THERMAL AUREOLE GOLD - vlcroRrA and ELSEWITERE by hjkuiw354

VIEWS: 21 PAGES: 4

									                              SelwynMemoriallzcture Series,        l99I
                                                          Melbourne,


             THERMAL AUREOLEGOLD - vlcroRrA                         and ELSEWITERE
                                        VJ. Wall* andJ.R.Taylor**

                               Dept.of EarthSciences,MonashUniversiry,
                      (now * MIM Exploration pty.Ltd.and ** petrogenesis Ltd.)
                                                                       Pty.


 BeforeSelwyn'stime geologists recognised
                               had             the     areappropriate gold precipitation
                                                                      for                could resultin
 associationof hydrothermaltin and tungsten            the formation of significant gold deposits.
 mineralizationwith granitesand suggested    links     Deformation anddisplacements therock mass
                                                                   of                 in
between   granibid plutonismand the forrrationof       surroundingthe pluton, associatedwith it's
 somegold deposits. HoweverA.W. Howitt in the          emplacement requiredto accommodate
                                                                     are                           the
early 1890's and O.A.L. Whitelaw in the late           magma volume, may provide the structurally-
 1890's- early 1900's,as part of their work wirh       controlledfluid parhways (Fig. 2). Given this
the Victorian GeologicalSurvey, developedthis          simple conceptual  framework, may be evident
                                                                                   it
conceptand provided excellentdescriptions       of     that thermalaureolesare potentially ore-foruring
goldfields which are spatiallyrelaredto granites.      systems.
We follow this pioneeringwork in examiningthe
natureand evolution of.thermal aureole gold, a          Although Victoria's gold productionhas (so far)
distinctiveassociation oredeposits
                      of             whichoccurs       come mainly from the slatebelt association, the
in the thermalaureoles granitoidbatholithsand
                       of                              temporally and parageneticallydistinct thermal
which canbe relatedto processes operating  during      aureolegold mineralisation morewidespread
                                                                                 is                 and
pluton emplacement    and contact metamorphism.        occursin association with granitesof severalages
Beforediscussing occurrence andcontlolson
                  the          of                      in severalPalaeozoic terrains. ln theStawell arca,
the localisationof some thennal aureolegold            for example,rhe Magdala  andWonga    gold deposits
deposits,we begin by examiningsome simple              occur in the thermal aureole of the Lower
spatial,kinematicandmass   balance relations ser
                                            to         Devonian, I-type Stawell Granodiorite  @romnitz,
t-hebroadercontext.                                    1986). The latter is a composite,discordant
                                                       barholith which intrudes, multiply deformed
 The heatreleased rie crystailisation cooling
                    by                  and            Cambro-Ordovician  volcanic metasedimentary
                                                                                    and
 of granitoid pluton createsa transientthermal         successionsof regional greenschist facies
 anomalyin the pluton'scounfryrocks,the volume         metamorphic grade.
 of the resultingtherrral aureole(Fig.1) depending
 primarily on the size of the intrusion and the       TheWongadeposit hosted massive,
                                                                          is      by           psammo-
 ambienttemperature the countryrocks (Jaeger,
                       of                             pelitic hornfelses high metamorphic
                                                                       of                   gradein the
 1969). Devolatilization theserocks and the
                            of                        inneraureole the granodiorite.
                                                                    of                 Mineralizationis
 crystallizationof the hydrous granitic magma         associated  with a quanz-hlledfracture network
 liberatessubstantial   quantitiesof hydrothermal     localised a shallow-dipping
                                                                by                 reversefault system
fluids which must vent throughthe aureole. For        which postdates the three major regional
example, the emplacementinto sedimentary              deformationevents. Goid occurs in quartz veins
countryrocks of a modelgraniticpluton of typical      and also disseminated wall rocks which exhibit
                                                                             in
sill/laccolithic form (20 kilometresin diameter,5     high temperature  alterationcomprising biotite +
kilometresthick and with an H2O contentof 2-3         pyrite + pyrrhotite + arsenopyrite plagioclase,
                                                                                         +
       -
wt.Vo Bumhan, 1979)produces        between170and      zoisite, amphibole, consistent   witb the meu-
240 cubic kilometresof fluid (Fig. l). If rhisfluid   morphic grade of the surrounding hornfels
contains10ppbgol( thenbemeen 1200and 1700             @romnitz, 1986). Retrograde    sericite-carbonate-
tonnes gold aremobilised. Even largermasses
         of                                           chlorite-sulphideassemblages quartzoverprint
                                                                                   and
of fluid may be circulated if hydrothermal            the high temperature   marerial,but it is unclear
convectionis possiblein and around the pluton         whether the retrogradeepisodeis mineralizing.
(Cathles, 1981) (Fig. 2). The focussingof             WatchomandWilson (1989)haveshownthat gold
relatively small portions of the hydrothemraifluids   mineralisation theMagdabdepositsis associared
                                                                    in
tbrough small volumes of rock in the thermal          with vein systems which,like Wonga postdate  the
