Environment and Structural Controls on the Intrusion of the ... - Kenex

ECONOMIC

GEOLOGY AND

BULLETIN

OF THE

VOL. 85

SOCIETY

THE

OF ECONOMIC

GEOLOGISTS

MAY, 1990

Environment

and Structural

Controls on the Intrusion

NO. 3

of the Giant Rare

Metal GreenbushesPegmatite,Western Australia G. A. PARTINGTON*

Key Centerfor StrategicMineralDeposits,Universityof WesternAustralia,Nedlands,WesternAustralia6009,Australia Abstract

The Greenbushespegmatitedistricthasbeen the main center of productionof alluvialtin in WesternAustraliasincethebeginningof thiscentury,andmorerecently,for the production of tin, tantalum,and lithium from one of the largestrare metal pegmatitedepositsin the world.The intrusiverocksin the Greenbushes pegmatitedistrictare concentrated alongthe ancientcrustal-scale, Donnybrook-Bridgetown shearzone, analogousto the presentSanAndreasfault system,and are characterizedby steeplydippingplanarmyloniticfabricswith horizontalstretchinglineations,asymmetricfolds,asymmetricpressureshadows,and shear bandssuggesting sinistralstrike-slipmovement.The Greenbushes pegmatiteoccursin a higher temperatureandhigherpressuremetamorphic terraincthanwouldbe expectedfor pegmatites containingrare elementmineralization.The pegmatitecontainsthe sameshearfabricsasits hostrocksand hasevidencefor syntectoniccrystallization of mineralssuchas tourmaline, fantalite,garnet,and cassiterite. It is proposedthat intrusionof the Greenbushespegmatitemagmawascontrolledby the Donnybrook-Bridgetown shearzone.Any meltsor fluidspresentduringmovementalongthe shearzonewouldhavebeenchanneled intoit, leadingto intrusionof the pegmatite.The fluid pressurein the pegmatitemagmamay then have increasedcausingfurther failure, zonesof structuralweakness, and further intrusionof pegmatitegenerallyparallelto the mylonitic fabric in the shear zone.

Accordingto presentmodelsandclassifications, the Greenbushes pegmatitegroupshould be unmineralized.However,it is apparentfrom this studythat giantrare metalpegmatites canoccurin highergrademetamorphicterranesandthat thesetypesof pegmatitesneednot haveobviousparentalgranitoids. They maycontaina varietyof mineralization andare likely, at leastin the Archcan,to be associated with tectonismalongcrustal-scale fault systems. Introduction

the alluvialore. Productionpeakedin 1908, then deTHE groupof pegmatitesat Greenbushes (hereafter clined until 1964 when GreenbushesTin NL. began termedthe Greenbushes pegmatitegroup)fallsinto dredgingoperations.Sincethat time the weathered the raremetalclassofpegmatitesasdefinedby Cerny pegmatitehasbeena majorspurceof ore,but recently (1982a) and, as such,representsan importantre- as a result of undergroundoperationsand opencut source of Sn, Ta, and Li. The Greenbushesmineral hard-rockmining, fresh pegmatitehas been mined field (hereafterreferredto asthe Greenbushes peg- for Sn, Ta, and Li. A total of 21,084 metric tons of matite district) is situated250 km southof Perth, in 72 percent Sn concentrateand 2,043 metric tons of the southwest of the Yilgarnblock,withinthe Western 40 percent Ta2Osconcentratewere producedfrom Gneissterrane (Fig. 1A). The firstcommercialtin de- the Greenbushespegmatite district until 1982 posits in Western Australia were found at Green- (Hatcher and Bolitho, 1982). Recent drilling in the pegmatitegroup bushesin 1888, andby 1900 tonnages hadincreased southernportionof theGreenbushes has identified a large reserve of high-grade spodusufficiently to allowa tin smeltertobebuiltto process mene. Proven reserves to date are reported by HatcherandElliot (1986) as28.3 X 106metrictons * Presentaddress:Northern Gold, RegionalExplorationOffice, at 2.8 percentLi20, whichincludesa high-gradezone C/o P.O., AdelaideRiver, NorthernTerritory 0846, Australia.

