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MINERALOGICAL AND CHEMICAL VARIATIONS IN CHRYSOTILE

VIEWS: 8 PAGES: 11

									 Canadian Mineralogist
 Yol.22, pp. 173-183 (1984)



     MINERALOGICALAND CHEMICALVARIATIONSIN CHRYSOTILE
                                                    VEINS
AND PERIDOTITE
             HOST-ROCKSFROMTHE ASBESTOSBELTOF SOUTHERN OUEBEC*

                                                   ERSEN COGULU
                      Department Geology,University Ottawa,Ottawa,Ontario KIN 6Ns
                               of                  ol
                                                  ROGER LAURENT
                        Ddpartement Gdologie,
                                  de                 Laval, Qudbec,
                                            Universitd                 GIK 7P4
                                                                  Qudbec


                       ABSTRACT                                pendantI'6pisodecontinental. II pr€sente      toutes les tran-
                                                                sitionsentrela p6ridotite saineet la veinede chrysotile.Dans
   Two types of serpentinites    from the Thetford Mines -      cettetransition, on observeles variations min€ralogiques
Black Lake areawerestudied:the first was selected        from   et chimiques             (l)
                                                                              suivantes: I'olivine esttransform6e     soit en
the Lake Asbestosmine and the second,from the Vimy              lizardite, soit en chrysotiledanslesmicroveines la p6ri-
                                                                                                                  de
Ridge deposit.The former, which has not undergonethe dotite. (2) La p6ridotite se trouve serpentinisee             dans une
oceanicepisodeof pervasiveserpentinization,displaysa dtroitezoneentourantlesveinesde chrysotile.(3) Les rap-
completetransition from fresh peridotite to serpentinite ports Mg/(Fe + Mg) and Mg/Si varient commesuit: a) ils
associated   with veins of chrysotileasbestos.   The latter is croissent dans les microveines de lizardite, et b)
tlpical ribbon-veinedasbestos     ore; the hosr peridotitehas Mg/(Fe + Mg) tend ir restercontant dansla zoneserpenti-
undergoneserpentinization      during both oceanicand con- nis6e,      tandis que MglSi diminue sensiblement. 6chan-
                                                                                                                 Les
tinental episodes. Samples were studied under the tillons de Vimy Ridgepr6sentent veinesrubanees    des                 typi
microscopeand analyzedby X-ray diffraction and electron ques.Cesrochesont 6t6 serpentinis6es              pendantles 6piso-
microprobe. In the Lake Asbestossamplethe following desoc6anique continental.Ellesmontrent lesvariations
                                                                                 et
variations can be seenasthe chrysotile asbestos    vein is ap- suivantes:(l) les rapports Mg,z(Fe+Mg) et MglSi crois-
proachedfrom the fresh peridotite: (l) Olivine is transform- sent de la bordure vers le centredesveinesde chrysotile.
ed into either lizardite or chrysotile in microveins in the (2) La roche encaissante fzardite montre de gtandesvaria-
                                                                                         a
fresh peridotite. (2) The peridotite host shows intense tions danslesrapports de Mg/(Fe + Mg) et Mg,/Si, sansrela-
serpentinizationin a narrow zone surrounding the asbestos tion dvidente        avecla distance jusqu'aux parois de la veine.
vein. (3) The Mg/@e+Mg) and Mg/Si ratios vary as
follows: a) they increasein the lizardite microveins of the Mots-cbs: p6ridote, lizardite, chrysotile,magn6tite,bru-
fresh peridotite; b) Mg,z@e Mg) tends to remain constant
                             +                                      cite, contrainte, fracture tensionnelle, veine, serpenti-
in the serpentinizedzone, whereas Mg/Si decreases                   nisation p6netrante, serpentinisation continentale,
significantly. Samplesfrom Vimy Ridge display the follow-           Appalaches    qu6b6coises.
ing variations:(l) Mgl(Fe+ Mg) and Mg,/Siratiosincrease
from the margin to the centrein the vein-filling chrysotile.
(2) The lizardite wall-rock of the chrysotjleveins showsma-                           INTRoDUcTIoN
jor variations in Mg/(Fe + Mg) and MglSi, apparently not
related to distancefrom the vein.                                  The chrysotile asbestos deposits of Quebec are
                                                               associated with serpentinized ultramafic rocks form-
Keywords: peridotite, lizardite, chrysotile, magnetite,
    brucite, stress,                                           ing the lower unit of ophiolites in the Eastern
                    tensionfracture,vein, pervasive   serpen-
    tinization, continental serpentinization, Quebec Ap-       Townships of Quebec. The principal asbestos
    palachians.                                                deposits are located at Thetford Mines, Black Lake
                                                               and Asbestos (Fie. l).
                                                                   The following characteristics are common to all
                         SoMvrerns                             these deposits (Cooke 1937, Riordon 1975,Lamar-
                                                               che & Wicks 1975). (1) They are situated near the
   Deux types de serpentinites la r6gion de Thetford lower tectonic contact of peridotite with Cambrian
                                   de
Mines - Black Lake ont €td €tudi6ssousle microscopeet country rocks. (2) Faults, shear-zones and fractures
analys€spar diffraction X et par microsonde6lectronique. control the location of asbestos veins within the
Le premier a 6t€ €chantillonn6 d la mine du lac d'Amiante      peridotites. (3) Sheets of granitic rocks are present
et le seconddansla rdgion de Vimy Ridge. L'dchantillon
                                                                                     vicinity of the deposits. (4) The
reprdsentatif de la mine du lac d'Amiante a 6chapp6d la within or in the
serpentinization ocdanique,et a 6td serpentinis6seulement peridotite host-rocks underwent two distinct episodes
                                                               of serpentinization.
                                                                   The ophiolites of the Appalachians occur in a nar-
 *Publication 2G83, Ottawa-CarletonCenuefor Geoscience row structural belt extending from Baie Verte (New-
 Studies.                                                      foundland) to Brompton Lake (southern Quebec).
                                                           173
t74                                  THE CANADIAN MINERALOGIST




