Metamorphism of the Ultramafic Rocks of The

Canadian Mineralogist Yol.22, pp. 77-91(1984)

METAMORPHISMOF THE ULTRAMAFICROCKS OF THE THOMPSONMINE, THOMPSONNICKEL BELT, NORTHERN MANITOBA*

A. DOGAN PAKTUNq Department of Geology, University of Ottawa, Ottawa, Ontario KIN 6N5

ABSTRACT d6formapar la suite.Les processus de recristallisation res- ponsablesde la formationde mosaiques dquigranulaires de Abundant sill-like bodies of serpentinized ultramafic n6oblastesd'olivine suivirent l'6pisode de d6formation. Le rocks, associatedwith nickel sulfide deposits,are found on cisaillementtardif et le m6tamorphismerdtrograde sont the western side of the Thompson Nickel Belt (Manitoba), marqudspar la chloritesecondaire et lesfilonnets de ser- near the Moak-Setting Lakes cataclastic fault-zone. An pentineque l'on trouveddveloppds localement parallble- unusually fresh ultramafic body, found on the 4000 level mente Ia fabrique,en borduredu massifultramafique. of the Thompson mine, is composedof dunitic, peridotitic, (Traduit par la R€daction) orthopyroxenitic and amphibolitic units. Tremolite is present as a Ca-bearingsilicate in most of theserocks. The Mots-cl4s:m6tamorphisme, roches ultramafiques, gise- Mg,z(Mg+Fe) ratio of olivine rangesfrom 0.83 to 0.90, mentsde sulfuresde nickel,Thompson, Manitoba. whereasthat of orthoplroxene rangesfrom 0.85 to 0.91 as a function of the bulk-rock chemistry,Spinel ranges in INTRoDUcTIoN compostion from Al-chromite to Mg-Al-spinel asthe gade of metamorphismincreasq. The partly ultramafic body was boundary altered before being progressively metamorphosed under The ThompsonNickel Belt lies on the upper-amphibolite-faciesconditions. The betweenthe Churchill and the Superior provinces in approximatem,D(- - imum temperature attained during metamorphism is north-central Manitoba (Fig. l). The Moak Set- estimatedto be 7ffi'C. The body was subsequentlydeform- ting Lakes cataclasticfault-zone definesthe north- ed. Recrystallization processes,which resulted in the for- westernboundary with the Churchill Province.The mation of equigranularmosaics of olivine neoblasts,follow- southeasternboundary, on the other hand, is partly ed the deformational episode. Late-stageshearing and a cataclastic fault-zone and partly a zone of retrognde metamorphismled to secondarychlorite and thin metamorphictransition (Cranstone& Turek 1976) parallel veins of serpentinedeveloped locally to the fabric, that definesthe border with the Pikwitonei Province. along the margins of the ultramafic body. In addition to the structural importance of this belt, Keywords: metamorphism, ultramafic rocks, nickel sulfide it contains important deposits of nickel sulfide; deposit, Thompson, Manitoba. therefore, the belt has been mapped in detail, and variousaspects have been studied. A comprehensive Soruuenn geologicaloutline of the belt is given by Peredery (1982). On trouve d'abondantsmassifs du tlpe filon-couchede Ultramafic rocks occur closeto the northwestern rochesultramafiques serpentinisdes, associ6s d desd6p6ts sideofthe belt. Individual bodieshave been studied de nickel sur le cot6 Ouest de la ceinture nick6lifdre de by various workers: the bodies of the Manibridge Thompson(Manitoba), prbsde la zonede faille cataclasti- mine by Coats (1960, those located on the south- que des lacs Moak-Setting. Un massif ultramafique tr0s westernedge ofthe belt, under the Paleozoiccover peu altdr6,observ6 au niveau4000 de la mine Thompson, (1973), Pipe II West Manasan se composed'unit6s dunitiques,p6ridotitiques, orthopy- by Bliss and the and roxdnitiqueset amphibolitiques.La tr6molite estprdsente ultramafic bodiesby Saboia(1978). Generally they commemin6ral calcifbredans la plupart de cesroches. Le are extensivelyserpentinized and sheared,retaining rapportMg/(Mg+Fe) del'olivine variede 0.83 i 0.90,alors very little relict texture. que celui de I'orthopyroxbnevarie de 0.85 i 0.91 en fonc- In this study, ultramafic rocks from the different tion du chimismeglobal de la roche, En composition,le levelsof the Thompson mine have beeninvestigated. spinelle passede la chromite alumineuseau spinelle-Mg- However, special attention is given to a large Al quand augmenteI'intensitd du m6tamorphisme.Le mas- ultramafic body found at the 4000 level sinceit is partiellement sif ultramafique fut altere avant d'6tre gra- remarkably fresh comparedto the other ultramafic duellementm6tamorphis6 au faciesamphibolite sup6rieur. Les roches ont atteint une temp€rature maximum d'envi- bodies found in the belt. ron 700oC au cours du mdtamorphisme.Le massif se GENERAL GEOLOGY *Publication 25-83, Ottawa-CarletonCentre for Geoscience Studies. The belt is underlain predominantly by migrnatitic 77

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gneissesthat mainly contain amphibolite-facies These supracru$tal rocks and the basement mineral assemblages.However, granulite-facies gneisseswere intruded by abundantultramafic and assemblagesare also commonin the belt and are in- mafic sills and dykes.Ultramafic bodiesare generally terpretedby Weber& Scoates(198), Peredery(1979) lensoid or tabular in shapeand vary in width from and Russell(1981) as relict Archean granulitesthat severalmetres to severalhundreds of metres.The survived the Hudsonian Orogeny. contact betweenthe ultramafic bodiesand the enclos- Near the westernmargin of the belt, the basement ing rocks is usually conformable, but the contact gneissesare overlainby thin (150-1500m) Aphebian rocks are so deformed and altered that their true supracrustal rocks (Peredery 1982) that include characteris uncertain (Peredery1982). Ultramafic metasedimentaryand metavolcanic assemblages. rocks rangein composition from peridotite to pyrox- Theseexhibit a range of metamorphicgrades from enile and are extensivelyaltered. Peredery(1979) also middle amphibolite to granulite. Two metasedimen- used a field term, 'ultramafic amphibolite', to tary assemblages,the Pipe and Thompson groups, describea variely of ultramafic rocks that rangesin have beendistinguished on the basisof lithology and corrrpositionfrom picrite to pyroxenite. metamorphicgrade @eredery 1982). The Thompson In the Thompsonmine, nickel sulfidesare found metasedimentsshow a higher metamorphic grade mainly as massive sulfide bodies together with and include quartzite, skarn, iron formation and ultramafic rocks in the metasedimentaryrocks. schist. Ultramafic rocks are mainlv in the form of inclu-

