The Ok Tedi porphyry and skarn copper-gold deposits on Mount Fubilan (2053 mASL) are located in the Star Mountains of far western Papua New Guinea, 18 km east of the Indonesian border. The pre-mining deposits contained approximately 4.8 Mt of copper and 490 tonnes of gold (actual metal production + full metal content of reserve). The mine is operated by Ok Tedi Mining Limited (52% PNG Sustainable Development Program, 18% Inmet Mining Ltd, 30% PNG Government).
Copper and gold had been reported from the district from as early as 1875. Reconnaissance exploration by Kennecott in 1968 recognised float of oxidized massive-sulphide creek float and traced this back to the skarn zone of Sulphide Creek at Ok Tedi. Initial drilling of the skarn was followed by recognition of the porphyry mineralisation. Kennecott was unable to agree to terms of mining with th PNG government and withdrew from the project in 1975. By late 1976 a consortium, headed by BHP had signed an agreement with the PNG Government, and mine construction commenced in 1981, followed by first production in May 1984. BHP Billiton withdrew from the project in 2002 and assigned their holding to the PNG Sustainable Development Program.
Ok Tedi is located within the Papuan Fold Belt that forms the spine of the island of New Guinea and marks the southern margin of the transition zone from the stable Australian plate to the Pacific plate in the north. This transition zone underwent rapid uplift from 2.6 to 0.7 Ma. The geology of the Star Mountains is characterised by generally shallow dipping Mesozoic and Cainozoic continental margin marine sediments, which have been forcefully intruded by middle Miocene to Pleistocene stocks, with coeval volcanism, and rest on a Palaeozoic metamorphics-granite basement exposed to the south. Deformation is principally expressed as large scale thrusting which was contemporaneous with intrusion of the calc-alkaline stocks and associated volcanism.
In the mine area, the sequence commences with 1300 to 1500 m of middle to late Cretaceous Ieru Formation marine mudstone to clayey siltstones and glauconitic sandstone, disconformably (or structurally) overlain by the Late Oligocene to Early Miocene Darai Limestone which comprises 300 to 600 m of massive dark grey limestone and conformably overlying thin bedded grey calcareous mudstone and siltstone with lenses of limestone, sandstone and marl. The Darai Limestone is in turn overlain by calcareous mudstone and siltstone with lesser, but prominent limestone horizons that comprises the Pnyang Formation. The earliest volcanic activity in the district is represented by tuffaceous sandstone in the Mid-Miocene Birim Formation. The latter is overlain by volcanoclastic sediments of the Awin Formation which is the eroded remnant of a Late Miocene to Pliocene stratovolcano.
Several calc-alkaline lithologies form stocks at Ok Tedi, the main phases on Mount Fubilan being:
i). The 2.6 Ma Sydney monzodiorite occurring as a 1.5 x 2.5 km stock with porphyritic to subporphyritic to equigranular textures, and is essentially to the south of Mt Fubilan. It contains andesine, clinopyroxene, orthoclase, hornblende and biotite in decreasing order of abundance, with accessory sphene, apatite and magnetite,
ii). The younger Fubilan Monzonite Porphyry, which occurs within and in contact with the Sydney monzodiorite, mainly as a downward tapering stock in the core of Mount Fubilan with a surface diameter of 850 m, and is the main host to mineralisation with alteration dated at 1.1 to 1.2 Ma. In the upper part of the Fubilan Monzonite Porphyry stock there is a downward tapering 350x125 m cone, surrounded by a series of hydrothermal and intrusive breccia dykes. It consists of phenocrysts of oligoclase, orthoclase, quartz, and hydrothermal biotite replacing hornblende and biotite, with accessory apatite, sphene, rutile after sphene, and magnetite in a felsic glassy matrix.
iii). Late phase, usually basic dykes which are up to 3 m thick and cut all rock types, constituting ~0.1% of the intrusive complex.
