The Gunung Bijih (or Ertsberg) mining district of West Papua (formerly Irian Jaya) contains a diverse group of large porphyry and skarn ore deposits. The district incorporates the super-giant Grasberg porphyry deposits associated with the 3.2 to 2.7 Ma Grasberg Igneous Complex, porphyry ores of the 4.4 to 3.0 Ma Ertsberg Diorite 2.5 km to the south, and a series of skarns deposits surrounding the latter and between the two intrusive complexes. Together these deposits account for near 80 Mt of copper and around 3900 tonnes of gold (including inferred resources). All are mined as part of a large integrated operation owned by PT Freeport Indonesia. The operation is one of the world's largest gold mines with an annual production (2007) of around 68 tonnes of gold and 560 000 tonnes of copper.
The mine is situated immediately adjacent to the 5030 m high Puncak Jaya, the highest mountain in Australasia, in the core of the Papuan Fold Belt that forms the spine of the island of New Guinea. The fold belt marks the northern margin of the stable platform of the northward migrating Australian continental plate, several hundred kilometres south of its convergent intersection with the current south subducting Caroline oceanic plate. The fold belt was initiated when the Australian plate entered the earlier north dipping subduction zone of the Melanesian Arc during the Miocene (at ~12 Ma) and deposited while that same northward subduction continued.
The mobile belt comprises thrust wedges of Proterozoic and Palaeozoic rocks overlain by Mesozoic marine clastics and Tertiary carbonates and platform sediments. In the mine area the Mesozoic is represented by >700 m of quartz sandstones, shales and the uppermost shale, sandstone and limestones of the Cretaceous Kembelangan Formation; overlain by the 1700 m thick Tertiary New Guinea Group. The New Guinea Group has been subdivided into the Paleocene Waripi Formation composed of ~300 m of grey to dark grey, thin bedded dolomite, dolomitic limestone, calcarenite, siltstone and white to light grey, usually calcareous sandstone with significant evaporite nodules and lenses; overlain by the Eocene to Lower Oligocene Faumi Formation which comprises a lower 70 m of grey-brown, locally sandy dolosparite and an upper 130 m of greyish-brown biosparite to biomicrite; followed by 50 m of Oligocene quartzose and calcareous sandstones and conglomerates and shales of the Sirga Formation; ~800 m of Late Oligocene to Miocene Kais Formation (or New Guinea Limestone), comprising a thick limestone succession. The Waripi Formation and to a lesser degree, the Faumi Formation are the principal hosts to skarn mineralisation in the district.
Within the Gunung Bijih mining district, the structure is dominated by an arc-parallel, WNW-ESE (110°) trending set of north dipping thrusts (with lesser strike-slip movement and strike-slip normal faults) and folds (limbs dipping at 50 to 80°), parallel to the major Mapenduma Thrust and anticline separating the Papuan Fold Belt and Fly Platform, 30 km to the south. A a second, NE to ENE trending set of steep faults also cuts the district.
Around 16 Pliocene hypabyssal intrusions have been delineated in the Gunung Bijih mining district, mainly dykes, sills or plugs with areas of outcrop ranging from a few to several hundred square metres. The Ertsberg Diorite and the Grasberg Igneous Complex (GIC) are the largest, with areas of a few square kilometres. These intrusions have similar chemistries, but the Grasberg Diorite is largely porphyritic whereas the bulk of the Ertsberg is dominantly equigranular. McDowell et al., (1996) reported 13 K-Ar ages from these intrusions that indicate magmatic activity in the district ranged from 4.4 to 2.6 Ma (mean 3.22 Ma). The Grasberg complex has returned dates of 3.2 to 2.8 Ma (K-Ar - McDowell et al., 1996) and 3.33 to 3.02 (Ar-Ar - Pollard et al., 2001).
