ARTICLE

Philip M. Scotney Æ Stephen Roberts

Richard J. Herrington Æ Adrian J. Boyce Æ Ray Burgess

The development of volcanic hosted massive sulfideand barite–gold orebodies on Wetar Island, Indonesia

Received: 25 January 2005 / Accepted: 7 February 2005 / Published online: 12 April 2005� Springer-Verlag 2005

Abstract Wetar Island is composed of Neogene volcanicrocks and minor oceanic sediments and forms part of theInner Banda Arc. The island preserves precious metal-rich volcanogenic massive sulfide and barite deposits,which produced approximately 17 metric tonnes of gold.The polymetallic massive sulfides are dominantly pyrite(locally arsenian), with minor chalcopyrite which are cutby late fractures infilled with covellite, chalcocite, ten-nantite–tetrahedrite, enargite, bornite and Fe-poorsphalerite. Barite orebodies are developed on the flanksand locally overly the massive sulfides. These orebodiescomprise friable barite and minor sulfides, cemented bya series of complex arsenates, oxides, hydroxides andsulfate, with gold present as <10 lm free grains. Linearand pipe-like structures comprising barite and iron-oxides beneath the barite deposits are interpreted asfeeder structures to the barite mineralization. Hydro-thermal alteration around the orebodies is zoned and

dominated by illite–kaolinite–smectite assemblages;however, local alunite and pyrophyllite are indicative oflate acidic, oxidizing hydrothermal fluids proximal tomineralization. Altered footwall volcanic rocks give anillite K–Ar age of 4.7±0.16 Ma and a 40Ar/39Ar age of4.93±0.21 Ma. Fluid inclusion data suggest thathydrothermal fluid temperatures were around 250–270�C, showed no evidence of boiling, with a meansalinity of 3.2 wt% equivalent NaCl. The d34S compo-sition of sulfides ranges between +3.3& and +11.7&and suggests a significant contribution of sulfur from theunderlying volcanic edifice. The d34S barite data varybetween +22.4& and +31.0&, close to Miocene sea-water sulfate. Whole rock 87Sr/86Sr analyses of unalteredvolcanic rocks (0.70748–0.71106) reflect contributionsfrom subducted continental material in their source re-gion. The 87Sr/86Sr barite data (0.7076–0.7088) indicatea dominant Miocene seawater component to thehydrothermal system. The mineral deposits formed onthe flanks of a volcanic edifice at depths of �2 km.Spectacular sulfide mounds showing talus textures arelocalized onto faults, which provided the main pathwaysfor high-temperature hydrothermal fluids and thedevelopment of associated stockworks. The orebodieswere covered and preserved by post-mineralizationchert, gypsum, Globigerina-bearing limestone, lahars,subaqueous debris flows and pyroclastics rocks.

Keywords Gold Æ Barite Æ Massive sulfide Æ BandaArc Æ Submarine hydrothermal systems

Introduction

Spectacular examples of asymmetric massive sulfidemounds, flanked by barite deposits, are preserved onWetar Island, Indonesia. The orebodies occur marginalto hydrothermally altered volcanic rocks, which overlyocean floor basalts, and are preserved beneath post-mineralization Globigeriena-bearing limestones, lahars,

P. M. Scotney (&) Æ S. RobertsSchool of Ocean and Earth Science,Southampton Oceanography Centre,University of Southampton,Southampton, SO14 3ZH, UKE-mail: philip.scotney@wrg.co.ukTel.: +44-1234-353996

R. J. HerringtonDepartment of Mineralogy,Natural History Museum,Cromwell Road,London, SW7 5BD, UK

A. J. BoyceIsotope Geosciences Unit,SUERC, East Kilbride,Glasgow, G75 0QF, UK

R. BurgessDepartment of Earth Sciences,University of Manchester,Manchester, M13 9PL, UK

Present address: P. M. ScotneyWRG, 3 Sidings Court,White Rose Way,Dancaster, DN4 5NU, UK

Mineralium Deposita (2005) 40: 76–99DOI 10.1007/s00126-005-0468-x

subaqueous debris flows and pyroclastic rocks (Sewelland Wheatley 1994; Herrington and First 1996; Scotneyet al. 1999). Associated hydrothermal alteration hasargillic characteristics and the sulfide mounds containpyrite, chalcopyrite, tennantite, covellite and low-Fesphalerite. Mineralization occurred at around 4.7 Ma(Herrington and First 1996) and, unusual for a volca-nogenic massive sulfide (VMS) system, only the preciousmetal-bearing barite resource was exploited. As a result,a subrecent VMS system is exceptionally preserved andexposed within open pits and associated drill core. Oreat Kali Kuning contained 1.9 Mt at 4.6 g/t gold, 151 g/tsilver and 60% barite, with 2.2 Mt at 4.0 g/t gold,146 g/t silver and 40% barite at Lerokis (Abadi 1996).

Volcanogenic massive sulfide systems are often sig-nificant repositories of gold and silver (see Hanningtonet al. 1999 for review). Various factors are recognized toplay an important role in the gold enrichment. Theseinclude the tectonic setting of the deposits, which in turninfluences the nature of the igneous basement, and thephysical and chemical characteristics of the hydrother-mal fluids, in particular temperature, salinity and oxygenfugacity (Hannington et al. 1999). The gold bearingcharacteristics of the sulfide assemblage and the argillicnature of the alteration at Wetar Island, has led tospeculation that the hydrothermal fluids responsible forthe gold mineralization contained a significant contri-bution of magmatic volatiles (Sillitoe et al. 1996). Thispaper describes the mineralization and alteration pre-served on Wetar, and the results of fluid inclusion andstable and radiogenic isotope studies. These new dataprovide a better understanding of the nature and originof the hydrothermal mineralizing system. Furthermore,as a relatively young system of Miocene age, the Wetardeposits provide an ideal opportunity to link observa-tions from active systems on the ocean floor with asystem only recently incorporated into the geologicalrecord.

Geological setting

Wetar Island forms part of the Inner Banda Arc, anarray of active and inactive volcanic islands surroundingthe Banda Sea, which are the result of the arc-continentcollision of the NNE moving (75 mma�1) Indian–Aus-tralian plate beneath the Eurasian plate (Audley-Charles1986; Masson et al. 1991) (Fig. 1). This zone of platecontact lies along the Java Trench to the west andcontinues into the Timor Trough. Seismic refractionsurveys indicate that the Timor–Tanimbar Trough(1,200 km in length, up to 70 km wide and 2–3 km deep)is presently underlain by continental lithosphere, vary-ing between 31 km and 40 km in thickness from west toeast (Audley-Charles 1986; Masson et al. 1991).

The Outer Banda Arc is dominantly non-volcanic inorigin, with Timor, to the south of Wetar preserving anaccretionary prism and central collision complex, whichwas accreted onto the front of the Australian continentalplate. Richardson and Blundell (1996) proposed that asubstantial part of the collision complex consists of amicro-continental fragment that lay some considerabledistance to the north of the Australian continentalmargin, and which collided with the subduction zone atapproximately 8 Ma. Seismic velocity and gravitymodelling suggests that the collision complex across theTimor profile is 37–60 km thick and 135–160 km wide(Woodside et al. 1989; Richardson and Blundell 1996).The frontal portion of this collision complex consists ofa number of high-angle thrusts imbricated from thesubducting Australian continental margin (Hughes et al.1996) (Fig. 2). Uplift rates in both the accretionary zoneand associated islands of the inner volcanic arc are high,with eastern parts of Timor presently situated 3 kmabove sea level (Snyder et al. 1996). Microfaunal andpalaeobathymetry studies, on the islands of Timor,Buru, Seram and Kai, show that continent-arc collisionhas produced episodic uplift of the outer islands at rates

Fig. 1 Location map of WetarIsland, Indonesia showingprincipal tectonic and volcanicfeatures of the Banda Arc.After Hamilton (1979),Varekamp et al. (1989), Breenand Silver (1989) and Massonet al. (1991). The Banda Arc isdivided into an Outer ‘‘non-volcanic’’ and Inner ‘‘volcanic’’arc. The extent of theAustralian continental crust isshown within the inset

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of between 500 mm ka�1 in a million years, to5,000 mm ka�1 in some hundreds of thousands of years(De Smet et al. 1989). These results indicate that upliftrates differ greatly along the arc, with some islandsexperiencing long episodes of submergence intermittentwith rapid pulses of uplift during the Pliocene–Quater-nary.

A complex zone of normal and strike-slip faultingoffsets the Inner Banda Arc between the islands of Alorand Wetar, to a distance of approximately 50 km(Fig. 1) (Masson et al. 1991). Recent GPS measure-ments, seismicity data and seismic reflection profilessuggest that the Wetar Thrust, located at the northernedge of the inactive segment of the Inner volcanic BandaArc, accommodates the majority of the present day75 mma�1 convergence, between the Australian marginand the Banda Arc (Silver et al. 1983; McCaffrey 1988;Genrich et al. 1994). This thrust may represent the site ofincipient arc reversal, due to the increased difficulty insubducting the buoyant Australian continental platepost-arrival of the Australian continental margin withthe collision zone at approximately 2.4 Ma (Richardsonand Blundell 1996).

