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Precambrian Research 160 (2008) 227–244

Tectonic setting, evolution and orogenic gold potentialof the late Mesoarchaean Mosquito Creek Basin,

North Pilbara Craton, Western Australia

L. Bagas a,b,∗, F.P. Bierlein a, S. Bodorkos b, D.R. Nelson c

a Centre for Exploration Targeting, School of Earth and Geographical Sciences, The University of Western Australia,35 Stirling Highway, Crawley, WA 6009, Australia

b Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australiac Department of Applied Physics, Curtin University, GPO Box U1987, Perth, WA 6845, Australia

Received 12 April 2007; received in revised form 5 July 2007; accepted 16 July 2007

bstract

The geology, evolution, and metallogenic potential of the Mesoarchaean Mosquito Creek Basin remains poorly understood, despite the presencef several orogenic gold deposits. The basin is dominated by medium- to coarse-grained, poorly sorted and chemically immature sandstone andonglomerates, characterised by very high Cr/Th, high Th/Sc, and low Zr/Sc relative to average continental crust. These features are consistentith the presence of significant mafic rocks in the source terrain(s), a limited role for sediment recycling, and deposition in an increasingly distalassive margin setting on the southeastern edge of the Palaeo- to Mesoarchaean East Pilbara Terrane.

New U–Pb SHRIMP data on 358 detrital zircons indicate a conservative maximum depositional age of 2972 + 14/−37 Ma (robust median; 96.1%onfidence). Zircon provenance spectra from conglomeratic rocks near the base of the unit are consistent with substantial derivation from the Eastilbara Terrane, but finer-grained sandstones higher in the stratigraphy appear to have been sourced elsewhere, as their zircon age spectra are notell matched by any of the exposed Pilbara terranes.The Mosquito Creek Basin was deformed before and during collision with the northern edge of the Mesoarchaean Kurrana Terrane, which resulted

n the development of macroscopic north-verging folds, thrust faulting, and widespread sub-greenschist to greenschist facies metamorphism. This
ollisional event probably took place at ca. 2900 Ma, based on two identical Pb–Pb model ages of 2905 ± 9 Ma from epigenetic galena associatedith vein-hosted gold–antimony mineralization. The metallogenic potential of the Mosquito Creek Basin remains largely unevaluated; however,
he possibility of a passive margin setting and continental basement points to relatively limited potential for the formation of major orogenic goldeposits.

2007 Elsevier B.V. All rights reserved.

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eywords: North Pilbara Craton; Archaean; Mosquito Creek Formation; Detrit

. Introduction

Turbidite-hosted orogenic gold deposits are well documentedhroughout the world. Examples are Palaeoproterozoic depositsn the Granites–Tanami Orogen of Northern Australia (Bagas
t al., 2007), and Palaeozoic deposits of southeastern Australiae.g. Bierlein et al., 2001, 2004), New Zealand (e.g. Christie andrathwaite, 2003), and North America (Goldfarb et al., 1997,
∗ Corresponding author at: Geological Survey of Western Australia, 100 Plaintreet, East Perth, WA 6004, Australia. Tel.: +61 8 9222 3221;ax: +61 8 9222 3633.

E-mail address: [email protected] (L. Bagas).

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301-9268/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.precamres.2007.07.005

on geochronology; Geochemistry; Orogenic gold

998; Bohlke, 1999; Bierlein et al., 2004). However, very littleas been published on Mesoarchaean deposits. Examples arehe epizonal deposits in the Mosquito Creek Basin (MCB) ofhe Archaean North Pilbara Craton that have produced over 8 tf gold (Ferguson and Ruddock, 2001).

Alluvial gold was discovered in the MCB near Nullagineownship in 1886 and gave impetus for further exploration forold and other precious metals in the region (Fig. 1), whichontinues to this day (Noldart and Wyatt, 1962).

Since the 1980s, various research groups have proposed
hat the structural history of the area involved either horizontalompressional tectonics, or invoked a range of diapiric mod-ls for the emplacement of Archaean granitic complexes (Vanranendonk et al., 2002; Blewett et al., 2002, and references
mailto:[email protected]

dx.doi.org/10.1016/j.precamres.2007.07.005

228 L. Bagas et al. / Precambrian Research 160 (2008) 227–244

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Fig. 1. Regional geological s

herein). Hickman (1984) interpreted the MCB to have formedhrough subsidence during the later stages of granitic diapirismn the East Pilbara Terrane (EP), whereas Eriksson et al. (1994)nterpreted it as a forearc basin and accretionary complex situ-ted to the north of a subduction complex. Tyler et al. (1992)nterpreted the Kurrana Shear Zone as a suture between twoistinct terranes (Fig. 2), which amalgamated between 3000
nd 2760 Ma. Krapez and Eisenlohr (1998) suggested that theCB was equivalent in age to the ca. 3240 Ma Gorge Creekroup in the EP. By contrast, Witt et al. (1998) interpreted
he MCB (Maitland, 1908; Noldart and Wyatt, 1962; Hickman,

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of the North Pilbara Craton.

983, 2001) to be contemporaneous with the Mesoarchaeane Grey Supergroup (Van Kranendonk et al., 2004, 2007;ig. 1).

This study presents new constraints on the age and prove-ance of the Mosquito Creek Formation based on SHRIMP–Pb dating of detrital zircons, which was documented as areliminary report including limited geochronological data by
agas et al. (2004). Zircons are durable and can survive therocesses of weathering and erosion, are largely unaffectedy metamorphism below amphibolite facies, and can surviveransport over distances of many hundreds or even thousands

L. Bagas et al. / Precambrian Research 160 (2008) 227–244 229

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Fig. 2. Simplified interpreted bedrock geological map of the M

f kilometres (e.g. the Amazon deltaic fans of South Ameri-
an with sediments derived from the Andes). Examples of theongevity of zircons are documented for the ca. 4370 Ma detri-al zircons from Palaeoproterozoic conglomerate at Jack Hillsn Western Australia (Wilde et al., 2001; Harrison et al., 2005),
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to Creek Basin and surroundings (modified after Bagas, 2005).

nd Archaean zircons in Carboniferous metasedimentary rocks
rom Patagonia in South America (Augustsson et al., 2006). Theistribution of U–Pb ages of detrital zircons can also give valu-ble clues to source regions for sedimentary rocks (e.g. Catalat al., 2004; Cawood et al., 2004), even though the rocks studied

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30 L. Bagas et al. / Precambria

re Archaean. This aspect is particularly interesting consider-ng that the rocks in the MCB are late Mesoarchaean in age andre generally metamorphosed to sub-greenschist facies, and thathe MCB might also represent one of the earliest examples of a

esoarchaean accretionary margin.The formation of orogenic gold deposits can be considered

n almost logical consequence of accretionary tectonics (e.g.oldfarb et al., 2001). However, there is enormous variation

n the overall endowment of orogenic gold provinces withinhis framework, and in the distribution of particularly largeeposits (e.g. Bierlein et al., 2006). A critical factor for anoma-ously well-endowed orogenic gold provinces is thickness of theubcontinental lithospheric mantle (SCLM) beneath a provinceuring gold mineralization, as they are more likely to developn orogens with high heat flux related to subducted oceanicr thin continental lithosphere, rather than thick continentalithosphere (Bierlein et al., 2006). Conversely, orogens with pro-racted pre-mineralization crustal histories are more likely to beharacterised by a thick SCLM that is difficult to delaminate and,ence, such provinces will normally be poorly endowed. Theemporal and spatial distribution of orogenic gold deposits inhe MCB can, therefore, contribute substantially to understand-ng late Mesoarchaean plate tectonic processes in the Pilbararaton. Such knowledge, in turn, will aid the formulation of con-eptual exploration strategies, and provide important insightsnto the evolution of the crust and deeper mantle processeshat controlled the formation of orogenic gold deposits in the

CB.

