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Petrology and chemical composition changes of diagnostic hydrothermal minerals within the Batu Hijau porphory copper-gold deposit


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).

Geologicalmap
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:

  1. formulate new reviewed hydrothermal alteration and mineralisation sequences based on the field petrologic observation and detailed petrography and geochemistry investigation
  2. document the changes in bulk-rock chemical composition that occur from one hydrothermal alteration zone to another
  3. recognise systematic changes in the major- and minor element composition of the diagnostic hydrothermal minerals
  4. evaluate the hydrothermal fluid composition and its evolution, leading to the formation of alteration zones
  5. calculate mass changes and geothermobarometry for the hydrothermal alteration and mineralisation evolution

The secondary objectives of this study are to:

  1. enhance better understanding on the geochemical characteristics of the Batu Hijau porphyry copper-gold deposit
  2. provide relevant information in order to compare with the results of other major porphyry copper-gold deposits around the world
  3. propose a possible exploration guide based on results of the lithogeochemistry and mineral chemistry studies

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.

 


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