IHOMPSONNICKEL BELT I Mctavolcanlc ,i,/ ll l*l Mctascdimant

l---l Migmatiticgnciss PIKWITONEIPROVINCE

[-Z uol.on dikcswarm

'f-----l ' Granutitafacias acidic ,ir and basicrocks !t\ --t lault ,tQ I at I gcologicalboundary Qn -: i'y'( tll, T E Ni deposit" ^.'.#, J

N.W.T. -i-'-

Ir | "t ll --{._i I Oni.rio u's'A' \ 36iov">

Ftc. l. Regionalsetting of the Thompson Nickel Belt.

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sions, which rangein sizeup to severalmetres across, Thompson and Pipe sedimentarysequences, accord- in massive sulfides. Although larger ultramafic ing to Peredery(1982). Cross folding, with an axial bodiesare not uncommon in the Thompson mine, trend closeto north-south, accountsfor the con- their volume is small relative to the amount of figuration of the Thompson Group of metasedimen- sulfides. tary rocks (Peredery 1982). The earlier folds are Nickel sulfide deposits of the Pipe II mine, subhorizontal, and their axial plane strikes north- Mystery and Moak localities mainly occur in the northeast. form of disseminatedsulfides in the ultramafic rocks. According to Peredery(1982), the distribution of MtNSRAI-ocvaNn PsrnocRAPHY sulfidesis largely the result of remobilization during the complextectonic and metamorphichistory The ultramafic body at the 4000 level of the of the belt. Thompsonmine, as seenalong two drill holes(Fie. The major structureof the Thompsonmine is an 2), is 130metres thick, probablylensoid in shapeand anticlinical fold striking northeast and plunging enclosedin a garnet-sillimanite-biotite schist.The steeplyto the south, accordingto Zurbrigg (1963). body is composedof peridotitic, orthopyroxenitic However,the fold could be synclinal,judging from and amphibolitic rocks. Using the schemeof Evans similaritiesin stratigraphicsuccession between the (1977), the mineral assemblagescorrespond to

FIc. 2. 3600N vertical sectionof the Thompson mine (T-3).

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tremolite peridotite and tremolite pyroxenite (Fig. Alternately, it may be magnetite formed during a 3), which are indicative of the amphibolite facies. previous episodeof serpentinizationand included in The unit named 'amphibole-rich pyroxenite' in metamorphic olivine during progressivemetamor- Figure 3 is mineralogically similar to the ultramafic phism (Arai 1975,Vance & Dungan 1977).A similar amphibolite of Peredery Q979). The main sulfide texture, consistingof black olivine megacrystswith mineralizationis found in the enclosingschist in the densetiny ilmenite granules, occurring in the outer- form of massiveand stringerore. Minor dissemina- most zone of the Bergell aureole in the Malenco tions of sulfidesare presentin the ultramafic body, serpentinite,is describedby Trommsdorff & Evans close to its contact with mineralized schist. As (1980).Fluid and fine solid inclusionsare common- observedin core from drill hole 64599, tremolite ly associatedwith the dusty material @9. 4). Pale peridotite and its serpentinizedequivalents amount brown to brownish yellow, euhedralto subhedral to one half of the body, whereas the tremolite solid inclusions are probably spinel. Some of the plroxenite makesup more than one third (Frg. 3). olivine megacrystsdisplay undulatory extinction and Tremolite-olivine pyroxenite is the main rock type kink banding (Fig. 5). seenalong drill hole 64816. Small light greengrains of olivine, on the other hand, do not contain opaque dusty material and are Tremolite peridotite strain-free.This variety forms an equigranularsugary mosaictexture that is well developedin zone 5b @ig. Olivine in the peridotitic zonesis of two types: 6). Among thesegrains, 120" interfacial anglesare megacrystic(2-4 mm) and fine grained (lessthan I commonly seen@ig. 7). Textural relationsindicate mm). Megacrystic olivine usually is black owing to that these fine grains of olivine are formed from fine opaquedusty material disseminatedor concen- megacrysticolivine by recrystallizationproce$ses. trated along irregular fractures in olivine (Frg. 4). Recrystallization starts along grain boundaries and This material, possiblymagnetite, may be the prod- dislocations @igs. 5, 8). This new generation of uct of high-temperatureoxidation of olivine (Hag- olivine is calledneoblasl, following the terminology gerty & Baker 1967,Champness 1970, Putnis 1979). advocatedby Mercier & Nicolas (1975)for deformed upper-mantleperidotites. Peridotite in zone 5a consistsof loosely packed olivine megacrysts,neoblastic olivine mosaicand a light green-greyorthopyroxene-amphibole matrix, giving the rock a 'spotted' appearance.Dunite in zone 5c, on the other hand, is composedof closely ffi rmuor-meenoorm packed megacrystsof olivine with a minimum of in- (occupied llllll rneuom ouvnroFrHopyRoxENm terstitial area by orthopyroxene, amphibole and aluminian chromite), with a texture very lilijilillrnevor-m onnropyRoxENrrE similar to the cumulus texture (Fig. 9). These llllll'Jitillllsenrer.nom rnEMoLrrE pEFrDonrE equigranular olivine megacrysts very commonly enclosetiny subhedralinclusions of amphiboleand ffi rumrnor.e-nrcHPYBoXENTTE chlorite, forming parallel oriented flakes or bunches El eunrnor-m I sn.rnrarneeorne sorsr ili

Fro. 4. Fine opaquedusty material concentratedalong irregular fractures in olivine. Also shown are fluid and Frc. 3. Nature of the ultramafic body, as observedin two fine solid inclusions disseninated and concentrated di"mond-drill holes. along microfracturesin olivine. Plane-polarizedlight.

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FIc. 5. Kink-banding (black and dark grey horizontal bands) in olivine megacryst.Parallel oriented flakes are amphibole and chlorite. Crossedpolarizers.