Materials and methods

Geological information was obtained from the open-pitmines, outcrops and exploration drill-core. Polished thinsections were examined in both reflected and transmittedlight. Standard XRD and FTIR methods were employedto characterize alteration mineralogical assemblages on135 samples.

Mineral chemistry was determined by electronmicroprobe analysis using a Cameca SX50 at the

Natural History Museum, London. Operating condi-tions were 15 kV (accelerating voltage), 20 nA (beamcurrent) and count time of 20 s.

Doubly polished fluid inclusion chips were preparedto a thickness of 100 lm. Microthermometric analyseswere completed on a Linkam TMS600 stage calibratedagainst a pure H2O–CO2 inclusion at low temperatures,and checked daily against internal standards. Accuracyis estimated at ±0.1�C for low-temperature phasechanges (�100�C to 0�C) with a precision ±0.1�C and±2�C, for homogenization measurements between100�C and 400�C, respectively.

Sulfides were prepared for conventional isotopicanalysis at SUERC by standard heavy liquid, magnetic,diamond micro-drilling and hand picking techniques.Barite was prepared by micro-drilling. In both cases,around 5–10 mg was used for isotopic analysis. Minorcontamination by non-S-bearing phases was tolerated,and has no effect on the final data. Sulfides were ana-lyzed by standard techniques (Robinson and Kusakabe,1975) in which SO2 gas was liberated by combusting thesulfides with excess Cu2O at 1,075�C, in vacuo. Bariteanalyses were performed by the technique of Colemanand Moore (1978), in which SO2 gas is liberated bycombustion with excess Cu2O and silica, at 1,125�C.Liberated gases were analyzed on a VG Isotech SIRA IImass spectrometer, and standard corrections applied toraw d66SO2 values to produce true d34S. The standardsemployed were the international standards NBS-123 andIAEA-S-3, and the SUERC standard CP-1. These gaved34S values of +17.1, �31 and �4.6&, respectively, with1r reproducibility better than ±0.2&. Data are re-ported in d34S notation as per mil (&) variations fromthe Vienna Canon Diablo Troilite (V-CDT) standard.Selected barite concentrates for sulfate oxygen analyseswere carefully cleaned by washing in Aqua Regia, andthorough rinsing in deionized water. Oxygen was thenextracted following the method of Hall et al. (1991). Theevolved CO2 was analyzed on a VG Sira 10 mass spec-trometer, with all results reported in standard deltanotation as & variations relative to the V-SMOWinternational standard. Replicate analyses of the NBS-

Fig. 2 Simplified cross-section of the Banda Orogen based ongeophysical data from Masson et al. (1991), Richardson andBlundell (1996) and Snyder et al. (1996). Timor was formerly anoutlier of Australian continental crust and is now trapped betweenthe Inner Banda Arc and the Australian continental margin. Thecollision zone is dominated by shallow, southward dipping faults,which have accommodated crustal shortening and thickeningwithin the zone of collision

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127 BaSO4 standard during these analyses gave+9.6±0.3&.

Sr isotopes were measured at Southampton Ocean-ography Center on a seven-collector VG Sector 54 massspectrometer with a separable-filament source. Isotoperatios were determined as the average of >100 ratios bymeasuring ion intensities in multidynamic collectionmode and fractionation corrected by normalization to86Sr/88Sr = 0.1194. Measured values for standard NBSSRM-987 were 87Sr/86Sr = 0.710242 ±13 (2 SD,n=42).

Stepped heating Ar/Ar data for biotite grains andillite separates (<2 lm), were analyzed at the Universityof Manchester, UK with analytical techniques followingthat of Kendrick et al. (2001). Samples of the syeno-granite and dacite were disaggregated by light crushing,and individual biotite grains (2–5 mm in length) werehand picked and cleaned in deionized water. Due to thefine-grained nature of the illite sample it was expectedthat there would be significant recoil loss of 39Ar duringirradiation, therefore, this sample was vacuum encap-sulated in a quartz vial prior to irradiation. The recoil39Ar gas was extracted using an ultra-violet wavelengthlaser to drill into the tube. The recoil 39Ar amounted toonly 5% of the total 39Ar released from the sample andwas recombined in the total age calculation.

Geology of Wetar Island

Wetar Island measures approximately 110 km by 40 kmand is composed entirely of Neogene volcanic rocks andminor oceanic sediments (Sewell and Wheatley 1994).

Submarine, basaltic–andesites, with local pillows, formthe volcanic basement to the island (Fig. 3). The basal-tic–andesites are intruded by rhyo-dacite domes (Ruxton1989) and overlain by dacitic lavas, tuffs and breccias,debris flows and lahar deposits (Fig. 4). Reef limestonesare evident around the perimeter of the island at varyingheights.

Radiometric dating, largely K–Ar, of the volcanicassemblages suggests that the basement volcanic rocks,to the south of the island, were extruded around 12 Mawith overlying dacites, diorites and basaltic–andesitesdeposited between 7.78 Ma and 3.03 Ma (Abbott andChamalaun 1981). A Globigerina-bearing limestoneoutcrops on Wetar Island, which locally overlies basal-tic–andesitic volcanism and mineralization hosted bycalc-alkaline andesites to rhyodacitic flows. This lime-stone yields a biostratigraphic age of between 5.2 Maand 3.9 Ma (Herrington 1993), and based on the ratio ofplanktonic:benthic assemblages (Table 1), is likely tohave formed in up to 2,000 m of water and possiblydeeper (J. Murray, personal communication). Upliftrates for Wetar based on these parameters are 420–570 mm ka�1, which are consistent with the lower ratescalculated for Timor in the outer arc (De Smet et al.1989; Audley-Charles 1986b). Based on these upliftrates, the localized areas of mineralization on the WetarIsland edifice would have emerged from the Banda Seabetween 0.5 Ma and 0.4 Ma. However, the central spineof Wetar (1,500 m, present height) would have emergedaround �3 Ma based on the current height differentials,and likely uplift rates.

Subaerial lahars generated along the central spine ofWetar developed into extensive subaqueous debris flows.These units, locally up to 250 m in thickness, coveredmineralized areas and infilled topographic depressions.After collision of the Australian margin with the OuterBanda Arc, at approximately 2.4 Ma (Richardson and

Fig. 3 Simplified geological map of Wetar Island, after Nash andSnodin (1992) and Farmer and Clifford (1993). The map shows theprincipal mineralized areas, structural lineaments, and key geolog-ical units

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Blundell 1996) uplift may have been substantially in-creased. In particular, the development of the Wetarthrust and subsequent back-arc thrusting may have ai-ded in the rapid exhumation of the Wetar volcanic edi-fice.

Geology of the sulfide deposits

Volcanic and structural setting of the deposits

The economic deposits of Kali Kuning and Lerokiszones 4 and 5 are located towards the central andnorthern part of the island (Fig. 3). These deposits, andthe majority of other recognized mineralized zones,which lie within the central part of the island, are boundto the west and east by extensive NW–SE and NE–SW

trending faults. The base of the volcanic stratigraphycomprises fine-grained basaltic–andesitic flows, whichdip around 10� towards 028�. Overlying the basalvesicular basalts and basaltic–andesites is a >450 mthick sequence of altered volcanic rocks, locally termedthe mine sequence (Fig. 4). At the base of this sequencegreen, chloritic altered, vesicular pillow lavas are wellpreserved. Up section andesitic to rhyodacite flow unitsand local breccias are preserved, and these are the hostrocks to the mineralization. Unconformably overlyingthis sequence are a series of post-mineralization laharsand debris flows, which appear geomorphologicallycontrolled by the palaeotopography. Local hydrother-mally altered dykes cross-cut the mine sequence, withclear evidence of post-mineralization dykes restricted tounaltered E–W striking andesitic dykes, which cut laharsand debris flows within coastal exposures.

The deposits are discordant to the local stratigraphyand are associated with faults. The Lerokis zone 5mound developed at the intersection of a northwest andwesterly trending structure and the Kali Kuning sulfidemound is located along a northwesterly trending fault.

