tl7 Thc Carwdian M ine ralo g ist Vol.36,pp. 117-135(1998)

COMPOSITIONOF IN THEUPPER CHROMITITE, MUSKOX ,NORTHWEST TERRITORIES

THOMAS A. ROACH ANDPETER L. ROEDER'

Department of Geological Sciences, Queen\ Uni,versity,Kingston, Ontario K7L 3N6

LARRYJ. HULBERT

Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario KIA 088

Arsrnacr

The texture, mineralogy and composition of chromite in the upper chromitite of the Muskox intrusion, in the Northwest Territories, have been studied in two O.s-meter sections of drill core. The principal rock-type is an onhopyroxenite that conrrins crrmulus , orthopyroxene and chromite, and the intercumulu5 minerals clinopyroxene and .The minor minerals and biotite are found, together with a number of accessory minerals, in pockets that are interpreted as sites of late intercumulus melt. The chromitite seam is up to 10 cm thick and contains chromite with a narrow range in composition: 0.64

Ketwords: chromite, cbromitite, orthopyroxene, layered intrusions, Muskox, Northwest Territories.

SoMMane

Nous avons 6tudi6 la texture et la composition de la chromite qui constitue la cbromitite sup6rieuredu complexe intrusif de Muskox, dans les Territoires du Nord-Ouest, en utilisant deux sections de carottes, chacune de 0.5 mdtres de longueur. La roche encaissanteest principalement une onhopyrox6nite contenant olivine, onhopyroxbne et chromite cumulatives, ainsi que les min&aux intercumulus clinopyroxbne et plagioclase. Les min6raux accessoires ilm6nite et biotite coexistent avec quelques autres min6raux plus rares dans des poches que nous interprdtons corrtme sites de magma intercumulus tardif. La couche de chromitite atteint une 6paisseurde l0 cm, et la chromite y possBdeun intervalle 6troit de composition: 0.64 < Crl(Cr + Al) <0.74,0.62

Mots'clds; chromite, cbromitite, orthopyroxbne, complexes stratiformes, Muskox, Territoires du Nord-Ouest.

I To whom correspondenceshould be addressed.E-mail address: [email protected]

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IxrnolucloN described in the present study is at the base of cyclrc unit22, which is near the bottom of megacycle 4. This Horizons of chromitite are prominent in the is in the part of the layered series where orthopyroxene sequenceof cumulatesin the Muskox layeredintrusion, becomes an important cumulus phase, and where locatedsouth of the CoronationGulf in theNorthwest DesRoches (1992) and Francis (1994) found Territories(Fig. l). The emplacementof the Muskox geochemical evidence, from elevated ratios of intrusion is consideredto be synchronousat about large-ion-lithophile [-IL) to high-fi eld-strength ([IFS) 1270Ma with the CoppermineRiver volcanicrocks elements, that there was enhanced contamination of the to the north and the Mackenzie dyke swarm magma by crust flrvine 1970).This contamination may (LeCheminant& Heaman 1989).The narrow dyke have resulted from roof melting, which increased southof the CoppermineRiver is consideredby most the silica activity sufficiently to cause orthopyroxene investigatorsto be a feederdyke that broadens into the to become a major liquidus phase. funnel-shaped Muskox intrusion north of the The chromitites in cyclic units 2l and22 areboth CoppermineRiver. The layersof the intrusiondip to situated above layers of and below the north and disappearunder cover rocks. A large orthopyroxenite (Irvine 1975), although the amount of gravity and magnetic anomaly to the north of the peridotite in cyclic unit 22 is small. There is only a intrusion suggests(Irvine & Baragar 1972) that limited amount of published chemical data on the thepresent outcrop of theMuskox intrusion represents chromite in these two horizons (Irvine 1967). We only a small portion of the whole intrusion. The undertook this study in order to describe the upper purposeof the presentcontribution is to characterize chromitite at the base of cyclic rtwt22 and to present the textureand composition of the chromiteassociated compositional data on chromite associated with the with theupper chromitite. chromitite and dissenminated in orthopyroxenite. The samples were taken from a half-meter section of two Bacrcnouvo lxponuxroN drill cores (MX3 and MX4) that intersect this horizon. Their location is shown on Figure 1 by the numbers 3 The intrusion was mappedby Smith (1962) and (MX3) and 4 (MX4). Smith et al. (1,963)and divided by Irvine & Smith (1969)into two marginalgroups, a layeredseries and ANALYT'IcALPRocEDtTRE granophyric roof-zone. The layered series of the intrusionis about1 800 m thick andis comprisedof 42 All mineral analyses were performed on polished mappableunits of 18 differentrock-rypes. These units thin sections using an Applied Research Laboratories dip gently to the north at about 5-8o and havebeen SEMQ Electron Microprobe in the Department of dividedinto 25 cyclic units.Each of thesecyclic units Geological Sciencesat QueenosUnivenity. The minerals was interpretedby Irvine & Smith (1967)and Irvine were analyzedusing energy-dispersionspectrometry at (1970)as due to the influx of a new pulseof magma 15 kV and a beam current of approximately 20 nA. The into the intnrsion.The order of crystallizationof the standards included glass from the National Bureau of major silicatesin thesecyclic units is variable,and Standards (NBS 470 K4l2), chromite from Tiebaghi changeswith heightabove the base(Fig. 2). Irvine (USNM 117075), and olivine (USNM 2566) provided (1970)divided the layeredseries into threeclasses by the Smithsonian Institution. Corrections were made basedupon ttre sequenceof crystallizingphases. The using the procedure ofBence & Albee (1968) and the crystallization sequence(Class I) for much of the alpha correctionsofAlbee & Ray (1970).The proportion Muskox intrusioninvolved olivine as the first phase, of Fe2*and Feh in the chromite was calculated assuming followed by clinopyroxene,plagioclase, and then a stoichiometric spinel strucfure. Seven chromite orthopyroxene.Higher in the layered series, the standards were analyzed a number of times, and orthopyroxeneappears earlier as a definite cumulus the secondary standard closest in composition to phaseand follows olivine (ClassIII) nearthe top of the the Muskox chromite was analyzed thirteen times. The layeredseries (Fig. 2). Chromiteoccurs tlrough much results of these analysesand a more complele description of the layeredseries (Irvine 1975)as a minor dissemi- of the analytical procedures are given by Roach (1992). natedphase (1-3Vo). The correlationof whole-rock nickel with the amount of chromite suggeststhat Pnrnocnapnv ANDMINERALocY chromiteand olivine coprecipitated(Irvine 1975).The only significantlayers of chromititeoccur at the base The focus of the petrographic examination was to ofcyclic units2l and22,which are three quarters of ascertain the relationship between chromite and the way up the layeredseries (Fig. 2). Francis(1994) silicates in the layer that contains chromitite. Figures combinedthe twenty-five cyclic units describedby 3 and4 illustrate the variation in rock type and compo- Irvine and Smith into four megacyclesthat are sition of chromite within the half-meter interval of consistentwith the original mapping conductedby the I\DG and MX4 drill core. These samples are from Smith et al. (1963).The layer containingchromitite within Class III of Irvine (1970), where the silicate

