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Weipa Bauxite Plateau - East Weipa, Andoon, Amrun, Aurukun, Bauxite Hills, Urquhart |
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Queensland, Qld, Australia |
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Super Porphyry Cu and Au
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The Weipa Bauxite Plateau hosts a series of both large and smaller bauxite and kaolin deposits on the west coast of Cape Yorke Peninsula in far north Queensland, Australia. The kaolin deposits are part of the same profile as, and occur below, the bauxite layer. The plateau which extends for 350 km, from Holroyd River in the south to Vrilya Point in the north, covers an area of 11 000 km2 with an average ~2.5 m thickness of bauxitic material.
The unusual 'red cliffs' of western Cape York Peninsula were first observed by western European explorers in 1606, when Willem Janszoon and Jan Roosengijn of the Dutch East India Company in the ship Duyfken recorded sighting them. Numerous subsequent sightings were recorded by seamen, including Matthew Flinders in 1802. In 1902, the geologist C.F.V. Jackson recognised that they were composed of pisolithic ironstone and bauxite and may be of commercial value, although his samples did not return encouraging compositions with clays associated with the mineralisation at Vrilya Point and Batavia River (now Wenlock River) respectively containing 32.64 and 42.53% SiO2; 16.0 and 6.72% Fe and 28.18 and 33.35% Al2O3. These two locations are ~150 and 60 km north of Weipa respectively. In 1955 the geologist Harry Evans recognised and investigated the bauxite potential of the area for Consolidated Zinc Proprietary Limited (CZP) while primarily in the area as part of a petroleum search. After receiving encouragent from initial sampling he returned to traverse and sample ~290 km of the coastline with the aid of local Indigenous people and identified the likelihood of a significant economic bauxite resource. An approximate resource of 2.50 Gt of bauxite was estimated and CZP formed the Commonwealth Aluminium Corporation Proprietary Limited (Comalco) to mine the ore and create an integrated aluminium industry in partnership with the US company Kaiser Aluminum and Chemical Corporation (KACC). In 1962, CZP and the Rio Tinto Company merged to form Conzinc Riotinto of Australia (later to become CRA Limited) who assumed CZP's share of Comalco. In 1970, the company became Comalco Ltd. In 1982 Kaiser divested its share of Comalco. In late 1995 CRA Limited and its sister company RTZ of the UK merged to become the dual listed Rio Tinto Limited and Rio Tinto plc which took control of Comalco. In 2006 Comalco Limited was re-named Rio Tinto Aluminium, and following the Rio Tinto merger with ALCAN, became Rio Tinto Alcan in October 2007. The Rio Tinto Weipa operations in 2019 comprised the East Weipa, Andoon and Amrun mines.
Increased interest in bauxite in more recent years has led to the delineation of resources and construction of new mining facilities including Glencores Aurukun, Metro Mining's Bauxite Hills, and Metallica Minerals' small Urquhart projects.
Geological Setting
Western Cape York Peninsula is essentially occupied by a series of connected younger sedimentary basins filled by rocks of the Mesozoic Carpentarian and Cenozoic Karumba basin sequences. These successions overlie and flank a basement of Proterozoic to late Palaeozoic rocks that outcrop along a north-south oriented spine that extends from the Lockhart River in the north, to well south of Coen. The rocks of this basement spine, which are not exposed on the western side of Cape York Peninsula, are composed of Palaeoproterozoic to Cambrian metamorphic rocks, Silurian to Devonian granites of the Cape York Batholith, Upper Devonian to Permian sedimentary and volcanic rocks, and Permian granites. The basement spine is overlain to the NE by the Devonian to Carboniferous sedimentary and volcanic rocks of the Pascoe River Basin, which is, in turn, overlain by the Upper Carboniferous to Permian Olive River Basin. These are both unconformably overlain by Jurassic to Lower Cretaceous sandstone of the Carpentaria Basin. To the SE, the older bedrock is overlain by Upper Carboniferous to Cenozoic sequences, including the extensive Mesozoic Laura Basin that is connected across the bedrock spine to the Carpentaria Basin (Taylor et al., 2008).
