Crustal-scale structures in the Proterozoic Mount Isa Inlier of north Australia: their seismic response and influence on mineralisation

TECTONOPHYSICS I£LSEVIER

Tectont)physics 28g I ]t)9S) 43 .z,(

Crustal-scale structures in the Proterozoic Mount Isa lnlier of north Australia: their seismic response and influence on mineralisation Barry J. Drummond '"+':,Bruce R. Goleby ", Alexey G. Ooncharov ". L.A.I. Wyborn ", C.D.N. Collins ", Tyler MacCready b ' lH~Iru/ian (;+'odxllamic+ ('+Jopdrallv+" R+'.xc+o'ch C('III/'(', .,tlt~lr(l/l+lll (~+'+~1(+~,'/('
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Abstract The Proter().,'t)i¢ Mount I'.+a lnlier m northern Australia in prospective I~+r base metals and gold. It contain,, a Western FnM Belt t including the l,eichhardt River Fauh T r o u g h ) a n d an Eastern FoM Belt, separated b). older basement rocks t)f the Kalkadoon Leichhardt Belt. Sediments and volcanics in both fold bell,+ were deposited in rifts v+hich '.~.ere ~ubsequentl3 shortened by up to 50+;. Mineralisation appears to be partitioned: large-tonnage lead anti zinc der~osits are mere pre,.alent m the I.eichhardt Ri',er Fault Trough. and most gold and copper (~.'currences are in the Fiastem [:old Belt. Cross-',ection,,, t)l the inlicr deri,.ed I+rom coincident seismic rellectioll and refraction pmiiling are dt)minated by the .,,ounpest tect~mic event,,. The refraction data imply a west-dippin~ lens t)f high-xelocit~ intennediate-to-malic rock in the middle to upper crust m the east of the inlier. It is collinear with another lens in the lower cru'.,t m the west t)t the miler The lenses form a belt oI high-veh)cit+,, rock c u t t i n g the crust l+t-t)tn It)p It) bottom and lrom east to ,.vest. The reflection data reveal diflSarent ,,tvles t)l +compression-related structures in the east and west o r the inlier. Thin-skinned tectorfics dominate in the Ea~,tern Fold Belt. The sediments and xolcanic,, are thrust to the ~ e s t alon.~ a number o f shallov, h e w , t-dippin.~ uppcr-cru,,tal detachments. r h e detachments in turn are cut b'. stecpl), east-dipping reverse fiJuJts which link into the ](m,." o f high-veh)city rocks defined by the rel+raction data. In contrast, faults in the Western Fold Beh are steel+ and penetrate to mM-crtlstal levels and pn+bablv also link into the bell o f higl;-velocity material deeper m the cruq. l+he partitioning into different tL'ctonic style,, occurs across the Kalkadoon Leichhardt Belt. which appears to have acted a~, a buttress during the cru' of fauhs ',~..ith a clt)',,e spatial ahxt)ciatit)n with mineralisati(m is attributed tt) alteration along the fault caused by rnigratirlg lluMs. Copper-gold minerali,,ation in the Ea,,tern Fnld Bell is scattered, but known major depo,~its lie along-trend from a thrust Rtult sht)~ n in the ,~ei,,mic data In be highl.~ reflective. This fimh links via the upper-crustal detachments and the Mgh-velocit> lense', into the nm.ldh.+ to Iov,er cruxt, and is seen as a likel) control on tluitt migration pathv..a).s l+rc.m h)~,er crustal le,,el~, intu the supracrtlxtal tLaxtern FtUd Belt. The partitioning of the tectonic styles seen m the ,,ei.~mic data and the mode of linking of faults int~ the middle to Io~er cru',,t are seen as primary factors in the partitioning ~)l rnineraJisation in the region. ( I()t)8 ILl,,e',ict-gciem.'e B.V. All right,, reser,.ed. A',"x u ++/~+/J+.. M o r t a l

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1. Introduction

The Mount Isa region in northern Australia is highly prospective for lead, zinc, silver, copper and gold. This zone of Proterozoic basement rocks crops out between the overlying, younger Georgina Basin in the west and south, and the Eromanga Basin in the south and east. Sediments of the Carpentaria Basin onlap in the north (Fig. 1). The region has the traditional name "Mount Isa Inlier' to describe its relation to the surrounding basins, but its true character is that of a composite Proterozoic fold bell. Although the area of exposure of Proterozoic r~xzks extends only 400 km in a north-south direction, regional gravity and magnetic coverage indicate a total extent, including that under the sedimentary cover. of approximately g(}O km. The area of exposure of Proterozoic rocks is about 2(X) km wide. Volcanic and sedimentary rocks of the Mount lsa Inlier may be subdivided simply into those of the Western and Eastern Fold Belts which are separated by a core of older basement rocks in the Kalkadoon Leichhardt Belt (Fig. I). Each of the Western and Eastern Fold Belts may be further subdivided into smaller zones on the basis of lithology, age and structural position: note for future reference that the l,eichhardt River Fault Trough lies within the Western Fold Belt. Although many factors inllucnce when and where ore bodies are formed, for example, suitable host rocks, the chemistry of mineralising ttuids, the presence of fractionating granites, heat input, perhaps through crustal thermal conduction or from granites, etc., nnost known deposits in tile Mount Isa region have a demonstrated spatial relationship with unajor faults. For example, the large Pb-Zn deposits at and near Mount Isa all lie close to the Mount lsa Fault in the Western Fold Belt. In the Eastern Fold Belt. a number of Cu-Au deposits lie in a linear belt between the MI McNamara and Osborne deposits, close to the Mount Dore Fault Zone (Fig. 1). Geodynamic processes are therefore believed to have played a major role in localising the ore bodies. l,ead and zinc deposits m the region are found in both the Western and Eastern Foil Belts. but demon-

