Physical and Chemical Constraints on Platinum-Group Element Behavior during Crystallization of a Basaltic Komatiite Liquid: Example of the Proterozoic Delta ...
Economic Geology Vol.90, 1995,pp. 2287-2302
Physical andChemicalConstraints on Platinum-Group ElementBehavior duringCrystallization of a Basaltic KomatiiteLiquid: Exampleof the Proterozoic Delta Sill,New Quebec,Canada C. PICARD,J. AMOSSI•, M. PIBOULE, Laboratoire de Geodynamique desChafnes Alpines (UPRES-A 5025),rueMauriceGignoux, 38031,Grenoble, France AND D. GIOVENAZZO
Falconbridge Limited,8, rueDoyon,C.P.1056,Rouyn-Noranda, Quebec, Canada]9X 5C8 Abstract
TheDeltasill,located in thesouthern partof the2.03to 1.92GaCapeSmithfoldbelt,northern Quebec, iscomposed of a peridotitc, pyroxenite, gabbronorite, andanorthosite suitederivedfroma basaltic komatiite parental magma (MgO= 13.58-15.66%). Despiteitsratherlimitedthickness (215-275m),it contains three platinum-group elements (PGE)horizons, twoslightly enriched horizons, onein theperidotitc andonein the lowergabbronorite, anda strongly PGE-enriched horizon(forminga reef)in the uppergabbronorite. ThehighPGEcontent in thereefimpliesa system withanR factor(volume of immiscible sulfideliquid/ volumeof silicated magma)of 100to 1,000.However,the smallvolumeof magmainvolved suggests that several otherprocesses controlled the PGE behavior. Someexperimental workshaveshownthatthe CaO andFeO contents in parentalmagmas influence the sulfidecapacity andconsequently the sulfurfugacity. Thelastparameter, aswellastheoxygen fugacity, havebeenpreviously recognized ashavinga fundamental influence onthePGE enrichment processes. Because of therelatively highCaOcontents (7.21-11.26%)of parentalmagmaandthe existence of PGE enrichment, the Deltasillprovides an opportunity to testthe effects ofvarying sulfurandoxygen fugacities ontheprocesses ofPGEenrichment. Thus,thePGEconcentrationprofiles observed in thetwosections transecting thesillarein goodagreement withtheexperimentally determined behavior of theseelements in silicate meltsasa functionof oxygen andsulfurfugacities. First, the relatively highCa contentof the initialmagmainduceda highsulfidecapacity in the liquid.Second, decreasing sulfide capacity (andcorrelatively, anincrease in sulfurfugacity) produced bytheprecipitation of calcicphases--notably clinopyroxene andplagioclase -- account for thePGE anomalies aswellasthelatestagecrystallization of sulfides in the reef. In the lowermost PGE anomaly, Ir andRu enrichment withinthe peridotitic layercanbe clearlyrelated to earlyferrochromite crystallization dueto increasing oxygen fugacity, whereas subsequent Pt enrichment canbe relatedto the formation of an isoferroplatinum phase(PtaFe)associated withferrochromite. In the second PGE anomaly, located in thelowergabbronorite, synchronous Ir andRu,Rh,Pt, andPd, andS enrichments aredueto theprecipitation of smallamounts of sulfides underlowoxygen fugacity. Both the Ir and Ru andthe Rh, Pt, and Pd, left overfrom the firstchromite-forming stage,are complexed by sulfurwhenthe sulfidecapacity increases, consequent to CaO removalfrom the melt by plagioclase fractionation.
Finally,the thirdanomaly in the PGE reef,appears to recordthe crystallization of a differentiated FeandTi-richresidual liquidcontaining fluidphases andsulfur.It reflects themigration of anS-bearing liquid following theimportant lowering of sulfidecapacity in theliquiddueto theincrease of SiO2andTiO2in the liquidandto theprecipitation of Ca-bearing minerals. Introduction
Naldrett and Barnes,1986; Naldrett, 1989; Peachet al., 1990;
MOSTof the world'splatinum-group elements(PGE) are producedfromtwo typesof deposits (Cabri,1981):PGEdominated deposits wherePGE are the mainproduct,and Ni-Cu sulfides wherePGE are a by-product. Bothtypesof deposit aredoselyassociated withlayeredintrusions (Cabri, 1981;Naldrett,1989;WhitneyandNaldrett,1989,andrefereneestherein).In thesedeposits, PGE distributions appear to be principally controlled by magmatie processes, suchas partialmelting,whichdetermines thePGE concentrations in the magma,and fractional crystallization and formationof immiseible sulfide-bearing liquidphases, whichhavean impoRanteffectontheprecipitation of PGE frommagmas and influence thelocationof mineraldeposits (Fleetet al., 1977,
Stoneet al., 1990).PGE behavioralsoappearsto be controlledbymajorelement composition ofthemagmas (particularlyFeO, CaO,A1203, andSiO2).Effectively, someexperimental data has shown that the CaO and FeO contents in
parentalmagmas influencethe sulfidecapacity(Buchanan and Nolan, 1979; Vannier, 1979; Schlakenatlas,1981; Saint-
Jours,1988)andconsequently the sulfurfugacity. Thislast parameter aswellastheoxygen fugacity hasbeenpreviously recognized ashavinga fundamental influenceon the PGEenrichment processes (Amoss6 et al.,1987,1990;Amoss6 and Allibert, 1993).