                                                   "'aa.----_




                                                             depositsin southern Quebec.
                    Frc. l. Location of the principal asbestos


They representparts of slicesof upper mantle and        beenpreserved    from the pervasiveserpentinization
Proto-Atlantic oceanic crust obducted onto the          of the oceanic episode. The Lake Asbestos and
North American plate during the Lower of Middle         British Canadian depositsof Black Lake locally con-
Ordovician (Church & Stevens    1971,Laurent 1975,      tain fresh peridotite cut by narrow serpentinized
1979).During their evolution the ultramafic rocks       zonesthat encase    veins of chrysotileasbestos   @ig.
were subjected to serpentinization in two main          2). This particular occurrenceprovides the oppor-
episodes: early episodewhen they were still part
          an                                            tunity for a detailed study of the genesisof the
ofthe oceanfloor and a later episode whenthey were      chrysotile asbestos  vein-association the absence
                                                                                             in             of
incorporatedinto the North American plate (Laurent      effects of the first, pervasiveserpentinization.  The
1975,Laurent & H6bert 1979).                            serpentinetextures and mineral associations    produc-
   The oceanicepisodetook placeunder conditions         ed by pervasiveserpentinizationof the peridotite
of relatively low fugacity of oxygen and at             have been describedin detail by Cressey(1979),
temperatures  lower than 340" C, giving riseto a per-   Wicks & Plant (1979),   Wicks&Whittaker (1977)     and
vasiveserpentinization  essentiallycharacterized   by   Wicks e/ al. (1977), but the textures and mineral
the pseudomorphicreplacement olivine and ortho-
                                of                      associations the chrysotile asbestos
                                                                     of                         veins havenot
pyroxene by lizardite t chrysotile and magnetite        beenas extensively  studied.The purposeofthis paper
(Laurent & H6bert 1979). The later, continental         is to describe and interpret the mineralogical and
episodeoccurredwhen the ophiolites weretectonical-      chemicalvariations of asbestos    veins and adjacent
ly emplaced their presentposition. During this late
            in                                          host-rocksproducedduring the singleepisodeof for-
episode commercialveins of chrysotileasbestos
        the                                             mation of chrysotile ore. In order to do this, we have
were formed. Oxygen-rich meteoric waters, perhaps       chosento study samples fresh peridotite bearing
                                                                                   of
mixed with connatewater, played an important role       chrysotile veins and the associatedserpentinized
during the secondepisodeof serpentinization,which       zone. This report is a sunrmtuyof our preliminary
hasoverprintedthe first. Serpentine  mineralsof the     work.
 oceanicepisodewerereworkedwhile chrysotilefibres
grew in dilation fractures and tension gashes  filled   Location of Samples
by serpentine-saturated  solutions.
   Someperidotite bodies contain zonesthat have                          samplefrom the Lake Asbestos
                                                          A representative
                             VARIATIONS IN CHRYSOTILE VEINS, SOUTHERN QUEBEC                                      175




FIo.2. Freshperidotite containing chrysotile asbestosveins with enclosingserpentinizedzone. Iake Asbestosmine, Black
   Lake. FP fresh peridotite, SZ serpentinized                                                                   5
                                               zone, CVI longitudinal vein, CV2 diagonal vein. Scalebar represents cm.



deposit(SE pit, elevation46 m; mine co-ordinates:           examinedper analysis.The data were correctedus-
N46.015, 871.2?n) contains fresh peridotite and             ing a ZAF program. Replicateanalyses standards
                                                                                                    of
chrysotileveinssurroundedby a serpentinized      zone       indicatethat the determinationshave a relative ac-
(SZ, Fig. 2). The chrysotileveinsoccupythe central          curacy of I I to 20/o for major elements.The
part ofthe serpentinized   zone.They fill a stockwork       serpentine-group minerals were identified in-
of longitudinal, crossand diagonaljoints, of which          dependentlyusing a Gandolfi 114.6-mm-diameter
longitudinal veins are the thickest and may reach 3         armera with nickel-filtered CuKa radiation. Six com-
cm across(CVl, Fig. 2). Diagonal veins (CV2) are            positionsgiven on Table I were determinedwith a
of medium thickness(0.5 cm), whereascrossveins              CambridgeMark V electronmicroprobe (Carleton
(CV3; seeFig. 4) are the thinnest (lessthan I mm).          University, Ottawa). Specimens  wereanalyzed    with
The latter are perpendicularto the longitudinal veins       15 kV acceleration potential and a beam current of
and only occur within the serpentinized     zone asso-      50 nA as measuredon pure iron. Raw X-ray data
ciatedwith the largeveins.The contact of the veins          were reduced using the computer program of
of chrysotile asbestos  with the wall rock is sharp,        Rucklidgeand casparrini (1969).Measuredmajor
although not always linear. Veins of picrolite are          elementsare accurateto I to 2o/0,
sometimes   presentwithin the longitudinal veins of           The terminology used in this paper relating to
chrysotile. Two samples from the Vimy Ridge                 serpentinizationfollows that of Wicks e/ al, (1977)
deposit(mine co-ordinates:N46.004,E 7 1,250)are             and Wicks & Whittaker (1977).
typical ribbon-veined asbestosore (Cooke 1937,
Riordon 1975).They contain setsof simple, paral-                                 MINERALOGY
lel veins of chrysotile, the width of the veins vary-
ing betweenI and 5 mm. The host peridotite has              The Loke Asbestos deposit
undergoneserpentinization    during both the oceanic
and continental episodes.                              The host peridotite has a harzburgitic composi-
                                                    tion, with olivine (65-9590),orthopyroxene  (5-3590)
Methods of Study                                    and chromian spinel (lessthan l9o). The represen-
                                                    tative sampleof this depositcontainsall transitions
    Samples weresfudiedpetrographicallyand analyzed from fresh peridotite to completely serpentinized
by X-ray powder diffraction and electron peridotite with associated           veinsof chrysotile asbestos
microprobe. The powder-diffraction method of iden- (Fre. 2). The dark olive-greenfresh peridotite is easily
tification used for serpentineminerals was that of distinguished from the strongly serpentinized
Whittaker & Zussman(1956).Microprobe analyses yellowish green zone surrounding the veins. The
were done with an ARL electronmicroprobe (EMX- thickness the serpentinized
                                                              of                  zonevariesin propor-
SM121000)    at20 kV and 10pA usingnatural olivine tion to the thicknessof the chrysotilevein, the ratio
and diopsidestandards   providedby the Geophysical of the thicknessof the serpentinized  zoneto that of
Laboratory, CarnegieInstitution of Washington. In- the vein varying between8 and 3. In the represen-
tegration time was ten seconds,and four points were tative sample of the Lake Asbestos deposit, the
t76                                                  THE CANADIAN MINERALOGIST


