May 1999 AGSO Research Newsletter 30 Deformation and gold mineralisation of the Archaean Craton, Western Richard Blewett1 & David L. Huston1 Early to late Archaean (3600–2780 Ma) Roebourne Group and 3120-Ma Whundo these structural and absolute-age data lithostratigraphy and structures in the Group — occur on either side of the Sholl constrain gold mineralisation temporally, north Pilbara granite–greenstone ter- shear zone, a multiphase strike-slip fault they need the support of information that rain are so well preserved over wide ar- (Smithies et al. 1999: Precambrian Re- discriminates the structural controls on eas, despite locally intense deformation, search, 94, 11–28). Locating a boundary mineralisation before we can effectively that we can link the chronology of or suture between the east and the west explore for deposits and understand the crustal/craton-scale events to episodes Pilbara is a continuing problem. This mineral systems. These controls are of mineralisation. Mineralising events search is frustrated by deep crustal summarised for a number of deposits in coincided with the formation, deforma- boundaries interpreted from gravity and Table 1. tion, and stabilisation of the Pilbara magnetic data not corresponding to faults The host rocks, structural controls, craton over its 800-Ma evolution, and with at the surface. and structural levels of gold mineralisa- the subsequent initiation of the We have established an event chro- tion in the Pilbara are varied (Table 1). Hamersley Basin (~2780–2400 Ma). As nology based on macro- and mesoscale Most deposits, including the largest ones, a contribution to the joint AGSO–GSWA structural observations (new work and are hosted by either mafic to ultramafic (Geological Survey of ) published compilations; details at http:// volcanics of the Warrawoona Group or ‘North Pilbara’ project for the National www.agso.gov.au/minerals/pilbara/ turbidites of the Mallina Formation or Geoscience Mapping Accord, we discuss table.html). In it, we identify eight phases Mosquito Creek Group. the tectonothermal and related mineral- of penetrative deformation manifested by Many of the gold deposits are asso- ising events of the north Pilbara gran- such features as folds, schistosity, and ciated with shear zones and the faulted ite–greenstone terrain (Fig. 11). shear zones (Fig. 12), and thus extend the contacts between units of contrasting fourfold division of Hickman (1983: op. competencies and lithologies. These Geological framework cit.). What caused the deformation? A shear zones and faults vary in strike Lithostratigraphic mapping in the north number of models have been proposed, length, throw, geometry, and structural Pilbara granite–greenstone terrain distin- including: level now exposed. The important con- guishes up to nine greenstone (volcanic– • intraplate (extensional and compres- trol of tensional zones in regional faults sedimentary) packages and at least 10 sional) tectonism with changes in far- is illustrated at the Withnell deposit, one largely coeval felsic intrusive events (e.g., field horizontal stresses controlling of the important new gold prospects Hickman 1983: GSWA Bulletin 127; Krapez the system; along the Mallina Fault Zone (Smithies 1993: Precambrian Research, 60, 1–45; • subduction-related marginal pro- 1998: Yule 1:100 000 geological map, Fig. 12). Recent lithostratigraphic and cesses and accretion similar to those GSWA) in the Mallina–Indee district geochronological studies have revealed operating during the Phanerozoic; (Fig. 11). Analysis of the regional struc- substantial differences between the east- and ture, coupled with mapping and logging ern and western parts of the terrain • diapirism and partial crustal overturn drillcore, indicate that gold mineralisation

