ORE DEPOSIT DESCRIPTIONS
Image: Dump trucks in the mist, Ok Tedi Mine, PNG.
The tour was divided into the following optional modules and visited eleven mines and projects. It was supported by full day overview seminars in each of Port Moresby, Manila and Jakarta, presented by renowned local experts and industry specialists.
Module 1 - Papua New Guinea - Sun 13 to Fri 18 April, 1997
Module 2 - Philippines - Sat 19 to Sat 26 April, 1997
Module 3 - Indonesia - Mon 28 April to Wed 7 May, 1997
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MODULE 1 - PAPUA NEW GUINEA
Ok Tedi is a large porphyry copper-gold deposit in PNG, approximately 500 km to the east of Grasberg in the same porphyry trend. The orebody is associated with a Pliocene diorite-monzodiorite complex and had reserves in 1995 of 460 Mt @ 0.71% Cu, 0.7 g/t Au. The main orebody being exploited is mainly within the intrusive Fubilan Porphyry stock, although high grade skarn mineralisation also exists. A low grade leached or un-mineralised zone is found at the surface, underlain by a generally flat lying higher grade zone. The main Fubilan Porphyry exhibits pervasive potassic alteration with a barren quartz-stockwork core. Potassic alteration comprises an early chalcopyrite-marmatised magnetite phase and a later chalcopyrite-bornite-molybdenite-gold episode. The mine is controlled and operated by BHP Minerals Ltd.
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The operation at Porgera, which was commissioned in 1990, was based on the lower grade, open pit, Waruwari bulk gold orebody (originally calculated at 54 Mt @ 4.3 g/t Au) and the high grade, underground, tabular Zone 7 deposit (5.9 Mt @ 27 g/t Au) at the time of commissioning. In 1997, the total measured, indicated and inferred resource at a 1 g/t Au cutoff was 196 Mt @ 2.9 g/t Au, following a production of more than 20 Mt. The orebody is associated with the middle Miocene Porgera Intrusive Complex of micro-gabbro to diorite and feldspar porphyry, which cuts late Jurassic to Cretaceous pelitic terriginous shelf sediments. The main host sequence comprises Cretaceous sediments including mudstones, siltstones and sandstones, which are often calcareous. Regionally, these sediments are broadly folded about east to ESE trending axes, but dip steeply around the intrusive contacts, adjacent to which they are altered. The earliest faults are flat dipping and may be related to early mineralisation. Steeper NE and EW faults post date the intrusive complex and are mineralised. The main gold ore is associated with the E-W trending, 60 to 75 degree dipping Romaine Fault and steeper associated footwall and hangingwall fractures. There is an early low silica, high sulphur mineralisation phase and a later high silica, and low sulphur stage. Early gold is in NE faults with iron and base metal sulphides, while the later economic mineralisation is in quartz veins containing roscoelite (a vanadium mica) associated with the Romaine Fault system. The majority of gold is present as extremely fine grains contained within pyrite. The Romaine Fault is around 4 km long, with the main mineralised zone being over a strike length of 1 km, with a width of up to 20 m. The highest grade ore has a vertical extent of 240 m, and is found below a depth of 400 m from the surface. At depth the roscoelite mineralisation gives way to a gypsum rich zone with less sulphide. Mineralisation is commonly associated with the altered contact zones of the intrusive complex. Porgera is operated by the Porgera Joint Venture, controlled by Placer Dome.
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Lihir is a major bulk gold deposit with mineralisation represented by an earlier un-economic porphyry stage, and a later, overprinting low sulphidation advanced argillic phase accompanying the bulk of the gold. The orebodies are located within an area of 5.5x3.5 km embraced by the breached Luise Caldera on the east coast of Lihir Island. The island is one of a series of volcanic islands rising from a submarine platform to the north-east of New Ireland in Papua New Guinea. The host sequence of the Luise Caldera comprises intermediate lavas, tuffs and volcanic breccias, the youngest of the five Miocene to Pleistocene volcanic sequences/events that constitute the island. Mineralisation is coincident with a north-east trending zone of hydrothermally altered volcanics and breccias intruded at depth by rocks of monzonitic and syenitic composition and some andesite porphyry. Intense alteration generally forms a horizontal layering, with siliceous and potassic assemblages at depth, grading up to epithermal argillic phases nearer surface. Alunite and opal fill veins and stockworks near the surface, grade to quartz and adularia at intermediate depths, underlain by anhydrite and carbonate. Sulphide and gold mineralisation generally mimic the horizontal alteration pattern, although some follows steep 'feeder' fractures. The horizontal zonation represents a surface oxidation regime, passing down progressively into argillaceous altered and silica clay, through to a boiling layer and then the anhydrite sealed zone. Grades are best from the silica-clay to the boiling zones. Gold is predominantly fine grained and contained within pyrite and marcasite. The overall sulphide or reactive sulphur content averages 6%. Geothermal activity is still taking place. Total measured, indicated and inferred resources in 1997 were 480 Mt @ 2.8 g/t Au, while proved + probable ore totalled 104 Mt @ 4.37 g/t Au. Lihir is owned by Lihir Gold Limited.
