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68 G,A, PAFTINGTON
LAWLERSDISTBICT
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E G r a n i t o d , s n e i s s
O 1 O O 2 O O k m
FlGURE l Maoof the Yiloarn Block showino the locatio n of the Lawlers and Greenbushes districts
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paper is to describe the tectonic environments forboth the gold deposits of the Lawlers district andlhe rare-metal pegmatites of the Greenbushesdistrict, and to suggest possible criteria to aidexplorat'on for both types of mineralization inArchaean terranes.
REGIONAL SETTING
IntroductionThe regional setting and mineralization styles
of both the Greenbushes pegmatite distric{ and thegold deposits ol the Lawlers dislricl hav€ beendescribed in detail by a number of authors (Platt eta/., 1978; Platt, 1980; Wilde & Walker, 1982, 1984;Partington, 1986; Partington et al, 1986; Bettenayel al, in press; B. Eisenlohr & G.A. Partington,unpubl. ms., 1986). A brief summary of the settingof both deposit lypes is given below.
LawlersGEOLOGY
The rocks of the Lawlers district are atfectedby lower- to upper-grade greenschist faciesmetamorphism (Binns et aL,1976). Struclures and
textures are heterogeneously preserved such thatprecursor rocks can be recognized and, thus, theprefix "meta" is omitted, although implied, in thefollowing discussion. The greenstones in thisdistrict consist of mafic volcanics, ultramaficrocks and interlayered gabbroic sil ls (Fig. 2).Overlying the sequence is a sedimentary unitconsisting of conglomerates, arkoses and silt-stones. The greenstone sequence is inlruded bythe following sequence of granitoids:(i) A voluminous granitoid intruded to the west olthe greenstone sequence. The €astern margin ofthis granitoid is incorporated in the WaroongaShear and lherefore intrusion is considered to bepre-sheanng.(i i) The Lawlers Tonalite intruded the axial regionof lhe Lawl€rs Ant ic l ine (F ig .2 ) , e i th€r dur ing orafter folding. Numerous xenoliths of the green-stone sequence occur in the tonalite, suggestingthai the exoosed oart of th€ intrusion is close tothe roof zone.(i i i) East-west aplite dykes and pods ol leuco-granite intruded between the tonalite and lhe green-stone sequence.
Extensive Rb/Sr isotope data, mainly for the
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70 G.A, PARTINGTON
surrounding granitoids and pegmatite dykes, areavailable for the region (Cooper et al., 1978:Roddick el al., 1976). Whole-rock Bb/Sr isochronsindicate ages ranging from 2.62 to 2.65 Ga for thebulk of the granitoids adjacent to the greenstones,with later pegmatite and aplite intrusions havingages between 2.58 and 2.47 Ga (Cooper et al,1978) .
STRUCTUBERegional setting
Recent regional and detailed mappingindicates that the siructures in the district are theresult of a greenstone-scale, non-coaxial deform-ation event (Eisenlohr, this volume; B. Eisenlohr &G.A. Partington, unpubl. ms., 1986). The majorstructural features of the region are large shearzones, kilometre-scale fold structures and a wide-spread north-northwest-trending foliation. Criteriaused to recognize ducti le shear zones are C-Smylonitic fabrics; asymmetric, relolded, andsheath lolds; and pressure shadows (Berth6 etat,1979; Bell & Hammond, 1984; Lister & Snoke,1984). Movement directions were deduced fromcriteria such as asymmetric pressure shadows andinclusion trails behind clasts in the conglomerates,and asymmetry ol C-S fabrics (Simpson & Schmid,1983) .
FoldsThe Lawlers Anlicl ine (Fig. 2) is the most
prominent of the regional fold structures, foldingthe greenstone sequence and clastic sedimenls oflhe Lawlers district. The Anticline plunges to thenorth at about 45" and the core region js intrudedby tonalite (Platt ef al, 1978). The regionat folds inthe Lawlers district were probably tormed byflexural slip: evidence includes the presence ofasymmetric folds associated with prominent C-Slabrics in the hinge region of the Lawlers Anticline.The C-S fabric paralJels stratigraphic layering, andis restricled to particular stratigraphic units. Inaddition the asymmetry of the folds is compatiblewith the regional iolds. The similar orientation ofthe stretching l ineation in the Waroonga Shear andplunge of the Lawlers Anticline further suggeststhat the shearing and folding are related.
