The development of volcanic hosted massive sulfide and barite gold orebodies on Wetar Island, Indonesia PDF

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Mineralium Deposita (2005) 40: 76–99 DOI 10.1007/s00126-005-0468-x A RT I C L E Philip M. Scotney Æ Stephen Roberts Richard J. Herrington Æ Adrian J. Boyce Æ Ray Burgess The development of volcanic hosted massive sulfide and barite–gold orebodies on Wetar Island, Indonesia Received: 25 January 2005 ...

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

A RT I C L E

Philip M. Scotney Æ Stephen Roberts Richard J. Herrington Æ Adrian J. Boyce Æ Ray Burgess

The development of volcanic hosted massive sulfide and 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 volcanic rocks and minor oceanic sediments and forms part of the Inner Banda Arc. The island preserves precious metalrich 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 cut by late fractures infilled with covellite, chalcocite, tennantite–tetrahedrite, enargite, bornite and Fe-poor sphalerite. Barite orebodies are developed on the flanks and locally overly the massive sulfides. These orebodies comprise friable barite and minor sulfides, cemented by a series of complex arsenates, oxides, hydroxides and sulfate, with gold present as 100 ratios by measuring ion intensities in multidynamic collection mode and fractionation corrected by normalization to 86 Sr/88Sr = 0.1194. Measured values for standard NBS SRM-987 were 87Sr/86Sr = 0.710242 ±13 (2 SD, n=42). Stepped heating Ar/Ar data for biotite grains and illite separates (450 m thick sequence of altered volcanic rocks, locally termed the mine sequence (Fig. 4). At the base of this sequence green, chloritic altered, vesicular pillow lavas are well preserved. Up section andesitic to rhyodacite flow units and local breccias are preserved, and these are the host rocks to the mineralization. Unconformably overlying this sequence are a series of post-mineralization lahars and debris flows, which appear geomorphologically controlled by the palaeotopography. Local hydrothermally altered dykes cross-cut the mine sequence, with clear evidence of post-mineralization dykes restricted to unaltered E–W striking andesitic dykes, which cut lahars and debris flows within coastal exposures. The deposits are discordant to the local stratigraphy and are associated with faults. The Lerokis zone 5 mound developed at the intersection of a northwest and westerly trending structure and the Kali Kuning sulfide mound is located along a northwesterly trending fault. Massive sulfides Two well-preserved polymetallic sulfide mounds at Kali Kuning 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

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Fig. 5 a View north–northwest of the Kali Kuning (KK3) deposit. The irregular nature of the sulfide mound is clear; pre-mining, the entire mound was covered by baritic ore. Hydrothermal alteration is evident around the deposit, post-mineralization lahars / debrisflows are shown. b Exposed sulfide mound at Kali Kuning. Height of the sulfide mound is approximately 60 m. c Lerokis zone 5 situated at a topographic height of 550 m on a prominent ridge (view approximately north). The host depression for the barite ore deposit is evident. Conformable, post-mineralization volcaniclasticsediment overlies the massive sulphide mound. Pre-mined and remediated zones 1, 2 and 3 are also shown. d Exposed sulfide mound at Lerokis zone 5, height of the sulfide mound above the pit floor is approximately 15 m. Extensive gossanous material surrounds both deposits

at the surface. In plan, the mounds are broadly arcuate. The sulfide mounds are blocky in appearance, with clasts of massive pyrite ranging in diameter from a few centimeter up to boulders some 30 cm across (Fig. 6). Talus and redeposited sulfides occur marginal to the mounds where matrix supported angular fragments of massive sulfide are held in a fine-grained sulfide mud. Minor evidence for seafloor reworking is evident at Lerokis zone 5, where a 30 cm zone of interbedded sulfide and volcaniclastic material overlies the mound. Chert, gypsum and globigerina-bearing limestone overlie the Kali Kuning sulfide mound. At the margin of the sulfide mounds, finegrained poorly consolidated granular pyrite marks the contact zone (0.2–2.5 m) between the sulfide mound and the associated barite deposits. The mineralogy of the massive sulfide mounds is dominated by pyrite, accounting for >98% of all sulfides present with minor amounts of chalcopyrite and sphalerite. Typical of seafloor sulfides, the pyrite and chalcopyrite often show ‘‘porous’’ textures as well as collomorphic growth zones up to 3 mm across (Fig. 7a). The collomorphic pyrite tends to nucleate on and around euhedral pyrite grains (Fig. 7b). This texture appears most frequently at the margins 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 the sulfide mounds and particularly at the base of mounds and in the underlying footwall. Occasional banding of pyrite and chalcopyrite is evident on a centimeter scale. A later fracture network permeates the pyritic mounds, with a sulfide assemblage dominated by covellite, Fe-poor sphalerite and lesser amounts of tennantite and tetrahedrite and tabular barite laths (Fig. 7d). Overall, typical sulfide abundance within the mounds are pyrite >> chalcopyrite > sphalerite > covellite/marcasite/tennantite/tetrahedrite and bornite. No sulfide mound is evident at the Lerokis zone 4 deposit despite drilling beneath the barite mineralization. All three deposits are surrounded by extensive gossanous material.

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

82 Fig. 7 Photomicrographs of polished sulfide sections: py pyrite; cpy chalcopyrite; sp sphalerite; ba barite; ten tennantite; cov covellite; si silica. a–e field of view = 5 mm, f field of view = 2.5 mm. a Collomorphic pyrite (sample 097056, Lerokis zone 5) within the massive pyritic sulfide mound. b Euhedral pyrite cores overgrown by collomorphic pyrite (sample 097009). c Chalcopyrite replacement of pyrite (sample 097059). d Fracture-fill sulfide assemblage (sample 097016). e Disseminated pyrite within an altered volcanic clast in the Lerokis zone 5 footwall, rimmed by pyrite and a fracture-fill assemblage of sphalerite and covellite (sample 097122). f Detail of fracture-fill sulfide assemblages in e, showing variation in tennantite composition

Stockwork zones

Barite deposits

The pyritic mounds at Kali Kuning and Lerokis zone 5 are underlain by stockwork zones which reach to a depth of >210 m below the Lerokis zone 5 deposit and >150 m below Kali Kuning. The stockwork zone is hosted by hydrothermally altered, locally vesicular, silicified volcanic rocks. Brecciated, angular volcanic footwall clasts are rimmed by sulfides up to 4 mm in thickness and also contain disseminated sulfides (Fig. 7e). The stockwork veins range from...