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Wn Coppermine {////) volcanics

ffi Granophyre, z Maficgranophyre 9 U) Granophyre- rich f, gabbro,Gabbro tr N F = Pyroxenites W x o peridotite, Y l:::::::::l Dunire a f, ffi Marginalzones, l;itriii:i::j,..ilFeeder dyke

DismalLakes : Group

HombyBay ffi Group

+. + rl HepbumIntrusive r Suite

Coronation m Supergroup ffi AkatachoGroup

l.-t-;l Fault

Ftc. 1. Generalized geological map of the Muskox intrusion (Irvine & Srnith 1969, Hoffrnan 1984). The drill core used in the present study was taken from the locations labeled "3" (MX3) and'"" MX4).

crystallization-sequenceis thought to be olivine - plagioclase. The other major rock-type in these sections orthopyroxene - clinopyroxene - plagioclase. is chromitite, defined as bands or stringers, >2 mm The principal rock-type in the two sections is thick, having greater than 3OVoby volume of chromite. medium-grained orthopyroxenite that contains euhedral The orthopyroxenite below or associated with the to subhedral orthopyroxene, with up to 20Vo olivine chromitite is coarseglainsd, verging on pegmatite.The and variable amounts of chromite, clinopyroxene and orthopyroxene, olivine and chromite are interpreted in

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cYcLtc ORDEROF is similar to that of UNITS SILICATECRYSTALLIZATION clinopyroxene. This texture pyroxenite associatedwith the UG2 chromitite in the Bushveld Complex (Cawthorn & Barry 1992) andthat s for the ulfiamafic zone of the Stillwater intrusion o (Jackson1961).

o OL-CPX-OPX-PL Chromite o chromitite chromititeOL-OPX-CPX-PL The mode of occurrence of chromite is classified as either massive, in chromitites, or disseminated, in orthopyroxenite. The disseminated grains are usually (r) OL-CPX.OPX-PL equant and fairly evenly distributed; however, in zones o containing significant modal olivine, ilmenite, and E biotite, chromite grains tend to be less regular in shape o (! and to cluster together, forming larger anhedral aggre- o gates. 7 illusffates a sample of orthopyroxenite E OL-CPX-PL-OPX Figure and best representsfypical disseminatedchromite. The individual grains are euhedral, less than 0.25 mm in diameter, and associatedwith cumulus orthopyroxene. Figure 8 is an example of a highly fracfured olivine orthopyroxenite. Note that the chromite grains are N :+h commonly clustered in aggregates between olivine o grainsorather than enclosed within them. The anhedral o considered to result from o OL-CPX-PL-OPX aggregates ofchromite are ct sintering of several small but separategrains (Hulbert o & von Gruenewaldt 1985) and may represent the location of pockets of late intercumulus melt (Roeder & Campbell 1985). There are all gradations between separatedisseminated grains to large amoeboid grains s ls are, in some instances,surrounded by biotite. o 1-' that that contains disseminated gl OL-CPX-PL-OPX Orthopyroxenite g chromite grades within u 1sw millimeters to chromitite orl12 ol (Fig. 9). The MX4 section contains a number of ^>l irregular, wispy stringers of chromitite, one of which is seen in the center of Figure 9, just above a more E-;-T...J-T| gabbro (troctolite,oFgabbro, gabbro-norite) massive chromitite. The main chromitite from MX4 (depth interval 29-38 cm) contains chromite evenly ffi orthopyroxenite distributed in a matrix of large poikilitic crystals of or (ol-websterite,websterite, pyroxene and plagioclase. Some WFPMwebsterite teldspathicol-websterite) plagioclase oikocrysts s4n s6nrain, in the plane of the grains. Characteris- ol-clinopyroxenite thin section, hundreds of chromite ffi tically, the chromite grains in the chromitite are not duniteor peridotite(or both) uniform in size (Fig. 10). Chromite is noticeably larger in plagioclase and along the boundaries of oikocrystic Ftc. 2. Summaryof Muskox cyclic units and megacycles. grains. These larger chromite grains are attributed Adaptedfrom DesRoches(1992) and Francis (1994). to recrystallization of smaller grains along silicate grain boundaries in the presence of a small amount of late interstitial melt. The difference in size between these sections as cumulus minerals, using the definition chromite in clinopyroxene oikocrysts and that in ofcumulus oflrvine (1982). Orthopyroxene has been adjoining plagioclaseoikocrysts suggeststhat the larger interpreted both as the major cumulus mineral in the grains of chromite in plagioclase were enclosed at a two sections and a major intercumulus mineral. later time, but still in the presenceof melt. Orthopyroxene commonly develops at the expense of The massive chromitite grades upward into olivine, leaving rounded or embayed aggregates, which chromitite that contains a few chromite-free areas have altered to a mixture of talc and serpentine (Fig. 5). (Fig. 10) and, at a higher level, into a cfuomitite with Orthopyroxene grains have euhedral terminations many irregular chromite-free areas that give the where in contact with plagioclase (Fig. 6), but anhedral chromitite a somewhat mottled appearance (not where intergrown with other orthopyroxene and shown). The chromite-free areasare 1-3 mm in length

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t.?+ * rf! !t!,"t { f' 4e,9lt!. '* t+ tz,,,,a|!:)L s,- 5.0 r I'T +' I II hF l.r+ fT+ l+ fr+'T+ +1 ilt, +]1* W i++$ *+$ T+ T+" ffi *, l, lL,+ ffiwfi ,1 ++ -* tI' tg Tl T 7' ffiw t L'.\t r+ ry l*,. x is aF n- .ii?4,1 f,i-* * t+,41 .T rt ,,f+ +f f IT' LI -+f Ti ,+ 1 . 20.0- +, l-+ T.]( 'r:t,jr , ilF Deplh +, $ J+ *r T +,' T (cm) ++ + #" +1,i+ ' [- +F-Ff'T - lT za.u T +l r,l I Ir f f =] II II t l.r t * I -fr |'rr' I Ii Irl ri I ril {F Tl +l + ,lr f .l+ +.L+ Jlh+ s $ri -* '+. .# g' -i-T+ --* T' it+ l + '+T f r. +

0.5 1.0 1.0 u.o 2 0. Fe/(Fe+Mg) Cr/(Cr+Al) FE3# Ti Ni

orMne-ortronvroxenite = cosBe{mined [EEl c ffi Orthopyroxenite + = Dlssernlnsted ffi ChDmlt&Onhopyrcxenlte I = chrcmttte r Chmmllile FIG: 3. Rock types and chemical composition of chromite, I!{X3 drillcore. Depth below drill collar is 56.01 meters plus listed depth in centimeters. Blank areas in section represents missing core. FE3# - Fe3+/(Fek + Al + Cr). Ti and Ni as cations per 24 atoms of oxygen.