. Regional geological setting

The Pilbara Craton has an exposed area of about 183 000 km2.he craton comprises Palaeo- to Mesoarchaean (3655–2830 Ma)ranite–greenstone successions of the North Pilbara Craton, andhe unconformably overlying Neoarchaean to Palaeoprotero-oic (2770–2400 Ma) volcanic and sedimentary formations ofhe Hamersley Basin (Thom et al., 1973; Lipple, 1975; Ingram,977; Hickman, 1983; Blake, 1993). Thorne and Trendall (2001)nd Van Kranendonk et al. (2002) summarized the stratigraphyf the North Pilbara Craton and Hamersley Basin, respec-ively. Hickman (1983) grouped the older granite–greenstoneelts of the North Pilbara Craton into the Pilbara Supergroup,nd Hickman (2004) and Van Kranendonk et al. (2007) sub-ivided the North Pilbara Craton into the ca. 3270–2920 Maest Pilbara Superterrane (WPS), ca. 2790 Ma Mallina Basin,

a. 3650–2830 Ma EP, MCB, and the Kurrana Terrane (Fig. 1).any of the successions are separated by regional unconfor-ities or disconformities, and generally dip and young away

rom granitic complexes that range in age from 3650 to 2850 MaSmithies and Champion, 1998; Nelson et al., 1999). The

allina Basin developed around 2970 Ma over the boundaryetween the EP the WPS (Fig. 1). The Hamersley Basin uncon-ormably overlies these successions in the south, or forms
utliers in the North Pilbara Craton (Trendall, 1990; Blake,001).
Hickman (1983) grouped the oldest volcano-sedimentaryocks across the North Pilbara Craton into the ‘Archaean Pil-

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earch 160 (2008) 227–244

ara Supergroup’. Hickman (1990) and Van Kranendonk et al.2004) again modified this stratigraphic nomenclature to includehe Mosquito Creek Formation, the Lalla Rookh Sandstone,nd the Mallina Formation in the ca. 3020–2920 Ma De Greyupergroup. The Mallina Basin contains turbidite-hosted lode-u deposits associated with sericite–carbonate–pyrite alteration

ssemblages, lode-Au deposits associated with pyrophyllite-earing alteration assemblages, and lode Sb–Au depositsHuston et al., 2002a). The mineralization in this basin formed atbout 2900 Ma. There are many similarities between these basinsncluded in the De Grey Supergroup. Similar ‘late-stage basins’re also present in other greenstone belts of Precambrian andhanerozoic ages throughout the world (e.g. Krapez et al., 2000).owever, as emphasised recently by Krapez (2007), it woulde “unwise to simply export either the physical characteristicsf these basins or their tectonic significance to other oro-ens”. Rather, detailed stratigraphic analysis of those orogens isequired before any generalisations can be made between themnd their proposed correlatives, or analogues. Consequently, were only just beginning to understand the significance of late-tage basins and their role in the sitting of orogenic gold depositsKrapez, 2007).

The De Grey Supergroup was deposited in a number ofsolated but broadly contemporaneous clastic sedimentary pack-ges across the North Pilbara Craton (Fig. 1). The supergroupncludes rocks of the Mallina Basin in the northwest, Paradiselains Formation in the north, Lalla Rookh Sandstone in theentral part of the EP, and the MCB in the southeast (Fig. 1)hat comprises the Coondamar Formation and the conformablyverlying ca. 2970 Ma Mosquito Creek Formation (Bagas, 2005;arrell, 2006).

In the southeastern part of the EP, the domical McPhee andilgalong greenstone belts shown in Fig. 1 comprise dominantlyreenschist facies volcanic rocks of the Warrawoona Group,hich are intruded by ca. 3315 Ma granitic rocks. To the southf the McPhee and Yilgalong greenstone belts, the Warrawoonaroup is unconformably overlain by, or faulted against, theCB. The EP and MCB are intruded (and stitched) by the

a. 2770 Ma Black Range Dolerite Suite and the ca. 1800 Mauartz syenite—quartz monzodiorite bodies of the Bridget Suite.hey are also both unconformably overlain by the ca. 2770 Maortescue Group of the Hamersley Basin.

The exposed part of the MCB is approximately 60 km longn a E–W direction and 30 km wide (Fig. 1). The Kurrana Ter-ane is exposed to the south of the MCB and locally includeshe ca. 3199–3178 Ma Golden Eagle Orthogneiss, and the post-ectonic ca. 2838 Ma Bonney Downs Granite (Bagas, 2005).he Golden Eagle Orthogneiss is a strongly deformed, amphi-olite facies, lithologically layered orthogneiss consisting ofoliated protoliths of biotite-bearing monzogranite, granodiorite,nd tonalite interlayered with lenses of amphibolite, ultramaficchist, and quartz–mica schist. The Bonney Downs Granites included in the Split Rock Supersuite that is character-
zed by post-tectonic tin–tantalum–lithium–beryl-bearing ca.890–2830 Ma granitic rocks in the North Pilbara Craton (Vanranendonk et al., 2004), and which are characterized by pro-ounced radiometric signatures.

n Research 160 (2008) 227–244 231

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. Mosquito Creek Basin

The Mosquito Creek Formation is a succession of Archaeaniliciclastic rocks that occupies much of the central part of the

CB in the northern part of the Archaean Pilbara Craton ofestern Australia (Fig. 1). The formation is multiply deformed

nd has been metamorphosed under low-grade (sub-greenschisto greenschist) conditions. The MCB is wedge-shaped in cross-ection (Fig. 2), as interpreted from a GSWA gravity study (S.hevchenko, written communication, 2004), and is separatedrom ca. 3200 to 2840 Ma orthogneiss and granites in the Kur-ana Terrane by the Kurrana Shear Zone (Bagas, 2005; Fig. 2).

The >1 km thick Coondamar Formation is exposed at thease of the MCB, its age is poorly constrained between ca.200 and 2938 Ma (Bagas, 2005), and it is exposed along theaulted northeastern and southern margins of the basin (Fig. 2).n the northeastern part of the basin, the formation consists ofithic sandstone containing subangular to subrounded basalticlasts. In the southern part of the basin, the formation consistsf chloritic metasedimentary rocks, metachert, metagabbro, andetamorphosed mafic and ultramafic volcanic rocks. The com-

osition of these rocks and their location at the base of the basinuggest that the Coondamar Formation formed during a periodf crustal extension.

The conformably overlying Mosquito Creek Formation rep-esents the remainder of the MCB succession (Figs. 1 and 2).he total thickness of the formation cannot be accurately deter-ined owing to the lack of suitable marker units, and because

ts top is not exposed. Hickman (1983) proposed that the entireCB succession is about 5 km thick, but this is probably an over-

stimation due to tight folding and structural repetition. Theseifficulties preclude construction of a rigorous lithostratigraphicolumn; however, Fig. 3 is a schematic representation of the maineatures of the succession.

