Evidence for structural repetition in the greenstones of the Kalgoorlie district, Western Australia

Precambrian Research, 37 (1987) 1-18 Elsevier Science Publishers B.V., Amsterdam h Printed in The Netherlands

1

EVIDENCE FOR STRUCTURAL REPETITION IN THE GREENSTONES OF THE KALGOORLIE DISTRICT, WESTERN AUSTRALIA JOHN E. MARTYN* Minerals Department, Esso Australia Ltd., P.O. Box 4047, Sydney, N.S. W. 2001 (Australia) (Received July 7, 1986; revision accepted January 27, 1987)

Abstract Martyn, J.E., 1987. Evidence for structural repetition in the greenstones of the Kalgoorlie district, Western Australia. Precambrian Res., 37: 1-18. Regional mapping and air-photographic interpretation of an area of about 20 000 km 2 centred on Kalgoorlie provided compelling evidence that some previously published polycyclic stratigraphies cannot be substantiated. Large areas of rocks formerly proposed as younger cycles represent repetition, mainly by faulting, of a somewhat simpler stratigraphic sequence. The main lines of support are: (1) the similarity of older and younger mafic-ultramafic succes sions, (2) the tendency of 'younger' sequences to merge with 'older' ones when traced along strike, and (3) the abundant evidence of faulting along critical contacts. The subtle concordant nature of some of the faulting is consistent with an origin by thrusting at an early stage in the tectonic history, especially where repeat sequences are folded around major upright structures. Later reactivation of sheared contacts, and initiation of new ones during upright folding and faulting, and transcurrent shearing, is believed to have widely occurred. Gravitational gliding is proposed as a possible mechanism for thrust generation, this being consistent with evidence of earlier instability in the sedimentation style of turbidites, debris flows and olistostromes, however, conclusive evidence of a mechanism is lacking due to incomplete field evidence. The repetition is viewed as a rearrangement of recognisable elements of the local stratigraphy, rather than the result of a major collisional event at a plate margin. This tends to favour an intracratonic rather than an oceanic setting for the local greenstones, though the characteristic geological features of modern continental rifts have not been observed,

Introduction

Stratigraphic interpretations of the Archaean supracrustals of the Kalgoorlie Subprovince (Williams, 1974) of the Norseman-Wiluna Greenstone Belt (Fig. 1 ) have varied from the early, simple, twofold division into older mafic

*Present address: 49, Monteith Street, Turramurra, NSW 2074, Australia.

0301-9268/87/$03.50

and younger sedimentary* assemblages (e.g. McMath et al., 1953), to several later polycyclic schemes (Table I). These essentially invoked two or three episodes of mafic-ultramafic vulcanicity each followed by felsic volcanic and/or clastic sedimentary rocks. *All the supracrustals of the district are metamorphosed to grades ranging from below greenschist to upper amphibolite (Binns et al., 1976), however, preservation of primary features is generally sufficient to establish the progenitor, and the term 'meta' is eliminated from the text for the sake of brevity.

© 1987 Elsevier Science Publishers B.V.

GRANITOIDS & GNEISSES -7

onstrates that many of the stratigraphic sequences postulated by proponents of polycyclic schemes do not stand up to close scrutiny or extrapolation. They can be shown to be incorrect over substantial areas by regional mapping and interpretation, and examination of the few critical contact zones exposed. The majority can be better explained by various forms of structural repetition. The evidence for structural repetition presented herein was gathered from regional mapping and air-photographic interpretation, and much of it concerns the mapped and interpolated disposition of belts of mafic and ultramafic rock (Fig. 2 ), the critical element of all the polycyclic schemes. Reliability was greatly enhanced by regional and larger-scale magnetic data.

GREENSTONE BELTS

Outline o f e v i d e n c e 0

,

2jO0 Km

Fig. 1. Location of study area in eastern Yilgarn block.

Such schemes have, however, been regarded with suspicion ( Burke et al., 1976; Archibald et al., 1981), and more recently Griffin et al. (1983a) proposed a single-episode mafic-ultramafic to felsic and sedimentary model for the Kalgoorlie-Widgiemooltha area, in which the younger mafic-ultramafic unit (Sequence 6) of Gemuts and Theron (1975) was interpreted as the result of structural repetition, mainly by folding. Martyn (1986) outlined a scheme in which imbricate thrusting was a major cause of repetition. The existence of diverse stratigraphic hypotheses for the same assemblage of rocks is partly a consequence of insufficient critical stratigraphic information, particularly on the contacts between major units. This is largely due to poor outcrop, but it also reflects a complex structural style which is poorly documented except in detailed studies of relatively small areas with better than average exposure (e.g. Archibald, 1979; Gresham and Loftus-Hills, 1981). The present study dem-

Model stratigraphic succession To discuss large-scale structural repetition, we need to establish a model stratigraphic succession in the study area, preferably in a locality of weaker deformation. Regional mapping has highlighted the Black Flag area, 30 km northwest of Kalgoorlie (Fig. 3), which contains a succession beginning with a regionally extensive mafic-ultramafic suite, and evolving up through a mixed intermediate to felsic volcanic and sedimentary succession, to the Kurrawang Conglomerate (Horwitz et al., 1967; Glikson, 1971b), generally acknowledged as the youngest unit in the Kalgoorlie area. Most elements of the sequence persist for 50 km along strike, the mafic-ultramafic portion persists much further, recurring 20 km south of Menzies (Fig. 2). The succession is an appropriate stratigraphic model for the following reasons. (1) The locality lies on the southwest limb of the broad Ora Banda Anticline and the rocks are relatively weakly deformed. Outcrop is better than the average for the district. (2) A number of features of the succession

TABLE I Summary of published stratigraphies for the Kalgoorliedistrict Gemuts and Theron, 1975

Glikson,1971a

Williams, 1970

McCall, 1969

( SW part of study area ) Sequence 8 Polymict conglomerate pebbly greywacke

(Coolgardie-Kurrawang)

( NE 3rd of study area)

(Yilmia area )