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suated reserves are heavily biased to the Leichhardt River Fault Trough. Most copper deposits are found in the Kalkadoon Leichhardt Belt and the Eastern Fold Belt, and gold deposits occur predominantly in the Eastern Fold Belt (Fig. 2). Exceptions occur. Some gold is found in the far west of the Western Fold Belt, and the largest copper body is found below the Pb-Zn ore body at Mount lsa in the Western Fold Belt. The Australian Geodynamics Cooperative Research Centre commissioned a seismic transect of the region in 1994 to image the third, depth dimension of many of the nnajor structures, particularly faults. The transect was part of a multidisciplinary study of the evolution of the inlier and its mineral deposits. Many of the results of the transect are now published or in press elsewhere (MacCready et al., 1997: Goleby et al., 199g: (]oncharo\' et al., 1998). In this paper, some of those results are drawn together to consider whether the apparent bias in the distribution of mineral deposit types across tile region is underpinned by structural information. 2. The transect 2. I. Work i,'ogram

Seismic reflection and refraction profilirlg were the main geophysical tools used on the transect (Fig. I). The reflection line discussed in this paper was 250 km long. The refraction line was kruger (5(X) kin) and overlapped the reflection line at each end to create sufficient offsets to ensure energy penetration to lower-crustal and upper-mantle depths ahmg the reflection line. The nettection data were the basis for interpreting the structure within the crust, particularly the upper crust. Relleetion data are not a good indicator of lithology and composition at depth, but seismic velocity from refraction proliling can be, if metamo,'phic grade can be estimated. Very high recording station and shot point densities are needed for refraction proliling to provide good structural images: because of cost and particularly logistic limitations with this transect, shot and receiver spacings wcrc

I:ig. 1. (.ieology of the Mount Isa lnlier, and h~.'alions of the transect, l~.~p left: mp right: mcga-clmncntx o f the Australian continent (Shaw el al.. 19L)6).

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Fig. 2...\pparerll zonalion o1" nlinclal dcpo~,ils III Itlk' I'Cgiol'l. {~1) %tluarcn. pb-Zl'l nul',crimpo,,c,.I on Ihc h.'chlllic pru,. in,.'c oLItIHIcn. I h c , c d e p o s i t s a r c c \ c n l \ dJnlrll',tlled in nlllllhCl helv~ccll lilt ~VcNIcII1 ;.ind t';.inlCrll Fold l:lclls bul dClllOnslralcd h)llllLIgCS dlC hell\ i h I~iascd I~ die W c n l e r n Fold |aJcll. (1",) (.'irclcs. ('u. l ' h c s c a r c inaJnl', ill lilt c;asl. N o t e boy, ever dial Ihc large'.t k n m ~ n ,..Icl'~'~sJl H1 Icrm.. ol c x l l a c l c d ( ' u in at M o u n l lsa In Ihc \Vc.,Icril Fold I:~cll. (c) "['rkmglcx. Au. "l'hcs¢ a r c heu\'ib, b i a s e d h~ Ihc cam iii ICl'nl~. o l I'uHh iltllllbCl alld dcIllOll>[ Filled 1(11111;.1~C>,. set to provide regional velocity information but little slructt, nd information. Seismic reflection results were reporled by Goleby el al. (1998). and d3c refraction results by Goncharov el al. (1998). Detailed structural mapping was conducled along the entire linc. aim the results inlcgratcd with regional gcological maps to provide constlaint ~, Oll the interpretation of the seismic rcllection data. The illlerprelalion of the upper 3 - 4 s of dalai used below ix essentially that of MacCready el al. ( 1997 ). 2.2. I, le£,r~Hi;zg the r¢'.sults in u 2.51) (;eo gr¢qdm h!lr~rmolio, Sv.slem Spatial relationships are usuall.\. used to assign a causal associalion between geological features; e.g.. tile close spatial relationship between ore bodies and mapped faults in the Mount Isa region implies that faults playcd a role in the formation of the ore hodies. Geographic Information System (GIS) software has been designed to exph)il the spatial characteris-

