Revista Brasileira de Geociências 12(1-3): 135-148, Mar.-Sel.. 1982 - Silo Paulo

THE ARCHAEAN AND EARLlEST PROTEROZOIC EVOLUTION AND METALLOGENY OF AUSTRALlA

DA VID I. OROVES'

ABSTRACT Proterozoic fold belts in Austrália developed by lhe reworking of Archaean base­ mcnt. The nature of this basement and the record of Archaean-earliest Proterozoic evolution and metallogeny is best prescrved in the Western Australian Shield. ln the Yilgarn Craton. a poorly-mineralized high-grade gneiss terrain rccords a complex,ca. 1.0 b.y. history back to ca. 3.6b.y. This terrain is probably basement to lhe ca. 2.9~2.7 b.y. granitoid­ -greenstone terrains to lhe east-Cratonization was essentially complete by ca, 2.6 b.y. Evolution of the granitoid-greenstone terrains ofthe Pilbara Craton occurred between ca. 3.5b.y. ano 2.8 b.y. The Iectonic seuing of ali granitoid-greenstone terrains rcmains equivocaI. Despitc coincidcnt cale­ -alkalinc volcanism and granitoid emplacemcnt , and broad polarity analogous to modem are and marginal basin systcrns. thcre is no direct evidencc for plate tectonic processes. Important diffcrences in regional continuity of volcanic scqucnccs, lithofacies. regional tectonic pauerns and meta1Jogeny of lhe terrains may relate to the amount of crusta! extension during basin formation. At onc extreme, basins possibly reprcsenting low total cxrensíon (e.g. east Pilbara l are poorly mi­ ncralizcd with some porphyry-stylc Mo-Cu and small sulphute-rich volcanogenic 01' evaporitic deposits reflecting the resultam subaerial to shaJlow-water environment. ln contrast, basins inter­ prctcd to have formcd during greater crusta! cxrcnsion (e.g. Norseman-Wiluna Belt) are riehly mineralized witb Ni-Cu and Au dcposits. duc lo widesprcad eruption of komatiitcs and rapid subsidencc of Ihe volcanic pile. Some" greenstone basins show intermediate character. Deposition of the eartiest Protcrozoic volcanics and shell' scdiments of the Hamersley Basin predated stabilization 01' lhe Y ilgarn Craton. Thc changc in Iectonic seuing heraldcda major ehange in mctallogencsis from dominantfy volcanic-related Arehaean deposits lo major sedimcntary ore associations. lhe earliesl of whieh were lhe iron ores 01' lhe Hamersley Group.

INTRODUCTION The Precambrian shield of Austrália ln Western Austrália. the term Arehaean has generally encompasses severaI major metallagenic provinces, Its tec­ becn applied to rocks older than 2.4 b.y., whereas 2.5 b.y. tonic evolution has been discussed recently by several au­ is more widely adopted (James, 1978). ln contrast, the Bu­ thors (e.g. Rutland 1976'; 1981; Gee, 1979; Plumb, 1979). reau of Mineral Resourees (Rutland, 1981) have used a It is not intended here to discuss in detail the entire shield time-rock systern, defining the base of the platforrn Fortes­ (e.g. Rutland, 1981), but to concentrare on the evolution cue Group in the Pilbara region of Western Australia as lhe and metallogeny of its Archaean and earliest Proterozoie lower Proterozoic boundary. With more geochronological segments. Emphasis is placed on the Western Australian data, Ihis age has increased from ca. 2.3 b.y. to the present Shield which ineludes the most extensive, preserved Archae­ estimare of > 2.7 b.y. Thus, platforrn cover sequenees were an bascment. and its oldest, gently folded and very low mela­ deposited on the Pilbara Craton before stabilization of the morphic grade cover sequenees (Gee, 1979); both eontain Yílgarn Craton, The Archaean-Proterozoic boundary, as major metallogenic provinces. This review firstly covers well defined in a tectonic sense and adapted here, is therefore established aspeets of lhe crustaI evolution of lhe Shield and diachronous. examines tectonic models in terms of constraints imposed by the nature and quality of available data. Metallogeny is then discussed in terms of tectonic and crustal evolution TECTONIC FRAMEWORK OF AUSTRALlA The and is shown to be an integral part of this evolution. gross structurc 01' Australia is dominated by Archaean A slightly modified terminology of Oee (1979) is used to cratons ln the.extreme wcst, thelate Precambrian-Phanero­ describe tectonic units. B/ock is used as a general term for zoic Tasman Fold Belt System in the east and the intervening a coherent outcropping part of the Precambrian ba..,ement. Early to Late Proterozoic provinces and basins ofcentral and Where ane 01' more of these blocks, 01' segments within northern Australia (Fig. I). thern, have tectonic characteristics that distinguish them The Pilbara and Yilgarn Bloeks represent the exposed from adjacent blocks ar segments they are termcd prol'inces. parts of Arehaean era tons. They represent segments of Ar­ The scale of provinces defined by various authors' (e.g. chaean crusi (> 3,6 to <'a. 2.7 b.y.) lhat were essentially Rutland, 1976, 1981; Oee 1979) is varied; the terminology stabilized by 2.8 to 2.6 b.y. and subsequently inlruded by in eurrent use by the Oeological Survey of Western Aus­ mafie dykes at ca. 2.5-2-4 b.y.; these bloeks largely eseaped o'alia is adopted. The larger scale provinces of Rutland Prolerozoic reworking that affected other Arehaean crus!. (1981) are, however, discllssed (Fig. I). The (erms craton, They represent major components of the Archaean super­ mobile bel, and basín are used in the normal tcctonic sense. province of Rutland (1981).

*Geology Depal'trnenl, University 01' Western Australia, Nedlands. Weslern Australia, Australia 6009 136 Revista Brasileira de Geocíências, Volume 12 (l·3), 1982

...... , Maior D,o;>vinclI bounda,y Approx;mau, boundary 01 ...... P'ohHozoic provinces delmed by Rulland (1981) ...... Subp'ovlnca bound.'y

....,_ ..... WeSlG,n Australi"" bordol

Q Lere ProtelOlo;c·PMnarow,c lold bells 13 P,oterolo,ç blocks (orogcnic doma,nsl ~ P,ote

Figure J -- The major tectonic units (~r Australia (odaptrd fram Rutland, /981)

Thc Proterozoic supcrprovince of Rutland (19811 eom­ from reactivation of Early Proterozoíc, and possibly Ar­ prises a nurnber of blocks overlain by plaiform cover se­ chaean, erust and partIy reflcct teetonie trends of the base­ quenees. Rutland (1981) suggests that unity of the super­ ment. provinee is demonstrated by a ubiquitons phase of pluto­ The marked eontrast in geophysieally derived strueture nism, commonly related to acid volcanism and associated of lhe extensively reworked Precambrian crust and that of with deformation and metamorphism, in the range 1.9 to the largely Phanerozoie Tasman Fold Belt System (e.g. 1.7 b.y., which pre-dated an early platform eover stage Finlayson, 1982) represents one ofthe fundamental teetonic (Central Australian Platform Cover). Mafic dykes at ca. features of Australia (Fig. I). It probably relates to the 1.1 to 1.0 b.y, mark stabilization of the superprovinee. longevity of global heat-flow deeay (e.g. Rutland, in prep.). Rutland (! 981) defined three main Proterozoie crustaI seg­ The Tasman Fold Belt System (Seheibner, 1978), whieh ments (his Northern Australian, Gawler and Gascoyne shows progressive stabilization frorn west to east and incor­ Provinees - Fig. I) separated by zones of crustaI reworking porates blocks of oceanic crust, was not cratonized until (Albany-Fraser Provinee and Paterson-Musgrave belt of the Triassic. There is current debate on whether this formed 'Fig. 3). Ali segments eontain relies of Arehaean basement or by lateral accretion or by vertical accretionary growth as inferred Archaean basement (sec summaries by Plumb et at., for the Proterozoie superprovince (e.g. Rutland, 1981). 1981; RutIand et al., 1981). The existenee of Arehaean erust beneath the Kimberley Basin has been suggested, but not THE WESTERN AUSTRALlAN SHIELD Theevolu­ substantiated. The North Australian and Gawler Provinces tion and metallogeny ofthe eariiest Precambrian ofAustralia of RutIand (] 98.1) have eontrasting styles of sedimentation is rccorded in the Western Australian Shicld. Its ovcrall and metamorphism/deforrnation. The former comprises tectonic history is briefly reviewed to placc in perspective trough successions, generally of low metarnorphic grade those aspects 01' its earlicst evolution discussed below. with loealized zones of higher grade, whereas the latter The most aneient teetonie units, the Arehaean Pilbara eomprises strongly deformed and high metamorphie grade and Yilgarn Blocks, are partly covered by Proterozoic basins thinner platform sequences. and partly surrounded by Proterozoie mobile belts (Fig. 2). The style of sedimentation, magmatism and tectonics of Gravity data define the margins of the Pilbara and Yilgarn the Proterozoie bloeks suggests that the lower Proterozoie Cratons and indicare a further craton in the south-east fold belts formed in an ensia1ie setting and developed lar­ (Fig. 3; Gee, 1979). The Pilbara and Yilgarn Cratons differ gely by reworking of Arehaean erust (e.g. Rutland, 1981). in i) erustal thiekness and structure, ii) relative heteroge­ Teetonie trends do not appear to mimie trends within adja­ neity, iii) exposure of high-grade gneiss belts, ir) age and, eent granitoid-greenstone terrains, but may be partly eon­ at least in part, stratigraphic continuity and gross tectonic trolled by strueturally elevated segments of high-grade pattern of their granitoid-greenstone terraíns, and v) metal­ gneiss terrains in the basement (e.g. Gee, 1979). A further logeny. They are interpreted as produets of independent stage of orogenie reworking is refleeted by linear belts of diachronous evolution. They appear to have -been in the high thermal and teetonie activity whieh appear to result same relative position since 2.4 b.y. (Embleton, 1978). RevistaBrasileira de Geocilncias. Volume 12 (1-3), 1982 137