            TABLE1.    LIZAiDITE IiIICROVEINS FRESH
                                            III    PERIDOTITES,             replacement.    The former type showssharp and linear
                            LA(E ASBESrcS DEMStr*
                                                                            contactsand cuts acrossthe olivine grain-boundaries;
              12345.57                                       I
sl02        42.36 40.54      q,20 41.17 41.29 42.1 40.17 4 . 0              thesemicroveinsare also parallel to the main vein
Alz0g        0.40 0.37        0.40 0.35 0.42 -       0.?2                   CVI and are filled by ribbon-type lizardite along the
lilS0       40.78 41.01      40.97 40.81 40.47 43.4 41.268 ci.g
                                                     ',t.3
Foo          1.94 r.57        1.62 1.67 r.53 1.40
                                                            l.,o
                                                                            margin and magnetiteat the vein centre (RL, Fig.
Cr20s        0.00 0.00        0.01 0.02 0.10 -       0,m
llno         0.06 0.06        0.06 0.04 0.07 0.I     0.05                   3a). Replacement     veins have serratedmargins and
Nt0          0.34 0.41        0.25 0.20 0.42 -       0.26
                                                                            display a gradual replacement olivine by serpen-
                                                                                                               of
             85.88 83.96 83.51 84.26 84.31 87.0 83.47 87.2
loal
                                                                            tine. There are two systems serratedveins, both
                                                                                                            of
us/(Fef,ls) 0,976 0.980 0.980 0.979 0.981 0.982 0.982 0.984
t'lg/sl     1.44 1.5I   1.52 1.48 1,47 1.54 r.54      r.56
                                                                            diagonal with respectto the main vein (CVl). System
*   Seples are aranged ln orier of dsruslng     dlstane frcD the ve{n
                                                                            I (Liz Vl) consists of simple lizardite replacing
    (see Flgs. 4 and 5). The sjdbol '-' slgnlfles 'belfi deteqtlm llnlt!.   olivine; it cuts the secondsystem2 (Liz Y2) at an
    Analyses I, 2, 3, 4, 5, 7 rere done on C@brldge librk V nlcrcprcbe
    (Carleton UnlveElty,  ottara), and analyses 6 ud 8 m the ARL nlcrc-     angleof 40-45o.The veins of the latter are thicker
    prcbe (unlveElt€ Laval, Qudbr).
                                                                            and zoned, with a central zone of chrysotile
                                                                            developedby replacementof the lizardite forming
                                                                            the border zone (Figs. 3a, 4). The contact between
thicknessof the serpentinized    zoneis 70 mm (twice                        thesetwo serpentine     mineralsis serrated.The veins
35 mm); the thicknessof the longitudinal chrysotile                         of the systemLizY2 areparallel to the diagonalveins
vein is 18mm; the ratio serpentinized   zone,/chrysotile                    of chrysotile (CY2).
vein is 3.89. The total volume of the serpentinized                            In somecases serratedveins of chrysotileare
                                                                                               the
zone has been increased by 250/oowing to the                                observedin fresh peridotite. Theseveins are also
developmenlof the chrysotile asbestos       vein.                                                                      of
                                                                            zoned,with a marginalzonecomposed chrysotile
   According to Cooke (1937),this ratio is relatively                       replacingthe olivine wall (Fig. 3b). The centralzone
constant in a given orebody, but varies from one                            is filled with crossfibres of chrysotileand magnetite.
orebody to the next (seeTables VII to X, Cooke                              The contact betweenthe marginal and central zones
1937).Our petrographic                    that
                           work suggests the ratio                          is sharp.
is mainly controlled by the degreeof serpentiniza-                             Peridotite gradesinto serpentinite    through a nar-
tion of the serpentinized   zoneadjacentto the vein.                        row transition zoneaveraging1 mm in thickness.The
Wherethis zoneis wholly serpentinized, ratio is
                                            the                             mesh texture is characteristicof this transition;
at a minimum (between and 4). Where the degree
                           3                                                otvine relics are surroundedby lizardite or chrysotile
of serpentinization                            the
                     ofthis zone decreases, ratio                           rims (Fig. 3c). The serpentinizedzone    consistsmainly
of the thicknessof the serpentinized    zoneto that of                      of lizardite, replacement chrysotile, brucite and
the chrysotilevein increases to 8 or more. How-
                               up                                           magnetite.
ever, the thickness of the vein can also vary in-                              The hourglass texture dominates in the ser-
dependentlyfrom that of the serpentinizedzone,                              pentinized zone; however, it is commonly over-
which indicatesthat the development a vein is con-
                                       of                                   printed by the replacementof lizardite by either
trolled by more than one factor. The stress    field and                    chrysotile or brucite and magnetite.The replacement
the chemicalparameters the environmentof for-
                           of                                               chrysotile has developedalong and oriented roughly
mation are considered be important in this respect.
                        to                                                  normal to microfractures, and produces a
Thosepoints cannot be handled statisticallyat this                          microscopic    vein with sharplyserrated  margins@ig.
stageof our work because cannot determineac-
                             we                                             3d). The microfracturesare irregular and tightly clos-
curately in the field the degreeof serpentinization                         ed, with no serpentine    developed the plane ofthe
                                                                                                                  in
of the serpentinized zonesadjacentto the veins. This                        fracture. Wherethe microfracturesare closelyspac-
is the main reasonwhy we selected suite of samples;
                                     a                                                                            of
                                                                            ed, almost completereplacement the lizardite oc-
we describein detail a repre$entative  exampleof that                       curs, which givesrise to a variety of massive of  ore
suite.                                                                      chrysotile asbestos    called mass fibre. Close to the
   Two types of chrysotile veins are distinguishedon                        main veinsof chrysotile, the microfractures are com-
the basisof their internal structure:simple or com-                         monly orientedparallel to veins, and the wall rock
posite.The simpletype consists a singlevein com-
                                  of                                        is completely transformed into replacement
posedof chrysotilefibres that are continuousfrom                            chrysotile.
wall to wall. Compositeveins consistof groups of                               In the chrysotileasbestos  veins,the growth of cross
several  veinsvarying in compositionand structure.                          fibres is uninterrupted from one wall to the other.
There are compositeveins composedof chrysotile                              Fibrous magnetitewas formed parallel to the chrys-
asbestos  with picrolite, the latter forming bands or                       otile fibres. In somecases magnetitecrystalsform
                                                                                                        the
lenses  parallel to walls. Another type of composite                        cone-shaped     groups whosetops lie at the centreof
vein contains veins of magaetite and brucite that cut                       the veins;in other cases,  magnetiteconesextendfrom
obliquely acrossveins of chrysotile fibres.                                 onewall to the other (Fig. 3e).Under the microscope
    Petrographicexaminationof the fresh peridotite                          the picrolite displaysone oftwo different textures,
reveals  two typesof microveins:fracture filling and                        crlptocrystalline or banded.X-ray powder-diffrac-
                             VARIATIONS IN CHRYSOTILE VEINS, SOUTHERN QUEBEC                                    177