Kurrawang Beds Polymict conglomerate Pebbly greywacke

Association 5, Kalpini Formation Mafic-ultramaficvolcanics

Cave Rocks Beds Cave Rocks Ophiolites

Sequence 7 Felsic volcanic rocks

Mungari Beds Greywacke, siltstone, argillite,including

Association4 Gundockerta

Merougil Beds Clastics

Sequence 6 Mafic-ultramafic volcanic rocks

Mt. Robinson and Red Lake Ophiolites -mafic-ultramafic volcanics

Association 3 Mulgabbie Formation Mainly mafic-ultramaficvolcanics & intrusives

Sequence 5 Mainlyclastic rocks

Coolgardie Ophiolites Mafic-ultramaficvolcanic and intrusive rocks, minor argillite

Association2 GindalbieFormation Felsic volcanic and clastic rocks

Lake Dam Beds Clastics

Association I MorelandsFormation Mafic-ultramaficvolcanic rocks

Wanda Wanda Beds Clastics, and Yilmia Ophiolites

Sequence 4 Felsic extrusive rocks clastic rocks

Formation Turbidite, conglomerate,acid volcanicrocks

Yilmia Dislocation

Sequence 3 Mafic-ultramafic volcanic rocks

Mandilla Beds Clastics, and Vesicular Ophiolite Belt

Sequence 2 BIF and clastic rocks

CausewayBeds Clastics, and Town Dam Ophiolite Belt

Sequence 1 Clasticsrocks and tholeiiticbasalt

(Fig. 3 ) suggest a normal progression younging from northeast to southwest with no significant structural breaks; these features are: (a) each major unit distinguished is lithologically unique within the section and is unlikely to be a structural repeat of another unit; (b) the mafic-ultramafic assemblage progressively changes from komatiite in the north, through high-Mg and tholeiitic basalts, to feldspar-phyric basalt as the highest unit in the

southwest. Two or more layered mafic sills young to the southwest; (c) bodies of feldspar-phyric basic rock, either flows, minor intrusives, or olistostromes, occur in the greywacke, shale, t u f f (turbidite) sequence, suggesting a stratigraphic connection between the mafic-ultramafic and turbidite assemblages; (d) the intermediate to felsic volcanic suite is overlain by sandstones and conglomerates

jini

.:."

¥il

.::::-:::i"" .....-."

N

I o 1o 20 km

Fig. 2. Simplified lithological map of the Kalgoorlie district. Mafic and ultramafic rocks - black. Intermediate to fetsic volcanics and sediments - stippled. Granitoid and gneiss - unshaded.

with clasts of the underlying volcanics. The clastic sequence grades up into the Kurrawang Conglomerate with increasing BIF, orthoquartzite, and felsic intrusive rock clasts. No unconformity or structural break of major significance is indicated. The importance of this locality in the context

of the study is that no younger mafic-ultramafic sequence is present.

Similarity o/'older' and 'younger' cycles In some published Archaean stratigraphies for Western Australia (e.g. Hallberg, 1983),

0

S

lOkm

~'~ granite&porphyry lit 6

°'~ Kurrawang Congl.

5

' ~ sandstone&congl.

/,

'-~ intermediate-felsic volcanics ~

greywacke,shale, chert, tuff

~0-] feldspar-phyric basalt

~

tholeiitic basalt

~

layered maficultramafic sills -~ omatiite,minor high M(j basalt J faults

Fig. 3. Simplified geological map of the Black Flag area near Kalgoorlie. This locality is considered to best represent the undisturbed stratigraphy of the overall study area.

significant differences have been distinguished between comparable lithological suites in older and younger subsections. In the Kalgoorlie district, however, such distinctions are not apparent. Williams (1969, 1970) (Table I) regarded his three major mafic-ultramafic volcanic units as differing slightly from one another, in relatively unimportant aspects such as the proportions of felsic intrusive rocks, and sedimentary intercalations. However, he remarked on their essential similarity. Likewise, the descriptions of Gemuts and Theron {1975) do not reveal any

gross differences between their mafic -ultramafic Sequences 3 and 6. The lithological assemblage of 6 is similar to the higher levels of the 'older' Sequence 3 in the Kalgoorlie area (e.g. Hannans Lake Serpentinite and above in the scheme of Woodall, 1965 ), and could be due to faulting or thrusting within the ultrabasic units, with only the overlying rocks being carried on the fault slices. Sulphide nickel mineralisation that generally occurred at a slightly lower stratigraphic level would only rarely turn up in the repetitions, in keeping with its absence

in Sequence 6 of Gemuts and Theron's scheme. Regional mapping during the study failed to find any consistent lithological signatures among the numerous tracts of mafic to ultramafic rocks in the Kalgoorlie district which could be used to make stratigraphic discriminations. Significantly, in areas" where a particular lithology is common in one of the previously proposed cycles, it is also common in the other. For example, mottled or variolitic high-Mg basalts:abundantly occur in both Sequences 3 and 6 of Gerputs and Theron (1975), in the KalgoorJm-Wldgmmooltha area. This is more consistent with structural repetition than with the less likely re-establishment of identical volcanic conditions after a long interval of felsic volcanism and sedimentation. The mafic plus minor felsic volcanic intercalations described by Glikson (1971a, b) in a thick sedimentary pile southwest of Kurrawang (Fig. 4), are strikingly similar to one another, and to elements of the underlying mafic-ultramafic and adjacent felsic suite near Coolgardie. On this basis alone, structural repetition by imbricate faulting is just as plausible as cyclic volcanism as an explanation, and regional structural configurations described below provide even stronger support for fault repetition. The repetition of a small-scale replica of a major sequence is also observed at Widgiemooltha (Fig. 5) where a splay of mafic -ultramafic rocks with lensoid sheared units along its western edge, diverges from the main rim of the Widgiemooltha Dome, and carries with it an identical sequence to that part of the northeast rim. This consists of an overlying black shale-chert unit followed by felsic volcanics, overlain by immature clastic sediments. Fuchsitic clasts occur in the felsic rocks in both the main and repeat sequences, and sulphide nickel deposits occur with ultramafic rocks in both cases (e.g. McQueen, 1981 ), enhancing the case for repetition. Such repeat sequences face the same way as their parent sequences, and an imbricate stacking is regarded as the main agent rather than folding.