PHANEROZOIC The Archaean granitoid-greenstone terrains behaved es­ o Sedimenta'y bas;n~ sentially as cratonic blocks during the Proterozoic, and are PROTEROZOIC SEOIMENTARY BASINS ~12b;Y~_:n: ly:u::_e~nd largely covered by gently folded Proterozoiccoversequences, hd,";,,:.:\i:;' õ youngar The earliest of these, the Fortescue and Hamersley Groups LU ca 2.6 b,y, and younger of the Hamersley Basin (Fig. 2) represent volcanic and ~ Proterozoic mobile beUs shelf facies sedimentary sequences of about 6 km total ARCHAEAN BLOCKS thickness. Zircon U-Pb ages (Compston et ai" 1981) indi­ r::::l G,an'loid-grcenstone bells cate that part ofthe Hamersley Group formed at ca. 2.5 b.y. U Gne,ss bells HALLS CREEK PROVINeE POr1 Hedland ~c:'"'~ and unpublished data suggest that the base ofthe Fortescue Group formed before 2,7 b.y., and that this group rnay be preceded by earlier, more-restricted, cover sequences (ToS, Blake, per, comm, 1981), A major zone of trough sedimentation, deformation, metamorphism, basement reworking and granitoid empla­ cement termed the Capricorn Orogen (Fig, 3) by Gee (1979) subsequently developed between the Pilbara and Yilgarn Cratons between ca. 2,0 and 1,6 b.y., its major components are shown in Fig, 2. According to Gee (1979), theCapri­ corn Orogen developed in an ensialic setting over a high­ grade gneiss terrain between the Pilbara and Yilgarn Cratons. Available exposure combined with gravity data (e,g. Gee, I 1979) indicate that the Archaean cratons and intervening Capricorn Orogen are enveloped by mobile belts, These ALSANY·FRASER I:,' PROVINCE lack the widespread trough sedimentation that typifies the Capricorn Orogen but contain extensively reworked base­ ment and voluminous granitoids. These belts include the Albany-Fraser Province (1.9-1.0 b.y.), the Patterson-Mus­ grave Belt (1.7-1.4 b.y.) and a belt inferred to have developed 400 km on the western margin of the Western Gneiss Terrain in the interval ca. 1.0 to 0.6 b.y. (Fig. 3). Gee (1979) compares Figure 2 - M(~iOI' tectonic units of Western Australia : adaptedfrom their development with arrested rifts in a continental setting. J:/ ,()()(),()()() map oI lhe Geoíogícal Survev of Wc'stern Austrália Apart from the Precambrian mineral deposits within the shield which are discussed below, its thick stable crust led to formation of diamond deposits associated with Mesozoic ~ Proterowic mobile bells kimberlites, and its protracted weathering history resulted

+ + . A,chaeal! 9,anilOidog,eenS,lone bells in surficial deposits including bauxite, lateritic Ni-Co, cal­ » A,chaaan gne,ss belts crete-associated U deposits, and 'Ti-rich beach sands. ~• + t + ""hacan bell (undtHe,entlalcd)

...... Limi1 01 Phane,oloic ccver PILBARA BLOCKAND HAMERSLEY BASIN The Pilbara Block and Hamersley Basin are linked spatially, the lattcr covering the southern half of the Pilbara Craton and extending to its southern margin (compare Figs. 2 and 3). There is also the possibility that their histories may be linked by morc-or-Iess continuous, if restricted, sedi­ mentation. Collectively, they occupy a similar time span to the Archaean terrains of the Yilgarn Block. They are accor­ dingly discussed sequentially below.

Pilbara block REGIONAL ASPECTS The Pilbara Block (ca. 60,000 km') with a crustaI thickness of 28-33 km (Drummond et ai" 1981) comprises a major, well-exposed INFERRED granitoid-greenstone terrain. Despite interfingeríng rela­ METAMORPHIC BELT tions, a regional stratigraphy lias been established and broad (1.0·0.6 b.y) correlations made over the entire block (Hickman, 1981; sec Fig. 4), Thc lower parts of greenstone belts are regionally conti­ nuous sequenccs of the Warrawoona Group, the lower units of which have well-defined ages between ca.. 3,5 and 400 km 3.4 b.y. (see Hickman, 1981, for summary). Thick associa­ Albany tions of basic to intermediate calc-alkaline lavas, sub­ Figure 3 - Reconstructíon of major Archaean and Proterozoic tec­ volcanic porphyry intrusíons and coarse pyroclastics repre­ tonic uníts of the Wesfern Austraíían Shieíd sent local elevated volcanic centres, Elsewhere, the succes- 138 Revista Brasileira de Geocíências, Volume 12 (1~3). 1982

IN(lIAN DCEAN

D POST ~ARCHAEAN ROCKS m WllIMCREEK GROUP E GORGE CRHK GROUP § WARRAWOONA GROUP UPPEA PART E3 TOWERS FORMATION (OASHEO "' AaSEN1) IIIIIIII WARRAWOONA GROUP lOWERPART L::3 POST --lECTO~IC GRANITIC ROCK O FOllAHO~GNEISSIC ORANiTlC ROCK 10., Dm

Figure 4 - Regional stratigraphv 01" greenstone sequem'es, Piíbara Block (fi'Ofll Hickman, 1981)