In theProterozoic CapeSmithfoldbeltof northernQuebec (Fig. 1), thereare a numberof PGE-enriched Ni-Cu sulfidedeposits located in ultramafic flowsandsills,particu1981; Naldrett et al., 1979, 1990; Barneset al., 1985, 1988; larly alongthe Lac Cross,Katinik,and Raglanhorizons
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FIG. 1. Simplified geologic sketchmapof the Ungava troughshowing location of the Deltasillandotherintrusions with occurrences of PGE.
(Barneset al., 1982, 1992; Dillon-Leitch et al., 1986; Giove- linites,andphonolites (Gaonac'h et al.,1992).In places, these lavas(Povungnituk Group;MgO nazzo,1991).Thesesillsarechemically comagmatic withthe lavasoverliefloodbasaltlike komatiitic basalts of theChukotat group(Francis et al.,1981, • 10%, TiO2 = 1.2-3.6%; Franciset al., 1981, 1983; Picard 1983; Picard et al., 1990). A number of differentiatedul- et al., 1990)whichwereeruptedin a within-plate environtramaficto maficsills(Povungnituk, Romeo,Delta,andNo mentsubject tohotspotvolcanism in a distended continental Name sills;Picard and Giovenazzo,1995) also crosscutthe regime(Picard, 1994).Geochemically, theDeltasillisrelated volcanic Povungnituk Groupunderlying theChukotat koma- to the overlying komatiitic basalts of the ChukotatGroup tiiticbasalts. Thesesillsarealsocomagmatic withthekoma- (MgO= 19-11%,TiO2•0.9%), whicharethemselves attribtiiticbasalts buttheydonotcontain PGEmineralization, with uted to ocean crust formation (Francis et al., 1981-1983; theexception oftheDeltasillwhichpresents a gabbroic zone Picardet al., 1990)duringmantleplumeactivity(Picard, with a sulfide-bearing reef (1 m thickness) displaying PGE 1994). enrichment. Althoughseveralother simfiarintrusions cut the volcaBecause oftherelatively highCaO(7.21- 11.26%)contents nosedimentary rocksof the Povungnituk Group,the Delta of itsparentalmagmaandof the extensive development of sillis theonlyoneto containa PGE reefof significant size the reef in relationto the totalthicknessof the intrusion(215- (1 m thick;seeFigs.2 and3). Otherintrusions ofperidotitic 274m), theDeltasill(Fig.2) appears particularly interesting or pyroxenitic type alsointrudethe Povungnituk Group, andprovides an opportunity to testthe effectsof varying sometimes cutting differentiated sillsoftheDeltatype.These oxygen andsulfurfugacity ontheprocesses of PGEdistribu- ultrabasic intrusions arealsocomagmatic withthekomatiitic tion.The presentstudysetsoutto describe the distributionbasalts oftheChukotat Group(PicardandGiovenazzo, 1995; of PGEacross theDeltasillandaimstodetermine thephysi- Barnesand Giovenazzo, 1990).However,they are distincochemical conditions thatgaveriseto localgeochemical en- guished fromthe differentiated sillsby thepresence of imrichments duringmagmacrystallization. Experimental runs portantnickelsulfidedepositsshowingPGE enrichment wereperformed to determine thevalues of thesulfidecapac- (Barneset al., 1992). itythatappliedduringthecrystallization of meltsrepresentativeof theDeltasillandto testtheeffects of sulfurfugacity Petrography on the precipitation of basemetalsaswell asthe closely TheDeltasill(Figs.2 and3) iscomposed of anultramafic associated PGE. to maficdifferentiated sheetvaryingin thickness from215 to 275m. Its basalchilledmargin(maxthickness, 30 cm)is Characteristics of the Delta Sill characterized by thepresence of olivinepseudomorphs (5Geologic setting 8%modal)in a chloritic groundmass, similartothekomatiitic The Deltasilloccurs in the southern partof the Cape basalts of the ChukotatGroup(Picardet al., 1990). Smith fold belt (2036-1922 Ma; Parrish,1989; Machadoet Theultramafic unitbegins witha websteritc layer(nomenal., 1990).It intrudes a succession of sedimentary rocksand clatureof Streckeisen,1976;thickness,50 m) whichbecomes sodic lavas ofalkaline affinity whichinclude basanites, nephe- enrichedin cumulusolivine(from5-13% modal)towardthe
PGE BEHAVIORIN PROTEROZOICKOMATIITE, DELTA SILL, CANADA
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FIG.2. Geologic sketchmapof the KentyandCeciliaLakesareain thevicinityof the Deltasill.