Physical relationships of 'older' and 'younger' mafic-ultramafic suites

As Griffin et al. (1983a, b) have pointed out, the evidence for two or more stratigraphically superposed mafic-ultramafic suites in the Kalgoorlie district is equivocal at best given the meagre outcrop evidence. In most of the polycyclic-sequence type areas there is a physical separation of hundreds of metres to several kilometres between the comparable sequences, but mapping by the author shows this to diminish to zero in a number of instances as 'younger' suites converge on 'older' ones. For example the 'younger' Mount Robinson Ophiolites of Glikson (1971b) (Table I), between Coolgardie and Kurrawang (Fig. 4), may be traced northwestwards as a linear belt of sheared mafic-ultramafic rocks, which gradually broadens, and merges with identical rocks traceable north from Coolgardie (Fig. 4) after initial discordance to them. There are felsic intrusions along the discordant contact. Further north it is joined by a second belt mainly consisting of komatiites and high-Mg basalts, which occupies a sheared-out anticlinal position on the west limb of the Kurrawang syncline (Figs. 2 and 4 ). Both of these belts are best interpreted as structurally controlled appendages, either fault splays or sheared attenuated anticlines, of the Coolgardie Sequence (older Sequence 3 of Gemuts and Theron, 1975 ). The structural splays resemble in form the narrow ultramafic to felsic fault-slice on the Widgiemooltha Dome (Fig. 5), which likewise unites with its parent sequence. There are several instances of interconnecting mafic-ultramafic sequences that conflict with published polycyclic stratigraphies. Sequences 3 and 6 of Gemuts and Theron (1975) join on the northwest termination of the Widgiemooltha Dome west northwest of Widgiemooltha (Fig. 2 ), and there seems no reason to regard them as separate. The Mulgabbie and Morelands Formations of Williams (1970) (Table I) join around the southern side of the Bulong Anticline and again southeast of Golden

S

2

'I

FAULTS & SHEARS ,.~-"~

SYNCLINE, ANTICLINE

POSSIBLE THRUST CONTACT

• °

° •

°

+

+

+

+

+

+

+

~. + + 4 + +~... KURRAWANG

?

I',+ + + -I- + 4. + +

4- 4+ + .I- 4-

0 '

'

'

15 Km I

INSON

,..~T :-'-I 4-

Fig. 4. Simplified interpretive geology of the Coolgardie area. Lithologies: 1 - intrusive granitoids; 2 - mafic-ultramafic volcanic and minor intrusive rocks; 3 - major mafic sills; 4 - sediments and minor intermediate to felsic volcanics; 5 polymict (Kurrawang) conglomerate. Narrow, fault-bounded mafic-ultramafic slices in Mt. Robinson-Kurrawang area and to south interconnect with broad areas of identical rocks. Shear zones are commonly layer-parallel and show evidence of folding around anticlinal structures.

Ridge (Fig. 2). It is therefore very doubtful whether they are separate units• Sheared contacts

The foundation for any stratigraphy is a type succession in which formations can be defined in normal relationship to one another. Should contacts be everywhere sheared or unexposed, then it is impossible to demonstrate such rela-

tionships convincingly• A sheared contact does not necessarily mean that major dislocation is present, but it invalidates that locality as a typesection• On this basis there can be very few valid type sections in Archaean greenstones of the Kalgoorlie district, since strong deformation and poor outcrop is the rule. Yilmia Hill, which is 12 km west of Kambalda (Fig. 2), has been the type locality for two proposed younger mafic-ultramafic sequences; the Yilmia

:1:

.',~,

°

• ".y.-,.

..-.:.;

A

~

FUCHSITIC CLAST LOCALITIES

•

• ..'.'.'."t:

N

I

.'..-

~'WIOGiEM'OO~

, ..... . _. _ , : , , . . j , . .r...:~:

• .,.*.'. , ..- : : :,;.-~'-"

....

REPEAT

,.

...:,::

•. ..:, . s-s. • • • ".'.'." • ~'J" . • .,. ..._ ..~,,:.~: ....-.'..::

-.t-

.+

'~

+~ +

~

.,,J

J

+

4

+ +

DORDIE~ ROCKS +

+

+~

+ 4-

+ +

+ +

--

"--" INTERPRETED FAULT +

~

GEOLOGICAL BOUNDARY

........

TREND OF LAYERING

+

, ;..::

NICKEL SULPHIDE LOCALITY

ii..."Y 0 I

I

'

'

'

5Km i

Fig. 5. Simplified geology of the N.E. Widgiemooltha Dome, adapted from McQueen (1981). Lithologies: 1 - mafic-ultramafic rocks; 2 - felsic volcanic rocks; 3 - black shale and chert; 4 - undifferentiated sediments; 5 - intrusive granitoids; 6 - L. Proterozoic mAfic dykes. The narrow belt of ultramafic to felsic rocks on the flanks of the dome represents a fault repetition of the main succession immediately to the west. O p h i o l i t e s o f M c C a l l (1969) ( T a b l e I ) , a n d Sequence 6 of Gemuts and Theron (1975). The u n e x p o s e d critical 'basal' c o n t a c t b e t w e e n sedi m e n t s a n d u l t r a m a f i c r o c k s is t r a n s g r e s s i v e to the primary layering of the mafic-ultramafic

s e q u e n c e , h o w e v e r , as is a p p a r e n t o n t h e m a p of Gemuts and Theron (1975). A distinct arcuate fault trace is e v i d e n t o n s m a l l - s c a l e airp h o t o g r a p h s , a n d t h e c l o s e s t e x p o s u r e s to t h e c o n t a c t (a few m e t r e s a w a y ) at Y i l m i a Hill, are

