GEOCHEMICAL CONSTRAINTS ON THE GENESIS OF THE ... - RRuff

Canadian Mineralogist Vol. 28, pp.451474 (1990)

GEOCHEMICAL CONSTRAINTSON THE GENESISOF THE MONTCALM GABBROIC ONTARIO COMPLEXAND Ni-Cu DEPOSIT,WESTERNABlTlBl SUBPROVINCE, C. TUCKER BARRIE* eNo ANTHONY J. NALDRETT Departmentof Geology,Universityof Toronto, Toronto, OntarioMsS 3Bl DON W. DAVIS JackSauerleyGeochronology Laboratory,Royal OntarioMuseum,100Queen'sPark, Toronto, OntarioMSS2C6 ABSTRAcT The Montcalm gabbroiccomplex(X02 + 2 Ma: U-Pb zircon), located in the western Abitibi Subprovincein Ontario, containsthreelithologic units: a basalpyroxenite zone(PZ), a gabbro - anorthositicgabbro zone(GZ) and a ferroan gabbro zone (FZ). Lower PZ rocks are orthocumulates,with orthopyroxene,clinopyroxene,olivine and minor plagioclaseas cumulusphases.They have Mg numbersof 68-72,and are enrichedin TiO2, P2Os,Zr, Y, and the REE relative to GZ cumulates,reflecting the presenceof abundantinterstitial liquid nearthe margin of the complex.They are ZREE-enriched,with La11/Ybpin the range2.4-3.8, and are slightly depletedin Ta, Nb, Ti, Zr, and Hf comparedto MORB or primitivemantle.Upper PZ rockshavecoarse-grained and pegmatitictextures,with cumulusclinopyroxene,orthopyroxeneand plagioclasethat are partly or completelyreplacedby postcumulushornblende, plagioclaseand Fe-Ti oxides. Pyroxenite and gabbro pegmatitedikeswith either sharpor gradationalcontactsarecommonin the upperPZ. Systematicenrichments in V, Cu, Ni, Co, Au and S and, to a lesserextent,the REE are found in upper PZ rocks. They are attributed to concentration by a zone-refining process, whereby partial pyroxenitewas fluxed by a melting ol hot (supersolidus?) volatile-richfluid medium,possiblyderivedfrom contemporaneousfelsic magmatism(granodiorite dike: 2700i5 Ma: U-Pb zircon). GZ rocks are composedof plagioclase - clinopyroxenemeso-to adcumulate,with localplagioclase subporphyritic intervals. GZ whole-rock Mg numbersrange from 73 to 81, and havelow incompatibletrace-element, sulfur and PGE contents,consistentwith low amountsof trappedliquid and sulfur-undersaturated conditionsduring GZ formation. The Montcalmdeposit(3.56million ronnes, | .44 wt.t/oNi and 0.68V0Cu, 50 ppb total PGE) is located within GZ cumulatesat the northem extent of the complex. High Pt,/Ir valuesand low PG.Econtentsin the NiCu depositand lower PZ rocks are consistentwith their derivation from a primitive magmathat had experienced 1) olivine and chromite fractionation, with Ir behavingcompatibly in the crystallizingphases,accompaniedby 2) continual removalof traceamountsof sulfide.The main event of sulfide segregationmay haveoccurredin a different part of the magma chamber or at depth, in a lower crustal chamber.The depositwasthen tectonicallyemplacedinto its presentlocation from a predominantlypyroxenitichost during the late stagesof consolidation. tPresentaddress:BP Resources CanadaLimited, 890West PenderSt., Ste.700,Vancouver,British ColumbiaV6C lK5.