and are mainly filled by orthopyroxene and (1961) and in the Bushveld complex by Eales & clinopyroxene. These chromite-free areas commonly Reynolds (1986), and have been attributed to the contain small anhedral grains of olivine or replacement of cumulus orthopyroxene and olivine, orthopyroxene (right side of Fig. 11) that are inter- and to retention of the original cumulus distribution of preted as relict cumulus grains. The clinopyroxene in chromite. the chromite-free areasgenerally extends well beyond Most chromite in chromitite is found as discrete the chromite-free area as a single-crystal oikocryst grains with a euhedral to subhedral, equant habit. (Fig. 11) containing many chromite grains. Some of Grains have an average diameter of 0.1 mm with a these chromite-free areas are flattened parallel to maximum diameter of about 0.5 mm. The majority of igneous layering, with a well-defined ovoid shape the chromite grains are enclosed within silicates other (Fig. 10); others are much less regul:u in outline. than olivine, and indeed chromite seems to be sparse in Despite the differences in shape, these features seem to olivine. Those grains enclosedin olivine or cumulus have resulted from a similar process. Features such as orthopyroxene are generally small (<0.1 mm). This is these were noted in the Stillwater Complex by Jackson expected, as chromite was an early cumulus phase

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| +T 'iffi ' Ta JT i1: .+ T !.+' + ,ffi r,+ + | +,, l?. r.:r..!.,!t4t R rq -i* f+ tT+ l-{' t F F t.F i+ ffi f 4 t r r+ f' I til'nr Wrnrl + + + '1+ ,* L .| 'r rlf'trr + ,T '{r t g+ +T i,+ + E. I t T #. # T J i+ *+-h '[l T II .Fl I fi . + f l, + 't+f t''Ti , rlf r+ +t{ Ir '+ +1 r*T ,'i+f I +fl fI' W gi I gl (cm) {rl Ti W I-I + I r+ f i,+ f+, 25.0- +!: +f .? ' # +i+ +Tr .*Fa E .tfl ffi f +'t +"Jr F ffi .51+ 30.0- tl Ir+ I+l II Ir Ir TI fl + T T +l+ I t.+ {

0.5 1.0 '1.00.0 u.o 0. Fe/(Fa+Mg) Crl(Cr+Ai) FE3# Ti NI

olivine-o ftropyroxenito C = Coarse-gEined m ffi = Disseminated EffiiIO rthopy rcxe n ite + I Chromidls I = Chromilite

Ftc. 4. Rock types and chemical composition of chromite, MX4 drillcore. Depth below drill collar is 95.64 meters plus listed depth in centimeters. Blank areas in section representsmissing core. FE3# = ps:"7(F.k + Al + Cr).

whose growtl would be restricted after enclosure by |O-2OVoof rocks designated olivine orthopyroxenite other early phasessuch as olivine or orthopyroxene. In and less than lUVo in orthopyroxenite. Some samples contrast, chromite found in plagioclase, the latest major contain alternating laminae of olivine orthopyroxenite postcumulus phase,is generally larger (up to 0.5 mm). and orthopyroxenite a few millimeters thick (Fig. 8). Some of the larger grains of chromite are irregular Alteration ofolivine is heterogeneouslydeveloped; in in outline, but crystal faces can be identified in the areasthat contain significant olivine, partial replacement outline. These grains seem as though they are by serpentine is the most cotlmon type of alteration. aggregates of many small crystals that have partially Serpentinization and volume change have produced recrystallized, and or sintered, into a larger grain. pervasive fractures in the olivine orthopyroxenite laminae that are nearly parallel to igneous layering. The Olivine fractures that extend through pyroxene or plagioclase do not contain alteration products. Olivine, a relatively minor modal constituent, The olivine grains in olivine orthopyroxenite are occurs in a variety of different habits and associations. typically euhedral to subhedral, with an averagediameter Tlpical olivine and its alteration products make up of 0.75 mm. and a maximum diameter of 2 mm

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Ftc. 5. Orthopyroxenesurrounding areas containing a mixture of serpentineand talc. Slide MX-303 (depth5-9 cm). Width of field of view: 1.8mm.

Frc. 7. Orthopyroxenitewith intercumulusplagioclase (clear) and disseminatedopaque grains of chromite. Slide I\D(-303 (depth5-9 cm).Width of field of view: 9.3 mm. Frc. 6. Orthopyroxene crystals in a matrix of clear plagioclase.Width of field of view: 1.8mm.

(Frg.12). Both olivineand orthopyroxene exhibit good clinopyroxene.Orthopyroxene is generallycumulus, cumulustextures. Some orthopyroxene contains small with associatedpost-cumulus overgrowth; on theother (<0.5mm), rounded,fine-grained aggregates enclosed hand,clinopyroxene is of postcumulusorigin. There is within the pyroxene(Fig. 5) that havea composition considerablevariation in grain size and texture of (No. 6, Table l) betweenthat of talc and serpentine. pyroxene.Orthopyroxenite above the chromititehas This compositionis closeto thatreported by Wordenet equantorthopyroxene,0.25-0.30 mm in diameter,that aL (1991)for intimately intergrowntalc andserpentine. varies from euhedral(Fig. 6) where in contactwitl The olivine associated with coarse-grained plagioclase to anhedral grains where in contact orthopyroxenitemay be quite large(up to 5 mm) and with other silicates. Orthopyroxenegrains contain hasa roundedmorphology suggestive of reaction. exsolution lamellae of clinopyroxene,and some The composition of most of the olivine in the orthopyroxenecontains rounded blebs of clinopy- orthopyroxeniteshows little variation. The average roxenethat are lesscalcic than the discretegrains of result of twenty analysesof olivine from MX3 and clinopyroxene(Roach I 992). MX4 is Fo76.7(SD = 1.0).Olivine locatedclose to The grainsof clinopyroxeneare generally smaller pocketsthat containedlate-intercumulus melt may be thanthe associatedgrains of orthopyroxene,except in morefayalitic (No. 5, Table1). somecoarse-grained orthopyroxenite or chromitite; the clinopyroxenegrains contain exsolutionlamellae. Orthopy roxene and clinopy ro xe ne Oikocrystsof clinopyroxeneup to 5 mm acrossare typically the mostabundant intercumulus phase in the Orthopyroxeneis the dominantphase in bothMX3 more massivepart of the chromitite (Fig. 11). The and MX4 and is commonlv associatedwith minor upward transition from more massivechromitite to

