hosted sumd6@ PGE-rlch sullldes. MNlv€ sulfrdeso Cp-rlch sulfidsq Unmetam. arglllller B€ddod pynhottte unlt

1 1 0 5 0

6 34s (z- )

Ftc. 7. Whole-rock 6:aS valte versus Pd + Pt of sulfidefraction for the five types of sulfide mineralization,bedded pyrrhotite unit and unmetamorphosed argilliteof the Virginia Fonnation. Broken lines join samples ofpyrrhotite-rich massive sulfide lenses and associatedchalcopyrite-rich disseminated sulfi des. Arrows showcompositional ftend expected during mss fractionation andmagma contamination. Symbols are the same as in Figure 6.

conttmination of the parental magma through partialmelting of the underlying Vrginia Formation. There isalso an apparent relationship between the relativeabundances of pyrrhotite and arsenide minerals and theextent of contamination, owing to the Cu-Ni-poor andAs-rich composition of the argillaceous country-rocksrelative to the parental mafic magma.

Furthermore, sulfur isotope measurements and S/Sevalues of the mineralized rocks clearlv demonstrate that

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S / S e

Frc. 8. Whole-rock S/Se ratio versus Pd + Pt of sulfide fractionfor the five types of sulfide mineralization forming theDunka Road deposit, bedded pyrrhotite unit andunmetamorphosed argillite of the Vrginia Formation,and various PGE-dominant magmatic sulfide deposits(1: Crystal Lake gabbro, 2: Merensky Reef of the BushveldComplex, 3: J-M Reef of the Stillwater intrusion). Data forother deposits t,ken from Naldrea (1981) and Eckstrand(unpubl. data 1988). Symbols are the same as in Figure 6.

an average of Tl%o of.the sulfur within the norite-hostedsulfides seems to have been derived from the footwallrocks, as opposed to 49Vo for the troctolite-hostedsulfides and only l3Vo f.or the PGE-rich sulfides.

The observed increase in 6sS values with degreeof country-rock assimilation is associated with aconcomitant increase in S/Se values (Fig. 6), as well asa decrease in the Pd + Pt content of the sulfide fraction(Fig. 7). The latter is also suggestive of a mixingprocess befween PGE-poor partial melts of the VuginiaFormation and metal-bearing mafic magmas. Lower Rfaetots (i.e., weight ratio of silicate magma to sulfide

Cu-Ni-PGE SULFIDES, DUNKA ROAD DEPOSIT 879

melt) attained in the more contaminated rocks alsocontribute to this gradual decrease in PGE (Th6riaultet al. 1997). Moreover, a plot of S/Se values versusPd + ft content ofthe sulfide fraction (Fig. 8) clearlyillustrates the contaminated nature of the norite-hostedsulfides, which have S/Se values above those estimatedfor mantle-derived rocks (2850-4350: Eckstrand &Hulbert 1987). The low S/Se value and high Pd + Ptcontent of the PGE-rich sulfide horizons have beenascribed in part to elevated R factors achieved at thetime of separation of the sulfide liquid (T\6iadt et al.1997), These values are not unlike those of maiorPGE-rich deposis (Frg. 8), whose origin also has beenlinked to very large R factors (Campbell et al. 1983).As for the spatially associated pyrrhotite-rich andchalcopyrite-rich sulfide samples, the difference inS/Se values and Pd + Pt content likely relates to adifference in the extent of sulfide liquid fractionation,considering that both samples within a pair seem tohave undergone comparable degrees of assimilationbased on their similar 6sS values (Table l).

Additional geochemical evidence (Ih5iault et al.1997) further confirm the importance of the assimilationprocess. po1 slample, electron-microprobe analysesof the sulfide phases suggest a higher proportion ofmonoclinic to hexagonal pyrrhotite in the norite-hostedsulfides, which was interpreted to be due to the greateravailability of locally derived sulfur from themetasedimentary country-rocks. Moreover, an increasein the incompatible elements such as Cs, Rb, Ba and Thwas noted in the more contaminated samples, as onewould expect these elements to be readily mobilized ina felsic partial melt.