sion is dominated by associations of pilJowed and/or vesi­ The tectonic pattern of the Pilbara Block is dominated by cular tholeiitic to high-Mg basalts, with onJy minor koma­ the domai structures of granitoid complexes and/or greens­ tiites. These interfinger with sheets of voleanic debris and tone roof zones with intervening stellate greenstone belts epiclastic sediments derived from calc-alkaline centres, gi­ (Figs, 4, 5 and 6l. Over large areas the greenstones have ving the impression of voleanic cycles .(e.g. Barley et al., suffered only low internal strain, low-grade metamorphism 1979; Barley, 1981). Sedimentological studies indicate an and brittle fracture, and in these arcas preserve stratigraphic extensive shallow-water depositional basin] (Barley et al., integrity: there is no evidence for regional disruption of 1981; Groves et al., 1981). There has been extensive early stratigraphy by carly structures, ln areas adjacenl to grani­ hydrothermal alteration ofthe voleanic sequences, and chert toid domes, the sequences have undergone more complex and chert-barite beds commonly represent replacement of ductile and penetrative deformation and higher grade meta­ original volcaniclastic, carbonate or evaporitic sediments. morphism. On lhe western margin of lhe Shaw Batholith 'The upper part of the greenstone belt is dominated by (Figs. 5 and 6I, for example the granitoid-greenstone belt thick clastic sedimentary associations, with some basalts, of is transitional lo a high-grade gneiss terrain (Bickle et a!., the Gorge Creek Group (Hickrnan, 1981 ). On the basis ofre­ 1980; Bettenay et ai.. 1981) in which early thrusting and gional reconnaissance studies, Eriksson (1981) argues that recumbcnt folding results in tectonic thickening which was this Group represents essentially contemporaneous plat­ followed by at least two phases of upright folding and deve­ form (fluvial) and trough (submarine fan) sedimentation. lopment of high-strain shear belts with considerable strike­ Despite complexities not recognized by Eriksson, fluvial -slip displacement. Diapiric uprise of lhe granitoid domes sediments partly deposited in fault-bounded basins do do­ was probably contemporaneous with, and partly the cause minate the northern part of the Block, whereas trough of', the later deformation events. Pb-Pb mineral ages and sediments dorninate the southern and western marglns reset Rb-Sr whole-rock ages in lhe range c{/. 3.0-2,9 b.y, (Fig. 5) and shallow marine shelf facies appear rare. The (see Hickrnan , 1981) probably provide a minimum estimare lowermost sequences of the Gorge Creek Group may have on these late deformation/metarnorphic events, been deposited as early as 3.4 b.y.. but evidence for the There is no unequivocal evidence for a sialic basement to Jongevity of Archaean c1astic sedimentation is conflicting, lhe greenstone belts. Geochronological and geochemical On one hand, sediments ascribed to the Gorge Creek Group data suggest that at least some gneisses are complexly defor­ were involved in folding events prior to ca. 3.0-2.9 b.y, med equivalents of intrusive calc-alkaline granodiorites and (see below), whereas fluvial sedimentation, albeit areally tonalites which are essentially contemporaneous (ea. 3.5 restricted, may have continued in the east Pilbara up until b.y.), and probably comagmatic, with calc-alkaline volea­ deposition of the basal Fortescue Group (B, Krapez and nics within the greenstone belts (Bickle et ai.. in prep. l. T.S. Blake, per. comm.. 1981). ln the west Pilbara, the Late intrusive granitoids have been dated at 2,7-2.6 b.y, Gorge Creek Group is overlain by the Whim Creek Group, (see Hickman, 1981 l, but geological constraints suggest that the age ofwhich is also uncertain (compare Hickman, 1981, these ages are reset. and Marston and Groves, 1981 l. , -

R~ \i istQ Bra.ril~irQ de Oeocu ncios, Volume 12 (I.), 1982 139

POrl Hedlan "

N

WES I P1l"tlAR>\ •

GO ROE CREEK G' OUP

e~ 'oo:::;, ~ m

Figure 5 - Solid lwo logy oIII/(, ('(ui Pi/hum showing distribut íon oI volcanic and sedimrntury lithofacies

M I NERAUZATlON The Pilbara Rloek eonl ains some nogenic massive Cu-Zn sulphide deposits in lhe west Pilba ra deposit types that typify gra nitoid-greenstone bells clsc­ (Fig. 6). There are also enriehed Fe ores derived from band ed whcre (Marston and Gr eves. 1981). Thcse include small iron format ion (RIF) of lhe Oorge Creek G rou p, for exam­ Sn-Ta deposits in pegrnatites in the east Pilhara and volca- pie Go ldsworthy, Shay Oa p (Fig. 6).

,,,"00'

c a VEA ROCK5 .. ~ P~ ' " " o , " ' c ~ P '/)< " O IOO<

11..... VOVNGUI e r ...... ()fI . HIC ....ec PlutONIC IlOC1(S L_~ ~ --"::'::"': --': -j

g:w "' ,,,.-. _ .._ "..d .... e...... ,... >. IlUERlIln fOIl VILGAR '" ...."'0 P1UAU Itoeu

...r r .... loIOR. .. OSED "'II( H"'("''' SUP" ACAUS1 Al ROCKS "'IIC H"'E "' ''' GRANITO' O "' NO G"' ElSSIC R OC~ S D S~P'O<'~"" h.II, G l' "''i'' ''''' 'o .d•..,..."., '''li' ,<<1 "",<. ".

Figure fi - Sotid 1:t'OliJ1:.I' oIIII (' Pilhara Btock sl/()I\'inK distribut íon afbase-mcta t mineralization in terms ofmajor Iithologies andstructurcs ífrom Marston and Graves. 198/) 140 Revista Brasileira de Geocténctas, Volume 12 (l-3), 1982

However, some mineral deposits are markedly anomalous, Problems with such a model include the apparent subdued These include stockwork Mo-Cu deposits associated with topography ofthe basin margins, the relative stability ofthe subvolcanic porphyry intrusives (e.g. Coppin Gap, Copper basin, andits potential longevity: the volcanic stage could Hills, Gobbos), and small voicanogenic massive sulphide have extended for more than 0.15 b.y. and total tectonic deposits with signicant Pb and sulphates (e.g. Big Stubby) evolution may have extended over ca. 1.0 b.y. (Fig. 7). An within associated calc-alkaline voicanics (Fig. 6). Bedded additional problem is introduccd by Eriksson's (1981) in­ barite deposits occur with silicified sediments within basait terpretation of the southern margin of the Block in the east sequences, and are either primary or replaced gypsiferous Pilbara as a rifted continental margin, rather than a conver­ evaporites (e.g. Groves e/ al., 1981). The only Ni-Cu sul­ gent margin, on the basis of a regional transition between phide deposits are subeconomic (Fig. 6), and are mainly platforrn and trough sedimentation (Fig. 5). More detailed associated with subvolcanic intrusive gabbroids (e.g. Mt combined stratigraphic-sedimentological studies are requi­ Sholl) or with Fe-rich sediments (e.g. Sherlock Bay) rather red to test the vàlidity of this interpretation. than komatiites 01' intrusive dunites (e.g. Marston et al., 1981 ). Epigenetic, probably metamorphogenic (Groves e/ al., in press), stratabound gold deposits are one of the most consistent features of Archaean greenstone belts. However, while such deposits are widespread in hydrothermally ai­ tered voicanic and sedimentary hosts, the total gold produc­ tion of the Pilbara is anomalously low, For example, the productivity of ca. 0.8 kg Au per km' ofexposed greenstones ~'. contrasts markedly with ca. 32 kg Au per km' for the Eastern OOlO Goldfields Province of the Yilgarn Block. Lack of an ade­ h quate source of Au may account for the lack, to date, of i significant gold discoveries in mature fluvial sedirnents of the Oorge Creek Group, """1'''"' .~.

'ROM

o•• TECTONIC SETTlNG Despite the lack ofdirect evidence ""S."0",,,,,o,,,, for a sialic basement to the greenstones, their regional con­ tinuity and evídence for ao extensive shallow-water environ­ ment throughout volcanism and sedirnentation argue for a relatively stable, intracratonic depositional basin 01' basins. Sulphates in synvolcanic stratiform mineral deposits reflect a shallow hydrosphere. It is difficult to evaluate the tectonic setting of the terrain, and whether there are modern piate­ -tectonic counterparts, because ofthe reconnaissance nature of data 00 volcanism, sedimentation, metamorphism, defor­ mation and geochronology. ln terrns of modern intracratonic settings, the terrain exhibits some characteristics of the are and marginal basin system ofan active continental margin (e.g. Windley, 1981). PwtON'C ln particular, the coexistence of contemporaneous Andean­ EvENTS -type calc-alkaline granitoids, volcanics and subvoicanic

intrusives (Bickle e/ ai., in prep.), with associated porphyry­ SOO"""'"'0<" OE.OS"'ON~C ~.a~",,,,,,.,,. ~v,NtS ,------, • -, 1 __ w",,,. GO"" r""," -style Mo-Cu and small Pb- and barite-rich (Kuroko-type?) "'ur<~"." 1 volcanogenic sulphide deposits are consistent with such a """0 0",'1 ... ,.

setting, Although individual low-strain greenstone seg­ PI(....~. IlOC«

ments between granitoíd domes show coherent stratigraphic nerONO· MET..... OR.."C relations, evidence of extensive strike-slip displacernents EVEN1S within high-strain belts make it impossible to confidently reconstruct the pretectonic distribution ofrnajor lithologies, PW'ON'C s,"O"'·""OO An additionallimitation is the cover ofthe Hamersley Basin. ,vEN!. -,-,-''::'.~ t "'"'''0''' i. O"""·, Nevertheless, the greeristone belts do show a crude polarity P..·.,••O"O·.O·.,.... ' --1-'-'-- with exposed calc-alkaline voicanics, together with highly OE.osmO>l~(. magnesian uitrarnafic intrusives, concentrated in the sou­ • EVENrs ,. ,. thern and central zones of the east Pilbara and the west " " .. Pilbara (Fígs. 5 and 6). This is at least consistent with a convergent margin along the southern margin of the Figure 7 -lnterpreted crustal evolution of the Pílbara and Yi/garn Block at the volcanic stage, Furthermore, there is local cratons and the distributíon of mineralizatíon with time in these evidence for thrusting and tectonic thickening, with tectonic cratons. Geochronological canstraintsare,díscussed inthe text, Com­ translation from the south-west, in the west Shaw belt bined resources of Cu and Zn are used to estímate temporal dístrí• (Fig. 6), near this proposed margin (Bickle et ai., 1980; butíon of Cu-Zn-Pb deposíts. Gold resources are shown tn terms of Bettenay et al., 1981). host-rock and prohable ore-formationaí ages Revista Brasileira de Geociências, Volume 12 (1·3), J982 141