The 2.5 x 1 km Ertsberg Diorite is mainly an even grained equi-granular quartz monzodiorite with lesser biotite-pyroxene diorite and porphyritic quartz monzonite dykes which cut the other two phases. This intrusive complex has been dated at 2.7 to 2.58 Ma (Pollard et al., 2005), and hosts, or is rimmed by, the Ertsberg skarn and the Ertsberg disseminated ores, the latter apparently related to a set of the quartz-monzonite dykes which are 5 to 15 m wide.
The Grasberg Igneous Complex (GIC), around 2.5 km to the NNW, is an upwardly flaring funnel shaped 1.7 x 2.4 km volcanic vent or diatreme (800 m diameter 1000 m below) with an intrusive core, composed of:
i). An early Dalam Igneous Complex (3.33 Ma; Pollard et al., 2005) comprising a lower Dalam Diorite phase and an upper andesitic phase which comprises matrix supported breccias, pyroclastics, volcaniclastic sediments and trachy-andesite lavas. On the margins, the upper andesitic phase is differentiated into a bedded volcanic unit, which is a thin band of bedded, water-lain volcaniclastic sediments that occur along the northeast and southwest edges of the complex, a unit of coarse andesite porphyry flows cut by dykes and sills, the Dalam Andesite and the core of breccias and pyroclastics, the Dalam Fragmentals. The Dalam Volcanic and Dalam Fragmentals are, respectively, polymict and monomict matrix supported breccias, typically containing 5% to 10%, but locally up to 30% clasts. The Dalam Igneous Complex represents an explosive, gas driven, space-generating eruption, which has produced a maar complex that has vented to the surface. It is now the un-eroded roots of a 2 to 3 km tall composite stratovolcano which has been removed by the strong uplift that produced the 5000 m Puncak Jaya and the surrounding mountain range.
ii). The Main Grasberg Intrusion (MGI) which is interpreted to have been passively emplaced as a non-venting plug in the centre of the Dalam phase rocks. It is characterised by coarse, relatively equigranular textures and a near lack of fragments. This is in turn divided into a). Early Main Grasberg - a 600 x 430 m stock of porphyritic quartz monzodiorite with a vertical extent of more than 2800 to 4250 m, dated at 2.83 Ma (?); b). Late Main Grasberg - a 900 m diameter stock of porphyritic quartz monzodiorite with associated dykes and a vertical extent of 2800 to 3800 m.
iii). Kali Intrusion - which occurs as a nest of quartz monzodiorite porphyritic dykes with crowded plagioclase-hornblende-biotite phenocrysts, and disseminated magnetite set in a finer groundmass of quartz, feldspar and biotite. The intrusion has been divided into a). Early Kali (3.16 Ma; Pollard et al., 2005) - an irregular 600 x 250 m stock-like dyke with a vertical extent of >4000 m, and b). Late Kali (3.13 Ma; Pollard et al., 2005) - mainly dykes and a 500 x 250 m stock with a vertical extent of more than 4000 m.
No volcanic deposits extend beyond the mapped limits of the GIC into the surrounding district.
The 1 km wide core of the Grasberg deposit has a strong associated potassic alteration suite of K feldspar-biotite-quartz-magnetite, grading out to a propylitic halo of epidote ±chlorite-magnetite-calcite represented by pockets and remnants. Strong magnetite (>8%) occupies a 600 x 300 m core to the potassic zone. The potassic alteration has been overprinted by intense phyllic alteration to form an exterior annular zone, ~500 m across, which is dominated by sericite+anhydrite+pyrite with small amounts of kaolinite, in places to within 400 m of the centre of the system. A 100 m wide zone of brecciated marble surrounds the GIC.
There are a number of pulses of mineralisation associated with the GIC. A major episode of pervasive alteration and disseminated Cu-Au mineralisation occurred in association with the Dalam Diorite that predated the intrusion of the MGI. A second major episode of Cu-Au mineralisation was post-MGI and pre-Kali emplacement. This event caused moderately intense alteration and ore mineralisation that was largely veinlet-hosted (Paterson and Cloos, 2005 and sources quoted therein).