Shear zonesThe dominant structure in the Lawlers district
(Fig. 2) is the 2 km-wide north-northeastlrendingWaroonga Shear described by platt ef al. (1978).The Waroonga Shear separates supracrustal
rocks to the 6ast and granitoid intrusjons to thewest, and extends to lhe north where it is obscuredby Recent sediments. lt has a vertical {oljation andsubhorizontal stretching l inealion, and movementcriteria indicate dextral strike-slip displacement(F ig .3a) .
Numerous smaller, subsidiary shear-struc-tures occur in the greenstone sequence marginalto the Waroonga Shear. These are impersistenthigh-strain zones, with two distinct orientations,and dimensions up to 100 m in width and 2 to g kmin length. The first orientation varies lrom north-south 10 northeast-southwest and the shearshave similar movement senses to the WaroongaShear (e.9., Emu and Donegal shear zones). Theyhave a steep mylonitic fabric which is commonlyfolded by a series of upright non-cylindrical folds,with fold axes paralleling the trend of the LawlersAnticline. Two sets of stretching l ineations aredeveloped. The early set parallels the fold axes,and is indicalive of strike-slip movement, whereasthe later set is steep, indicating dominantly verticalmovement.
The second type of subsidiary shear (e.9.,the Caroline and Mcoafferys shear zones) has adominantly west-northwest orientation with avertical foliation and dip-slip l ineation. In somezones, for example in the Great Eastern openclt,there is evidence for strike-slip movement which isoverprinted by a late normal dip-slip movement. Afeature common to all these subsidiary structuresrs the presence of some component of dip-slipmovement late in the history of the structure.
MINERALIZATIONGold occurrences in the Lawlers district are
located jn structures subsidiary to lhe major,regional shear zones. The latter are hundreds oimetres wide, whereas the mineralized structuresare metres 10 tens of metres wide. The formationand extent of subsidiary shears is also controlledby lhe mechanical propenies of rocks. Composi-tional and competency contrasts dictate whether arock detorms in a britt le or ducti le manner (Fig. 3b-d). Curving of shear planes, feathering, andbrecciatjon all tend to occur at l i thologic contacts.In addition, the presence of prior weaknesses(e.9., foliation planes) funher constrains theorientalion and termination of structural sites. lt isimportant that, in many instances, rne rocKs arernechanically anisotropic and, therefore, do not failalong the theoreticaily predicted orientations(Donath , 1961) . The tens i le s t rength o f fo l ia led
FIGURE 3 (see opposite page) Examples ot the fabrics observed in rocks of the Lawlers district. A: Ducti leshear fabrics in the regional Waroonga Shear. Asymmetric pressure shadows on conglomerate clastssuggest a dextral strike-slip sense of movement on the shear zone (width of photograph is about 0.5m). B: Evidence for britt le delormation in the Great Eastern gold mine. The conjugat; fractures containgold-bearing quartz veins (width of photograph is about 2 m). C: Evidence lor britt le deformation in theGreat Eastern gold mine. En echelon tension gashes are fi l led with gold-beafing quartz veins (width ofphotograph is about 2 m). D: Ducti le-britt le subsidiary, sinistral, antithetic strike-slip shear zone in theLawlers tonalite (width of photograph is about 0.5 m)
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72
FIGURE 4 Regional map of southweslern Australiashowing the location of the Greenbushesdistricl wlth respect to major structures
rocks is weakest parallel to lhe foliation, lhuspredisposing the rocks to lail in a britt le manner inthis orientation. Most deposits in the area arehosted by volcanic/intrusive rocks of sub-amphibolit ic metamorphic grade. Within this class,two structural styles are recognized: (i) alterationhaloes t quartz vein systems in shear zones, and(ii) thick, persistent, laminated quartz veins infractures or shear zones. ln all cases golddeposition postdated peak metamorphism, as thealteration assemblages are not melamorphosed.Whlle structure is an imDortant lactor in locatingthe alteration and gold deposition, the distributionof gold within a locality is also subject to host-rockchemical conlrols; the predominance of golddeposits in iron-rich host-rocks has previouslybeen noted by Groves et a/. (1985). An example ofhosfrock chemistry influencing the site of golddeposition occurs in the Donegal Mine, wherestratabound oreshoots are located within alholeiit ic unit in the mine sequence, and at theGreat Eastern mine, where the iocations of high-grade oreshoots ar€ influenced by mafic xenolithcontacts in the tonalite.