Along the northwestern and northern edge of the basin,he basal ∼200 m of the Mosquito Creek Formation is faultedgainst the EP and consists of a succession of interbedded con-lomerate, coarse-grained sandstone, siltstone, and shale thatnconformably overlies, or is in faulted contact with, the EPFig. 3). Individual conglomerate beds are lenticular channeleposits around 10 m thick, and contain rounded greenstonelasts of rocks that are commonly found in the EP. Theselasts include vein quartz, chert, and basalt, and rare felsicolcanic rocks up to about 200 mm in diameter, in a poorlyorted sandstone matrix. The conglomerate beds are coarserained, and are sometimes interbedded with cross-bedded peb-ly sandstone. The sandstone is moderately sorted, containingubrounded chert and quartz pebbles, and well-rounded to sub-ounded grains of quartz, chert, white mica, feldspar, and rareiotite.

Conglomerate beds in the southwestern part of the MCB areenticular. They are interbedded with, and overlie, wacke, silt-tone, and shale successions. These beds do not appear to be
lose to the base of the Mosquito Creek Formation, as suggestedy Noldart and Wyatt (1962), and possibly represent depositionssociated with syndepositional movement along the Middlereek Fault (Fig. 2). The conglomerate fines upwards and grades
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ig. 3. Partial and simplified stratigraphic section of the Mosquito Creek For-ation (modified after T.S. Blake, quoted in Eriksson et al., 1994).

nto a succession comprising interbedded wacke, siltstone, and
hale, which forms the bulk of the upper part of the Mosquitoreek Formation.
The progradational character of the basal conglomerate andandstone was interpreted by Eriksson et al. (1994) to be con-

2 n Research 160 (2008) 227–244

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istent with a fan-delta depositional system, but no evidencef wave- or tidal-influences has been documented. Eriksson etl. (1994) further argued that the progradational trend of theosquito Creek Formation as a whole suggests that it repre-

ents the response to either gradual, step-like falls in sea level,r increasing tectonically driven source-region uplift and basinubsidence.

The upper part of the Mosquito Creek Formation consists ofhinly bedded sandstone interbedded with siltstone and shale,nd has been metamorphosed at sub-greenschist to lower green-chist facies (Bagas, 2005; Farrell, 2006). The sandstone beds areypically fine- to coarse-grained, well graded, and the only trac-ional structures observed are a weak horizontal lamination andare ripple marks. Sandstone beds typically have a sharp base,ith localized scour structures, and the lack of hummocky cross-edding suggests a deep-water depositional environment. Theontact with overlying siltstone and shale is commonly grada-ional, and full Bouma cycles are locally present. The sandstoneontains a variety of angular to subrounded clasts of quartz,eldspar, chert, intraformation sandstone and shale, and rare fel-ic igneous rocks and quartz–sericite schist, in a finer-grainedelitic matrix. The matrix characteristically consists of meta-orphic white mica, chlorite, carbonate, rutile, very fine-grained

isseminated pyrite, and quartz, with detrital grains of quartz,hite mica, and feldspar. The sandstone beds are commonlyraded and form either thin interbeds with siltstone and shaler massive graded beds up to 2 m thick. Siltstone and shale areaminated or thinly bedded, and well cleaved. They constitute

significant, although poorly exposed, part of the formation.n places, these rocks contain fine cross-bedding and solearks (Fig. 4), and elsewhere graded beds have irregular basesith flame structures associated with hydroplastic disruption of

he underlying shale (Fig. 5). The sandstone is recrystallizedo micaceous hornfels, close to intrusive rocks of the Brid-et Suite, with cordierite and andalusite porphyroblasts in a
ranoblastic matrix containing quartz, biotite, muscovite, andeldspar.
Eriksson et al. (1994) classified the sandstone of the Mosquitoreek Formation as lithic wacke and lithic sandstone.

ig. 4. Finely cross-bedded, sole marked, and fine-grained sandstone interbed-ed with siltstone, and shale of the Mosquito Creek Formation (MGA 203775E571020N). Photograph taken looking southward.

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ig. 5. Graded and cross-bedded sandstone with a flame-structure at its irregularase. The flame structure is associated with a hydroplastic disruption of thenderlying dark-coloured shale (MGA 203775E 7571020N).

.1. Deformation events in the Mosquito Creek Basin

At least five phases of deformation (D1–5) are recognized inhe MCB (Table 1). Structures in the basement and deformationvents that predate the formation of the MCB are discussed byan Kranendonk et al. (2004) and are not considered in the

ollowing.

.1.1. D1–2 (between ca. 2972 and 2905 Ma)A strong layer-parallel schistosity or cleavage (S1) is present

n all rocks of the MCB. The schistosity in the Coondamarormation trends east to northeast with a bedding–cleavage

ntersection lineation (L1) that plunges steeply to the north.weak down-dip mineral lineation (L1) is present in places,

nd this commonly plunges about 50◦ northeast (e.g. at MGA12350E 7567800N). The intensity of the D1 deformation andrade of regional metamorphism decrease northward towards theiddle Creek Fault (Fig. 2). These features and the observation

hat S1 is parallel to compositional layering suggest that D1 mayepresent an extensional deformation event in a northerly direc-ion involving westerly striking normal shearing at the contactetween the Kurrana Terrane and the MCB along the Kurranahear Zone.

Rare isoclinal and reclined F2 folds are preserved in theoondamar Formation (Fig. 6). Where observed, the axes of

he F2 folds, which are defined by folded S0 and S1 (Fig. 6),lunge shallowly northwards, and the folds commonly verge tohe east, suggesting west-over-east thrusting. The age of thesevents are not well constrained, but are bracketed by the max-mum age for deposition of the Mosquito Creek Formation2972 + 14/−37 Ma; see below) and the ca. 2905 Ma age of the

3 event (see below).

.1.2. D3 (ca. 2905 Ma)Structures assigned to the D3 event include regionally sig-

ificant, east-northeasterly trending, shallowly and commonlyoubly plunging, open to isoclinal folds and chevron-like orngular folds (Fig. 7), penetrative slaty cleavage (S3), SSE-over-NW thrusting and reverse faults, and mineralized faults that


Table 1Deformational events recognized in the Mosquito Creek Basin (after Bagas, 2005; Farrell, 2006)

Event Age (Ma) Structures Tectonics Comments

D1 Between ca. 2972and 2905

S1 fabric, steeply plunging mineral lineation Early ?extensional faults trendlingwestward

Metamorphic peak between D1 and D2

D2 Between ca. 2972and 2905

Rare isoclinal and reclined F2 folds plungingshallowly northwards and commonly vergeto the east

West-over-east thrusting

D3 ca. 2905 Major ENE–WSW folds, S2 penetrativeslaty cleavage, NNW–SSE thrusting,crenulation lineation, weak NNW-plungingmineral lineation

SSE-over-NNW thrusting andreverse faults

Steepening of faults in centre and south(back ?rotation). Probably correlates withthe main fabric in the Mallina Basin. Faultsassociated with this event host Au and basemetal occurrences in MCB (Blewett et al.,2002). Disruption of metamorphic isogradslate in D2. Possible reactivation of late-D2

thrust faults. Post-dates BDG(a) in KT(b)

D4 Between 2905 and2765

North–south kink folds and kink bands,upright local crenulation cleavage (S4)

East–west shortening Last widespread event. Conjugatecrenulation cleavages (trending 000–020◦and 040◦)

D5 <2717 Steeply dipping faults Probably in response to NE–SWcompression

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re generally parallel to the axial trace of F3 folds. During thisvent, F2 folds are refolded by folds related to D3 (Fig. 8) thateform the Kurrana Shear Zone and modify the S1 foliation.he F3 folds are associated with vertical to steeply dipping axiallanar fabrics defined by a spaced cleavage (S3) in sandstonef the Mosquito Creek Formation, a weak and shallow plung-ng crenulation (Fig. 8), and mineral lineation (L3) parallel tohe fold axes. The general orientation of these structures indi-ates a north-northwesterly–south-southeasterly compressionalegime. Based on the age of mineralization hosted by D3 struc-ures (Table 2), the age of this deformation event is ca. 2905 Madiscussed below).