At this stage, there are insufficient data to adequately conglomerates (e.g. Goode, 1981). Smith (1976) pointed out constrain or test detailed tectonic models. Given the length lhe exploration potential of basalt sequences for Cu minera­ of time available, the volcanic and early sedimentary stage lization of Keweenawan-type, but no significant minera. may even represent different tectonic settings from the late lization has been discovered, Small Mn deposits also occur sedimentary stage. These could mark evolutionary steps bet­ in a variety of environrnents in the Hamersley Basin, and ween initiation of the basin and late gravitationally driven crocidolite has been mined from the Hamersley Group, rise of granitoid diapirs through tectonically thickened, The enriched Fe-ores of the Hamersley Group make the dense greenstone cover. Hamersley Basin a major rnetallogenic province (e.g.. Blo­ ckley, 1975). The hematite-rich ores, extending in places to a depth of400 m, are controlled hy both stratigraphy and Hamersley basin REGIONAL ASPECTS The Ha­ structure, and appear to have formed in the Proterozoic mersley Basin (ca. 100,000 km") contains thc Fortescue and by deep circulation of groundwaters (Morris et ai., 1980). Harncrsley Groups which developed as intracratonic vol­ The greater size of the Fe deposits relative to those of the canicand shclf-facies cover scqucnces frorn prior to 2.7 b.y. Archaean greenstone belts (Fig. 7) mirrors their vast regional to at least 2.5 b.y. The basin now comprises shallowly extent, in turn retlecting the greater stability and possibly dipping. essentially undeformcd sequenccs in the north more consistent water depth of the Proterozoic basins, which becornc more deformed and metamorphosed towards thc Ashburton Trough (Fig. 2). The Fortcscue Group (e.g. Goode, 1981) consists of ex­ YILGARN BLOCK The Yilgarn Block (650,000 km') is tensive flows ofsubaerial to submarine, commonly vesicular one of the largcst intact segments of Archaean crust in the or amygdaloidal basalt intcrlayered with thick rnafic tuffs world. Its regional continuity makes it ao important area and volcaniclastic units, and more rarcly felsic lavas and to test hypotheses of tectonic and crustal evolution and volcaniclastic rocks and carbonate sediments. The lateral metallogeny. The mean crustal thickness of ca, 30 km extent of basal units is restricted to palaeolows developed (Drummond et at., 1981), is exceeded (40-50 km) on those "over Archaean greenstones, whereas upper units are re­ margins which correspond to surface exposure ofhigh-grade gionally more extensive shallow-shelfsequences. The pattern gneisses of the Western Gneiss Terrain (Figs, 2 and 3). of restricted basin developrnent over greenstone is also The remainder ofthe Block eonsists ofgranitoid-greenstone evident in earlicr (pre-Hamersley Basin) deposits (T.S. terrains, subdivided into the Murchison, Southcrn Cross Blake, per. comm., 1981), suggesting a transition from late and Eastern Goldficlds Provinces (Figs. 2 and 3) on the Archaean to early Protcrozoic evolution through protracted basis of contrasts in lithofaciesand structural patterns (e,g. uplift of granitoid domes. This postdates the main Archean Gee et ai. 1981, following Williams, 1974). The tectonic tectonic events. histories of ali components of the Block are linked by the The sedimentary components of the Hamersley Group intrusion of voluminous granitoids at ca, 2.7 ±0.1 b.y. form a broadly alternating sequence of shale. or shale and Exposure is generally poor due to prolonged weathering dolomite, and BIF, generally with subordinate shale. Igneous of the essentially tlat surface, and this severely reduces the rocks form over 40% of the Group, mainly as thick dolerite confidence with which regional correlations can be made. sills and overlying acid voleanics in the upper part of the Gee et ai. (1981) suggest that the surface is an exhumed sequence. A feature of the Group is the lateral continuity Proterozoic unconforrnity implying that present crusta! of the internal stratigraphy (e.g. Trendall, 1975) suggesting layering is an inherited Arehacan fcature. exceptional stability of the deeper shelf environment. Des­ pite this, isopach and sedimentological data suggest that granitoid domes and greenstones in the basement continued High-grade gneiss tarrain The Westcrn Gneiss Terrain to exert some influence on sedimentation (D,M, McConchie, forms an arcuate zone around the western periphery of the per. cornm., 1982). Basement intluence is also indicated by Y.ilgarn Block (Figs. 2 and 3). It comprises multiply deformed coincidence of synclines and anticlines with greenstone belts and metamorphosed scdimentary sequences intruded by and granitoids respectively in the Archaean basement (Gee, mafic and ultramafic bodies including minor anorthositic 1979). Similar controls on deforrnation of covo r sequences and chrornitite layers, and infolded with granitic gneisses. are evident in the Nabberu and Bangemall Basins (Gee, Both shallow-water shelf facies sediments (including con­ 1979). glomcrates, quartz sandstones, shales and calcareous sedi­ The original extent of the depositional basin of the For­ ments) and trough facies, greywacke-dorninated sediments tescue and Hamersley Groups is unclear. The present boun­ are recognized (Gee et al., 1981); BIF is also present. Large daries of the Hamersley Basin may, in part, be erosional thickness of ultramafic and mafic voleanics that dominate (northern margin) and, in part, tectonic (southern margin), the greenstone belts to the east are absent: felsic volcanics so that only part of the original basin is preserved. are difficult to recognize. The consistently high metamorphic grade, upper amphibolite facies with local granulite facies, MINERALlZATlON Mineralization styles of the early and the possible occurrence of both moderate and high Proterozoic cover sequences, in which major mineral de­ pressure metamorphic belts (Oee et al., 1981) also contrast posits (Fe, Mn, Au, U) are sediment-hosted, contrast with with the metamorphic patterns in the granitoid-greenstone those ofthe Archaean granitoid-greenstone belts where most terrains (Binns et al., 1976). The structure of the terrain is important deposits are associated with vo1canic rocks (e.g. complex and no regionally consistent pattern has yet emer­ Lambert and Greves, 1981). ged. However, early recumbent folding is likely in the Mineralization in the Fortescue Group is sparse, being Toodyay area (Gee et ai., 1981). restricted to uneconomic tluorite, Pb, Zn, Ag, Au and Cu Suggestions that the high-grade gneisses represent the veins, minor stratiform Cu mineralization in pyritic shales coeval root zones of the granitoid-greenstones (e.g. Glikson and minor Au ·and U occurrences in near-basal imrnature and Lambert, 1976) are refuted by the contrast in Iithofacies 142 Revista Brasileira de Geociências, Volume 12 (1-3), 1982 and by available age data.Detailed geoehronologieal studies leiites, high-Mg basalts and komatiites, with associated vol­ in the terrain are at an early stage, but Sm-Nd and U-Pb canogenic sediments and intrusives. Felsic volcanic associa­ in zircon ages indicate that some supracrustal rocks and tions are mostly confined to discrete submarine to subaerial some precursors to granitic gneisses formed in the interval volcanic centres surrounded by subaqueous tuffs, epíclastic ca. 3.3 to 3.6 b.y. (de Laeter et aI., 1981b; Nieuwland and sediments and eherts that interfinger with mafic-ultramafic Cornpston, 1981). The age is thus eomparable with the sequences in distal areas. Some of these volcanics are cale­ Pilbara granitoid-greenstone terrains. Rb-Sr isotopie data -alkaline (e.g. Hallberg et al., 1976; Giles, 1981). Clastie indieate resetting of this system to about 2.5 b.y., and eon­ sedimentary associations occur predorninantly, but not firrn the prolonged evolution of the terrain (Fig. 7). exelusively, at higher stratigraphie leveis. Fluvial to shal­ The contact between the high-grade gneiss terrain and the low-rnarine sequences of conglomerate, sandstone or shale granitoid-greenstone belt is poorly defined, and there are mark loealised, in plaees fault-bounded, troughs in whieh numerous disrupted belts of gneiss within the latter. AI­ predorninant1y immature clastic sediments derived from though to date these gneisses have not yielded ages in exeess both greenstones and granitoids were deposited. of ca. 2.9 b.y, (see Groves et al., 1982) geophysically based These associations are common to ali three provinces erustal profiles suggest that the high-grade gneisses extend (Figs. 2 and 3). ln eaeh the lowermost, normally volcanie­ as a basement beneath granitoid-greenstone belts (see Gee -dorninated, sequenees probably extended over large areas et al., 1981 ; Archibald et ai., 1981). Low metamorphie grade whereas upper sediment-dominated sequences formed in greenstone belts do also oeeur within the high-grade gneiss more restrieted basins (Fig. 9). However, there are impor­ terrain (Gee et al., 1981) but relationships are equivocaI. tant differenees in stratigraphy and lithofaeies between the provinees (Gee et ai.. 1981) whieh suggest their development as separate, large basins (Fig. 9). Reeonnaissanee geoeh­ Granitoid-greenstone terrains Individual greenstone ronology (see summary in Greves et ai., 1982)", also suggests belts consist 01' thick volcano-sedimentary sequences. dis­ that volcanies of the Murehison Provinee may be older playing the sarne rock associations as the Pilbara Bloek (sec (ea. 2.9 b.y.) than those of the Eastern Goldfields and pos­ Gee et ai., 1981; Greves et ai., 1982). The most extensive is sibly Southern Cross Provinee (ea. 2.8 b.y.). the rnafic-ultramafic associations of submarine, low-K tho- ln the Murehison Provinee, the regionally persistent stra­ tigraphy comprises a lower mafic sequence, a felsic volcanic­ -sedimentary sequence, an upper rnafic sequenee and an unconforrnably overlying dominantly sedimentary sequence (Gee et al., 1981). BIF is comrnon, but high-Mg basalts and kornatiites are rare, Decreasing stratigraphic continuity with time probably reflects inereasing instability in the basin. A regionally persistent, but eontrasting, stratigraphy is recorded from the Southern Cross Provinee. Here a thin , lower eross-bedded orthoquartzite unit is overlain by a thick sequence of tholeiitie ando high-Mg basalt and a thiek vol­ caniclastic sequence, which in places is unconformably over­ lain by clastie sediments. BIF is again eommon and koma­ tiites are rare. These two provinces show some similarities to the east Pilbara Bloek in their degree ofstratigraphie inte­ grity and teetonie style with a distinetive pattern of grani­ toid domes and intervening greenstones. The eastern part of the Eastern Goldfields Provinee eon­ tains abundant BIF and is similar to the previously des­ cribed provinces. However, the Province is transected by a highly mineralized linear zone up to 200 km wide and almost 1,000 km long termed the Norseman-Wiluna Belt. It is eharaeterised by i) abundant komatiites, ii) an extensive belt of intrusive dunites along its north-eastern margin, ih) numerous felsic volcanic centres, tv] an abundance of sulphidie eherts and shales and eorresponding paucity of oxide-facies BIF, and v) development of late-stage restrieted basins of immature clastie sediments (Figs, 8,9 and 10; see Gee et al., 1981; Groves et ai., 1982). The volcanics are the youngest (ea. 2.8 b.y.) greenstone sequenees in Western Australia, Facies variations are rapid andextensive areas of shallow-water volcanism have not been reeognized. Eruption of mueh of the mafie-ultramafie association in deep water is inferred, and sueh sequenees show little evi­ denee of the extensive early alteration of equivalent litho­ iUb'oV 200km .~-~, ",'00 faeies in the Pilbara. There is controversy about the extent of stratigraphie correlations in the belt, and loeally ereeted Figure 8 - Solid geology of the Yilgarn Block showing distributíon stratigraphic sequenees are more eomplex than those ofother of base-metal mineratízation. The reference is the sarne as Fig. 6 greenstone belts. The Belt displays the eharaeteristie domai ([rom Marston and Groves, /981) granitoid pattern on a small scale, but on the regional seale Revista Brosileira de Geociêncías, Volume 12 (1-3), 1982 143