Magmatic origin of the sulfides

A number of studies pertaining to the Babbittdeposit have emphasized the role of hydrothermalfluids in remobilizing the PGE (e.9., Ripley 1990b,Mogessie et al. 7991, Mogessie & Stumpfl 1992,Ripley et al.1993, Ripley & Chryssoulis 1994), on thebasis of the common presence of platinum-groupminerals (PGM) and secondary sulfides (bornite,valleriite and violarite) in strongly altered silicaterocks. However, studies carried out in the Dunka Roaddeposit report only minor evidence of hydrothermalalteration, and relate the origin of mineralization toprimary magmatic processes (e.g., Ripley 1981, Rao &Ripley 1983, Severson & Hauck 1990, Geerts 1994,Th6riault et al.1997). In the present study, field andpetrographic evidence suggest that the bulk of themineralization (norite-hosted, troctolite-hosted, andPGE-rich sulfides) formed by the separation of sulfideliquid within a partially crystallized magma, whichmay explain the general lack of massive sulfides alongthe base of the intrusion. Where a relatively largevolume of sulfide liquid did accumulate, it appears tohave undergone fractional crystallization, producing an

C u / l r

Ftc. 9. Cu/Ir yerszs Ni/Pd diagram for the five types ofsulfide mineralization, bedded pynhotite unit andunmetamorphosed argillite of the Virginia Formation.Symbols are the same as in Figure 6.

Fe-rich cumulate of monosulfide solid-solution (mss)and a Cu-rich fractionated sulfide liquid.

On a plot of Cu/h yersas Ni/Pd (Fig. 9), nvo distinctfeatures can be recognized: l) a gradual decrease inNi./Pd and Cu/Ir values from the norite-hosted tothe troctolite-hosted sulfides, reaching a minimumin the PGE-rich sulfide horizons, and2) a decrease inNi/Pd ratio with concomitant increase in Cu./Ir ratiofrom the massive sulfide lenses to their associatedchalcopyrite-rich disseminated sulfides. In the flustcase, the systematic decrease in metal ratios is dueto an increase in the PGE content of the sulfides,brought about by a rise in the R factor as the effect ofcountry-rock assimilation diminishes away fromthe basal contact. This relationship was recentlydemonstrated by Th6riault et al. (1997), who modeledthe composition of the above three rypes of disseminatedsulfide mineralization using the equation for equilibriumfractionation, and obtained average R factors of.175,1300 and 8000 for the norite-hosted, troctolite-hostedand PGE-rich sulfides, respectively. A similar increasein PGE content (hence R factor) with distance awayfrom the intrusive margins was also observed byBarnes & Francis (1995) in the Muskox intrusion,in the Northwest Territories. As for the associatedmassive sulfide lenses and chalcopyrite-rich sulfides,we explain the observed variation in metal ratios by thefractionation of k and, to a lesser extent, of Ni into nss,with Cu and Pd behaving as incompatible elements andbeing concentrated in the residual sulfide liquid. Themetal ratio of the massive sulfide pod sample (DC-75)in proximity to the field of norite-hosted sulfidessuggests that it has not undergone significant fractionalcrystallization, which is substantiated by the apparentlack of chalcopyrite-rich sulfides in the immediate area.

(!

z

t07ro61 G L

105

oa Tr@tolit+h@bd sulffd6@ PGE.dchsulllds. Maslvo gullldogo Cp'rlch sulfldogtr Unmom. argilllter B€dded pyrhotite unil

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880

The whole-rock metal content of all mineralizedsamples was recalculated to IOOEI sulfides for thepurpose of comparing the composition of the sulfidefraction of each rype of mineralization (Table 2). Thenormative composition of the sulfide fraction was thencalculated. Results clearly support the petrographicobservations, and show that the proportion ofbase-metal sulfides (1.e., chalcopyrite, cubanite, andpentlandite) increases relative to pyrrhotite as the effectof country-rock assimilation wanes away from tlebasal contact. Furthermore, a striking increase inthe normative proportion of Cu-rich sulfides is observedwithin the disseminated mineralization surroundingmassive pyrrhotite-rich lenses, as predicted by thefractional crystallization model.

TABLE 2. AVEMGE METAL CONCENTRATIONS ANDNORMATIVE MINERALOGY OF THE SULFIDE FMCTION

NOR TROC MSl CP ARGZ

36.3 35.5 36.460.2 42.7 63.21.39 19.9 0.081.93 1.75 0.08

3 14 <554 9 1 3 446 48 <370

'140 50 1126 7400 190

1300 10000 18047 3600 88

140 27 491500 900 1200

90 190 11002 21 2aO

46 110 ' t2

E Norite-hosted sulfidesZ Troctolite-hosted sulfldesI PGE-rich sulfidesE Argillito (DC-70)

Flc. 10. Mantle-normalized metal patlerns showing the rangein composition of the sulfide fraction. A. Norite-hosted,troctolite-hosted and PGE-rich disseminated sulfides, andthe bedded pyrrhotite unit of the Virginia Formation. B.I-enses of pyrrhotite-rich massive sulfide and associatedchalcopyrite-rich disseminated sulfides. Normalizationfactors are from Barnes et al. (1988).

