An AMF-PGC International Study Tour
Developed & Managed by Porter GeoConsultancy
Pacific Gold '99
Major Gold Deposits of the Western Pacific
September, 1999
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This joint tour, supported by the Geological Society of South Africa, the Mineral Deposits Division of the Geological Association of Canada, ADIMB (Brazil) and the Australian Mineral Foundation, was managed by PGC on behalf of all parties.

[Luise Bay - Lihir Island, PNG]
Image: Luise Bay - Lihir Island, PNG.    
DESCRIPTION of ORE DEPOSITS VISITED

This tour of 16 days, commenced in Port Moresby, Papua New Guinea at Noon on Sunday 12 September, 1999, and finished in the late afternoon of Tuesday 28 September, 1999 in Kagoshima, Kyushu, Japan.

The deposits on the itinerary were:

PART A PART B

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MODULE 1, PART A

LIHIR Au

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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PORGERA Au-Ag

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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GRASBERG Au-Cu

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 bt @ 1.27 g/t Au, 1.06% Cu and 0.67 bt @ 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.

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KUCING LIAR & BIG GOSSAN Au-Cu

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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MODULE 1, PART B

MESEL Au

The Mesel sediment hosted disseminated gold deposit in North Sulawesi, Indonesia, was discovered by Newmont geologists in 1988 and comprises a mineable reserve of 7.8 mt @ 7.3 g/t Au using a 2 g/t Au cutoff. The deposit occurs in a structurally complex zone within the North Sulawesi magmatic arc. The predominant lithologies are a carbonate siliciclastic succession deposited in an extensional middle Miocene basin. In the vicinity of the mine the sediments are intruded by sills, dykes and laccoliths of hornblende-plagioclase phyric andesite. The distribution of hydrothermal alteration is controlled by two steeply dipping major faults, and intersecting shallowly dipping lithologies. Gold bearing carbonate rocks have characteristically undergone decalcification, dolomitisation and silicification, with ubiquitous brecciation textures. The breccias are due to sedimentary, tectonic and hydrothermal dissolution processes. Insoluble residues (clay and organic matter) and pyrite line stylolites and breccia clasts in alteration zones, while clay is present in some rock types. The gangue mineralogy includes quartz, chalcedony, calcite and dolomite. Sulphides such as pyrite, realgar, orpiment, stibnite and cinnabar accompany the gangue and precious metals. Trace elements Ag, As, Sb and Hg are zoned through the deposit, with Ag coinciding with high grade gold and silica, while As, Sb and Hg are found on the peripheries. Gold prefers carbonate lithologies adjacent to and beneath the andesite intrusive. The orebody has a central near vertical high grade feeder system flanked by tabular, shallowly inward dipping, moderate grade zones. The 100gm-m contour outlines a 600x250 m zone in plan, with gold extending to a depth of 150 m. Gold is mid to late stage in the paragenesis and sub-micron in size, locked in very fine (< 10 micron) disseminated and fracture controlled arsenical pyrite. Three major ore types are recognised, namely silica-pyrite, carbonate-pyrite and oxide.

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LEPANTO-FSE-VICTORIA Au-Cu

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). The recently discovered Victoria gold orebody is a low sulphidation quartz-gold vein deposit hosted by Pliocene dacites and volcani-clastics. About 80% of the ore minerals comprise sphalerite, galena and chalcopyrite within steeply dipping 2 to 8 m wide quartz-gold-base metal tensional vein systems that extend for strike lengths of up to 600 m and 300 m vertically. Gold is mostly associated with the quartz. The predominant gangue minerals are quartz, carbonate and gypsum. The current resource is estimated at 11 mt @ 7.3 g/t Au, with a potential for 5 to 10 million ounces of contained gold. These veins may be a late stage over printing both the FSE and Lepanto enargite stages.

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ACUPAN-ANTAMOK-BAGUIO DISTRICT FIELD WORKSHOP Au

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.The workshop was organised by Benguet Corporation.

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HISHIKARI Au

The Hishikari is a giant epithermal gold orebody, with one of the highest grades of gold recorded form such deposits. In 1995 the total contained gold, both mined and in reserve, totalled 260 t at an average grade of 60 g/t Au. The geology of the deposit comprises a basement sequence of Cretaceous sediments and overlying Quaternary volcanics. The concealed vein system of the ore deposit is localised by this unconformity. The Cretaceous basement is made up of shale and sandstone, with minor tuffaceous shale and chert. These sediments are heavily hematite stained up to 20 m below the unconformity with the overlying volcanics. About 60% of the orebody is contained within these sediments which form a restricted basement high in the vicinity of the orebody. The Quaternary volcanics comprise sub-aerially deposited alternating andesites and dacites, both lavas and pyroclastics, which are calc-alkaline in composition and have magnetic susceptibilities typical of the magnetite series. They are mostly Pleistocene in age and make up 5 units, with a basal andesite and an upper-most andesite, separated by two dacite and one andesite unit. They are in turn overlain by younger pyroclastics and alluvial accumulations. The orebody is an epithermal gold-silver bearing quartz-adularia vein deposit. The ore veins have been dated as Pleistocene, closely related to the lower andesites of the sequence and the first overlying dacite. The individual veins are steeply dipping and 300 to 400 m long, composed of crustiform banded quartz-adularia with subordinate smectite. They are generally 1 to 3 m thick, to a maximum of 8 m. Chlorite and sericite are the principal alteration minerals surrounding the high grade Honko-Sanjin veins, while the lower grade Yamada vein extends into a smectite-mixed layer clay alteration phase. The veins only contain a few percent sulphides, including prominent chalcopyrite, sulphosalts, selenides and electrum. The original discovery is credited to the Metal Mining Agency of Japan, with the deposit being enlarged, proven and developed by the Sumitomo Metal Mining Co Ltd.

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KYUSHU VOLCANIC DISTRICT FIELD WORKSHOP

The Kyushu volcanic district comprises a series of semi-active volcanoes and active geothermal fields surrounding the Hishikari and other gold mines of the district. The purpose of the field workshop is to understand the volcanic and geothermal architecture, mineralisation and alteration systems of the district and the relationship between the geo(hydro)thermal activity and the recent gold mineralisation.It was presented by Professor Sachihiro Taguchi of Fukuoka University over three daysand included a visit to the Kasuga high sulphidation gold deposit.

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For more information contact:   T M (Mike) Porter, of Porter GeoConsultancy   (mike.porter@portergeo.com.au)

This was another of the International Study Tours designed, developed, organised and escorted by T M (Mike) Porter of Porter GeoConsultancy Pty Ltd (PGC) in joint venture with the Australian Mineral Foundation (AMF).  While the reputation and support of the AMF contributed to the establishment of the tours, after it ceased trading at the end of 2001, PGC has continued to develop, organise and manage the tour series.

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