Keywords: Montcalm gabbroic complex, Abitibi Subprovince,Ontario, nickel, platinum, magmaticsulfide, zone refining. SOMMAIRE Le complexegabbroiquede Montcalm, dansle secteur de l'Abitibi en Ontario, a un dge Ouestde la sous-province U-Pb obtenusur nrcon de2702+ zMa, et comprendtrois unit6slithologiques,une zone inf6rieure de pyroxenite(PZ), une zonede gabbro et de gabbro anorthositique(GZ), et une zone de gabbro riche en fer (FZ). Les rochespris de la basede la zonePZ sont des orthocumulatsd'orthopyroxdne, de clinopyroxbneet d'olivine, avec plagioclase accessoire.Ces roches ont une valeur de Mg# [00Mg/(Mg+Fe)] entre 68 et 72, et sont enrichiesen TiO2, P2Oj, Zr,Y et lesterresrarescompar6esalrx cumulats GZ; ces caracteressont I'expressiond'une fraction importante de liquide interstitiel prbs de la bordure du massif, Elles sont enrichiesen tenes rares l6gbres(2.4 < Lap/Yb1 < 3.8)et ldgirementappauvriesen Ta, Nb, Ti, Zr et Hf compardes aux basaltesMORB et au manteauprimitif. Plus haut dans I'unit6 PZ. les rochesont une texture grossidrea pegmatitique,et sont descumulatsde clinopyroxdne, d'orthopyroxdne et de plagioclasequ'ont remplac6ir un stade postcumulus,complbtementou en partie, homblende, plagioclaseet oxydes de Fe-Ti. Les filons de pyroxenite et de gabbro pegmatitique y sont r€panduset montrent une 6pontesoit franche, soit floue. syst6matiquesen V, Cu, Nous signalonsdesenrichissements Ni, Co, Au et S, et desterres rares. Nous attribuons ces 2run processusde purification par fusion enrichissements en zones,c'est-i-dire une fusion partielled'une pyrox€nite prEsde son soliduspar I'addition d'une phasefluide, li6e peut etre i un magmatismelelsique contemporain (un filon de granodiorite a un dge U-Pb obtenu sur zircon de 2700:45Ma). Les rochesde la zone GZ sont desm6socumulats et des adcumulatsir plagioclase+ clinopyrox0ne qui montrent despass6es localementsubporphyriquesir plagioclase.Leur valeur de Mg# estsitu€eentre 73 et 8l; elles incompatimontrent de faiblesteneursen 6lements-traces bles,en soufreet en 6l6mentsdu groupedu platine (EGP), cequi concordeavecune faible proportion de liquide intersen soufre.Le giseritiel et desconditionsdesous-saturatiot-r en ment Ni-Cu de Montcalm(3.56 x 10otonnes,1.440/o poids de Ni, 0.6890de Co, 50 ppb desEGP) est situ6 au seindescumulatsGZ dansle secteurNord du massif.Les valeurs6levdesde Ptllr et les faibles teneursen EGP du minerai et desrochesprbsde la basede l'unit6 PZ seraient l'expressiond'une d6rivationd panir d'un magmaprimitif

451

4s2

THE CANADIAN MINERALOGIST

qui aurait subi un fractionnementdeI'olivine et de la chromite, le Ir ayant agi de fagon compatibledanscet assemblage, et d'une perte continue de faibles quantit6sde sulfures. L'6v6nement principal de s6paration de sulfures pourrait avoir eu lieu ailleursdansla chambremagmatique ou a plus grandeprofondeur, dansla crorlteinf6rieure.Le gisementa ensuite6t6 mis en place tectoniquementpar le d6placementd'une roche-h6teA caractdresurtout pyrox6nitique au cours d'un stadetardif de la cristallisationdu complexe. (Traduit par la R6daction) Mots-clds: complexe gabbroique de Montcalm, sousprovince de I'Abitibi, Ontario, nickel, platine. sulfure magmatique,affinage en zones. INTRODucrroN The Montcalm Ni-Cu deposit was discovered in 1976. when the first drillhole to test an airborne electromagnetic anomaly intersected 14.4 m of massive and disseminated sulfide mineralization (Blecha et al. 1977). Further drilling outlined 3.56 million tonnes of reservesto a depth of 300 m, with an average grade of l.44Vo Ni and 0.6890 Cu. The deposit