of strongly sheared komatiite and silicified black slate. The contact is undoubtedly faulted, and does not demonstrate a mafic-ultramafic suite overlying a sedimentary one, whatever the stratigraphic facing evidence. The 'younger' Yilmia suite or Abattoir Line mafic-ultramafic belt of McCall (1969) is seen to be faulted against metasediments (graded greywackes) 1.8 k m west of Mount H u n t near Kalgoorlie (Fig. 6 locality B ). Here the facing of both greywackes and mafic sills is generally to the west ( Griffin et al., 1983b), but the actual contact is a shear, which is locally quartz filled. Facing reversals, doubtless due to a drag effect, are observed in the greywackes, and gabbro to the west has been converted to an amphibole-chlorite-clinozoisite schist. The mafic suite thus does not overlie the greywackes in the stratigraphic sense. Evidence from broad stratigraphic discordances (Fig. 6 ), and shearing, suggests that most, if not all, of the contacts between mafic-ultramafic assemblages and felsic volcanic-sedimentary suites in the Mount H u n t area are faulted. Whether this is obvious depends on where an observation is made, or how much interpretation beneath cover is attempted. The nature and position of the Mount H u n t and Boulder Faults (Fig. 6) have been determined from good exposure near Mount Hunt, and the locality is regarded as a faulted, folded repetition of the Kalgoorlie Goldfield sequence (e.g. Travis et al., 1971). Further north on the Mt H u n t Fault, however (Fig. 5 locality A), it appears on the ground that mafic-ultramafic rocks follow in sequence above a west-facing felsic volcanic to clastic sequence. Here, there is no fault or shearing exposed, and no apparent strike discordances. Since this locality is more representative of the norm in exposure in the Kalgoorlie district it is easy to see how critical faults can be overlooked. There are many other critical faulted contacts in the Kalgoorlie district, of which the following two are an illustration. (1) Between the Golden Ridge-Paddington mafic-ultramafic belt (immediately east and

northeast of Kalgoorlie, Fig. 2) and sediments to the northeast: subvertical linear bands of chrome-stained tectonic breccia containing altered clasts of sediments, basalt, felsic porphyry, and komatiite are accompanied by schistose rocks with moderately SSE-plunging lineations. Broad strike discordances are apparent from air photographs. Williams (1969, 1970) regarded this contact as an unconformity, but the evidence is more consistent with faulting. There is no evidence that this sequence is younger than similar rocks at Kanowna to the northeast as interpreted by Williams. (2) A narrow mafic-ultramafic belt along the west shore of Lake Lefroy, near Pilbailey Hill 12 km southwest of Kambalda (Fig. 2), has a well exposed, intensely sheared and interleaved western contact with semipelitic sediments. A strong stretching lineation plunges about 30 ° to the north. The deformation is overprinted by metamorphism. This is one of many narrow mafic-ultramafic suites in sedimentary terrain that have been regarded as volcanic intervals in thick sedimentary piles (e.g. McCall, 1969 ) and the locality is analogous to the Coolgardie-Kurrawang area described by Glikson (1971a, b). Where traced over distances of kilometres, contact shears are usually broadly parallel to stratigraphy in the horizontal sense, and close to conformable in the vertical, over the short distances normally available for observation. They are usually marked by topographic lows, and quartz veining is not especially prominent. Subtle discordances that confirm the faulted character are often apparent from air-photographs, detailed mapping, and interpretation. It is this general conformity which has misled many earlier workers into assuming normal successions.

Misinterpretation of stratigraphic facing directions In an area that has suffered two or more phases of isoclinal folding (Archibald et al., 1978) plus complex faulting, it would be expected that frequent stratigraphic facing

10 • 1

Kalgoorlie 6km z~

v\

. .\ ,, \v

v

,,

\v

:.:\ • -\

2

g~

I V~

ix A

Z~

,,

~\'..:~

N I

ix Z~

,,l'.'.

,X

!

...~A

Z~

A \..

Z~

~A/'~." HANNAN

%, " . ". " . -

•

LAKE

SHEARED CONTACT "~ EXPOSED

/

\ / /

"\!!!i!!!:

/

0 I

1 |

--

2 Km I

I

.

\

.

i'

/

~

.

.

.

Kambalda 35 k m

1 --

2

3

4

5

6

FAULT -- ESTABLISHED

~

~

...-.~.-,,,~ "1--70

,, - CONJECTURAL DIP OF BEDDING

"-~"""~"

- ~ ' - 75

OVERTURNEDBEODING

LITHOLOGICAL BOUNDARY APPROX,OR INFERRED

STRATIGRAPRIC FACING

Fig. 6. Mt. Hunt and vicinity: geological interpretation showing faulted nature of most contacts between mafic-ultramafic and felsic volcanic-sedimentary assemblages. At locality A, evidence for faulting is subtle and is inferred from observations elsewhere. At 'B' a sheared contact is exposed. Lithologies: 1 - komatiites; 2 - high-Mg basalts; 3 - dolerite, gabbro, pyroxenite; 4 - black slate; 5 - intermediate to felsic volcanics and coarse volcaniclastics; 6 - graded greywacke, siltstone, shale.

reversals will be present. Surprisingly there is often a consistency of f a c i n g d i r e c t i o n in measurements over substantial areas (cf. Glikson, 1971b) (though possible measurement sites are generally rather few in n u m b e r ) . This has c o n -

tributed towards interpretation of thick polycyclic successions especially where strike faults have been overlooked. In the case of the Kalpini Formation (Table I ), of W i l l i a m s (1970) to the east of K a l g o o r l i e ,

11 pillows and sedimentary structures have been recorded (Williams, 1970) that support a regional syncline of mafic-ultramafic rocks flanked by older clastic sediments. However, basalt pillows and graded bedding in an exposed, overturned, but undeformed major contact, 2.5 km east northeast of Gundockerta Hill (Fig. 7 ), support the opposite sense of facing. The mafic-ultramafic suite actually appears to be older than the clastics, and is here assumed to be a repetition of an older mafic-ultramafic suite outcropping to the northeast, and around the Bulong Anticline to the southwest (Figs. 2 and 7 ). Facings in the area that contradict this direction may represent preserved isoclinal fold limbs, the rarity of 'normal' directions being the result of poor outcrop and the shearing-out of opposite limbs. A similar situation was encountered by Archibald (1979) at Eundynie southeast of Widgiemooltha (Fig. 2) where careful measurements of sedimentary facing directions revealed a previously unrecognised isoclinal F1 fold set suggesting that mafic rocks assigned by Gemuts and Theron (1975) to a younger cycle are actually older than the sediments of the locality.

