Journal of Geochemical Exploration 106 (2010) 110–120

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Journal of Geochemical Exploration

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The Mt. Mulga barite–magnetite–copper–gold mineralisation, Olary Domain,South Australia

Martin Griessmann a,⁎, Andreas Schmidt Mumm a, Thomas Seifert b, Colin Conor c

a Center for Tectonics, Resources and Exploration, School of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5005, Australiab Department of Economic Geology and Petrology, Institute of Mineralogy, TU Bergakademie Freiberg, Brennhausgasse 14, D-09596 Freiberg, Germanyc Geological Survey Branch, Primary Industries and Resources, South Australia, GPO Box 1671, Adelaide, South Australia 5001, Australia

⁎ Corresponding author.E-mail addresses: martin.griessmann@adelaide.edu.a

andreas.schmidtmumm@adelaide.edu.au (A. Schmidt Mthomas.seifert@mineral.tu-freiberg.de (T. Seifert), conor(C. Conor).

0375-6742/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.gexplo.2009.11.008

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 May 2009Accepted 24 November 2009Available online 13 January 2010

Keywords:BariteCu–Au mineralisationWillyama SupergroupOlary DomainREEIsotopes

The Mt. Mulga barite Cu–Au deposit in the central part of the Olary Domain is hosted by thePalaeoproterozoic Willyama Supergroup of the Curnamona Craton. The lenticular, stratiform ore bodiesare hosted by the Ethiudna Subgroup at and near the contact to the underlying Wiperaminga Subgroup. Theore bodies consist of massive, coarse crystalline barite up to 6 m thick interspersed with granular (b2 cm)magnetite and quartz. In the southern section the massive barite contains Cu–Au mineralisation consisting ofchalcopyrite, bornite and native gold. Along strike the mineralised unit grades into a stratiform quartz–magnetite layer which can be traced for several kms. Several smaller (b1 m thickness) barite lenses occurabout 1.5 km north of main barite body in the upper parts of the Ethiudna Subgroup close to the contact tothe Saltbush Subgroup.A pronounced positive Eu-anomaly of the mineralisation hosting unit indicates a reducing character of themineralising fluids and is characteristic for exhalative sediments. 87Sr/86Sr-isotope ratios of barite fall into anarrow range of 0.708 to 0.713 suggesting an origin of the mineralising fluids from the underlying LowerWillyama sequence. The δ34S values of barite from the main ore body range from +13 and +16‰, while thebarites of the other ore bodies range from +6 and +11‰. Chalcopyrite revealed δ34S values of −1.3 to+4.5‰. Barite is suggested to be formed by mixing of a hydrothermal, reduced, hydrothermal Ba-rich fluidwith oxidised, sulphate-rich water of a non-marine basin. Although an epigenetic formation of the Cu–Aumineralisation during the Olarian Orogeny is possible, we suggest a cogenetic formation with the barite–iron-oxide mineralisation.

u (M. Griessmann),umm),.colin@saugov.sa.gov.au

ll rights reserved.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The Olary Domain in the south-western part of the ProterozoicCurnamona Craton in South Australia contains several occurrences ofbarite mineralisation, commonly associated with ironstones andelevated Cu–Au values. The largest one of these, showing the highestCu–Au grades, is the Mt. Mulga deposit (see Fig. 1). It is located110 km west of Broken Hill and 20 km north of the Olary township.Mining of the deposit started on a minor scale for copper and goldtook place at the beginning of the 20th century, but the major miningperiod was between 1962 and 1985. 18,502 t of low quality barite wasproduced by open cut mining and sold as drilling grade barite afterprocessing (Ashley et al., 1998a). The resource of the deposit wasestimated by Olliver (1974) to be about 89,000 t of barite ore.

Several other small scale mining operations for Cu, Au and feldsparare found within a 5 km radius around the Mt. Mulga barite mine. Atthe beginning of the 1980s the area was explored for base metals anduranium by CEC/ESSO without the discovery of economicallysignificant mineralisation (ESSO, 1981).

To date, mineral deposits of economic value in the Olary Domainare mainly industrial minerals (barite and albite) and the uraniumdeposit at Radium Hill (Conor et al., 2006). In recent years,encouraging discoveries of Cu and Cu–Au mineralisation have beenmade at Kalkaroo, Portia or theWhite Dam gold–copper project whichclearly shows the economic potential of the Olary Domain (Skirrowand Ashley, 2000; Cooke, 2003; Burtt et al., 2004; Johnson et al., 2006;see Fig. 1).

2. Regional geology

The Curnamona Province is the Palaeo- toMesoproterozoic nucleusof theWillyamadepositional basin, inwhich theWillyamaSupergroupwas deposited between 1.72 Ga and 1.64 Ga in the late Palaeoproter-ozoic (Conor and Preiss, 2008). The Willyama Supergroup comprises

Fig. 1. Simplified geological map of the Olary Domain with the location of Mt. Mulga Mine: the map shows the distribution of the lower Willyama Supergroup, major igneous unitsand significant mineral occurrences.

111M. Griessmann et al. / Journal of Geochemical Exploration 106 (2010) 110–120

metamorphosed clastic and chemical sediments as well as felsicvolcanics, in which plutonic rocks of different types and ages intruded(see Fig. 2). The Curnamona Province is subdivided into several do-mains. The Olary Domain in the south-western part of the CurnamonaProvince represents the western extension of the Broken Hill Block,host to the super-giant Broken Hill Pb–Zn–Ag deposit. Detailed strati-graphic work in the past three decades has unravelled the complexgeological setting and depositional environment of the Palaeoproter-ozoic Willyama Supergroup (Clarke et al., 1986; Conor, 2000; Pageet al., 2005; Barovich and Hand, 2008; Conor and Preiss, 2008) and thesequence can now be confidently traced and correlated regionally inboth domains.