Massive sulfides

Two well-preserved polymetallic sulfide mounds at KaliKuning and Lerokis zone 5 have exposed dimensions of�150·100·70 m and �120·90·30 m, respectively(Fig. 5a–d). Pre-mining, no sulfide mounds were exposed

Fig. 4 Stratigraphic columnand summary of tectonic andgeochronological data from theKali Kuning, Lerokis andMeron areas of Wetar Island.Age constraints are: (1) Scotney2002 (Ar/Ar); (2) Herrington1993 (K/Ar); (3) Abbott andChamalaun 1981 (K/Ar); (4)Herrington 1993, abiostratigraphic age for a post-mineralization Globigerinabearing limestone from the KaliKuning deposit. The Zanclianand Messinian aged strata arelocally referred to as the minesequence

Table 1 Planktonic and Benthic foraminifera assemblages identi-fied within post-mineralization limestones at Kali Kuning

Planktonic assemblage includes Benthic assemblage includes

Orbulia universa CibicidoidesGlobigerinoides conglobatus GlobocassidulinaGlobigerinoides sacculifer FavocassidulinaGloborotalia menardiform group FontbotiaSphaeroidinella dehiscens Pleurostomella

StilostomellaUvigerina

Planktonic:Benthic ratio = 99:1

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at the surface. In plan, the mounds are broadly arcuate.The sulfide mounds are blocky in appearance, with clastsof massive pyrite ranging in diameter from a few centi-meter up to boulders some 30 cm across (Fig. 6). Talusand redeposited sulfides occur marginal to the moundswhere matrix supported angular fragments of massivesulfide are held in a fine-grained sulfide mud. Minorevidence for seafloor reworking is evident at Lerokis zone5, where a 30 cm zone of interbedded sulfide and volca-niclastic material overlies the mound. Chert, gypsum andglobigerina-bearing limestone overlie the Kali Kuningsulfide mound. At the margin of the sulfide mounds, fine-grained poorly consolidated granular pyrite marks thecontact zone (0.2–2.5 m) between the sulfide mound andthe associated barite deposits. The mineralogy of themassive sulfide mounds is dominated by pyrite,accounting for >98% of all sulfides present with minoramounts of chalcopyrite and sphalerite. Typical of sea-floor sulfides, the pyrite and chalcopyrite often show‘‘porous’’ textures as well as collomorphic growth zonesup to 3 mm across (Fig. 7a). The collomorphic pyritetends to nucleate on and around euhedral pyrite grains(Fig. 7b). This texture appears most frequently at themargins and upper parts of the sulfide mounds. Chal-

copyrite frequently rims and locally replaces pyrite(Fig. 7c) and is more apparent at the margins of thesulfide mounds and particularly at the base of moundsand in the underlying footwall. Occasional bandingof pyrite and chalcopyrite is evident on a centimeterscale. A later fracture network permeates the pyriticmounds, with a sulfide assemblage dominated bycovellite, Fe-poor sphalerite and lesser amounts of ten-nantite and tetrahedrite and tabular barite laths(Fig. 7d). Overall, typical sulfide abundance withinthe mounds are pyrite >> chalcopyrite > sphaler-ite > covellite/marcasite/tennantite/tetrahedrite andbornite. No sulfide mound is evident at the Lerokis zone4 deposit despite drilling beneath the barite mineraliza-tion. All three deposits are surrounded by extensivegossanous material.

Fig. 5 a View north–northwest of the Kali Kuning (KK3) deposit.The irregular nature of the sulfide mound is clear; pre-mining, theentire mound was covered by baritic ore. Hydrothermal alterationis evident around the deposit, post-mineralization lahars / debris-flows are shown. b Exposed sulfide mound at Kali Kuning. Heightof the sulfide mound is approximately 60 m. c Lerokis zone 5situated at a topographic height of 550 m on a prominent ridge(view approximately north). The host depression for the barite oredeposit is evident. Conformable, post-mineralization volcaniclastic-sediment overlies the massive sulphide mound. Pre-mined andremediated zones 1, 2 and 3 are also shown. d Exposed sulfidemound at Lerokis zone 5, height of the sulfide mound above the pitfloor is approximately 15 m. Extensive gossanous material sur-rounds both deposits

Fig. 6 Blocky sulfide talus at the base of the Lerokis zone 5 sulfidemound

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Stockwork zones

The pyritic mounds at Kali Kuning and Lerokis zone5 are underlain by stockwork zones which reach to adepth of >210 m below the Lerokis zone 5 depositand >150 m below Kali Kuning. The stockwork zoneis hosted by hydrothermally altered, locally vesicular,silicified volcanic rocks. Brecciated, angular volcanicfootwall clasts are rimmed by sulfides up to 4 mm inthickness and also contain disseminated sulfides(Fig. 7e). The stockwork veins range from <1 mm upto 4 cm in width and contain pyrite, chalcopyrite andsphalerite. Intense vein and disseminated mineraliza-tion occurs within an extremely silicified zone imme-diately beneath the sulfide mound at Lerokis zone 5and appears to be related to a fault intersection.Disseminated pyrite is also abundant in this zone andis generally euhedral to subhedral (usually <1 mm).Gold was observed in association with a zone of in-tense clay (illite) alteration in the stockwork beneaththe Lerokis zone 5 sulfide mound (Herrington 1993).A late sphalerite, tennantite and covellite-rich fracturenetwork (Fig. 7e, f) locally permeates the stockworkzone.

Barite deposits

The mined gold-rich barite deposits have been termedbarite sands by Sewell and Wheatley (1994) and Scotneyet al. (1999) due to their unconsolidated friable nature.At Kali Kuning and Lerokis zones 4 and 5 the depositscontain on average �60–70% barite, and up to 90%barite where more massive. The barite consists of a fri-able mass of barite crystals, which shows variable de-grees of cementation and colour variation, such thatchaotic bedding with evidence of slumping are locallydefined. Individual barite crystals typically range in sizebetween 2 mm and 4 mm, up to a maximum ofapproximately 7 mm in length. Barite crystals typicallyshow euhedral, rectangular, rhombohedral and polyhe-dral forms (Fig. 8a). Cross-bedding within the barite isreported by Sewell and Wheatley (1994), suggesting thelocal reworking of barite on the seafloor; however, thesefeatures were largely evident pre-mining and within theupper parts of the deposits. At Kali Kuning, bariteoverlies and locally surrounds the sulfide mound. Thecontact zone between the massive sulfide and baritedeposit is gradational, with a zone of granular pyrite,clay and barite up to 2 m thick.

Fig. 7 Photomicrographs ofpolished sulfide sections: pypyrite; cpy chalcopyrite; spsphalerite; ba barite; tentennantite; cov covellite; sisilica. a–e field ofview = 5 mm, f field ofview = 2.5 mm. aCollomorphic pyrite (sample097056, Lerokis zone 5) withinthe massive pyritic sulfidemound. b Euhedral pyrite coresovergrown by collomorphicpyrite (sample 097009). cChalcopyrite replacement ofpyrite (sample 097059). dFracture-fill sulfide assemblage(sample 097016). eDisseminated pyrite within analtered volcanic clast in theLerokis zone 5 footwall,rimmed by pyrite and afracture-fill assemblage ofsphalerite and covellite (sample097122). f Detail of fracture-fillsulfide assemblages in e,showing variation in tennantitecomposition

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The matrix of the poorly consolidated barite largelycomprises collomorphic Fe-oxide, traces of clay materialand arsenates (Table 2), with increased cementation to-wards the footwall contact. Arsendescloizite, arsen-brackebuscite and mimetite are all found intergrown,cementing tabular barite (Fig. 8b). Anglesite is observedpseudomorphing galena and as a cementing phase totabular laths of barite and brecciated fragments(Fig. 8c). Arsenbrackebuscite occurs as rhombic crystals(<20 lm) showing compositional zoning (Fig. 8b,d),with cores depleted in As and Pb and enriched in Sb andS compared to the margins; this phase also occurs asamorphous cement. Euhedral barite laths are cementedby jarosite and a complex intergrowth of karibibite andarsenbrackebuscite (Fig. 8d). Gold occurs in the bariticore unit as free grains and in one instance attached to abarite lath (Fig. 9). An association between gold and Fe-oxides is also noted by Herrington (1993). The Hgbearing phase moschellandsbergite occurs at the strati-graphically highest position of the Kali Kuning bariteore zone near to the overlying chert and limestone cover,whereas, the Ag-bearing mimetite phase along with themajority of the complex arsenates tend to occur at thebase of the Lerokis zone 4 barite deposit; which coin-cides with the highest Au and Ag grades reported to-wards the base of other ore zones, e.g. Lerokis zone 1and Kali Kuning (Sewell and Wheatley 1994).