is unusualin two respects.First, the sulfide mineralizationis hostedwithin cumulategabbroic rocks. Generally,Ni-Cu sulfide mineralizationis found at the baseof the host intrusion, a feature commonly attributedto gravity settlingof a densesulfideliquid through a lessdensesilicateliquid. Barrie& Naldrett (1989)interpretedthe depositashavingbeentectonically emplaced into its present location from a predominantly pyroxenitic host, during or shortly after consolidationof the Montcalm Gabbroic Complex (MGC). The second unusual feature of the depositis its very low PG,Etenor (averageof 50 ppb total PGE), despiteNi and Cu contentsthat areconsiderednormal for gabbro-hostedmagmaticsulfide deposits(e.g., nearly two ordersof magnitudelower than typical Sudburydeposits:Naldrett l98l). An initial explanationfor the low tenor of the PGE at Montcalm wasprovidedby Naldrett & Duke (1980). They modeledthe averageNi, Cu and Pd contents of the depositas a product of a suddeneventof sulfide segregationfrom a picritic magma, after the magma had undergonecontinuous segregationof traceamountsof sulfide during the fractionation of olivine, clinopyroxeneand plagioclase.

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Ftc. l. Location map, and regionalgeologyin the westernAbitibi Subprovince.The contactwith the KapuskasingStructural Zone representsa west-dippingthrust fault.

453

IHE MONTCALM GABBROIC COMPLEX. ONTARIO

The geology and strucrural history of the MGC and Ni-Cu deposit have been describedin a complementary study (Barrie & Naldrett 1989). The presentstudy usesa detailedpetrographic,mineral and whole-rock geochemicaltraverse across the stratigraphy to interpret the magmatic and postmagmatic processesthat formed the MGC and Ni-Cu deposit. High-precision U-Pb zircon ages are presentedfor the MGC and a granodioritedike that cutsthe Ni-Cu deposit,to placetheserockswithin the temporal framework of magmatism and tectonism in the southernSuperior Province. In addition, neodymium isotopeanalysesfrom a regional study(Banie& Shirey,submitted)areusedto characterize isotopic signatures of the MGC parental

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REcToNALGeot-ocrcet- SErrrNc The Abitibi Subprovince,a major lithologic - tectonic block of the southernSuperiorProvince,is characterizedby predominantly east-trendinglowgrademetavolcanicand metasedimentary rocks,and massiveto gneissicintermediateand felsic plutons. The KapuskasingStructural Zone is a northeasttrending elongatebelt (500 x 59 km) ofhigh-grade metamorphicrocks at the westernterminus of the Abitibi Subprovince.The Montcalm areacomprises

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Zone refining in the upper Pyroxenite Zone Upper PZ rocks havetexturesthat suggestpartial or completeremeltingin the presenceof a volatilerich phase.Theseinclude: l) coarse-grained and pegmatitic textureswith intercumulushornblendeand Fe-Ti oxidesafter cumulus phases,2) gabbro pegmatite gradational with pyroxenitecumulates;and 3) pyroxenite and gabbro pegmatite dikes. The presenceof a volatile-richphaseduring postcumulus cooling can lower the solidustemperatureof pyroxe[ite cumulatesand causepartial melting.The newly formed melt will migrate through the cumulates, leavingbehind a residuerich in compatibleelements (e.9., pyroxenite dikes), while the melt becomes increasingly enriched in incompatible elements, analogous to zone refining in the metallurgical industry (Harris 1957,McBirney 1987).Enrichment in incompatible elements will occur where the volatile-rich melt crystallizes(e.9., gabbro pegmatite dikes). As in hydrothermal systems,repeated pulses of volatile-rich melt may follow the same direction and ultimately leavea complexsignature. This processis similar to infiltration metasomatism