lies of inlbrnmtion. Because o1 tile cxisling digital GIS covera,,e c,f Ihe X'|ounl Isa Mineral Prmincc (Jagodzmski cl al.. 1993). and new spatially hascd sample and structural mapping inforn3alion from the Mount lsa seismic transects, a GIS system w.as chosen to integrate the results from the transect and investigate the spatial relalionship belwcen strucl.urcs lotnld ill the seismic data and kllow:n mincralisution. Prcsenl-day GIN systems c a n hLilld]c on]'v t\vo :-,p~.ltial di|nensions. The pre-existing GIS o1 the geology of the region conhlincd separa|e layers o1 gcosciencc infornlalion in nlap vJev,; e.g.. outcrop geolllCll}.. airborne magnetics, gruvily, fauhs, location o1 orc bodies, etc. The seismic transccl produced lavcrs of reformation thai v~crc csscnlialh xe|tical depth slices. Depth slices that were axuilable included sexeral images of rellcction data (nligralcd and uninigrated amplitude sections, energy stack), refraction results in the form ()t" IeflLlClOl holiZOllS Lllld is()velocity lines. ;.nld interpretations of the rctleclion data which included bodies at depth and lhulls. l h c

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challenge v,,as to include these layers in the same GIS system as the horizontal map layers in such a way thai the ilfformation fmrn the depth slices could be related spatially to the information from the map layers using the GIS software. The approach taken also allowed visualisation of the vertical depth slices froln the seismic sections together with tile surface maps and images as a series of simple 2.5D nlodels (Wyborn et ill.. 19961. "Fhrce hnportant modilications to the data were tlnderliikell Ill eilStlie effective integration into the 2.51) GIS. Firstly, the seismic line was crooked, although it had an overall east-west trcnd. The seismic trimscct with all of its bends was projected onto a straight easi-v+esi-oriented plane along the ilVCl+age norttling (51"the seismic line. The surface trace of this plane then prmided a line along which tht-" surf)ice images and data could be integrated with the vertical ilnages. Secondly, to integrate the various data sets \isutilly, the 2D lllap hiyers were bilinearly transfornlcd (i.e.. "tilted') to give an impression of the top surface of a rectiliriear volume. These surface maps iuld images were "split" along the trllce (51"the vertical plane onto v+hich the seismic data had been pr(~jccted It) gi,. e a northern and a soulhern projection. The ', crtical ,,co(ions. including the seismic reflection data. seismic refraction data and their geol.:,gical interpretuticu+,,, v+ere then matched to the "split line', iht-'se \ crlical sections then give an impression of the front surf)ice tll" tile rectilinear voltune. This technique enabled a clear visualisation of connections bel\~l.'eil lilt_" faults in the plan vicv+ with those imaged in the seismic scctit)il. Finally, the "(]iS natning" (51"faults and geological unit>, was necessary if the ability of the GIS svstenl to exploit spatial relationships yeas to be used. Each fault and geological unil was coded identically ill both lhc seismic sections and the surface laver data scls. This allowed the GIS io learn that faults and geological units in the depth sections wcrc the saint Ieaturc,4 as those in the map layers. Because the ulineral deposits could bc spatially rehlted to geological units lind faults in the map layers, this identical t-'llding could then be used It) relate nlincralisation tt) l+ealures in the seismic sections, end lherebv delernline which faults were more inlporhull ill conlrlllling the ValiOLIS styles (51"inincralisation al depth.

2
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47

An example of the output of the (;IS devcloped for the Mount Isa transect is shown in Fig. 3. Part
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2.3. ]. R+?/i+nl/+m ru.,,tdl+ ]'hc tefiilcthm data v+ere hlterpretcd h,, {Mnchart)'., et ;.d. (lt.Jt-)g). Their veh)citv nlodcl ix shtlv,+n as vch)cit+,, contours in Fig. 4. The seisnlic ',eh)cit,,. distribution in the crLIst and crust-mantle transition .,:one ix complicated and varies si.b'ni(icantl,, along II1¢ line. l+ov+.-velocity layers are quite utmmlon in the crust and in tile cruM-lllantlc tram, ition / o n e . Relati',d,. + 1(5,.,, velocities (5.7 fl.I kill + i) arc typical v, ithin the tippet 15 kill of the crust t)l the ~kstern Fold Belt. In conlparison, ',eh+,cities hi the upper crust of the I-astern Fold Belt are typically highel-(ft. I kill t., i and higher pre,,ailing in tht-' upper 15 kill). .-%high-\eh+cit) (6.9-7.3 km s i) body oCCUltS in the middle crust at the centre of the transect. It i,, collinear v+ith a high-vehscity protuberance <6.9-7.3 kill n I) v, hich extends fronl the Moho hi the v+est of the prolile upv+ards towards the caM. Ahhough collinear, the Iv+'() bodies nla) rlol hc conlinuous because rocks v+ith lower velocities appear Io separate ihenl. Along the whole transect tile Moho is transitional riiltler ltliiil sharp. Velocities iilcieiise lroin

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2..?.2. Rclh'ction resuh.s Both unmigrated and migrated data are uscd in the following discussion. Individual retlections in unmigrated data lend to be stronger and have greater lateral continuity than those in the migrated data. Therefore Lmmigrated data are sometimes better for demonstrating the presence and form of structures {Figs. 5-.71. Migrated data are used when it is important to demonstrate thc correct spatial relationships of structures (Fig. 8). The interpretation of MacCready ctal. ( 19971 for the entire prolile is shown as the vertical faces on the rectilinear volumes in Fig. 3. Figs. 5--8 show the nature of key structures in more detail. The locations of the sections in Figs• 5-8 are sho,a.n in Fig. 3. Thc sediments and volcanics in the upper 4 - 6 km (to - 2 s reflection time) of the Western Fold Belt dip steeply west in the vicinity of the Mount Isa Fault (Fig. 5). The reflection data suggest that they.'