Variable amounts of sulfide phases, predominantlypyrite and galena are included within the barite laths (see

Fig. 8 a SEM image ofrectangular, rhombohedral andpolyhedral interlocking baritecrystals from the Kali Kuningore zone (sample 096879). bRelationship ofaresendescloizite (ad),arsenbrackebuscite (ab) andmimetite (m) as cementingphases within the Lerokis zone4 barite ore zone. c Secondarynature of anglesite (an)pseudomorphing galena andcementing barite laths. d Baritelath rimmed by jarosite (ja) andsubsequently rimmed /cemented by karibibite (k) andarsenbrackebuscite (ab)

Table 2 Barite ore cementing and inclusion phases identified atWetar Island

Minerals identifiedcementing barite

Minerals identifiedincluded within barite

Anglesite: PbSO4 Sphalerite: ZnSRomerite: Fe2+ Fe2

3+

(SO4)4Æ14H2OGalena: PbS

Silica/Jasper: SiO2 Chalcopyrite: CuFeS2Mimetite: Pb5(AsO4)3Cl Argentite: Ag2SMoschellandsbergite: Ag2Hg3 Acanthite: Ag2SJarosite: KFe3

3+ (SO4)2(OH)6 Pyrite: FeS2Fe-oxides Bournonite: PbCuSbS3Arsendescloizite: PbZn(OH)(AsO4)

Bornite: Cu5FeS4

Arsenbrackebuscite: Pb2(Fe,Zn)(AsO4)2ÆH2O

Jordanite: Pb14(AsSb)6S23

Karibibite: Fe23+ As4

3+ O9 Boulangerite: Pb5Sb4S11Plumbojarosite: PbFe6

3+

(SO4)4(OH)12

Geochronite: Pb14(SbAs)6S23

Perroudite: (Hg5Ag4S5Cl4) Stibio-Luzonite: Cu3SbS4Clorargyrite: AgCl Enargite: Cu3AsS4Cerussite: PbCO3

Dusserite: BaFe3(AsO4)2(OH)Æ5H20Cinnabar: HgSCopiapite: Fe2+ Fe4

3+

(SO4)6(OH)2Æ20H2ORomerite: Fe2+ Fe2

3+

(SO4)4Æ14H2O

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Table 2). Included phases range from euhedral toanhedral in form and the size of inclusions are generally<50 lm in diameter. Significantly more sulfides are in-cluded within barite laths from the barite orebodiescompared to barite inclusions from within the sulfidemound and stockwork structures.

Barite also occurs as disseminated laths withinstockwork zones (up to 1 cm), as vug fill within sulfidemounds, associated with the fracture-fill sulfide assem-blage and as coatings on external sulfide mound faces.Vein barite occurs both proximal and distal to miner-alization with individual crystals up to 5 cm in longdimension.

Iron oxide–barite pipe structures

Distinct iron oxide–barite structures are preserved atKali Kuning and Lerokis zones 4 and 5. These structuresare pipe-like in appearance and, at Lerokis zone 4(Fig. 10), are 1–1.5 m high and �1.5 m in diameter andare located towards the base and central part of themined barite deposit. At Lerokis zone 5 two irregularstructures (1 m ·12 m) are present (Fig. 5d) and at KaliKuning two iron oxide–barite pipe features are ob-served, one at the base of the mined barite ore zone, theother directly underlying the chert and limestone coverrocks. All these structures comprise abundant anasto-mosing layers of barite separated by layers of iron oxi-des, (goethite and limonite). Locally, incorporated clastsof vesicular and highly altered volcanic rocks are noted,which were probably derived from the volcanic footwall.

Mineral paragenesis

The mineralization on Wetar can be subdivided intoparagenetic stages based on observations from the three

major components of the systems: stockwork zone,sulfide mounds and barite deposits (Fig. 11).

Stage I Pyrite (+ chalcopyrite + sphalerite + mar-casite). Forming the massive sulfide mounds, and asso-ciated stockworks.

Stage Ia Barite as poorly consolidated barite oredeposits, typically with inclusions of mound sulfides,flanking the massive sulfide mounds. Gold is evidentlyassociated with barite precipitation. Notably, sulfideinclusions in the proposed feeder structures to the baritedeposits only contain inclusions of sulfides typically re-ported for the massive sulfide event, e.g. pyrite, sphal-erite, chalcopyrite, with no evidence of sulfosalts asinclusions.

Stage II Multiple fracture events reactivate the stock-work zone and the sulfide mounds. These veins showchalcopyrite/sphalerite intergrowth, Fe-poor sphalerite,Zn-poor tennantite, Zn-rich tennantite ± tetrahedrite,covellite, bornite, digenite and barite. Tabular baritelaths up to 7 mm long are associated with this event.

Stage III Late barite veins with individual crystals upto 5 cm developed both proximal and distal to miner-alization and coating external faces of the sulfidemounds.

Stage IV Cementation of the barite deposit by oxides,arsenates and sulfates other than barite (see Table 2)and partly through the local oxidation of sulfides. Therelationship of this event to stage III is uncertain.

Alteration

Intense zones of alteration are present within the footwallto Kali Kuning, and Lerokis zones 4 and 5. At the KaliKuning orebody, the immediate footwall to the sulfidemound and barite mineralization exhibits intense silici-fication, including microcrystalline silica and cristobalite,

Fig. 10 Examples of Fe-oxide barite pipe structures, approximately1.5 m in height, preserved at the base of the Lerokis zone 4 bariteore deposit

Fig. 9 Anhedral, <20 lm gold grains adhering to barite laths

84

and a clay alteration assemblage of illite, pyrophyllite,kaolinite and minor alunite. This alteration gives waylaterally and vertically to an illite–smectite assemblage,which passes outwards to zones of chloritic alteration(Fig. 12a). At Lerokis zone 5, a similar zonation is ob-served, with the most intense alteration associated withthe stockwork that underlies the sulfide mound. Beneaththe massive sulfide mound is a �25 m thick zone of

silicification, microcrystalline silica, and extensive argillicalteration, illite, pyrophyllite and kaolinite. Beneath thiszone the predominant alteration is chloritic. At Lerokiszone 4 (Fig. 12b), which lacks an exposed sulfide body,the immediate footwall to the barite deposit is a zone ofquartz–illite rich alteration and microcrystalline silica.Residual silica and local jarosite veins are observed(Fig. 12c, d). This alteration passes outwards into a zoneof quartz–illite, illite–smectite and finally chloritic alter-ation. Intense silicification is observed within the foot-wall adjacent to all the barite deposits. More peripheralfootwall zones to the barite deposits show predominantlychloritic alteration, which is well exposed in road cutsaway from the mineralization.

The alteration around the Wetar mineralization isthus extensive, intensive and zoned. The dominantalteration assemblages comprise silicification proximalto mineralization, moving outwards and downwards to

Fig. 11 Paragenetic sequencefor the mineralization events atthe Wetar Island ore deposits

Fig. 12 a Illite–smectite and chloritic alteration zonation aroundthe Kali Kuning ore following removal of the barite. b Zonedalteration sequence around the Lerokis zone 4 orebody, majorphases are shown determined by XRD, FTIR and initially byPIMA: qtz microcrystalline quartz; ill illite; ja jarosite; goe goetite;chl chlorite; sm smectite; kao kaolinite; al alunite. The rivers S.Koreng, S. Kelapa Tiga and the mine road are shown. Located atthe centre of the ore zone are the Fe oxide/barite pipe structures. c

85

illite–smectite and finally to distal chlorite. Althoughspatially restricted, the presence of pyrophyllite, kaoli-nite and local alunite suggest that low pH fluids atmoderate temperatures have contributed to the alter-ation in the immediate footwall to the sulfide moundsand the barite ore deposits (Reyes 1990).

Mineral identification

Pyrite

Pyrite is ubiquitous in all associations except barite oresand the associated pipe structures where iron is generallypresent as oxides. Stage I pyrite, forms the bulk of thesulfide mound contains a distinct arsenian pyrite. Thispyrite often shows complex grains or collomorphicbanded zones (Fig. 13a, b) and may contain up to6.7 wt% As (Table 3). The arsenian cores to complexgrains often contain idiomorphic inclusions of barite.Overgrowth pyrite is characteristic of the later part ofstage I, immediately pre-dating or coincident with mainstage chalcopyrite. This later pyrite (stage Ia) is char-acteristically arsenic-poor.

Chalcopyrite

Chalcopyrite is associated with the overgrowth pyriteand locally replaces the earlier stage I pyrite. Composi-tional variation in the chalcopyrite is negligible withrespect to paragenetic association. Chalcopyrite oftenoccurs intergrown with later stage II barite.

Sphalerite

Sphalerite is associated with the chalcopyrite and as alater phase with sulfosalts and late barite. Earlier chal-copyrite-associated sphalerite is generally opaque totranslucent in thin section, reflecting an elevated ironcontent of up to 6 wt% Fe. Later sulfosalt-associatedsphalerite (stage II) is generally iron-poor, often lessthan 0.1 wt% Fe (Table 3).

Sulfosalts

Tennantite is a minor accessory of the chalcopyrite–sphalerite stage of mineralization and is more abundantin stage II cross-cutting veins containing covellite, dig-enite, bornite and barite. In the earlier stage (chalcopy-rite + sphalerite, Fig. 7d), the tennantite is generallyzinc-poor with between 0 wt% Zn and 1 wt% Zn,�31 wt% S, �17 wt% As and 0.1–3.9 wt% Sb. In thelater association (+covellite), both zinc-poor and zin-cian tennantite are found (Fig. 7f). Zn-rich tennantitecontains up to 8.3 wt% Zn, �27 wt% S and �19 wt%As and between 0.4 wt% and 6.5 wt% Sb. Zincian tet-rahedrite is often seen as inclusions in Fe-poor sphal-

erite. Bournonite and jordanite are common as tinyinclusions in larger barite laths.

Minor sulfides

Covellite and digenite are common in the stage II veinsand bornite plus argentite are common inclusion phasesin barite laths. The covellite from stage II cross-cuttingveins in the sulfide mound and stockworks contain be-tween 0.2 wt% and 1.3 wt% arsenic.

