Regional Geology

Batu Hijau Porphyry copper gold

The Batu Hijau porphyry copper-gold deposit is situated in southwest Sumbawa Island, Indonesia that it is a world-class deposit of copper and gold, containing mineable reserves of 914 million tonnes grading 0.525 % Cu and 0.403 g/t Au (Clode et al., 1999). The Batu Hijau deposit lies along the tectonically active east-west trending Sunda-Banda magmatic arc. The western part of the Sunda-Banda arc is mostly mineralised by low-sulphidation epithermal systems, e.g. Mangani, Lebong Tandai and Lebok Donok in Sumatra as well as Gunung Pongkor, Cikidang and Cirotan in West Java. The mineralisation style changes to the east (e.g. Central-East Java and Sumbawa Island), which is is marked by discovery of the Porphyry-type at Selogiri area (Central Java) (Imai et al., 2007) and the Batu Hijau porphyry deposit in Sumbawa Island (Maula and Levet, 1996). The southwestern part of Sumbawa Island is underlain by Late Oligocene to Middle Miocene low-K calc-alkaline to weakly alkaline andesitic volcaniclastic rocks, associated with low-K intermediate intrusions and minor shallow marine sedimentary rocks and limestones. The low-K intrusions include porphyritic andesite, hornblende diorite, quartz diorites and tonalite porphyries.

Copper and gold mineralisation is closely related to the emplacement of the tonalite porphyry intrusions. During the emplacement and cooling of the causative tonalite porphyry intrusions, the magmatic hydrothermal fluids are exsolved and interact with the wall rocks, mixing with meteoric waters and form multiple stages of alteration zones. The nature of ore-related fluids is an essential issue in the study of mineral deposits. Three approaches are commonly used to constrain the characteristics of the ore fluids: stable isotopes, fluid inclusions and mineral chemistry. Stable isotope study is mostly employed to determine the source of ore fluids. Mineral chemistry and fluid inclusion are utilised to evaluate the physicochemistry and evolution of the ore-related hydrothermal fluids (e.g. Walshe, 1986; Zhu and Sverjensky, 1992 and Yang and Lentz, 2005).

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