THE BATU-HIJAU PORPHYRY COPPER-GOLD DEPOSIT, SUMBAWA-ISLAND, INDONESIA

The Batu Hijau porphyry Cu-Au deposit lies in southwest Sumbawa Island , Indonesia. It is a world-class porphyry Cu deposit in an island arc setting, and is typical of this deposit type in most features, includi ng igneous association, morphology, hydrothermal alteration and minera lisation style. The region was not previously recognised as a porphyry Cu province; disseminated Cu sulphides were first recognised in float samples in southwest Sumbawa in 1987. Associated stream sediment samp ling identified a broad area of anomalous Au and Cu in an area of grea ter than 5 km2 around Batu Hijau, including 169 ppb Au in BLEG samples and 580 ppm Cu in stream silts 1 km from the deposit. Mineralisation in bedrock at surface contains > 0.1 wt % Cu and > 0.1 ppm Au over an area of 0.6 km x 1.2 km, including a zone 300 m x 900 m containing > 0 .3 wt % Cu. Areas with elevated Mo ( > 30 ppm) form a distinctive annu lus around this Cu-rich zone. Batu Hijau mineralisation is hosted in a tonalite intrusive complex, and diorite and metavolcanic wallrocks. T here are no post-mineralisation igneous intrusions or breccia pipes wi thin the deposit. The main tonalite intrusion forms a stock in the cen tre of the deposit, where it generally displays intensely pervasive po tassic (biotite with magnetite-quartz) alteration and hosts most of th e higher grade mineralisation. Younger tonalite dykes intruding the ce ntre of this stock are generally less altered and mineralised than the older tonalite.The core zone of potassic alteration grades outward in to extensive propylitic alteration (chlorite-epidote), with both varia bly overprinted by widespread fracture controlled intermediate argilli c alteration (sericite-chlorite), and minor phyllic (sericite-pyrite) and sodic (albite) alteration. Argillic (sericite-kaolinite) and advan ced argillic (kaolinite-alunite-pyrophyllite) assemblages occur near s urface. Copper and Au grades within the orebody show a positive correl ation with quartz stockwork intensity, although disseminated Cu sulphi des are also common. Chalcopyrite and bornite are the principle hypoge nal minerals, with minor chalcocite. Oxidation extends to a depth of 5 m to 85 m below surface across the deposit, and is underlain by weak supergene mineralisation. Drill testing of the deposit down to 650 m b elow surface reveals a single cylindrical to conical orebody of 334 mi llion tonnes grading 0.8 wt % Cu and 0.69 gm per tonne Au; the depth e xtent of mineralisation is unknown.