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Frc. 8. Disseminated chromite in alternating laminae of orthopyroxenite and olivine orthopyroxenite. The olivine Frc. 9. Orthopyroxenitewith wispy layer of chromitite orthopyroxenite contains serpentinized aad fractured (center) and more massivechromitite (bottom). Slide olivine. Slide MX-404 (depth 1G-12 cm). Width of field MX4O7 (depth 14-19 cm). Width of field of view: of view: 10.8mm. 9.9mm.

chromitite with inegular chromite-freeareas is marked almost completely controlled by the shape of the by the upward appearanceof orthopyroxeneas a interstices remaining among the other phases(Fig. 6). significant intercumulusphase, as well as of relict In the lower part of the two sections, plagioclase is cumulusolivine. volumetrically less abundant than clinopyroxene, but Representativeresults of analysesof orthopyroxene the amount of plagioclase seems to increase toward and clinopyroxeneare given in Table 1. Coexisting the top of both sections, where it is more abundant orthopyroxeneand clinopyroxene in ten different than clinopyroxene. Tlpical plagioclase grains show sampleshave a narrowrange in Fez+/(le2++ Mg), but twinning and vary in composition from Anu6to Anr6. variable level of Ca. The average composition (standarddeviation in parentheses)of the orthopy- Accessory minerals in pockcts roxenein termsof wollastonite,enstatite and ferrosilite is Wo+<0.:Fnzsro.oFslro.aand that of the clinopyroxeneis There are a number of discrete patches that contain Worr,r,En5,6,Fsr2@).Part of the variabilityin measured a mixture of biotite, ilmenite, chromite, chlorite and calciumfor the clinopyroxeneis believedto be dueto minor minerals such as pectolite, apatite,rutile (No. 13, the variable size and distribution of the exsolution Table 1), armalcolite (No. 14, Table 1), hematite (No. lamellaerelative to thesize and position of theelectron 10, Table 1), monazite, Fe-Ni{u sulfides, baddeleyite beamduring analysis. and alkali feldspar. One good example, shown by the arrow in Figure 12 and enlarged in Figure 13, is taken Plagioclase from 100 cm below the massive chromitite in MX3. This patch is enclosedby olivine in the plane ofthe Plagioclaseis generallythe last major intercumulus thin section (Fig. 13), whereas the much larger patch phaseto crystallize;the morphologyof the crystalsis shown in Figure 14 is bounded by olivine, pyroxene

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and plagioclase. The position of some of the labels for the minerals in Figures 13-15 indicate the location of the spots analyzed (data given in Table 1). These patches of accessoryminerals are very similar to those found by Roeder & Campbell (1985) in the Jimberlana intrusion and by Hanski (1992) in olivine from the Pechengaferropicrite. In both cases,these were inter- preted as the sites of late interstitial melt. The major silicates are commonly zoned in the vicinity of these patches; for example, plagioclase of composition Anu6-rsmay be zoned to almost pure albite, and some grains of pyroxene look ragged in the vicinity of these patches. Ilmeniteo a minor phase in the orthopyroxenites, increases in modal abundance toward the top of both sections.This distribution parallels the overall upward increase of Ti in chromite and the greater modal abundance of associatedpale to deep brown, pleochroic biotite that contains 2 to 8 wt.Vo TiOr. The biotite s6mmonly surrounds large anhedral grains of ilmenite (Fig. la). The iknenite shown in Figures 14 and 15 is separatedfrom the biotite by a rim ofhematite that is connected to plates of hematite that extend into biotite. Another example of the complex assemblageof oxide minerals found in rs6e samplesis shown in FigUre 16 where ilmenite, rutile and armalcolite are in contact with a large grain of chromite. The compositions of these oxides are given in Table I (No. 12-15). The composition of armalcolite is consistent with the defi- nition given by Bowles (1988) and is similar, except for Frc. 10. Chromitite with two chromite-free areas of higher Cr and Ca, to that reported in experimental runs clinopyroxene and orthopyroxene. Slide IID(-410B (depth by Cawthom & Biggar (1993). Their experiments were 3l-34 cm). Width of field of view: 11.6mm. designed to test the crystallization of armalcolite,

Flc. 11. Enlarged view of the left-hand part of the upper chromite-free area in Figure 10. Large oikocryst of clinopyroxene with good cleavage encloses many crystals of chromite. Slide I\D(-410B (depth 3l-34 cm). Width of field of view: 1.5 mm.

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ND(4 sections;the compositionprobably depends upon very local conditionsafter initial crystallization.The disseminatedchromite varies in diameterbetween 0.04 and 0.5 mm; no correlationwas found betweenthe diameterof the grainsand the compositionalpaftrmeters shownin Figures3 and4. Thedisseminated chromite is consistently higher in Fez+l(Fe2++ Mg) than the chromitein chromitite,which probablyreflects both reactionwith intercumulusmelt andsubsolidus equili- bration with nearby silicates (kvine 1967). The disseminatedchromite has higher Fel'/(Fer* + AI + Cr), Ti andNi, whereasthe averageCrl(Cr + Al) is similar to that of the chromitein the chromitite.The average Crl(Cr + Al) is relatively constantacross the half-meter intervalshown in Figures3 and4, but is morevariable in disseminalsdchromite. The Fe3+lCFe3++ Al + Cr) and amountof Ti andNi in disseminatedcbromite show a pronouncedincrease with stratigraphicheight above the main chromitite bandin both IIDG and I!D(4. The range in composition of the chromite in chromititeand disseminated chromite is alsoshown on Figures17 and 18.The disseminatedchromite has a higher Fez*/@ez*+ Mg) and Fe:* + 2Ti than the chromitein chromitite.The disseminatedchromite that is in contact with very fine-grained, irregular intergrowthsof chlorite and alkali feldspar,or less commonly,albite, is generallyhigher in Crl(Cr + Al) (opentriangles in Figs. 17, 18).Some of the dissemi- natedchrornite (open circles in Figs. 17, 18)that is in contactwith, or included within, what appearto be olivine relics now altered to a mixture of talc and Ftc. 12. Onhopyroxenitewith two large grains of olivine, serpentine(Fig. 5), hasa lower Crl(Cr + Al) thanmost one above the other. Arrow without label indicatesa of the disseminatedchromite. mineral assemblagewithin the lower crystalthat may have formedfrom late intercumulusmelt. SeeFigure 13 for a more detailedview of this area.Slide MX-332B (depth DlscussloN 126-132cm). Width of field of view: 4.6mm. The processesthat contributedto the evolutionof the chromititein the Muskoxintrusion can be grouped into three general categories:1) primary cumulus titanium-rich chromite and ilmenite in titanium-rich crystallizationof olivine, orthopyroxeneand chromite, magma, such as that involved in the Mount Ayliff 2) modificationin themagmatic environmento 3) post- intrusion in South Africa. magmatic alteration. These are equivalent to the cumulus,postcumulus and subsolidusstages used by CnetvtrcAr-CoMposITroN oF Cm.oMrrE Cameron(1975) to describethe evolutionof chromite andsilicates in the BushveldComplex. Representative results of chemical analyses of The appearanceof chromitite in layered basic chromite are given in Table 2. Figures 3 and 4 display intrusionsis faidy common;however, the chromitite in the correlation between the element ratios of chromite the Muskox intrusion is unique in that it occurs and the rock type for each of the two drill-cores. The relativelyhigh up in the stratigraphicsection (Fig.2). chromite in chromitite bands shows little chemical The two chromitite horizonsin the Muskox are in a variation within, or among bands, and is similar in cyclic unit, with peridotite at the base,followed by IID(3 and MX4. There is no significant comFositional chromitite, orthopyroxenite,websterite and, in the case variation of the chromite in the chromitite related to ofcyclic unll 22, by gabbro.The rock typesfound in stratigraphic height. The chromite in the chromitite our study are consistentwitl tle identification by likely represents the closest approach to primary Irvine & Smith (1967) of peridotite grading to magmatic chromite. orthopyroxenite;however, at the small scale of the The disseminatedchromite is as variable in compo- presentstudy, there is no obviousboundary between sition within a thin section as between the MX3 and theserock types.Indeed, at this scale,it mightbe better

