Super Porphyry 2003-04 - Module 2, Western Pacific

MODULE 2 - WESTERN PACIFIC, Mongolia, China, Indonesia & Australia

Erdenet ...................... Saturday 24 & Sunday 25 April, 2004.

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The Erdenet porphyry Cu-Mo deposit is located some 250 km WNW of Ulaanbaatar, the capital of Mongolia within the northern Mongolian magmatic belt which is characterised by late Palaeozoic to Mesozoic calc-alkaline volcanism. This belt is believed to have formed as a consequence of the collision of the Siberian craton with the Mongolian-North China block to the south and subduction of oceanic crust from the intervening Mongol-Okhotsk basin.

Country rocks in the deposit area include Precambrian and early Palaeozoic basement intruded by 290 to 260 Ma granites, granodiorites and gabbro-norites of the late Permian Selenge Complex which is in turn cut by the upper Permian to Mesozoic ore bearing porphyries of the Erdenet Complex. Post mineralisation trachyandesites and syenite porphyry dykes which both intrude and overlie the Erdenet and Selenge complexes, are of upper Triassic to lower Jurassic age (182 ±6 and 177 ±6 Ma). Hypogene mineralisation has been dated as lower most Jurassic (207 ±2 Ma).

The Erdenet Complex comprises the following stages: 1). diorite porphyry and microdiorite (approx 250 Ma), 2). granodiorite porphyry (approx 230 Ma) and 3). granodiorite and associated breccias.

The Erdenet ore deposit, which covers an area of 2 x 1 km, has produced some 1.5 Mt of copper from a 100 to 300 m thick chalcocite blanket containing bornite-covellite-chalcocite (coating and replacing chalcopyrite and pyrite) with an average grade of 0.75% Cu within a zone of sericitic alteration. This secondary enrichment overlies 0.4% Cu hypogene mineralisation which persists to depths of 560 m within a broader halo of K feldspar altered Erdenet Complex porphyries. The hypogene assemblage includes chalcopyrite, bornite, molybdenite and pyrite within quartz veinlets with muscovite halos and as disseminations within the host porphyries in the following paragenetic order: a). magnetite, b). quartz-pyrite, c). quartz-molybdenite d). chalcopyrite-pyrite-quartz, e). pyrite, f). pyrrhotite-chalcopyrite ±cubanite, g). hypogene chalcocite-bornite and h). galena-sphalerite-tennantite.

While potassic alteration is widespread within the complex, the mineralisation appears to be largely associated with sericitisation.

The deposit, which has been exploited since 1977, is quoted as containing a total of 9.2 Mt of copper and 0.27 Mt of molybdenum in reserves and historic production.

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Oyu Tolgoi ...................... Monday 26 to Tuesday 27 April, 2004.

The Siluro-Devonian Oyu Tolgoi high sulphidation porphyry copper-gold-(molybdenum) deposit is located in the Gobi Desert of southern Mongolia, 550 km due south of the capital, Ulaanbaatar and 80 km north of the Chinese border. In 2003, the deposits was estimated to contain at least 12 Mt of copper and 570 tonnes of gold in estimated resources and is owned by Ivanhoe Mines.

Oyu Tolgoi falls within the Barga terrane, part of the South Mongolian tectonic unit, near the contact with the South Gobi block. It lies within a region dominated by Palaeozoic volcanics, sediments and intrusives with Mesozoic sedimentary cover and represents a zone where a series of magmatic/volcanic island arc and continental blocks, have been accreted, with associated rift basins and continental margin arc settings. The district is dominated by coarse Silurian-Devonian terriginous sediments and intermediate to felsic volcanics (including rhyolitic & dacitic ignimbrites), with lesser basic volcanism. These rocks are intruded by Devonian syenite and granite and Carboniferous diorite, granite, granodiorite and syenite, ranging from dykes to batholiths, and by a Permian per alkaline complex.

The four main mineralised centres, Hugo Dummett, Central, South and SW Oyu lie within a 5 km long structural corridor and appear to represent three porphyry centres at South and Central Oyu and near Hugo Dummett. Mineralisation and alteration are associated with small plugs, dykes and hydrothermal breccias and occur as multiple porphyry Cu-Au centres with high sulphidation zones partially telescoped onto underlying porphyry systems. Alteration includes K silicate (quartz-K feldspar-biotite) and overprinting sericite-chlorite at South Oyu, while several advanced argillic and quartz-sericite-illite associations are dominant at Central and North Oyu, over printing and obliterating the earlier K silicate and quartz-sericite stages, particularly in association with hydrothermal breccias. Peripheral, magnetite stable propylitic alteration of calcite, chlorite and epidote is weak, low in pyrite and fringes the advanced argillic alteration at Central and Hugo Dummett.