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MODULE 2 - PHILIPPINES
Santo Tomas II
Santo Tomas II is an underground, block caving mine exploiting a pipe like deposit developed at the contact of a diorite plug and a suite of meta-volcanics. Reserves are 205 Mt @ 0.3% Cu, 0.6 g/t Au . The mine is owned by Philex Mining Corporation.
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Guinaoang is a porphyry copper prospect some 6 km to the south-east of Lepanto. The deposit was investigated by Gold Fields Asia, but not developed. It is part of the same porphyry system as Lepanto-Far Southeast. A later diatreme has partially destroyed the Pliocene intrusive porphyry. A substantial advanced argillic lithocap covering an area of 1.5 km2 and thickness of 200 to 300 m overlies the porphyry copper mineralisation. Quartz-alunite alteration has been recorded to a depth of 650 m. The resource is estimated at 326 Mt @ 0.4% Cu, 0.5 g/t Au. The project is currently being managed by Pacific Falkon Resources Corp.
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Lepanto - Mankayan District
The Mankayan Mineral District in northern Luzon, Philippines embraces three major gold bearing orebodies, all held by Lepanto Consolidated Mining Company. The first of these was the Lepanto enargite-luzonite-gold orebody which produced 36.3 Mt @ 3.4 g/t Au, 2.9% Cu from 1945. Ore occurred as high sulphidation vein and unconformity controlled replacement mineralisation hosted by Miocene andesite and Pliocene dacite adjacent to, and as a late stage of the Far South-east (FSE) Au-Cu porphyry deposit. Far South-east is a blind, deep seated, bell-shaped porphyry Au-Cu deposit hosted by volcani-clastics, and centred on a late Miocene quartz-diorite intrusive complex. The porphyry ore is overprinted by a late stage breccia which is in turn mineralised with Cu-Au. The porphyry system has a core of potassic alteration accompanied by bornite-chalcopyrite-magnetite mineralisation, fringed by a zone of chalcopyrite-magnetite-hematite-pyrite-molybdenite in chlorite-illite alteration, surrounded by chalcopyrite-pyrite-hematite in an outer propylitic regime. The porphyry is capped by advanced argillic quartz-alunite alteration. The breccia has an upper sericite-illite-tourmaline-chlorite zone, passing down into chlorite deficient advanced argillic products. Gold throughout the deposit is present as fine native metal, associated with chalcopyrite-bornite. The current geological resource at FSE is estimated at 650 Mt @ 1.33 g/t Au, 0.65% Cu (or a reserve of 105 Mt @ 2.02 g/t Au, 0.86% Cu at a 1.8% Cu equiv cutoff).
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Acupan - Antamok (Baguio District)
The Baguio gold district has produced approximately 800 t of Au and 900 t of Ag from several epithermal and porphyry systems. The district is located within the Central Cordillera of Luzon, near the major sinistral Philippines Fault. The Cordillera has an ophiolitic basement, overlain by a 7 km thick volcano-sedimentary pile, extending, with breaks from the Cretaceous to late Quaternary. The sediments are mainly marine, while the volcanics are basaltic to andesitic. These are intruded by a series of mid Tertiary to Quaternary gabbroic to dioritic and granodioritic bodies. The gold mineralisation is associated with both porphyry and epithermal fluid types. The porphyry mineralisation is related to sub-volcanic porphyry dacites of Pleistocene-Pliocene age, accompanied by K-silicate biotite-magnetite-anhydrite alteration, fringed by white sericite and propylitic zones. The superimposed low sulphidation epithermal mineralisation occurs as up to 1 m thick quartzose veins with associated adularia-sericite-silica alteration. Calcite, telluride and base metals are found in the latter phases of mineralisation, followed by barren anhydrite. The most recent manifestation of the hydrothermal system is the multiphase 200x300 m Balatoc diatreme.
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The Dizon porphyry copper-gold deposit is associated with a small Miocene quartz-diorite porphyry stock on the margins of a strato-volcano. A post-ore diatreme cut and partly destroyed the orebody. Initial reserves were 140 Mt @ 0.43% Cu, 0.93 g/t Au. The mine is owned by Benguet Corporation.
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MODULE 3 - INDONESIA
Mt Muro Gold Mine
Mt Muro is one of the principal deposits of the Central Kalimantan Gold Province, which also includes Kelian and Busang(?). Gold production commenced in Dec 1994. It then had proven+probable reserves of 8.5 Mt @ 3.9 g/t Au 86 g/t Ag. The mineralisation differs from that at Kelian by having a greater association with hydrothermally altered faults and fracture zones, cutting Eocene to Miocene sediments and andesitic volcanics. These are similar host rocks to those found at Kelian, where the mineralisation is probably of Miocene age. The mine is majority owned by Aurora Gold Ltd.