Ftc. 6. Neoblastic olivine occurring with megacrystic olivine from which it was derived. The megacrystic olivine com- pletely recrystallized into neoblasts, forming a fine-grained mosaic texture on the right-hand side of the photomicrograph.Crossed polarizers.

Ftc. 7. Fine-grainedneoblastic olivine occurring together with onhopyroxene megacrysts,Idioblastic flakes are chlorite. Plane-polarized light.

Ftc. 8. Large olivine megacryst containing olivine neoblasts, presumably developed along dislocation planes. Crossed polarizers.

Ftc. 9. closely packed olivine megacrystsforming cumulus texture. crossed polarizers. Ftc. 10. Secondarychlorite developingalong a preferred crystallographicdirection of olivine megacryst,probably as a result of olivine and spinel reaction. Small relics of spinel are enclosed in chlorite. Crossed polarizers.

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(Fig. 5) that are similar to those constituting the 11), and enclosingmedium-grained amphibole and matrix. Closeto the serpentinizedtremolite peridotite chlorite. Amphibolesare mainly colorlesstremolite (zone 6), there are anhedral chlorite inclusionsin and cummingtonite. They generally form small megacrysticolivine. It seemsthat they developalong fibrous and prismatic crystals,coexisting with olivine certain crystallographicdirections of the host olivine neoblasts. and usually enclosegreenish yellow spinel(Fig. l0). Two types of spinel are present. One is pale Tremolite peridotite in zones 2 and 6 is ser- brownish yellow chromian spinel, which is cornmon- pentinized, the degree of serpentinization varying ly seen as fine disseminated erains in the olivine throughout the zones.Thin serpentineveins form orthopyroxenite and orthopyroxenite units. It is ran- planar systemsof veinletsprobably parallel to the domly disseminatedthroughout the peridotitic zones. fabric of the body and the foliation of the enclosing In someplace$, however, it is preferentiallyconcen- schist. They cut megacrystic and neoblastic olivine trated along the grain boundaries of olivine mega- and tremolite. Magnetiteoccurs in stringersfollow- crysts. The spinel usually forms fine anhedral zon- ing serpentineveins and along the cleavagesof ed grains, the core of which is darker than the rim. chlorite. Minor carbonateis also presentin thin veins The other type of spinel is opaque aluminian of serpentine. chromite, which is coarserthan the first type. It oc- Medium-grainedorthopyroxene is found intersti- curs interstitially to the olivine megacrystsin zone tially to olivine megacrysts,enclosing xenomorphic 5b, in which the olivine grains are closelypacked, islandsof olivine showing communal extinction @g. forming a cumulus texture (Fig. l2).

Frc. 11. Orthopyroxene(at extinction) replacingolivine megacryst.Clossed polarizers.

Frc. 12. Euhddratand subhedralolivine megacrysts.Al-chromite and minor orthopyroxeneare interstitial to olivine. Crossedpolarizers. Frc. 13. Elongate orthopyroxenemegacrysts enclosing fine inclusionssimilar to those constituting matrix. Crossed polarizers.

Frc. 14. Deformation lamellaein orthopyroxene.Crossed polarizers'

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Tremolite ort hopyroxenite common as small subhedralto anhedral crystals; thesecommonly are zoned (brownish core, yellow Orthopyroxene forms coarse, slightly elongate rim) disseminatedgrains, preferentially concentrated equigranulargrains. Its grain sizeranges from 0.3 in the intersticesof orthopyroxenemegacrysts or in to I cm, and it is pale brown. Abundant fine-grained orthopyroxeneclose to its margins. Sulfides are pres- amphibole, phlogopite and brown spinel are dissem- ent as disseminated anhedral grains. inated in someof the orthopyroxenemegacrysts. The intersticesof the orthopyroxenemegacrysts are oc- Tremolite olivine orthopyroxenite cupied by fine-grained light green amphibole and phlogopite, which are similar to those enclosedin Texture is quite uniform in this unit. The grain the orthopyroxenebut coarserin grain size@ig. l3). size of olivine is 4 mm and less, and it forms Thin lamellaeparallel to (100)of orthopyroxene;ue xenomorphic grains that are usually enclosedin observedin somegrains and characterizedby a very coarser(l-1.5 cm) orthopyroxene.Some of these low Ca content(Fig. I 1, 14).Strained crystals of or- orthopyroxenegrains have triple junclions showing thopyroxene are cornmon. The orthopyroxene 120' interfacial angles. megacrystsin zone I are elongate,probably along the foliation planeof the enclosingschist. Tremolite Amphibolite is common as medium- to coarse-grainedxenoblasts, whereas anthophyllite forms randomly oriented, The unit in zone 8 is composedof amphibole, radiating prismatic bladelike crystals in a finer- phlogopite and minor plagioclase.Amphibole forms grainedmatrix. Phlogopiteoccurs as strainedplaty a medium-grained(l mm) granular texture with a and fine grains. It is partly replacedby orthopyrox- weak fabric. Two amphibolesare present:magne- enein zonel. In zone12, on the margin ofthe body, sian cummingtonite and anthophyllite. Medium: phlogopite is recrystallized and replaced by a grained phlogopite forms the groundmass for pseudomorphof green chlorite. Close to zone 3, equigranular amphibole, and shows a foliation minor olivine is presentas small inclusions in or- parallel to the margin of the body. Plagioclase,on thopyroxene. Brown to greenishyellow spinel is very the other hand, is fine- to medium-grained,occur-