aureolein which the physicochemical      conditions   regional deformation. Thesesystemsappear to
relateto thereactivationofpre-granite          in
                                     structures,               extremelyfine grained(<0.5 mm), and
                                                      associated,
the Stawellfault zone at plutonemplacement  stage            generally comprisebetweenl-57o of the
                                                      together
in its outer tbermal aureole, the Magdala             rock.
mineralizationforrring at lower temperaturesthan
the Wongaprograde   mineralization.                   Perseverance   Corporation'sNagambie     depositis a
                                                       broad zone of gold bearing,quartz stockwork
In central Victoria, gold mineralizationoccursin       forming a large low grade resourcehostedby
zones of alteration and brittle deformationof         relativelymassive   Siluro-Devonian  metasandstones
metasediments the Melbourne Trough in the
                of                                    and metasiltstones.  Although the nearest   exposure
aureolesof Late Devonianplutons such as the           of the Strathbogie      Batholith is almost 10 km
StrathbogieBatholith (Fig. 3). The larter is a        away,recrystallisation thequartz-richsediments
                                                                               in
large, discordant, S-type batholith intruding at      and the presence spottingin silt-rich lithologies
                                                                         of
comparativelyhigh crusrallevels @hillips, Wall        indicatesthat rie deposit lies within rhe outer
and Clemens,   1981). Broadzonesof hornfelsing        aureoleand roof zoneof the Strathbogie     Batholiti
areevidentup to 30km to the northand westfrom         (Fig. 3). The gold mineralizationoccurs in the
outcroppinggranite,indicatingthat theseregions        hinge zone of a broad,open,upright anticlinorial
fepresent batholith roof zones. In contrastthe        fold, whichhasan E-W trending     axial surface a
                                                                                                      and
relatively narrow (1-2kmwide) aureole that            sub-horizontal  plunge.The most.  extensive  fracture
characterisesmuchof the southern  contactreflecrs     systemis developed     along the crest and down the
the steeplydippingplutonsidewallin this area.         northern limb of the fold structure (Fig. a).
                                                      Silicification and minor disseminated       sulphide
Tbe GoldenMountain prospecr (Fig. 4), an              mineralisation  (traceto 3Vo, pyits andarsenopyrite
historicalgold producer,lies within 100m of the       or their pseudomorphs, subordinatestibnite)
                                                                                 and
southern contacr the Srrathbogie
                of                Batholith. The      accompany gold mineralisation. The higher
                                                                   the
workings lie in tbe hinge zone of a steep,            Sb/Asof Nagambie      mineralization relativeto that
northerlyplunging (-601 regional fold closure         of GoldenMountainreflectsthe lower depositional
around a north-southfault. Mineralisationis           temperatures Naganbie and is consistrent
                                                                    at                               with a
developedas wall rock disseminations    arounda       broadregionalzoningexhibitedby thermalaureole
subtlenetwork of fracturesystems high-grade
                                   in                 mineralization centralVictoria.
                                                                     in
semi-massiveto massivepelitic and psammo-
pelitic hornfelses.Distinct, broadly stratabound      Thermalaureolegold association not restrictedto
                                                                                      is
zones of largely undeformed,(micro) fracture-         thePalaeozoic Victoria andN.S.W. Othermajor
                                                                     of
controlled mineralisationappearto be steeply          gold deposits (e.9. Telfer,l7.A., Goellnichret al.
plunging to the north into a near vertical granite     L988;The Granites,N.?", kelandandBryce,1990)
contact (Fig.3).                                      of tbermal aureole associationoccur in several
                                                      provincesof the AustralianProterozoic. Anong
At Golden Mountain two major generations         of   thesethe Pine Creek(N.T.) region represenrs   the
micro-vein/vein development evident (i) An
                               are                    most intensely explored whicb exhibits thermal
assemblagecontaining biotite + pyrite +               aureole gold deposits (e.g. Enterprise, Cosmo
pyrrhotite + arsenopyritetquartzcomprisesthe          Howley, Goodall, Woolwonga,Moline, Mt. Todd,
narrow(<1 andup to 2 mm) auriferous     veinlets of   Mt Bonnie, Tom's Gully, etc.) aggregatingover
theearly,high-temperature  phase.This assemblage      250 tonnesof gold. Thesedepositsoccur in a
is clearly in thermal equilibrium with rtre high-     rangeof lithologicaland stratigraphic settingsand
grade contactmetamorphic     minerals since such      exemplify a variety of mineralisation styles,
veinscrosscut do not retrogress
                but                their cordierite   includingstockworks sheeted
                                                                           and         veins,dissemina-
+ andalusite biotite + k-spar bearingtrosts. (ii)
              +                                       tions and replacements.  However,on the basisof
A secondgeneration veinscomprisingquartz+
                     of                               their structural
                                                                     hisories andalterationparageneses,
sericite + pyrite + arsenopyrite + chlorite +         the mineralization may be relatedto deformation
carbonalefonn a crosscuttingnetwork <lf veins         and contact metamorphismaccompanyingthe
which result in extensiveretrogression phase (i)
                                       of             emplacement the CullenBatholithinto the Early
                                                                    of