Barite

Barite appears as a ubiquitous phase throughout themain sulfide mound, sulfide talus breccias, barite ores,stage II stockwork veins and stage III veins. The mineralchemistry of the barites is variable with both Sr-poorand Sr-enriched types present in samples from thestockwork and the ore.

Barite cementing phases

The phases cementing the barite ores are largely ironoxides, hydroxides, complex arsenates and sulfatephases, many of which are amorphous. Distinct phasesanalyzed include mimetite, arsenbrackebuscite, arsen-descloizite, anglesite and moschellandsbergite. Mimetitecontains significant silver, along with the minor ar-gentite as inclusions in barite, and was probably thesource for the silver recovered in the vat-leachprocessing of the barite ore. Along with cinnabar,metacinnabarite, perroudite and tiemannite (De Roever1991), moschellandsbergite is a major host for mercuryin the barite ore, the presence of which was discoveredduring early gold production at Wetar and subse-quently recovered during processing (Sewell andWheatley 1994).

Fluid inclusion studies

A representative suite of barite samples, encompassingall stages of ore paragenesis, were analyzed using mic-rothermometric techniques. These included samplesfrom the stockwork zone, barite ore, late vein barite andiron oxide–barite feeder structures to the barite ore. Themajority of analyzed samples (7 of 11) were collectedfrom the Lerokis zone 5 deposit. Optical examinationindicates that >95% of all barite hosted inclusions areassociated with secondary and pseudo-secondary inclu-sion trails (Fig. 14a), approximately 5% appear primary.The solitary bi-phase primary inclusions are two phase(L + V) and range in size from 12 lm to 25 lm; theseare described type 1A. Type 1B are also two-phase liquidplus small amounts of vapour inclusions (L + V), whichoccur as pseudo-secondary and secondary trails. Rec-ognition of a few mono-phase (L) inclusions (by first andfinal ice-melt phase changes) are described as Type 2.

86

Inclusions are concentrated towards crystal marginsparticularly in stockwork associated samples (Fig. 14b).

Microthermometry results

For all Type 1 fluid inclusions, the degree of fill (F)ranges from 0.6 to 0.95, typically around 0.9–0.95, nodaughter phases are observed and many inclusionsshow evidence of decrepitation or leakage (Fig. 15a).Homogenization of inclusions (Th) is consistentlyL + V to L and occurs between 144�C and 314�C witha mode of 238�C (n=206) (Table 4, Fig. 15b). First ice-melt temperatures (Tfm) occur between �20�C and�24.4�C (n=35), with final ice melting (TMice) between�0.9�C and �3.1�C (mode �1.6); corresponding tosalinities of 1.6–5.1 wt% equivalent NaCl (Bodnar,1993) (Fig. 15c and d). This suggests that salinitiesspan the salinity of seawater (�3.2 wt% equivalentNaCl).

Although the petrographic data allows the inclusionsto be subdivided into Type 1A, 1B and Type 2 inclu-sions, they show similar Th and salinity ranges

throughout the paragenesis, which suggest no significantevolution of the hydrothermal fluid throughout thebarite crystallization and that a single-phase fluid wasresponsible for the precipitation of barite within thesystem. However, the variable salinities above and belowthat of seawater suggest that super-critical phase sepa-ration may have occurred deeper in the system (Delaneyand Cosens 1982; Bischoff and Pitzer 1985). The Globi-gerina-bearing limestone caps the mineralization at KaliKuning and preserves fauna, including Favocassidulina,and benthic:planktonic foraminifera ratios, which sug-gest deposition occurred at least 2,000 m below sea-leveland possibly considerably deeper (J. Murray, personalcommunication). These results contradict those of Se-well and Wheatley (1994), who suggested that the min-eralization occurred in water depths of <600 m. Basedon a minimum depth estimate of 2,250 m and a maxi-mum estimate of 3,000 m, the confining pressure equatesto a homogenization temperature correction of +23�Cand +40�C (Bodnar and Vityk 1994). If the maximumcorrection is applied, the Wetar Island average trappingtemperature will be 275�C. The fluid inclusion datasuggest that the hydrothermal fluids responsible for thebarite deposition throughout the Wetar hydrothermalsystems were similar in salinity and temperature to thosecurrently venting as white smoker fluids on active ventsites at mid-ocean ridges (Rona et al. 1993).

Fig. 13 Back-scattered electron microphotographs of As zonationwithin massive pyrite samples from the Lerokis zone 5 sulfidemound. Point locations correspond to the microprobe traverse, thegraphs illustrate the variation of As along each sample, a sample097009; b sample 099903

87

Stable isotopes

d34S

d34S analyses of 56 samples, comprising 29 sulfides, 25sulphates, 1 gypsum and 1 native sulfur from thedeposits of Kali Kuning, and Lerokis zones 4 and 5,significantly expand on earlier data and ranges reportedby de Ronde (1995). Sulfides were collected from allstages of mineral paragenesis and sulfates were recov-ered from barite bodies, within the mound sulfides andfrom stockwork zones (Table 4).

Sulfides

The d34S values of pyrite within the massive sulfidesrange from 6.2& to 11.7&, stockwork sulfides rangebetween 3.3& and 9.8& (Fig. 16 and Table 4). Thepoorly consolidated granular pyrite from the margins ofthe mounds, show a similar range of d34S to the massivesulfides of between 8.5& and 11.7&. No significantvariations are observed for the different sulfide phases,sphalerite, chalcopyrite and a mixture of sphalerite,tennantite and covellite. The similarity of the sulfidedata from both the underlying stockwork and sulfide

Table 3 Representative analysis of Wetar Island mineral phases from the Kali Kuning and Lerokis orebodies, Indonesia

py py ten bor sph sph tet ten bour ga cpy arg jor py Mim arsStage I Ia Ia Ia Ia II II II Ia Ia Ia Ia Ia Ia IV IV

Sulfide phases Sulfide inclusion phases within barite Cementingphases

S 48.01 52.60 31.23 30.46 32.61 32.29 25.16 26.56 19.36 13.74 34.32 12.00 17.94 51.64 Cl 2.09 0.20Ag – 0.19 0.05 0.10 – – 0.01 – 0.06 0.26 – 80.34 – 0.02 SO2 0.65 0.46As 6.71 0.19 15.91 – – 0.07 7.02 17.79 1.83 0.07 0.10 0.06 11.12 1.42 FeO 0.25 14.02Ba – – – 0.46 0.12 – – – 0.16 0.79 0.87 2.63 0.42 0.67 ZnO 0.13 0.07Cd – – 0.07 – – 0.48 – – – – – – – – As2O5 22.60 30.67Cu 0.03 0.20 45.80 62.21 0.23 1.56 38.56 41.24 12.78 0.02 34.62 0.06 0.03 0.23 SeO2 0.03 0.02Fe 44.37 46.48 0.04 7.15 6.12 0.10 2.20 0.11 0.05 – 29.07 1.26 – 44.86 Ag2O 0.79 0.15Hg – – – 0.03 0.06 – – – – – 0.14 0.65 – – PbO 73.05 50.65Pb – – – – – – – – 41.08 84.37 – – 69.48 – Sb2O3 0.09 0.04Sb – – 3.95 – 0.03 0.19 19.87 3.10 22.57 1.31 0.12 – 0.64 0.04 BaO 0.04 –Se 0.04 0.38 0.13 0.03 0.03 0.04 0.05 0.06 0.03 0.01 – 1.20 0.05 0.04 HgO 0.00 –Zn 0.04 – 0.16 – 58.28 64.98 5.22 8.30 – 0.16 0.07 0.01 – 0.18 Bi2O3 0.15 0.33TotalWt%

99.20 100.04 97.34 100.43 97.48 99.71 98.07 97.16 97.92 100.73 99.30 98.21 99.68 99.08 Totalcompound%

99.85 96.60

Sulfide phases (wt %): py pyrite; sph sphalerite; tet tetrahedrite; ten tennantite; bour bournonite; ga galena; cpy chalcopyrite; arg argentite;jor jordanite; bor borniteCementing phases (compound %): mim mimetite; ars arsendescloizite

Fig. 14 a–b Photomicrographsof barite hosted fluid inclusionsfrom Wetar Island. a Pseudo-secondary and secondary trailsin samples 097084, decrepitatedinclusions are also evident (d).b Abundant 2 phase inclusionsin sample 097106, a pronouncedreduction in number ofinclusions is evident toward thecrystal core

88

mounds suggests a common source of sulfur at eachdeposit. The mean value of d34S for this study is 8&,close to d34S values of +5 and +7& reported forIndonesian arc lavas by De Hoog et al. (2001). It issuggested that arc-related lavas are enriched in d34S as aresult of the significant incorporation of reduced sea-water sulfate during petrogenesis. In general, dissemi-nated sulfide in the stockwork system has a similar rangeof d34S to the mineralization (mound and vein stock-work, Table 4). This consistency of d34S suggests a ge-netic correlation between the volcanic sulfide andmineralization. Anomalously heavy d34S is a commonoccurrence in these settings (e.g. Arribas 1995).