'I. TABLE REPRESENTATIVECOMPOSITIONs OF OLIVINE, ORTHOPYROXENEANDSPINEL rJltvlne 0rthopyroxene Spinel 234 1?341 23

si0z 39.t9 39.64 40.48 39.68 57.59 56.75 58.05 56.30 0.0 0.0 0.0 0.0 0.0 Ti02 0.0 0.0 0.0 0.0 0.0 0.0 0.06 0.09 0.0 0.0 0.0 0.05 0.?7 A',t203 0.0 0.0 0.0 0.0 0.57 1.36 0.68 1.06 58.78 51,22 49.30 37.84 14.58 'I Crr0, 0.0 0.0 0.0 0.0 0.0 0.10 0.1t 0.18 9,12 5.36 19.54 30.75 53.28 'l z5 0.20 0.57 0.0 0.98 .t8 '-2"3 0.11 0.20 0.39 0.15 'I Feo I4.23 |2.89 0.85 I0.19 6.59 9.48 7.11 8.65 10.76 20.30 I4.20 17.77 23.37 'I MgO 45.70 47.20 47.86 48.64 34.88 32.7 I 34,'17 33.09 19,72 2,80 I6.46 12.65 6.82 Cao 0.0 0.0 0.0 0.0 0.13 0.15 0.13 0.19 0.71 0,0 0.0 0.0 0.0 l{n0 0.28 0.18 0.21 0.24 0.20 0.30 0.30 0.35 0.14 0.23 0.07 0,23 0.54 Nio 0.08 0.12 0.27 0.27 0.07 0.0 0.0 0.0 0.30 0.0 0.0 0..09 0.0 Kzo 0.0 0.0 0.0 0.0

Na20 0.0 0.'ll 0.05

TOTAL 99.48100.03 99.67 99.20'100.03100.92 100.72 99.96100.08 100.59 gg,s7100.76.100.19

ttn 0.851 0.867 0.887 0.895 0.904 0.860 0.895 0.812 0.766 0.529 0.674 0.559 0,342 'Cr 0.094 0.167 0.210 0.351 0.709

F€0 representstotal iron for olivine and orthopyroxene.Ferric iron is ca]culated assumingperfect stoichiometrj for spinel. 0livine l, 2: neoblasts:3, 4: megacrysts.Spine'l l: brown,2: yellowishbrown, 3: red, 4: semiopaque, 5: oDaoue.

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ring in the intersticesbetween phlogopite and amphi- Geology, DalhousieUniversity. The instrument is bole. This unit is cut by severalthin (up to I cm) equipped with an Ortec energy-dispersionspec- pegmatiteveins. trometer automated to produce simultaneous multielement analysesand reducedata using the pro- MINERAL CHEMISTRY gram'EDATA2' of Smith& Gold (1979).Natural and synthetic minerals were used as standards. Mineral analysiswas performed with a Cambridge Several representative compositions of olivine, Mark V electronmicroprobe at the Depafiment of pyroxene and spinel from the ultramafic rocks describedabove are listed in Table l. The complete set of analyticalresults, together with the precision of the analysesobtained from replicatedetermina- r olivinc on requestfrom the Depository . Orthopyrorcna tions, are available r Amphibota of Unpublished Data, CISTI, National Research Council of Canada, Ottawa, Ontario KIA 0S2,

F Olivine o E The compositionof olivine in the dunitic zone5b Forr. However, it tt is uniform, ranging from Fo6rto :C' is not uniform in zone5a, and exhibitsa rapid change Fos5toward zone4, In zone 3, it is CD from Foseto slightly more magnesian,Fose.5. The olivine from drill hole 64816is more iron-rich, showing a compositional range from Fo63to Fo6r. This compositional variation is controlled mainly by the bulk-rock composition (Fig. l5). The NiO content of olivine rangesfrom the limit of detectionto 0.40 wt. q0,increasing with the magnesium content (Fig. lO. However,this relationshipis not the sameas shown Mg/(Mg+Fel.*1 by magmaticolivine. The olivine compositionswith Frc. 15. Relationshipof bulk-rock chemistryto the con- low nickel content fall outside the range defined by stituting minerals, olivine in cumulatesand mantle peridotites. The MnO content rangesfrom 0.1 to 0.35 wt. Vo and shows weak negative correlation wilh forsterite content. 0. The compositional difference betweenmegacrystic I -<' l. and neoblasticvarieties is small, although neoblastic : 0. i.{. to be enrichedin Ni and Mn compared e : olivine seems z to the megacrysticolivine from which it formed (Fig. a^ 1 grains of olivine megacrystsseem '\' 16). The individual ' ' to be compositionallyhomogeneous.

Orthopyroxene

The Mgl(Mg+Fe) ratio (Xsr) of orthopyroxene rangesfrom 0.84to 0.91. It is 0.89in zone5b and g decreasesto 0.865in zone5a, showinga similar pat- , aa tern to the Xr, value of olivine. In zone 4, this ratio o 0. showsan increasetoward zone3. ' is about 0.85 and 0. ' d '''-'/ It reachesa maximum value of 0.905in zone 3 (close ^.t../ to zone4) and decreasestoward zone2. The ortho- pyroxenefrom zones ll and 12 is more iron-rich, the coexisting 45 46 t;7 48 49 5b ranging from En65to E[s7, similar to Mgo(wt'"/') olivine (Forr-Fos). The Xr" of both olivine and or- thopyroxeneis probably controlledby the bulk-rock Frc. 16. Plots of MgO versusNiO and MnO (wt. 9o) in chemistry(Fig. l5). olivine. Tie-linesconnect neoblastic olivine (triangles) to megacrysticolivine (solid circles) from which it was The distribution coefficient for Mg and Fe between derived. coexisting olivine and orthopyroxene [Ko:

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lx.rys/XFJ'x (XFe/XM)oelis 0.985(Fig. l7). This tinites, ferritchromite in antigorite serpentinitesand is similar to the value for'regionally metamorphos- chromite in talc-olivine rocks. ed peridotitesfrom the Alps (0.96: Trommsdorff & The ratio Cr,/(Cr+Al+Fe3*), labeledI",, falls Evans 1974)and smaller than those of zone IV of between0.60 to 0.75 in the dunitic part of zone 5b the Tari-Misaka contact-metamorphosedultramafic and rangesfrom 0.10 to 0.75 in the rest of zone 5. complex(1.03: Arai 1975),Totalp serpentinites(1.05: Someof this wide rangeis defined by zonedgrains Peters1968) and metaperidotitesof Paddy-Go-Easy and by grainsin the samesection. The rim of zoned Pass(1.10: Frost 195). Accordingto Medaris(1969), grains is enrichedin Al and Mg relative to Cr and Mg and Fe partitioning between olivine and or- Fd+ (Fig. l9). Similar paths from core to rim are thopyroxene is not sensitiveto temperature over the range 700oC to 1300.C. However, recent ther- modynamic formulations (Sack 1980) show that it is potentially a useful geothermometer. Ca concentrations in orthopyroxene are very low . Orthopyrdo€ Qessthan 0,21 wt,Vo CaO), similar to orthopyroxene ^ Amphlboto from contact metamorphosed (Arai 1975) and regionally metamorphosedperidotites @vans & Trommsdorff 1974), The Al content of orthopyroxene,here less than 1.75wt.9o Al2O3,has been h shown to be essentially a function of temperature F @obretsov 1968,Fujii 1976,Mercier 1976)and of coexisting minerals. Its pressure dependenceis restrictedto temperaturesabove 9(X)oC according to 0 the T-P phase diagram of Gasparik & Lindsley (1980).Low contentsof aluminum and calciumare 4l'ory=o.gg characteristicsof orthopyroxenein the amphibolite facies, though there is a gradual increasein these 4L'AMP"o.$l componentsas the higher-temperatureparts of the faciesare reached@vans 1977).Highly aluminous orthopypyroxene,on the other hand, is well known in alpine spinel peridotites equilibrated in the granulite facies @vans 1977). Fto. 17. Distribution of Mg and Fe betweenolivine and Amphibole coexisting silicates.