vein assemblages    and the high-grade contact        Proterozoicmetasedimentary  succession.
metamorphicminerals.The retrogradeveins are
wider (av. 1-2mm), significantlymore quartzrich       The Cullen plutons, of I-type affinities (Stuart-
and contain coarsersulphide aggregates. Gold          Smith and Needham,1984) exhibit a range of
appe:m to occur with both veining episodes,    and    emplacement stylesanddepthCandwereappalently
bas beenobservedas an essentiallypure mehl, as        intrudedneartheend or during a p(otractedregional
electrum, and also in tbe forrr of tellurides. The    sbortening. Emplacementof the Burnside,
sulphides and gold-bearing   phases intimately
                                    are               Shoebridge Fentongraniteswas accompanied
                                                                and
by penetrativedeformationof their aureoles,           lithologies, implying either that sulphide
whereasthe contactsof otier plutons (e.g. in the      depositioneffects gold depositionor that similar
southernand easternparts of the batholith) are                                      for
                                                      mechanisms responsible contemporaneous
                                                                    are
strongly discordantand tleir aureoles  show only      sulphide  and gold deposition.Such mechanisms
localised brittle deformation. The very broad         includesulpbidation iron rich and/orcalcareous
                                                                           of
therrral aureolesdeveloped  (e.g.Stuart-Smith
                                            and       hostrocks,and   in the caseof somevein/stockwork
Needham,    1984), indicatethat much of the Pine      systems,  fluid mixing and also simplecoolingof
Creek region is underlain by granite at shallow       the fluid. Although fluids at gold-forming stages
depth and constitutes roof zoneof the Cullen
                      the                             may be inferredgenerallyto havebeenon cooling
Batholith. Most of thegold deposits situated
                                    are       in      paths, simplemodelsinvolving forcedventingof
these roof zones, someoccurin plutonsidewall
                 but                                  magmaticvolatilesor convectivefluid circulation
environments, arelocalised a limited variety
               and             in                     aboutcooling intrusivesdo not adequately  explain
of structuralsettings. The Pine Creek region          severalaspects tiermal aureole
                                                                       of                 gold deposits:
exhibitsgold deposits formedat high temperatures      Much remainsto be learnt about the provenance,
in inner aureoles   (eg. Enterprise and Cosmo         path history and chemicalevolutionof the ore-
Howley)andalsolower temperature    mineralisation     formingfl uidsinvolved.
in outer aureoles(eg. Goodall), the nature of
associatedalteration and mineral parageneses          The evolutionof thermalaureole   gold is a subset
                                                                                                      of
reflecting tlte host rock lithology and also the      more fundamental problems involving the
metamorphic   grade. Mineralisationhas occurred       mechanics kinematics granitoidintrusionas
                                                                  and             of
under near peak to retrograde meumorphic              well as deformation and fluid migration in the
conditions.                                           Earth's upper crust. Theseare fertile fields for
                                                      research  which requiremulti-faceted    approaches
From the aboveand other examples is apparent
                                     it               integratingfield observations,  petrological and
that thermalaureole  gold may occuraroundS-, I-,      geochemical with modelsfor coupledtiermal,
                                                                    data
and (?) M-type granitoids that its occurrence
                          and                    is                and
                                                      mechanical fluid transport     systems.This work
not readily correlated with the stateof differenti-   will also lead to a better understanding thermal
                                                                                               of
ationof theassociated  plutons.However,thedepth       aureolegold, an ore association which, in Victoria
andmechanisms pluton emplacement well as
                 of                     as            and elsewhere,is both prospectiveand under
the natureof their country rocks appearto havea       prospected.
greater bearingon tie occunence gold deposis.
                                  of
The thennalaureole   gold association transitional
                                     is
to the regionalmetamorphic    gold association and
also to epithermal  styles.                           Aclotowledgements
                                                      The AustralianResearchCouncil partially funded
A key factor in the localisation thermalaureole
                                of                    this study. We are especiallygrateful to the
gold deposits the evolutionof local and broader
               is                                     companies whichprovidedsupport and/oraccess to
scale,structurally-controlled  plumbing systems.      their deposits well as to the geoscientists
                                                                    as                           who
The structuresthat channel fluids may be              havecontributed.
reactivated,  pre-existingstructures(eg. thrust-
           in
anticlines PineCreck)or those    that formedat the
time of granitoidemplacement. eitherof these
                                 In
cases,  suchstructures are systematicallyrelatedto    References
the   pluton emplacementmechanismand the              Supplied request.
                                                              on
contemporaneous    regional deformationfrcld. In
particular, pluton emplacementlargely by roof
Itlttng (FiS. 2 and Fig.5) and, to a lesserextent,
pluton lateral expansion contribute to the
development b'rittleandbrittle-ductile
              of                         structures
with whichtbegolddeposits associated.
                             are