Sulfates

The d34S of sulfates recovered from the Barite depositsof Wetar varies between 22.4& and 26.9&, with theheaviest d34S values, up to 31&, recorded from the stageIII barite veins distal to the ore deposits (Fig. 16).Ohmoto et al. (1983) reported similar d34S barite valuesfrom Kuroko deposits, whereas, Goodfellow andFranklin (1993) report barite d34S values from the BentHill system, which are considerably less than seawater,by as much as 10&. The Wetar sulfate data reflect or areheavier than the predicted Miocene seawater sulfatevalue of d34S 22& (Claypool et al. 1980). Ohmoto et al.(1983) suggested that mixtures of seawater sulfate andhydrothermal fluid sulfate, with equilibrium d34S valuesof between 29& and 34& between 280�C and 200�C,could account for the values observed in Kuroko sys-tems. However, only limited mixing of seawater andhydrothermal fluid would return the d34S sulfate valuesto seawater, as modern vent fluids contain extremely lowlevels of dissolved sulfate (Scott 1997). Chiba et al.(1998) suggested that increases in d34S observed foranhydrite in the TAG mound is due to the partialreduction of seawater sulfate by ferrous iron in thehydrothermal fluid. Given that the formation of bariterequires the mixing of seawater and hydrothermal fluid asimilar interpretation is favored. The heaviest d34S sul-fate values are reported from the distal stage III bariteveins and suggest that seawater and hydrothermal fluidmixtures are subject to closed system reduction, high-lighted by the peripheral vein at +31& (Table 4, sample099938). Notably, coexisting pyrite and barite pairscommonly give unrealistic isotope equilibrium temper-atures (usually higher), which corroborates ourobservations that the pyrite and barite is seldomco-precipitated.

Goodfellow and Franklin (1993) account for therelatively light d34S data at Bent Hill, by the mixing ofbarium in vent fluids with sulfate formed by the oxida-tion of pre-existing sulfides, or H2S from the hydro-thermal fluid, either within chimney structures or in theunderlying sulfide mound. There is no evidence for thisat Wetar. However, the gypsum and native sulfur d34S

Fig. 15 Fluid inclusion microthermometric data fromWetar Islandbarite samples

89

Table

4Isotopic

andmicrothermometricdata

obtained

fortheWetarsamples

Sample

Material

analyzed

d34S

(CDT)

d18O

(SMOW)

87Sr/86Sr

±2SE

Th(�C)

TFm(�C)

Tmice(�C)

NaCl

(wt%

)Description

Location

Range

Av

nRange

Av

nRange

Av

n

111943

Barite

22.78

0.70764

7196–262

235

16�22.1–22.8�22.5

4�1.4–2.3�1.9

15

2.5–3.8

Barite

asso.with

sulfides

KK

100134

Barite

24

Barite

asso.with

sulfides

KK

096877

Barite

24.23

Barite

ore

KK

096878

Barite

26.98

Barite

ore

KK

096879

Barite

23.65

10.9

0.70773

7209–289

242

21�21–22.5

�21.9

4�1–1.9

�1.6

17

1.7–3.2

Barite

ore

KK

096889

Barite

22.39

Barite

ore

KK

100133

Barite

9.1

Barite

ore

KK

100103

Gypsum

12.8

13.3

0.70909

7Post-m

ineralization

cover

KK

096890

Pyrite

11.55

Granularpyrite

KK

096858

Pyrite

8.55

Granularpyrite

KK

096879

Pyrite

8.44

Sulfidemound

KK

111943

Pyrite

9.52

Sulfidemound

KK

111943

Chalcopyrite

10.25

Sulfidemound

KK

100134

Pyrite

11.69

Sulfidemound

KK

096857

Pyrite

10.37

Sulfidemound

KK

097228

Pyrite

10.78

Sulfidemound

KK

111942

Pyrite

8.47

Sulfidemound

KK

100144

Nativesulfur

15.8

Veinfrom

thefootw

all

KK

097333

Pyrite

9.89

Vein/disseminated

pyrite

KK

111979

Barite

25.18

6.5

0.70827

Barite

ore

L4

111931

Barite

26.3

7.7

0.70774

8189–267

228

17

–�21.1

1�1.1–2.1�1.6

61.9–3.6

Barite

ore

L4

096948

Barite

24.6

8.7/9.4

0.70801

8197–309

240

20

–�23

1�0.9–2.4�1.6

16

1.6–4.1

Baritic

feeder

structure

L4

099917

Barite

24.2

8.4

0.70829

7183–237

220

15�20.4–21.8�21.1

3�1.6–2.6�2.2

12

2.8–4.3

Barite

asso.withsulfides

L5

099939

Barite

23.9

6.8

0.70836

8199–269

233

18

–�2.6

1�1.2–1.8�1.6

17

2.1–3

Barite

ore

L5

099938

Barite

31

11.3

0.70786

8144–212

194

15�22–24.4

�22.7

11�1.4–2.7�2.2

22

2.5–4.5

Barite

vein,

distalto

sulfide

mound

L5

099933

Barite

27.8

10.5

0.7088

8Barite

vein,

proxim

alto

sulfidemound

L5

099923

Barite

24.5

7.8

0.7083

8Bariticfeeder

structure

L5

097106

Barite

27.5

7.7

0.70807

8217–302

255

24�20–21.8

�20.7

5�1.8–3.1�2.1

17

2.5–5.1

Disseminated

stockwork

barite

L5

097106

Pyrite

3.33

Disseminated

stockwork

sulfides

L5

097104

Sphalerite

6.1

Disseminated

stockwork

sulfides

L5

90

097098

Pyrite

6.7

Disseminated

stockwork

sulfides

L5

111997

Pyrite

7.25

Pyrite

vein,

distalto

sulfidemound

L5

099902

Pyrite

6.92

Sulfidemound

L5

099917

Pyrite

6.26

Sulfidemound

L5

097219

Pyrite

8.4

Vein/disseminated

pyrite

L5

097122

Barite

28.1

8.6

0.70836

186–314

269

20�22.1–22.5�22.4

4�1.9–2.8�2.2

16

3.2–4.1

Barite

asso.with

cov-spha-ten

L5

097059

Barite

22.7

5.8

0.70809

8Barite

asso.withsulfides

L5

097024

Barite

24.6

11.3

0.70873

210–288

245

18

––

–�1.6–2.4�2.1

11

2.8–4.1

Barite

asso.withsulfides

L5

097023

Barite

24.43

Barite

asso.withsulfides

L5

097098

Barite

25.04

8.9

0.70836

Disseminated

stockwork

barite

L5

097084

Barite

27.02

8.6

0.70854

8187–286

230

22

–�21.4

1�1–2.1

�1.8

14

1.7–3.6

Disseminated

stockwork

barite

L5

097084

Pyrite

9.03

Disseminated

stockwork

sulfides

L5

097084

Sphalerite

4.26

Disseminated

stockwork

sulfides

L5

097122

Pyrite

8.02

Stockwork

sulfides

L5

097122

Cov-spha-ten

6.91

Cov-spha-ten

stockwork

sulfides

L5

097041

Pyrite

6.56

Stockwork

sulfides

L5

097024

Pyrite

8.57

Stockwork

sulfides

L5

097059

Pyrite

8.01

SulfideMound

L5

097059

Chalcopyrite

7.44

Sulfidemound

L5

097023

Pyrite

7.52

Sulfidemound

L5

097009

Pyrite

7.4

Sulfidemound

L5

096920

Whole-rock

0.70773

17

Coastalbasalt

sample

–

096921

Whole-rock

0.70773

18

Coastalbasalt

sample

–

096922

Whole-rock

0.70775

17

Coastalbasalt

sample

–

096884

Whole-rock

0.70834

21

Altered

volcanic

rock

KK

91

values, suggest oxidation of excess H2S in the hydro-thermal fluid and local oxidation of sulfides may beimportant in their formation.

d18O

The sulfate oxygen data vary between d18O of 5.8& and11.3& (Table 4), with a mode at 9&. As the barite d18Odata are shifted to values both higher and lower than thecomposition of seawater, the sulfate oxygen data sug-gests isotopic exchange has occurred. Nevertheless, thed18O mode of 9& coincides with the d18O value of sea-water sulfate and suggests that the bulk of barite pre-cipitated in equilibrium with seawater.

The d34S and d18O stable isotope data suggest thepredominant source of sulfur in the sulfides was derivedfrom the volcanic rocks in the basement, with the sulfurin the sulfate largely derived from seawater and reaffirmsthe importance of hydrothermal fluid and seawatermixing in the formation of VMS systems (Teagle et al.1998a, 1998b; Roberts et al. 2003).

87Sr/86Sr

Whole-rock 87Sr/86Sr analyses were completed on vari-ably altered host volcanic rocks, and barite separatesfrom the massive barite bodies, fractures within the sul-fide mounds and iron oxide-barite pipe structures. Thebarite data range between 87Sr/86Sr 0.7076 and 0.7088with no systematic variation according to setting(Fig. 17). The 87Sr/86Sr ratios of the barites are consid-ered to reflect mixtures between Miocene seawater(87Sr/86Sr 0.70849, Farrell et al. (1978)) and hydrother-mal fluid. However, the majority of the data plot close tothe value of Miocene seawater suggesting a significantcontribution of seawater to the barite formation.

