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Significance of Dumortierite in an Aluminosilicate.Rich

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Significance of Dumortierite in an Aluminosilicate.Rich t37 CanadianMineralogist Vol. 3 I, pp. 137-146 (1993) SIGNIFICANCEOF DUMORTIERITE IN AN ALUMINOSILICATE.RICHALTERATION ZONE, LOUVICOURT,OUEBEC MEHMETF. TANER BP Resources Canala Limited, lzs Mines Selbaie' Case Postale j70, Joutel, Qulbec JOY IN0 ROBERTF.MARTIN Departmentof Eanhand Planetary Sciences, McGill Universiry' 3450University Street, Montr,1al, Qutbec H3A 2A7 ABSTRACI A dumortierite-bearing,aluminosilicate-rich zone of hydrothermalalteration has been found in the northeasternsector of Louvicourt Township, in-the Abitibi greenstonebelt, Val d'Or area Quebec.The affected schists(metavolcanic rocks of &e Malartic Group) are andalusite-bearing;they are located near the southerncontact of^the Bevcon pluton. The presenceof dumonieritemakes the rocks pinkish. Its a andb cell parameters,I 1.822(5) nd20.251(7) A, arelarger than usually reported, and mayreflect the presence of vlMg andlvAI. Thec diminsionis 4.698(I ) A. Theaverage chemical composition of thedumortierite in two samplesdiffers slightly: (A16.sA4g0.r2Ti0.mFeo.oootro.zs)x.0rB(Si2.8lAl0.r4Po.oa)>z.seorz.0e(oH)q.q.l (surfaceexposure) and (A16.62Mg6'67Ti6.orF"o.orio.r)*.ril$ir-A1o*i,o.oJo-6"-(OH)1.16 (subsurface).The dumortieritereplaces andalusite, whi;ii:fodA diiring iii6 miitari<irphis;';d aafom;ri;nbf the intenselyleached metavolcanic rocks. The introductionof boron thus occunedlate. The presenceof dumortierite,here described for the fint time in theAbitrbi greenstonebelt, provides.avaluable targetfor further explorationbecause of the link betweenperaluminous altered rocks and zonesof Au and Cu mineralization. Keywords:dumortierite, ce]l dimensions,electron-microprobe dat4 andalusite,alteration zones,boron, Louvicour{, Val d'Or mining district, Abitibi belt, Quebec. Souuens Nousd6crivons une zone d'altdration riche en aluminosilicateet contenantde la dumorti6ritedans le secteurnord-est du canton de Louvicourt, ceinturedes roches vertes de l'Abitibi, r6gion de Val d'Or, Qudbec.ks schistesaffectds, d l'origine desroches m(tavolcaniquesdu Groupede Malartic, contiennentde I'andalousite;ils sont situdsprds du contactavec le pluton de Bevcon- l,a or6sencede dumorti6ritedonne aux rochesune teinte rose violac6e.ks dimensionsa et b de la maille de la dumortidrite' ll.3226) et20.251 (D A, sontsup6rieures i celle-squi sontdocumentdes dans la littdrature,et seraientI'expression de la pr6sence de uMg'et IvAl. La'dimensionc est 4.698(l)A.'La compositionchimique de la dumorti6ritediffbre l6gbrementdans deux dchantillns: (A16.56Mge.12Ti6.s?Fe6.006tr6.25)x.0tB(Si2.81A10.14P0.04)r2.eeor7.0e(OH)0.er(6chantillon de surface) et (Al6.62Mg6.s7Ti6.*F.o.ooirjo."l;.qrstSirLAt';po.orlooooroBotoqf'ltcarotte). l,a dumorti6riteremplace l'andalousite, qui s'6ff;fdil; p;-reds:riini'atiori'inetamorphiriueOiiibatres m6tavolcaniques lessivdes et ddform6es.L'introduction du boreserait donc tardive.i,a pr6sencede la dumortidriie,signal6e pour la premidrefois dansla ceinturede rochesvertes de l'Abitibi, aurait une significatioti co1n-" indice de cibles d'exploration I cause du lien entre les roches alt6r6eshyperalumineuses et la min6ralisationen Cu et Au. Mots-cl6s: dumortifrite, dimensionsde la maille, donn6esi la microsondedlectronique, andalousite, zones d'alt6ration, Louvicourt, campminier de Val-d'Or, ceinturede I'Abitibi, Qu6bec. INTRODUCTION LouvicourtTownship, Val d'Or area Quebec(Fig. l). The occunenceofdumortierite sheds light on the locally A dumortierite-bearing,aluminosilicate-rich zone of intense hydrothermal transformations that affected hydrothermally altered rocks has been found in the meta-igneousrocks in partsof this belt. Our objectives Abitibi greenstonebelt, in the northeasternsector of in this paperare l) to characterizethe dumortierite,frst 138 TIIE CANADIAN MINERALOGIST Ftc. l. Simplified geologicalmap of the Val d'or mining district, showinglocation of the dumonierite-bearingzone and of the areacovered by Figure 2. The mapis modifiedfrom a compilationof the Ministbrede l'Energie et desRessources du Qu6bec, andImreh (1984). notedin a fine-grainedassemblage ofalteration products Sharpe(1968), Dimroth et al. (1982,1983),and H6bert byTaner(1984),2)to examineitsrelationship with other et al. (1991). aluminosilicatephases, and 3) to exploreis possible associationwith Au- and Cu-bearingfluids responsible llcer Gsorocv for mineralizationin this part of the Abitibi belt. Severalgeological units can be recognizednear t}re Gsot-ocrcnt.Snrmrc dumortierite-bearingzone (Fig. 2); undifferentiated mafic to felsic rocks of the CentralPyroclastic Belt of The dumortierite-bearingzone is located in the Sharpe(1968) are the most widespread.These rocks extremesoutheastern part ofthe Abitibi