Sedimentation in the Willyama half-graben was suggested to bediachronous as rifting of the graben commenced in the western OlaryDomain with initial deposition in a lacustrine or shallow marineenvironment, including possible Sabkhas (Cook and Ashley, 1992), anddeepening upwards in the sequence. The Willyama Supergroup in theOlary Domain is subdivided, from bottom to top, into the Curnamona

Group, Saltbush Group and Strathearn Group (Conor and Preiss, 2008,see Fig. 2). The Curnamona Group is subdivided into the WiperamingaGroup as its lower unit, which is represented by the albitised schists andgneisses of the GeorgeMine Formation in the study area. The upper partof the Curnamona Group is formed by the Ethiudna Subgroup whichhosts the mineralisation in this study. The basal unit of the EthiudnaSubgroup is the Cathedral Rock Formation which unconformably over-lies the clastic,meta-pelitic, biotite schist of theGeorgeMine Formation.The quartzite units vary in composition from clean and cross-bedded,through clean and recrystallised to ferruginous and volcanoclastic.Occurrences of barite andmagnetite ironstones are commonwithin theunit (Conor et al., 2006). In the study area, the Cathedral Rock Formationis a beige to red feldspathic (K-feldspar) quartzite with intercalated,sometimes massive, magnetite layers. The upper part of the EthiudnaSubgroup is formed by various mica-schist and gneisses with somequartzites of volcano-sedimentary origin in the northern part of thestudy area. The transitional section of the Ethiudna Subgroup to theoverlying Bimba and Plumbago Formations in the lower SaltbushGroup

Fig. 2. Simplified stratigraphy of the Olary Domain and study area with associatedmagmatic and deformational/metamorphic events; based on data from Conor andPreiss (2008).

112 M. Griessmann et al. / Journal of Geochemical Exploration 106 (2010) 110–120

is believed to be the western equivalent to the Broken Hill Group,hosting theBrokenHill deposit in theBrokenHill Domain to the east andhas been amajor exploration target in the Olary Domain. This sequence,possibly interrupted by a hiatus (see Fig. 2) consists of immature clasticsediments with intercalated volcaniclastics of the Ethiudna Subgroupand chemical sediments of the Bimba Formation with regionally ano-malous contents of Pb, Zn, Cu and Co.

The entire sequence underwent two events of deformation andmetamorphism, the Olarian (∼1.6 to 1.5 Ga) and the PalaeozoicDelamerian (514 to 490 Ma, Foden et al., 2006). During the OlarianOrogeny the units in the area of study underwent upper greenschist toamphibolite facies metamorphism and multiple regional deformation(summarised after Conor et al., 2006): Development of layer-parallelfoliation and local isoclinal folding is the initial phase of deformationin the Olary Domain (ODf). Tight to isoclinal folding and developmentof regional nappes/thrusts with fold axes of variable orientation isrelated to the ODm deformation phase. The mineralisation at Mt.Mulga is hosted by an overturned limb of a west-verging ODm fold.The third deformation (ODc) included open to tight folding with foldaxes trending in similar directions to ODm but at steeper angles. Thelast phase of deformation during the Olarian Orogeny is the develop-ment of regional shear zones and open folds with east-trending foldaxes (ODw).

Recent investigations of the structural andmetamorphic evolutionof the Olarian Orogeny (Clark, 2005) suggest clockwise P–T–t pathswith metamorphic peak conditions of 260 to 340 MPa and 500 to600 °C in the south-western part of the Olary Domain.

Minor additional thermal events are postulated at about 1200 to1100 Ma during the Musgravian Orogeny and the emplacement ofmafic dykes during the Adelaidean extension at about ∼820 Ma. TheDelamerian Orogeny (515–490 Ma, Foden et al., 2006) with twodeformational events (DD1 and DD2) affected the Willyama Super-group by low amplitude folding and reactivation of Olarian shearzones (Clark, 2005; Conor et al., 2006).

Several Palaeo/Mesoproterozoic magmatic events have been iden-tified in the Olary Domain. The earliest record of magmatic activity isthe syn-Willyama felsic Basso Suite (1719±3 Ma to 1703±3 Ma,zircon U–Pb SHRIMP) with intrusive (Ameroo Subsuite) and extrusive(Abminga Subsuite) equivalents (Conor and Preiss, 2008). TheEthiudna Subgroup hosting the mineralisation of the study areacontains Abminga volcanics, while the Drew Hill meta-granite bodybelonging to the Ameroo Subsuite underlies the Wiperaminga Sub-group east of the study area. Emplacement of the Lady-Louise Suitemafic dykes took place after the deposition of the WillyamaSupergroup at about 1685±4 Ma (Conor and Fanning, 2001), nearestoccurrences to the Mt. Mulga deposits are within the aforementionedDrew Hill meta-granite and near the Woman-In-White mine 3 kmwest of Mt. Mulga Mine. The most intensive magmatism took placeduring the Olarian Orogeny with the emplacement of the ‘NinnerieSupersuite’. Igneous rocks belonging to this event are absent in thestudy area with the exception of abundant pegmatites and migma-tites, which formed during Olarian peak-metamorphism (Conor et al.,2006).