Notably, one analysis shows a 87Sr/86Sr ratio of0.70644 (Herrington 1996), from the stockwork of anundeveloped mineral prospect, Batu Kapal (Fig. 3).This value suggests that locally the hydothermal endmember value may be less than 0.70644, and alsoindicates that less radiogenic basement may be in-volved in the source of Sr to this hydrothermal fluid.

The whole-rock data of variably altered basementshow 87Sr/86Sr values between 0.7074 and 0.7116. Thesevalues are typically more radiogenic than MORB, andare consistent with data reported from surrounding is-lands (Whitford et al. 1977; Margaritz et al. 1978;Varekamp et al. 1989; Vroon et al. 1993, 2001). Themore radiogenic values of 0.71165 and 0.71106, fromdacitic flows, suggest a significant contribution of con-tinental crust and or sediment in the generation of thesepost-mineralization magmas (Vroon et al. 2001; Elburget al. 2002). Values of up to 0.72227 are reported byMcCulloch et al. (1982), however, the nature of thesesamples is uncertain.

The unaltered volcanic samples tend to show 87Sr/86Srratios between 0.70748 and 0.70781 and Sr valuesT

able

4(C

ontd.)

Sample

Material

analyzed

d34S

(CDT)

d18O

(SMOW)

87Sr/86Sr

±2SE

Th(�C)

TFm(�C)

Tmice(�C)

NaCl

(wt%

)Description

Location

Range

Av

nRange

Av

nRange

Av

n

096168

Whole-rock

0.71166

20

Altered

post-m

ineralization

dacite

Meron

096167

Whole-rock

0.71107

12

Post-m

ineralization

dacite

Meron

111914

Whole-rock

0.70747

17

Altered

volcanic

rock

KK

097106

Whole-rock

0.70772

7Brecciatedstockwork

volcanic

L5

100129

Whole-rock

0.70789

9Altered,vesicular

footw

allvolcanic

KK

111935

Whole-rock

0.70819

23

Altered

volcanic

rock

L4

111999

Whole-rock

0.70803

21

Syeno-granite

Meron

111994

Whole-rock

0.70781

7Basaltic–andesite

KK

096926

Whole-rock

0.70748

9Basaltic–andesite

–

KK

KaliKuning;L4LerokisZone4;L5LerokisZone5;cov-spha-ten

covellite,sphalerite,tennantite

composite

sample

92

>160 ppm (160–388); which are substantially changedduring alteration. The progressively altered samplesshow a significant decrease in whole-rock Sr concentra-tion to <60 ppm with a concomitant increase in the87Sr/86Sr value from 0.70746 to 0.70833. The most highlyaltered samples show Sr concentrations and 87Sr/86Srsignatures that suggest that the samples have undergonecomplete isotopic exchange with seawater (Fig. 18).

Ar–Ar age determinations

Three samples were chosen for Ar–Ar age determinationin order to better constrain the age of mineralization and

volcanic events of the area. Samples of the followingintrusive and volcanic rocks were analyzed: (1) a syeno–grantite intrusion (sample no. 111999) collected from theKali Lurang river in the Meron area; (2) a post-miner-alization capping dacite flow (sample no. 097167) fromMeron; and (3) a fine-grained illite (sample no. 056896,<2 lm size fraction) collected from the hydrothermallyaltered footwall volcanic rocks at Lerokis at a depth of26 m below the mineralization. Herrington (1993) re-ported a K/Ar age of 4.7±0.16 Ma for the illite sample.

Ar–Ar data are given in Table 5 and shown asapparent age spectrum diagrams in Fig. 19. Errors arequoted at the one standard deviation level and includethe uncertainty in monitor age (Hb3gr 1072±11 Ma).An isotope correlation diagram of 39Ar/40Ar versus36Ar/40Ar (not shown) reveals that the trapped40Ar/36Ar ratios are lower than the present day atmo-spheric ratio (295.5) constrained most precisely by thesyeno–granite sample to be 284±3. Previously, Nagyet al. (1999) determined similar anomalously low

Fig. 16 Comparison of sulfur isotope data from the Kali Kuning,Lerokis zone 4 and 5 sulfide barite deposits

Fig. 17 Histogram summarizing the range in 87Sr/86Sr for WetarIsland sulfate and whole rock data. Data from Herrington (1996) isincluded. s syeno-granite; d dacite

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40Ar/36Ar ratios from dacite flows and attributed themto either a minor contaminant at m/z 36 in the massspectrometer, or fractionated atmospheric argon withinthe samples. The age obtained from the illite sample of4.93±0.21 Ma is within error of the K/Ar age of thesame sample (4.7±0.16 Ma) reported by Herrington(1993). The age of the syeno–granite intrusion and daciteflow are 4.73±0.16 Ma and 2.39±0.14 Ma, respectively(Fig. 19).

The age data indicates that the spatially related sye-no–granite intrusion (proximal to the Meron prospect)and the mineralization are the same age and thereforeimplies that this intrusive event supplied heat to thehydrothermal system. The Ar–Ar age of the illite iswithin error of the previously published conventionalK–Ar age (Herrington 1993) and indicates mineraliza-tion of the Lerokis deposit occurred between 4.7 Ma and4.9 Ma. The age of the post-mineralization dacite flowindicates that volcanism continued at least as recently as2.4 Ma, which is the proposed age for the collision andaccretion of the Australian continental margin with theOuter Banda Arc in the region of Timor (Richardsonand Blundell 1996). The sample may record a period ofextension within the Inner Banda Arc as a direct resultof compressional tectonics to the south in the Outer Arc.The age of the dacite flow also indicates that the debrisflow that overlies the Meron deposit also occurred post-2.4 Ma. This new age data coupled with the Sr dataconfirms an increasingly contaminated source regionunder Wetar Island, progressively modified by sub-ducted continental material (SCM) related to tectonicevents further to the south.

Evolution of the hydrothermal system

The data collected suggest that the Wetar massive sulfideand barite deposits were formed on the flanks of a vol-canic edifice during the development of the Inner BandaArc. Observations of the volcanic stratigraphy and tec-tonics suggest the Wetar edifice initially formed around12 Ma due to extensive rifting and associated volcanismwithin oceanic crust. The mineralization is associatedwith bimodal volcanism, on a basement of basalts andbasaltic–andesite, which most likely formed around5 Ma, given the dates of the overlying mine sequence.The major sulfide mounds show talus textures and arelocalized on faults, which provide the main pathway forhigh temperature hydrothermal fluids and the develop-ment of associated stockworks (Fig. 20a). Within themassive sulfide mound much of the pyrite is arsenian (upto 6.7 wt%), and given the established relationship be-tween arsenic and gold content of pyrite (Cook andChryssoulis 1990; Cline 2001; Pals et al. 2003) mayrepresent an initial reservoir for Au subsequently re-mobilized by later hydrothermal fluids responsible forthe barite–gold ore. The pyrite d34S data suggest that thesulfur is sourced from basement arc volcanic rocks,modified by a subduction zone component, which is alsosuggested by the whole rock 87Sr/86Sr data. The slightlyelevated d34S values, compared to arc values, indicates acomponent of reduced seawater sulfate during pyriteprecipitation. The hydrothermal fluids responsible forsulfide precipitation produce a well zoned, intensivealteration sequence (Fig. 12b) with illite–smectite cen-tered on the mineralization and chlorite alteration dee-

Fig. 1887Sr/86Sr v ppm Sr

from variably altered Wetarvolcanic rocks. Increasedalteration coincides with lowerppm Sr and more radiogenic87Sr/86Sr ratios, reflectingincreased interaction withMiocene seawater. The syeno-granite and post-mineralizationdacites are omitted from thisfigure

Table 5 Stepped heating Ar/Ar data for biotite grains and illite separates (<2 lm), Wetar Island, Indonesia. Amounts of Cl and Kobtained from measured 38ArCl and

39ArK using the Hb3 gr monitor and the parameters of a = 0.542±0.01, b = 4.37±0.03 and J =0.017093±0.000026

Sample No. Location Sample type Weight (mg) Cl (ppm) K (Wt%) 40Ar*·10�6 cc/g Age (Ma)

111999 Meron area Biotite 12 2,288±6 7.35±0.01 1.35±0.4 4.73±0.16097167 Meron Biotite 11 9,404±7.1 8.73±0.01 0.75±1.2 2.39±0.14056896 Lerokis Illite 9.1 13.88±0.90 7.90±0.01 1.38±0.05 4.93±0.21

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per and distal to the ore zones. The heat source drivingthe hydrothermal convection is most likely intrusivesyeno–granite bodies at depth, which from Ar/Ar datingare known to be coeval with mineralization.