On the western Cape York Pensinsula, the basement is overlain by the Carpentaria Basin that is, in turn, unconformably overlain by the Cenozoic Karumba Basin. The Weipa Sub-basin of the Carpentaria Basin extends from the Coen inlier westwards across the Weipa region and into the Gulf of Carpentaria, and was filled with ~830 m of sedimentary rocks that range from Mid-Jurassic to Cenomanian (lowermost Upper Cretaceous). These comprise 250 m of Jurassic to Cretaceous fluvial sandstone, followed by 600 m of early Cretaceous shallow marine mudstone, siltstone, sandstone and volcanolithic rocks (derived from andesitic source rocks) of the Rolling Downs Group. The latter are exposed along the western Cape York Peninsula. The Carpentaria Basin at Weipa is unconformably overlain by the Tertiary Karumba Basin, which onshore includes the Paleogene Bulimba Formation (previously the Weipa Beds), a fluvial, marginal to open-marine feldspathic quartz sandstone with minor siltstone, eroded from metamorphic and acid igneous rocks, with organic, peat-rich beds. Sedimentation still continues within the Karumba Basin with quartzose fluvial accumulations along river courses in the vicinity of Weipa and as large Paleogene to Holocene alluvial fans farther to the south (Taylor et al., 2008).
East Weipa, Andoon and Amrun
In 2019 bauxite was being mined by Rio Tinto Alcan from three operations in the Weipa area, East Weipa on the Weipa Peninsula, where production commenced in 1961, but in 2019 was nearing exhaustion; Andoon which is ~10 km to the NW, across the Mission River estuary, on the Andoon Peninsula; and Amrun commissioned in 2109, ~40 km south of the East Weipa mine and 40 km north of the centre of Aurukun.
The sequence in the Weipa area is as follows, from the base, after Taylor et al. (2008):
Basement - Proterozoic metamorphic and Palaeozoic granitic and volcanic rocks of the Coen Inlier
Unconformity
Jurassic to Cretaceous Rolling Downs Group
• Garroway Sandstone - Jurassic quartz sandstone with minor siltstone, claystone, coal and carbonaceous shale;
• Gilbert River Formation - the basal Cretaceous unit composed of coarse to medium grained quartz sandstone with minor siltstone and claystone;
• Wallumbilla Formation - argillites with some coarse to fine grained quartz sandstone toward the base, including glauconite and pyrite;
• Toolebuc Formation - carbonaceous to calcareous silt to claystone with minor sandstone;
• Allura Formation - medium grained glauconitic sandstone, grading to interbedded siltstone and sandstone;
• Normanton Formation - labile medium- to fine-grained glauconitic sandstone with locally common carbonaceous intercalations and siltstone;
Unconformity
Cenozoic
• Paleogene Bulimba Formation - quartzose sandstone, siltstone, coal and claystone with <10% quartz. These sedimentary rocks are interpreted to have been deposited by meandering and braided streams flowing across the relatively flat Cretaceous regression surface. At Weipa it commences with 6 to 8 m of coarse greyish-white angular, poorly sorted, quartzose to feldspathic sandstone (with a pebble conglomerate at the base) overlain by reddish brown interbedded clayey sands, sandy clays and clay lenses, fining upwards to kaolinitic clays and quartz sands in the lateritic weathering profile. Heavy minerals include zircon, leucoxene, rutile and tourmaline with rare ilmenite, anatase, titanite, siderite, magnetite, apatite, monazite, andalusite, spinel and staurolite. These assemblages show that the Bulimba Formation was derived from the Proterozoic and Palaeozoic bedrock spine to the east, in contrast to the underlying Rolling Downs Group which also had addition from andesitic volcanic source rocks. Similar assemblage are found in the bauxite (Schaap, 1990) and respective underlying sedimentary units, which is taken to demonstrate in situ formation of bauxite.