W

lsa Fault has no, ir~tuinsic reflccti,.it,, and is traced

frOTll its surface outcrop to depth on the basis of trur~cations of rellecti(ms associated with the I{astern Creek Volcanics ( E ( ' V ) which crop out near the fault. The Adelheid Fault. however, is strongly reflective. and dips more shallowly to the west 111all the .'k'l(Rlnl lsa Fault. Both faults arc planar ill the upper crust. and cut Ihe stratigraphy of thc fold hell at shallow angle. The strong rellection labelled "S" het~,,..ecn the Adelheid and Moun! lsa Fauhs is tl~e S-~
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1"i~2 7. tllllllgralcd data ,hov, mg It~c intcrprctalion of MacCwcad,, cl al. (ItJ97) in [he region ol the l\.la.lrilllOhuh l'hl, hlghl,, rcllcclcd Ml'tlCltllt" ttlls alld dispkiccn the basal delachlnenl, anti appears Io link al depth \~ itII l ' t ' J i c c l i t l n s illkl-cru~inl hi,,_'h-~ cloch.~ bottx in Ihe ~eh~_'it~. inodcl (Fig. 41.

The %vhella Fauh ix interpolated into the seismic section from the surface based on its relationship in outcrop to the Eastern ('reek Volcanics. The tktult intcrpretcd dipping east from the Sybella Fault separates east-dipping strong reflections at 1.5-3 s m the eastern part of the section from weaker west-dipping reflections in the upper part of the section. The origin of the dcepcr cast-dipping retlections at the eastern end of Fig. 5 remains uncertain. They may be part of the Western Fold Belt which has been thrust eastwards under the basement rocks of the Kalkadoon l.eichhardt Belt. In the western end of Fig. 6, the highly reflecfix c rocks clip east under the unreflective upper crust atlributcd to the Kalkadoon Leichhardt Belt. The Rail,aa 3 Fauh ix named in both Figs. 5 and (~ to facililatc linkin,, the tv,o lieures. The Gort, e Creek Fault is the mapped boundary between the Kalkadoon l.eichhardt Belt and the Western Fold Belt in the region of the transect. The Kalkadoon Lcichhardt Belt is b e l i e \ c d to he representative of basement below the supracrtlstals of the foh.I belts to the west and east.

inlcrprelcd I{~ U'lllllC Irluli Iht3 h~p Ill Ihc

The seismic reflection data are consistent with this. The upper crust in the Kalkadoon l.eichhardl Belt i,, poorly reflective, consistent with a region dominated by granitic and metamorphosed telsic rocks. Those rellections which are observed within the Kalkadoon Leichhardt Beh are interpreted as shear zones. In contrast to the western side of the Kalkadoon l.eichhardt Belt. rocks of the Eastern Fold Belt are thrust westwards across the eastern side or the Kalkadoon l.eichhardt Belt. In Fig. 6, this is illustrated by three Ikiults which dip east from Ihc surface and Ihlk inlo a single east-dipping fault which underpins Eastern Fold Belt supracrustals. The rellection marked "S" ix an S-wave ret|ection from this fault. Whereas the rocks in the Western Void Belt generally have steep dips (Figs. 5 alld 6). those in the l-astern Fold Belt generally are morc fiat lying. Fig. 7 shows a portion ol +unmigrated data ltc)ul the upper crust in the region near the Marimo Fault ( ' M ' ) . and Fig. 8 shows migrated data for a broader region which puts Fig. 7 into a more regional context. "l'he interpretation is a stlnlnl;.ir\: l'1o111 M n c ( r c a d v el al.

P,

I : : A q T I : : ~ I M I:f'~l 13 I~.1::1T

IkAnHni' I ~

\ M

5 km

161F541188

V/H = 1

~4

Fig. 8. Seismic reflection ~,,..'clion (nligralcd)fi'om lhc Ea,~tcrn Fold Bch. R = rcllcclic.~m, l'n.ml |he high-velocit,, hod.,, in the middle crul:,;l interpreI.cd in ~.h¢ rel'raclion dal~L: I) = thl" h11u'rpn.'Icd basal delachmcnl Io lh¢ Easlcrn l-old l'~ch supracru~lal~,: .~I = lhc M a r i m o FaLIII. Is~.~-',clocily lines ~in." from Fig. 4. L~/L = Io~.,.-,, cl~x..il.,, )a)'cr.

n-"