Ir(18), Au(21), ft(31), Rh(36), and Pd(49). The higherenrichment-factors of the Pd-group of PGE (Rh, ft,and Pd: Barnes et al.1985) possibly relates to ttreirhigher partition-coefficient into the sulfide liquid. Thegradual increase in metal content observed fromthe norite-hosted to the PGE-rich sulfides probablyreflects a number offactors, including the degree ofcountry-rock assimilation, the ratio of silicate meltto sulfide liquid, and the partition coefficient of metalsinto the sulfide liquid.

The mantle-normalized patterns of the massivesulfide lenses and associated chalcopyrite-rich dissemi-

'to5

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IF

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too

104

s (%)Fe (%)Cu (o/o)Ni (%)Os (ppb)lr (ppb)Ru (ppb)Rh (ppb)Pt (ppb)Pd (ppb)Au (ppb)Re (ppb)Co (ppm)As (ppm)Sb (ppm)Se (ppm)

cp+cb (%)Pn (%)Po (%)

36.1 35.7 35.257.9 45.'l 33.94.45 15.3 22.51.26 3.77 8.14

14 40 16020 80 370

170 260 130058 310 2'tO0

470 2900 150001500 11000 72000330 1700 700082 160 180

2300 1700 2000280 '140 3266 74 1641 83 140

t03C)

6

2 r c 2@

= -

too

to '1

1 3 4 4 & . 4 5 7 04 1 2 2 6 6 6 0

83 44 10 90 37 100

NOTES: comDosition of the sulfide ftaction calculatedallowing 200 ppm Ni and 150 ppm Cu in the silicatecomponent of the intrusive rocks. Details of thecalculations in Bames & Francis (1995). 1 : includesonly massive sulfide lenses (i.e. DC-73 and DC-76);2: includes both argillite and bedded pynhotite unit.

The recalculated composition of the sulfide fractionof each type of mineralization was plotted on mantle-normalized variation diagrams to better compare theirlevel of metal enrichment. As shown in Figure l0A, thePGE-rich sulfides are shongly enriched in metalsrelative to the troctolite-hosted and norite-hostedsulfides, particularly in PGE and Au. In contrast, thesample from the bedded pyrrhotite unit shows a strongdepletion in all metals, and plots closest to the field ofnorite-hosted sulfi des. The following enrichment-factorswere calculated between the average PGE-rich andnorite-hosted sulfides: Cu(5), Ni(6), Os(l1), Ru(16),

f] Masslve sulfide lenses

I Chalcopyrite-rich sulJides

Cu-Ni-PGE SULFIDES. DIINKA ROAD DEPOSIT 881

nated sulfides show a clear antithetic relationship(Fig. 10B), with the former having higher contenrs in Ir,Ru and Rh, whereas the chalcopyrite-rich sulfides arestrongly enriched in the more incompatible metals(i.e.,Pt, Pd, Au and Cu). A striking characteristic ofthe massive sulfides is their strong negative anomaliesin Pt and Au relative to the smoother pattern ofthe chalcopyrite-rich assemblages. Similar patternshave also been described from sulfides of the Noril'sk-Talnakh District (Zientek et al. 1994, Barnes et al.7997a), the Cape Smith Fold Belt (Barnes et al. 1997a)and the Stillwater Complex (Zientek et al. 1994), wrd,illustrate the highly variable partitioning of meralsbetween /r?rs and the sulfide liquid.