sequences, followed by syn-deformational water infiltration and metamorphism related lo uprise of hotter granitic ma­ terial from deeper in the crust (e.g. Archibald et al., 1981). The granitie terrain comprises banded gneisses and mig­ matized equivalents, plus intrusive synkinematic and post­ -kinernatic granitoids. The regional extent ofgranitoid types is constrained by poor exposure; for example, Arehibald et al., (1981) argue for widespread gneisses in largely unex­ posed areas, whereas Gee et ai. (1981) show these as large zones of post-kinematic granitoids. Ali granitoid types are dominated by biotite adamellites and granodiorites; horn­ blende granodiorites or tonalites that dominate many Archaean granitoid terrains are rare. Rb-Sr isotopic data from throughout the granitoid-greenstone terrains suggest emplacement ages of ca. 2.7 to 2.5 b.y. (e.g. de Laeter et al., 1981 b), àlthough some younger ages may be reset (e.g. Chapman et al., 1981). Recent Pb-Pb whole-rock and Sm-Nd data (e.g. McCulloch and Cornpston, in press; Biekle et al., in press) suggest that the granitoid terrain resulted from CROSS melting of ca. 2.8 b.y. gneiss precursors at ca. 2.75-2.7 b.y. to produce residual gneisses and synkinematic granitoids BASIN in agreement with field observations (e.g. Archibald et al., 1978). Emplacement of post-kinernatic granitoids continued lholeiile, BIF, to ca. 2.65 b.y., and possibly to 2.6 b.y. Rb-Sr isotopic data kOmaliiticbesatt alico.nmon suggest that metamorphism and subsequent cooling occur­ red in the interval 2.7 to 2.5 b.y. As in the Pilbara Block, the earliest granitoids (now gneisses) of the Eastern Goldficlds Province have a similar age (ca. 2.8 b.y.) to the lower greenstone sequences. The gneisses are geochemically dissimilar to the felsic volcanies of the greenstone belts but this may be due to subsequent ::;2~Okm '- modification rather than original differences. Essentially • serstc votcantc complexes synchronous development of granitoids and greenstones in m StratigraphicaUy high eorcreeuc sedlments Archaean granitoid-greenstone terrains appears a common feature as more geochronological data become available Figure 9~· Regional distributíon of Iíthofacies and reconstruction of (e.g. Barton, 1981), and perhaps explains the controversy major greenstone basíns in lhe Yilgarn Block. The lightly shadedarea that surrounds field interpretations of the relative ages of is the Norseman-Wiluna Belt (adapted from Gel' et ai.. 1981) granitoids and greenstones. Some greenstones of the Mur­ chison Province dated at ca. 2.9 b.y. are older than the oldest gneisses so far recorded in the granitoid-greenstone terrain, exhibits a marked NNW-trend which is partly related to but neither Pb-Pb nor Sm-Nd isotopie data are available disruption of the terrain into tectonic slices by major for gneisses from this Province. strike-faults (Figs. 8 and 10). A major contrast between the granitoid-greenstone ter­ Although detailed structural and metamorphic data are rains of the Yilgarn Block and those of the Pilbara Block only available from the Norseman-Wiluna Belt, it appears and the Western Gneiss Terrain is the longevity of iectonic that ali provinces .have a ralher similar tectono-rnetarnor­ evolution (Fig. 7). The Eastern Goldfields Province, apart phic history (e.g. Archibald et al., 1978; Platt et al., 1978). from late granitoid emplacement and metamorphie cooling, As in the Pilbara Block , there are abundant low strain and appears to have evolved over ca. 0.1 b.y. between 2.8 and low metamorphic grade zones within areally extensive gre­ 2.7 b.y., and evolution ofthe Murchison Province may have enstone belts. Near margins Dfsuch terrains with regionally occupied twice this time span. ln contrast, there is evidence extensive granitoids, and particularly in narrow greenstone that volcano-sedimentary and tectonic history of the gra­ belts, there are zones of higher grade metamorphism and nitoid-greenstone terrains of the Pilbara Block extended higher strain. Along the western margin of the Norseman­ over at least 0.5 b.y. from 3.5 to 3.0 b.y. and their total -Wiluna Belt, linear zones ofgranitoid diapirs are developed tectonic history may have been as extensive as ca. 0.9 b.y. within such high-grade zones. Estimates ofthermal gradients (see Hickman, 1981). Similarly, there is evidenee for cornplex derived from metamorphic assemblages range from ca. evolution of the Western Gneiss Terrain from earlier than 60 'C/km in high-grade zones to ca. 30 'C/km in low-grade 3.6 b.y. to cü. 2.7 b.y. zones (Archibald et ai" 1981). Two phases ofupright folding, the latter essentially synchronous with metamorphism, affec­ Mineralization The range of mineral deposits occuring ted the greenstone belts. Early recumbent folding is inferred within the Yilgarn Block overlaps that of many other Ar­ from structural studies (e.g. Archibald et al., 1981), but its chaean terrains, although nickel deposits are exceptionally regional significance is debated (e.g. Gee et al., 1981); abúndant and volcanogenic Cu-Zn deposits are scarce, there is little related regional disruption of stratigraphy particularly when compared to the Canadian Shield Major (e.g. Platt, 1980): Integration ofmetamorphic and structural deposits (e.g. Marston and Greves, 1981) relate to the vol­ data suggests rapid burial ofessentially unaltered greenstonc canic and syn-volcanic intrusive stage (e.g. Ni, Ni-Cu, Cu-Zn, 144 Revista Brasileira de Oeocunctes, Volume 12 (1-3), 1982