.1.3. D4 (between 2905 and 2765 Ma)The fourth deformation event (D4) resulted in the devel-

pment of upright, northerly trending open macroscopic F4

ig. 6. Reclined fold (F2) in foliated (S1) chlorite–actinolite schist of theoondamar Formation (photograph taken facing northward at MGA 234400E575250N). Partly annotated to highlight the reclined fold.

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p down to east (oblique slip)istral (north side up)

olds (e.g. at MGA 230300E 7593900N), a rare spaced andteeply to vertically dipping crenulation cleavage (S4), andmall-scale kinks and crenulations. Dextral movement along theast-northeasterly trending Sandy Creek, Blue Spec, and Mid-le Creek faults (Fig. 2) is also attributed to this event (Bagas,005), and the orientation of the folds and faults indicatespproximately east–west shortening. This deformation event didot affect the ca. 2765 Ma Hardey Formation in the overlyingamersley Basin, indicating that it took place between 2905 and765 Ma.

.1.4. D5 (<2717 Ma)Southeasterly striking, dextral, and southwesterly striking,

inistral F5 faults in the MCB, Kurrana Terrane, and the ca.717 Ma Maddina Formation in the Fortescue Group (Nelson,998, pp. 133–135) define the D5 event. These 10–30 km longaults crosscut F3 and F4 folds (e.g. around MGA 230300E

ig. 7. Folded (F3) banded pyritic siltstone interbedded with fine-grained poorlyorted sandstone of the Mosquito Creek Formation (MGA 203100E 7570100N).


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Table 3Gold production in the Mosquito Creek Basin (after Ferguson and Ruddock,2001)

District/deposit Production (kg) Au resource

Middle Creek ‘Line’ 1053 8.97 Mt at 1.96 g/tGOLDEN EAGLE 15 17 Mt at 1.96 g/tBARTON 239 0.331 Mt at 2.5 g/tHOPETOUN 8Blue Spec ‘Line’ 2953 2.60 Mt at 3.36 g/tBlue Spec/GOLDEN SPEC 2356 0.167 Mt at 21.4 g/tGOLDEN GATE – 2.43 Mt at 2.12 g/tMosquito Creek area 525EN

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ig. 8. Foliated (S1) and crenulated (D3) fine-grained ferruginous schist of theoondamar Formation (MGA 235850E 7568950N).

593900N and 211900E 7692000N; Bagas, 2005), and theverall orientation of these faults is consistent with north-orthwesterly–south-southeasterly shortening.

.2. Gold deposits in the Mosquito Creek Basin

Major D3 shear zone host epizonal to mesozonal orogenicu(–Sb) deposits in folded, cleaved, and locally crenulatedelitic and psammitic units of the Mosquito Creek FormationFig. 2). Most of the deposits are located in the arcuate Mid-le Creek (predominantly Au with rare Sb) and the subparallellue Spec (Au–Sb) fault zones, both of which are orientatedpproximately east–west (Fig. 2). Several placer deposits are
ituated west of the Blue Spec Fault Zone, north of Nullagineownship, and in the Neoarchaean Hardey Formation of theamersley Basin (Fig. 2). Production and resources of depositsosted by the Mosquito Creek Formation account for around
iail

able 2b–Pb model ages on epigenetic galena from mineral deposits in the Mosquito Creek

eposit Location MGA, Zone51K

Description

osquito Creek in theMosquito Creek Formation

237565E 7586430N From gold-bearing quhosted by D3 structureschist

oondamar Creek in theCoondamar Formation

258931E 7572090N From a sulfide depositsphalerite, chalcopyritminor galena) interbedshale hosted in a D3 st

astern Creek area 358ullagine Common 374

2 t Au, or about 50% of the total gold production from thentire North Pilbara Craton (Table 3; Ferguson and Ruddock,001).

The Middle Creek Fault Zone hosts many small-cale deposits, including Golden Eagle, North Dromedary,hearers–Otway, Barton–Hopetoun, and Little Wonder (Fig. 2).ince the 1880s, this group of deposits has produced about 1 tu from over 32 000 t of ore averaging 23 g/t Au (Ferguson anduddock, 2001).

The Golden Eagle deposit (Fig. 2) is located in the northernpright limb of a reclined F3 anticline that plunges gently tohe east (Blewett et al., 2002). The quartz vein-hosted deposit isstimated to contain a resource of over 600 000 oz (or 17 t) AuD.J. Carmichael Pty Ltd., 2006). Mineralization is associatedith veined and disseminated pyrite, lesser disseminated rutile,isseminated magnetite, chalcopyrite, gold, sphalerite, galena,nd feldspar (Blewett et al., 2002). Faults associated with D3 and3 cleavage intersecting the anticline appear to control the goldineralization, and the historical workings followed narrow fer-

uginous (pyritic) quartz veins with grades reaching 145 g/t AuFerguson and Ruddock, 2001).

The Shearers and Otways deposits are located about 11 kmortheast of the Golden Eagle deposit (Fig. 2). The mineraliza-ion at the Shearers deposit is in a stockwork zone of quartz veins,hich are up to 1 m wide and hosted by dextral faults trend-

◦
ng about 020 that crosscut the Middle Creek Fault (Fergusonnd Ruddock, 2001). The mineralization at the Otways deposits irregular and follows subvertical quartz veins that are paral-el to, and north of, the Middle Creek Fault Zone. No grades
Basin

Analyticaltechnique

Age (Ma) Reference

artz veinss in pelitic

Conventional 2905 ± 9 I.R. Fletcher, quoted inThorpe et al. (1992)

(containinge, pyrite, andded with blackructure

Conventional 2905 ± 9 Huston et al. (2002a)

n Res

od

pTihoaaidsnv

OtNfa(no

B(pio1dc

oflsadtprrc1(t(

3

rlatd

crsda

8aAsaaFpAdca

1

uo

gdatz2

tsoCyi

11hAsp

ymztt2


r production figures are available for the Shearers and Otwayeposits.

The Barton and Hopetoun deposits have been the biggestroducers of gold along the Middle Creek Fault Zone (Fig. 2).he deposits are at or near the intersection of two major faults

n the Middle Creek Fault Zone, and have a complex structuralistory involving folding and refolding, with several generationsf shearing (Blewett et al., 2002). About 7000 t of ore with anverage grade of 36.5 g/t was mined between 1898 and 1926 forrecorded production of over 240 kg of gold. The mineralization

s hosted in anastomosing quartz veins that trend north-northeast,ip steeply southeast at about 60◦ near the surface, and becomehallower dipping with depth (Hickman, 1983). The ore plungesortheast at about 40◦, coinciding with the intersection of quartzeins with cleavage in the country rocks (Hickman, 1983).

The Latest Surprise, Ard Patrick, Galtee More or Galteemore,ff Chance, and Lands End 1 Au (–Sb) deposits form part of

he Mosquito Creek Mining Centre situated about 40 km east ofullagine (Fig. 2). Over 230 kg of gold was produced in this area

rom about 7000 t of ore until 1913 (Hickman, 1983). The miner-lization is in easterly trending, quartz veins up to 1 m in widthHickman, 1983). The veins are vertical or dip steeply to theorth, and contain gold, minor pyrite, and local concentrationsf scheelite and stibnite (Hickman, 1983).