Morphology of mafic-ultramafic belts In the Kalgoorlie district, mafic-ultramafic volcanic rocks form an almost continuous outcrop up to 10 km wide along the northern, northeastern and northwestern margins of the greenstones (Fig. 2 ). This suggests that a continuous blanket of these lithologies existed at a lower stratigraphic level over a wide area. Many of the narrower, linear, fault-affected mafic-ultramafic belts to the south are joined to this outcrop, and appear to result from its tectonic disruption. Where this disturbance has been most severe, along the southwestern side of the Kalgoorlie district, there is a complex interleaving of mafic, and felsic volcanic-sedimentary assemblages. Here sediments commonly border the granitoid batholiths suggesting that stacking and repetition took

place before the emplacement of the batholiths. Instances where superposed mafic-ultramafic sequences separated by sediments are wrapped around granitoid domes or other major folds suggest a layer-parallel or thrust-type stacking. Such is the case in the northwest of the areafeatured in Fig. 4, where a 'higher' mafic -ultramafic suite can be traced southwards into the main 'older' body of mafic -ultramafic rocks along strike. Its domed western contact is locally discordant to photogeologically-interpreted strike trends in the adjacent sediments and is interpreted as of thrust origin. Other strongly arcuate and deformed mafic-ultramafic belts in sediments, such as the Republican-Bluebush belt of Gresham and Loftus-Hills (1981), 15 km east of Widgiemooltha (Fig. 2 ), may be refolded early thrust slices. Some mafic-ultramafic belts are extremely narrow, hundreds of metres or less, but since they still contain a full varied suite of normally massive volcanic rocks they appear to be structurally attenuated rather than stratigraphically thinned. This is often confirmed by observations of strong heterogenous deformation and discordances along contacts. Where they are traceable along strike into much broader and less deformed sequences, the situation is clearer, as from Paddington to Golden Ridge (Fig. 2). The tectonised nature of these belts is consistent with structural emplacement. Many belts are strongly linear and occupy major upright deformation zones that include transcurrent shears which truncate major upright folds. Whether they were generated in such zones, or modified by them from thrust-sheet precursors is considered below. Discussion There is a variety of processes by which structural repetition could have been brought about, the main ones being thrusting, reverse, transcurrent, and normal faulting; evolution of similar, overlapping, but essentially separate

12

!

?

11

FAULT UPRIGHT ANTICLINE OVERTURNED FOLDS STRATIGRAPHIC FACING SEDIMENTARY FEATURES IGNEOUS FEATURES DiP OF LAYERING OVERTURNED ,, VERTICAL

Y X

0

5

!

I

A

10Km a

B

"~~ ~ : : , : . . . ..'.:.-..~ . : . : ~ • :.::....:.:-:.;..." --.:.:.-..:2:-.2.:--:.-.:::......-++++ + ÷ +-k•~.'.:'-~:...:..:i-.".:..y + + + + + +

+

+ ~ 4.

4 +

4-

+ 4-

÷

+

÷:÷÷ + +

+ + + 4-

4-

-~-

Fig. 7. Kalpini - Gundockerta Hill area showing location of key contact in south. This west-facing contact is evidence for the anticlinorial nature of the 'Yindarlgooda Syncline' of Williams (1970) and the probable correlation of Williams' Kalpini and Mulgabbie formations. Legend: 1 - intermediate to felsic volcanics and sediments of Bulong Anticline; 2 - mafic-ultramafic rocks; 3 - intermediate to felsic volcanics and fine-grained sediments; 4 - conglomerate, arkose, turbidites; 5 - intrusive granitoids; 6 - Proterozoic mafic dyke.

sequences in parallel rift zones followed by reactivation of the controlling faults; and folding.

Thrusting Several workers in the Kalgoorlie Subprovince have published evidence for an early ( D 1 ) layer-parallel deformation event involving such

processes as recumbent folding, thrusting, and low-angle ductile shear, e.g. Archibald et al. (1978), Platt et al. (1978), Archibald (1979), Gresham and Loftus-Hills (1981), Chapman (1982), Spray (1985), M a r t y n and Johnson {1986). These studies generally relate to localities where strong north-to-northwest-trending upright folding and faulting is subdued or absent. T h e y include one well-documented case

13 of a narrow mafic-ultramafic belt generated by an overthrust in an area of east-west striking rocks (Chapman, 1982). In the Menzies area (Fig. 2), Martyn and Johnson (1986) described a suite of rocks which remain in near-horizontal attitude in places, and carry an intense layerparallel schistosity and minor recumbent folds. Since most of the Kalgoorlie district is affected by northwest to northerly-trending upright folds and faults, there are relatively few areas where D1 structures can be confidently discriminated. Where D 1 is expressed only as folding, fabrics may not have developed {e.g. Archibald, 1979). In other cases, notably at formerly deeper crustal levels near to some granitoid/greenstone contacts, broad zones of layerparallel foliation or schistosity have been generated (e.g. Platt et al., 1978; Spray, 1985; Martyn and Johnson, 1986). Comparable zones have been described from the Archaean Pilbara Block (e.g. Bickle et al., 1980; Bettenay et al., 1981). The examples of mafic-ultramafic slices folded around major anticlines described above are good reason to suspect an origin by thrusting for at least some of the structural repetition. It is quite possible that many linear, fault-controlled belts of mafic-ultramafic rock in sedimentary terrain also had an early thrust origin, but were later subjected to vertical or transcurrent shear stress. This would have exploited the original thrust planes, plus any volcanic/sedimentary contacts or other competency contrasts. Many thrust planes probably evolved in interflow argillite units and within the ultramafics, since shale and talc-carbonate schists occur on many sheared contacts. Generation of thrusts within the nickel-bearing komatiite suite at a level above most of the nickel sulphide lenses may explain why Gemuts and Theron's 'younger' Sequence 6 contained n o nickel deposits. Thrusting does not appear to have occurred on a scale comparable with many Phanerozoic convergent plate boundaries. There is no evidence of juxtaposition of strongly contrasting

domains, or of high-pressure metamorphism despite similarities in rock deformation to such provinces (e.g. Meneilly and Storey, 1986). The tectonics can be viewed more in terms of a rearrangement of familiar elements of the local stratigraphy, a situation more consistent with a closed or intracratonic setting, rather than an open plate margin. As such, intrabasinal gravity gliding is a likely mechanism (Fig. 8). This is consistent with the sedimentation style which is dominated by turbidites, and includes debris flow deposits. Olistostromes have also been reported ( Taylor, in Gee and Groves, 1982). In some respects t h e style resembles that proposed by de Wit {1982) for the Barberton Greenstone Belt. Felsic volcanism was intimately associated with the sedimentation, and it is possible that concomitant granitic intrusion into a dense sheet of mafic-ultramafic volcanics may have triggered the instability that first led to the sedimentation and later to gravity gliding tectonics.