greenstonebelt, consistof l) a complexsequence ofdiorites, which host in the SuperiorProvince of CanadianShield, about 28 the dumortierite-bearingaluminosilicate-rich assem- km eastof Val d'Or, nearthe junction of Highways1 17 blage,a largesericite-enriched zone (Fig. 2), anda facies and 113 (Figs. 1, 2). The affectedrocks are considered containing "opalescent"quartz ooeyes",such as recog- ultimately to havebeen volcanic in origin. nized in the Dumagami mine area by Marquis er aL The geologyof the Val d'Or miningdistrict (Fig. l) (1990),2) undifferentiatedmalic to felsicvolcanic rocks is dominated by suites of fuchean mafic to felsic of the Malartic Group, 3) a horizon of mafic volcanic volcanicrocks, including pyroclastic facies; the major rocks containing disseminatedmagnetite, 4) horn- cyclesof volcanicactivity were separated by periodsof blende-rich diorite porphyry, similar to the diorite clastic sedimentation.The volcanic sequenceswere porphyry of the Annamaqueshowing (Fig. 1), and 5) a intrudedby severalintermediate to felsic syntectonic horizon of chloritoid-bearingmetavolcanic rocks. and late-tectonicplutons of trondhjemitic to tonalitic The Bevcon pluton consists of tonalite and character, of which the Bevcon pluton @g. 2) is trondhjemite,typical of the intrusive complexesin the consideredtypical. All volcanic and sedimentaryrocks Val d'Orminingdistrict.It is stronglyaltered andlocally havebeen metamorphosed, generally to the greenschist deformed;silicification and albitizationhave led to the facies,and locally deformedalong numerouszones of developmentof a micrographicintergrowth of quartz mylonitization. Further information on the geological and albite (Ianer 1990).The pluton hoststwo gold setting of igneousactivity in the area is provided by deposits(Bevcon and Buffadisson, both former produc- DUMORTIERITE IN AN ALUMINOSILICAIE-RICH ALTERATION ZONE 139 0 q 2.05o/oCu,6g/t Ag { cHLoRrrolD-EEARING f,fl orrelse ovxe El -----'FAUC| [! rorurre-rnoxorrJEMtrE Sl courlex DronrrcaEolENcE O MINERALIZATION zoNE (aEBlclrE) bjal HyDRoTHERMALLyALTEFED ALUMNoalLlcarE zoNE SHAFT l=l DUMoRllEn|TE-BEARINO Z urotrrsnelnArED MAFrc ro FELsrc volcAilrc BocKg uxe I @ sunentsr DronrrEpoRpHyRy ffi "onreatnot-Frcx v,^ 53115!;1""^' DlsgEMtNAtED MAoNETTTE-BEARTNoMAFrc voLcANtc RocKg =l oeaano ro aRAxopHyRE (vrcouR arLL) ffi Frc. 2. Schematiclocal geology,from field observationsand interpreted from aeromagnetic survey(Taner 1990). ers) near its highly shearednofihem border (Sauv6 distributed ovet abotrt2}Eoof the area.The intensityof 198s). the color varieswith the proportionof dumortierite.The To the south of the dumortierite-bearingzone, the regionally developedschistosity strikes roughly east- Vicourgabbroicsill, whichhoststhe Sigma-2 goldmine, w6st anddips steeplyto the north. The axial plane of has local granophyricsegregations (Hdben et al. 1988' minor foldJ parallels the schistosity.Deformation af- 1991).This sill andthe map-unitsmentioned above are fecteda dumortierite-bearingfelsic dyke whosestrike is cut by relatively undeformeddiabase dykes. perpendicular- to the schistositY The dumortierite-bearingschist consistsof alternat- ing grey andalusite-richand white quartz-rich bands THa DuMonrnnrrE-BEARn{cZol.IE (Fig. ). The grainsof andalusite,0.5to 0.8mm across' locally containeuhedral inclusions of dumortierite,with The dumortierite-bearingrock, an andalusite- du- rare rutile. Where enrichedin dumortierite,the rock is mortierite- rutile quartziticschist, with localkyanite and pinkish and generallycontains 607o qiwu, up to 20Vo pyrophyllite, is unevenly developedover an area of andalusite,up to 207o dumortierite, with 2Vo ruttle' about 180x 300 m to the southof the pluton. The same sericite and rare epidote.Zircon and apatiteare acces- lithological unit wasencountered in drill coreat a depth sory phases.Kyanite occursexclusively associated with of.2i75m; on this basis,the map-unitis inferredto dip quartzin veins. Pyrophyllite is found in the samearea, 40o to the north (Fig. 2), and seemsto have a roughly in dumortierite-freerocks. constantthickness. The outcropsare mottled (Fig. 3)' The pinkish schist is presentas "knots" that range with the strikingly pinkish dumortierite-bearingrocks from l0 to 40 cm in diameter(Fig. 3)' andas conform- r40 THE CANADIAN MINERALOGIST Flc. 3. Main dumortierite-richourcrop. The purplishpink color variesin intensity with abundanceof dumortierite. Ftc. 4. Andalusite(An) phenoblastwith inclusionsofthe euhedral,rectangular dumortierite @u) and rutile (Ru) grains (opaque).We interpret this as rextural evidenceof the replacementof andalusiteby dumortierite. DUMORTIERITE IN AN ALI]MINOSILICATE.RICH ALTERATION ZONE t4l ablelayers up to onecm thick. Thepresence of deformed (estimated),y 1.686(2). These values are somewhat paramorphsafter p-quartz, with the amoeboidshape lowerthanthose given
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