(Hickman 1997: GSWA Annual Review, caused by density contrasts between postdated D7, and formed under a moder- 76–81). the granitoids and greenstones. ate- to high-level structural (e.g., brittle) In the eastern, older part of the craton, Instead of one single mechanism, all and fluid regime. The deposit appears to younger granitoid phases tended to be these processes may have been involved be hosted in a tension-gash array devel- intruded into older granitoid rocks (al- in shaping the Pilbara over the 800 Ma of oped during normal faulting with though not exclusively), forming large its development. Such interpretations downthrow to the north (Fig. 13A). (100+ km diameter) granitoid complexes. have an impact on metallogenic models Sheared contacts between units with Contemporaneous greenstones accumu- in terms of linking deposits in time and a marked competency contrast control- lated episodically in synforms between space (e.g., far-field or marginal tectonic led mineralisation in the Lynas Find dis- the developing granitoid complexes, most forces) rather than them being random, trict (Fig. 11), where the sheared (D4) across unconformable contacts (e.g., isolated occurrences (diapirism). contacts between ultramafic rocks and Buick et al. 1995: Nature, 375, 574-577; Van highly altered mafic schists typically host Kranendonk 1998: GSWA Annual Review Implications for gold gold deposits. As some of the shear zones 1997–98, 63–70). These observations mineralisation have displacement indicators of high-an- have led some workers to suggest that Gold mineralisation was episodic in the gle reverse-faulting, blind (stacked) crustal overturning and/or passive grav- Pilbara, in concert with magmatism and orebodies may have developed in the ity-driven tectonics was the driving force deformational events (Fig. 12). Lead-iso- footwalls of thrust-planes (Fig. 13B). behind the crustal evolution of the east tope data suggest that epigenetic gold A pervasive subvertical lineation

Pilbara (Hickman 1997: op. cit.; Collins et deposits formed at ~3410 Ma (D2), ~3200 formed by the intersection of two al. 1998: Journal of Structural Geology, 20, Ma (D3), ~2990 Ma (D4), and ~2900 Ma orthogonal foliations (S2 and S5?) is promi- 1405–1424). (D7), and structural relations suggest fur- nent in the complex geology of the In the west Pilbara, two distinct ther events at <3000 Ma (D4 or D5) and Klondyke district in the Warrawoona greenstone packages — the 3270-Ma <2880 Ma (D8; Fig. 12, Table 1). Although Syncline (Figs. 11 and 13C). Boudinaged

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and sheared sulphide-bearing veins (S2?) to the east (Fig. 13D). The timing of min- 41, 125–130), gold mineralisation devel- developed with long axes plunging gen- eralisation is unknown, but this ‘favour- oped at many periods in the evolution of tly in a steeply dipping fabric. Other able’ structure continues eastwards with the . In addition, much of boudin axes plunge steeply. Vearncombe a shallow plunge. Shear zones and high- the mineralisation appears to have been

(1995: Report to CRA Exploration, p. 45) strain zones (D5) control the Middle Creek syntectonic rather than post-tectonic. An identifies four phases of deformation; the line (Fig. 11). Their late dextral (D5) kin- understanding of these structural and main schistosity was associated with min- ematics is consistent with movement on temporal controls on mineralisation is es- eralisation and S-directed thrusting. An the regional Kurrana Shear Zone, which sential for gold exploration in the Pilbara. alternative view (Collins et al. 1998: op. marks the southern margin of the belt. cit.) is that the deformation is a function Study of these individual districts Conclusions of partial crustal overturning and granitoid demonstrates the variability and impor- • Regional structural analysis combined diapirism. tance of structural control on gold miner- with district- and deposit-scale stud- At Golden Eagle, in the Mosquito alisation in the Pilbara. Moreover, it ies have improved our understanding Creek Belt (Fig. 11), gold is stratabound indicates that, unlike the temporally con- of the Pilbara gold mineral systems. in the upward-facing limb of a reclined an- strained Yilgarn gold events (see Yeats & Further detailed and regional studies ticline (F4) that plunges subhorizontally McNaughton 1997: AGSO Record 1997/ are clearly needed.

Port Hedland I N D I A N O C E A N 2

Dampier 1A 21° 3

B C 4 D

6 22° 5 7

117° 118° 119° 120° 121°

16/WA/176 Cover Greenstone belts A Mallina-Indee district 1 Withnell deposit 5 Golden Eagle deposit Late Archaean B Lynas Find district (abandoned) 2 Mallina Fault Zone 6 Middle Creek line Mine Hamersley Basin C Warrawoona Syncline 3 Lynas Find mine 7 Kurrana Shear Zone Deposit Granitoids D Mosquito Creek Belt 4 Klondyke deposit

Fig. 11. North Pilbara Craton generalised geology, and localities discussed in the text.