The thernal aureolegold association includesa
wide rangeof deposittypesand it is unlikely that
any singlechemicalmechanism responsible
                             was             for
ore formation. Howevergold is generallyspatially
and paragenetically related to sulphides
(arsenopyrites,pyrite/ pyrrhotite), the latter
precipitated veinsor as replacements reactive
            in                       in
 Crcss sdron




  T.hrt-      86d0r


   50km. long
   20kn. w€.
    5kn. rh'd


    Sill-lik. lom

 Rotlitrtr!    ollrcnent


                                                             fieronr   lnrru   166   $l cou^rye
 Figure 1 : Schemadc     crosssectionshowingrhe
 typical dimensions a large,tabularbatholithand
                     of                              Figure 4 : Cartoonof the strucruralsettingof
 it's thermalaureole  based both geological
                             on               and    some gold deposisin the Strathbogie
                                                                                       Aureoleand
 geophysicaldara- Lower rigni uoi shows              the distibution of auriferous veining in the
 conserva[ve   esdmates the average
                          of            volumesof    metasedimentary   host rocks. Vein/stockwork
 hydrothermalfluid generated      from within and    systemsshownin black.
 arounda 20km diameter,              plutonand the
                            Skrn.rhick
 total massof gold mobilised,assuminglOppb
 aqueous  gold in rhefluids.




                                 patterns
during granitoid emplacerrentand thermal
metamorphism.                                                                                 @q   !0rt




                                                     Figure 5 : Schematic  crosssectionshowingthe
                                                     settingof someProterozoicgranitoidplutonsand
                                                     strain variations relative to regional pluton
                                                     geometry. Inset shows evidencefor syn-pluton
                                                     emplacement   defonnation,associatedwith pluton
                                                     roof lifting. See also figure 2. From Wall &
                                                              (in
                                                     Valenta prep.)




Figure 3 : Locationplanand regional     geological
setting.ofthe StrathbogieGraniteanOsurrounOing
               (approximately
1!e9! aureole                  delimitedby dashed
line). Modified from phillips, Wall and Clemens,
1981.

								
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