The program included
IRON OXIDE COPPER-GOLD DEPOSITS,
Thursday 7 to Wednesday 13 December, 2000.
The tour participants assembled in ADELAIDE SOUTH AUSTRALIA on the evening of Wednesday 6 December, 2000. The tour started on the morning of the following day.
It comprised the following mine visits, field and other workshops, in the order as detailed below:
Data Metallogenica - Iron Oxide Cu-Au & Related Deposits of the World
The AMF-AMIRA Data Metallogenica Centre in Adelaide houses more than 50 000 rock and ore samples collected from 3500 minerals deposits/occurrences from 100 countries on all inhabited continents. These are all mounted and catalogued, with generally more than 20 (up to 60+) representative samples per deposit. More than 1000 specimens were studied from 35 key examples of Iron Oxide Copper-Gold deposits from around the world, supported by geologic diagrams and sections showing the location of individual specimens, with literature back-up and experts on hand to answer queries. The tour group was divided into four syndicates who progressively studied and discussed deposits laid out from the following districts or regions, and the deposits selected as follows:
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- The Americas, Phanerozoic: Candelaria, Punta del Cobre District, Mantos Blancos, El Soldado, Manto Verde, El Romeral, Las Adrianitas, La Estrella, Ovalle, El Tofo, Cerro Iman, Cerro Negro Norte.
The Americas, Precambrian: Salobo, Igarape Bahia, Pilot Knob, Pea Ridge, Iron Mountain (Missouri), Cerro Mercado (Mexico), Wernecke Breccias (Canada).
Scandinavia, Africa & Asia: Kiruna, Viscaria, Malmberget, Aitik, Gruvberget-Svappavaara, Bergslagen, Grangesberg (Scandinavia), Palabora (South Africa), Bayan Obo (China).
Australia: Olympic Dam, Ernest Henry, Osborne, Eloise, Selwyn, Tennant Creek.
Dam - South Australia
The Olympic Dam ore deposit lies within the outer margins of the Lower Proterozoic Gawler Craton in northern South Australia. It falls within the Stuart Shelf where 300m of flat lying, barren, Upper Proterozoic to lower Paleozoic sediments unconformably overlie both the craton and the ore deposit. To the east this sequence expands into the thick succession of the Adelaide Fold Belt. Mineralisation is hosted by the Olympic Dam Breccia Complex which is developed within the Mid Proterozoic (1600-1585 Ma) Roxby Downs Granite. The Olympic Dam Breccia Complex comprises a downward narrowing funnel shaped body of fractured, brecciated and hydrothermally altered granite which has resulted in a great variety of granitic, hematitic and siliceous breccias. The outer margins of the complex are diffuse, grading outwards from heterolithic breccia, through granite breccia, to crackle breccia to fractured granite. Minor volcaniclastics are found in diatreme structures at the top of the
complex. The resource occurs as up to 150 separated bodies distributed within an annular zone up to 4 km in diameter surrounding the central barren hematite-quartz breccia. The highest grade mineralisation occurs where an up to 40 m thick, shallowly inward dipping, irregularly developed, chalcocite-bornite 'zone', cuts hematite rich breccia zones. This undulose layer grades progressively downwards and outwards into chalcopyrite and then pyrite rich zones. The chalcocite-bornite 'zone' is overlain by a barren sulphide deficient interval extending to the overlying unconformity. As at December 1999 proven+probable reserves totalled 605 Mt @ 1.8% Cu, 0.5 g/t Au, 0.5 kg/t Uranium oxide; within a total resource of 2320 Mt @ 1.3% Cu, 0.5 g/t Au, 0.4 kg/t U3O8. The current expansion should result in an annual production rate of 200 000 t of contained copper and 4600 t U3O8 from 9 Mt of ore. The mine was 100% owned by WMC Ltd through WMC (Olympic Dam Corporation) Pty Ltd.