The bulk of the Cu-Au-Mo mineralisation at South and SW Oyu is present as porphyry style heavily stockworked and sheeted veining and is pyrite poor and magnetite rich, dominated by quartz, chalcopyrite, bornite and trace molybdenite in andesite and feldspar-hornblende porphyry. The upper 30 to 60 m is characterised by a mixed sulphide-oxide zone. Only the roots of an original high sulphidation system remain.

At Central Oyu copper is present in a supergene chalcocite blanket that formed at the expense of a pyrite rich, hypogene chalcocite-covellite-tennantite (arsenosulvanite, sulvanite, chalcopyrite, bornite) suite that accompanied the advanced argillic alteration. The jarosite-goethite leached capping is 25 to 50 m thick, overlying a chalcocite blanket and a mixed supergene-hypogene zone to depths of 100 to 200 m. The upper 20 to 30 m of the enriched blanket (the main supergene enrichment) has steely chalcocite and minor covellite and digenite and carries from 0.6 to 1.9% Cu. The distribution of gold is erratic and not well defined. High sulphidation systems are partly telescoped into underlying porphyry systems at Central Oyu and Hugo Dummett. At Central Oyu covellite-pyrite is related to an upwardly flared intense quartz-sericite zone, centred on a porphyry style quartz veined dyke swarm.

At Hugo Dummett high grade mineralisation is mainly bornite, chalcocite and chalcopyrite, with subordinate pyrite, enargite and tetrahedrite-tennantite to the south, and hydrothermal breccias at a depth of around 100 m below surface. Although some supergene chalcocite is present most high grade is associated with millimetric to centimetric massive sulphide veins.

In February 2003, at a 0.3% Cu equivalent cut-off, the four deposits had:
- an indicated resource of 508.9 Mt @ 0.4% Cu, 0.59 g/t Au + an inferred resource of 1.602 Gt @ 0.63% Cu, 0.17 g/t Au.
In February 2003, at a 0.6% Cu equivalent cut-off, the same deposits yielded:
- an indicated resource of 266.9 Mt @ 0.53% Cu, 0.86 g/t Au + an inferred resource of 811.7 Mt @ 0.90% Cu, 0.21 g/t Au.
The indicated resource was all at SW Oyu.

For updated information see update

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Dexing ...................... Wednesday 28 & Thursday 29 April, 2004.

The Dexing copper district is located in Jiangxi Province in south-eastern China. It contains three large porphyry copper deposits, namely Tongchang, Fujiawu and Zhushahong aligned over an 8 km northwest-southeast trending interval. The Guanmaoshan gold deposit on the same line, lies between Fujiawu and Tongchang. Only Tongchang, which is operated by Jiangxi Copper Company, is in large scale (90 000 tpd) production with an output of 120 000 tonnes of copper in 2003, although a smaller scale 1250 tpd operation centred on the Fujiawu deposit is owned by Dexing County.

The deposits of the district are associated with a NW-SE trending cross structural corridor within the major NE-SW aligned South China Fold System on the margin of the Yangtze Para-platform. The country rock sequence comprises Sinian (1400 Ma) low grade metamorphics made up of alternating beds of phyllite, slate, dacitic meta-tuff and meta-sandstone, overlain by thin Carboniferous to Permian marine sediments. These sequences were cut and overlain by 5 stages of Jurassic to Cretaceous Yanshanian magmatic activity, commencing with 193-190 Ma mafic to ultramafic intrusives, the ore related 196 to 172 Ma granodiorite, diorite, quartz-diorite and granite porphyries, 145 to 125 Ma dacitic volcanism and sub-volcanic intrusives, 127 to 103 Ma intermediate to acid volcanics and 100 to 96 Ma post mineralisation granite, granite porphyry, quartz porphyry and quartz-diorite.

Three pipe like, NW plunging cupolas of granodiorite porphyry, ranging in size up to the largest at Tongchang which is some 1300x300 to 800 m at surface, host the three porphyry deposits. Each is surrounded by a 100-400m wide hornfels zone and has a core rich in K feldspar, chlorite and hydro-muscovite. The alteration is centred on the contact zone, grading inwards and outwards from strong quartz-sericite, to chlorite-sericite-(epidote)-carbonate-anhydrite to chlorite-epidote-illite-albite-anhydrite. Ore is principally associated with strong quartz-sericite alteration, occurring as stockwork sulphide and quartz veins and disseminations of chalcopyrite and molybdenite with minor tennantite, tetrahedrite, bornite, chalcocite and electrum. At Tongchang the orebody forms a 2500 m diameter cylinder with a barren core and extends down plunge for 1000 m.