~g

W

,c

_=

B..I. I h ' t . n m m u t

el al. / l i ' ( ' l m u q d D ~ i (

(1997), who used delai led surface structural rnapping as a control on the interpretation. The rocks of lhe Eastern Fold Belt have been thrust westwards on several sub-horizontal detachments ('D'). Both detachmcnls would have been active at the same time. The basal detachment surface of the supracrustals is nlarked by strong reflections, and appears to lie ahmg the interface between the supracrustals and basement rocks. The thin unlabelled lines in Fig. 7 are form lines which illustrate the gentle east dips in the rocks in this region. The detachnlent surfaces have been cut by more-steeply easbdipping reverse faults. Most of these reverse faults are generally unrellective, and their positions in the interpretation tire controlled by their locations in surface outcrop and offsets on the basal detachment. In contrast, the Marimo Fault ( ' M ' ) had not been identified in outcrop prior to the seismic survey, and was discovered on the basis of its strong intrinsic retlectivily as well as the offset it caused on the basal detachment. The reverse faults link into the zone of high-vchvcitl, rocks in the middle to upper crust (Fig. 8). Fig. 8 (migrated data) shows the continuation of the basal detachment to the west. Higher-level dctachmen( surfaces and t'ornl lines for the strata are not shown to avoid clutter. Also shown in Fig. 8 are the ,,eh>city contours from Fig. 4. Goncharov e[ al. (1997) suggested that the high-velocity lens in tile middle to upper crust was intermediate to malic in composition. It lies below the basal detachment.

k

Western Fold Belt

+

~ 2,'~','~'1190,% 43- ~,0

53

a,ld reltections marked "R' lic near its upper surface. The dip of these reflections is usualb, steeper than the iso-velocity contours. This suggests that the high vehu_'ities result from the comhined effects of a con]posite feature consisting of layers with alternating high and low impedance. Thesc individual layers are oriented tit a steeper dip to the overall distribution of the composite body and cannot be resolved in the refraction model due to the lower fiequencies of the rel+raction data.

3. Discussion The results of the transect are SUnlnlarised in carloon fornl in Fig. 9. Tile detailed structure in the upper crusl hi the face of the rectilinear volume is drawn to emphasise the nature of the fokl belts: hence the faults within the basement of the Kalkadoon Leichhardt Belt (Fig. 61 are not drawn. The fault which truncates the Mount [sa and Adelheid Faults (Fig. 5) is drawn penetrating to mid-crustal levels, lit the east. tile Marinlo and other reverse faults link down into the upper of the high-velocity lenses. Other deeply penetrating fault,s, including the Pilgrim and Mav l)owns Faults. are probably vertical or close to vertical. They are nuuor strike-slip fauhs. are probably younger than n|ineralisation in the region {Wyborn et al., 1996), and have therefore been ignored in the previous discussion. The Moullt Isa region lies on the botlndarv beKalkadoon Eastern Fold Bell Le'chhard"Be!t + Z>t

,~30

I'i~. L]. ('dFloon ~,LiinlllLlri~,Jn~ the key x|rLicturcs from the detailed interpretation of tile rCI]CCIiOTI data hi the LlppCr L'I'LI~.J mcrg,..'d v, ith an itltL'l'pl+clatitH10J the rc|+ra,.:tJon model t'rtml Fig. 4. and ~,ho,.vn ~ +th the 111uiorL'k'lllCnt boulldarJc'~ l'rolll l'ig. I

54

I¢,..I. I ) v l l l l t l m J m /

+'l +ll. / li'< I¢+mllJil', ",ll ", +._,S,", i /i)t+,'.,'j 4.¢ .~(~

twecn two of the basic building bk~,'ks of thc ,,\ustralian continent, the North Australian (NA) ;.uld North Queensland (NO) mega-elenlents (see inset. Fig. 1; Shay+ c t a l . , 1996). "l'hc Palace- to Mcsoproterozoic tectonic history of the Mount lsa region indicates that numerous ensialic rifting events occurred prior to the Isan Orogeny (between 1620 Ma and 1500 Ma). Most of these events are associated with bimodal volcanism, and some of the rifting may have been close to creating new oceanic crust. Crustal sht)rtenhlg dtll+ing the [san Orogen) ttlickcned the previously thinned crust back io high continental values of about 55 kln. The collmear high-velocity bodies near the crustmantle boundary and in the middle crust project to the surface near the mega-element boundary. The high-vehmity bodies can therelk)re be interpreted as a major crustal boundary that dips to the west. The Western Fold licit lies above the thickest (in today's terms) part of the hanging wall block, whereas the Eastern Fold Belt lies in tile thinnest part of tile hanging wall block. Heinrich et al. (1995) proposed a mineral)sat)on model in which tluids circulating at shallow, high crustal levels are responsible for both the world-class Cu and Pb-Zn deposits at Mount Isa in the Western Fold Belt. This ccmcept is supported by the lact that the nlaxinlunl nletainorphic grade of host rocks to the maior deposits of the Western Fold Belt is upper greenschist, Heinrich et al. (1995) inferred that the Pb Zn deposits l'omled at fairly shallow water depths and that the Pb-Zn orcs at Mount lsa were similar to thc sedimentary or early diagenetic ores found in other provinces (Cam 1981; l,ogan et al.. 1990) where Hinman et al. (1994) demonstrated early diagenetic ore fo,'mation probably within tens of metres below the sediment-water interface. In support of this syn-sedimentation or early diagenetic timing, Sun et al. (1994) argued that the Pb isotopic composition of the ore systems at Mount [sa and McArthur River is similar to that of the surrounding host sediments. Likewise. the preferred interpretation of the stable isotope and fluid inclusion data is that the fluids that formed the Mount lsa copper ore body were derived from low-grade metamorphic fluids or basinal brines, rather than ftonl deep-scatcd metamorphic or magma(it lluids (Heinrich el al., 1995t.