Deposits hosted by the Blue Spec Fault Zone include thelue Spec, Golden Spec, Branchis, Billjim, and Parnell deposits

Fig. 2). Of these, the Blue Spec deposit has been the mostroductive with almost 2 t of gold, and concentrates contain-ng 1500 t of antimony, produced from more than 110 000 tf ore with an average grade of over 18 g/t Au (Hickman,983; Ferguson and Ruddock, 2001). These deposits differ fromeposits in the Middle Creek Fault Zone by containing signifi-antly elevated concentrations of stibnite.

Lodes in the Blue Spec Fault Zone generally dip verticallyr steeply to the south, and appear to be located in shearedexures in the fault zone. This field relationship suggests thathearing was at least in part synchronous with the miner-lization (Hickman, 1983). Mineralization at the Blue Speceposit is located in an east-trending, 15–20 m-wide shear zonehat hosts quartz veins containing stibnite, aurostibnite, gold,yrite, pyrrhotite and carbonate, with minor scheelite, arsenopy-ite, marcasite, sphalerite, chalcopyrite, magnetite, calaverite,ickardite, and gudmundite (Hickman, 1983). Also present areervantite in the oxidized zone and traces of mercury (Hickman,983). This mineral association is characteristic of epizonal<6 km) orogenic Au deposits (Groves et al., 1998), suggestinghat significant Au–As–Te deposits might be present at depthGroves et al., 1998).

.3. Age constraints for the Mosquito Creek Basin

The age of the structural evolution of the MCB remainselatively poorly constrained due to a general lack of geochrono-
ogical data. In order to better constrain both (1) the maximumge for deposition of the Mosquito Creek Formation and (2)he nature and evolution of potential sediment source terrains,etrital zircons were separated from eight samples of medium- to
gT1C

earch 160 (2008) 227–244 235

oarse-grained sandstone (Fig. 2), and dated via sensitive high-esolution ion microprobe (SHRIMP). The procedures used forample preparation, instrument setup, and data reduction areescribed in Compston et al., 1984; Pidgeon et al., 1994; Staceynd Kramers, 1975, Appendix 1.

A total of 358 analyses were obtained from 351 zircons, fromsamples over 10 analytical sessions between 12 August 2002

nd 26 October 2006 (details of which are summarised in Table1). Data for individual samples (with 1σ uncertainties) are pre-

ented in Appendix 2 (Tables A2.1–A2.8). In each table, the datacquisition sequence is defined by the spot number, and multiplenalyses of a single grain are indicated by the “grain.spot” label.or each analysis, f204 is the proportion of common 206Pb as aercentage of total measured 206Pb (based on measured 204Pb;ppendix 1), and 238U/206Pb* and 207Pb*/206Pb* ratios andates (where “*” indicates data corrected for common Pb) werealculated using the decay constants recommended by Steigernd Jager (1977). Percentage discordance is defined as

00 ×[

207Pb ∗ /206Pb ∗ date − 238U/206Pb ∗ date207Pb ∗ /206Pb ∗ date

]

All ages quoted in the text are 207Pb*/206Pb* ages, and theirncertainties are quoted at the 95% confidence level, unlesstherwise indicated.

Within each table, every analysis has been allocated to aroup. Group D contains all analyses characterised by significantiscordance (>10%) or high common-Pb contents (f204 > 1.5%),nd these analyses are not used in the geological interpreta-ion. Group 1 constitutes the youngest population of detritalircons (defined by ranking the analyses in order of increasing07Pb*/206Pb* date, and calculating the weighted mean date ofhe largest group for which all constituent analyses lie within 2.5tandard deviations of the weighted mean). Group 2 includes alllder detrital zircons (without further subdivision), and Group

(sample 177131 only) denotes analyses of an anomalouslyoung grain attributed to contamination during sample process-ng.

Complete datasets for samples 169199, 169200, 177131, and78010, and partial data for samples 169187 (spot numbers–32; Table A2.1) and 169194 (spot numbers 1–32; Table A2.2)ave previously been reported by Geological Survey of Westernustralia (2004, 2005), but have been reprocessed for pre-

entation here. The remaining data have not previously beenublished.

Maximum depositional ages for each sample (based on theoungest group of detrital zircons as defined above) are sum-arised in Table 4. The sample with the youngest detrital

ircon population is 177131 (a conglomerate near the base ofhe Mosquito Creek Formation), for which two analyses ofwo zircons yielded a weighted mean 207Pb*/206Pb* age of930 ± 19 Ma (1σ; Table 4). However, this age is not distin-
uishable from the weighted mean age of 2941 ± 11 Ma (1σ;able 4) yielded by two analyses of a single zircon from sample69200 (a finer-grained lithic sandstone high in the Mosquitoreek Formation succession).


Table 4Summary of sample-specific constraints on the maximum age for deposition of the Mosquito Creek Formation, based on youngest detrital zircons (each Group 1 inAppendix 2)

GSWA sample no. Data table Number of analyses Number of zircons 207Pb*/206Pb* age (Ma) Confidence level MSWD

169187 A2.1 11 11 3039 ± 6 95% 0.97169194 A2.2 1 1 2999 ± 10 1σ –169199 A2.3 5 5 3290 ± 14 95% 1.8169200 A2.4 2 1 2941 ± 11 1σ 0.30177131 A2.5 2 2 2930 ± 19 1σ 0.16177252 A2.6 1 1 2972 ± 13 1σ –177254 A2.7 1 1 2977 ± 8 1σ –1

A

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“ta1dswfi“cas(oCiia(MmMc

iawMam

2atbmwetitmFei2

4

msFgaAr

4

bdtFic

sM

78010 A2.8 4 4

ll samples See text 9 8

The validity of these sample-specific constraints on the max-mum depositional age of the Mosquito Creek Formation areebatable, as they are based in each case on one or two analysesf one or two zircons. The fact that the absolute constraint is pro-ided by the sample lowest in the stratigraphy means that it is notossible to infer a maximum duration for base-to-top depositionf the entire succession. However, the similarity in maximumges for deposition at opposite ends of the stratigraphic columnntroduces the possibility of pooling the data from all eight sam-les, in an attempt to determine a single, statistically valid (andonservative) maximum depositional age for the entire Mosquitoreek Formation.

Combining the data from all eight samples yields a youngestgroup” comprising nine analyses of eight zircons, spanninghe Group 1 data from samples 177131 and 169200 describedbove, the Group 1 data from samples 177252 (one analysis) and77254 (one analysis), together with three analyses of “olderetrital zircons” (with post-2990 Ma 207Pb*/206Pb* ages) fromample 177131. These nine analyses from four samples yield aeighted mean 207Pb*/206Pb* age of 2971 ± 15 Ma (95% con-dence; MSWD = 2.1), potentially representing a conservativebest estimate” of the maximum depositional age. However, theonstituent data are not distributed evenly about this mean value,nd the associated MSWD value indicates discernible internalcatter, so we consider the robust median of the nine analyses2972 +14/−37 Ma; 96.1% confidence) to be a better definitionf a conservative maximum age for deposition of the Mosquitoreek Formation (Table 4). The siliciclastic turbidites that dom-

nate the southern and eastern part of the Malina Basin locatedn the western part of the North Pilbara Craton (Fig. 1) werelso deposited at or after ca. 2970 Ma and before ca. 2955 MaSmithies et al., 2001). The maximum ca. 2972 Ma age for the

osquito Creek Formation therefore confirms the suggestionade by Hickman (1983) that the turbiditic succession in thealina Basin in the western part of the North Pilbara Craton is

oeval with and a correlative of the Mosquito Creek Formation.A probable minimum age for the Mosquito Creek Formation