Reserves at Tongchang in 2003 amount to 1.17 Gt @ 0.47% Cu, 0.01% Mo, 0.19g/t Au at a 0.3% Cu cut-off for 5.2 Mt of copper and 215 tonnes of gold.

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Batu Hijau ........... Sun. 2 to Tue. 4 May, 2004.

The Batu Hijau porphyry copper-gold deposit is located on the south-western corner of the island of Sumbawa in central Indonesia. It is a joint venture between the Sumitomo controlled Nusa Tengarra Mining Corporation of Japan and PT Pukuafu Indah. Newmont is the operator and holds a 45% equity in the mine.

The deposit lies within the east-west trending Sunda-Banda magmatic arc at the convergent intersection of the Australian-Indian and the Eurasian plates. The northern half of Sumbawa is occupied by recent volcanoes, while the southern segment, where Batu Hijau is located, comprises oceanic crust overlain by low K calc-alkaline to weakly alkaline andesitic volcanics and volcaniclastics, associated intermediate intrusives and minor shallow marine sediments and limestones. In the mine area the sequence is represented by andesitic volcanic lithic breccias, volcaniclastic sandstones and mudstones and hypabyssal porphyritic andesites, with a younger thick sequence of quartz diorite in the east. Multiple tonalite porphyry intrusions were emplaced along the contact between the andesitic volcaniclastics and the quartz diorite. These tonalites, around which the mineralisation is zoned, are divided into the Old, Intermediate and Young Tonalites. Each has associated quartz veining and Cu-Au mineralisation, with the Old Tonalite having the highest grades and most intense associated alteration. The two following phases have progressively lower grades, vein densities and alteration.

Alteration and mineralisation has been divided into five temporally and spatially overlapping stages, namely: 1). Early pervasive biotite, secondary magnetite and plagioclase with fine 'A' type stockwork veining and bornite-digenite-chalcocite mineralisation, 2). Transitional oligoclase/albite-sericite-quartz±vermiculite with planar 'B' veins containing chalcopyrite±bornite (representing 50 to 70% of the Cu in the deposit) and rare 'C' veining, 3). Late feldspar destructive sericitic + other minerals (propylitic) alteration with associated 'D' veins of pyrite and quartz±chalcopyrite, 4). Very Late feldspar destructive alteration producing smectite and chlorite with associated sphalerite, galena, tennantite, pyrite and chalcopyrite, 5). Zeolite alteration, a low temperature phase of open space filling, The final influence was oxidation to depths of 210 m with weak supergene sooty chalcocite enrichment in a thin 15 to 60 m thick layer.

Based on the feasibility study prior to the commencement of production in 2000, the Batu Hijau deposit had a resource of 1.1 Gt @ 0.525% Cu, 0.37 g/t Au. (Newmont website)
Proven+probable reserves at the end of 2003 were stated as 570 Mt @ 0.55% Cu, 0.37 g/t Au, representing 2.9 Mt of Cu and 215 tonnes of Au.

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Grasberg / Ertsberg ........... (Tues. 4 - travelling Sumbawa via Denpasar to Timika) ........... Wednesday 5 to Friday 7 May, 2004.

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 3.3 to 2.7 Ma Grasberg Igneous Complex, porphyry ores of the 4.4 to 3.3 Ma Ertsberg Diorite 1.5 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 30 Mt of copper and around 2700 tonnes of gold. All are mined as part of a large integrated operation owned by PT Freeport Indonesia. The operation is currently the worldÕs largest gold mine with an annual production (2002) of around 95 tonnes of gold and 770 000 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 ~12Ma).

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 quartz sandstones, shales and the uppermost shale, sandstone and limestones of the Cretaceous Kembelangan Formation, overlain by the Tertiary New Guinea Limestone, comprising basal shale, dolomite-evaporite-limestone-siltstone-sandstone overlain by a thick limestone succession. The Ertsberg Diorite and the younger Grasberg Igneous Complex (GIC), both of Pliocene age, cut these sediments.