To assess the degree of compatibility of metalsin mss, the recalculated composition of each lensof pyrrhotite-rich massive sulfide was divided by thatof its associated chalcopyrite-rich zone of disseminatedsulfide mineralization @g. I l). Results show that Re,Ir and Rh appear to partition strongly into zss, whereasPd, Sb, Cu, Au and Pt are highly incompatible andshow an increasing affinity for the sulfide liquid. Theaverage ratio of concentration in pyrrhotite-richassemblage to that in chalcopyrite-rich assemblage wascalculated for each metal, and yielded: Re(5.3), Ir(3.8),Rh(2.8), Co(1.6), N(1.1), Ru(0.96), As(0.47), Se(0.40),Pd(0.13), sb(O.10), Cu(0.07), Au(0.01) and ft (0.004).These values are quite similal to those obtained forother deposits (e.9., Zientek et al. L994, Li & BarnesL996), and compare favorably with experimentallydetermined partition-coeffi cients (e.g. Fleet et al. 7993,Li et al.1996, Barnes et al.1997b), considering thatthe lenses of massive sulfide are not perfect mssadcumulates (i.e., they sontain some fractionated liquidcomponent, manifested as a narrow Cu-rich rim). Theapparently incompatible nature of Se with respect tonss could explain the lower S/Se of the chalcopyrite-rich sulfides relative to the lenses of massive sulfide(Table l, Figs. 6, 8).

Genesis of the Dunka Road deposit

The origin of the Dunka Road Cu-Ni-PGE deposithas been interpreted in terms of tbree main processes,which operated in sequence. Each of the five typesof sulfide mineralization may be explained by thecombined action of an externally derived process,country-rock assimilation, and two internal magmaticprocesses, namely the interaction between the sulfideliquid and the silicate melto and fractional crystalliza-tion of the sulfide liquid. In light of rhe above, weinterpret the sequence of events as follows (Fig. l2):

l) Early during emplacement of the Partridge Riverintrusion, the surrounding sedimentary rocks werecontact-metamorphosed, leading to their partialdehydration and devolatilization. A light-sulfur-enriched H2S vapor phase was released into the magma

.001

Flc. I l. Variation diagram showing the degree of compatibilityof metals in the pyrrhotite-rich massive sulfide lensesrelative to their associated chalcopyrite-rich disseminatedsulfi de mi neralization.

through the breakdown ofpyrite into pyrrhotite, andeither homogenized with the resident juvenile sulfur orescaped to surface via overlyrng flows of flood basalt(Fie. l2A).

2) The restitic, light-sulfur-depleted pymhotite-bearingmetasedimentary rocks were subsequently assimilatedby the magma, releasing a granitic partial melt andimportant quantities of sS-enriched sulfur near the baseof the intrusion and in proximity to count4r-rockxenoliths. Owing to intense contamination by thegranitic melt, the hybridized noritic magma was coolerthan the surrounding hoctolitic magmq which led to itsearly crystallization and fonnation of the norite-hosteddisseminated sulfide mineralization (Fig. l2B). Thesulfide liquid crystallized near its source and had lesstime to interact with the magma, resulting in lowR factors. This hypothesis explains the metal-poor andpyrrhotite-rich nature of the norite-hosted sulfides.Furthermore, the sulfide liquid was As-rich andSe-poor owing to the large contribution of sedimentarymaterial to the magma, which is shown by the relativeabundance of arsenide minerals and elevated S/Sevalues.

3) A significant proportion of 3aS-enriched sulfur alsodiffused into the surrounding foctolitic magm4 mixingwith juvenile igneous sultur (Fig. l2C). The lesscontaminated and thus hotter troctolitic magma hadlonger to cool than the noritic magma. Therefore, thesulfide liquid had longer to interact with the silicatemelt, which led to higher R factors. This findingexplains the significant increase in the PGE content ofthe troctolite-hosted disseminated sulfi des.

O 1 ^

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hcr@lng@mpadbllity

882 THE CANADIAN MINERALOGIST

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v v v v v v v v v v v v v v v v

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S -".----

-----*:---,/ I

Cu-Ni_PGE SULFIDES, DTINKA ROAD DEPOSIT 883

4) A fresh input of relatively uncontaminated hoctoliticmagma was later injected above the basal mineralizedsequence. The low volume of sulfide liquid presentnear the base of this new injection was free to swirlwithin the turbulent magma, achieving very elevated Rfactors. Eventually, this 3aS-depleted sulfide liquidpercolated downward into the partly consolidatedtroctolitic rocks, and crystallized to form a PGE-richdisseminated sulfide horizon (Fig. 12D). As thesesulfides were largely derived from a mafic magm4 theyhave low As concentrations and S/Se values close tothose typical of the mantle.