Ti-V, Fe), the intrusive granitoid stage (Sn-Ta-W-Mo, Li) important to the north of the Windimurra Intrusion in the and the metamorphic stage (Au). The type ofmineral depo­ Murchison Province (Fig. 8), enriched iron ores (e.g. Koo­ sits and their relative importance contrast markedly with Iyanobbing near Southern Cross), and small lithium, eme­ those ofthe Pilbara Block (Figs. 6, 7 and 8). There is a hete­ rald and Sn-Ta deposits associated with pegmatites. Orei­ rogeneous distribution of mineralization within the Yilgarn sen-, contact metasomatie- and stoekwork W.,Mo deposits greenstone belts, and the Western Gneiss Terrain, like most at Mulgine in the western Murchison Province (Fig. 8) high-grade gneiss belts (e.g. Lambert and Greves, 1981), represents unusual mineralization for an Arehaean terrain contains few important deposits. The major exception is the (cf. Lambert and Greves. 1981). Grcenbushes Sn-Ta-Li deposit hosted by deformed pegma­ tites in amphibolites (Fig. 8). There are also small Fe-Ti-V Tectonic evolution The earliest events in the Yilgarn deposits near Perth (Fig. 8), and Fe ores and small strata­ Block are rccorded from the Western Gneiss Terrain at ca, bound Cu deposits occur at Koolanooka and Wongan Hills 3.6 b.y. There are insufficient geochronological and structural respectively, probably in relics of greenstone belts (e.g. studies to define the complex and long-lived evolution ofthis Gee et al., 1981). terrain (Fig. 7). The nature of supracrustal rocks, however, Within the granitoid-greenstone terrains, Ni and Au depo­ suggests both shallow, shelf and deeper trough sedimenta­ sits are economically important, and both are concentrated tion without significant volcanism. Mature clastic sediments within the Norseman-Wiluna Belt. The nickel deposits containing ca 3.3 b.y. detrital zircons (Nieuwland and represent metamorphical1y-modified magmatic mineraliza­ Compston, 1981) suggest the existence of some stable base­ tion associated with komatiitic volcanism (e.g. Marston ment at this stage. et al., 1981). They can be subdivided into voleanic peridotite­ Although no unequivocal basement highs of this ancient -associated deposits that accompany thick komatiite flow high-grade gneiss terrain have been detected within the sequences (e.g. Kambalda) and intrusive dunite-associated granitoid-greenstone terrains, there is indirect evidence deposits (e.g. Agnew, Mount Keith) in which the dunites that the terrain extcnded as a basemcnt beneath them. This rnay be subvoleanic equivalents of overlying komatiites. includes the continuity of the geophysically-derived erustal Despite their probable close genetic links, there is a marked profile aeross the Block, the relative stability ofsome greens­ spatial separation of the two deposit types. Voleanic tone basins as deduced from regional continuity of volcanic peridotíte-associated deposits cluster in the southern part and BIF sequences and the presence of mature clastic sedi­ of the Norseman-Wiluna Belt whereas intrusive dunite­ ments at the base of the greenstones in the Southern Cross -associated are virtually restricted to a ca. 200 km long Province (Gee ct ai" 1981). The possibility of such base­ lineament marking its north-eastern boundary and to a ment extending between the Yilgarn and Pilbara Blocks is more restricted linear zone at Forrestania in the south­ also indicated by basement relics within the Capricorn -eastern part ofthe Southern Cross Province (Figs. 8 and 10). Orogen (Fig. 3). There is no positive evidence ofoceanic crust Most gold deposits within greenstones represent epigene­ within the greenstone belts, nor evidence for the partial tic, but stratabound, mineralization. This formed from melting of simatic crust in the present crustaI pro file. channeling of supercntical.llow salinity, H,O,CO, fluids Despite differenees betwecn greenstones ofthe Murchison derived by metamorphic dehydration and decarbonation and Southern Cross Provinces and the north-eastern seg­ of deeper, high-metamorphic grade parts of the greenstone ment of the Eastern Goldfields Province, ali appear to have sequence into lower metarnorphic grade zones where preci­ been deposited in large, initially relatively stable basins pitation ofgold from reduced sulphur complcxes took place. (Fig. 9). ln contrast, features of the Norseman-Wiluna Belt Greves et aI. (in press) describe the controls, which include suggest that it was a more active, fault-controlled trough preservation ofAu in source rocks to the metamorphic stage (Gee et al.. 1981; Archibald et ai., 1981), perhaps reflecting and a tectonic history involving late metamorphic uplift. The greater crustal -extcnsion. lnsufficient geological and geo­ Norseman-Wiluna Belt is the most highly mineralized chronological data are available to determine whether this (ca. 32 kgAu per km' of greenstone) and the deposits are was i) a separate basin, ii) superimposed on a preexisting selectively hosted by tholeiitic basalts or dolerites. Major basin now represented by flanking greenstones, or iii) the deposits are exposed along the axial zone of the Norseman­ more rapidly subsiding axial zone of a single large basin. -Wiluna Belt or along extensions of the linearzones contai­ The tectonic setting of granitoid-grecnstone formation ning intrusive dunite-associated nickel deposits (Fig. 10). in terrns of plate-tectonic settings. more specifícally are and Banded iron-formation (ca. 50% of production) is an im­ marginal basin systerns, or intracratonic basins above mantle portant additional host rock in the north-eastern Eastern plumcs has becn much discussed. Most authors working Goldfields Province and in the Murchison and Southern within the terrain find no positive evidence for subduction­ Cross Provinces (m. 14 kgAu per km' of greenstone). -related processes (see Greves et ai., 1979; Platt, 1980; Volcanogenic Cu - Zn ± Pb deposits within dominantly Gee et ai., 1981; Archibald et ai., 1981). They poínt out i) felsic voleanics (cf Sangster and Scott, 1976) are so widely the lack ofdetectable progressive age differences in greens­ dispersed in the Murchison Province (Fig. 8) that no regional tones, ii) the discrete nature of the identified felsic volcanic structural and stratigraphic controls are evident. The only centres, iii) the apparent lack of polarity across the green­ major deposit occurs at Oolden Greve within a dominantly stone belts in terms ofmagma types, iv) the absence ofdetec­ pyroclastic and voleaniclastic felsic pile. There are no table spatial and temporal variation in granitoid types, v) significant deposits in the Southern Cross Province, Within the contrasts in structural style, and vi) structural problems the Eastern Goldfields Province, a number ofsmall deposits with generation of a number of' essentially synchronous, including Teutonic Bore oecur in mixed felsic-mafic sequen­ subparallel greenstone basins by second-order convective ces within a zone containing undoubted calc-alkaline volca­ rolls during subduction. However, the recent indications nics (Giles, 1981) along the north-eastern margin of the from reconnaissance Sm-Nd geochronology that volcanic Norseman-Wiluna Belt (Figs. 8 and 10). Other deposit types sequenees in 'lhe Murchison Province rnay be older than include Fe-Ti-V deposits in layered gabbroids that are most those of the Norseman-Wiluna Belt , combined with recent Revista Brasileira de Geociênctas, Volume 12 (l~3). 1982 145

extension and related komatiite extrusion withín the mar­ ginaI basin. The duration of volcanism and tectonism of about 0.1 b.y. is .permissible in such a model (cf. Pilbara evolution). The tectonic signiftcance ofthe eastern boundary of this zone is further emphasized by the broadly coincident distribution of post-greenstone «2.5 b.y.) felsic alkaline intrusives (Libby et al., 1978). These suggest continued thermal activity (hot spots and possible rifting along this zone late in its history, but do not directly support a sub­ NI duction model for earlier greenstone development. A number of features are still difftcult to recoincide with '----__~'OO "rn an are-marginal basin model for this belt and for the Yilgarn terrain in general. The volume of intrusive granitoids that can be demonstrated to be produced by melting mantle or PHA~EAOZO'C basaltie crust is small; intrusive granitoids formed largely