s provided by a Pb–Pb model age of 2905 ± 9 Ma (Thorpe etl., 1992; Table 2), derived from epigenetic galena associated
ith lode gold mineralization hosted by D3 structures in theosquito Creek mine area (Fig. 2). An identical Pb–Pb model
ge of 2905 ± 9 Ma on epigenetic galena that is associated withineralization within the Coondamar Formation (Huston et al.,

acRt

3295 ± 10 95% 0.31

2972 + 14/−37 (robust median) 96.1% –

002b; Table 2). Gold in the Mallina Basin also has a similarge of ca. 2900 Ma (Huston et al., 2002b), which suggests thathis mineralizing event was widespread across the North Pil-ara Craton. In addition, the Pb evolution curve upon which theodel ages for the MCB are based is constrained by severalell-dated, syngenetic sulphide deposits Pilbara-wide (Thorpe

t al., 1992), and the Pb isotopic compositions measured inhe MCB galenas lie very close to the model curve, support-ng the contention that these model ages approximate the trueiming of D3-related mineralization, and provide a real mini-

um constraint on the depositional age of the Mosquito Creekormation. This event was also broadly synchronous with themplacement of 2897 ± 6 Ma monzogranites and granodioritesn the Cooninia Inlier (Geological Survey of Western Australia,006), about 100 km to the south of the basin (Fig. 1).

. Provenance and tectonic setting studies

Eight samples of sub-greenschist to greenschist facies,edium-grained sandstone were collected from surface expo-

ure and drill core in the northern part of the Mosquito Creekormation for dating (Appendix 2), and of these four wereeochemically analysed (Table 5, Eggins et al., 1997; Norrishnd Chappell, 1997; Norrish and Hutton, 1969; Pyke, 2000,ppendix 3). Despite variable degrees of recrystallization, the

ocks show relatively limited compositional variation.

.1. Preliminary geochemical discrimination studies

The assumption is made here that sandstone classificationsased on Phanerozoic systems can be applied to sedimentseposited during the Mesoarchaean. Similar discriminationechniques have been used on Archaean metasandstones (e.g.ralick and Kronberg, 1997). It is also acknowledged that an

nterpretation at a basin-scale on only four samples is not statisti-ally reliable, and the interpretations made here are preliminary.

Applying a geochemical classification plot (K2O/Na2O ver-us SiO2/Al2O3; Wimmenauer, 1984), the sandstones of theosquito Creek Basin are quartz-rich wackes and quartz-rich

rkoses (Fig. 9). The sandstones show a spectrum of chemi-al compositions resembling that of the bulk crust (Fig. 10a;udnick and Gao, 2004), and have geochemical compositions

hat are free of anomalously high concentrations of Ti, Zr, Hf


Table 5Whole-rock geochemical data for sandstone samples from the Mosquito CreekFormation

GSWA no.

169187 169199 169200 177131

Major elements (%)SiO2 90.59 88.87 92.53 86.21TiO2 0.13 0.27 0.20 0.17Al2O3 2.99 5.76 4.44 3.69Fe2O3 1.11 1.24 0.28 1.72FeO 1.59 0.51 0.06 2.60MnO 0.02 0.02 0.01 0.04MgO 1.09 0.57 0.13 2.08CaO 0.08 0.06 0.03 0.07Na2O 0.17 0.32 0.26 0.11K2O 0.08 1.13 1.13 0.05P2O5 0.04 0.04 0.03 0.03

Trace elements (ppm)Ba 46 302 272 123Ce 10.94 29.49 23.76 20.14Cr 83 257 174 157Cs 0.14 0.85 0.86 0.12Dy 2.13 1.78 1.74 2.24Er 1.19 1.20 1.17 1.37Ga 3.6 7.3 5.2 4.8Gd 1.85 1.99 1.64 2.08Hf 1.2 3.5 3.4 1.4Ho 0.44 0.42 0.39 0.49La 5.96 17.39 13.95 11.86Lu 0.15 0.23 0.27 0.22Mo 0.8 1.1 1.1 1.7Nb 1.7 4.2 3.6 2.0Nd 5.13 12.62 8.97 9.26Ni 50 61 16 117Pr 1.21 3.2 2.33 2.22Rb 12.3 33.1 33.1 15.2Sb 0.6 2 1.2 2.3Sc 4 6 4 22Sm 1.37 2.37 1.69 2.01Sr 19.6 36.0 30.6 19.5Ta 0.3 0.5 0.7 0.3Tb 0.33 0.29 0.27 0.35Th 3.5 8.8 7.9 3.5Ti 761 1625 1187 1031Yb 0.91 1.28 1.41 1.19Zr 46 129 119 57

Fig. 9. Archaean sandstones of the Mosquito Creek Formation in the geochem-ical classification plot after Wimmenauer (1984).

FnM

ahtt

BacAhof(eyt

ta(pss

aihdttd

ig. 10. (a) Bulk-earth normalized (Rudnick and Gao, 2004) and (b) chondriteormalized (Sun and McDonough, 1989) REE profiles for sandstones of theosquito Creek Formation.

nd Y (Fig. 10b). These elements would be concentrated byydraulic sorting of heavy minerals during the processes ofransportation and deposition of detrital material obscuring rela-ions to the sources of such material (e.g. Cullers et al., 1987).

Studies by, among others, Bhatia (1983, 1985a, 1985b),hatia and Crook (1986), Cullers et al. (1987), and McLennan etl. (1993) have related systematic variations in the geochemicalompositions of sandstone to different tectonic environments.ccordingly, the chemical compositions of sedimentary rocksave commonly been used to help constrain the tectonic affinitiesf provenances. Examples are the studies of the tectonic settingor Palaeozoic flysch deposits in eastern Australia by Bhatia1985a, 1985b), and Palaeozoic sediments in New Zealand andastern Australia by Roser and Korsch (1986). Using such anal-ses, the sandstones from the Mosquito Creek Formation plot inhe passive continental margin field in Figs. 11 and 12.

La/Sc versus Ti/Zr ratios of sandstones have also been usedo discriminate between oceanic island arc, continental islandrc, active continental margin, and passive margin environmentsBhatia and Crook, 1986). In Fig. 13a, the MCB sandstoneslot in the continental island-arc and passive margin fields. Aimilar result is obtained in the Sc, Th, and Zr triangular plothown in Fig. 13b.

Most of the plots in Figs. 11–13 suggest a passive marginffinity, but some also suggest a continental island-arc affin-ty. Discriminant function analysis is another method used toelp determine the provenance of sediments using geochemical
ata. This statistical technique defines two values (discrimina-ion functions) that separated samples into groups defined by theectonic settings of the source regions. Following Bhatia (1983),iscriminant function analysis is used to distinguish basin tec-


Feg

tw(ts

Ff

ig. 11. Sandstones plotted on selected published discrimination diagramsmphasizing major element chemical variations (after Bhatia, 1983), which sug-est a passive margin setting during deposition of the Mosquito Creek Formation.

onic settings for sandstone samples from the MCB (Fig. 14a),
hich plot in the passive margin field. The Roser and Korsch
1988) diagram (Fig. 14b) is also based on a discriminant func-ion analysis and is useful to discriminate the basin tectonicetting of sandstones and mudstones. The MCB samples plot in

ig. 12. K2O/Na2O vs. SiO2 plot (from Roser and Korsch, 1986) of sandstonesrom the Mosquito Creek Formation.