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Cloncurry Field Workshop - Queensland
Noted international expert, Dr Pat Williams of James Cook University in Townsville, Queensland conducted a two day field workshop in the Cloncurry Terrane, including a visit to the Mount Fort Constantine magnetite occurrence near Ernest Henry. The workshop also included lectures in Cloncurry on the geology and granites of the terrane, the regional and deposit scale alteration and implications for exploration, study of a suite of rocks illustrating the lithologies and alteration, accompanied by geological maps. The visits to the Selwyn Mines (see below) illustrating the diversity of mineralisation encountered in the terrane were also conducted in conjunction with the field workshop. Pat has considerable experience working on the Proterozoic members of this family of deposits, particularly in the Cloncurry Terrane which hosts a variety of significant examples. He has published widely on the subject and is well known for his clear and comprehensive presentation of his accumulated knowledge and experience.
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Selwyn Mines - Queensland
The operations of
Selwyn Mines Australia Pty Ltd lie approximately 100 km to the south of
Cloncurry in the Cloncurry-Selwyn zone of the Mid Proterozoic Eastern Fold Belt (Cloncurry Terrane) of the Mt Isa Inlier. The main assets include the Mt Elliot, Mt Dore and the Starra/Selwyn plant and mines.
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The Selwyn/Starra mines (6.9 Mt @ 1.65% Cu, 4.8 g/t Au) are associated with magnetite-hematite ironstones forming a zoned system in meta-siliciclastic and calc-silicates. Barren quartz-hematite ironstones are found to the east, while massive magnetite±hematite ironstones are found within the broad high strain zone known as the Starra Shear to the west, broadly defining the boundary between sediments and a granite mass to the west. This high strain zone contains a series of discrete, steeply plunging magnetite ironstone bodies aligned parallel to the main lineation of the shear. The ironstone lenses are flanked by breccias with highly strained albitised rock in a matrix of magnetite and iron silicates, representing an extensive earlier albite (±actinolite) phase being overprinted by a local potassic-iron alteration (biotite-magnetite). The Cu-Au mineralisation is predominantly confined to the ironstones. Seven discrete ore zones were identified over a strike length of 5.5 km. Pyrite, chalcopyrite, anhydrite, barite, calcite and gold are interpreted to have been precipitated from late oxidising solutions in association with the brecciation of the magnetite ironstone, chloritisation of biotite and hematisation of magnetite.
The Mt Elliot
mine (2.9 Mt @ 3.33% Cu, 1.47 g/t Au) is some 20 km to the north and is hosted by carbonaceous meta-pelites and amphibolites. The deposit is present in two zones controlled by NW trending, NE dipping brittle faults in a 200 m wide zone of intense post peak metamorphic alteration., comprising an older, outer, system of albitised meta-sediments (bleached by the loss of biotite and carbonaceous material). This is over-printed by a "skarn" zone of diopside-hedenbergite veins and replacement features with abundant scapolite, apatite and calcite. Sulphides and magnetite (from outer pyrrhotite-pyrite to chalcopyrite-pyrrhotite-pyrite to chalcopyrite-pyrite-magnetite to magnetite-pyrite±andradite in the core) occur within the skarn, veining and replacing clinopyroxene and intergrowing with calcite.
At Mt Dore, some 26 Mt @ 1.1% Cu, predominantly as oxide ore, is hosted by a fault bounded slice of mica schist, sandwiched between calcareous and calc-silicate rocks to the east and the Mt Dore granite to the west. Mineralisation and accompanying alteration are localised by tabular dilational breccia bodies associated with post-metamorphic and post-granite fault movement and hydrothermal alteration. Alteration comprised an early K feldspar-biotite-muscovite-quartz stage to a later dolomite-calcite-apatite-chlorite phase with pyrite, chalcopyrite and minor sphalerite-galena.