The 2.5x1 km Ertsberg Diorite is mainly an even grained equi-granular quartz monzodiorite with lesser biotite-pyroxene diorite and quartz monzonite dykes. The GIC is a funnel shaped 1.7x2.4 km volcanic vent or diatreme composed of matrix supported breccias, pyroclastics, volcaniclastic sediments, trachyandesite lavas and several quartz-monzodiorite stocks and dykes. The intrusives, which are all within the GIC diatreme, are porphyritic quartz-monzodiorites in composition and are known over a vertical interval of up to 1500 m, as follows: 1). Early Main Grasberg - a 600x430 m stock, 2). Late Main Grasberg - a 900 m diameter stock with associated dykes, 3). Early Kali - an irregular stock of 600x250 m, and 4). Late Kali - mainly dykes and a 500x250 m stock.

The GIC has a strong associated potassic alteration suite of K feldspar-biotite-quartz-magnetite, grading out to a propylitic halo of epidote ±chlorite-magnetite-calcite. Strong magnetite (>8%) occupies a 600x300 m core to the potassic zone. The potassic alteration has been overprinted by intense sericite-pyrite±quartz (phyllic) alteration to within 400 m of the centre of the system. A 100 m wide zone of brecciated marble surrounds the GIC. The bulk of the copper ore is present as a chalcopyrite stockwork and disseminations that postdate the potassic alteration, but predates the phyllic phase. Mineralisation extends from the surface at an elevation of 4200 m, to below 2700 m ASL.

In the Ertsberg Diorite, mineralisation and alteration comprises: 1). early feldspar stable potassic alteration with hairline bornite veining, 2). transitional green sericite veins with chalcopyrite and chalcopyrite-pyrite veins (and endoskarn development) and 3). late quartz-sericite-pyrite±chalcopyrite. The Ertsberg stockwork contains a reserve of 122 Mt @ 0.54% Cu, 0.90 g/t Au. The skarn mineralisation, includes the: 1). GB (33 Mt @ 2.5% Cu, 0.8 g/t Au) surrounded by Ertsberg Diorite near its NW margin, 2). 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, 3). 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, 4). 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, 5). Kucing Liar between the two intrusive complexes with >225 Mt @ 1.42% Cu, 1.57 g/t Au.

Hydrothermal fluids related to mineralisation and alteration appear to have been introduced mainly along intrusive margins, a NW-SE trending axial fracture zone and on cross structures. Mineralisation is 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.

The total proven+probable reserve at the Grasberg/Ertsberg operation at the end of 2002 was quoted at 2.584 Gt @ 1.13% Cu, 1.04 g/t Au.

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Field Workshop ........... (Sun. 9 - travelling Timika to Sydney) ........... Mon. 10 May, 2004.

A field workshop was run to provide an overview of the geological setting of porphyry copper and gold mineralisation in the eastern Lachlan Fold Belt of New South Wales.

The workshop commenced with an expert briefing in Sydney by Dick Glen of the NSW Department of Mineral Resources outlining the tectonic, geologic and metallogenic framework of the Lachlan Fold Belt and the porphyry copper and gold mineralisation it hosts, particularly that segment that embraces the Cadia and Northparkes mines.

A forty five minute flight after the briefing (which was held at a venue near the airport in Sydney) took the group direct into the field from the city of Orange.

The field component of the workshop was hosted by Max Rangott of Rangott Mineral Exploration and comprised visits to exposures that demonstrated the basics elements of the region's geology, particularly the lithologies that host significant mineralisation, both proximal to/within and removed from mineralisation.

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Cadia & Ridgeway ........... Tues. 11 May, 2004.

The Cadia and Ridgeway porphyry gold-copper deposits are located 20 km south of Orange in the central tablelands of New South Wales, Australia, some 200 km WNW of Sydney. Cadia Hill and related adjacent resources are low grade, bulk mining, porphyry style Au-Cu deposits while Ridgeway, 3 km to the north-west of the Cadia Hill open pit and 500 m below surface, comprises quartz veins, sheeted and stockwork quartz and quartz-sulphide veins and disseminated mineralisation with higher grade gold and associated copper mineralisation.

The Cadia district falls within the Molong Volcanic Belt in the eastern part of the lower Palaeozoic Lachlan Fold Belt of south-eastern Australia where a number of relatively undeformed, shoshonitic, Ordovician volcano-intrusive complexes host porphyry and high sulphidation epithermal gold mineralisation. The largest such deposits are in the Cadia district. The Cadia-Ridgeway cluster of deposits are principally associated with a 3 x 1.5 km late Ordovician composite quartz-monzonite to dioritic porphyry stock and its probable co-magmatic volcanic wall rocks and intercalated volcaniclastics that together form part of an Ordovician volcano-intrusive complex. Overall the stock has an alkaline composition, with mineralisation and alteration being associated with porphyritic quartz-monzonite phases that are altered over an area of 5.5 x 3 km and to a depth of up to 1.6 km, defining a NW trending corridor that encloses the known deposits.