GreenbushesGEOLOGY
The Greenbushes pegmatite group (ca 2.54Ga) is located on the Donnybrook-Bridgetownshear zone (Fig. 4). The pegmatite is a north-nonhwest-trending,400 m by 6 km, dyke-like bodywith subsidiary dykes and pods at its periphery(Fig. 5). Recent exploration in the northern 1 km olits strike lenglh indicates lhat it contains theworld's largest resource of Ta and Li. The original
orientation of the pegmatite,, and the spatialrelationships of zones within it, are equivocalbecause ol intense heterog€neous delormation.The pegmatite varies internally lrom pod-likebodies of coars€ pegmatite with igneous structure,such as large radiating tourmaline suns, throughzones of fractured and aligned sil icate minerals toultramylonite. Even where igneous structuresarepreserved, the pegmatite is characterized bygranular, recrystall ized aggregates or granularsubgrains. Deformation is most intense alongcontacts, where there are complex relationshipsbetween pegmatite and country rocks similar tothose present along the contacl zones of somesynkinematic granitoids (e-9., Cowan Brook Damolulon).
The Gre€nbushes pegmatite group is hostedby a sequence of diorit ic gneisses, amphibolitesand sediments, and occurs at or close to a contactbetween dominantly mafic-ultramalic amphibolitesand sedimentary granofels (Bettenay et al., inpress). The occurrence of pil low-like, vesicular-l ikestructures, and an association with banded iron-formations suggest lhat the amphiboli les evolvedin a deepwater-volcanic environment. The sedi-ments resemble greywacke sequences whichcommonly occur in Archaean greenstonesequences and have a mixed granitoid-mafic prov-€nance, as defined by geochemistry and zirconmorphology. The sequ€nce in the psgmatitehangingwall and lootrval! is dominated by darkgreen, tholeiit lc, marse- and fine-grainedamphibolites after dolerite and basalt precursors,respectively. However, within the zone occupiedby the pegmalite, lhere are a variety of pale-greenamphibolites and ultramafic schists that havekomatiit ic affinit ies and, in this respect, theyresemble greenstone s€quences. All l i thologiesare deformsd by narrow high-strain zones withassociated biotitetphlogopit€:lquartz-{arbnatealteration.
The metamorphic history ol the district iscomplex, with geochronological evidence for threemetamorphic episodes (Partington et al., 1986).However, the metamorphic grade within the shearzone is mainly upper amphibolite facies, wilhtemperatures of approximately 560'C beingremrded, in contrast lo low- to mid-amphibolitefacies melamorphism on a regional scale. Thehigher-grade metamorphism along the shear zonesappears to be related to migmatization andseleclive granitoid emplacement, alihough it ispossible that viscous heating within these zonescaused the coincidence of all these leatures. Theoccurrence of siaurolite-kyanite assemblagesnear Bridgetown suggests relatively high-pressureconditions during metamorphism, as does theoccurrehce of spodumene rather than petali le as aprimary phase in the pegmatite.