Fig. 13. (a) La/Sc vs. Ti/Zr (after Bhatia and Crook, 1986; modified by Bahlburg,1fa

tmcc

dmcu1

998), and (b) Sc–Th–Zr/10 (after Bhatia and Crook, 1986) discriminant plotsor sandstone suggesting that the Mosquito Creek Formation was deposited incontinental island-arc to passive margin tectonic setting.

he recycled field, which corresponds to quartz-rich sediments ofature continental or recycled provenance. This provenance is

ommonly interpreted as representing clastic compositions typi-al of passive margin tectonic settings (Roser and Korsch, 1988).

Trace elements such as Cr are useful in identifying accessoryetrital components such as chromite, commonly derived from
afic to ultramafic sources. The average Cr content of the upper
ontinental crust is 83 ppm (McLennan, 2001). Chromium val-es of the studied sandstones range from 83 ppm (for sample69187) to 257 ppm (for sample 169199). The Cr/Th ratios of


Fig. 14. Plot of sandstones from the Mosquito Creek Formation in the dis-criminant function diagrams: (a) Discriminant function diagram (modified afterBhatia, 1983) suggesting a passive margin tectonic setting during deposition and(b) for the provenance signature of sandstone-mudstone suites based on majores

bavcssta

bigsaM

Fig. 15. Plot of sandstones from the Mosquito Creek Formation on a Th/Sc vs.Ztts

rpuZnt(acMtiatt

t2czhTscfstte

lement chemistry after Roser and Korsch (1988) suggesting various degrees ofource rock reworking.

etween 22 (for sample 169200) and 45 (for sample 177131)re higher than 7.8, which is the average upper continental crustalue (McLennan, 2001). Sample 177131 was collected fromlose to the northern edge of the MCB and the high Cr/Th ratiouggests that it was derived from mafic or ultramafic sources,uch as those located in basement rock located to the north ofhe basin, which would also account for the continental islandrc affinity of this sample.

A good tracer of mafic source components is the compati-le element Sc, particularly when compared with Th, which isncompatible and enriched in felsic rocks. Both elements areenerally immobile under surface conditions and therefore pre-
erve the characteristics of their source, making the Th/Sc ratiorobust provenance indicator (Taylor and McLennan, 1985;cLennan et al., 1990). In sandstones from the MCB, the Th/Sc
4

v

r/Sc diagram (after McLennan et al., 1993), suggesting that the sample closero the basin margin (177131) is sourced from more ‘primitive’ material, whereashe other samples are from less ‘primitive’ material or relatively more recycledources.

atios increase from 0.88 for sample 169187 to about 2 for sam-le 169200 (Fig. 15), whereas the Th/Sc ratio for the averagepper continental crust is 0.79 (McLennan, 2001). Th/Sc andr/Sc ratios can reveal compositional heterogeneity in prove-ances, if the samples show Th/Sc and Zr/Sc values alonghe trend from mantle to upper continental crust compositionsMcLennan et al., 1993). The Zr/Sc ratio is commonly used asmeasure of the degree of sediment recycling leading to the

oncentration of zircon in sedimentary rocks (McLennan, 1989;cLennan et al., 1993). Such a trend is present in Fig. 15, with

he lower Th/Sc value from sample 177131 suggesting a strongernput from a less evolved source, while the other samples suggestmore evolved source. This suggestion is strongly supported by

he distribution profiles of U–Pb ages of detrital zircons fromhe samples (discussed below).

Zirconium values range from 46 to 129 ppm, which are lowerhan the average upper crustal value of 190 ppm (McLennan,001). Zr/Th ratios are another measure of the degree of recy-ling. Thorium is commonly abundant in the heavy mineralsircon, monazite, titanite and epidote. Concentration of theseeavy minerals during recycling leads to an increase in Zr andh abundances. Zr/Th ratios of between 13.14 and 16.29 forandstones in the Mosquito Creek Formation are below the upperrustal average of 17.76 and results from high Th concentrationsor samples 169199 and 169200, and lower Zr concentrations foramples 169187 and 177131. These observations also suggesthat sample 177131 from the northern edge of the MCB hashe least evolved source, whereas the other samples have morevolved (variably recycled) sources.

.2. Detrital zircon studies

Relative cumulative-probability plots are commonly used toisually assess the statistical similarities or differences between

2 n Res

sTc2

F1(u


amples and potential source regions (e.g. Camacho et al., 2002).he plots present summed probability density curves for con-ordant analyses (i.e. those analyses for which the calculated38U/206Pb* date and 207Pb*/206Pb* dates agree within 10%),

aaet

ig. 16. Probability density diagrams of SHIMP Pb–Pb zircon ages from the Mosq998, 1999, 2000, 2001, 2002; Geological Survey of Western Australia, 2006) for (ae) 169200; (f) sample 169194; (g) sample 169199; and (h) sample 169187. Samplespper part of the formation. Only concordant data (discordance < 10%) are used in th

earch 160 (2008) 227–244

ssuming that the probability density of each analysis followsGaussian distribution. The horizontal spread in the graphs forach peak relates to the standard deviation, and generally reflectshe accuracy of the data.

uito Creek Formation and the East Pilbara Terrane (EP) (Nelson, 1996, 1997,) sample 177131; (b) sample 178010; (c) sample 177252; (d) sample 177254;(a–d) are from the basal part of the formation, and samples (e–h) are from thee construction of these graphs (n = number of analyses).

L. Bagas et al. / Precambrian Res

Fig. 17. Relative cumulative-probability diagrams of pooled SHRIMP Pb–Pbzircon ages from the Mosquito Creek Formation compared to: (a) East PilbaraTerrane (EP) and (b) West Pilbara Superterrane (WPS), both compiled fromdata sourced from the Geological Survey of Western Australia (2006). Onlyc(

ttzdt

tpAdFtihP3

t(pfra3

pctH3sd

pda3tEpzognf

5

csKDtsrtI

ccm(usit3tQdta1tmw

the sediment now forming the bulk of the Mosquito Creek For-

oncordant data (discordance < 10%) are used in the construction of these graphsn = number of analyses).

Fig. 16 displays the distribution of ages for the samples fromhe samples from the Mosquito Creek Formation and illustrateshat the MCB had a provenance supplying ca. 2938–3730 Maircons. By comparing data against the pooled data for all rocksated from WPS and EP, it is apparent that the provenance forhe basin was not entirely the North Pilbara Craton (Fig. 17).

A significant proportion of the detrital zircons dated fromhe Mosquito Creek Formation are appreciably older than theooled zircon data for the WPS (Geological Survey of Westernustralia, 2006; Fig. 17), and the detrital zircon age distributionata do not correlate well with the pooled zircon data for the EP.or example, prominent peaks at ca. 3540, 3360 and 3090 Ma in

he profile for the Mosquito Creek Formation are not representedn the plot for the EP (Fig. 17). The implication is that the maininterland for the MCB was an area other than the exposed Northilbara Craton and one that contains zircons with ages of ca.540, 3360, and 3090 Ma.

When the detrital zircon data for the samples fromhe Mosquito Creek Formation are considered individuallyFig. 16), two groups emerge. One group comprises four sam-les (GSWA samples 177131, 178010, 177252, and 177254)rom the base of the formation, and the other group includes the
emaining samples (GSWA samples 169200, 169199, 169194,nd 169187). The basal samples have distribution peaks at ca.470 and 3425 Ma and a trough at ca. 3370 Ma, similar to the age
mtc

earch 160 (2008) 227–244 241

rofile for the EP. Peaks in these basal samples ca. 3290 Ma areonsistent with new data for the Yilgalong Granitic Complex inhe EP (Geological Survey of Western Australia, 2006; Fig. 1).owever, most of the samples also have peaks at ca. 3540 and200 Ma that are not represented in the plot for the EP. Thisuggests that the basal fan deposits had a mixed provenance thatid not include the EP alone.