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TABLEI. [email protected]

| 2. 3. 4. 5. 6. ?. 8. 9. lO. lL t2_ 13. 14. 15, 16. op& opr cp& cp& ol. Tlo.+SD ltm. Bt Ilo Hm Br Chr. Rr Am lld pL & |\/m w4l4 l/m w w @ @ rc rr'M w @ w@ w@ w@ w@ @ Ic. @ rio Frg. 13 Frg. 13 FE 13 Frg. 15 Frg, t5 FA. 15 Eg. 16 nS. 16 FrS 16 FA 16 siq ss24 55.t0 s2.st st.a 37.13 g 03s 38.78 0.16 t.4s 3833 O2t 05 O.l8 O_19 54.06 Tio, 021 0 0.66 0.81 0 0 47.85 2.6 48.9 03 6.84 5J9 9554 6631 52.01 0.12 AIp' 0..18 0.82 2.89 2A4 0 1.53 0.2, ls3o O.O9 O2I 12.14 4.m 0.06 t.ls 0.16 2A.n crrq 0 0.O7 0.91 0.?5 0 0.15 0.56 O 0.60 O 037 A27 l.lo jS1 tZ, 0.09 F€or ls,4s 1457 6.78 6.80 a.a 92 46.7t 12Ai 45.1s 9o3o 9a9 53.91 o3i t6zi qg 0.45 Mno 0.36 025 0 0n 03s 029 0.53 O.l1 036 O O.lO 0.19 0 O.l5 038 O Lrco AJS 28.91 162A rc24 37J9 24.4 3.55 162 421 O3j 11.70 3.85 O l92 4J3 O.l5 CaO 0.47 0A1 2l.U 20.88 0.t5 0.81 0 3.4S O O 0.13 0.08 OZ, 393 0.06 ll37 Ne,o000.520380OOlJlOOOj2O.t20O.2lO4.48 KrO000000.08062900.05929000.0300.59 srJM l(o.96 [email protected] 100J5 10021 [email protected] n.r3 9n 96.19 9952 92.69 953t i2 n.9 C1.72 Lts tm3

sf 7.881 7.885 7571 7.624 5.930 8313 0.069 6.14 0.t32 0335 6.@0 0.(X5 0.M2 0.q13 0.03t 73n Tf 0.023 0.000 0.073 0.@0 0.000 0.(n0 1.036 0246 7.t88 0.058 0.818 0941 il.799 9.05t 7.672 0.012 0.0$ 0.139 0363 0.425 0.@ 0m 0.051 2.85 0.021 0.058 2i€5 tx2 0.012 0247 0.037 4-619 H 0.0e 0.@ 0.000 0.000 0.@0 0.(m t.725 0.000 1.481 [.s96 0.@0 3.rn 0.000 0-0m 0383 0.@ ci 0.000 0.0s 0.113 0,088 0.000 0.019 0.0a7 0.0@ 0.093 0.000 0.u7 4.916 0.143 1.088 0..189 0.0t0 Fd l.u4 1.744 0.834 0.840 3263 I.182 5,9t4 t.6s2 5.8a7 5.821 t t 5.651 0.05t 2.472 6,2a 0.051 Mn3 o.UA 0.031 0.@ 0.034 o,M1 0.038 0.0E8 0.015 0.060 0.0@ 0.014 0.035 0.000 0.u24 0_w 0.@ Ltd 6.114 6.166 3.568 3.514 8.806 5573 1.035 3.828 1242 0.12t 4.t 2 r2l9 0.@ 0.520 nA 0.030 csr 0.072 0.073 33t5 3303 0.@s 0.133 0.000 0.5t6 0.@0 0.m 0.0a 0.019 0.039 0.?65 0.013 t.651 N&' 0.0@ 0.000 0.149 0.109 0.000 0.000 0.@ 0.& 0.@ 0.@ 02 0.050 0.m0 0.M4 0.m t2g2 Kr 0.000 0.000 0.@0 0.@ 0.0@ 0.016 0.0@ l27t 0.@ 0,01s t.8{3 0.000 0.000 o.{xr' 0.000 0.1d2

Tb mlytimt mlt re r€pqEd in wigh % dides @dcrtir. per24 ffiyga im. FeOT: total Fe qressed m F6O. Am rmslolite. lc.: Figrre shwing the gro thst n€re mtFed

Ftc. 13. Enlarged view of pocket of accessory minsrals in olivine (OL) indicated by arrow in Figure 12. The minerals include biotite @T), ilmenite (ILM), serpentine, chlorite, clinopyroxene and sulfide. This is considered !o have been the location of a pocket of late intercumulus melt. See Table I (Nos. 5, 7, 8) for the composition of the labeled minerals. Slide I\D(-332B (deprt 126-132 cm). Width of field of view: 0.9 mm.

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Fra. 14. Orthopyroxenirewith olivine (OL), Ilmenite (ILM), biotite (BT), plates of hematitei.n biotite and an acicularcrystal of apatite(AP). Slide MX-323 (depth 85-89 cm).Width of field of view: 6;7 mm.