There are five components to the Cadia porphyry system within the mineralised corridor, namely:
1). Intrusion- and volcanic wall rock hosted sheeted veins at Cadia Hill. Alteration is principally propylitic with little recognised potassic developments, while a late stage phyllic phase was restricted to zones of faulting and is followed by late carbonates. Mineralisation is mainly chalcopyrite and pyrite with lesser bornite within and disseminated around sheeted 1 to 20 mm thick quartz veins in a 100 to 350 m wide, 65° dipping zone that is 1 km long and has not been closed at depth;
2). Volcanic wall rock hosted disseminated and sheeted vein mineralisation at Cadia East within moderately to strongly altered lavas and volcaniclastic breccias. Alteration and mineralisation is centred on a steeply dipping, 300 m wide, east plunging core of steeply dipping sheeted quartz-calcite ±chalcopyrite ±bornite ±molybdenite ±covellite ±pyrite ±magnetite veins within a disseminated envelope of chalcopyrite, bornite and pyrite. This core persists down plunge for at least 1.6 km. Alteration types include weak propylitic, weak sericite-silica-albite, moderate to strong silica-albite flooding with hematite and K feldspar, and strong sericite-albite with silica-albite flooding ±tourmaline;
3). Intrusion hosted sheeted veins at Cadia Quarry, developed as a 1 km long by 200 m wide package controlled by faulting and fracturing;
4). The up to 70 m thick distal, stratabound hematite-magnetite skarns at Big and Little Cadia. Chalcopyrite is the dominant sulphide, with pyrite and calcite interstitial to the magnetite and hematite blades;
5). Probable late stage distal veins.

The Ridgeway deposit is an upright, bulbous body of stockwork quartz veining zoned about a 50 to 100 m diameter plug of porphyritic Cadia Hill Monzonite where it intrudes Ordovician andesitic volcanics to the west of the main Cadia multiple intrusion. The highest grade gold accompanies the most intense alteration and stockwork development immediately adjacent to the monzonite porphyry, with the best being localised directly above the plug compared to grades on its lateral margins. Grades decrease laterally outwards and inwards from the intrusive contact. The ore minerals are predominantly veined, but are also disseminated and comprise native gold, chalcopyrite and bornite, with accessory magnetite. The more intense mineralisation is accompanied by a potassic alteration suite of orthoclase, albite, actinolite, magnetite and biotite, overprinted by a propylitic assemblage of epidote, chlorite, iron carbonates, calcite and hematite dusting. NW-SE trending pre-mineral faults appear to have exerted a strong control on mineralisation.

The total pre-mining resources were:
Cadia Hill in 1977 - 352 Mt @ 0.63 g/t Au, 0.16% Cu for 221.3 t of contained Au;
Cadia Quarry in 2003 - 50 Mt @ 0.40 g/t Au, 0.21% Cu for 21.7 t of contained Au;
Ridgeway in 2002 - 54 Mt @ 2.5 g/t Au, 0.77% Cu for 132.6 t of contained Au.
Cadia East was un-mined in 2010.

The remaining proved+probable reserves in August 2010 (Newcrest website) were:
Cadia Hill - 116 Mt @ 0.60 g/t Au, 0.14% Cu;
Ridgeway underground - 101 Mt @ 0.81 g/t Au, 0.38% Cu;
Cadia East underground - 1073 Mt @ 0.60 g/t Au, 0.32% Cu.
The total measured+indicated+inferred resources at the same date were:
Cadia Hill - 408 Mt @ 0.42 g/t Au, 0.12% Cu;
Cadia Extended - 83 Mt @ 0.35 g/t Au, 0.20% Cu;
Ridgeway underground - 155 Mt @ 0.73 g/t Au, 0.38% Cu;
Big Cadia - 42 Mt @ 0.38 g/t Au, 0.40% Cu;
Cadia East underground - 2347 Mt @ 0.44 g/t Au, 0.28% Cu.

The total declared measured+indicated+inferred resource in the Cadia district was estimated in 2010 to contain 1360 tonnes (43.7 Moz) of gold and 7.99 Mt of copper. The Cadia-Ridgeway mines are operated by Newcrest Mining Ltd.

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The summaries above were prepared by T M (Mike) Porter from a wide range of sources, both published and un-published. Many of these sources are listed in the Literature Collection citation page for this Module.

		Another PGC International Study Tour Developed & Managed by Porter GeoConsultancy Super Porphyry 2003-04 The Super Porphyry Cu/Au Deposits of the World October 2003 & April-May 2004
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