Frc.3. a) Lizarditemicroveins freshperidotite.RL: ribbon lizardite.Liz Vl diagonalvein of systeml;Luy2 diagonal
                              in
   vein of system2. Lake Asbestosmine. Polarizedlight. Scalebar represents mm, b) Senatci chrysotilevein replacing
                                                                          0.4
   olivine. The cenfralpart of the vein consists chrysotileand magnetitefibres. British Canadianmine, Black Lake.
                                                of
   Polarizedlight. Bar scalerepresents mm. c) Meshtexturein the transitionzone.Meshcentreof olivine surrounded
                                       0.2
   by meshrim of lizardite. Lake Asbestosmine. Polarized light. Bar scalerepresents mm. d) Serratedveins of
                                                                                       0.4
   replacement  chrysotilein the serpentinized zone. Replacement  chrysotilewas partly replacedby later brucite and
   magnetite.Lake Asbestosmine, Polarized light. Bar scalerepresents mm. e) Compositevein. Right-hand side:
                                                                        I
   magnetiteand chrysotilefibres. Left-hand side: bandedpicrolite. Lake Asbestosmine. polarized light. Bar scale
   represent mm. f) Compositevein. Chrysotile-magnetite
             I                                             fibres (upperright and lower left). Bruciteand nonfibrous
   magnetite-bearing  vein (centre).Lake Asbestosmine. Polarized light. Bar scalerepresents mm. 4


tion analysis reveals that the cryptocrystalline posite veins the contact betweenveins of magnetite-
picrolite is composedof lizardite, and the banded brucite and of chrysotileasbestos very sharp@ig.
                                                                                  is
picrolite, which is madeup of ,,apparentfibres,' of 30.
Tserpentine lying perpendicularto the banding @ig.
3e), is composedof clinochrysotile. In deformed The Vimy Ridge deposit
compositeveinsthe picrolite veinsare bent or broken
and, in someca$es,   form en dchelonlenses. com-
                                           In        Samplesfrom the Vimy Ridge deposit contain
178                                                    THE CANADIAN MINERALOGIST