The second group contains samples from the upper turbiditicart of the Mosquito Creek Formation. The samples have slightlyiffering zircon age profiles and some are very closely spacedpart (Fig. 2), but there are common peaks at ca. 3540, 3490,360, 3280, 3220, 3140, 3040, and 3000 Ma in some or all ofhe samples, none of which are common in the plot for theP. Conversely, peaks at ca. 3470 and 3430 Ma in the sam-les from the base of the formation are not represented in theircon distributions for the upper part of the formation. Thesebservations suggest that the EP contributed detritus to the strati-raphically lower part of the Mosquito Creek Formation, but didot form a significant source of detritus for the upper part of theormation.

. Discussion and conclusions

Various studies have observed that turbidity currents typi-ally flow parallel to the long axis of elongate confined basins,uch as the MCB, parallel to basin-controlling normal faults (e.g.neller et al., 1991; Flottmann et al., 1998; Haines et al., 2001).espite the lack of palaeocurrents in the MCB, the long axis of

he basin trends in a northeasterly to easterly direction; thus, theource of the material in the basin is likely to be an unknown ter-ane to the east (Fig. 1), or under the Hamersley Basin coveringhe EP and to the southwest, west of the Mesoarchaean Sylvanianlier.

About two thirds of the Sylvania Inlier (Tyler, 1990, 1991)onsists of ca. 2793 Ma granites, and the remainder consists ofa. 3500–3450 Ma greenstone successions, and dykes of ultra-afic rocks, gabbro, anorthosite and ca. 3450 Ma monzonite

Hollingsworth et al., 2001). The greenstone belts compriseltramafic to intermediate volcanic rocks, conglomerate, peliticchist, quartzofeldspathic schist, quartzite, chert, and bandedron-formation (Hollingsworth et al., 2001). The granites con-ain xenocrystic zircons with SHRIMP U–Pb ages of ca. 3481,023, 2912, and 2832 Ma (Hollingsworth et al., 2001). One ofhe dolerite dykes has been dated by Wingate (1999) at 2747 Ma.uartzite from the greenstone succession has SHRIMP U–Pbetrital zircon ages of ca. 3665, 3612, 3600 and 3560 Ma. Allhese ages are represented in various proportions in the prob-bility density plots for samples 169187, 169194, 169199, and69200 (Figs. 2 and 16). Although the Sylvania Inlier may not behe exact source of the upper part of the MCB, the basin shares

ore similarities in zircon profiles with the Sylvania Inlier thanith the EP.A combination of evidence including the large volume of

ation (<5 km thick), the immaturity of the sediments, andhe fine-grained nature of the sandstone in the formation indi-ate derivation from a tectonically active but distal source (e.g.

242 L. Bagas et al. / Precambrian Res

F2

Hct

atfaimssgctltsibbftatDbdfbtAdsAagaagtBdlcg

aeta2lehba

M2cpipOtmuSFnoM

A

CAmaitAtc

A

i

R

A

A

Bagas, L., 2005. Geology of the Nullagine 1:100 000 sheet. West. Aust. Geol.

ig. 18. Preliminary model for the proposed passive margin settings of the ca.972 Ma Mosquito Creek Formation in the Mosquito Creek Basin.

aines et al., 2001). From the observations made above it islear that the EP did not form a significant source of detritus forhe upper part of the formation.

The detrital zircon and limited geochemical data discussedbove suggest that the MCB formed in a passive margin set-ing (Fig. 18) that developed before 2970 Ma (the maximum ageor the Mosquito Creek Formation). Passive continental marginsre usually the result of ocean floor spreading in a developingntracontinental rift-system (e.g. Wilson, 1997). Such passive

argins are characterised by thick wedge-shaped deposits ofediments in a post-rift phase, which is also the interpretedhape of a cross-section of the MCB (Fig. 2) based on a recentravity survey by the GSWA (S. Shevchenko, written communi-ation, 2004). The massive post-rift subsidence associated withhe thickened sediments may be due to factors such as sedimentoading (e.g. Allen, 2004) or gravitational collapse following theermination of asthenospheric upwelling. In a model that invokesubsidence of the MCB due to ongoing sediment loading, thenterbedded conglomerate and sandstone along the edges of theasin were derived from the EP as fan or delta deposits. The tur-iditic succession of sandstone, siltstone and shale further awayrom the basin edge was probably derived from an exotic sourcehat is not exposed in the North Pilbara Craton. The change tocompressional setting between ca. 2938 and ca. 2905 Ma led

o the development of an accretionary margin system during3. Metamorphic reactions in deeper parts of the MCB duringasin inversion resulted in large-scale fluid migration into major,eep-seated conduits such as the Blue Spec and Middle Creekault zones. At shallower crustal levels, these sulphur- and gold-earing hydrothermal fluids migrated into proximal second- andhird-order fault systems, with the formation of epizonal (i.e.u–Sb) to mesozonal (i.e. Au–As) lode systems late during theeformation, shortening and exhumation of the inverted MCBequence. The close association of anomalously large (i.e. ≥30 tu) orogenic lode gold deposits with meta-sedimentary rocks

bove ‘primitive’ oceanic crust in the majority of major lodeold provinces implies that these oceanic rock sequences playn important role in the ore-forming process (e.g. Bierlein etl., 2002). Moreover, giant gold provinces and deposits areenerally sited in geodynamic settings involving lithospherichinning just prior to, or synchronous with, the gold event (e.g.ierlein et al., 2006). As indicated by the geochemical and
etrital zircon data presented above, development of the MCBikely occurred on thickened, previously dehydrated crust ofontinental origin that had also been punctuated by voluminousranite intrusions during extensional tectonics. These conditions
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earch 160 (2008) 227–244

re generally considered unsuitable for the formation of well-ndowed orogenic lode gold provinces as they do not favourhe release of massive fluid volumes from underplated juvenilend hydrated rocks within a relatively short time (Bierlein et al.,002). Therefore, the potential for the formation of anomalouslyarge orogenic gold deposits in the MCB is considered low. How-ver, considering that relatively small-scale economic depositsave been and are being mined in the basin, there is a possi-ility that concealed similar-sized deposits are present in therea.

Geochronology and detailed mapping have identified that theosquito Creek Formation is likely to have an age between ca.

972 and 2905 Ma, which is similar in age to turbiditic silici-lastics in the Malina Basin (Fig. 1). The basal, conglomeraticart of the Mosquito Creek Formation has a provenance thatncludes the EP, but the WPS and EP cannot represent the mainrovenances for the upper part of the Mosquito Creek Formation.ur preliminary geochemical and geochronological data suggest

hat the Mosquito Creek Formation was deposited in a passiveargin tectonic setting, and the provenance for the turbiditic

pper part of the formation is either to the south towards theylvania Inlier, or the east under the Hamersley Basin (Fig. 1).urther work will endeavour to shed more light on the exactature and provenance of the source region for the upper partf the Mosquito Creek Formation and will further compare theCB with the Mallina Basin.

cknowledgements

This contribution is a product of collaboration between theentre for Exploration Targeting at the University of Westernustralia, and the Geological Survey of Western Australia. Theanuscript also benefited from discussions with Brian Krapez,

nd constructive reviews and helpful comments by Paul Duur-ng and Richard Blewett. Bagas and Bodorkos publish withhe permission of the Director, Geological Survey of Westernustralia. Zircon analyses were performed on the Western Aus-

ralian SHRIMP II, operated by a WA University-governmentonsortium with support from the Australian Research Council.

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at doi:10.1016/j.precamres.2007.07.005.

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