Frc. 15. Back-scattered electron image of part of the large FIc. 16. Back-scattered electron image of a large crystal of grain of ilmenite in Figure 14. The minerals include cbromite (CHR) in plagioclase (PL) and orthopyroxene ilmenite (ILM), biotite (BT), chromian (MAG), (OPX). See Table 1 (Nos. L3,14, L5) for the composition sulfide (top center) atrd plates of hematite (HEM) of the labeled rutile @I), armalcolite (ARM) and ilmenite extending from the ilmenite into the biotite. See Table 1 (ILM). Slide !D(-408A (depth 19--22 cm). Width of field (Nos. 9, 10, 11) for the composition of some of the labeled of view: 0.4 mm. minerals. Slide lD(-323 (depth 85-89 cm). Width of field of view:0.5 mm.

to describethese rocks as orthopyroxenites, with varying 1982);however, our interpretationof the texturesis amountsof olivine, that include a bandof chromitite consistentwith thatof Irvine & Smith(1967). We have 10 cm thick anda few thin wispy bandsof chromitite. identifiedthree c'mulus, or primarycrystalline phases; The interpretationof cumuhs versus intercumulus olivine, orthopyroxeneand chromite. Postcumulus phasesis fraught with danger(Jackson 1961, Irvine processesin the presenceof melt have changedthe

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I 45 67 8910 ll12t314 Smp lvD<-3 lrD(-3 ltD<-3 IrD(-3 }vD<-3 !O<-3 Ii/D(-3 !,D<4 lvD(4 lvD<4 }o<4 MX4 MX4 !O(4 Dis Di$. Dis. San S@ Sqn Dis Dis. Di!s. Dis. Die S@ S@ S@ DEpd 03 83 t43 29.t 299 33.0 37.5 43 9.6 14.1 It-9 33.1 U2 353 sior 0.60 0.?l 0.56 038 0.40 0.53 030 025 026 0 032 053 olt 0.i18 TrO. 8J5 4n ?.18 3.59 3.68 3.55 3.19 9.91 1.65 5.41 3.4 3A9 3.63 332 Al,o, 5.60 3.55 7:r3 12.80 l23t 1235 15.61 6.58 A26 ll.l9 3.98 10.89 il.61 10.66 '36.46 cr,q 17.95 34.50 3l 36.0t 3631 .& n.@ n 35 25.49 3tA2 37.& 3729 3957 FcO 60.91 s438 51.51 3838 38.93 39.t2 45.& s4.70 49i6 50.73 5?.$ 31,41 37.88 31.42 l\{no 036 0.52 033 0 0.05 0.19 0.74 0.70 0.% 03t 0.50 0.16 0.13 024 lvlgo 4.60 2.t0 5.51 7.6 1.sl 1.55 538 539 J.4l 4.r2 0.90 72. 732 731 CsO 0.08 0.08 0.t0 0.15 0.x 0z 025 0.1I 0.05 0.t8 ora 02 0.18 02s Nro 0.55 0.s9 035 o.42 0.49 039 0.44 054 0.48 0.60 oJl 035 033 0.8 zlro 0.16 0 0.06 0 0.08 0.t7 024 0 024 0.09 0.07 0.08 0.09 0.19 v,o, 0.61 0.8t 0.58 0.60 0.63 0.61 0.,16 0s1 0.66 0.65 0.91 0.51 0.59 0.54 srJM 99.n lof.5t 10f,2 [email protected] 100.70 l0l.l5 9.62 1m.94 100-6t 99J5 [email protected] 9a26 94i3 10023

sf 0.021 0.025 0.02 0.0t3 0.014 0.018 0.0t0 0.009 0.009 o.q)9 0.012 0.018 0.010 0.016 Ti: OD, 0.113 0.1t4 0.069 0.091 0.088 0.095 0253 0.195 0.13t 0.w2 0.089 0.@2 0.0&l Alr 02a 0.t47 0310 0.497 0.419 0.416 0.609 0214 0330 0.445 0.168 0434 0.457 0.418 Fd 0.808 0.632 0.5&7 0360 0367 0.371 0.483 0-612 0.525 0.576 0.722 035t 0351 0341 cl 0.490 0.9s6 0.107 0939 0.944 0.942 0.696 0.593 0.132 0.6t0 0.88E l.mo 0.985 1.0,10 Fd 0.948 0.962 0.871 0.691 0.701 0.692 0.785 0s42 0.n2 0.t56 1.005 0.701 0.101 0594 Mr2 0.01I 0.0t6 0.010 0 0.002 0.006 0210 0.021 0.028 0.011 0.0t6 0.m5 0.004 0.007 Md 0237 o.ft 0279 0316 0368 0368 02(6 02'73 0n3 or,n 0_048 03dr 0365 0352 ca, 0.003 0.@4 0.@t 0.@6 0.008 0.009 0.009 0.004 0.002 0.@7 0.011 0.@8 0.@7 0.010 Nz 0.016 0.017 0.01 0.012 0_013 0.01r 0.012 0.015 0.0t4 0.017 0.0t5 0.010 o_q}9 0.008 zsl 0.005 0 0.002 0 0.002 0,005 0.006 0 0.006 0.003 0.m2 0.w2 0.m3 0.005 \P 0.01? 0.023 0.016 0.016 0.01? 0.016 0.013 0.019 0.018 0.01? 0.t27 0.014 0.016 0.015 FE4 0.800 0.t98 0.139 0.650 0.656 0.653 0:747 0.n5 0.162 0.176 0.954 0.658 0.@ 0.65? cRr 0.6E3 0.t67 0-695 0.654 0.663 0.64 0.533 0.693 0.690 0.604 o.Ml 0397 0.683 0.713

Tbs arslyticol 6ults re rcport€din wEigbt % ffiids md cdiG pq 4 oxy$n im. D€Elh is exprBsed in m in the lvD(-3 q I\rX-4 sratigraphio stim Chonite is eithq fron c,bronitite se@ cr dimiraled (Dis.). FE'?#= FCY(F* + Md. CR# = Cr(Cr + At).

morphology of all three phases, with the volume coarseningdid not significantly affect the composition proportion of olivine having decreased by reaction and of the chromite in the chromitite. the formation of both postcumulus orthopyroxene One method to test whether the minerals that are and clinopyroxene at the expense of olivine and melt. present have retained the composition at the time of The phases, textural associations and sequence of primary magmatic crystallization is to see whether crystallization in these Muskox samples are very these compositions can be used to calculate a similar to those describedfor other intrusions (Jackson reasonableset of initial conditions (Irvine 1975). Irvine 1961,1970, Roeder & Campbell 1985, Hulbert & Von (1967) suggestedthat in order to test the conditions of Gruenewaldt 1985). initial magmatic crystallization of chromite, olivine and The very low solubility of chromium in silicate pyroxene, it is best to use the composition of the melts (Roeder & Reynolds 1991) probably means that minerals in adjacent units, where they occur at there has been little significant change in the volume maximum concentration. The average composition of proportion of chromite in the chromitite during chromite in chromitite is Crl(Cr + Al) = 0.681, Fe2tl postcumulus processes.On the other hand, the number (Fe2*+ Mg) = 0.662 and Fe3+/(Fe3++ AI + Cr) = 0.205, of chromite crystals has decreased, and the size of and the average olivine in the orthopyroxenite is Foru,r. crystals has increased, owing to sintering and The 1993 version of the equations described by Sack & recrystallization. The small size and amount of Ghiorso (1991) have been used to calculate a tempera- chromite in the remaining olivine and the larger size ture of 1146oCat which this chromite and olivine can of chromite grains along silicate grain-boundaries, and coexist at equilibrium. The oxygen fugacity has been of those associated with intercumulus plagioclase, calculated for the chromite - olivine - orthopyroxene suggest that grain coarsening occurred in the presence equilibrium using the equations of O'Neill & Wall of late intercumulus melt. Indeed, in the Muskox (1987). The calculatedlog"(O) is -9.2, which is essen- chromitite, some of the largest grains of chromite are tially the same as that for the quartz - fayalite - enclosed by plagioclase. The composition of the magnetitebuffer [logflO) = -9.07] at 1.146"C.T\ere chromite in the chromitite does not vary depending on are many assumptions implicit in these calculations, the size of the chromite grains or the type of enclosing and thus the resulting temperature and oxygen fugacity silicate. This suggests that the process of grain can be used only as a general guide. We conclude that