Age (Ma)

3500D2 3300 3100 2900 2700 ? Sedimentation

Volcanism

Plutonism East Pilbara

Au mineralisation ? D1

Sedimentation Volcanism

Plutonism West Pilbara

Au mineralisation ?

D3 range D4 D5 D6 D7 D8 16/WA/179 Fig. 12. Pilbara Craton event chart, showing correlative penetrative deformational, plutonic, volcanic, and mineralising events from 3500–<2700 Ma.

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Table 1. Host and structural and age parameters of gold mineralisation at selected locations in the Pilbara Craton District/deposit Host rocks Structural controls Structural Absolute age Structural age level (Ma)

Bamboo Creek Komatiite, E–W-trending mylonitic Moderate to 3410 Syn-D2 Warrawoona shear zones (sinistral deep Group NE up shear)

North Pole/Normay Mafic schist, E–W-striking, N- 3405 Syn-D2 Talga Talga dipping veins Subgroup

Warrawoona/Klondyke Mafic and Subvertical lineation Moderate to 3400 Syn-D2 ultramafic schists, caused by intersection deep Warrawoona of S2? and S5? Group

Lalla Rookh/Lalla Basalts, basal part Dilational zones in axial Moderate? 3200 Syn-D3? Rookh of Salgash regions of E–W- Subgroup oriented ‘kink’ folds North Shaw/Big Bertha Metabasalts, Moderately east-dipping 2990 D4? Warrawoona veins Group

Nullagine/Golden Turbidites, Upright limb of Moderate to 2905? Syn-D4 or -D5 Eagle Mosquito Creek reclined, south-verging deep (3000–2950 Ma) Group F4 fold Lynas Find Amphibolite and Highly sheared contact 2890 Syn- or post-D4 talc schist, (thrusted?) between Warrawoona amphibolite and Group ultramafic schist Indee–Withnell Turbidites, Tension-gash array Moderate ? Post-D7 (<2880 Mallina developed on E–W Ma) Formation normal faults (S side up)

Indee–Peawah Turbidites, Tension-gash array Moderate ? Post-D7 (<2880 Mallina developed on E–W Ma) Formation normal faults (S-side up)

Indee–Becher Turbidites, N–S veins Shallow ? Post-D7 (<2880 Mallina Ma) Formation

• The recognition of late (e.g., post-D7) diapirism have been proposed. It is tive reviews, and acknowledge that they high-level gold associated with nor- possible that all three mechanisms may not necessarily agree with all the mal faults has implications for greater have occurred at some time in the statements in this paper Pilbara prospectivity, especially close craton’s long history. (in time and structural level) to the 1 Minerals Division, Australian Geological Sur- Acknowledgments vey Organisation, GPO Box 378, Canberra, Hamersley unconformity. ACT 2601; tel. +61 2 6249 9713 (RB), • There is currently little consensus on We thank Arthur Hickman, Martin Van +61 2 6249 9577 (DLH); fax the tectonic evolution of the Pilbara Kranendonk, and Hugh Smithies (GSWA) +61 2 6249 9983; email Richard.Blewett@ Craton, for which models including and Kevin Cassidy, David Champion, and agso.gov.au, [email protected]. intraplate, marginal/subduction, and Shen-su Sun (AGSO) for their construc-

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Fig. 13. (A, top) Stylised diagram of tension-gash veins developed in a competent unit with bedding-parallel

faulting (viewed north), where s1 is vertical (normal Andersonian faulting). Vein tips intersect the surface and strike east–west. The geometry predicts ‘stacked’ veins, each of which has a limited vertical extent. (B, upper middle) Sheared altered ore zone in McPhees pit in the Lynas Find district (viewed southeast). (C, lower middle) Intense intersection lineation at the Klondyke deposit (viewed southeast). (D,

bottom) Reclined parasitic F4 fold in the centre of the Golden Eagle deposit (viewed east).

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