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Elang

Sumbawa, Indonesia

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The Elang porphyry Cu-Au deposit is located in south-central Sumbawa in Indonesia, ~60 km due east of the Batu Hijau porphyry Cu-Au mine (#Location: 8° 58' 1"S, 117° 22' 48"E).

Epithermal vein mineralisation was discovered in the Elang area during a regional stream sediment sampling and mapping program in 1987 and 1988. Subsequent reassessment for porphyry Cu potential revealed a large low grade resource. Further exploration from 2002 significantly added to the resource to the south under a 200 m thick lithocap.

As at Batu Hijau, the Elang deposit lies within the east-west trending Sunda-Banda magmatic arc at the convergent intersection of the Australian-Indian and the Eurasian plates. The northern half of Sumbawa is occupied by recent volcanoes, while the southern segment, where Batu Hijau andElang are located, comprises oceanic crust overlain by low K calc-alkaline to weakly alkaline andesitic volcanics and volcaniclastics, associated intermediate intrusives and minor shallow marine sediments and limestones.

The main Elang deposit is located within a cross-cutting NNE trending structural corridor that also embraces the Kuda Mati and Gerbang prospects to the north.

Copper gold mineralisation is associated with Pliocene ~4 to 2.7 Ma quartz-diorite and tonalite intrusions within a thick pile of andesite volcanic, volcanogenic marine sedimentary rocks and hornblende diorite. Mineralisation is associated with three recognised porphyry intrusive phases, although the contacts of each phase are not clearly defined. The non-outcropping Elang Quartz Diorite, is interpreted to be the oldest and best mineralised of the three, and is characterised by a porphyritic texture and abundant large feldspar phenocrysts set in a fine grained mafic groundmass (Hoschke et al., 2013; Maula and Levet, 1996).

Two intra-mineral intrusive episodes are recognised, known as the Delta and Echo tonalites respectively. The Delta Tonalite is characterized by a porphyritic texture with 'quartz-eye' phenocrysts in an equigranular groundmass. The Echo Tonalite has a more porphyritic texture, with abundant 1 mm size micro phenocrysts, and is less strongly altered and mineralised than the other two porphyritic lithologies. These intra-mineral porphyries contain less magnetite than either the Elang Quartz Diorite or the immediate wall rocks, resulting in a relative weak signature in the ground magnetic data (Hoschke et al., 2013; Maula and Levet, 1996).

Diatreme breccias occur along the contact of a late stage dacite porphyry intrusion, cutting the earlier intrusives. These breccias include clasts of the dacite wall rocks and tonalite and quartz diorite, but do not host significant copper-gold mineralisation. The ridges to the west are capped by rhyodacite volcanic rocks, which are strongly altered and leached, almost obliterating the primary textures (Hoschke et al., 2013; Maula and Levet, 1996).

Prograde potassic alteration in the form of quartz, biotite, magnetite and minor actinolite, is recognised in the sub-surface, in the centre of the deposit, but does not outcrop. This core is surrounded by a broad outer halo of propylitic alteration, and overlain by retrograde intermediate argillic and argillic alteration above the centre of the system. The intermediate argillic assemblages comprises quartz, clay, sericite, chlorite, magnetite and hematite which outcrops at lower elevations and defines a NNE trending zone. This zone is commonly capped by an intense argillic alteration zone at higher elevations, comprising assemblages of clay, pyrite, quartz and sericite, which is the most widespread alteration exposed in the district. A broad zone of advanced argillic alteration also forms a crescent shaped cap over the ridges to the west of Elang, where the primary mineralogy has been completely destroyed by a replacement assemblage of vuggy sillica, pyrophillite, diaspore, haematite, alunite and minor clay (Hoschke et al., 2013; Maula and Levet, 1996).

Phyllic alteration assemblages of quartz, sericite and pyrite are poorly represented in outcrop, occurring as envelopes to fractures and veins, overprinting other alteration types (Maula and Levet, 1996).

Vein densities, and to a lesser extent, pyrite and chalcopyrite ratios, provide strong vectors towards the mineralising centre of the porphyry deposit (Maula and Levet, 1996).

The deposit, as defined by the 0.3% Cu contour, is elongated in a NNE-SSW direction, with a long dimension of 1900 m, comprising a southern rectangular section that is ~1250 x 1200 m, centred on the Echo and Delta tonalites, which are surrounded by intrusive breccias, and a narrower 250 x 650 m 'handle' to the NNE, centred on a diorite, with lesser Delta and Charlie tonalites. A body of dacite and diatreme breccia lies along and outside of the eastern margin of the main zone of mineralisation. The alteration system occurs as an elongated NNE-SSW to NE-SW corridor that covers a much larger area than the 0.3% Cu contour, mainy composed of argillic and advanced argillic alteration over a width of ~2 km and length of >5 km (Hoschke et al., 2013).

An initial resource of 600 Mt @ 0.4 g/t Au, 0.35% Cu was quoted by Maula and Levet (1996), whilst Ball (2011) quotes an estimate of 2425 Mt @ 0.33 g/t Au, 0.31% Cu.

Ore reserves and mineral resources at 31 December 2015 for Newmont's 48.5% share were (Newmont, 2016):
    Measured + indicated resource - 715.90 Mt @ 0.35 g/t Au, 0.34% Cu,
    Inferred resource - 182.0 Mt @ 0.24 g/t Au, 0.24% Cu,
  Measured + indicated + inferred resources, Newmont's 48.5% share - 897.9 Mt @ 0.33 g/t Au, 0.32% Cu,
  TOTAL resources, 100% of deposit - 1851.3 Mt @ 0.33 g/t Au, 0.32% Cu.

The most recent source geological information used to prepare this summary was dated: 2013.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Hoschke, T., Schmeider, S. and Kepli, S.,  2013 - Geophysics of the Elang Cu-Au Porphyry Deposit, Indonesia, and Comparison with Other Cu-Au Porphyry Systems: in Vearncombe, J., 2013 Bali 2013 East Asia: Geology, Exploration Technologies and Mines, 27-29 May 2013, Bali, Indonesia, Australian Institute of Geoscientists, Symposia, Extended Abstracts,   Bulletin No. 57 - 2013, pp. 34-35.
Maula, S. and Levet, B.,  1996 - Porphyry copper gold signatures and the discovery of the Batu Hijau deposit, Sumbawa, Indonesia: in   Porphyry Related Copper and Gold Deposits of the Asia Pacific Region, Conf Proc, Cairns, 12-13 Aug, 1996 AMF, Adelaide    pp 8.1-8.13.


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