Large parts of the Willyama Supergroup are also affected byvariable styles of alteration. Especially sodic, calc-silicate and K–Fe-alteration are important features in parts of the Curnamona Groupincluding the study area. Timing of the alteration is not wellunderstood yet, but it is thought that both syn-sedimentary/diagenetic and syn-metamorphic styles occur. Geochemical studieson theMount Isa Block and geochemical modelling (Oliver et al., 2001,2004) indicate that large amounts of Fe, K, Ca andMg are mobilised bysodic alteration. Albitised rock will further loose most of its Ba, Rb, Sr,Co, V, Mn, Pb and Zn, while Al, Si, Ti and Ga behave conservatively. Theelements lost during albitisation are those which are commonlyenriched in the proximal K–Fe alteration zones around Cu–Au orebodies and in several unmineralised ironstones. Oliver et al. (2004)conclude that sodic alteration is not the source for Cu observed inthose Cu–Au ore bodies but either it was initially contained in thefluids responsible for the alteration process or it was introduced intothe ironstones in a later process. However, Perring et al. (2000)describe a distinct loss of Cu in albite–clinopyroxene (sodic–calcic)altered monzodiorite from the Lightning Sill Complex, CloncurryDistrict in northern Australia. This rock was also substantiallydepleted in Ba. In this context, Perring et al. (2000) concluded thatthis type of alteration has the potential for providing importantamounts of ore forming metals including Cu to magmatic hydrother-mal fluids. Thus, mobilisation of various elements by sodic alterationcould have played a vital role for mineralisation in the study area.

3. Geological setting of the Mt. Mulga mineralisation

The Cu–Au mineralised barite in the study area is hosted by anoverturned sequence of meta-sedimentary rocks of the CurnamonaGroup in the lower part of the Willyama Supergroup (see Figs. 2, 4).Strike direction of the sequence is north–south in the northern part ofthe study area turning to northwest in the southern part. The dipangle of the units is about 45° to the east, turning northwest in thesouthern section.

The stratiform barite–quartz–magnetite ore body at Mt. MulgaMine is hosted by the Cathedral Rock Formation at the base of theEthiudna Subgroup, directly overlying the George Mine Formation(see Figs. 2, 3 and 4). This body has a length of about 200 m alongstrike of outcrop and a maximum true width of about 5 m. Themineralisation at Mt. Mulga extends from the main ore body at themine to the southeast in form of two “seam”-like ore bodies. Thesmaller one is completely contained within the Cathedral RockFormation quartzite south of the mine. It has a total length of about400 m and an average true thickness of about 0.5 m.

The larger barite unit can be followed for at least 1.7 km along striketo the southeast of Mt. Mulga Mine gradually changing into a nearly

Fig. 3. a) Massive barite ore with black magnetite in the Mt. Mulga open pit; b) reflected light image of polished section from an ore sample from the Mt. Mulga open pit: bnt —bornite, brt— barite, ccp— chalcopyrite, go— native gold, mgt—magnetite, qtz— quartz; c) outcrop of the barite-free extension of the mineralisation hosting strata southeast of themine: this rock consists of milky massive quartz and magnetite; d) transmitted light thin section image of a sample from c): layers of coarse crystalline magnetite (mgt) and chlorite(chl) are hosted in quartz; e) barite schlieren (brt) within Ethiudna Subgroup gneiss 200 m west of the Mt. Mulga open pit; f) barite-quartz (brt-qtz) layer near Mulga Bore Creekwithin Ethiudna Subgroup micaschist.

113M. Griessmann et al. / Journal of Geochemical Exploration 106 (2010) 110–120

barite-free quartz-ironstone. The immediate, hosting albitised gneiss ofthe Ethiudna Subgroup also contains schlieren of barite. South of themine the stratigraphically uppermost ore body at Mt. Mulga forms theboundary between the Cathedral Rock quartzite and the covering gneissof the Ethiudna Subgroup. The Cathedral Rock quartzite dies out about500 m southeast of the mine. From this point on the mineralisationforms the boundary between the George Mine Formation and theEthiudna Subgroup. The true thickness of thismineralisation reaches upto 5 m about 100 m south of the mine but is mostly much lower witharound 0.5 m. At the place with the highest thickness, there is a verticalzoning in the mineralisation with a barite-rich base, a quartz-richcentral unit and a layered quartz-ironstone on top. In some parts themineralisation and the overlying wall rock are Ca-altered with highcontents of epidote. With increasing distance to the mine quartzbecomes increasingly abundant while the barite content decreases thusgradually turning this layer into a quartz-ironstone.

The small barite lenses in the upper part of the Ethiudna Subgroupat the Mulga Bore Creek about 1.5 km north of Mt. Mulga Mine occurwithin muscovite micaschist (see Fig. 4). The thickness of these lensesis 20 to 50 cm and they can be followed over a strike length of about400 m parallel to bedding.

Immediately above, the barite–quartz–magnetite layers are overlainby pelitic schist and calc-silicates comprising the uppermost units of theEthiudnaSubgroup. Further up the sequence theBimbaFormation formsthebase of theoverlying SaltbushGroup. In the investigated area this is asulphide-rich meta-pelite which is locally enriched in base metals.

A quartzitic volcanoclastic unit is present at theMulga Bore Creek aswell stratigraphically between the small Cathedral Rock Formation andthe barite layers at the creek. This unit contains schlieren of chalcopyriteand is highly anomalous in gold. It is related to the Abminga Subsuiteigneous event and zircons ages of 1718±3 Ma and 1714±3Ma aregiven (Conor et al., 2006).

Fig. 4. Geological map of the study area showing chondrite normalised REE-patterns for the different sections: Total REE values increase along strike in south-easterly direction awayfrom Mt. Mulga Mine while Eu-anomalies gradually disappear. Barite mineralisation at Mulga Bore Creek has a low total REE content together with a distinct positive Eu-anomaly.