                                  IT
                           LArc ASBESTOS
                                                   ZONE.
                 TASIT2. LIZAROIIT THESTRPSIIIIIIZSO
                                      DEPOSITT
                                                                              (l) the Me/Ge+Mg) and Mg/Si ratios of the lizard-
                'lz                       45             67            I
                                                                              ite microveinsin the fresh peridotite, the lizardite and
                                                                              the replacement    chrysotilein the serpentinized   zone
stq            43.2   42.8 a ;          42,0   42,3    43.2 42,4
Alz0s
llg0           qr.o   q'it
                            0.4          0.3
                                        39.6
                                                0.1
                                               39.1
                                                        0.1  1.0
                                                       38.9 38.5      qz:,a   all are distinctly different @igs. 4' 5). Thus' the
                            't.0
Fd              1.2    T.0              r:r    r:o      1.0 1.0
                                                         -
                                                                      t:t     chemicaldata confirm that thesephasesconstitute
llno            0.1    0.1  0.1                              0.1
                                                                     w.7
                                                                              distinct paragenetic groups. (2) The following
Total          85.5 85.6                &3.0   82.5    83,2 83.1
                                                                              changes   occur as the chrysotileasbestos     vein is ap-
i{o/(Fe[tq] 0.985 0.986 0.985 0.986 0.986 0.986 0.986 0.986
iltsr
         -'
            l.4l  r.4s  1.37 1.41 1.38 1.35 1.36 1.45                         proached from point A within the wall rock: a)
                                                                              Mg/(Fe+Mg) and Me/Si increasein the lizardite
     (seir Flgs. 4 ild 5). Analyses
     nlcrcprcbe  (Unlvereltd
                                   rere donem the ARL
                            Laval' Ql6ec).
                                                     elstrcn
                                                                              microveinsof the freshperidotite(Figs.4, 5). b) The
                                                                              ratio Mg/(Fe + Mg) tendsto remain constantin the
                                                                              lizardite of the intenselyserpentinized  zone(SZ) (Fig.
           TABLE REPLACB,IENI
               3.                 lN THE
                          CHRYSoTILE                Zol{E'
                                        SERPEIIIINIZED
                                                                              4). c) The sameratio remainsconstantin the replace-
                      LAKE       DEPOS1TT
                           ASBESTOS                                            ment chrysotileof the SZ (Fig. 4). d) The ratio Mg/Si
                I      2345                              674                  tendsto decrease the lizardite and the replacement
                                                                                                  in
s t02
Al203
               43.0   44.4 44,1         43.1   41.6    4't.9 4Z.O
                                                        0.4
                                                                      43.7     chrysotile of the SZ (Fie. 5). e) Each petrogenetic
ilgo           43:r   qzlo      4z'.2   soi    3s-.2   39.2 39.2      41.8
                                                                       0.9
                                                                               group of serpentine chemicallydistinct from the
                                                                                                     is
Fe0                    0.9       0.9     0.9    0.9     0.9   0.9
tln0            0.1    0.1       0.1                    0.1 0.1                othersin term of their respective   Mg:Fe:Si propor-
tftal          87.I   8 7. 4    87.3    U.7    81.7    82.5   52.2    86.4     tions, as shown on a variation diagram of Mg/Si ver-
Mg/ (Ferfig)   0.990 0.990 0.990 0.990 0.990 0.990 0.990 0.990                 sas Ms/(Fe+Mg) (Fig. 6).
ilg/sl         1.50 1.M    1.42 1.46 1.40 1.39 1.39 1.42
r    sanples are arrsnged ln order of decreaslng distan@ fr@ the veln
     (see Flgs. 4 ond 5). Analyses tere done on the ARl- electrcn nlcrc-       Vimy Ridge
     probe (unlYe6lt€ Loval, Qrdb4).

                                                                                 Microprobe analyses  wereperformed at the margin
                                                                              and centreof the asbestos   chrysotileveinsaswell as
numerousparallel simpleveins.Microscopeex€unina-                              in the lizardite wall-rock. Data are given in Tables
tion revealsthat the wall rock betweenchrysotile                              4 and 5 and in Figure 6. Thesedata indicate that:
asbestos  veins was completelyserpentinized.     Rock                         a) In vein-filling chrysotile, the Mg/(Fe+Me) and
texturesare similar to thoseobserved the serpen-
                                        in                                    Mg,/Si ratios increase from the margin to the centre
tinized zone of Figure 2, but the main constituents                           (Fig. 6). This trend is clearly shown by the veins 3
are lizardite, bastite and magnetite.The hourglass                            and 4. which have a thicknessof about 5 mm. A
texture is dominant, and serratedveinsare commonly                            similar trend in Mg/Si is apparent for veins I and
observed.   The relict textureof serratedreplacement-                         2, which arethinner. In this case points analyzed
                                                                                                                 the
chrysotilewas transformed almost completelyinto                               with the microprobe wereseparated no more than
                                                                                                                    by
lizardite during the secondepisodeof serpentiniza-                            0.5 mm. The observedtrend is analogousto the
tion. Pyroxenemay be pseudomorphically        replaced                        developmentstagessincethe chrysotile fibres started
by either lizardite or chlorite. Magnetite forms either                       forming at the margin (M) and grew toward the cen-
subhedral disseminatedgrains or a vein-filling.                               tre (C). The changein chemicalcomposition appears
Sharp-walledasbestos      veins commonly cut bastite                          to be regular and identical in veins 3 and 4 of sam-
and lizardite grains into two halves that can be                               ple 114779 and in veins I and2 of sample3828772.
observedon both sidesof the veins. The growth of                              b) The lizardite forming the wall rock of the
chrysotile is uninterrupted betweenthe two walls.                              chrysotile veins shows major variations in
Fibrous magnetite was in some instancesformed                                 Mg/(Fe+Mg) and Me/Si (Fig. 6), but thesevaria-
parallel to chrysotile.                                                       tions are apparently not related to the distance
                                                                               separatingthe lizardite from the chrysotilevein. c)
                               Curulcar Dere                                   The values of Mg,/(Fe+ Mg) in the vein-filling
                                                                               chrysotileat Vimy Ridgearemuch lower than those
 Lakes Asbestos                                                                of the replacementchrysotile in the SZ aI Lake
                                                                               Asbestos.Values of the Mg/Si ratio are similar.
      Systematic   microprobeanalyses  weredoneon the
    sampleshown in Figures2 and 4 betweenpoints A                                       DISCUSSION AND INTERPRETATION
    and B, wherea completetransition betweenthe fresh
    peridotite to completelyserpentinized  peridotite with                       Field observationsand structural studiesof the
    associated  chrysotileasbestosveinsis displayed.Data                       chrysotile veins in the asbestosbelt of southern
    are given in Tables I to 3.                                                Quebec  indicatethat they fill a stockwork of expan-
      The cation Mg/(Fe+Mg) and Mg/Si ratios (Figs.                            sion fractures(Cooke 1937,Riordon 1955,Riordon
    4, 5,6) were calculated the basisof the major
                              on                                               & Lalibert6 1972,Laurerr/.. 1975,Laurent & Hdbert
    oxide data (Tables I to 5). Thesevaluesindicate that                       1979). The material cementingthe fractures and
                                                                                                               179