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described for pegmatitic hornblendites at Duke Island in Alaska by Irvine (1974,1987). The origin of the volatile-rich fluids is not well understood.The pegmatitichornblenditesat Duke Island, exposedover hundredsof squaremeters,are most prominentnearthe margins(Irvine 1987),with the inferencethat the infiltrating fluids were derived from surroundingrocks. At the SkaergaardIntrusion and severalof the Tertiary layeredintrusions of East Greenland,gabbro pegmatitedikes, veins, and pods are especiallyabundant in the Marginal BorderSeries,and the volatile-richfluids responsible for their formation are believedto be derivedfrom adjoining country-rocks (Taylor & Forester 1979, Irvine 1987,Bird et ol.1988).This fluid infiltration would have to take place while the cumulateswere still hot enoughto be remeltedunder hydrous conditions. Geothermometersindicatethat calcicamphiin subsoliduspegmabole + pyroxeneassemblages titic veins up to 2 cm thick at Skaergaardformed at525"C to 825oC,under hydrostaticconditions at 0.5 kilobars (Bird el a/. 1988).Experimentson the melting of magnesian basalt with P(H2O) = P(total) indicatethat at2 and4 kilobars, the solidus

468

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THE CANADIAN MINERALOGIST

matitic pyroxeniteand gabbro pegmatitedepletedin incompatible elements, and precipitating an residuum,partly due incompatible-element-enriched of saturation of incompatible-element-enriched phases.Given this interpretation, a V-rich phase, probably a Fe-Ti oxide, becamesaturatedin the volatile-rich melt before a sulfide phasethat contained high Cu and Au contents. In many respects,the upper PZ is similar to the Robie Zone of the Lac des Iles Pt-Pd-Au-Cu-Ni depositin northwesternOntario (Macdonald1987). Gabbrosof the Robie Zqne with texturessimilar to upper PZ rocks host significant concentrationsof Pt-Pd mineralizationthat hasbeenattributedto the zone-refiningprocess(Briigmann et al. 1989).Unfortunately,resultsof semiquantitativePt-Pd-Au analysesfor PZ pyroxenitesand gabbro pegmatitesdo not indicate any significant enrichmentlike that at Lac des Iles (Barrie, unpubl. data). HiCh Pt/Ir values, and derivotion of the metol content in the Ni-Cu deposit

o s

The high Pt,/Ir valuesof the lower PZ cumulates (144,n = 4) arecomparableto thosefound in pyroxenitic and gabbroic rocks of the Thetford ophiolite (71and 265,respectively: Oshin& Crocket1982)and gabbrosassociatedwith Munro Township komatiites (55:Crocket& MacRae1986).Thesevaluesaresignificantly higher than the chondritic ratio of 1.9 (Nalo.oo1 drett & Duke 1980),or Pt/lr valuesfound in ultrattttttl pt pd Os lr Ru Rh Au mafic tectonites and cumulates at Thetford Ftc. 15.PGE andAu average abundances for Montcalm (0.05-11.7:Oshin & Crocket 1982)or at Munro Ni-Cu depositin comparison to typicalkomatiite-hosted Township(7.8: Crocket& MacRae1986).Further(Langmuir,MountEdward,Kambalda), deposits and more. the Ptllr value of the Montcalm Ni-Cu orhergabbro-hosted deposits. ModifiedafterNaldrett deposit(90, n : 20) is similar to valuesfor the Min& Duke(1980)andNaldrett(1981). n€rmaxand Great Lakes Nickel deposits of the Duluth GabbroicComplex(55-75:Naldrett l98l), but much higherthan for komatiite-hosteddeposits temperatures are 700 and 825'C (Holloway & (3.3-14.4:Naldrett & Duke 1980).Partial melting Burnham 1972).Thesetemperatureswould represent and fractional crystallizationare consideredviable minimum temperaturesfor the fluids responsiblefor mechanismsto producehigh Pt,/Ir values(or Pd/Ir development of the upper PZ pegmatitic rocks. values,as Pd and Pt behavesimilarly) in primitive Interestingly,this is within the upper limits of fluids magmasand derivativeNi-Cu deposits(Barnese/ that have evolvedfrom granitic plutons (Burnham al. 1986,Naldrett & Barnes1986).In both cases,the 1979,Beane&Titley l98l). Ifthe contemporaneous argumentsare basedupon the assumptionsthat Ir felsic dikes that cut the GZ and PZ representthe behavescompatibly in olivine and chromite, whereas upperreachesof a largerfelsicpluton at depth,then Pt and Pd are incompatible.Bri.igmannet al. (1987) the fluids could be derivedat leastin part from such determinedan olivine - komatiitic liquid partition a pluton. coefficient for Ir of about 2, and there is petrologIn the upper PZ, theREE arelowestnear the GZ ical evidencethat chromite fractionation removesIr and progressto higher levels down-section;this is by nucleatingaround laurite and Os-Ir-Ru alloys particularly prominent for S, Cu and Au. These (e.9., Stockman& Hlava 1984). trendswould suggestthat a volatile-rich fluid origiIt is difficult to envision partial melting from a natednearthePZ-GZ contact,possiblyfrom a con- primitive mantle as the principal mechanism to duit near one of the contemporaneousfelsic dikes. increase the Ptllr value at Montcalm, for two The migrating fluid may havecausedpartial melting reasons.First, clinopyroxeneand whole-rock Mg of pyroxenitecumulates,leavinga residuumof peg- numbersup to 82 and HREE contentsin the lower o