114 M. Griessmann et al. / Journal of Geochemical Exploration 106 (2010) 110–120

This study investigates the Mt. Mulga barite–magnetite–Cu–Aumineralisation to determine its geological setting, the physical andchemical characteristics and possible sources of fluids and metalsof the involved mineralising system(s) and in conclusion deduces agenetic model for the deposit.

4. Methods

For this study, samples from the Mt. Mulga and Mulga Bore Creekbarite occurrences were analysed for their whole rock geochemistry.Samples were crushed down using a jaw crusher and milled in atungsten carbide swing mill. Whole rock geochemistry was carriedout by Amdel Analytical Laboratories in Adelaide, Australia. Theanalytical procedure for major elements was fusing with lithium

metaborate and dissolution afterwards. The resulting solution wasanalysed using ICP-OES. Trace elements including REE wereprepared using a HF/multi element digestion with the final solutionbeing analysed by ICP-MS. REE were subject to additional qualityassurance and control. For gold, a subsample of the analytical pulpwas fused in a lead collection fire assay. The resultant prill wasdigested in aqua-regia and the gold content of the sample wasdetermined by AAS.

SEM/EDX and electronmicroprobe (EPMA) studies were carried outat the TU Bergakademie Freiberg, Germany, using polished sections andpolished thin sections. For SEM/EDX a JOEL JSM-6400 scanningmicroscope was used with an acceleration voltage of 20 kV and acurrent intensity 0.6 nA. For EPMA analysis we used a JOEL JXA-8900workingwith an acceleration voltage of 20 kV for Cu-sulphides and gold

115M. Griessmann et al. / Journal of Geochemical Exploration 106 (2010) 110–120

and 15 kV for barite. The sample current was 30 nA for barite and Cu-sulphides and 40 nA for native gold.

Barite and chalcopyrite dedicated for isotope analyses were pickedout by hand under a binocular from the crushed sample. The mineralswere milled using an agate mortar. For the 87Sr/86Sr-measurments,the samples were leached in 6 M hydrochloric acid at 140 °C in 15 mlSavillex PFA vials for a time of 48 h. The strontiumwas then separatedfrom the leachate by ion-chromatography using 2 ml BIORAD AG50W-X8 200–400 mesh resin in Poly-Prep columns. Measurementwas carried out on a Finnigan MAT262 thermal ionisation mass spec-trometer (TIMS) in static measurementmodewith 100 integrations of16 s. The reference standard is SRM987 87Sr/86Sr=0.710237±10(SD) for 15 measurements.

The δ34Smeasurements were carried out by the isotopic laboratoryof the Mineralogical Institute, TU Bergakademie Freiberg and by GNSScience, New Zealand. δ34S analyses of sulphides were measured bymeans of combustion isotope-ratio mass spectrometry (C-IRMS)using a Carlo Erba EA 1110 elemental analyser connected to a FinniganMAT Delta plus mass spectrometer via a Finnigan MAT Conflo II split

Table 1Geochemical data of (barite)–magnetite–quartz mineralisation of the study area.

Sample 1 2 24.1 24.2 29a 29b 9

Locality MBC MBC MMM MMM MMM MMM MM

Al2O3 0.59 1.75 0.31 0.35 0.05 0.07 0.74CaO 0.13 1.52 0.06 0.05 0.06 0.10 0.08K2O 0.19 0.04 0.05 0.07 b0.01 0.01 0.07Fe2O3 tot 0.13 19.20 3.22 5.02 3.26 3.11 8.87MgO 0.07 0.03 0.07 0.05 0.02 b0.01 0.41MnO b0.01 0.04 b0.01 b0.01 b0.01 b0.01 b0.0Na2O 0.04 b0.01 b0.01 b0.01 b0.01 b0.01 b0.0P2O5 b0.01 b0.01 0.01 0.02 b0.01 b0.01 0.02SiO2 2.95 3.01 2.68 3.61 1.98 4.00 12.4TiO2 0.02 b0.005 b0.005 b0.005 b0.005 b0.005 b0.0LOI 0.7 0.52 0.7 1.16 0.33 −0.04 2.53Ag b0.1 b0.1 0.2 b0.1 b0.1 0.2 0.1As 1 b0.5 0.5 b0.5 b0.5 0.5 0.5Au b0.01 b0.01 1.22 0.43 0.25 0.57 1.3Ba 539,000 416,000 536,000 519,000 534,000 540,000 443Bi 0.3 0.6 9 6 2.6 2.6 6Cd b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1Cs 0.4 b0.1 0.2 0.2 b0.1 b0.1 0.1Cu 29.5 8.5 5050 82 1300 2750 170Ga 1.3 11 4.2 4.3 1.6 1.9 9In b0.05 0.3 0.6 0.35 0.15 0.35 0.4Mo 1.9 2.5 0.7 0.6 0.6 0.3 0.7Pb 1.5 2.5 b0.5 b0.5 b0.5 b0.5 b0.5Rb 10 1.1 3.4 4 0.5 0.7 3.3Se 1 1.5 3 1 3.5 2.5 1.5Sr 3450 2650 1350 1650 1950 1400 150Th 0.04 0.04 0.17 0.21 0.09 0.09 0.13U 0.82 0.91 0.27 0.31 0.15 0.07 0.6Y 2.5 3 0.25 0.3 0.3 0.25 1.3Zn 1 7.5 3.5 3 1.5 1.5 11.5Zr b20 b20 b20 b20 b20 b20 b20La 7 1.5 0.5 1 1.5 1 b0.5Ce 14 1.5 0.5 0.5 1 0.5 b0.5Pr 1.95 0.2 0.1 0.1 0.1 0.1 b0.0Nd 5.5 0.8 0.32 0.34 0.32 0.33 0.11Sm 1.6 1.15 1.0 0.79 0.77 0.98 0.75Eu 1.6 0.99 0.33 0.45 0.41 0.33 0.38Gd 1.05 0.7 0.25 0.3 0.15 0.25 0.2Tb 0.09 0.06 b0.02 b0.02 b0.02 b0.02 0.03Dy 0.65 0.44 0.11 0.13 0.1 0.12 0.29Ho 0.08 0.05 b0.02 b0.02 b0.02 b0.02 0.05Er 0.2 0.15 b0.05 b0.05 b0.05 b0.05 0.15Tm b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.0Yb 0.1 0.1 b0.05 b0.05 b0.05 b0.05 0.15Lu 0.03 0.02 b0.02 b0.02 b0.02 b0.02 0.03REE total 33.82 7.64 3.11 3.61 4.35 3.61 2.11