                                                               + Replocanent drysotile
                                               r+ rt+i           in the serpentiniad
                                                                                   zone

                                                               r   Uzordite the
                                                                           in
                                                                   serpentinized
                                                                               zone
               o.985
                                                               x   Uzordite microrcins in
    Mg/Fe+ Mg                                                      fhe fresh peridotite
               o.980




                                       from the vein (cm)
                                distonce
FIc. 4. Variation diagram showing Me/Fe + Mg) yersasdistancefrom the longitudinal vein of chrysotile. Representative
   sampleof the Lake Asbestosmine (seeFigure 2).



forming the chrysotile veins has a similar chemical       previously by Lamarche & Wicks (1975), Wicks
composition to the host rock and presumablywas            (1979)and Wicks & Plant (1979).Lizardite,replace-
derivedby solution or diffusion from the wall rock.       ment chrysotile, magnetite and brucite make up the
As the processof serpentinizationis promoted by           bulk of the strongly serpentinized zone adjacentto
a highly mobile fluid phase,there is a clear poten-       the chrysotileasbestos  veins.The asbestos veinsare
tial here for mass transfer; the development of           usually composedof chrysotile and fibrous magnet-
chrysotileveinsillustratesthe mobility of the major       ite. Brucite is very rare exceptin compositeveins,
elements  Mg, Si and Fe during serpentinization.ln        where it is associated  with nonfibrous magnetite.
this study, we have attempted to document the main        This observationhasan obviousbearingon the prob-
stepsof this mobilization.                                lem of masstransfer. It documentsdepletionof Si
   The paragenetic mineral assemblages the wall
                                         of                                  of           of
                                                          and the existence an excess Mg and Fe in the
rock and asbestosvein are similar and mainly con-         systemat a relatively late stageof the serpentiniza-
sist of serpentine, magnetite and brucite. This           tion process.         Mg
                                                                         Excess is taken up by brucite, and
similarity in mineral compositionis bonsistent with       excess by magnetite.
                                                                 Fe,
synchronousformation and closecogeneticrelation-             Has all eassss msfilized Mg and Fe finally been
ship of the two assemblages, has beenpointed out
                            as                            precipitated and kept in the system,or have substan-
180                                   THE CANADIAN MINERALOGIST




                                                                  + Roplacemont chtyaotllc
                                                                    in tho acrpontiniz€d zono

                                                                   a Lizardlto inth6
                                                                     acrpcntlnlrod zono

                                                                   x Llzardltc mlcrovelna
                                                                     in lhe lrcah peridollto

                          Dlst6n6 lrom tho voln (cm)
Frc.5. Variation diagramshowingMg/Si rersa; distancefrom the longitudinal vein of chrysotile,Lake Asbestosmine
   (seeFigure 2).



tial amounts of Mg and Fe been removed from the         peridotite and the sum of the R2* cations in the
system? The preciseamount of brucite and magnetite      rock-forming olivine and orthopyroxene. This
formed during serpentinizationcannot be ascertained     theoreticalconsiderationand the late occlurenceand
becauseof their irregular distribution, so that it is   irregular distribution of brucite and magnetitein wall
not possible to make direct mass-balancecalcula-        rocks and veins strongly suggest    that an averageloss
tions. Another approach is possible on the basis of     is around l0 (oxide volume) 9o of MgO and FeO,
volume changes.The volume occupied by a vein is         which have been removed from the system. The
equal to the increasein volume of the serpentinized     country rock in contact with peridotite is strongly
zone adjacent to the vein. This measurement in-         chloritized, and this chlorite-bearingreaction zone
dicate volume increasesvarying between l0 and           could be a by-product of the transfer in solution of
3590.In the caseof the samplefrom Lake Asbestos         Mg and Fe from the peridotite into the nearby silica-
mines, the increasein volume is 25t/o. Calculations     rich rocks.
predict volume increasesof about 4090 when the             At the sample scale, the microprobe work
serpentinization reaction produces chrysotile (88       documents the main stages of mobilization and
volume 9o) and magnetite (120/o);    the increaseis     definesthe chemicaltrends. Embryonic serpentiniza-
about 5090when chrysotile (82V0)and brucite (l8Vo)      tion starts with the development of lizardite and
are produced. When both magnetile and brucite are       magnetitemicroveins in the fresh peridotite. Within
producedby serpentinization,the increase volume
                                          in            the strongly serpentinized zone adjacent to a
must vary between 40 and 5090. There is a large         chrysotileasbestos   vein, lizardite developsextensively
discrepancy between the observed and calculated          from the lattice of microveinspreviously formed.
changa in volume. The observedchanges volume
                                          in             Replacement    chrysotile and magnetiteconstitute the
are not sufficiently large, even when one takes into     following phase; finally, brucite and magnetite
consideration the orthopyroxene content of the          replace the former minerals in the last phase of
                                             VARIATIONS IN CHRYSOTILE VEINS, SOUTHERN QUEBEC                                                                              181
  M e / 3 1'
     1.OO




                                                         o.e70


                                                                                        xg /rc + ns
                                             VIIIY RIDCE DEPOSTT                                                              L A K E A S B E A T O SD E P O S I T
               E    Chrtsotll.    tltlltlg                             O   Llzardlto   wall   locl                             I    Lkardltr mlc@Elna ln ths
                    Yalna                                                                                                           lbah p.rldotlto