G.A. PARTINGTON
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A
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FIGURE 7 Oi"gr"r -rplring structures in awrench tectonic regime with those of theKathleen Valley-Lawlors region (Eisenlohr,this volume, lig.6). A: Predicted orienlation ofstructurss in a wrench tec'tonic regime, B:Measured orientation of structurss in theKathleen Valley-Lawlers region
similarit ies. The dominant structures in th€ Anger-Lanvaux shear zong. France. are a horizontalstretching l ineation which parall€ls told axes and aloliaiion or slaty cleavage which wraps the axis ofthe shear zon€. As in the Greenbushes pegmatitedistric't, the lolds in this shear zone hav€ axesparallel to the gen€ral trend of the sh€ar zone (Fig.8). This is atlributed to crustal shortening in adirection oblique to the boundaries of the shsarzone. In the Montagne Noirs shear zone, Franc€,high-grade metamorphism is centred on ths axialregion of lhe shear zone, and lower{radeassemblages occur symmetrically away from theaxial region. A similar metamorphic bilateralsymmetry sesms to b€ present in thg major shearzones in lhe Greenbushes pegmalits district:zones ol migmatization and anatexis ars pressntonly in lhe ar€as of highest strain (Fig. 8), whereaslower-grade ass€mblages without migmatizationoccur in lower-slrain zon€s. A major sequence oforthogneiss and late- to post-teclonic granitoidintrusions occurs in the core of the Donnybrook-Bridgetown shear zone. Th€ axial regions ol thsseshear zones display clear evidence for partialmslting at various scales within the deformedgneisses, and a general associalion of pegmatiteand granitoid intrusion. Similar variations in plungeand trend ol the l inear structures in theGreenbushes pegmatite district are described lromthe Mayadan, Lanvaux-Angsrs and MontagneNoire shear zones, ln some areas, ihe l ineationscompletely encircle lhe main axial orthogneissregion, suggesting diapirism was synchronous withtranscurrent shearing (Den Tex, 1975). Such arelationship would €xplain lh€ halo€s of down-dipplunging l ineations that are pros€nt along themargins of all the granitoids in the distric't and alsoalong the intrusivs contacls of th€ Greenbushes^6^nr t i t6 ^ r r r ^
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----------7
ffi rr .on" ffi r .on" f ru" {sn, ,") ,on"_':-i- Shear zone boundary
FIGURE 10 Schematic diagram showing the intru-sion sequ€nce of the Greenbushes pegmatitegroup. A to C indicate progressive shearingduring pegmatito intrusion
pI€SSUfe = lithostatic pressure) the containingstress in ths host rocks must docrease to allow themagmatic pr€ssurss containsd within thspsgmatite melt to €xceed the host-rock confiningpressures. Experimenlal work and computermodell ing indicate that during movements in ducli leshear zones, high strains build which evenluallyresult in strain hardening (White st aL,1980 ). Thisprocess continues until failure o@urs and movs-ment is lnit iated within the shear zone. Spaces wil lnot actually open in lhe ducti ls l ield but, aspredicted by computer modell ing (Mclntyre, 1985),major pressure reduction occurs in certain sites,such as in the lee of ridged bodies or along curvedshear planes. Sinistral movements along theDonnybrook-Bridgetown shear zone created majornorth-northwest-south-southeasl anisotrooiesand secondary east-wsst anisolropies which,combined with the anastomosing charact€r of thshigh strain zones, rssulted in irregular shearplanes within the regional shear zon€. Latermovements caused ths shear to "open', rgsultingin zones of intsnss pressur€ reduction. Any meltsor f luids in ths system at this time would havemigrated towards this zone, thus init iating intrusionof lhe pegmatits (Fig. 10). Then th€ hydrostaticpressure in th€ pegmatite magma would haveincreased, forcing further tailure and associatedzones of intense pressure reduction, andpermitting intrusion of p€gmatite magma along theshear zone. Furth€r movoment in lns snear zone
was triggsred by th6 intrusion of ths hot magma,which would have lubricated the shear system andaided turther ducti le deformation. This. in turn.attracls more melt into the syslem, allowing thep€gmatite to increase in size until ths site coolssutficiently and further movement c€ases.
This type of loctonism tapped large expans€sof the lower crust, mixing melts, hydrothermalfluids and/or metamorphic fluids. Such processesmay account for lh€ large size ol the Greenbushespegmatite group and explain ths obvious controlsthat th6 Donnybrook-Eridgetown shear zonesxerts on the intrusion of not only pegmatile, butalso associated granitoids and later mafic dyk€s.The structural control on igneous activity in thedistrict is emphasized by the similar intrusion some2.1 Ga lat€r ol the Ferndale (Kepert, 1985) andMalulyup (S6et, 1986) pegmatite groups.
GUIDES TO EXPLORATION
A comparison of the struclural and metamor-phic features of ths two types of minsralization isgiv€n in Tabl6 1, and is shown schomatically inFigure 1 1. Ths pr€senc€ of rare-metal p€gmatitesin similar structures to gold mineralizationsuggests that th€se magmatic fluids utl l izedsimilar structural channelways to the Au-bearingore fluids. This similarity emphasizes theimportance of greenston€-scale snear zones tnlocalizing l luid flow in Archa€an terranes. How€ver.although the struclures which mntrol thedeposition oI Au and intrusion of rare-metalpegmatites are the same, the metamorphic gradeand style ol delormation associated with €ach typeof mineralization ar€ quite differont. cold depositsgenerally occur in structures within the britt le-ducti ls f ield associated with greenschist to lowamphibolite lacies metamorphism (Fig. 1 1). Incontrast, rare-metal pegmatites tend to occur inslructures formed in the ducti le f ield associatedwith mid- to upper amphibolite facies meta-morphism (Fig. 1 '1).