Table X — Geochemical data of barite-rich rocks and related quartz-ironstones from the studtrace elements including REE in ppm; MBC — Mulga Bore Creek area; MMM — Mt. Mulga M

(method according to Giesemann et al., 1994). All samples weremeasured at minimum threefold. For routine measurements an inter-nal standard (SO2) was used, which is calibrated against the inter-national IAEA-standard NBS 127.

5. Results

5.1. Mineralogy

Barite, quartz and magnetite are the main constituents of the Mt.Mulga mineralisation with grades of nearly 90 wt.% barite in the mainore body (see Table 1, Fig. 3). The barite forms crystals with a size ofmostly around 1 mm, but there are also coarsely recrystallisedsections in which grain size may exceed 1 cm. Quartz is subhedralwith grain sizes up to 5 mm. Both quartz and barite show undulatoryextinction under crossed polarized light. Crystals of magnetite aremostly irregular in shape with sizes of up to 1 cm. They are partlymartitised and show fissures and inclusions filled with barite and

18 33 34 7 37 39

M MMextended

MMextended

MMextended

MMextended

MMextended

MMextended

0.10 1.66 0.15 3.97 2.99 0.230.04 0.31 1.04 3.63 0.02 0.04b0.01 0.56 0.04 0.11 1.88 0.057.20 48.60 5.42 13.10 78.70 27.20b0.01 0.81 0.16 0.21 3.26 0.44

1 b0.01 0.10 0.02 0.04 0.05 0.021 b0.01 0.03 0.01 b0.01 0.02 b0.01

b0.01 0.05 0.80 0.16 0.07 0.070 39.40 42.90 90.20 6.47 8.39 70.1005 b0.005 0.14 0.02 0.07 0.09 b0.005

1.7 1.89 1.05 1.18 2.11 0.64b0.1 b0.1 b0.1 0.2 0.1 b0.10.5 1 2 1 2 40.02 b0.01 b0.01 b0.01 b0.01 0.11

,000 301,000 21,100 3300 413,000 260 60b0.1 0.4 0.2 0.9 0.2 0.3b0.1 b0.1 b0.1 b0.1 b0.1 b0.1b0.1 0.7 0.2 0.2 3.8 0.328.5 430 19.5 21.5 24 8.51.6 9 1.2 11.5 33 1.80.1 0.8 b0.05 0.3 0.1 b0.050.7 2 1 2.6 0.5 2.30.5 2 2 2 1 0.5b0.1 19.5 2.5 5.5 175 4.3b0.5 2 3 2.5 1 3.5

0 1450 260 62 2850 33 640.09 4.1 0.42 0.23 4.7 0.520.24 2.8 0.54 1 11 2.11.95 6 3.5 8 4.8 3.4b0.5 32.5 3.5 11.5 24 1b20 60 b20 b20 b20 b201.5 3 1.5 5 15 9.51.5 5 2.5 5.5 24 15

5 0.25 1 0.4 1 3.7 20.88 3.6 1.5 3.7 11.5 70.96 0.03 0.48 1.65 2.8 3.10.68 1.7 1.0 1.85 1.3 0.720.65 1.3 1.0 1.9 2.3 20.06 0.12 0.09 0.17 0.17 0.120.49 1.3 0.83 1.55 1.3 0.860.06 0.23 0.13 0.24 0.16 0.110.2 0.9 0.45 0.85 0.5 0.35

5 b0.05 0.1 b0.05 0.1 b0.05 b0.050.15 0.7 0.35 0.65 0.4 0.20.03 0.14 0.07 0.11 0.08 0.047.38 18.98 10.23 24.16 63.13 40.96

y area; samples are ordered along strike from north to south; major elements in wt.%;ine; MM extended — Mt. Mulga extended.

116 M. Griessmann et al. / Journal of Geochemical Exploration 106 (2010) 110–120

sulphides. Smaller (b2 mm) magnetite crystals may show sub- toeuhedral shapes.

The disseminated sulphides in the Mt. Mulga mineralisationconsist of chalcopyrite and bornite, which are locally replaced bychalcocite and covellite. Individual sulphide grains are anhedral andmostly below 1 mm. Rare sub- to euhedral chalcopyrite occurs withinindividual barite crystals. Bornite frequently shows Widmannstättentextures caused by the exsolution of chalcopyrite, while chalcopyritemay show complex exsolution textures of bornite. Some of the chal-copyrite grains contain tiny blebs of a (Cu,Fe)–Bi-sulphide, the onlyadditional sulphide observed. Fe-sulphides were not observed.

Native gold grains are mostly contained within the sulphides butsome gold was also observed within quartz grains. The maximumobserved size of native gold is 0.1 mm. Composition of the gold grainsas determined by EPMA falls into two discrete populations: a low-Agpopulation with 0 to 2 wt.% Ag and a high-Ag population containing12 to 17 wt.% Ag.