Following massive sulfide development, barium richfluids are discharged as white smokers, from thehydrothermal system and in particular at the margins ofthe sulfide mound (Fig. 20b). The distribution of baritesuggests these fluids exploit many of the fracture systemspreviously employed to develop the massive sulfidemound. For example, barite infiltrates the base of thesulfide mound and fills any voids and fractures present.The d34S barite data suggest that the sulfate in the bariteis predominantly seawater derived, whereas, the bariumis most likely derived from the destruction of feldspars,within the andesites and felsic volcanic rocks of thebasement. Fluid inclusion data show that the hydro-thermal fluids were at around 250–270�C, with no evi-dence of boiling. However, the salinities are greater andless than seawater, suggesting super-critical phase sepa-ration may have taken place, prior to egress on theseafloor. The form, location, isotope and fluid inclusiondata of the iron oxide–barite structures, strongly suggest

Fig. 19 Age versus cumulative 39Ar released during steppedheating for the Wetar Island samples

Fig. 20 Schematic evolution of the Wetar deposits: a T1, the Wetardeposits initiated as typical volcanogenic massive sulfides with azoned footwall alteration predominantly propylitic to argillic incharacter (Kuroko like). b T2, the barite deposits originate as aperipheral ‘vent’ system, with fluids circulating through the sulfidemound and undergoing significant mixing of seawater. As thesystem evolves conductively cooled hydrothermal fluids circulatebeneath the massive sulfide mound generating the alteration andreflecting the passage of more oxidized and acidic fluids. Thisresults in the argillic to advanced argillic alteration observed. Thisis also the major Au-precipitation phase. c T3, the sulfide andbarite system is preserved beneath limestones and lahars, prior toexhumation from the ocean floor, due to continued collision of theAustralian continental margin and the Outer Banda Arc

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they are the palaeofluid conduits for the barite deposits.Gold is significantly enriched in the barite mineralizationand is closely related to its formation and the most in-tense phases of alteration. The presence of high levels ofarsenic in the barite ore matrix suggest that ‘zonerefining’ of the initial arsenian pyrite may be important.It is questionable whether black smoker mineralizationwas still occurring at this time.

The limited nature of the covellite dominatedassemblage, and accompanying d34S, d18O data, suggestthat no significant contribution of magmatic volatileswas involved in the formation of the Wetar orebodies.For example, no isotopically light d34S values wereobserved for sulfides or sulfates compared to Hine Hinaor Conical Sea-Mount (Herzig et al. 1998) at least nocontribution that could be detected beyond the copiousamounts of seawater that must have circulated in thesystem. There is no compelling evidence that the fluidsresponsible for mineralization boiled at the sites ofdeposition, however, they may have been subject tosupercritical phase separation, which is becoming anincreasingly recognized phenomenon in modern ventsystems.

The sulfide and barite orebodies are preserved on theseafloor by the subsequent precipitation of chert, gyp-sum and limestone and, perhaps most significantly, bythe accumulation of lahars and debris flows (Fig. 20c).A dacite flow, within the lahars and debris flows gives anage of 2.4 Ma and records continued volcanism on theWetar edifice post-3 Ma.

Discussion and comparisons

Wetar deposits in comparison with modern systems

The fluids responsible for barite precipitation withassociated gold and sulfosalt assemblages show strongsimilarities to late stage hydrothermal fluids reportedelsewhere in modern seafloor deposits. For example, latestage hydrothermal fluids rich in Pb, As, Sb, Ag, +Auwere reported from Axial Seamount (Hannington andScott 1988); with up to 6.7 ppm Au reported from anassociated sulfosalt assemblage. Furthermore, galena,anglesite and sulfosalts of Pb, Ag, As, and Sb are fre-quently associated with low-T venting fluids, and themineralogy of white smokers are typically enriched inbarite, sphalerite and consistently report elevated Auvalues (Koski et al. 1984).

The Spire at Axial Seamount shows both high- andlow-Fe sphalerite, with the higher values associated withthe main stage ore formation (Hannington and Scott1988). This is also the case for Wetar with low-Fesphalerite restricted to the later fracture network. Thesedata are consistent with a fluid evolution that evolvedfrom a reducing fluid at low pH and high T (350�C) to arelatively oxidizing fluid at high pH and lower T due tocooling and mixing with seawater.

Gold is commonly associated with low-T fluids inmodern hydrothermal systems. High concentrations ofgold are reported in low-T Zn–Ba–SiO2 precipitates(Hannington et al. 1986; Hannington and Scott 1988;1989). In the Zn-rich chimneys of Snake Pit, Cd, Pb,Sb, Ag and Au are considered to have directly pre-cipitated in the Zn-sulfides, with the highest Au con-tents (>500 ppb) observed in the Zn-rich chimneysand massive Zn-rich sulfides at the surface of the de-posit (Foquet et al. 1993). At the JADE hydrothermalfield, Au enrichment correlates well with the baritecontent of the samples, with minute rounded Augrains observed between barite crystals (Halbach et al.1989, 1993). The Au-rich samples also showed higherconcentrations of As, Ba, Sb, SiO2 and Ag, similar toWetar. The similarity between the Wetar gold miner-alization and observations from active white smokersystems is striking, suggesting there was a significantrole for such fluids in the origin of the mineralization.

Au-rich volcanogenic massive sulfides and Wetar

The mineralogy of the ore deposits at Wetar is highlyanalogous to that reported from back-arc spreadingcenters, e.g, Lau Basin, where visible gold was firstdocumented in a white smoker chimney (Herziget al.1993). In particular, the mineralogy and preciousmetal content of the Wetar deposits are stronglycomparable to hydrothermal vent fields developed onisland-arc or continental crust, e.g. Okinawa Trough(Halbach et al. 1989, 1993). The importance of the Au-composition of the igneous basement in the generationof Au-rich VMS is debated. Herzig and Hannington(1993) suggest that back-arc lavas are not significantlyenriched in gold compared to MORB, and that a gold-enriched source is not a prerequisite to the developmentof gold-rich VMS systems. However, Moss et al. (2001)investigating the Manus Basin, suggest that the Au-enriched arc lavas, typically at 6 ppb compared to1 ppb and below for MORB, may have an importantinfluence on the Au-forming potential of the system.Although not developed directly on continental crust,the isotopic data for Wetar provide strong evidence fora significant component of continental crust and orsediments in the generation of the volcanic edifice thathosts the VMS mineralization. Using simple massbalance equations, the amount of basement required tobe stripped of gold to produce the Kali Kuning depositsignificantly increases from 0.5 km3 for a basement of6 ppb Au to �3 km3 for a basement with only 1 ppbAu. These values climb to 0.8 and 5 km3, if extractionefficiency rates more in keeping with experimental workare assigned (Moss et al. 2001). Such a dramaticreduction in the rock volumes required to generate theAu-mineralization suggests that the Au content of thevolcanic basement may well play an important role inthe generation of Au-rich VMS. Herzig and Hanning-

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ton (1993) note that gold appears most abundant insulfides associated with immature seafloor rifts incontinental or island-arc crust, settings dominated bycalc-alkaline volcanic rocks, including andesites, dacitesand rhyolites. Notably, the Au-rich VMS system atBoliden is thought to have developed within calc-alkaline to dacite rocks within an island-arc located oncontinental crust or a thin continental margin (Vivalloand Claesson 1987; Allen et al. 1996; Billstrom andWeihed 1996). Similarly, the Au-rich Eskay Creek de-posit formed within a mid-Jurassic arc of calc-alkalineto dacitic rocks that developed on an earlier Triassicarc and Palaeozoic volcanic and sedimentary rocks(Macdonald et al. 1996).

Summary and conclusions

The massive sulfide and barite–gold mineralization ofWetar Island, Indonesia, provides a significant per-spective on the formation of Au-rich barite in arc situ-ations. The mineralization at Wetar relates to arcvolcanism triggered as the Banda Arc and Australiancontinental plate collide. Progressive stages of mineral-ization are recognized, dominated by the early arsenianpyrite with associated chalcopyrite and minor barite.Later sulfosalts, low-Fe sphalerite and further stages ofbarite and then oxide mineralization developed, leavinga complex mineralization over intensely altered footwallvolcanic rocks showing argillic and local advanced arg-illic alteration close to mineralization. The presence ofoxide-cemented barite ores and iron oxide-cementedbarite ‘pipes’ are evidence of the later, oxidized hydro-thermal fluid dominated by seawater sulfate. Theseoxidized fluids released arsenic and gold from earlierarsenian pyrite precipitating free gold with barite andcomplex iron-bearing arsenates with significant con-tained silver and mercury.

Acknowledgements PS acknowledges the generous financial sup-port of BHP-Billiton, The Natural History Museum and TheUniversity of Southampton. The logistical and financial support ofBHP-Billiton through Chris Farmer, David Hopgood, David Firstand James Macdonald is particularly acknowledged. Tony Fallick,John Murray, Andy Barker, Robin Armstrong, Ernie Rutter andDamon Teagle provided key insights throughout the project. TheSUERC is funded through support of the Natural EnvironmentResearch Council (NERC) and the Scottish Universities. AJB isfunded by NERC support of the Isotope Community SupportFacility at SUERC. We acknowledge careful review of the manu-script by Donna Sewell and an anonymous referee.

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