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The Geologic Setting of Intrusion-Related Hydrothermal Systems near the Batu Hijau Porphyry Copper-Gold Deposit, Sumbawa, Indonesia

By
Steve Garwin
Steve Garwin
Centre for Global Metallogeny, Department of Geology and Geophysics, University of Western Australia, Nedlands, Western Australia 6907, Australia
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Published:
January 01, 2002

Abstract

The Batu Hijau deposit, 914 million tonnes at an average grade of 0.53 percent copper and 0.40 g/t gold, lies within a 12 by 6 km district that contains several mineralized centers. The oldest rocks exposed consist of an Early to Middle Miocene andesitic volcaniclastic succession. This sequence has been cut by several phases of intermediate to felsic intrusion, Middle Miocene to the mid-Pliocene in age, and a late andesitic diatreme and dike complex. The volcaniclastic rocks and intrusions in the district, typically of low K, calc-alkaline affinity, form part of the Sunda-Banda magmatic arc, which is underlain by oceanic crust near Sumbawa.

The margins of easterly elongate quartz diorite plutons focused brittle deformation, dike emplacement, and quartz vein deposition throughout the Neogene to Holocene. The reactivation of preexisting faults and zones of crustal weakness, and the intersection of fault and fracture zones with the complex margins of premineralization composite plutons, are inferred to have localized emplacement of felsic intrusions associated with porphyry-style mineralization at Batu Hijau and three peripheral porphyry systems.

Hydrothermal alteration centered on the Batu Hijau deposit and peripheral porphyry systems shows a progression of alteration from central biotite-magnetite ± oligoclase through proximal actinolite and distal epidote-chlorite to background chlorite-calcite. Late, structurally controlled feldspar-destructive alteration that overprints all types of early alteration includes intermediate argillic, sericitic/paragonitic, illitic, and advanced argillic types. The geometry and clay-mica mineral assemblages of these zones indicate higher temperatures and more acidic fluid conditions near Batu Hijau than indicated for late alteration zones elsewhere in the district.

At Batu Hijau, hypogene chalcocite, digenite, bornite, and native gold compose part of early A-veinlets, and chalcopyrite ± bornite occur in transitional B-veins. Late pyritic D-veins contain chalcopyrite ± minor bornite. The peripheral porphyry systems lack the early copper-sulfide assemblages and are characterized by late-stage pyrite-chalcopyrite in reopened parts of early, copper-poor A- and B-veins. Elevated gold and silver values are associated with pyrite ± sphalerite ± galena ± chalcopyrite ± tennantite-bearing, comb and banded quartz veins that occur in late structurally controlled zones. The general patterns of metal zoning with respect to the porphyry centers indicate central Fe, Cu, and Au, proximal Mo, and distal Pb, Zn, Ag, Au, and As anomalies. Silver/gold ratios also display a systematic increase from ~1 to 2 in the Cu-Au core to >50 in the Pb-Zn halo to each center.

SHRIMP 206Pb/238U geochronology indicates four major felsic intrusive episodes, each separated by 0.6 to 0.9 m.y. and related to a distinct porphyry center with ages that range from 5.9 to 3.7 Ma. The duration of the Batu Hijau hydrothermal system is 80 ± 80 k.y. (2a), as determined from the 40Ar/39Ar closure of early biotite and late sericite (3.73 ± 0.08 Ma vs. 3.65 ± 0.02 Ma). The causative tonalite intrusions at Batu Hijau were emplaced at a depth of <2 km (± 0.5 km) and hornblende phenocrysts formed in magma chambers that reached at least 9 km below the mid-Pliocene paleosurface, as indicated by the results of amphibole-plagioclase thermobarometry.

The Batu Hijau deposit displays many of the characteristics of global porphyry copper systems. However, significant differences are related to the low K, calc-alkaline composition of the tonalitic melts at Batu Hijau. The resultant magmatic-hydrothermal fluids are responsible for the K-poor alteration types that distinguish this deposit from others, particularly those porphyry systems associated with quartz monzonite in continental settings. At Batu Hijau, secondary oligoclase supplants K feldspar in the central biotite-magnetite zone, and locally paragonite proxies for sericite.

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Special Publications of the Society of Economic Geologists

Integrated Methods for Discovery: Global Exploration in the Twenty-First Century

Richard J. Goldfarb
Richard J. Goldfarb
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Richard L. Nielsen
Richard L. Nielsen
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Society of Economic Geologists
Volume
9
ISBN electronic:
9781629490335
Publication date:
January 01, 2002

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