Accessory minerals in the Mt. Mulga barite are muscovite, biotite,albite, zoned K–Ba-feldspar (K-rich rim, Ba-rich core), siderite andvery rare uranium-pitchblende.

A sample taken from the extension of theMt. Mulgamineralisationabout 1.7 km along strike southeast of the mine consisted mainly ofquartz with layers of subhedral magnetite crystals and chlorite (seeFig. 3). SEM studies showed that barite is only present as tiny platycrystals in cavities of the quartzmatrix. Monazite occurs as a rare tracemineral.

The Mulga Bore Creek barite mineralisation in the upper part ofthe Ethiudna Subgroup contains coarse crystalline barite and quartzas main components with crystal sizes of up to one centimeter. Thebarite content decreases rapidly toward the schistose part of thepolished thin section, but there are also some single barite crystalswithin the quartz–feldspar–muscovite–biotite schist. Magnetiteoccurs as bedding-parallel layers together with chlorite in the rimzone of the barite–quartz-layers, while the central part of the mine-ralisation is magnetite-free. The mineralisation contains very rarechalcopyrite and aggregates of various secondary copper mineralscontaining native gold. The gold has a composition between the twotypes observed at Mt. Mulga containing about 4.4 wt.% of Ag.

5.2. Geochemistry

Whole rock geochemical analyses of samples from the main orebodywithinMt.MulgaMine showBaO values between 33 and 60 wt.%with a median of 55 wt.%. The median of total Fe2O3 is at 6 wt.% and8.5 wt.% for SiO2. The rest is mainly accounted for by sulphur, otherelements together make up less than 2 wt.% of the analysed samples.

Copper values are highly variable in the main ore body rangingfrom 28.5 to 5050 ppm with a median of 650 ppm (n=13 includingdata of Olliver, 1974; Millar, 1994; Lottermoser and Ashley, 1996). Allother base metals including Pb and Zn show values below 20 ppm.Gold grades range from 0.02 to 1.3 ppm with a median of 0.57 ppm(n=8; including data of Millar, 1994; Lottermoser and Ashley, 1996).Silver is below the detection limit of 0.1 ppm inmost sampleswith thehighest measured value being 0.2 ppm.

Trace element geochemistry of the barite-rich rocks is very similarto other barite mineralisation within the Olary Domain. Ashley et al.(1998b) determinedmean values for 19 samples fromdifferent barite-rich rocks including one sample fromMt.MulgaMinewhich show thatthese rocks are generally enriched in copper (mean491 ppm) and gold(mean 119 ppb), but with very low values of other base metals likelead, zinc or molybdenum.

Total REE contents are generally low in barite-rich samples of themineralisation at Mt. Mulga and Mulga Bore Creek with less than5 ppm (Table 1). Chondrite-normalized patterns show a slightnegative Ce-anomaly and a strong positive Eu-anomaly as well as anenrichment of LREE relative to HREE. Over the entire extent of the

mineralisation, the total REE content increases in south-easterly di-rection along strike, away from the mine. Samples taken at about500 m from the mine also show a distinct positive Eu-anomalyincluding samples with relative low Ba-values. Samples taken about1 km and 1.7 km from themine show the highest values of REE of with60 and 40 ppm, respectively. However, the overall REE characterchanges markedly (see Fig. 5). LREE are still enriched compared toHREE, but the positive Eu-anomaly is suppressed, in one sample it iseven slightly negative. The barite-rich rocks and ironstones of thestudy area share their REE signature with similar mineralisation of theOlary Domain discussed in Lottermoser and Ashley (1996) andBierlein (1995).

5.3. Stable and radiogenic isotopes

To obtain insight into the possible sources of fluids and metals,barite and chalcopyrite were analysed for their δ34S composition (seeFig. 6), barite was further analysed for 87Sr/86Sr-ratios.

Chalcopyrite from the main ore body shows δ34S in the range of−1.5 and +3.1‰ (vs. CDT, n=6). Lottermoser and Ashley (1996)give an additional value with +4.5‰.

Barite from the main ore body shows δ34S values from +13.0 to+16.3‰ (n=7) which is consistent with values given in Lottermoserand Ashley (1996) for Mt. Mulga Mine (14.4%, 14.8%) and otherstratiform barite-rich rocks of the Olary Domain. Barite from thesouthern section shows δ34S values of +6.8 to +10.8‰ (n=3),samples from the Mulga Bore Creek barite range from +7.5 to+10.6‰ (n=2).

87Sr/86Sr-ratios of barite from the Mt. Mulga Mine range from0.7089 to 0.714 (n=12; main ore body and southern extensions) andfrom 0.709 to 0.710 for the Mulga Bore Creek barite. This includes twovalues for Mt. Mulga Mine given in Lottermoser and Ashley (1996)with ratios of 0.7094 and 0.7089. (see Fig. 7).

6. Interpretation and discussion

The detailed mapping of lithologies and facies variations of the Mt.Mulga Barite deposit and the host sequence show a distinct stratiformrelationship with the deposit hosting Cathedral Rock Fmtn. Alongstrike the barite (+magnetite+quartz±Cu-sulphides) bearing ho-rizon gradually changes to a distinct layer of mostly coarse grainedmagnetite and quartz, showing sedimentary textures such as gradedbedding. This field evidence together with the distinct REE patterns ofthis horizon with a well expressed, positive Eu-anomaly is evidencefor its exhalative origin; the slight negative Ce-anomaly furthersupports the hydrothermal nature of the mineralising fluids.