The seiSllliC reflection data show the rocks of the Western |:old Belt today to be contained between basement buttresses to the west and the east (Kalkadt'~on l.eichhardt Belt). "File strata are folded, commonl+,,, with steep dips. Faults which cut the strata at Iov< angle no'a, have steep dips. The seismic data filet+eft)re shc~vv the rocks of the Western Fold Bell to be constrained within a narrow basement-bounded container, probably of low permeability. The lluid circulation patterns of Iieinrich el al. (1995) would have acted within that container: faults which today exhibit high rellcctivity probabl) played a in,tier role in lhcilitaling the fluid circulation patterns. The slrongl) rellcctivc Adelheid I;ault We(lid have becll one t)f those i].iulls. M~tior ore bodies near Motlnl Isa ;.ire adiaceni It> the Adclheid and Mount lsa Fatllts and their local splay fauhs. High rellcetivity of fauh zones Call be caused by a broad /..one oi+ alteration along the fault trace (Jones and Nur, 1984). In contrasl It) the shallow crustal sellings of the in~tior nlinelal deposits of the Western Fold Beh. the Cu- Au deposits in the I-]astern Pold Belt ;.ire hosted by rtu.'ks of higher metanlorptlic grade. The seismic data shl.)~ the [{astern Fold Bell to be dtnninated by thin-skinned thrusting fronl the cast, allowing deep btn+ial ,aith liinited tilting of the strata. The [:.astern Fold Belt was a lnore open syslel'il than the Western Fold Belt, with no h)w-perlneable lateral basement buttresses within the region of the transect. The Cu-Au deposits have a strong spatial correlation with oxidised gralliles of the Williams and Naraku Ilatholiths (tiJlv'yborn and I Icinrich. 1997: ~,lvbl)lil, 199g). Although the associated granites are beli0ved to have cr\.stallised at J 5 kbar (Williams et al.. 1995). for example. Ihe host rocks to the ()sborlle Cu Au deposit are tipper amphibolile grade (,,\dshead, 1996). file actual deposits theinselves fllrnled at shallower crustal levels (Perkins and Wyboi'n. 1998). Individual deposits are structurall), controllcct (c.g., Williams ci al.. 1995). The Osborne deposit is one of several which lie along-strike to the south of where the scisnlic line imaged the highly reflective Mariino Fault (Fig. I), which is linked to tile high-vclocit,~, body in the middle to upper crust. Thus a significant difference between tile niftier Pb-Zn and ('u deposits el tile Western Fold Belt and the ('u--Au deposits of the Eastern Fold Belt is thai the I(.)llller are associated v,.ith Io,,~,-grade nletanlor-

B..L I ) r , , , m m U

('! e l . / 7 < ' + l < , , , l ~ h w / ( .~ 2
phic fluids and/or basinal brines and are hosted hy grem~chi,q-gradc

55

References

rocks or lower, and the latter are

associated ~ith deeper rnagmatic and higher-grade mctamorphic systems and hosted by rocks that are metamorphosed tip to uppcr amphibolitc grade. If any simple difference in structural setting betv+'ecn ore bodies in the Western Fold Belt and those in the Eastern Fold Belt has become evident in the seismic data, it is that the ilature o f the d e f o r m a t i o n in the west is f u n d a i n e n l a l l y d i f f e r e n t f r o m that in the

east. h:ading to a discrete container between latefal buttresses in the west and broader open structures in the east w h i c h a l l m + e d deep burial. The nature o f the d e f o r n i a t i o n was p r o b a b l y c o n t r o l l e d by the disI;.uice lrOlll the l l l e g a - e l e l l l e n i boundary. The megae l e n l e n l b o u n d a r y under lhe We.stern Fold Beh lies llear Ihe base o f the citlSl. "]'he Western Pold Beh wa~ rernt)vl+,tl some distance f r o m Ihe leading edge o f d e l t > r m a l i t m associated ~vith the boulldafy, aild orogenesis w i t h i n the h a n g i n g w a l l invol~.ed w h o l e o f cru,d deft~rmalion. In conlrasl, the lliega-elelllt.'nl

boundary wan at shallov, Ic\els below the l-:asLern Fold Belt. v, hich lay neal+ the leading edge of dcforlllalit)n, l ) e l o r m a t i o n affected o n l y a thin layer of crusl in the hanging

wail. and thhi-skirmed

te¢Ituiics

resulted.