0361-0128/90/1052/437-2053.00

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STRUCTURAL CONTROLS ON INTRUSION OF A RARE-METAL PEGMATITE

quently, a lithium milling and treatment plant was recently commissioned. Many attemptshavebeen madeto classifyand describerare metal pegmatitesusingmineralogicaland geochemical criteria.Becauseof the mineralogical and geochemicalbias inherent in most classification schemes,manystudieshave not addressedthe timing and controls on intrusion of rare metal pegmatite magmas.Recentstudiesindicatethat the Greenbushes

pegmatite.group, potentiallyoneof the largestrare metal resourcesin the world, differs from other rare

metalpegmatitedepositsin severalimportantaspects (Table 1; Bettenay et al., 1985, 1986; Partington, 1986), particularlyin its tectonicsettingand syntectonic emplacement.To understandthesedifferences it was necessaryto resolvethe structuraland metamorphichistoryof the Greenbushes pegmatitegroup with respectto the structuraland metamorphichistory of its hostrocks.The aim of this paper is to describethe environmentandtimingof intrusionof the Greenbushes pegmatitegroup.A preferredmodelfor the intrusionof the pegmatiteis alsopresented,and the implicationsfor explorationfor this type of rare metal pegmatitediscussed.For the purposesof this paper moreemphasisis givento the regionalsetting andstructuralevolutionof the pegmatitegroup,with descriptionsconcentratingon metamorphismand structuralgeologyof the pegmatitesand their host rocks.Detaileddescriptions of the petrographyof the hostrocks,internalgeology,andgeochemistry of the pegmatitesare presentedin Bettenay et al. (1985, 1986) andPartington(1986, 1988), andisotopicdata are givenin Partingtonet al. (1986), Seet (1986) and Partington (1988).

RegionalSettingof the PegmatiteGroup The Greenbushes pegmatitedistrictoccurswithin a 15- to 20-km-wide, north- to northwest-trending regional lineament between Donnybrook and Bridgetownand hasa strikelength of approximately 150 km (Fig. lB, C, andD). This structuremaybe a splayfrom a regionalnorth-southArcheanstructure whichmarksthe traceof the Proto-Darlingfault (Fig. 1D), asdefinedby Blight et al. (1981) and White et al. (1986), or it may be a remnant of an Archean structurewhich is offsetby post-Archeanmovements along the Proto-Darling fault (Fig. 1D). A seriesof parallellineaments,which maycontainsimilarstructures andbe of a similarage, occursnorth and south of the Donnybrook-Bridgetown lineament(Fig. 1D). A seriesof shearedgneisses,orthogneisses, amphibolites,and migmatitescropsout alongthe lineament (Figs.1Band2), andthe structures in theselithologies suggestthat thislineamentmarksthe traceof a ductile deformation zone which had a noncoaxial deformation

history(Fig. 3). Furthermore,the rotationof an eastwest-strikingsequenceof gneisses, in the vicinityof