Calcic amphibole compositions plot between lieumorpnic tremolite and pargasite end-members,forming a [Jll omphibote tuAl-(Na+ linear trend on the K) diagram Grg. lf) llfffl Mosmoticomphibole in the field of metamorphicamphiboles defined by Jamieson(1981). Al2O, content rangesfrom 3.3 to 8.4 9o (wt.), increasingwith an increasein alkali content. The CaO content is relatively constant at about Y 12.5(wt.)90. The Mgl(Mg+Fe) t rario of calcicam- o phibole rangesfrom 0.90 to 0.94 and is higher than z. that of the coexistingolivine and orthopyroiene @ig. 17) and of the bulk rock (Fig. l5). Edenite'{ru[J -:,1,,".;r Spinel

a-'- a ^ aa.' In the peridotitic parts ofthe body, spinelranges 176 ^t^/.' Tschermdkite in composition from aluminian chromite to chro- t.0 rv 2.0 3:0 mian spinel, whereasin the pyroxenitic parts it ranges Al.. from chromian spinel to Mg-Al-spinel. According Fro. 18. Plot of tvAl yer,rzsNa+K (number per to Evans(1977), these compositional ranges of atoms are con- 23 oxygenatoms) in calcic amphibole. Location fined close granulite of end- to the transition betweenthe and membercompositions are after Deer et al. (196O. Fields pyroxene-hornfels facies.Evans also indicatedthat of metamorphicand magmaticamphibole are those of the spinel is chromian magnetitein low-gradeserpen- Jamieson(1981).

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reportedby Evans& Frost (1975).This ratio is rather GEoISI\4pSRATURE ESTIMATES uniform in zone 4, ranging from 0.10 to 0.20. The effect of temperatureon Y6, of spinel and Olivine-spinelgeothermometry based on the par- the Mg-Fd* partition between spinel and co- tition of Mg and Fd* betweencoexisting spinel and existingsilicates was first studied by Irvine (1965)' olivine has been applied to pairs from Thompson. then by Evans & Frost (1975)and Frost (1975).In Two different calibrations of this geothermometer general, spinel becomes more AI- and Mg-rich havebeen used. One of them, that of Roederet ol. relative to Cr and Fe2*, similar to the path seenin (1979),is basicallythe geothermometerof Jackson Figure 19,as the gradeof metamorphismincreases. (1969)except that a different value for the free energy Minor elementssuch as Ti, V and Mn are enriched of FeCrrOa has been used. The other is that of in Cr-rich varieties@ig. 20). No suchcorrelation ex- Fabriis (1979), which is based on the tentative ists for Ni. TiO2 goesup to 0.30 wt.Vo, whereas graphical calibration of Evans & Frost (1975). VrO3is lessthan 1.90wt.Vo. Temperaturesobtained from thesetwo calibrations are different. Those obtained from the equation of Roederet ol. (1979)vary from 400oCfor Cr- rich spinel to ll00'C for Al-rich spinel. These temperatures are ulueasonable for the mineral assemblages.The applicability of the Roedere/ a/. (1979)geothermometer has been challenged,since their isothermsare inconsistentwith recentlydeter- mined reversed experimental data at low

F a t ] .. 3!. do. a.' F aa aa a

i rot tL a + a + *' a o ' tt . zo c) so t a a I ar t at ' : . .tt ! |

E ^^^^ 2 o l^ -c ^t^

Mg/(Mg*Fd'l Frc. 19. Plot of X1,4nversus Ys, in spinel. 3b ao sb 6b * rb lo Xvn = Mg/(Mg + r'd+ )' Yc, = Cr/(Cr + AI + Fe3 ). Cr2O3(wt.%) Cor-eand rim compositionsof zoned spinelgrains are connected.Spinel changesfrom opaque to gxeenas the Frc. 20. Plots of Cr2O3versus TiO2, V2O3and MnO (wt. temperatureincreases. Yo)in spinel.

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temperaturesand with trends from metamorphicand Other geotemperaturedata are obtained from the .igneousrocks @ngi & Evans 1980)and sincethe free- T-P phasediagram of Gasparik& Lindsley (1980). energy values for the end-member spinels are not The diagram is a reconstruction of aluminum known accurately@abrids 1979). solubility in orthopyroxenecoexisting with olivine In contrast to the isothermsof Roederet ol. (lW9) and spinel in the systemMgO-AlrOr-SiO, after Fu- @ig. 2l), thoseof Fabrids(1979) coincide with the jii (1976)and Danckwerrh & Newron (1978), and regressionline for the olivine-spinel pairs from representsan approximation to the model of ideal Thompson. They plot linearly with a correlation solution. Extrapolatedtemperatures using the con- coefficient of 0.996 rn the lnKo-Y.. diagram @ig. tent of the Mg-Tschermakcomponent (mol. go) in 2l). The regressionline lies betweenthe 600 and the orthopyroxenesolid-solution are approximately 800oCisotherms and intersectsthe 700.C isotherm 640oC for orthopyroxenecoexisting with Mg-Al- where Is, is 0.17. This may indicatethat not all the spinel.The reasonfor the low temperatureis prob- sampleshave equilibratedunder the samethermal ably because of the limitation of the geother- conditions, but some of them, with I", < 0.17, mometer. This geothermometer in the system have re-equilibratedduring progressivemetamor- AI2O,-MgO-SiO, is an approximation to the ideal phism. Thus, the regressionline may representthe solid-solutionmodel. According to Danckwerth & path of progressivemetamorphism. Newon (1978),corrections for idealsolution involv- The presenceof zonedspinel indicatesthat com- ing FeO and CrrO, in coexisting spinel species plete equilibrium betweenolivine and spinel was not would raise temperatureestimates. achieved at the high grades. In the lnKD-yc, diagram, the core composition of two zoned crystali PETROGENESIS of spinel plots near the 660oCisotherm. If we assume that the olivine is in equilibrium with spinel rims, As deducedfrom the mineral assemblagesand tex- the assemblageplots very closeto the 7@oC isotherm tures, the ultramafic body has experienceda series (Fie. 2l). of complex hydration and dehydration reactions