COV~A by melting of previously formed granitic crust which is now considerably modified (e.g. Archibald et 1978).Available SEQUENCE ai. data on K/Na ratios -ofintrusive granitoids show no signifi­ cant polarity across the Belt (Libby et al., 1978). Metamor­ phic facies (Binns et al., 1976) show no obvious relationship to the felsic volcanic belt, except that felsic centres are generally in low-grade dornains, and do not resemble the metamorphic fades associated with subduction zones, Struc­ tural patterns and timing of granitoid emplacement do not match the marginal basin model (Platt , 1980). The Norse­

c;RANIlOI[l,GIlHN$TQNE 3ElIS man-Wiluna Belt is llanked by apparently more stable • fO""_."~ '01""'"'0''','"''''''''' '0<" greenstone basins on each margino Although no definitive [.::J UM,"o,""""o~. '0"'''," ,,~ mM,,_v"'.""'" geochronological data are available, the belt on the eastern side cannot be much younger than the Norseman-Wiluna Belt on the basis of constraints on ages of intrusive grani­ ___ M.",·, ,,,,,'o,,"""" toids. No equivalent flolarity is obvious elsewhere in the Figure /0 ~ Solid geotogy vI lhe eastcrn par! af file Yitgam Block Yilgarn granitoid-greenstoúe terrain (Figs. 8 and 9), although illustrating polarity S/UHl'n by Iíthofacíes, ore deposíts and aíkaííne the possibility that this is.due to extensive fragmentation of intrusíves. ln the rejerence VPA = volcanic perídotíte-assocíated and the other greenstone belts must be considered, IDA = íntrusíve dunite-associated; base map adapted from Thus, the teetonic setting of the greenstone belts cannot 1:/ ,000.000 rnap of lhe Geologica/ SUrI'l'Y of Western Austrália be defined with any confidence at present. There is no compe­ lIing evidence within the terrain for subduction-related pro­ cesses, but the possibility of an overall decrease in age of recognition that calc-alkaline volcanics (and possibly gra­ supracrustal rocks from west to east plus the polarity dis­ nitoids) are more abundant than previously considered played within the Norseman-Wiluna Belt and its north­ (Giles, 1981) necessitates re-examination of this questiono -eastern margin should be investigated further. What is ln the Eastern Goldfields Province, the greenstone belts urgently required to resolve tectonic models are more geo­ are sufftciently eontinuous to define broad trends in litho­ chronological data on the greenstone belts so that age faeies aeross the belts, but the presence of major strike variations can be established across the Yilgarn Block and faults and uncertainties about stratigraphic position of se­ lithofacies variations then examined within temporally-equi­ quences provide important constraints on the confidence valent' belts. Better constraints on the extent and directíon ofinterpretation. Some polarity is evidcnt inthe distribution of rnovement on major lineaments would also assist basin of selected rock and ore types within the Eastern Goldfields reconstructions. Province (Fig. 10). Most felsic volcanic centres, including undoubted calc-alkaline suites, occur as a belt along the north-eastern margin of the Norseman-Wiluna Belt: ali OISCUSSION OF CRUSTAL EVOLUTION ANO significant volcanogenic Cu-Zn deposits occur in this CONTR01S ON METALLOGENESIS Thc oldest belt. Hornblende-bearing granodiorites and tonalites are events recorded in the Precarnbrian shield of Australia are generally rare in the terrain, but several bodies occur within the Western Gneiss Terrain of the Yilgarn Craton at in this belt (Fig. 10) raising the possibility of a calc-alkalinc ca. 3.6 b.y., and it is possible that evolution of this terrain intrusive and volcanic suite. Immediately west of this zone, commenced even earlier. The terrain had a long (ca. I b.y.), ali significant intrusive duníte-associated nickel deposits of cornplex, as yet largely unresolved, teetonic history (Fig. 7) the Province occur along one of a nurnber of major linea­ involving repeated reworking ar granitoid and intrusive ments that typify the zone. ln contrast, the volcanic perido­ mafic-ultramafic rocks, migmatization and interleaving or tite-associated nickel deposits are dominant in the axial zone infolding of supracrustal rocks. The latter suggest largely of the Norseman-Wiluna Belt. On the regional scale, these non-volcanic shallow-shelfand deeper-trough sedimentation features are superficially consistent with the polarity of an not unlike that ofthe Proterozoic fold belts. There is indirect are-marginal basin system with the possible calc-alkaline are evidenec for stablc sialic crust older than 3.3 b.y. The lack systernin the northeast and the volcanic peridotite associated of significant volcanism probably led to the paucity of nickel deposits coinciding with the axial zone of maximum mineralization within supracrustal sequences. Significantly, , 146 Revista Brasileira de Geociências. Volume 12 (l~3), 1982 major Sn-Ta mineralizauon in this terrain, and smaller largely by the Geologieal Survey of Western Australia is an deposits in granitoid-greenstone terrains, relate to relatively excellent foundation, but to date there have been insufficient YOUng (ca. 2.8-2.6 b.y.) pegmatites and/or late granitoids detailed studies of criticai areas and criticai problems to derived from rnultiply reworked granitic crust. resolve many of the important questions. ln particular a Independent diachronous development of granitoid-gre­ rigid geochronological framework is urgently required to enstone terrains of the Pilbara and Yilgarn Cratons oc­ constrain the temporal and tectonic evolution of individual curred in the intervals ca. 3.5 to 2.8 b.y. and ca. 2.9 to belts and define temporal variations across the terrains: 2.65 b.y. respeclively. Greenstone basins appear to have the recent emergence ofsuch data has already changed pers­ developed 00 sialic crust in ao intracratonic setting; the pectives on evolution of the shield. For these reasons, the Wcstern Gneiss Terrain probably forms the basement in tectonic settings of the greenstone belts remain unclear. the Yilgarn B1ock. Des.Jte differences between greenstone The association of calc-alkaline volcanics and granitoids belts, they ali show a broadly similar evolution. They deve­ and broad polarity shown by some belts support analogues loped through an early voleanic stage dominated by wide­ to modem are and marginal basin systerns, but no direct spread mafic (± ultramafic) volcanism and more restricted evidence for subduction has yet been recorded from the felsic, in part calc-alkaline volcanism and associated sedí• greenstone belts. Because of this model involving develop­ mentation. Some íntrusive granitoids, now largely strongly ment of intracratonic basins above mantle plumes are also deformed gneisses, were synchronous with early volcanism. attractive. The subsequent history was dorninated by clastic sedimen­ Despite the limitations discussed above, discrete greens­ tation, involving both greenstone and granitoid detritus, tone basins defined on the basis ofdistinctive lithofaciesand in more restricted depositional basins. Early subhorizontal stratigraphy also have distinctive gross tectonic patterns deformation involving thrusting, recumbent folding and and metallogeny, suggesting a fundamental relationship crustaI thickcning, was followed by at least two phase of between these parameters (Fig. II I. It is suggested that upright folding. These were accompanied by low to moderate contrasts in these pararneters (lithofacies, tectonic partem. pressure rnetamorphism. widespread and crustal melting metallogeny) may relate to variations in crustal extension and granitoid emplacement, and gravity-driven diapiric during basic development (cf. McKenzie, 1978; de Wit uprise of granitoid and gneiss diapirs, Large strike-slip et aI., 1981I. A possible temporal progression is suggested displacements accompanied the later events. Although the between ii old (ea. 3.5 b.y.), long-lived, poorly-mineralized regional tectonie patterns of the greenstone belts are com­ basins of the east Pilbara formed under tectonic regimes monly dominated bythe distribution of diapiric granitoid involving a low degree 01' extension, ii) transitional basins domes, many ofthe structures and much ofthe strain within 01' the Murchison and Southern Cross Provinces, and greenstone sequences relates to earlier deformation events. iii) young (ea. z.s b.y.): relatively short-lived, exceptionally The distribution of rnineralization is extremely heteroge­ well mineralized basins of the Norsernan-Wiluna Beit rela­ neous between and within greenstone belts, but ali contain ted to greater extension. mineral deposits associated with volcanism and syn-volca­ Active extension during formation of the greenstones of nic intrusion and sedimentation (e.g. Cu-Zn ± Pb ± Ba; the Norsernan-Wiluna Belt is interpreted to have produced Mo-Cu; Ni-Cu ; Ti-V; Fe), late granitoid emplacernent (e.g. an active, initially rapidly subsiding, fault-controlled basin. Sn-Ta ; W-Mo; Li) and metamorphism (e.g. Au). Varíations in subsidence rate and water depth from the Severe constraints are provided on the erection and tes­ margins to the centre of the basin would account for abrupt ting of any tectonic mode by the reconnaissance nature of facies variations resulting in lack ofstratigraphic continuity, much ofthe relevam data. The regional framework provided The linear, fault-bounded nature ofthe basin also inlluenced