top.It contains disseminated crystals of pyrrhotite(0.5-1% The PGE-bearingDelta reef is locatedat the interface modal)with exsolutions of pentlanditc, amongpoikilitic betweenthegabbronorite andtheheterogeneous gabbrolaypatches ofpyroxene andbrownhornblende. Thislayerpasses ers. It corresponds to a discontinuous pegmatitichorizon upwardsharply intoa werhliticheteradcumulate containingcomposed of melanocratic gabbronorite (maxthickness =4 ferrochromite(1-2% modal,diam = 0.2 ram) and cumulus m, ca.2 km long)characterized by largepatches(diam_ olivine (Fo82_sa; 55-72%modal), whichareincluded in large 2 cm) of poikiliticFe-richclinopyroxene (60-70% modal), patchesof diopside(18-20% modal)and intercumulusorthopyroxene (5-10% modal),quartz(1-2% modal),and bronzite (2-3% modal). Interstitial lamellae of biotite (1- sphene whichincorporates allotriomorphic crystals of oligo3% modal),aswellasa fewcrystals ofpoikilitic brownhorn- clase(diam+_3-4 ram,20-35% modal).In section D2 (Figs. blende,arecrystallized aroundthe pyroxene (2-4% modal). 3 and4), the reefcontains I to 2 percentmodalpyrrhotite Theperidotitc giveswayto a second websteritc layer(thick- occurring assmallpockets in the cleavage planesof pyroxness,6 m), whichis composed of diopsidic augitc(42-55% enes,but thereis no evidence for significant PGE enrichmodal),ferrochromite (1-2% modal)]cumulus olivine(15- ment. However, in section D3 and at several localities be25%modal),andorthopyroxene (10-13%modal)withbrown tweensections D2 andD3, highPGE contents arerecorded hornblende(2-4% modal)and intercumulusbiotite (1-2% (Fig.4). The observed variations maybe correlated withthe modal). presence of nickeliferous pyrrhotiteaswell aspentlanditc The olivinewebsteritc thenpasses upwardinto a 10-m- andchalcopyrite (upto 20% modal)occurring asinterstitial thickmelanocratic gabbronorite layerwithintercumulus pla- grainsbetweenthe silicated minerals. gioclase (up to 15%in the mode)fillingthe interstices beThe pegmatitic horizonwedges outto thewestof section tweencrystals ofclinopyroxene (66%modal) andorthopyrox-D2 anddisappears entirelyonthenorthern flankoftheintrnene (13% modal).This layergradesup into a 62-m-thick sion(Fig.3). Southof DeltaLake,boththeuppermost anorgabbronorite with rare olivine(1-5% modal)and cumulus thositeunit and the PGE-enriched pegmatitichorizonare plagioclase (45-60% modal)whichareincluded in patches missing fromthe sill.Here,the intrnsion contains an upper of clinopyroxene (30-45%modal,diam_+I mm),orthopy- zonecomposed of leucoxene-bearing quartzgabbro. Missing roxene(5-16% modal,diam= 2-10 mm),andsphene (1% in the easternpartof the Deltasillaroundthe D2 andD3 modal).Bycontrast, grains ofpyrrhotite (1-2% modal)form sections, thisunitnormally formstheupperpartof theother smallpockets in thecleavages oforthopyroxene nearthetop differentiated sillsof theregion. of theunit.In turn,thegabbronorite is overlain bya 32-mAnalyticalmethods thicklayerofheterogeneous gabbro containing olivine (4-5% Geochemistry: modal)andcumulus plagioclase (50-75% modal)included Samples werecollected foranalysis alongtwocross sections withinintercumulus clinopyroxene (24-36% modal)andor- (D2 andD3, seeFig.2) located vertically beneath theminerthopyroxene (3-4%, diam-- 4-5 ram).Opaqueminerals ahzedreefof the Deltasill.Thepresentgeochemical study makeup a verysmallproportion (
