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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