439

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G. A. PARTINGTON

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Bridgetown,into this lineament(Fig. 1C), suggests indicate that these are probablythe oldestin the that deformation occurred as a result of sinistral Greenbushes pegmatitedistrictandassuchareprobto thesupracrustal sequences (Fletcher transcurrentmovementalonga major crustalshear ablybasement zone (hereafterreferred to as the Donnybrook- et al., 1983; Partington et al., 1986; Partington, Bridgetown shearzone;Partington, 1986, 1988).The 1988). Two regional shear-boundedsupracrustalbelts Donnybrook-Bridgetown shearzoneis truncatedto metasedimentary the southby theProterozoie Albanymobilebelt (Fig. (Fig. lB) containingamphibolites, andgranofelsoccurin the Greenbushes peglB), suggesting thatdeformation occurred duringthe schists, Arebeanandthat thiszonewaslargelyinactiveduring matRedistrict(Wilde andWalker, 1979, 1981). The the Proterozoie. supracrustal sequenceto the northeastof the Donshearzoneoccurs asanortherly A sequence of gneisses (3,100 Ma; Fletcheret al., nybrook-Bridgetown of schists, granofels, 1983)occursin thesouthernpartoftheGreenbushes trending,3-km-wide,sequence pegmatitedistrictin thevicinityofBridgetown (Figs. andquartzites.The sequenceisboundedto the west shearzoneandto the lB and 2). The easternandwesternmarginsof the by theDonnybrook-Bridgetown (Fig. lB). The supracrustal segneisses aretectoniccontacts against twosupracrustaleastby oldergneisses of the Donnybrook-Bridgesequences to the eastandwest.Later shearing and quenceto the southeast granitoidintrusionare concentrated in thesecontact town shearzone,in contrast,containsultramafic-mafic felsicschists,andbandediron-formazones,and the deformationof theseyoungergranit- amphibolites, oidsproducedorthogneiss very similarin character tions (BIF). This supracrustalsequencereachesa to the older gneisses. Structuraland intrusiverela- maximumwidth of 30 km and is boundedby older to thewest,by theDonnybrook-Bridgetown tionshipscombinedwith geoehronologieal evidence gneisses

STRUCTURALCONTROLSON INTRUSION OF A RARE-METALPEGMATITE

441

FIG. 3. A. Relationshipof granitoid podsto D• shear zonesfrom the contactof the Cowan Brook Dam Granitoid. Note the complexconjugatefolds in the lee of the granitoidpods (of. Fig. 4). B. Migmatite from the axial zone of the Donnybrook-Bridgetown shearzone. Note the asymmetricpods of leucosomewhich suggestsinistralshearing(cameracap is 5 cm in diam). C. Asymmetricpressure shadowsin the lee of K feldsparaugenfrom a D• shearzone,whichcutsthe DalgarupBrookGranitoid, stiggestingsinistralmovement(cameracapis 5 cm in diam). D. C-S structuresdraggedinto C' structures from a D• shearzone deforminggranitoid,which suggestssinistralmovement(cameracap is 5 cm in diam).

shearzoneto the eastandolder gneisses to the south (Fig. lB). The amphibolitesandmetasediments in the transition zone between the supracrustalbelts are sheared,resultingin heterogeneous high-strainzones containingmigmatiteswith transpositionstructures (rig. 3:B). Granitoidsin the Greenbushes pegmatitedistrict aresubdivided onthebasisof geochronology andtectonicassociation. A seriesofyoungergranitoids occurs as stocksand lens-shapedbodieswhich intrude the gneisses, amphibolites,and metasedimentary lithologlesof the supracrustal sequences (Figs.lB and2). The granitoidsare associatedwith felsic sheetsand dikes and elongatemigmatitezones (Fig. 2). The migmatitesnot only occuraselongatenorth-trending zonesaroundthe granitoidplutonsbut alsoaslinear

occurred synchronouslywith deformationas sug-. gestedby the (1) gradationfrom uncleformedstromatic migmatitesto ptygmaticmigmatitesfrom the marginsto the centersof the regionalshearzones, (2) the veinsof leucosomecrosscutting the regional myloniticfoliation,whichwere alsodeformedby later movementsalongthe foliationplanes,(3) the asymmetric form of the boudinagedpodsof leucosome, and(4) the similaritybetweenthe asymmetryof mineral porphyroclasts fromthe regionalshearzonesand the asymmetryof the porphyroblastic podsof leucosomefrom the migmatitezones(cf. Fig. 3B andC). Although somegranitoidscrosscutthe regionalmylonitic foliation, many containgradationsin strain from uncleformedzones with igneoustextures to highly deformedorthogneiss with myloniticfabrics belts in shear zones outside the immediate contact (Fig. 3C andD). Many granitoidsare ovalshapedand zonesof the granitoids(Fig. 2). Migmatiteformation are cut, but are alsowrapped on a macroscopicand