Transcurrent, transpressional and reverse faulting Much of the faulting which is now recognised in the Kalgoorlie district is the result of compressional and transcurrent shear events, which may have had a long history of reactivation. These are marked by zones of sheared, phyllonitic, or brecciated rock, and varying degrees of strike discordance. The scale, direction, and chronology of movement of these structures are not fully understood, and their relationship to thrusting is not clear. For example, shear and breccia zones in the mafic-ultramafic belt from Paddington to Golden Ridge (Fig. 2) are nearvertical, but the trend swings to the southeast into an area where low-angle structures prevail (e.g. Chapman, 1982). In the absence of comprehensive outcrop evidence, the relationship of the two styles is conjectural. Either (1) the steep portion of the belt is a modified former thrust complex, or (2) thrusting and upright. shear concurrently evolved in a transpressional

14

Fig. 8. Possible sequence of events leading to tectonically stacked sequence. (1) Eruption of widespread komatiitic to tholeiitic volcanics followed by local felsic vulcanity and related granitoid intrusion. (2) Unstable tectonic phase involving localised uplift, erosion, and rapid deposition of turbidites, conglomerates, and debris flows, accompanied by further intermediate to felsic vulcanicity. (3) Increasing uplift and instability triggers gravity gliding and thrusting at mid to upper levels of pile, and broad zones of ductile deformation at deeper levels. A stacked sequence results. (4) Compression, and vertical and transcurrent faulting leads to upright structures, further repetition, and the illusion of a folded polycyclic stratigraphy.

regime similar to t h a t p r o p o s e d for p a r t of t h e Abitibi Belt b y H u b e r t et al. (1984). T h e first case is c o n s i s t e n t with s t r u c t u r a l histories proposed by A r c h i b a l d et al. (1978), a n d G r e s h a m a n d L o f t u s - H i l l s (1981), b u t c o m p r e s s i o n a l faulting a n d folding a s s o c i a t e d with t r a n s c u r r e n t fault s y s t e m s in general are also well doc-

u m e n t e d (e.g. S y l v e s t e r a n d S m i t h , 1976). T h e r e is a m a r k e d similarity b e t w e e n some of t h e n a r r o w e r m a f i c - u l t r a m a f i c slices in t h e Kalgoorlie area, a n d the C a m b r i a n g r e e n s t o n e belts of Victoria, Australia. T h i s r e s e m b l a n c e e x t e n d s to t h e turbiditic n a t u r e of flanking sediments. According to C r a w f o r d a n d K e a y s

15 (1978) the Victorian greenstone belts evolved from a marginal sea setting and were emplaced along reverse faults. The chronology of many of the major faults in the Kalgoorlie district may have included reverse fault movements, perhaps predating transcurrent shear. Hopefully structural analysis will be carried out in many of the newly established gold mines of the district on or near these structures, to obtain senseof-shear information.

Normal faulting The characteristic imbricate tilt block pattern of extensional continental rifts may produce numerous repetitions of stratigraphy, however, the individual blocks are usually bounded by faults approximately perpendicular to the dip of strata (e.g. Chapman et al., 1978). This is not observed in the present area where faults and shear zones are normally parallel or subparallel to primary layering. It is theoretically possible that repetition by block tilting may have occurred during extension in the depositional phase of the greenstones, but no obvious examples are preserved. A partial analogy could be drawn between layer-parallel ductile shears around granitoid margins (e.g. Spray, 1985 ), and ductile shears bordering Cordilleran metamorphic core complexes associated with Basin and Range tectonics (e.g. Wernicke, 1985), but major geologic differences between the two terrains inhibit close comparison. Nevertheless strongly layer-parallel foliated granitoid/greenstone margins may be a result of gravitational sliding off rising granitoid domes ( cf. Hickman, 1984), which is an extensional rather than compressional process.

Parallel rift systems Groves and Batt (1984) envisaged a more riftlike character for the greenstones of the Norseman-Wiluna Belt similar to adjacent terrain. Extension is implicit in the nature of much of

the vulcanism in the Kalgoorlie district. This is likely to have been most rapid during eruption of komatiites, and other high magnesium lavas showing high temperature petrographic features (e.g. Redman and Keays, 1985). It is unlikely that these were erupted in a rift which was well-defined topographically, since the sequences incorporate mainly fine-grained pelitic and chemical sediments. The continuity of mafic-ultramafic rocks around both east-west and north-south margins of the greenstone belt terrain in the northern half of the Kalgoorlie district also suggests a blanket-like development rather than discrete linear belts. Eruption was conceivably from zones of contemporaneous parallel fissures or 'hot lines' as suggested by Groves et al. (1978). Variations in mafic-ultramafic stratigraphy and mineralisation, e.g. between the Kambalda and Widgiemooltha Nickel Fields ( Groves and Batt, 1984) may relate to these separate fissure zones, but there is no strong indication that the events took place at grossly different stratigraphic levels in unrelated rift zones, at least until later coarse clastic sedimentation ( e.g. Kurrawang Conglomerate).

Folding Early-generation refolded isoclinal folds have been recognised in a number of localities in the Kalgoorlie district (e.g. Archibald, 1979; Griffin et al., 1983b) as discussed above. These have led to repetition of units, and an apparent thickening of successions. Thickening of the sequence by major isoclinal folds is apparent in the mafic-ultramafic succession around Coolgardie (W.M. Hunter, personal communication, 1983) ). Lack of recognition of refolded isoclinal folds at Eundynie near Widgiemooltha, led Gemuts and Theron (1975) to misinterpret an older mafic-ultramafic suite for a younger one, according to Archibald (1979). A major anticlinorial area misidentified as a syncline led to the creation of a new but apparently superfluous unit, the Kalpini Formation

16 by Williams (1970), as discussed in this study. Folding, however, does not seem to have contributed greatly to producing the imbricate-type fault slice repetition in areas such as Coolgardie to Kurrawang, where repeat sequences face the same way as parent ones. Resolution of the complexities of isoclinal folding in the D1 and D2 events of Archibald et al. (1978) would clearly lead to a greater understanding of the true thicknesses of units in the Kalgoorlie district if it could be carried out over a wide enough area. This is unfortunately impossible due to poor outcrop. Given the presence of these structures, and of numerous strike faults and other complicating factors, an accurate modelling of greenstone belt evolution for the area remains beyond reach, and only qualitative attempts are possible. General model

A simplified evolutionary model to explain the essential processes in structural repetition in the Kalgoorlie district is featured in Fig. 8, and the stages are discussed in the figure caption. The processes involve a transition from extension, through uplift, to upright compressional phases. The model views the main period of repetition to result from uplift and gravitational instability. Unfortunately this interpretation is poorly constrained by a lack of directional data on the proposed thrust structures. An alternative model which places thrusting at the early stages of compression is also plausible.