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Ernest Henry - Queensland
The Ernest Henry
ore deposit lies to the east of the Cloncurry Overthrust, within the Cloncurry-Selwyn zone of the Eastern Fold Belt (Cloncurry Terrane), of the Mt Isa Inlier. It is concealed by 35 to 60 m of an extensive Phanerozoic cover and does not outcrop. While the exact stratigraphic position of the host rocks is not known, they have been tentatively correlated with the 1730±10 Ma Mount Fort Constantine Meta-volcanics towards the top of Cover Sequence 2. The only other outcrop in the district is the 1480 Ma Mount Margaret granite some 12 km to the east. Within Cover Sequence 2 volcanism is common between 1790 and 1780 Ma, and 1760 to 1720 Ma, with later 1540 to 1450 Ma granitoids. Within the immediate orebody area the principle lithologies encountered from hangingwall to footwall are a diorite intrusive, altered felsic volcanics, brecciated volcanic rocks which host the orebody, footwall siltstone, marble matrix breccia and more altered felsic volcanic rocks. The brecciated volcanic mass that hosts the ore forms a plunging elongate body, some 250 m thick, 300 m average length and extending at least 1000 m down plunge to the SSE where it has not been closed by drilling. The breccia ranges from the unbrecciated volcanics, to crackle fracture veining to clast supported and matrix supported breccia to total clast digestion (massive matrix). The matrix is a magnetite-carbonate-sulphide assemblage with variable associated biotite, chlorite and barite. The sulphide assemblage is dominated by chalcopyrite within a magnetite-carbonate gangue. The bulk of the economic mineralisation is restricted to breccia zones with more than 10% matrix. The total reserve + resource in 1997 was 166 Mt @ 1.1% Cu, 0.54 g/t Au. The operation is controlled by Ernest Henry Mining Pty Ltd, a 51% owned subsidiary of MIM Holdings Ltd, 49% Pasminco Ltd.
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Osborne - Queensland (Group 1)
The Osborne copper-gold deposit is located some 195 km south east of Mt Isa in a complex sequence of metamorphic, igneous and metasomatic rocks belonging to the Mid Proterozoic Eastern Fold Belt (Cloncurry Terrane) of the Mt Isa Inlier. In 1993, prior to mining, the total measured and indicated mineral resource was 11.2 Mt @ 3.5% Cu, 1.5 g/t Au within a larger global resource of 36 Mt @ 2% Cu, 1 g/t Au. The predominantly meta-sedimentary host sequence comprises feldspathic psammites ± thin layers of pelite, stromatitic migmatites and local pre-metamorphic banded ironstone units and schists. Sheet intrusions of amphibolite and post metamorphic pegmatites are also present. The psammites are characterised by sodic plagioclase comprising >95% albite and/or oligoclase + quartz. The ironstones are predominantly composed of varying proportions of magnetite, quartz and apatite. The deposit area is divided into two domains by the Awesome Fault. The bulk of the high grade Cu-Au mineralisation is focused along the contacts of the upper banded ironstone with feldspathic psammite in the Western domain. Both ironstones in this domain contain disseminated Cu-Au mineralisation along their 1.3 km strike lengths and the gross distribution of ore mirrors the shape of the ironstone. The Eastern Domain is largely devoid of ironstones but embraces a prolate high grade lens of ore in pegmatite, feldspathic psammite and meta-mafic intrusives. The majority of the Cu is present as chalcopyrite hosted by zones of coarse silica flooding. Within the ironstones, mineralisation is present with secondary hematite-pyrite-magnetite, whereas well mineralised silica flooded zones outside of the ironstones contain only magnetite-pyrite. In the Eastern Domain ore is characterised by pyrrhotite-pyrite-magnetite. Peak amphibolite facies metamorphism is dated at 1595 to 1568 Ma, while the Cu-Au is 1538 Ma. Fluid inclusion studies indicate the orebody formed at around 300 degrees C from immiscible hyper-saline brines and carbon dioxide rich components. The Osborne mine is 100% owned by Placer dome Inc, through Placer Pacific Limited.