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469

THE MONTCALM GABBROIC COMPLEX, ONTARIO

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FIo, 16. REE diagrams,normalizedto averageCI chondritic valuesof Evensenel al. (1978).a. PyroxeniteZone, with lower PZ (vertically ruled) and upper PZ (horizontallyruled).b. GabbroZone,with lower GZ ( ltr) and similar to one another,so that sulfide derived from these magmas would have similar Pt,/Ir valuesof 5-10. Thesevaluesare much lower than thoseof the lower PZ cumulatesand the Montcalm Ni-Cu deposit. Second,the low PGE tenor of the depositcannot be reconciledwith the base-metaltenor after segregationof an immiscible sulfide liquid from a primitive picritic magma, for any ratio of silicate magma to sulfide liquid, as

pointedout by Naldrett& Duke (1980).Whereasa large batch of sulfide liquid in equilibrium with a magnesiantholeiitic melt (a low "R" factor: Campbell & Naldrett 1979)could accountfor the low PGE tenor. the Ni and Cu contentswould be too low. Partial melting from a mantle sourcethat had undergone a previous event of partial melting that left behind trace amounts of sulfide, as suggestedfor boninitic magmas(Hamlyn et al. 1985),could also lead to a higher Ptllr value; however,this scenario is not consideredapplicableto Montcalm in view of its tholeiitic affinities. Fractional crystallizationwith continual segregation of sulfidefrom a primitive magma,asoriginally proposedby Naldrett& Duke (1980),is a reasonable alternative.They modeledthe sulfide that would be in equilibrium with a magnesiantholeiitic liquid after fractional removal of 50Voolivine, clinopyroxene, plagioclase, and continuous removal of trace amountsof sulfide. By this method, they achieved a sulfideliquid with 4.0 wt.9o Ni, l.3Vo Cu and 17 ppb Pd in l00Vo sulfide, very similar to sulfide-

470

THE CANADIAN MINERALOGIST

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FIc. 17. Montcalm metal contentsin comparisonto Thetford Ophiolite Complex data (Oshin & Crocket 1982),MontcalmPZ rocks: opensquares,MontcalmGZ rocks: opencircles,Thetford lherzolites,dunites,and pyroxenites:closed circles,Thetford gabbroicrocks: closedsquares,a. Ni/Co versusPI.b. Ni/Co versusb. c. Ni/Co versusPt/b. Star representsMontcalm deposit average.d. Ni/Co yersasS.