Summary and conclusions Substantial structural repetition and stacking has taken place in the southwest part of the Kalgoorlie Subprovince of the Norseman-Wiluna Greenstone belt. The evidence may be briefly summarised as follows: (1) there is a gross tithological similarity between previously proposed successive 'cycles',

which is especially apparent in the mafic-ultramafic elements; (2) the merging along-strike of supposedly younger sequences with their 'older' counterparts; and (3) strong shear deformation is apparent in many of the repeated sequences, especially along critical 'basal' contacts. The repetition has mainly come about as a result of faulting, although folding is implicated in some instances. The result has been illusory stratigraphic sequences consisting of repeated cycles of essentially similar rock types which had been proposed by several previous workers using the mafic-ultramafic elements of the succession as benchmarks. A feature of the repetition is long, linear or curvilinear to arcuate, rather narrow mafic-uttramafic slices. These in many cases merge with broader areas of identical rocks accepted as belonging to an older succession hosting most of the sulphide nickel deposits of the district (e.g. Sequence 3 of Gemuts and Theron, 1975). A number of tectonic processes may be involved in the repetition. A major contribution is thought to be from thrusting at an early stage in the tectonic history. This may have been in the form of gravity gliding, possibly from incipient rising granitoid diapirs or gneiss terrain to the west. The linearity of many of the mafic slices, however, suggests that upright shearing has also exerted an important influence on their deposition. The hindrance of lack of outcrop renders many of the structural problems implicated herein insoluble, but it is hoped that this study will stimulate further discussion of specific problems and more general aspects of greenstone belt evolution.

Acknowledgements Much helpful discussion took place with a number of people, notably Drs. T.J. Griffin, W. Keats, and W.M. Hunter of the Geological Survey of Western Australia, Associate Prof. D.I. Groves of the University of Western Australia,

17

who also critically read the manuscript, Dr. M.A. Etheridge of the Bureau of Mineral Resources, Canberra, and many former colleagues in the Minerals Department of Esso Australia Ltd. Dr. R.D. Gee of the Geological Survey is thanked for his comments on an earlier draft of the manuscript. Most of the 'fieldwork which contributed to the paper was carried out while the Author was employed by Esso Australia Ltd., whose support is gratefully acknowledged. References Archibald, N.J., 1979. Tectonic-metamorphic evolution of Archaean terrain. Ph.D. Thesis, University of West. Aust, (unpublished). Archibald, N.J., Bettenay, L.F., Binns, R.A., Groves, D.I. and Gunthorpe, R.J., 1978. The evolution of Archaean greenstone terrains, Eastern Goldfields Province, Western Australia. Precambrian Res., 6: 101-131. Archibald, N.J., Bettenay, L.F., Bickle, M.J. and Groves, D.I.,1981. Evolution of Archaean crust in the Eastern Goldfields Province of the Yilgarn Block, Western Australia. In: J.E. Glover and D.I. Groves (Editors), Archaean Geology. Spec. Publ. No. 3. Geol. Soc. Aust. Inc., pp. 103-120. Bettenay, L.F., Bickle, M.J., Boulter, C.A., Groves, D.I., Morant, P., Blake, T.S. and James, B.A., 1981. Evolution of the Shaw Batholith--an Archaean granitoid-gneiss dome in the Eastern Pilbara, Western Australia. In: J.E. Glover and D.I. Groves (Editors), Archaean Geology. Spec. Publ. No. 7. Geol. Soc. Aust. Inc., pp. 361-372. Bickle, M.J., Bettenay, L.F., Boulter, C.A., Groves, D.I. and Morant, P., 1980. Horizontal tectonic interactions of an Archaean gneiss belt and greenstones, Pilbara Block, Western Australia. Geology, 8: 525-529. Binns, R.A., Gunthorpe, R.J. and Groves, D.I., 1976. Metamorphic patterns and development of greenstone belts in the eastern Yilgarn Block, Western Australia. In: B.F. Windley (Editor), The Early History of the Earth. Wiley, London, pp. 303-313. Burke, K., Davey, J.F. and Kidd, W.S.F., 1976. Dominance of horizontal movements, arc and microcontinental collisions during the later permobile regime. In: B.F. Windley {Editor), The Early History of the Earth. Wiley, London, pp. 113-129. Chapman, D.M., 1982. Structural analysis of a volcanic associated Fe-Ni-Cu sulphide deposit: Carnilya Hill, Western Australia. B. Sc. Hons. Thesis, University of West. Aust. (unpublished). Chapman, G.R., Lippard, S.J. and Martyn, J.E., 1978. The