LOW EHENSION • SLOW SUBSIDENCE INTERMEOIATE EXTENSION HIGH EXTENSION - R/lPIO SUBSIOENCE

EXAM,PLE PILBARA BLOCK MURCHISON ANO SOUTHERN NORSEMAN·WILUNA llELT CROSS PROVINCES 200' km ca 200 km BASIN FORMATlON VOlCANIC ~:::::::::::;:§:,;~l STAGE ••• ++" ••••• +•••• '," ++' •••••••• +., •• , •• Reg'anal conlinuily 01 .olcaniç Regional cOnlinuily oJ volcanic sequoncos soquences Complox .olcano_sedimenta,y st,alior"('hy Basalta domlnanl - komaliites 'ere Basalls domin.nt - komallitesrarc Komalillos and basalts cammon LITHOFACIES Calc_alkaline VOIC8',lcs .a,lable Isolaled calc-al~alin" I'olcanics Calc·alkalino 'otcanics common . Cher! o, e.apo'il"S dominaM Blf O' cher! dominanl Sulphidic chens and al\ales dominant

INTERMEOI/lTE STYLE .....+ .... REGIONAL '''''' TECTONIC G'eenstone eens ,... ' . STYLE .,." .. Low melamarPh,c g,ade ...... , LZJ l1igl1 n1Ul.mofphic grade ...... , .. ,...... [ITIj.G,anilaid. and gMisses ...... " , '''''''' ...... " ...... ,+ ... " " " + ...... , ...... , ...... ""''''''+" Poo'ly mlnelallzed: Mode,alely mine,allzed W.. II minefalized Bodded ba,ilo Blf & volcanic-hosled Au dOposits Volcanlc·hosled Au deposito Small yolcanaganic Cu·Zn·Pb-Ba depoalts Volcanogonic Cu-Zn deposita Komallile-I\osted Ni-Cu depOsilG METALLOGENY Sn-Ta pegmatlles Sn-T" Or W·Mo in o,anftoids Volcanooonic Cu-Zn dOPOGilG NI·Cu oabb'oida fe_Ti_V in gabb,oids Li·Sn·Ta in poomalilOG Figure 11 - Schematic diagram shawíng tithofacies, tectoníc patterns and mineral deposíts ofgreenstone beíts [ormed under contrastíng exten­ sional regimes. An íntracratonic basin, unreíated to a convergem margín is shown for conveníence. Some [auíts would inevitablv be presem in rhe Pííbara-type bastn Revista Brasileira de Geocténcías, Volume 12 (1·3), 1982 147

late r deforrnation and hence the gross tectonic pattern of tone basin. The marked scarcity of gold deposits relative to the granitoid-greenstone terrain (Fig. II). Linear belts of other granitoid-greenstone terrains probably relates to higher grade metamorphism and granitoid diapirs are rela­ early, intense hydrotherrnal alteration of the slowly sub­ ted to control of broad crustal uprise and more local grani­ siding voleanic pile and leaching ofAu from potential source toid diapirism by edge effects related to initial fault contacts rocks, perhaps coupled with a less favourable plumbing sys­ between greenstones and sialic basement (e.g. Archibald and tem for metamorphic Au-bearing fluids due to relative lack Bettenay, 1977): variable wavelength of diapirs relates to of major reactivated faults. variations in thickness of greenstone cover. The greenstone belts of the Murchison and Southern Distribution of rnineralization can also be explained in Cross Provinces may represent transitions between the extre­ terms of such a mode!. Extensive faulting in the central part me situations discussed above, Their lithofacies, tectonic of the basin led to extensive deep-water eruption of koma­ styles and metallogeny ali show interrnediate character tiites carrying immiscible sulphide liquids that formed the (Fig. 11) and at least the Murchison Province may have voleanic peridotite-associated Ni-Cu deposits (Fig. lO). evolved over an intermediate time interva1. From the view­ Low c1astic input and reducing conditions led to develop­ point of metallogenesis, the most important factor is the ment of largely exhalative, sulphidic, interflow sediments. development of oxide-facies BIF's. These are not only hosts The spatially separated intrusive dunite-associated Ni-Cu to Fe ores, but also are the preferred host rocks for metamor­ deposits (Fig. lO) may represent subvolcanic equivalents of phogenic Au deposits, providing an abundant Fe source for the volcanic peridotite-associated deposits in marginal, less reaction with Au-bearing sulphide complexes to precipitate active and shallower water environrnents. The rapid sub­ pyrite and hence induce deposition of Au (e.g. Greves sidence and consequent rapid accumulation of volcanics in et al., in press). the axis of the basin restricted fluid influx and thereby hy­ The progressive coupled change in evolution and metallo­ drothermal alteration at the volcanic stage. Thus original, geny ofgreenstone belts with time, as shown in the Western probably sulphide-bound, Au contents of thick sequences Australian Shield, rnay be a world-wide feature. Certainly ofvolcanic rocks were retained as a Au source for metamor­ the marked concentration of major base-metal and Au mi­ phogenic deposits, Such deposits are best developed in the neralization within the younger (ca. 2.8-2.7 b.y.) terrains axial zone (Fig. lO). Additional factors influencing localiza­ so clearly shown onFig. 7 is a factor common to most tion of major gold deposits along linear zones (Fig. lO) Archaean cratons. include fault reactivation, and hydraulic fracturing related The change from Archaean to early Proterozoic evolution to differential uplift along these faults. Thus the major faults appears transitional, with earliest sedimentation and sub­ provided channelways for metamorphic Au-bearing fluids. aerial to shallow-watcr volcanism restricted to paleolows The scarcity of volcanogenic Cu-Zn deposits is somewhat on greenstones due to continued uprise of granitoid domes. surprising in this environment, but the subaerial nature of The more extensive supracratonic shelf sequences of the many of the felsic centres concentrated along the rnargin Hamersley Basin signalled a major change in metallogenesis of'the active basin probably inhibited forrnation and/or . from Archaean volcanic-associated ore deposits to major preservation of such deposits, stratiform to stratabound, sedirnent-associated deposits of ln marked contrast, the longer lived, relatively stable the Proterozoic. The earliest of these comprises the giant basin of deposition for greenstones in the east Pilbara is Fe ores within basin-wide oxide-facies BIF ofthe Hamersley intcrpreted to result from lower crustal extension. This re­ Group, constituting one of the major Fe ore provinces of sulted in developrnent of a regionally continuous volcanic the world. pile 01' relatively uniform thickness in an initially slowly subsiding sballow-water depositional basin. The subse­ quent tectonic pattern ar relatively uniformly-spaced and Acknowledgments The author is indebted to R.W.R. randornly-oriented granitoid-gneiss diapirsis a conse­ Rutland for his assistance and useful discussions concer­ quence of uniform thickness of greenstone cover over sialic ning Precambrian crustal evolution. R.D. Gee provided basement. Due to a low degree ofextension, thick komatiite much useful information on the Western Australian Shield. sequences were not developed, and magmatic Ni-Cu depo­ Much ofthe data and many ofthe ideas developed here have sits were restricted to gabbroid intrusions. The subaerial evolved in collaboration with colleagues at the University of nature of felsic volcanic centres, and the extensive shallow­ Western Australia. Contributions, discussion.- and criticai -water environments, were not conducive to formation comment by M.E. Barley, W.D. Batt, L.F. Bettenay, M.J. and/or preservation of volcanogenic massive sulphide de­ Bickle, T.S. Blake, J.S.R. Dunlop, B. Krapez, C.M. Lesher. posits. Small barite-rich, volcanogenic deposits and bedded D.M. McConchie, P. Morant, G.N. Phillips and J.R. (evaporitic) barite deposits within basaltic sequences are the Thornett are particularly acknowledged. Financial assis­ consequence of a shallow relatively oxidised hydrosphere, tance from A.R.O.S. grant no. E78-15665 is acknowledged. Porphyry-style Mo-Cu mineralization. further reflects the The paper is a contribution to LO.C.P. Project 91 "Metal­ subaerial to shallow-water, continental setting ofthe greens- logeny of the Precambrian",

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