Comparison of the REE-pattern of the barite-rich parts of themineralisation in the study area with those of recent submarine hydro-thermal fluids and hydrothermal marine sediments suggests fluidtemperatures above 230 °C as well as a highly saline and acid (pH b6)character of the fluidwhile those from the distal barite-poor parts of themineralisation are similar to fluids with higher pH and lower tem-peratures (Lottermoser, 1989; Lottermoser and Ashley, 1996). Thiswould be consistent with a formation of the barite-rich parts of themineralisation proximal to the hydrothermal ventwhile the barite-poorparts were deposited more distal.

87Sr/86Sr-ratios of the barite from0.709 to 0.714 donot reflect the LatePalaeoproterozoic seawater which was distinctly lower, in the range of0.705 to0.707 (Veizer, 1989) but rather reflect the Sr-isotopic signatureofthe underlying units of the Curnamona Group. Because of the extremelylow Rb/Sr ratio of the barite mineralisation below 0.003 only very minorradiogenic 87Sr additions after barite formation, thus ratios are still veryclose to the numbers of a potential source during the Late Palaeoproter-ozoic rather than today's ratios.Measured 87Sr/86Sr-ratios of the 1660 MaPotosi Gneiss, a clastic metasediment in the upper Broken Hill Group ofthe BrokenHill Domain, are 0.75 to 0.82with calculated original values of

Fig. 5. Chondrite-normalized REE pattern of a) stratiform ironstones in the Olary Domain comparable to recent submarine low temperature hydrothermal fluids (from Lottermoserand Ashley, 1996), b) stratiform barite mineralisation in the Olary Domain comparable to recent submarine high temperature hydrothermal fluids (from Lottermoser and Ashley,1996) and c) recent high temperature hydrothermal fluids on the East Pacific Rise (from Lottermoser, 1989).

117M. Griessmann et al. / Journal of Geochemical Exploration 106 (2010) 110–120

0.709 to 0.719 (Etheridge and Cooper, 1981). Although the Potosi Gneissis younger than the rocks in the study area, it shows that clastic meta-sedimentary rocks of theWillyama Supergroup have initial 87Sr/86Sr as inthe observed range atMt.Mulga. In comparison similar 87Sr/86Sr-ratios oftheMt.Mulga and the distinctly epigenetic barite veins at theMulga BoreCreekmineralisation indicate that both received their strontium from thesame source.

δ34S values of barite mineralisation within the study area (6.8‰ to16.3‰ CDT) are inconsistent with those of Late Palaeoproterozoic

seawater given in Strauss (1993) with 18‰ to 25‰, supporting a non-marine character of the lowerWillyama basin as suggested in Cook andAshley (1992)or Lottermoser andAshley (1996).Mixingof the reduced,exhalative fluid with the oxidised, sulphate bearing Willyama basinwater triggered the precipitation of barite and iron-oxides.

Isotopic data, REE patterns and field observation suggest thatbarite–iron-oxide mineralisations in the study area formed as a resultof mixing between a reducing metal-rich fluid from a crustal sourceand non-marine water column of the lower Willyama basin. The

Fig. 6. δ34S (vs. CDT) values of barite and chalcopyrite from the study area (includingdata of Lottermoser and Ashley (1996) for Mt. Mulga Mine: chalcopyrite: +4.5‰;barite: 14.4%, 14.8%).

Fig. 8. Systematic variation of the Eu/Eu* anomalism with apparent distance to theassumed source at the Mt. Mulga barite deposit. Apparent distance calculated based onGPS coordinates as direct line between points.

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bedding-parallel layering of the Mt. Mulga mineralisation furthersupports a stratiform, SEDEX-style model of exhalative fluids mixingwith theWillyama basin water during sedimentation of the CathedralRock Formation. Suchmodels are applied for several barite deposits inthe world as the Proterozoic Aberfeldy mineralisation in Scotland(Hall et al., 1991) or the giant barite ore bodies in the Brooks Range,Alaska (Johnson et al., 2004). The hydrothermal vent was most likelylocated near the main ore body, which is supported by the decrease ofbarite content and the systematic change of REE patterns of themineralised unit with increasing distance to the main ore body. Asystematic decrease of the Eu/Eu* intensity with distance to themassive barite occurrence at Mt. Mulga demonstrates the progressivemixing and dilution of the vent fluids with the waterbody into whichthey were expelled (see Fig. 8).

The unconformity between the George Mine Formation was thelikely seafloor during the active period of the exhalative vents in theMt. Mulga area. This hydrothermal activity was contemporaneouswith the sedimentation of the Cathedral Rock Formation, the basal

Fig. 7. 87Sr/86Sr-ratios of barite from the Mt. Mulga mineralisation and Mulga Bore Creek(including data of Lottermoser and Ashley (1996) for Mt. Mulga Mine: 0.709369±17;0.708944±13).

unit of the Ethiudna Subgroup. Volcanoclastic input within theCathedral Rock Formation as well as the volcanoclastic units at theMulga Bore Creek is proof of active magmatism during the sedi-mentation of these unit (Abminga Subsuite of the Basso Suite event)providing a heat source for hydrothermal activity during that time.

Based on the chemical affinities of Ba and Sr it can be suggestedthat the strontium source for both barite occurrences should also bethe source of barium. Geochemical data of the GeorgeMine Formationfrom Geosciences Australia show that this highly albitised rocksequence is strongly depleted in barium and strontium. Oliver et al.(2001, 2004) and Perring et al. (2000) point out that sodic alteration(albitisation) is an efficient process to mobilise Ba and Fe as well asmany other elements from a source rock. Considering this, the GeorgeMine Formationwould be a suitable source for the elements containedin the mineralisation of the study area. Unfortunately, the timing of thesodic alteration of the George Mine Formation is unknown. Sodic alter-ation is very commonwithin the Olary Domain and has been suggestedto be both, syn-diagenetic and syn-metamorphic processes (Conor et al.,2006).