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1.0 dots) from various geological environments. These f Chromitite data are taken from the spinel databaseoftwenty-one thousand compositions initially described by Roeder Disseminatedin: (1994). The Muskox chromite in chromitite has a a Chlorite higher Fe2./(Fer* + Mg) and FeI + 2Ti than those in the chromitites from the Bushveld, Stillwater and Great o AlteredOl Dyke Complexes, but a similar Crl(Cr + Al). If it is + Misc. assumedthat the composition of the Muskox cbromite 0.9 in the chromitite reflects primary crystallization, then we can conclude that the melt compositions that gave rise to this chromite were more evolved than those of the other intrusions. The most common composition of chromite in the Bushveld chromitite horizons is shown in Figures 19 and 20. Local examples exist, 0.8 however, where the chromite can become very rich in o Fe and Ti (Cameron & Glover 1973, Scoon & Mitchell 1994). Such Fe- and Ti-rich chromite in the Bushveld is + usually associatedwith mafic pegmatites and the pipes O that are generally recognized (Merkle 1988) as having -\ formed later than the chromitites by the action of O late intercumulus melt (Kinloch & Peyerl 1990, Scoon & Mirchell 1994) or a water-rich fluid (Stumpfl & 0.7 Rucklidge 1982). As explained earlier, the chromitite horizons at Muskox are also unique in their relative position so high within the intrusion. Irvine (1979) and Francis (1994) showed how the Mg/(Mg + Fe) for the whole-rock pyroxenite layers in the Muskox go from ooo 0.81 at the base of megacycle I to a maximum of 0.87 0.6 in megacycle 2 to abottO.76 at the chromitite horizons o in megacycle 4. Inthe opinion of Francis, tlis trend was produced by a combination of primary magma contaminated in megacycle I by the selective melting .r-o of the country-rock gneiss and the increasing influence up-section of fractional crystallization of the magma and mixing with new additions of primitive magma. 0.51 I The change from megacycle 2 to megacycle 3 is 0.5 Fe2l(Fe2+Mg) 1.0 considered to involve mixing of new magma with fractionated magma within the chamber. By the time that most of megacycle 3 had formed, orthopyroxene Ftc. 17. Cr(Cr + AI) versusF*+l(Fe2* + Mg) for disseminated became the second silicate phase to crystallize after chromite and chromite in chromitite from the Muskox olivine. This may have been due to a combination of intrusion. The compositions of disseminated chromite fractional crystallization within i6s chamber, the addi- have been divided into categories three depending upon tion of new magma and a component of high-silica the minerals associated with the chromite. melted roof-rock. The relatively high Fez*/(Fez*+ Mg), Fe3+/(Fe3++ Al + Cr) and Ti of the chromite in the Muskox chromitite suggest that the magma was the calculatedtemperafure and oxygen fugacity are relatively rich in Fe and Ti by the time megacycle 4 reasonablefor the primary crystallization of these crystallized. The very presenceof chromitite suggests mineralsoas shown by experimentalsrudies (e.9., that much of the chromium was introduced by a new Roeder& Reynolds1991). pulse of a more primitive magma. The magma at tlte The compositionof chromitein chromitite from the time of deposition of the upper portion of megacycle 4 Muskox intrusion, the Bushveld Complex, the was quite evolved, as shown by the magnetite- and StillwaterComplex and the GreatDyke are shownin ilmenite-rich oxide layers near the roof (Irvine & Smith Figures19 and2O.The variationin thecomposition of 1967). chromite from thesefour layered intrusionscan be There have been three models suggested for the comparedin thesefigures to the generalvariation in formation of the Muskox chromitites. These models compositionshown by the spinel-groupminerals (small were developed mainly to explain the problem of

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Fe3+2Ti r Chromitite Disseminatedin: a Chlorite o AlteredOlivine + Misc.

Cr AI

Ftc. 18. Composition in terms of Cr - Al - (Fe:+ + 2Ti) for disseminated chromite and chronite in chromitite from the Muskox intrusion. The compositions of disseminated chromite have been divided into three categories depending upon the minerals associatedwith the chromite.

developing a seam of nearly pure chromite that Francis (1994) suggested that the appearance of contains little or no cumulus silicates such as olivine or orthopyroxene as the third silicate ia megacycle 3 and orthopyroxene. Irvine & Smith (1967) suggestedthat its appearance as the second phase in megacycle 4 the chromite concentrations were mainly the result of coincides with spikes in LILIHFS element ratioso cotectic crystallization of olivine and chromite, indicating contamination at the level of the upper crust, followed by a period of little or no crystallization, but as originally suggestedby Irvine (1970). differential settling of olivine and chromite. Under The available evidence is compatible, in our these conditions, the small crystals of chromite would opinion, with a combination of 1) contamination by settle at a much lower rate and thus could become roof rocks, 2) evolved magma from within the intrusion concentrated above, and separatedfrom, the silicates and, 3) a pulse of new magma to bring more chromium that accumulated in a lower peridotite layer. Irvine into the chamber and bring olivine back onto the (1974,1975) suggesteda different model that involved liquidus. The introduction of a new pulse of magma the mixing of a new pulse of magma with a roof- into the chamber at afurly late stage in the evolution of contaminated magma. Because of possible curvature of the Muskox chamber may have meant that the new the cotectic between olivine and chromite, this mixed magma had the opportunity of mixing with both roof- magma was considered to have a composition entirely contaminated magma and evolved magma. This could within the chromite primary phase-field. Irvine used have led to a period of negligible crystallization of the presenceof alkali- and silica-enriched inclusions in silicate and thus concentration of chromite by settling. chromite in the chromitite as evidence to support this On the other hand, the small amount of peridotite model. Irvine (1977) then suggestedthat "subsequent below tlie chromitite in cycle 22 does not support this work has revealed that this mechanism is probably not alternative. The detailed modeling of processeswithin adequate". His more viable alternative involves the the Muskox intrusion in order to explain the chromitite same curved olivine--chromite cotectic, but the mixing layers is questionable,specially since the exposed part of a more primitive melt with a melt already in the of the intrusion may be such a small part of the chamber that had evolved by fractional crystallization complex (Irvine & Baragar 1972).Tttus the scale of within the chamber. The evidence from the present the processes that led to the chromitites is largely study as it bears on these models is not definitive. unknown.

