Batu Hijau is a calc-alkaline, island are porphyry copper-gold deposit located in the southwest corner of Sumbawa Island in the Banda archipelago of southern Indonesia. Proven and probable reserves are 913 million tonnes at an average grade of 0.53 per cent copper (10.631 billion pounds Cu) and 0.41 g/t gold (389 tonnes Au). Construction of the deposit infrastructure began in October 1996, and formal approval for the project mobilisation was granted by the Indonesian government in May 1997. Preoperational testing and start-up of the first grinding line is scheduled for mid-year 1999. Copper and gold mineralisation are genetically related to at least three tonalite porphyry dykes and small plugs which intrude a quartz diorite stock that was emplaced into andesitic to dacitic pyroclastic rocks and lava flows. Faults are uncommon, and where they occur they do not significantly displace hydrothermal alteration and mineralisation zones. The general sequence of hydrothermal rock alteration and sulphide mineralisation at Batu Hijau are similar to those features of other island are porphyry copper deposits, with a few notable exceptions. The most obvious differences are that sodic plagioclase (albite, oligoclase) supplants K-feldspar which is typically present in the early, high-temperature portion of porphyry copper deposits, and that late-sulphidation of the early, low-sulphur copper assemblage digenite-magnetite is much less developed at Batu Hijau than at many other porphyry copper deposits. The temporal progression of hydrothermal rock alteration and concomitant sulphide and oxide minerals is: black biotite-quartz-digenite-magnetite +/- bornite double right arrow green biotite-sodic plagioclase-quartz-chlorite-bornite-magnetite double right arrow sodic plagioclase-green mica (chlorite-sericite)-quartz-bornite-chalcopyrite double right arrow white mica (sericite +/- chlorite)-quartz-chalcopyrite-pyrite double right arrow quartz-sericite-andalusite double right arrow pyrophyllite- kaolinite +/- dickite, alunite, barite, pyrite, chalcopyrite and bornite. Weathering has resulted in the development of a weak secondary copper horizon which blankets the top of the deposit. Although quartz veinlets formed throughout copper-gold mineralisation, four types of quartz veins can be distinguished. The quartz content of early ('A' and 'A family') veinlets is highly variable (<10 to >90 per cent), but they are characteristically comprised of quartz, black biotite and magnetite +/- sodic plagioclase and copper sulphide minerals. Halos to these early veinlets are to <1 to similar to 2 mm wide, have irregular margins, and have mineralogy similar to the veinlets, but with less quartz and sodic plagioclase. Some later 'A' veinlets contain green instead of black biotite. 'A' and 'A family' veinlets are characteristically irregular and discontinuous to whispy, and probably formed prior to complete solidification of the intrusion. 'AB' veinlets have characteristics which are transitional between 'A family' and 'B' veinlets. They are generally thicker than 'A family' veinlets, are dominated by quartz without a center line, and are less linear than 'B' quartz veinlets. In addition to quartz, 'AB' veinlets and their halos contain sodic plagioclase and chlorite +/- biotite in association with bornite and chalcopyrite +/- magnetite. They post-date the 'A family' veinlets associated with sodic plagioclase, and are younger than the quartz-chalcopyrite +/- molybdenite veins with sodic plagioclase envelopes. 'B' veinlets are less common than 'A family' and 'AB' veinlets. They are characteristically 2 to 3 mm wide, milky white quartz veinlets, but vary from 1 to >15 mm in width and may be clear or have a light pink hue. 'B' veinlets have continuous planar walls and commonly have irregular center lines filled with sulphide +/- carbonate and sulphate minerals, with chalcopyrite as the dominant sulphide phase. 'B' veinlets may contain bornite in higher grade, central portions of the deposit and pyrite in lower grade, peripheral portions. Magnetite generally is not present. Some 'B' veinlets contain sodic plagioclase which may be concentrated along the margins of the veinlet or, less commonly, along the vein center. Cross-cutting relationships document multiple periods of 'B' veinlets, each of which formed after the causative intrusion had solidified to where continuous linear fractures could be maintained. Sulphide-dominant 'D' veinlets developed late in the history of the porphyry copper system. These veinlets are dominantly pyrite, but may include bornite and chalcopyrite. Feldspar destructive alteration halos are well developed and contain silicate, sulphate and oxide minerals characteristic of sericitic or advanced argillic alteration, indicating that the sulphide minerals precipitated from a moderately to strongly acidic fluid. Within the halos of the late pyritic veinlets feldspar is replaced by sericite and clay minerals, biotite is altered to chlorite or sericite, and magnetite is partially to completely converted to pyrite +/- specularite. Where 'D' veins pass through moderate-to high-grade copper mineralisation, they may enrich the bulk rock copper grade, whereas 'D' veins identified in the peripheral parts of the system tend to be barren of copper-gold mineralisation, and may have leached copper from the rock. 'D' veins are most common in the upper 400 m of the deposit. In areas of intense 'D' veinlet development, the veinlets have outer halos where rock texture is destroyed and replaced by andalusite, sericite quartz and pyrite, and inner halos of pyrophyllite, quartz and pyrite. The inner halos commonly have a high-sulphidation state copper assemblage of pyrite, chalcopyrite and bornite. Post-'D' veinlet age, silver-and locally gold-rich base metal-sulphosalt veinlets and veins constitute the final phase of copper-gold mineralisation. These veinlets are comprised primarily of sphalerite, galena, chalcopyrite, pyrite and tennantite, and are accompanied by very weak argillic alteration.