ln Ko

Yr"r", =0.00

,Crv

Ftc.2l. Plot of /nK2 versus.Yg,.K,= (Xp1r/Xp)o1x (Xp"/X;4)ro. yc, = Crl(Cr + Al).The compositionof two sprnelcores (stars) and rims (fllled stars)-shownfor comparisoi.Dashed Ene (2.97Yc.-laKD+0.755:0) is the regressionline of plots with a correlationcoefficient of 0.996.Solid linesare the isothermi calculatedirom the equation ofFabribs (1979):dash-dot lines are calculatedusing the equation of Roederet at. (1979),precision is wiihin r 50'c.

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before, during and following progressivemetamor- thesestringers of relict magnetite,amphibole and phism. The dominant mineral assemblageis olivine chlorite inclusionsforming parallel oriented flakes + tremolite + orthopyroxene+ aluminousspinel, optically continuouswith grainsadjacent to enclos- $uggestingupper-amphibolite-facies conditions. The ing olivine megacrystsindicate metamorphic growth presenceof tremolite as the Ca-bearingsilicate in- of olivine. Similarly, very irregular grain-boundary stead of diopside and the absenceof talc and cor- relations of anhedral olivine with other metamo.rphic dierite constrainthe pressureand temperaturecon- mineralsstrongly suggestthe same.Subhedral and ditions for the ultramafic rocks in the systemMgO- euhedral olivine megacrysts, in part forming a CaO-AI2O3-SiO2-H2O(Fie. 22). The sporadic oc- cumulus texture, may be interpreted as recrystalliz- curenceof mineralsrepresenting lower grades,such ed grains of igneousolivine with minor metamor- as aluminian chromite, chlorite and Ca-poor am- phic modification. This is supportedby the relatively phibole, together with olivine and orthopyroxene, regular content of forsterite in the olivine within in- may be explainedby polymetamorphismor locally dividual specimensand uniform MgO-NiO and varying fluid-phase composition, accordingto Evans MgO-MnO relationshipsin subhedraland euhedral (1977).The fine opaquematerial may representrelict olivine. magnetite stringers formed during serpentinization Low CaO and AlrO3 content of the ortho- and enclosedin olivine during deserpentinization,or pyroxenemegacrysts and their poikiloblasticnature alternately,it may be a result of high-temperature indicate that they are metamorphic, and not igneous oxidation of olivine. Sincehigh-temperature products relics.Minute inclusionsof amphibole,chlorite and of oxidation usually form more regular patterns phlogopite are similar to the matrix material but (Haggerty& Baker 1967),pyroxene and magnetite smaller in size. They probably resulted from high usually defining a dendritic intergrowth (Putnis growth-rateof orthopyroxenerelative to the rate of 1979),the former possibility is favored for the for- diffusion of componentsof the inclusionsthrough mation of the fine opaquematerial. In addition to the host orthopyroxene.According to the phaserela-

iP""*'*-1

el N

CL ,,.#'

800 T('C) Frc. 22. Phasediagram showing mineral assemblagesin ultramafic rocks as a functron of temperatureand water pressure. Solid lines are experimentally determined curves; dashedlines are theoretical (calculated) curves, after Evans (1977) and Jenkins (1981).Also shown is the interpretedpath of metamorphismfor ultramafic rocks at the Thompson mine. Abbreviations:Amp amphibole,Anth anthophyllite,Antig antigorite, Chl chlorite, Co cordierite,Dio diopside, En enstatite,Fo forsterite, Gt garnet, Serp serpentine,Sp spinel, Ta talc, Tr tremolite, w water.