stratigraphy and structure of the Kamasia Range, Kenya Rift Valley. J. Geol. Soc. London, 135: 265-281. Crawford, A.J. and Keays, R.R., 1978. Cambrian greenstone belts in Victoria: marginal sea-crust slices in the Lachlan fold belt of southeastern Australia. Earth Planet. Sci. Lett., 41: 197-208. De Wit, M.J., 1982. Gliding and overthrust nappe tectonics in the Barberton Greenstone Belt. J. Struct. Geol., 4: 117-136. Gee, R.D. and Groves, D.I.,1982. Kanowna felsic volcanic complex. In: D.I. Groves and C.M. Lesher (Editors), Regional Geology and Nickel Deposits of the Norseman-Wiluna Belt, Western Australia. University of West. Aust. Geol. Dept. and Extension Service, Publ. No. 7: C3-C10. Gemuts, I. and Theron, A.C., 1975. The Archaean between Coolgardie and Norseman - - stratigraphy and mineralisation. In: C.L. Knight (Editor), Economic Geology of Australia and Papua New Guinea, I. Metals: Aust. Inst. Mining Metall., Mon., 5: 66-74. Glikson, A.Y., 1971a. Structure and metamorphism of the Kalgoorlie System southwest of Kalgoorlie, Western Australia. In: J.E. Glover (Editor), Spec. Publ. No. 3, Geol. Soc. Aust. Inc., pp. 121-132. Glikson, A.Y., 1971b. Archaean geosynclinal sedimentation near Kalgoorlie, Western Australia. In: J.E Glover (Editor), Spec. Publ. No. 3, Geol. Soc. Aust. Inc., pp. 443-460. Gresham, J.J. and Loftus-Hills, G.D., 1981. The geology of the Kambalda Nickel Field, Western Australia. Econ. Geol., 76: 1373-1416. Griffin, T.J., Hunter, W.M. and Keats, W., 1983a. Geology of the Kalgoorlie-Widgiemooltha District. In: P.C. Muhling (Editor), Eastern Goldfields Geological Field Conference 1983. Abstracts and Excursion Guide. Geol. Soc. Aust., and Eastern Goldfields Discussion Group, pp. 7-8. Griffin, T.J., Hunter, W.M., Keats, W. and Quick, D.R., 1983b. Description of excursion localities. In: P.C. Muhling (Editor), Eastern Goldfields Geological Field Conference, 1983. Abstracts and Excursion Guide. Geol. Soc. Aust., and Eastern Goldfields Discussion Group, pp. 28-50. Groves, D.I., Archibald, N.J., Bettenay, L.F. and Binns, R.A., 1978. Greenstone belts as ancient marginal basins or ensialic rift zones. Nature, 273: 460-461. Groves, D.I. and Batt, W.D., 1984. Spatial and temporal variation of Archaean metallogenic associations in terms of evolution of granitoid-greenstone terrains with particular emphasis on the Western Australian Shield. In: A. KrSner et al. {Editors), Archaean Geochemistry. Springer-Verlag, pp. 73-98. Hallberg, J.A., 1983. Geology and mineral deposits of the Leonora-Laverton area, north eastern Yilgarn Block, Western Australia. Geol. Surv. West. Aust. Rec., 1983/8: 134 pp.

18 Hickman, A.H., 1984. Archaean diapirism in the Pilbara Block, Western Australia. In: A. Kroner and R. Greiling (Editors), Precambrian Tectonics Illustrated. E. Schweizerbart'sche Verlagsbuchhandlung (Nagele u. Obermiller), Stuttgart, pp. 113-127. Horwitz, R.C., Kriewaldt, M.J.B., Williams, I.R. and Doepel, J.J.G., 1967. A zone of Archaean conglomerates in the Eastern Goldfields,Western Australia. Geol. Surv. West. Aust. Ann. Rept. for 1966: 53-56. Hubert, C., Trudel, P. and Gelinas, L., 1984. Archaean wrench-fault tectonics and structural evolution of the Blake River Group, Abitibi Belt, Quebec. Can. J. Earth Sci.,21: 1024-1032. Martyn, J.E. and Johnson, G.I., 1986. Geological setting and origin of fuchsite-bearing rocks, Menzies, Western Australia.Aust. J. Earth Sci.,33: 373-390. Martyn, J.E., 1986. Evidence for structural stacking and repetition in the greenstones of the Kalgoorlie district, Western Australia. In: M.J. de Wit and L.D. Ashwal (Editors), The Tectonic Evolution of Greenstone Belts. Lunar and Planetary Institute,Houston (Abstract). McCall, G.J.H., 1969.The Archaean successionwest of Lake Lefroy. Proc. R. Soc. West. Aust., 52: 119-128. McMath, J.C.,Grey, N.M. and Ward, H.J., 1953. The geology of the country around Coolgardie, Coolgardie Goldfield,W.A. Bull. Geol. Surv. West Aust. No. 107. McQueen, K.G., 1981. Volcanic associated nickel deposits from around the Widgiemooltha Dome, Western Australia.Econ. Geol.,76: 1417-1443. Meneilly, A.W. and Storey, B.C., 1986. Ductile thrusting within subduction complex rocks on Signy Island,South Orkney Is.J. Struct. Geol.,8: 457-472. Platt, J.P., Allchurch, P.D. and Rutland, R.W.R., 1978.

Archaean tectonics in the Agnew supracrustal belt, Western Australia. Precambrian Res., 7: 3-30. Redman, B.A. and Keays, R.R., 1985, Archaean basic volcanism in the Eastern Goldfield Province, Yilgarn Block, Western Australia. Precambrian IRes., 30: 113-152. Spray, J.G., 1985. Dynamothermal transition zone between Archaean greenstone and granitoid gneiss at Lake Dundas, Western Australia. J. Struct. Geol., 7: 187-203. Sylvester, A.G. and Smith, R.R., 1976. Tectonic transpression and basement-controlled deformation in the San Andreas Fault zone, Salton Trough, California. Bull. Am. Soc. Petrol. Geol., 60: 2081-2102. Travis, G.A., Woodall, R. and Bartram, G.D., 1971. The geology of the Kalgoorlie Goldfield. In: J.E. Glover (Editor), Spec. Publ. No. 3, Geol. Soc. Aust. Inc., pp. 175-190. Wernicke, B., 1985. Uniform simple shear of the continental lithosphere. Can. J. Earth Sci., 22: 108-125. Williams, I.R., 1969. Structural layering in the Archaean of the Kurnalpi 1:250 000 sheet area, Kalgoorlie region. Geol. Surv. West. Aust. Ann. Rept. for 1968: pp. 40-41. Williams, I.R., 1970. Explanatory notes on the Kurnalpi 1:250 000 geological sheet, Western Australia. Geol. Surv. West. Aust. Record No. 1970/71, 43p. Williams, I.R., 1974. Structural subdivision of the Eastern Goldfields Province, Yilgarn Block. Geol. Surv. West. Aust. Ann. Rept. for 1973: pp. 53-59. Woodall, R.W., 1965. Structure of the Kalgoorlie Goldfield. In: J. McAndrew (Editor), Geology of Australian Ore Deposits. Aust. Inst. Mining. Metall., Melbourne, 2nd ed., pp. 71-79.