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Ertsberg Mining District - Grasberg and Ertsberg
Grasberg, in the Ertsberg Mining District of Irian Jaya, Indonesia, is one of the largest and richest Au-Cu porphyry deposits in the world. It was discovered by PT Freeport Indonesia in early 1988 and went into production in late 1989. The current open pit and block cave reserves (Dec 1997) total 1.09 Gt @ 1.27 g/t Au, 1.06% Cu and 0.67 Gt @ 1.09 g/t Au, 1.22% Cu, respectively. The porphyry orebody is present as hypogene disseminated and stockwork mineralisation, contained entirely within the 1.7x2.4 km oval shaped, 3 Ma old Grasberg Igneous Complex. Mineralisation extends from the surface at an elevation of 4200 m, to below 2700 m elevation. The Grasberg Igneous Complex is a funnel-shaped volcanic vent or diatreme composed of matrix supported breccias, pyroclastics, volcaniclastic sediments, trachyandesite lavas and several porphyritic quartz-monzonite stocks. The system was apparently formed at a shallow depth. Hydrothermal fluids related to mineralisation appear to have been introduced mainly along intrusive margins, a NW-SE trending axial fracture zone and on cross structures. Mineralisation appears to be late stage, independent of rock type, and postdates almost all intrusive phases, except a late dyke phase with which it is believed to be coincident. Mineralisation in upper levels is predominantly fracture controlled and the distribution of high grade is principally permeability controlled. Intense K-feldspar-biotite-magnetite-quartz alteration is paragenetically early, while gold and copper mineralisation is late. The preserved potassic zone extends up to 700 m outward from the centre of the system. The outer 150 m of the igneous complex is strongly propylitically altered with epidote ± chlorite- magnetite- calcite. Strong over-printing phyllic (sericite-pyrite ± quartz) replaces the outer sections of the potassic core. In the central ore zone the phyllic alteration is structurally controlled and sericite degrades to clay.
In addition to the Grasberg porphyry deposit, high grade gold-copper ore is found in sediment hosted skarns fringing both the Grasberg Igneous Complex and the adjacent, marginally older Ertsberg Diorite 2 km to the south-east. These skarns include the Kucing Liar deposit adjacent to the Grasberg Igneous Complex, the DOM, IOZ and DOZ, GBT of the East Ertsberg Skarn System and the Ertsberg skarn, all fringing the Ertsberg Diorite, and the Big Gossan orebody west of the latter. With the exception of the original Ertsberg Skarn of 32 Mt @ 0.46 g/t Au, 2.2% Cu, all are underground mining operations. In the Ertsberg Mining District, the intruded sequence comprises a lower, 2000 m thick Jurassic to Cretaceous sequence of sandstones, shales and limestones. These are overlain by 1600 m of Miocene age limestones and dolomites of the New Guinea Limestone Group, commencing with the 290 m thick Paleocene Waripi Formation, which includes an upper evaporite band, and the overlying 200 m Eocene to Oligocene Faumai Formation. These are followed in turn by younger sandstones and carbonates. Skarn style mineralisation in the Ertsberg Mining District is found in both Mesozoic and Tertiary sequences. At both Kucing Liar and Big Gossan ore is hosted by both the Palaeocene Waripi Formation and the sandstones and limestones at the top of the Jurassic-Cretaceous sequence. In the East Ertsberg skarns of GBT, IOZ and DOZ the hosts are the limestones at the top of the Jurassic to Cretaceous, the Waripi Formation and the Faumai Formation, while DOM is exclusively within the Faumai. The different protoliths have produced different skarn mineralogies, which in turn also vary laterally. There are both prograde and retrograde assemblages, and quartz vein stockworks, as well as over-printing hydrothermal breccias. The reserve remaining in the skarn orebodies is over 400 Mt @ 0.4 to 1.6 g/t Au, 1.2 to 2.7% Cu. Of this, Kucing Liar accounts for 221 Mt @ 1.57 g/t Au, 1.42% Cu, and Big Gossan 37 Mt @ 1.02 g/t Au, 2.69% Cu.
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Batu Hijau is a recently discovered porphyry copper-gold orebody on the island of Sumbawa. Quoted reserves in 1995 were 930 Mt @ 0.54% Cu, 0.39 g/t Au. It is the first major porphyry Cu-Au deposit discovered in the Sunda-Banda Arc, on the southern margin of the Sundaland cratonic plate. Mineralisation is hosted by the central stock of a tonalite intrusive complex, and by diorite and volcanic wallrocks. The central stock has associated intense pervasive potassic alteration that hosts most of the higher grade mineralisation. This merges outwards into an extensive propylitic zone. Both of these zones are over-printed by widespread intermediate-argillic and minor phyllic and albite alteration. Argillic and advanced-argillic assemblages occur near surface. Cu and Au are largely hosted by quartz stockworks, but also as disseminations. Chalcopyrite and bornite are the main ore minerals. The property is owned by Newmont Indonesia.
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