               11   vcln    1 maigln         Ct   vcln    I   c.nlar
                                                                                                                               a Llrordlig ln tho
               l.22C22
                                                                                                                                 aeapanllnircd rom
               X33Ct3
               X44Ca4
                                                                                                                               + RGplaclment ch.yrolll9
                                                                                                                                 In tho aorportlnlzod
                                                                                                                                 t@
Ftc.6. Variation diagram showing Mg/Si rersas Mgl(Fe+Mg).


serpentinizationwithin the wall rock. During this                                            of oceanic serpentinization and was later reserpen-
evolution, the composition of serpentine varies                                              tinized during the late episodethat led to the develop-
systematically  (Figs. 4, 5). The ratio Mg/(Fe + Mg)                                         ment of the chrysotileveins.Multistagelizardite, as
increases from the lizardite microveinsto the replace-                                       defined by our data, has a Mgl(Fe+ Mg) ratio in-
ment chrysotile,whereas    Mg,/Si decreases.
                                           This trend                                        termediatebetweenthat of the Lake Asbestoslizard-
showsthat the Fe content of the successive    serpen-                                        ite and the vein-filling chrysotile.Therefore,the wall-
tine minerals decreases.                                                                     rock lizardite of Vimy Ridge represents      multistage
   This evolution is reproducedeven at the much                                              serpentinization,in contrastto the single-stage lizard-
smallerscaleofsingle crystals,as illustrated by the                                          ite of Lake Asbestos(an observationin agreement
data on the chrysotileveins from Vimy Ridge. The                                             with the model of evolution).
chrysotile fibres of Vimy Ridge have relatively low                                             The processof formation of asbestos initiated
                                                                                                                                         is
valuesof the low Mg/(Fe + Mg) ratio at the begin-                                            by the reaction of water with the olivine and ortho-
ning of their growth (margin), and higher valuesof                                           pyroxeneof the peridotite. Thesemineralsare pro-
the Mg/(Fe+ Mg) ratio in their final stageof growth
(centre).From our microprobe data it is clear that
the tendencyfor Fe to enterinto the serpentinestruc-                                                     4.         ASSESToS
                                                                                                     TABLE CHiYS0TILE                          TIPEVEIilST
                                                                                                                          FILLltlo Vll'rl RIDCf,
ture decreases during serpentinization. This                                                             Voln I   V€ln I   Veln 2   Veln 2   Veln 3    Veln 3   Veln 4   Veln 4
necessarily  leads to Fe enrichment in the residual                                                      ihrcln   Cmtre    tursln   Catre    l{areln   Cqtre    ihrqln   Cqtm
                                                                                         s,loa           40.5     40.3     4t.3     40.7     M,0       44.9     43.?     42.3
fluid during the late stagesof the process.                                              41203            1.2      1.0      1.0      l.l       0.4      0,7      0.5      0.6
                                                                                         t'lg0           39.2     39.3     39.1     39.2     39,2      40.9     40.0     39.7
   The lizardite formrng the wall rock of the Vimy                                       Feo              2.5      2.5      2,4      2.3       2.6      2.6      2.6      2.5
Ridge chrysotileveins showslarge chemicalvaria-                                          irro             0.08     0.m      0.03     0.1       0.05     0.t      0.04     0.04
                                                                                         ilto             0.08     0.1      0.13     0.05
tions @g. 6), with no clear trend with respectto                                         Total           &.63     83.28 84.01 8it.5l         6.25      89.2     86.84 85.4
either mode of genesis the lizardite or distance
                          of                                                             t'lg/(Ferilg)   0.965    0.966    0.969 0.969 0.964 0.967 0.065 0.969
                                                                                                                           '1.41
from the vein contact.This largechemicalvariation                                        ns/sl           1.44 1.46               1.43 1,32 1.36 1.37 lt39
may result from multistage serpentinization.The                                          .    [slns I and 2. fm suple 3829779  Vluy Rldge. VElns3 and 4, fm
                                                                                              snnple 114779 Vlry Rldge. Analysesrere doneq the ARLelstrcn
rock was first pervasively altered during an episode                                          Elcrcprcbe (UnlveFlt6Laval, qu€bec).
t82                                                   THE CANADIAN MINERALOGIST