5) In areas where significant amounts of sulfide liquidcollected (e.9., likely along the base, in proximity to thebedded pyrrhotite unit), it was filter-pressed upwardto accumulate as subvertical injections (Fig. 12D). Thesulfide liquid subsequently underwent fractionalcrystallization, forming an Fe-rich cumulate of rzss thatcrystallized along the cooler margin of the lenses. Theresidual Cu-rich sulfide liquid, which would haveaccumulated within the central part of the lenses, waseventually expelled along the outer margin of thelenses as well as into the surrounding mush of crystals(Fig. l2E). This fractionated sulfide liquid ultimatelycrystallized as chalcopyrite and cubanite alongthe outer margin of the lenses, as well as within theadjacent intrusive rocks, to form the chalcopyrite-richdisseminated sulfide mineralization. With furthercooling, the mss exsolved into pyrrhotite andpentlandite, forming the inner part of the observedlenses of massive sulfide (Fig. l2F).

Post-crystallization hydrothennal remobilization ofthe metals appears to have been minimal, on the basisof the lack of significant alteration throughout thedeposit.

CoNcr-ustoNs

Compositional variations in magmatic sulfides of theDunka Road Cu-Ni-PGE deposit are best explained byvarying degrees of country-rock assimilation, which infurn affected the amount of interaction between thesulfide liquid and the silicate melt (R factor). Fractionalcrystallization of sulfide liquid subsequently occurredwithin localized lenses of massive sulfide.

In this study, we have confirmed the significantrole of country-rock assimilation in localizing themineralization; this aspect had been emphasized inprevious studies (e.9., Ripley 1981, Rao & Ripley1983, Geerts 1991). In addition, however, we have beenable to monitor systematic variations in the intensity ofthe process through detailed field, petrographic,geochemical and isotopic studies of the mineralization.

AcrNowLepceNlpNTs

We gratefully acknowledge the significantcollaboration throughout this study of Steven Hauckand Mark Severson of the Minnesota NaturalResources Research Institute. Special thanks areextended to John McGoran of Fleck Resources for hisperrnission to publish the results, Penny Morton forher logistical support, Roger Eckstrand for allowinguse ofunpublished data, and Richard Lechasseur forhis assistance with the analytical work. Ed fupley(Indiana University) aad Gilles St-Jean (Ottawa{arletonGeoscience Centre) are also thanked for their 6sSanalyses and for informal discussion of results, Fundingfor this work was provided by a Fonds pour la Formationde Chercheurs et I'Aide I la Recherche research grantto R.D. Th6riault and by a Natural Sciences andEngineering Research Council of Canada operatingsrant to S.-J. Barnes.

Frc. I2. Schematic diagram illustrating the processes involved in the formation of the five types of sulfide mineralizationfrom the Dunka Road deposit. The figure is not meant to be to scale. A. Early during contact metamorphism, a light-sulfur-enriched HrS vapor phase is released into the troctolitic magma through the breakdown ofpyrite to give pyrrhotite. B. Therestitic, light-sulfur-depleted metasedimentary rocks are later as5imil2@d !y ftg magma, releasing a granitic partial melt andimportant quantities of sS-enriched sulfi:r near the base of the intrusion and in proximity to country-rock xenoliths. Thehybridized magma and metal-poor sulfide liquid eventually crystallize, forming the norite-hosted disseminated sulfidemineralization. C. A significant proponion of:+S enriched sulfur also diffuses into the surrounding troctolitic magma,mixing with juvenile igneous sulfur. The immiscible sulfide liquid interacts with a larger volume of silicate magma, andcrystallizes to form foctolite-hosted disseminated sulfides having moderate PGE contents. D. A ftesh input ofuncontaminatedfoctolitic magma is injected above the basal mineralized unit. Sulfide liquid from this new injection is free to swirl withinthe turbulent magma, and eventually percolates downward into the partly consolidated rocks to form a PGE-richdisseminated sulfide horizon. Meanwhile, small pools of sulfide liquid near the base of the intrusion are filter-pressed upwardto form subvertical injections. E. The sulfide liquid undergoes fractional crystallization, forming a cumulate of mss thatcrystallizes along the lens margins. Residual Cu-rich sulfide liquid accumulates in the central pan of the lens, and is even-tually expelled along the outer margins as well as into tlre surrounding crystal mush. F. With further cooling, the fractionatedsulfide liquid cryst'llizes into chalcopyrite and cubanite while the m.ss exsolves into pyrrhotite and pentlandite, forming theobserved pyrrhotite-rich massive sulfide lenses and associated chalcopyrite-rich disseminated sulfide mineralization.

884

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