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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 pres- ent 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 Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/22/1/77/3435463/77.pdf by guest on 23 September 2021 78 THE CANADIAN MINERALOGIST 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. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/22/1/77/3435463/77.pdf by guest on 23 September 2021 METAMORPHOSED ULTRAMAFIC ROCKS, THOMPSON, MANITOBA 79 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 dur- ing 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). Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/22/1/77/3435463/77.pdf by guest on 23 September 2021 80 THE CANADIAN MINERALOGIST 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
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