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Muskox There is siglificant evidence that late-stage reaction tr of intercumulus melt with disseminated chromite, and + Bushveld later subsolidus reaction, were responsible for the A stitlwater spread of Fez*l(Fez"a Mg), Fe3*,Ti and Ni. The local O GreatDyke variability in disseminated chromite and the upward increase in the above compositional parameters for disseminated chromite above the main chromitite band suggest some upward percolation of intercumulus melt. This upward change did not seem to significantly affect the smaller bands of chromitite (Figs. 3, 4). Irvine (1980) described the cyclic change in a number of compositional parameters through the and olivine clinopyroxenite layers of cycles 4--7 near the base of the Muskox layered series. He showed that the base of a cyclic rnit comrtroilly repres€ntsa sharp modal change, such as olivine clinopyroxenite to dunite, and an increase in modal disseminated chromite. Thus the start of a cyclic unit may represent a new influx of magma into the intrusion, Irvine described how chemical changes such as Fe2+/(Fe2++ Mg) in olivine, and Ni in both olivine and chromite, + were displaced meters above modal discontinuities at L (J tle base of a cyclic unit. He explained this displace- E ment by a process of infiltration metasomatism where o compaction of the cumulates resulted in upward movement of melt and displacement of the chemical changes from the modal discontinuities. The increase in Fe?*lFe* + Mg), Fe3*/(Fe3++ Al + Cr), Ti and Ni in the disseminated chromite above the thickest chromitite in MX-3 and MX-4 is attributed to the same kind of infi ltration metasomatism. The concentration of late interstitial melt in some locations (Figs. 13, 14) gave rise to a complex assemblageof silicates and oxides. The decreasein temperature that caused melt reaction and crystalliza- tion may have also caused local re-equilibration in Fe?+l(Fe2++ Mg) of disseminated chromite, olivine and pJrroxene.This re-equilibration would have continued 0.5 1.0 to increase the FezV(Fez++ Mg) of disseminated chromite at subsolidus temperatures by the mechanism Fe2l1Fe'+Mg; described by Irvine (1967). The textural evidence suggeststhat the last minor phasesto crystallize from the melt are plagioclase, biotite and ilmenite. The last FIG.19. Crl(Cr + Al) versusFe2*l(Fe2+ + Mg) for cbroqritein remnantsofmelt are consideredto have occupied those chromititefrom: Bushveldintrusion (van de Walt 1941, sites where there is biotite plus ilrnenite, local strong Cameron& Ernerson1959, Cameron l9&, 1977, deWaal zoning in plagioclase, fine-grained alteration assem- 1975,Mclaren & De Villiers 1982,Engelbrecht 1985, blages and, in some instances,a ragged appearanceof Hanon& VonGruenewaldt 1985, 1987, Eales & Reynolds orthopyroxene. 1986,Nicholson & Mathez1991, Scoon &'teigler 1994), the Stillwater intrusion (Stevens l9zl4, Howland 1955, Subsolidus alteration seems to have changed the Sampson1966, Beeson & Jackson1969, Nicholson & composition of disseminatedchromite that is presently Lipin 1985),the GreatDyke (Worst1964, Mlson 1982), associatedwith chlorite and other hydrothermal minerals. andthe Muskox intrusion(present study). The smalldots This is distinct from the subsolidus equilibration of representspinel compositionstaken from the spinel Fe2+/(Fe2++ Mg) with olivine and pyroxene that was databaseinitially describedby Roeder(1994). described above. These chromite grains associated with chlorite may have lost some aluminum during the formation of chlorite, and thus increased the Cr/(Cr + Al) of chromite. This trend is similar to the trend shown by chromite altering to'Territchrcmit"

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tr Muskox + Bushveld A Stillwater o GreatDyke

Cr AI

Ftc. 20. Composition of chromite in terms of Cr - Al - (Fe:* a 2Ti) for chromite in chromitite from: Bushveld Complex, Stillwater Complex, the Great Dyke, Muskox intrusion and spinel database.See Figure 19 caption for details.

(Roeder 1994). This is only a minor trend for the chromitite in both drill-core sections.This is explained Muskox disseminated chromite, and no real by infiltration metasomatism of late-intercumulus melt. "ferritchromit" with low Al has been found. The only 5) The texture and oxide mineralogy are described for other subsolidus trend that has been noted, but not pockets of minerals, which are consideredto have been explained, is that some chromite associated with altered the location of late-intercumulus melt. olivine shows a tendency to have a lower Crl(Cr + Al). 6) The composition of chromite from the Muskox chromitite is compared to the general range in compo- Suvnaeny auo CoNcLusloNs sition of spinel-group minerals, and to the range in composition of chromite from chromitites of the 1) The chromite in two 0.5-m drill cores of the Bushveld Complex, Stillwater Complex and the Great chromitite and orthopyroxenite in cycle 22 of Dyke. The higher Fe2*/(Fe2*+ Mg), Fe! and Ti of the the Muskox intrusive bodv has been described and Muskox chromite in chromitite are considered to be analyzed. due to its formation late in the history of the magma 2) The chromite in the massive and wispy chromitite chamber from a magma that combined fractionated bands shows a nzurow range in composition, magma with roof-contaminated magma and an influx Crl(Cr + Al) in the rangeO.M4.74, Fez*/(Fez*+ Mg) of a more primitive magma. in the range 0.62-0.69, and Fe3+/(Fe3++ Al + Cr) in the range 0.18-{.26. The range is the same in the two ACKNOWLEDGEMENTS drill cores. 3) The disseminated cbromite in the orthopyroxenite We thank Equinox Exploration for making this and olivine orthopyroxenite has a higher and more study possible by donating the drill core from the variable Fe2+l(Fe2++ Mg), in the range 0.644.95, Muskox intrusion. We also thank David Kempson and Fe3+/(Fe3++ Al + Cr) in the range O.l4-0.54, and has a Roger Innis for their help with the research, and Joan similar, but more variable Crl(Cr + Al), in the range Charbonneau for her help in preparing the manuscript. 0.53-0.88, than the chromite in the chromitite. Financial support was provided by the Natural Sciences 4) The Fez./@et* + Mg), Fe3+/(Fe3++ Al + Cr), Ti and and Engineering ResearchCouncil of Canada.We are Ni of the disseminated chromite increases with very grateful to Dogan Paktunc, Grant Cawthorn and strati$aphic height above the massive 10-cm seam of Robert F. Martin for reviewing the manuscript.

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