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Eloise - Queensland (Group 2)
The Eloise deposit is contained within the mid Proterozoic Eastern Fold Belt - the Cloncurry Terrane - of the Mt Isa Inlier in North-west Queensland. The pre-mining resource was 3.1 Mt @ 5.5% Cu, 1.4 g/t Au, 16 g/t Ag. It comprises a number of steeply plunging, structurally complex mineralised zones and is characterised by very high grade chalcopyrite-pyrrhotite rich mineralisation hosted by mafic silicate alteration. The host sequence is concealed below some 50 to 70 m of flat lying mudstone and unconsolidated Mesozoic sediments and comprises a steeply dipping sequence of interlayered meta-arenite and micaceous quartz-schist, intruded by amphibolite and cut by faults of various ages. At the southern end of the deposit the host rocks are characterised by the presence of magnetite schists and magnetite carbonate rock. Ore mineralisation comprises coarse grained intergrowths of pyrrhotite and chalcopyrite, generally in close association with quartz and minor carbonate stockwork veins. Sulphide textures vary from massive aggregates of pyrrhotite and chalcopyrite, through to flame textured blebs and accumulations. The mineralised zone forms a steeply dipping, tabular body more or less conformable with the schistosity. The mineralised body is approximately 600m long, with a maximum thickness of 30 m, and extends to a depth of at least 500m. Alteration is pervasive silica-carbonate flooding, with a lesser mafic over-print of course grained biotite and amphibole. The contact between the mineralisation and the enclosing country-rock is gradational over several metres. Patches of course grained carbonate with layers and disseminations of magnetite are common within the alteration envelope.
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Creek - Northern Territory
The Tennant Creek gold field is located in the central province of the Tennant Creek Inlier in central Northern Territory, surrounded by flat lying lower Palaeozoic carbonate. The oldest rocks of the Inlier are the metamorphosed greywacke, siltstone, mudstone and hematitic siltstones/mudstones of the Warramunga Formation (maximum age 1860 Ma). Authigenic and detrital magnetite is characteristic of the Formation. This unit was intruded by 1850 Ma granitoids, then unconformably overlain by 1845 to 1825 Ma felsic volcanics, with accompanying intrusives, and a siliciclastic molasse like succession which includes minor dolomitic rocks and basalts, followed finally by the intrusion of an unfoliated granite. All of the Tennant Creek mineralisation is associated with iron oxides, in particularly, ironstone bodies. Over 650 ironstone bodies are known in the Inlier, although only 25% contained any ore grade Cu, Au or Bi, many of which were only very minor in size, and only 100 of these have been mined in any way. Individual lodes vary from a few tens of tonnes up to 15 Mt of ore. The individual orebodies are irregular, but overall ellipsoidal in shape and generally pipe like, with near vertical and near horizontal long axes, the latter trending east-west. The principal primary gangue minerals are magnetite, quartz, chlorite, talc, hematite, dolomite, sericite, jasper, pyrite and pyrrhotite. The most common ore minerals are chalcopyrite, native gold, native bismuth and bismuth sulphosalts, with lesser bornite, galena, sphalerite, cobalt, uraninite and scheelite. The deposits exhibit a spectrum of mineralogic associations, with the end members being more reduced and oxidised respectively. Two main hydrothermal stages are recognised in the field. These are an early iron oxide phase producing magnetite-hematite-chlorite-quartz epigenetic ironstones during the 1860-1840 Ma Barramundi Orogeny, localised in dilational zones generated during the D1 reverse shearing and folding, and concentrated by hematitic shale seals in shears, faults and fold closures. Gold-copper-bismuth mineralisation followed during D2 at or before 1830-1825 Ma and overprinted the pre-existing ironstones where favourable D1 structures were re-activated. To date the Tennant Creek gold field has produced more than 160 t Au, 240 000 t Cu and 5000 t Bi.
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For more information contact: T M (Mike) Porter, of Porter GeoConsultancy (firstname.lastname@example.org)
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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