Acknowledgements Robyn Gallagher linked the seismic data item thc transect to thc existing AGSO Mount Isa GIS. Brucc Kilgour and Shaska Martin provided invaluablc assistance in cornbinit+g the digital geology with the seismic interpretation. BJD, BRG. AGG. I.AIW. anct ('DNC publish with the pcrrnission of the Execulixe Director of the Australian Geological Surx.e\ ()rgaliisation tAGS()). All authors publish with

.4~~

the i~ermission o f the D i r e c t o r o f the Aus-

traliall Geodynamics Cooperative Research (Tentrc IAGCR('I. The AGCRC is an unincorporated joint venture ccmiprisiug AGSO. the Division of Exploration and Mining of the Commonwealth Scientific and Industrial Research ()rganisation. Monash and LaTrobe t'niversities and Digital Equipment Corporation (Australia) l.imited, and is funded by the Department of Industry, Science and Tourism under the Commonwealth Government's Cooperative Research ('entres Program.

Ad,hcad. N.I). ItJt~b. "l'ht.- role ol hyper>,ulmc h\'drt~lhcrmal Iltlid>. in the Iol'lllalion I.II Ihc ()sht~rnc ('tl ,\tl dcl~,~,il. C'hm CHll+.~ dinlliCI. Nxil%,' Ouecn',land. M l ( ' " . l b : Nc'a dc'..clot'llllcllls HI

mclallo~cnic rcncarch in the McAl'dlur Mount Ina (+torte'err\ tnit1¢raln l',ro\m,,:c, iEctm. (tool. Re, Linii. Junw>, ('t~t+k t!nl,, uf North Quu'cnnland. ('untrih. 55. I"P. I -I.

('err+ (;.R.. IOXI. The Mh+lcralog+',.Pctroh+,.z', and C~eocl+u.'mi,lr\ 'M" lit,++'ZIIIC l.ca,,l--SilxcrOre,, and H~)nl Su.'dilllClll+.ir\Rt~'kn +.ii I.ad\ I.orcIla. N\V Quccn,lun,,I. t'ni',uI`.IPhD tile,d>,.I. hi,, ,u %.\~flhmgong. 431 I;P. (i~flcl',\. l:IR.. Ma~.'('rcad\. "[.. l)rtmummcl, l.l.J..(hmcharm..\. I")"JX The ~tlItlnl Is,i ~cOd~ll~iilli~. ll~iilxt2cl ',:ru',talilll[`.JlL'~lli~m',. In: l'haurl..I, l)oolc\..I.('.. (h~h.,Iv, ILR.. :ml ,,Icl Ilil,,1.

R.I).. Kh,ol\~Uk. ( ' 1 Ihd>,.I. Struclurc uml I'.~dutlon t~l the ..\Llqlall~lll ( "online'lit+ AIII. (~c<+pll~,-, [ "tHtul. ( b+.+t,tl\ 11. <,el

2(i.

I<}O-II,X (hmu'hurt+~,...\.(L. l+i/m,,k\. MD.. ('ollin~. (" l).N+. Kdhmi. K+,\.. FumiN. f.N++l)run+intmd+ l'iJ. (hflcl+,:. B.R. l+lah+n¢llkt,\a. I.N.. ItJt),'4 Intra-cru'qal "Nci~lnic i:,tixldx\" ill II1," Ilalllc' Shield dlid .,~lti~lialllul l~rcCdllibritln c.+l'+.llllll~, I'ftllll tlCl'l+l ",Ci>qliiC prtllilc~ alid Ihc KOiCl ,'qLlpCltlccp hl+ilt+ hole dala. hi: l+*iCiUll..I.. I)(~(ih_'\. } ( ' . (it>lob\. B.R.. '~Lilldcr ttil~t. RI).. I~hult~iik. (.T. II:d,.i. ~lltlt.'lUl'c

~illd t!\oltllitlll

(]onch:.iro\..\.(l+.

Ill Ihc .\tl',llahiln

<,till. ~-,,.. ~,~.~.horll. 1._.\.

('olilinclll. IO07.

.\111.

Ilalarlct'tl

pcirolt~g3 t~l the' t'ltl,.I in Ihc i'%tOlllll I,u ic.,_,itui \tiq. (lord. 5Jtll\. ()r,.2anisdlltln. Rc+ Nc\~h.'ll. 2fl. 13 If+ Iicinrich. ('+,\.. Ilain..1+11+('.. \lCilla'.2h. l i t . \~,\btuil I..,\+1.. ItJtJ~ Iquid Cilld IllilnS II'clll',lt.'l" thll'ili~ iilClaba~all ci]lclalil~ll and ctippcr iliiilc'lclli/illitlll ill Mtitilll Ix,i..\tlnlllihH Itvtlll (k',,I
7ii5-7.+m. Hilitiian. Nl.( .. \'+all. %<'..I..Itcmrich. (L..\.. Iut)4. |'he Intcrplu.~ bcl\~.CCll "+edilllc'lll:.ilil)ll. dclorlli++ilil~ll alltl h~clltilhcrllial iiCll\ il\ /.it Iho M c . \ I l h u r Ph. ~ l i i ('tll dcpo,il. (ic<,l 5,oc ,\tl-d. -\b~ll .+,7. 17f, 177. .lagodzin~,ki. t....\.. \~'.%born, I...,\J.. (Tallu.,_'hct. R.. ]~)u.+~, ~ltltlnl I>,a \'lillCl;il pltl\ into..-\Lt>,t. (IC~I. 5;Lil\ ()l,,_'ani~dlilln. \lctalh>7ClliC Allan ~u'l I +hlllC~. "[+., ~LII, ..\.. I t)~4. l h c