The bulk of the copper ore is present as stockwork veins and veinlets and disseminations that postdate the potassic alteration, but predates the phyllic phase. Mineralisation occurs as a horseshoe shaped mass surrounding the Kali intrusive contact and extends from the surface at an elevation of 4200 m, to below 2700 m asl. In general, it comprises an inner annulus around 100 to 150 m outward from the barren core of >3% (much in excess of 4%) Cu equivalent, surrounded by a 150 to 250 m wide interval of 1 to 3% Cu equivalent ore with an outer diameter of 500 to 800 m. Copper minerals are dominated by chalcopyrite and bornite. Gold is closely associated with chalcopyrite and bornite and some is with digenite. High Au grade zones always coincide with the high Cu grade zone. A rind of strong pyrite forms at the contact between the GIC and the enclosing limestone, with no sulphides extending further into the limestone except as skarns in faults that the GIC intersects.
In the Ertsberg Diorite, mineralisation and alteration comprises: i). early feldspar stable potassic alteration with hairline bornite veining, ii). transitional green sericite veins with chalcopyrite and chalcopyrite-pyrite veins and endoskarn development and iii). late quartz-sericite-pyrite±chalcopyrite. The bulk of the ore at Ertsberg has been produced from high grade magnetite-rich, calcium/magnesian skarns which are characterised by magnetite and high temperature assemblages including forsterite, monticellite and minor melilite, as well as diopsidic clinopyroxene, anhydrite and phlogopite. At least 3 garnets have been recognised which decrease with depth. Talc, serpentine, tremolite-actinolite and chlorite are common retrograde minerals. Copper is texturally associated with early retrograde alteration. Chalcopyrite dominated in some bodies (eg. GB and Dom), while bornite is the principal copper sulphide at GBT. The endoskarn of this complex has been dated at 2.71 Ma, while the exoskarns have returned ages of 2.58 Ma (Pollard et al., 2005).
Mineralisation associated with the Ertsberg intrusive includes: The Ertsberg stockwork which contained a resource of 122 Mt @ 0.54% Cu, 0.90 g/t Au in 2005.
The skarn mineralisation, which includes the: i). GB (Gunung Bijah) - 33 Mt @ 2.5% Cu, 0.8 g/t Au (the original reserve on which mining in the district was commenced), which is surrounded by Ertsberg Diorite near its NW margin; ii). GBT Complex (the vertically stacked GBT, IOZ & DOZ), 1.5 km east of GB on the northern contact, with reserves of >230 Mt @ 1% Cu, 0.8 g/t Au, iii). Dom Skarn, 0.5 km south of GBT, partially enclosed by the intrusive near its SE margin, with >70 Mt @ 1.4% Cu, 0.4 g/t Au, iv). Big Gossan within a fault zone cutting sediments to the west of the Ertsberg Diorite with 33 Mt @ 2.81% Cu, 1 g/t Au, v). Kucing Liar (dated at 3.42 Ma, the oldest mineralisation in the district, predating the Dalam Diorite) is associated with a fault zone between the two intrusive complexes, but close to the Grasberg complex, contains >225 Mt @ 1.42% Cu, 1.57 g/t Au.
The total proven+probable reserve at the Grasberg/Ertsberg operation at the end of 2007 were:
2.712 Gt @ 1.04% Cu, 0.90 g/t Au (Rio Tinto, 2008).
The total measured + indicated + inferred resource (which are in addition to the reserves) at the same date were:
3.049 Gt @ 0.55% Cu, 0.49 g/t Au (Rio Tinto, 2008).
Production in 2006 from the Grasberg open pit totalled 57.5 Mt of ore, while an additional 19.5 Mt was from underground in the Ertsberg operation.
(Source: Porter Geoconsultancy, http://www.portergeo.com.au/, 2008)