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tions in Figure 22, enstatiteforms from forsterite and This fact is further supported by the unrecrystalliz- anthophyllite at temperaturesabove 700oC,below ed grains of oliving. 8 kbar pressure. Recrystallization-accompaniedor followed defor- During progressivemetamorphism, spinel chang- mation. It seemsto havebeen an annealingor static ed in compositlon from Al-chromite ttyouglt pi"oiii" type of recrystallization becauseof the large grain- size, equant polygonal grain- and Cr-spinelto Mg-Al-spinel,as suggesbd'bi Su*, . ,grain-shapes, boundaries,and absenceof featuresof intragranular (1977).Equilibration of coexisting-atAl-chromite and olivine was probably achieved approximatety strain (Green et,al. 1970),as opposedto inequant 660"C. This opaqueAl-chromite ctrarftea to a dari erain-shapesand strong preferred orientation, which brownish red s-erniopaquevariety at its margin, which are characteristics.of syntectonic recrystallization. presence becamemore Al- and tvtg-ricniehtive to b, a"O f" The of a bimodal texture seenin zone 5a, recrystallizationis syntectonic during an increase of temperature. pale brownish lgwever,llggoj: ||ut yellow and yellowish green spinels, on the othei (Goetze1975)..If this is the case,some features such grain-size Land, probaLly formedat temperaturesof 685 and as,eqg.anl andlarge may havebeen form- (Green 720"C,respectively. Spinel, together with olivineanJ ed withthe help.of a small amount of water orthopyroiene, muy be formed from chlorite accor- et al. 1970)during the recoverystage. The driving ding io the dehydiation reaction: clinochlore = forceforthetype-ofrecrystallizationthatdeveloped foriterite + 2 enstatite + spinel + 4Hro. This alonggar.nboundariesanddislocationsoftheolivine reactionin the systemMgO-C;O-AI2O3-SiO2-H2O megacrystsis probably strain energy stored in @vans1977, Jenkins pdt; tatcespta6e Uet*e6n ft-O megacrysts..Onthe other hand, the driving force for and 825.C at 4-9 kbar pressure.The upper-pressure th^erecrYstalljzation that resultedin the formation limit wasestimated from the intersectionof ihe reac_ of equigranularolivine mosaicis more likely grain- tion abovewith the reactionFo + Tr : En + Dio oounqary ano surlace energy. + Sp + w (Fig. 22), whereasthe lower limit was estimatedfrom the reactionEn + Sp = Fo + Co. Addition of iron into this 5-component system, CoNcrusrous however, shifts the reaction curves toward lower temperatures.Furthermore, reaction boundaries in _The_ultramafic rocks in the Thompsonmine were the systemwill be shiftedto lower temperaturesand altered prior to metamorphism,as indicated by 1) pressuresif the activity of HrO in the ambientfluid relict.magnetite.stringersin olivine megacrysts,2) is not closeto unity (ienkins l98l). As a result, a Parallel oriented amphibole and chlorite flakes in temperatureof appioximately7?.0"t,obtained from olivine megacrysts,and 3) abundant minute inclu- MgiAl-spinel and olivine pairs, would be realistic sions of amphibole, chlorite and phlogopite in or- for the formation of spinel.Anhedral inclusionsof tnopyroxenemegacrysts' chlorite (chromium-bearing clinochlore), which Olivine megaqystsmay haverecrystallized from developedalongcertain cryJtalographic airectioni partly altered olivine during progressivemetamor- in the host olivine, are interpretedai a product of phism. Orthopyroxenemay have formed at the ex- pense of olivine and (probably) anthophyllite the reactionbetwein the host olivine and spinel' in- at clusions@ig. l0). temperaturesabove 700oC. Serpenrinizationseenin zones2 and6is a late- .#fi[*tif*"j$?r3:Ufffi}}ff"?:1#;-il stageeYent, developed following metamorphismanq n- Meri.h-rpinei ;*gitt relative to the cr- and Fe- deformation. This is indicated by thin veins of ,Lf, .*" li p.oOaUfVtook place at higher serpentinecutting olivine neoblasts as well as other gr-"0* i"..e metamorphism.This could metamorphicminerals. ;r"gessive On the margin of the body f.;;;il;;;-d'.i, i'p"rti"f re-equiribrarion,in terms (zone l2), recrystallizedpllogopite is larer replaced ;i ; M;_F"r;".iifr*g. reaction betweenolivine by a green chlorite pseudomorph. and spinel at higher temperatures. Deformation of the body is indicatedby the fabric, Temperatures estimates and the mineral kink banding and undulatory extinction of olivine assemblageof olivine + tremolite + orthopyrox- and orthopyroxenemegacrysts. According to Poirier ene + Al-spinel correspondto the upper amphibolite & Nicolas (1975),spinel in peridotitenodules is pro- facies of metamorphism. During or following gressively scattered with increasing flow since it is metamorphism, the ultramafic body was deformed lessductile than the silicates.By their interpretation, and subsequentlyrecrystallized. the scatterseen in Mg-Al-spinel may be due to in- Locally developedsecondary serpentinization is creasingdeformation. However, the situation is dif- the latest event observed.This secondaryserpen- ferent in zone 5b, where the spinel is Al-chromite tinization probably correspondsto the extensive occupying the intersticesof olivine megacrysts.This serpentinizationof the peridotitic rocks elsewherein zone is probably the leastdeformed part of the body. the belt.

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ACKNOWLEDGEMENTS & Fnosr,B.R. (1975):Chrome-spinel in progressivemetamorphism - a preliminaryanalysis. This work wasdone as part of a Ph.D. thesisunder Geochim.Cosmochim. Acta 39. 959-972, the supervisionof Dr. G. Armbrust. The research & Tnovusoonrr, V. (1974): Stability of grant was supported by a from the University of Ot- enstatite + talc, and CO2-metasomatism of tawa and a scholarshipfrom the Mineral Research metaperidotite, Val d'Efra, Lepontine Nps. Amer. and Exploration Institute of Turkey MTA). Inco J. Sci. 274,274-296. Metals Ltd., Manitoba Division, supported field work. The author is greatly indebted to Dr. W. FanruEs,J. (1979): Spinel-olivine geothermometry in Peredery for suggestingthe topic and for his peridotites from ultramafic complexes. Contr. generous help in the field. Drs. R. Kretz, W. Mineral. Petrology 69, 329-336, Perederyand R. F. Emsliereviewed the earlierver- Fnosr, B.R. (1975): Contacl sion of the paper gave metamorphism of serpen- and constructivesuggestions. tinite, chloritic blackwall and rodingite at Paddy- The final versionof the paper wasreviewed by Drs. Go-Easy Pass, Central Cascades,Washington. ,I. C. Pride and O. R. Eckstrand. Petrology 16,272-313.