iv). Hydrothermal and intrusive breccia dykes
Two phases of potassic alteration (characterised by micas) are observed, centred on the quartz-stockwork core of the Mt Fubilan Monzonite Porphyry stock. The central quartz stockwork comprises silica flooding with quartz stockwork veining ±sericite-clay that forms a carrot like mass. The first phase of potassic alteration is characterised by dark brown to greenish brown primary igneous mica, K-feldspar, rutile and commonly accompanies chalcopyrite and martitised magnetite. The second phase is more intense and extensive with phlogopite, red brown micas, K-feldspar and rutile and is commonly associated with chalcopyrite, bornite, molybdenite and gold. The potassic zone is flanked by argillic alteration and very limited propylitic alteration in the adjacent Sydney monzodiorite. The argillic alteration is often only weakly developed and comprises an assemblage of kaolinite and montmorillonite impregnated with martitised magnetite, iron oxides, minor secondary sphene and rare sulphide minerals.
Although the orebody is defined by grade boundaries, the great bulk of the ore is within the intrusives and is divided into the:
i). Leached cap - a remnant cap varying from 40 to 290 m in thickness with an average of 0.05% Cu,
ii). Oxide copper - an irregularly shaped zone, averaging around 0.5% Cu, with deep roots to depths of >150 m in fault zones, and comprising cupriferous goethite, copper oxide, copper sulphide with lesser copper phosphates or carbonates, cupriferous clays and native copper,
iii). Enriched copper - mainly chalcocite and digenite on fracture and in veinlets replacing chalcopyrite and pyrite to grades of 1 to 4% Cu over variable thicknesses of from 50 to 300, and locally 400 m,
iv). Hypogene ore - composed of chalcopyrite, pyrite, molybdenite and minor bornite, with grades of 0.35 to 0.45% Cu, 0.011% Mo, 0.2 g/t Au, but decreasing to 0.1 to 0.2% Cu, where greater than 400 m thick, although Mo increases to 0.02% at depth and towards the periphery,
v). Gold cap - in an annulus around the quartz stockwork core within the leached cap, decreasing with depth from a grade of 5 g/t Au near the surface as electrum in secondary iron oxides, to 0.5 g/t in the enriched copper zone where subhedral inclusions of gold are within chalcocite and digenite grains, and
vi). Skarns - a series of endo- and exo-skarns are recognised including calc-silicates (diopside, pyroxene, garnet), massive magnetite, massive sulphide (pyrite, pyrrhotite and some chalcopyrite) with sulphides post-dating magnetite. In general the paragenesis of the skarns comprised prograde garnet-pyroxene assemblages with retrograde epidote-actinolite-tremolite were replaced by magnetite and overprinted by sulphides. In the Edinburgh and Sulphide skarns, there is a lateral change outward from the intrusive from massive magnetite to massive sulphide. Sulphide mineralisation postdates the calc-silicate alteration and magnetite
Mining commenced in 1984, with an original resource of:
700 Mt @ 0.63% Cu. 0.011% Mo, 0.63 g/t Au.
The reserve at start-up in 1984, totaled 415 Mt of ore, comprising:
Leached cap - 18 Mt @ 0.05% Cu, 2 g/t Au,
Sulphide - 348.9 Mt @ 0.7% Cu, 0.56 g/t Au, 0.11% Mo,
Skarn - 28.8 Mt @ 1.25% Cu, 1.58 g/t Au,
Oxide copper - 19 Mt @ 1.74% Cu, 1.34 g/t Au.
At December 31, 2003, the remaining measured + indicated + inferred resource totaled:
663 Mt @ 0.76% Cu, 0.85 g/ Au, and
the proven + probable reserve was:
246 Mt @ 0.87% Cu, 0.93 g/t Au.
At December 31, 2008, the remaining measured + indicated + inferred resource totaled (OTML Annual Report, 2009):
391 Mt @ 0.58% Cu, 0.77 g/ Au, and
the proven + probable reserve was:
123 Mt @ 0.83% Cu, 1.19 g/t Au.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2009)
