tl7 Thc Carwdian M ine ralo g ist Vol.36,pp. 117-135(1998) COMPOSITIONOF CHROMITE IN THEUPPER CHROMITITE, MUSKOX LAYERED INTRUSION,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 olivine, orthopyroxene and chromite, and the intercumulu5 minerals clinopyroxene and plagioclase.The minor minerals ilmenite 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 <Cr/(Cr+Al)<0.74,0.62<Fe2+l(Fe2*+Mg)<0.69,and0.18<Fe3+/(Fe3*+Al+Cr)<0.26.Theaveragecompositionof chromite in the chromitite, and the olivine and orthopyroxene in the orthopyroxenite, were used to calculate a temperature of 1146'C and logflO) = -9.1. The disseminatedchromite in the orthopyroxenite shows a much gteater range in composition, and increases in Fe2+/(Fe2++ Mg), Fe:*/(Fe3" + Al + Cr), Ti and Ni with stratigraphic height above the maisive chromitite. The chromite in the Muskox chromitite is significantly higher in Fe3+,Ti and Fez+l(Fe2++ Mg) than chromite in the Bushveld, Stillwater and Great Dyke chromitites; furthermore, the Muskox chromitites formed much higher in the stratigraphic section of the layered series than in tlese other intrusions. The Muskox chromitites are considered to have formed late G the magmatic history of the intrusion as a result of mixing of a fractionated magma with a more primitive magma and a component due to wall-rock assimilation. 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 <Fez*(Fez" + Mg) < 0.69, et 0.18 < Fe3+/(Fe3++ Al + Cr) < 0.26. La composition moyenne de la chromite dans la chromitite, et celle de I'olivine et de I'orthopyroxdne dans I'orthopyrox6nite, ont servi pour calculer la temperature, 1146"C, et la fugacit6 de I'oxygbne, logflO) = -9.1. En revanche, la cbromite diss6min6e dans I'orthopyrox6nite montre un intervalle beaucoup plus grand de valeurs de Fe2+/(Fe2++ Mg) et dg psr+(Fsr+ + Al + Cr), er des teneurs plus 6lev6es en Ti et Ni selon l'6l6vation stratigraphique au dessus de la chromitite massive. La chromite de la chromitite de Muskox est nettement plus riche en Fe3*et Ti, et possbdeun rappoft Fe2+/(Feh+ Mg) plus 6lev6, que la cbromite des cbromitites des complexes de Bushveld, Stilltlater et Great Dyke; de plus, d Muskox, les niveaux de chromitite se sont form6es beaucoup plus haut dans la section stratigraphique que dans ces auEes complexes intrusifs. A Muskox, les chromitites se seraient formdes tardivement au cours de l'6volution magmatique suite d un mdlange de magma fractionn6 avec un magma plus prirnifif et un composant cont'min6 par assimilation des roches encaissantes. (Traduit par la R6daction) Mots'clds; chromite, cbromitite, orthopyroxbne, complexes stratiformes, Muskox, Territoires du Nord-Ouest. I To whom correspondenceshould be addressed.E-mail address: [email protected] Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/36/1/117/3420523/117.pdf by guest on 02 October 2021 118 THE CANADIAN MINERALOGIST 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 peridotite 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