RsrsRENcss Fu;rr, T. (1976): Solubility of Al2O3 in enstatite coexisting with forsterite and spinel. Carnegie Inst. ll'ash. Yearbook 15, 566-571. Aner, S. (1975): Contact metamorphoseddunite- harzburgitecomplex in the Chugokudistrict, western Japa* Contr. Mineral. Petrology 52, l-16. Gespanrr, T. & Lrrosr-rv, D.H. (1980): Phase equilibria at high pressure of pyroxenes containing Blrss, N.W. (1973):A ComparativeStudy of Two monovalent and trivalent ions. /z Pyroxenes(C. T. Ultramafic Bodies at the SouthwestEnd ol the Prewitt, ed.). Mineral. Soc. Amer., Rev. Mineral. ManitobaNickel Belt, with SpecialReference to the 7.309-339. ChromiteMineralogy. Ph.D. thesis,McGill Univ., Montreal, Que. Goerzs, C. (1975): Shearedlherzolites: from the point of view of rock mechanics. Geology 3, 172-173. CnaupNsss,P.E. (1970):Nucleation and growth of iron oxidesin olivines.Mineral. Maq.37,790-800. GnreN,H.W., Gnrccs,D.T. & Cnrusrrs,J.M. (1970): Synthetic and annealing recrystallization of fine Coars, C.J.A. (1966):Serpentinized Uhramafic Rocks grained quartz aggregales. In Experimental and of the Manitoba Nickel Belt. Ph.D. thesis,Univ. Natural Rock Deformation (P. Paulitsch, ed.). Manitoba, Winnipeg,Man. Springer-Verlag, New York. CnaNsroNe,D.A. & Tunx, A.T. (1976):Geological and geochronologicalrelationships of the Thomp- Heccnnrv, S.E. & Baxrn, I. (1967): The alteration of son nickel belt, Manitoba. Can. J. Eqrth Sci. 13. olivine in basaltic and associated lavas. I. High 1058-1069. temperature alteration. Contr. Mineral. Petro logy t6,233-257. Darcrwrnnr, P.A. & NEwroN, R.C. (1978): Ex- perimentaldetermination peridotite of the spinel to (1965): petrogenetic garnet peridotite reaction in the systemMgO- Invrre, T.N. Chromian spinel as a AI2O3-SiO2in the range900'C-l100'C and Al2O3 indicator. I. Theory. Can. J. Earth Sci.2,648-672, isoplethsof enstatitein the spinel fi,eld. Contr. Mineral. Petrologlt 66, 189-201. JacrsoN, E.D. (1969): Chemical variation in coexisting chromite and olivine in the chromite zones of the Dren, W.A., HowrB,R.A. & Zussrr4aN,J. (1966):An Stillwater Complex. In Magmalic Ore Deposits Intrcduction to the Rock-Forming Minerals. (H.D.B. Wilson, ed.).Econ. Geol., Mon.4,4l-71. Longman,London. JnvrrsoN, R.A. (1981): Metamorphism during DosRErsov,N.L. (1968):Paragenetic types and com- ophiolite emplacement - the petrology of the St. An- positionsof metamorphicpyroxenes. J. Petrology thony Complex, J. Petrology 22,397-M9. 9, 358-3',77. JrNrrNs,D.M. (1981): Experimental phase relations of Eror, M. & EvaNs,B.W. (1980):A re-evaluationof the hydrous peridotites modelled in the system H2O- olivine-spinelgeothermometer: discussion. Corzlr. CaO-MgO-Al2O3-SiO2. Contr. Mineral. Petrology Mineral. Petrologlt 73, 201-203. 77,166-t76,

EvaNs,B.W. (1977):Metamorphism of alpine Meoenrs, L.G. (1969): Partitioning of Fe+ + and peridotiteand serpentinite.Ann. Rev.Earth Planet. Mg+ + 6.t*..n coexisting synthetic olivine and or- Sci.5, 397447. thopyroxene.Amer. J. Sci. Xi1,945-968.

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Mrncrsn, J.-C. C. (1976):Single pyroxene geother- Sesoh,L.A. (1978):The ll/estManasan and thePipe mometry and geobarometry.Amer. Mineral. 61, Mine: 2 Ultramalic Bodiesof the ThompsonNicker 603-615. Belt.M.Sc. thesis, Univ. WesternOntario, London, Ont. & Nrcons, A. (1975):Textures and fabricsof upper-mantleperidotites as illustrated by xenoliths Secr, R.O. (1980): Some constraintson the ther- from basalts.J. Petrology16,454-487. modynamicmixing propertiesof Fe-Mg orthopy- roxenesand olivines.Contr. Mineral. Petrology71, PBnronny, W.V. (1979): Relationship of ultramafic 257-269. amphibolites to metavolcanic rocks and serpentinites Srrarrn,D.G.W. & Gorp, C.M. (1979):EDATA2: a in the Thompson belt, Manitoba. Can. Mineral. 17, program processing 187-200. Fortran IV computer for wavelength- and,/or energy-dispersiveelectron (1982): microprobeanalysis. In Proc. MicrobeamAna-. - Geology and nickel sulphide depositsof (San the Thompson belt, Manitoba. .lz Precambrian Soc., l4th Ann. Conf. Antonio; D.E. Sulphide Deposits (R.W. Hutchinson, C.D. Spence Newbury,ed.), 273-27 8. & J.M. Franklin, eds.). Geol. Assoc. Can., Robin- (1974): son VoL, 165-209. Tnovvrsoonrr,V. & Evars, B.W. Alpine metamorphism of peridotitic rocks. Schweiz. PBrBns,T. (1968): Distribution of Mg, Fe, Al, Ca and Mineral. Petrog. Mitt. 54, 333-352. Na in coexisting olivine, orthopyroxene and (1980): clinopyroxene in the Totalp serpentinite (Davos, & - Titanian hydroxylclinohumite: Switzerland) and in the Alpine metamophosed formation and breakdown in antigorite rocks (Malenco, 72, Malenco serpentinite (N. Italy). Contr. Mineral. ltaly). Contr. Mineral. Petrology Petrology 18, 65-75. 229-242. (1977): PornrEn,J.P. & Nrcoras, A. (1975): Deformation- VaNcn,J.A. & DuNcaN,M.A. Formation of peridotites induced recrystallization due to progressivemisorien- by deserpentinizationin the Darrington Washington. tation of subgrains, with special reference to man- and Sultan areas, CascadeMountains, 1497-1508. tle peridotites. J. Geol. E3,707-720. Geol. Soc. Amer. Bull. 88, (1978): Purus, A. (1979): Electron petrography of high- Wsnsn, W. & Scoarns,R.F.J. Archean and temperature oxidation in olivine from the Rhum Proterozoic metamorphism in the northwestern layered intrusion. Mineral. Mag. 43,293-296. Superior Province and along the Churchill-Superior boundary, Manitoba. 1r? Metamorphism in the Canadian Shield (J.A. Fraser & W.W. Heywood, RonoEn, P.L., Caurnerr, I.H. & JaMresoN,H.E. eds.).Geol. Surv. Can. Pap.78-10,5-16. (1979): A re-evaluation of the olivine-spinel geother. mometer. Contr. Mineral. Petrology 68, 325-334. ZunsRrcc,H.F. (1963):Thompson Mine geology. Can. Inst. Mining Metall. Bull. 56,451-460. Russsr.r-,J.K. (1981):Metamorphism of the Thompson nickel belt gneisses:Paint Lake, Manitob a. Can. J. Received May 20, 198i, revised manuscript accepted Earth Sci.18. l9l-209. August 1, 1983.

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