normalized values calculated for the Montcalm deposit.This model also accountsfor higher Ptllr values,asolivine (andchromite)fractionationwould depletethe magmain Ir. A schematicillustration of this model, involving silicatefractionation accompaniedby sulfide segregation from a primitive magma, is given in Figure 17,whereMontcalm data areplottedalongwith data from from the Thetford Ophiolite, Quebec(Oshin & Crocket1982).The Thetford suiteincludesupper mantleharzburgiteand peridotitetectonites,in fault contactwith a cumulatesequenceof partly layered cumulates. The cumulates are chromite-bearing dunites at the base, overlain by pyroxenites and gabbros. Ophiolite suites including Thetford are generallyregardedas the product of open-system fractionalcrystallization(e.g., O'Hara I 977,Pallister & Knight 1981).Ni/Co is usedasa generalindexof fractionationfor thesecumulaterocks. Ni is favored over Co by 2-4 times in mafic silicatesand spinel (seeIrving 1978),and 3-5 timesin sulfide(Rajamani & Naldrett 1978),so that Ni,/Co decreases systematically with fractionation. The Montcalm traverse samplesaresimilar to Thetford pyroxeniticand gabbroic rocks in their Pt, Ir and S contents(exceptfor the high S contentin upperPZ rocks),and their high Pt,/Ir values;the Ni-Cu deposithas a similar Pt,/Ir value (Fig. 17c).

Relativetiming of the immisciblesulfide eventduring the evolution of the MGC The timing of the immisciblesulfideeventthat led to the formation of the Ni-Cu depositis constrained by the following considerations:the presenceofsulfides in thePZ, but very low S and PGE concentrations in the GZ, implying sulfur undersaturation during GZ formation; similar, but slightly higher, more fractionatedPt/b valuesin thePZ than in the Ni-Cu deposit;6345and S/Sevaluesconsistentwith a magmaticderivation of sulfide; and depletedisotopic signaturesfor the PZ and GZ , but an enriched signaturefor the FZ. Theseconsiderationsare consistent with the following sequenceof events: l) ponding of a primitive partial melt from the mantle in the lower crust,and fractionationof olivine,chromite and trace amounts of sulfide; 2) migration of the residualliquid from the lower crustal chamber to the MGC chamber, to form pyroxenites represented by thePZ and the pyroxenitefragments in the inclusion-brecciasulfide; 3) formation of the main immisciblesulfideliquid from the PZ parental magma,eitherin a lower crust chamberor the MGC chamber;4) continuedfractionation into the stability field of plagioclase,accompaniedby influxesof new magma,and the formation of GZ cumulatesat2702 + 2Ma;5) assimilationof an isotopicallyenriched

THE MONTCALM GABBROIC COMPLEX, ONTARIO

qustal componentduring FZ formatioa; 6) emplacement of the sulfide massinto the GZ after consolidation of GZ cumulates, during tectonism and regionalcrustalrotation into a subverticalposition; 7) emplacementof the main granodiorite dike at 2700105Ma and subsidiary felsic dikes; 8) volatile fluxing causingremelting and recrystallizationof upperPZ rocks; and 9) deformationassociatedwith the emplacementof granitic plutons to the eastat 2696-2692 Ma. CoNcI-ustoNs The principal conclusionsdrawn from the geology, geochronology,petrology, and geochemistryof the MGC and Montcalm Ni-Cu depositare summarized as follows: l. The MGC was emplacedat2702 t 2 Ma in an activetectonicsetting,and wassubjectedto rotation of the crust into a near-verticalposition during or shortly after consolidation.The MGC and associated Ni-Cu depositwere cut by a subverticalgranodiorite dike at 2700!n5Ma. 2. In the vicinily of the Ni-Cu deposit, the lower PZ orthocumulatesprovide the best estimateof a liquid composition. They have relatively high concentrationsof the incompatibletrace-elements,are LREE-enriched,and havePtllr valuessimilar to but slightly higher than the Ni-Cu deposit. The majority of upper PZ rocks havecoarse-grainedand pegmatitic textures,and havebeenextensivelymodified by postcumulus recrystallization due to a flux of volatile-rich fluids, possibly derived from contemporaneous felsic magmatism. GZ rocks are predominantlyplagioclase- clinopyroxenecumulates with low levelsof incompatibletrace-elementsand sulfur. There is limited evidencefor multiple pulses of replenishingmagma during GZ formation. 3. The deposithas Ni, Cu and Ptllr valuestypical for gabbro-hosteddeposits,and yet a very low PGE tenor. Values of 63aSand S/Se are consistentwith a predominantly magmatic sourcefor sulfur. The high Ptllr value and the low tenor of the PGE are consistentwith sulfide segregationfrom a primitive magmaafter fractionationof olivine, clinopyroxene and probably chromite, along with trace amounts of sulfide. This fractionation occurred during or before PZ formation, as suggestedby high Ptllr values for PZ rocks, in the Montcalm magma chamberor in a chamberdeeperin the crust. 4. The parentalmagmawasderivedfrom an isotopically depleted mantle, typical of tholeiitic and komatiitic magma sourcesin the southernAbitibi and similar to the depletedMORB mantle at 2700 Ma. Assimilation of an enrichedcrustalcomponent occurred during FZ crystallization.