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G. A. PARTINGTON

mesoscopicscaleby a regional mylonitic foliation and/or major oxide chemistrycompatiblewith tin (Figs.3A and 4). This suggests that, like the migma- granites.However, none have comparablecontents tites,the granitoids wereintrudedsynchronously with of Li, Rb, or Sn nor the typicallylow K/Rb ratiosof the regionaldeformationin the Greenbushespeg- traditionally accepted source granites (BlockIcy, matitcdistrict(of.Hanmeret al., 1982; Castro,1986). 1980). Geochronologicalstudiesindicate emplaceMostof the granitoids in the districthavemineralogies ment of these granitoidsbetween 2,610 and 2,580

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STRUCTURAL CONTROLS ON INTRUSIONOFA RARE-METAL PEGMATITE

Ma, some 50 Ma before the intrusion of the Green-

443

areasare completelyrecrystallized andmylonitized

bushespegmatitegroup(Blightet al., 1981; Parting- (cf. Fig. 6A andF). Fourmajorandfoursubsidiary compositional zones ton et al., 1986; Partington,1988). Two separateeppegmatitegroup. isodesof pegmatiteintrusionfollowed granitoidin- are recognizedin the Greenbushes trusionin the Greenbushespegmatitedistrict. The Asnotedby Bettenayet al. (1985), the macroscopic olderpegmatites(ca.2,530 Ma; Sect,1986; Parting- zonationin the Greenbushespegmatitegroup is unton, 1988) include the mineralized Greenbushes usual,andperhapsunique,in that thosezones(e.g., pegmatite group and the unmineralizedMaranup lithiumzones)normallyexpectedto crystallizelast, Ford pegmatitegroup.The youngerpegmatites(ca. andhence,occurin the centerof the pegmatite(Jahns, 700 Ma) include the Ferndalepegmatitegroup and 1982; Norton,1983), occurasfootwallandhangingthe Mullalyup pegmatitegroup(Kepert, 1985; Sect, wall zonesin the Greenbushespegmatitegroup(Fig. 5B). Many smallersubzonevariationsoccurwithin 1986). the broad zonal sequence,e.g., muscovite-apatiteMine Geology beryl in the K feldsparzone,tourmaline-rich layers

Pegmatitesof the Greenbushes pegmatitegroup in the albite zone, and quartzlayersin the lithium 1983;Bettenayet al., 1985, occur as a seriesof linear dikes, varyingin length zone(Fig.5B;Paterson, from 2 to 3 km and 10 to 300 m in thickness, to in- 1986; Partington,1986, 1988). The highestgradetin-tantalumore shootsoccur dividualpodsof a few metersacross.The pegmatite in the albitezonesin the pegmatiteand dikes,anden echelonpodsemanatefromanintrusive exclusively subzones. Tin and center (Fig. 5A). The pegmatiteand its subsidiary generallywithintourmaline-rich dikesand podsare concentratedwithin shearzones tantalumoxidesare associatedwith uraniniteand apwith tourwhichmarkthe boundariesbetweenmajorsequences pear to havecrystallizedsynchronously of granofels, ultramarie schists, andamphibolites (Figs. maline. Cassiteriteis the main tin-bearingphaseoccrystalswhich 2 and5A). Primarymagmatictexturesandstructures curringas euhedralswallow-tailed texin the Greenbushes pegmatitegrouphavebeenmod- whendeformedhavepulled-apartandcataclastic occurasincluified to varyingdegreesby later deformationand tures.Earlyformedtantalumminerals metamorphism (Fig. 6; Bettenayet al., 1985; Par- sions(mainlywodginiteandixiolite)withincassiterite andtourmaline crystals. In contrast, thelater tington,1988). Asthe deformationis heterogeneous, crystals tantalumphases(microlite,tantalite,and someareasretain primary featureswhereasother coexisting

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