Preliminary data suggest that striks-slipmovement occurred both prior to and during peakmetamorphism botween ca 2.8 Ga and 2.S Gaacross the Western Australian craton. Theteclonism would have allowed fluids or melts tobecome minsralized because of: (i) access to largevolumes of rock through craton-wide faultsyst€ms, (i i) increased solubil ity ol Au at high€rP-T conditjons, and (i i i) lhe @ncentration ol rare-metals by anatexis deep in ths crust (cL Groves efa/., 1985). Lower metamorphic conditions preval-ent within lhe subsidiary structures outside themalor shear structures (Fig. 11) allowed tluidlocalization and Au deposition under a suitabletemperature regime (e.9., Seward, 1984). Incontrast, rare-metal pegmatites generally intrudedinlo arsas of the crust where higher tomperaturesand prsssures were presenl (Fig. 11), associated
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80 G.A. PARTINGTON
Original Archaean surface
Limit of greenschist facies metamorphism
Zone of bri t t le-ducti le deformation
zone of ducti le-bri t t le deformation
Granitoid intrusion and shear-heatingprecludes gold deposit ion and widensthe zone of ducti le deformation
Ducti le transitaon
Upper amphibol i te facies metamorphism,anatexis and granitoid intrusion
Zone ol ducti le delormation
FIGURE 11 Crustal cross-s€c1ion showing the potential zones of gold and rar€-metal pegmatitemineralization with respect to metamorphic and struclural rsgim€s in an Archaean greenstonesequgnce
serpentrnetalcanthophyll itetremolitehornblendequartzcummingtonrtegarnetCa-plagioclasebiotitecarcrlemuscovite
staurolrtecordieriteandalusitekyanitesil l imanitegedrite
TABLE 3 Structural and metamorphic leaturssindicalivs of ductile reoimes
Metamorphic minerals Structures
types rarely occur together bscause of lheditferent tempsraturs-prsssurs regimes underwhich they form. Ths lypes of structures andmetamorphic conditions associated with each typeof minsralization is distinctiv€. Gold depositsoccur in britllsiuctil€ structures which wereassociated with low- to medium-gradg metamor-phism. In contrasl, rare-mstal magmas occur inoarts of the crust in which ductile delormationaloroc€sses dominat€d and which wer€ associatedwith medium- to high-grade metamorphic condi-tions. Thersfore, it should be possible to use thecrileria outl ined abov€, in conjunction with detailedstructural and metamorphic mapping, to identifythe most prospective parts ol these fault or shearsystems Jor gold or rare-melal mineralization.
ACKNOWLEDGEMENTS
This paper represents a portion of a structuralstudy undertaken on the Greenbushes pegmatiteas part of a doctoral thesls, and detailed structuralinvestigations for Forsyth N.L. in the Lawlersdistrict. lwould liks to thank Dr L.F. B€ttenay for allhis help and €ncouragement in the initial stag€s ofthe doctoral thesis, and Grsenbushes Tin N.L. lorproviding financial and logistical support. I wouldalso like to thank the statf of Forsyth N.L. tor theirco-operation and permission to publish data on theGreat Eastern mine and ths Lawlers district. lalsoacknowledge lellow students and stalf, sspeciallyDrs L.B. Harris and D.l. Groves, for suggestions
layeringfeldspar augenrecrystallizationmicaJichpressure-shadowsC-structuresS-structuresshear bandsflattened grainsquartz ribbonsstretching l ineationpull apart structuresconjugate loldsasymmetric foldssheath foldsnon cylindrical foldshelicit ic structures
CONCLUSIONS
Faults and shgar zones were a dominantcontrol of mineralizaiion in Archasan terranes.These structurss acted as channelways tor bothAu-bearing ore fluids and highly lractionalsdmagmatic melts. However, these mineralization
FAULT ZONE
Potential zone ofgold/mtrr�et alizalion
Rare-elementmineral izat ion
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G,A. PARTINGTON
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