471

ACKNOWLEDGEMENTS

We thank Dr. M. Blechaand Teck Corporation for accessto the drill core of the Montcalm deposit and to unpublishedgeochemicaland aeromagnetic data; Dr. A. H. Green,J. Cecchettoand Falconbridge Ltd. for logistical support; Dr. T.E. Krogh for use of the analytical facilities at the Jack Satterley Laboratory of the Royal Ontario Museum;S. Kamo for helpwith U-Pb chemistry;Dr. S. B. Shirey for helpwith Nd isotopedatacollection;Drs. G. E. Brtgmann and M. P. Gorton for fruitful discussions about the geochemicaldata; Dr. R.F. Martin and two refereesfor their editorialand scientificreviews. This work has been supported by grants from the GeologicalSocietyof Americaand SigmaXi to CTB and NSERC Grant A-4244to AJN. RrrrnsNcss of, composition Alr-rn, L. H. (1967):Earth,chemical andits comparisonwith that of the sun,moonand planets..In Compositionof the Earth I (S.K. Runcorn, ed.).PergamonPress,New York. BanNrs,S. J. (1986):The effectof trappedliquid crystallization on cumulus mineral compositionsin layeredintrusions.Contrib. Mineral. Petrol. 93, s24-531. BnnNes,S.-J., Nar-onrrr,A. J. & GonroN,M. P. (1985):The origin of the fractionationof platinumgroupelements in terrestrialmagmas,Chem,Geol. s3.303-323. and TectonicEvo' Bennrr,C. T. (1990):Petrogenesis lution of the Kamiskotiaand Montcalm Gabbroic Complexesand Adjacent Granitoid - Greenstone Belt Terrane,WesternAbitibi Subprovince,Ontario, Canada.Ph.D. thesis,Univ. Toronto, Toronto, Ontario. & Devts, D. W. (1990):Timing of magmatism and deformation in the Kamiskotia - Kidd Creek area, westernAbitibi Subprovince,Canada.Precambrian Res. 46.217-240. & Nernnerr, A.J. (1989): The geology and tectonic setting of the Montcalm gabbroic complex and Ni-Cu deposit. In Magmatic Sulphides, The Zimbabwe Volume (M.D. Prendergast& M.J. Jones, eds.).Inst. Min. Metall., London (l5l-165). BaeNE,R.E. & TrrLrv, S.R. (1981):Porphyry copper deposits. II. Hydrothermal alteration and mineralization. Econ. Geol., 75th Anniv. Vo|.,235-269. BeNNrrr, G. (1966a):Belford Township, District of Cochrane. Ont. Dep. Mines Prelim, Geol, Map P344. (1966b): Montcalm TownshiP, District of Cochrane. Ont. Dep. Mines Prelim. Geol. Map P346.

472 -

THE CANADIAN MINERALOGIST

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DuenE, B,, CHeuvEL,C. & Anuor, N.T. (1984): Pb and Nd isotopic study of two Archean komatiitic flows from Alexo, Ontario. Geochim. Cosmochim. Acta 48. 1965-1972.

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