The explanation of the Cu–Au mineralisation within the barite–magnetite rocks is more controversial than the model for these rocksitself. Two models can be proposed: a) an all-SEDEX-style model inwhich the Cu–Au minerals formed together with the barite–mag-netite mineralisation or b) a combined model of syngenetic baritedeposition with an introduction of Cu and Au at a later stage, e.g.during the Olarian Orogeny.

Comparisonwith other epigenetic Cu–Aumineralisation in theOlaryDomain shows that theMt.Mulga deposit shares theK–Fe-rich lithologyof its host rocks (Cathedral Rock Formation) with other epigenetic Cu–Au(±Mo) deposits (Burtt et al., 2004), e.g.WhiteDam. These epigeneticmineralisations have an age of 1630 to 1612 Ma (Re–Os-dating; Skirrowand Ashley, 2000) and are associated with the ODc event of the OlarianOrogeny. In this epigeneticmodel, δ34S values of barite and chalcopyritecould be the result of fractionation during the Olarian Orogeny.Although barite is a very insolublemineral, under higher P–T conditionsand in the presence of highly saline fluids it can be mobilised in smallamounts (Blount, 1977). The sulphate of remobilised barite could havebeen reduced to form sulphides with the elements carried by themetamorphic fluids. The δ34S values of barite (6.8‰ to 10.8‰) from thesulphide-poormineralisations (Mt. Mulga extended,Mulga Bore Creek)in this case represent the original isotopic composition of the barite. Thecalculated equilibrium fractionation temperatures of δ34S of barite(+13.0‰ to +16.3‰) and chalcopyrite (−1.5‰ to +4.5‰) from Mt.Mulga Mine are 414 °C to 474 °C, which is consistent with proposed

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formation temperatures of epigenetic Cu–Au mineralisations in theOlaryDomainbetween300 °Cand550 °C (Williams and Skirrow, 2000).

In a cogenetic barite–Cu–Aumodel the barite of the main ore bodyrepresents the original composition of the water column deliveringthe sulphate. The chalcopyrite formed near the hydrothermal ventfrom reduced sulphur species carried by the hydrothermal fluid.Under the reducing conditions near the vent, the reduced sulphurcould not be oxidised and contribute to the formation of barite, thusthe δ34S values of both minerals represent the original composition oftheir sulphur sources. Distal to the vent, the hydrothermal fluid mixedup with basin water and the hydrothermal reduced sulphur oxidisedto sulphate contributing to the formation of barite explaining thelower isotopic values and the general absence of sulphides outside themain ore body. For the cogenetic barite–iron-oxide–Cu–Aumodel, theδ34S composition of chalcopyrite around 0‰ fits well with a proposedcrustal reservoir which was source of hydrothermal fluids during theWillyama Supergroup deposition as well as the Olarian Orogeny(Bierlein et al., 1994). Based on δ34S data of pyrite frommany syn- andepigenetic mineralisations within the Olary Domain a δ34S signatureof 0‰ to +4‰ for this reservoir was suggested by Clark et al. (2004).This makes it likely that the chalcopyrite data represent the δ34S signa-ture of that source and not one resulting of a fractionation processwithin the ore body. It is proposed for other stratiform barite-sulphidemineralisations that the δ34S values of barite represent the signature ofsulphate from the basin water while the sulphides kept the δ34S signa-ture of the hydrothermalfluid (Hall et al., 1991;Ohmoto andGoldhaber,1997; Johnson et al., 2004).

The Cu–Au mineralisation in the study area is related to the mostintensive barite mineralisation within Mt. Mulga mine is the proposedposition of the hydrothermal vent site. The extension of Mt. Mulgamineralisation andMulga Bore Creek is still locally enriched in Cu andAu,but values are much lower down to background. The sulphides at Mt.MulgaMine are not related to fissures or other structures in the ore bodybut occur disseminated in the barite matrix. Mineralogical studies ofsamples from theMt. MulgaMine show recrystallisation of minerals andthe presence of barite and chalcopyrite on fissures within magnetiteindicating that they are younger than the magnetite. However,recrystallisation and local remobilisation during the Olarian Orogenyhave to be taken into account which likely erased the originalmineralogical texture of the ore. In general, stratiform barite mine-ralisations in the Olary Domain are enriched in Cu and Au with the Mt.Mulga main ore body showing the highest values (Ashley et al., 1998b).This supports a cogeneticCu–Aumineralisationwith thebarite indicatingthat mineralising fluids responsible for barite mineralisation in the OlaryDomain were generally enriched in Cu and Au. In contrast, Mo which isoftenpresent in elevated amounts in epigenetic Cu–Aumineralisations ofthe Olary Domain occurs only in the low ppm range at Mt. Mulga.

7. Conclusions

Barite–iron-oxide mineralisation within the study area formed bymixing of a reduced, metal-rich crustal-source fluid with non-marineoxidised, sulphate bearing water of the lower Willyama Basin duringthe initial deposition of the Ethiudna Subgroup. Formation of theassociated Cu–Au mineralisation can be explained by formationtogether with the barite-rich host rock or as an epigenetic processduring the Olarian Orogeny. Considering the δ34S values of sulphidesand related barite in the Mt. Mulga Mine and the regional enrichmentof similar barite-rich rocks of the Olary Domain with Cu and Au, acogenetic SEDEX-style barite–iron-oxide–Cu–Au model is favoured.

Acknowledgments

The authors want to thank the reviewers Katharina Pfaff(University Tuebingen) and Joel Brugger (The University of Adelaide)for their advice on this paper.

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