IldltilC ~ll ",¢l~llilt" It.'llccliOlln IIt)lll

dccp ctti~la[ I~.iull /oiles. J. (lcl~ph)>,. Re> ,'4%.+,15.+, 3171. I,llQan. R.(;.. Murfci\. W J.. ~,li'illicilti~. N . It)till I h c t t Y ( " dc D~-,il. Mc.,\llhur Ri~cr. ~ o r l h c i n Tc'frlllU\ .\u,,tiillci~. In,,I. \lin..'%lclall. \ltuil~gr I-L.I)(i7 tJll

Mac('rcud\. "1.. (iolcb). ILR. (hmcllaltl~. A. l)rulnm~md, ll.J.. l.i,,Icr. (i ,q.. IO07...\ IrtlliiC\\lllk td tl\crl~rlnlin,_' llttl,,.2,c'll~ based illl inlcflllu'l~llioll tll Ihc lX,'ltllllll I~,t dccp xClxllllC Irall',ctl t~u'tm. (;~.'l+'l . 111 l+lcss+

l~¢rkinn. ('.. \V\l`.orn. ].A.I.. IOUX..\go el ('u.-\u tnim_'rali,,alltm. ('ltllICl.ll'F~. I)ixtl'ic'l. ¢:.lsldlll .~i|tll.llll [rill Inlicr. Qu¢cn,,lancl. tin ,.tctcrlnincd I',\ a+:\r/'<'.-\r d a t i n g . . \ u n t r . . I I a M h ~cl.. it1 plc',n.

Shax~. R.I). Welhnau. P. (;unn. p.. \Vllltlakcl \..I.. l'arh+\\,.ki+ ( ' . Molnc. M.. lCJCJb.(iuidc to u~ing tile ..\Li,ilalitlil ('ru>,lal P.lcliIOIll~ Mal+J. Au~,I. (tool. 5;tire. Or t_'ani~alioli. Rc'c. IOt)f~/.l(i. ~tlll. ~hqll-,+Ll. I);J~t.". 14.. ('al-f. (i.. it)t)4. I.cad-isotopc-ba>+cd ',lliill 7raphic morftqtlliOli,, and il~2t'~, tq I)rolu'lll/Olc ~Cdillic'nl ho,+lcd

5(~

B.,I. l ) r u , , m , ~ d

~'1 al. / 7 i ' c , , , ~ l ~ h w i c ~

ph Zn depot, its in the M~unt b,a Inlier. :\unl. (ie~l. Surv. ()r[2imi~..alion. Ile~,. NewMctl. 2(1. I - 2 . Williams,. RJ., Adshead, N I ) . . Blake. K.I... tic jong. (i.. Mark. (}.. Rt)therham. J.F.. 1995. Magnetite-('l.i-Au deposit,, in deeply eroded inaglllalJc arcs: lennon', l'roltl l'rotcrozoic ten rains, l"roc. Pacilic Rim (_'tmgrcsn. Australasiarl Institute of Mining and Mclallurg,.. Melbourne. pp. 631-636. W.vborn. I..A.I.. 1998. The younger ---15(X) Ma granile~, ~I" tile Williams. and Naraku Batholiths. ('l,.~ncurry Db.trict. Mount [nit Inlier: gcochernistr.,,, origin, metalh~genic si~nilicancc and

2,~,'~ I Ig~,~,~J 4¢ .=,6

exph~ration indicatorn. Aust. j. Earth Sci.. 45 13). in prenn. Wyhorn. ]..A.I.. Ilcinrich. (.'.A.. j993. The relalmnship bet,,~,een latc-t,,:clonic feb, it nltrunr, en alld (.'LI AI.I mineralisation in the ['a~,lcrn I:old Belt. Mount [sa [slier. Au~,t. Inst. Gcosci. Bull. 13. 2"7 30 lalso published in t[Ic ,&l.lsl. [lInt. (]¢o,.,cj. Bull. 14. 51--541. W.,, horn. 1... (;olcb,,. I:l.. I)runlnlond. B.. Gallagher. R.. It}06. Tile Mo~.lllt 1~,~1gc{~,,]~nanlic transect. A 2.5 dimensional illctalh ~¢lliC illlal}Ms. Atlnl. (ieol. Stir',. Organisalion. Re>,. Ncv, qcll. 24. 11)-12.