Team members: F.M. Meyer, J. Kolb, A. Arifudin
Introduction
The Batu Hijau porphyry Cu-Au deposit is located in southern Sumbawa Island, Indonesia (Fig. 1).
Fig. 1 Map of the Indonesian archipelago, which shows the location of the Batu Hijau deposit on the Sumbawa Island
The Batu Hijau deposit is being mined by Newmont Nusa Tenggara company, a joint venture between Newmont Mining Corporation (45%), Sumitomo Corporation (35%) and PT Pukuafu Indah (20%). It is a world class deposit which contains 914 million metric tonnes of ore at an average grade of 0.53% Cu and 0.40 g/t Au. Batu Hijau has some unique features, although it shares many similarities with other porphyry Cu-Au deposits worldwide.
Geology and hydrothermal alteration
The wall rock units of the deposit consist of interbedded andesitic lithic breccia and fine grained volcaniclastic rocks as well as intrusions of porphyritic andesite and quartz diorite bodies. These units are intruded by multiple phases of tonalite porphyry stocks (Fig. 2).
Fig. 2 Geological map of the Batu Hijau copper-gold porphyry deposit (after PT. NNT, 2002).
Hydrothermal alteration and mineralisation developed in four temporally and spatially overlapping stages. 1) The early alteration stage is divided into biotite (potassic), actinolite (inner propylitic), chlorite-epidote (outer propylitic) and skarn-related calc-silicate zones. Copper sulphides consist of bornite, digenite-chalcocite and chalcopyrite and are closely related to the biotite alteration zone. Gold is associated with the copper sulphides within early “A” and “B” veinlet types mainly as inclusions in the sulphides and less frequently as free gold. 2) The transitional alteration stage is typified by a chlorite-sericite (intermediate argillic) zone. 3) The late alteration stage is characterised by destruction of feldspar and the formation of pyrophyllite-andalusite (advanced argillic) and sericite-paragonite (argillic) zones. 4) The very late alteration stage is characterised by illite-sericite replacement of feldspar. Copper and gold mineralisation are directly related to the emplacement of the tonalite porphyries. The tonalities have unusually low potassium contents and are therefore classified as low-K calc alkaline tonalities. The low K reflects the general absence of alkali feldspar within the tonalities (biotite alteration zone). This may indicate a lack of magma contamination by crustal or sea floor sediments.
Present project on the Batu Hijau deposit
The present research project is concentrated on “Petrology and chemical composition changes of diagnostic hydrothermal minerals within the Batu Hijau porphyry copper-gold deposit”. The primary objectives of this research are to:
The secondary objectives of this study are to:
This Ph.D. project was carried out by Arif Arifudin under the supervision of Professor F. Michael Meyer and Dr. Jochen Kolb, and supported by the DAAD (Deutscher Akademischer Austausch Dienst) and Newmont Nusa Tenggara company, Indonesia.