           TABLE
               5.        LIZARDITE THEWALL
                                 IN       ROCK
                                             OF                       4. Removal of major elements occurred during
                         VIMYRIDGE   VEIN*
                                  TYPE                                serpentinization;Mg, Fe and Si were removed in
                    l23                                       5       solution from the peridotite and precipilatedin the
  SiOu            4l.l       41.3      40.0        435      4J.   I   tension fractures to form fibrous chrysotile asbestos
  Alz0:            0.8        0.9       1.0         0.6      0.7      and magnetite.
  MgO             40.8       40.3      41.3        4 0 .I   40.8
  Feo              2.0        1.8       1.8         4.5               5. Excess Mg and Fe crystallized later to form
  Mno              0.08       0.08      0.05        0.03     0.03     brucite and nonfibrous magnetite-bearing     veins.
  Nio              0.1         -        0.2
  Total:          84.88      84.38 84.38
  M s / ( F e + M g0 . 9 7 4 0 . 9 7 6 0 . 9 7 7
                   )                               U.Y1U    U.YbJ                    ACKNOWLEDGMENTS
                                                   'I
  Ms/si            1.48      1.45 1.54               .37    I .41
  *   Analysesl, 2 and 3 from sarnple3828772 Uiny                        We thank Dr. F.J. Wicks, Royal Ontario Ontario
      Ridge. Analyses4 and 5 from sample114779                        Museum, for constructive comments on this
      Vimy Ridge. Analyseswere done on the ARL                        manuscript, and Dr. B.R. Rust, Universityof Ot-
      eleitron microprobe(Universit6 Laval, Qudbec).
                                                                      tawa, for reviewingthe paper. This study was sup-
                                                                      ported by the grant 48293 from the Natural Sciences
                                                                      and EngineeringResearch     Council of Canadaand
gressively replacedpseudomorphically variable
                                           by                         by a joint grant from the Direction G6n6ralede
amountsof lizardite and chrysostile( + lnrsit. an4                    I'Enseignement   Sup6rieur,Qu6bec.
magnetite).A lattice of interconnected      microveins
oflizardite or chrysotile(or both), as shownby the
Lake Asbestos s€rmple,appeuusto representthe                                             REFERENCES
pathwayof introduction of serpentinizing     solutions,
making possiblechemicaltransfer. The microveins                       CnuncH, W.R. & SrevrNs, R.K. (1971): Early
result from a combination of processes       involving                  Proterozoic ophiolite complexes of the
                                                                        Newfoundland Appalachians as mantle-oceanic
replacement, dissolution and precipitation. The                         crust sequences.J. Geophys. Res.76, 1460-1466.
development the microveinlattice combinedwith
                of
the progressive    replacementof the mineral grains                   CooxE, H.C. (1937): Thetford, Disraeli and eastern
causes systemto expandin volume.
        the                                                             half of Warwick map-areas, Quebec. Geol. Sum.
   The fibrous texture of the chrysotileand magnetite                   Can. Mem,2ll.
results from incrementaldilations in tensilefractures
(Durney & Ramsay 1973)for the cross fibres, or                        Cnrsssv, B.A. (1979): Electron microscopic studies of
sometimes shearfracturesfor the slip fibres. The
            in                                                          serpentine textures. Can. Mineral. 17' 741'756.
growth ofthe chrysotile fibres is parallel to the direc-
                                                                      Duruev, D.W. & Rausav, J.G. (1973): Incremental
tion of the minimum shear-stress.     When the local
                                                                        strains measured by syntectonic crystal growth. In
stress-field modified, the openingof the fracture
             is                                                          Gravity and Tectonics (K.A. Dejong & R. Scholten,
and the growth ofthe fibres cease change. sim-
                                    or          A                        eds.). J. Wiley & Sons, New York.
ple vein may remain preservedat this stage, or
reopening of the fracture may occur, and a new                        LauancHB,R.Y. & Wrcrs, F.J. (1975):Where to look
generationof fibres may form as a distinct vein, or                     for new asbestosdeposils. Third Int. Conf, Physics
in continuation of the previously formed fibres.                        and Chembtry of Asbestos Minerals (Univ. Laval)
Which caseoccursdependsupon the stressfield and                         (abstr.).
the chemical environment. Significant changesin
                                                                      LeunsNT, R. (1975): Occurrences and origin of the
thesefactors will promote a new and distinct genera-                     ophiolites of southern Quebec' northern
tion of veins.                                                           Appalachians. Can. J. Eqrth $ci.72,443-455.

                          CoNcLUSIoNS                                    -  (1979):Environmentof formation, evolution
                                                                         and emplacement the Appalachianophiolites
                                                                                         of
l. The development of chrysotile veins in the                            from Quebec. /n Ophiolites (A. Panayiotou, ed.),
asbestosbelt of southern Quebec illustrates the                          Proc. Ophiolite Symp. (Cyprus), 628-636,
mobility of the major elements   Mg, Fe and Si dur-
ing serpentinization.                                                         & HEasnr, (1979):,
                                                                                      Y.      Paragenesis serpentine
                                                                                                       of
                                                                         assemblagesin tectonite and dunite cumulate from
2. The similarity in mineral compositionbetween the
                                                                         the Qu6bec Appalachians. Can. Mineral, 17,
wall rock and the veinsimplies that the assumption                       857-869.
of synchronous   formation and closecogenetic rela-
tionship betweenthe two is valid.                                      RronpoN, P.H. (1975): Geology of the asbestos
3. Each petrogenetic  group of serpentine chemical-
                                         is                               deposits of southern Quebec. Ministdre Rich. Nat.
ly distinct in terms of its Mg:Fe:Si proportions.                         Qudbec,  .E.S.lE.
                       VARIATIONS IN CHRYSOTILE VEINS, SOUTHERN
                                                                QUEBEC                           r83
       & LausrnrE R. (1972):Asbestosdepositsof            & Pram, A.G. (1979):Electron-microprobe
   southern Quebec. .fu1. Geol. Congr. 24th,          and X-ray-microbeam       of
                                                                          studies serpentine
                                                                                           textures.
   Guidebook,Excarsion B-08.                          Can. Mineral. 17, 785-830.
Rucrrrncs, J.C. & Gnspannru, E.L. (1969):                  & Wnrrrarsn, E.J.W. (1977): Serpentine
  Specifications a computer
               of           progrnmfor processing     texturesand serpentinization.
                                                                                  Can. Mineral. 15,
  electronmicroprobeanalyticaldata:EMPADR VII.        459-488.
  Dep. Geol., Univ. Toronto.
                                                                   & Zussvar, J. (19'77): idealized
                                                                                         An
Wnrrrarrn, E.J.W. & ZussrrtaN, (1956): The
                                  J.                  model for serpentinetextures after olivine. Can.
  characterization serpentine
                  of            mineralsby X-ray      Mineral. 15. M6-458.
  diffraction. Mineral, Mag. 31, 107-126.
Wtcrs, F.J. (1979): Mineralogy, chemistry and
  crystallography chrysotile.Mineral. Assoc.Can.
                of                                  Received
                                                           October10, 1983,
                                                                          reyised         accepted
                                                                                 manuscript
  Short Course Handbook4.35-78.                       January 11,1984.

								
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