• Neogene Wyaaba Beds - limestone, sandstone, siltstone, mainly offshore;
• Quaternary - quartzose gravels, sands and minor muds;
At Andoom and south of East Weipa, bauxite overlies the glauconitic sediments of the Normanton Formation, the uppermost unit of the Rolling Downs Group, whilst on the Weipa Peninsula bauxite is developed over the Bulimba Formation, which itself unconformably overlies the Rolling Downs Group. The bauxites formed over these two different units can be distinguished on the basis their contents of total iron (<8% at East Weipa, >10% at Andoom), quartz (generally >2% at East Weipa, <1% at Andoom) and some trace-element chemistry, although they are otherwise difficult to differentiate. At Amrun, the top of the bauxite strata is generally <1m below the surface and the average bauxite ore thickness across the deposit area is ~3.4 m. It is hosted by coastal dunes, estuarine and delta deposits of the Bulimba Formation.
Three major and a fourth minor regolith landscapes have been defined (Bain & Draper 1997) within the Weipa Bauxite Plateau:
• Bauxite and ferricrete over completely weathered saprolite, the most extensive of these. This landscape extends from south of Aurukun, and continues for ~210 km to the north, to where it is within ~50 km of the tip of Cape York. It averages ~40 km in width. Bauxite remnants persist east of the main outcrops as relict mesas isolated from the main plateau (e.g. the Embley Range at Amrun). The regolith mainly consists of sandy and silty red- to yellow-coloured earths with local zones of fine grained bauxite ('red soils') overlying, or incised in channels cut into either a thicker (up to 10 m), or thinner (<1 m) layer of pisolitic bauxite. This in turn overlies a very widespread 'lateritic profile' capped by variably developed ferricrete. This regolith formed in both the Rolling Downs Group and the Bulimba Formation, whichever was the uppermost preserved at surface.
• Moderately weathered saprolite on Cretaceous (Rolling Downs Group) sandstone and siltstone;
• Moderately weathered saprolite on Early Tertiary (Bulimba Formation) sandstone; and
• Sand, tidal flat mud, coquina, salt pans and dunes of the coastal margin.
On the Weipa Peninsula the Bulimba Formation is up to 20 m thick and dips gently west. A 20 to 35 m thick weathering profile has been imposed on this sequence, comprising four gradational zones from the surface down as follows:
Laterite zone - comprising from top to bottom:
• topsoil,, which is 0.5 m thick and organic rich.
• redsoil, which is 0 to 5 m thick and comprises fine bauxite that has been transported and deposited in channels and basins incised into the bauxite horizon.
• bauxite, averaging 3 but up to a maximum of 12 m thick, which forms a gravel layer that is continuous over many kilometres. It is mostly composed of loose pisolites, with minor cemented bauxite. It comprises loose, fawn to dark brown coloured pisolites from 1 to 20 mm in diameter in a red brown and sandy matrix. It is mainly composed of gibbsite [Al(OH)3] and boehmite [AlO(OH)] which constitute 70 to 80% of the bauxite with 5 to 15% hematite, 0 to 20% kaolinite, 2 to 4% anatase, <1% zircon and 0 to 5% quartz, with 1 to 10% silica as both combined silica and free quartz. The composition of the bauxite varies significantly between pisoliths and within the bauxite horizon, and is characterised by lateral continuity and strong vertical zonation. Boehmite is strongest near the top of the bauxite horizon, whilst gibbsite increases downwards. The two distinct parent rocks has produced two main bauxite styles (Clements, et al., 2017):
i). Wiepa style formed from the Bulimba Formation, characterised by a higher silica derived from the parent rock, with quartz grains being common within pisoliths;
ii). Andoom style formed from the upper Rolling Downs Group, reflecting the higher Fe content of the glauconitic parent unit, producing pisoliths with a darker red colour, and with silica mainly within the kaolinite lattice.
Pisoliths range from q to 15 mm in diameter, median of 5 mm. They often exhibit complex histories of formation with a core that may include voids and fissures, coated by concentric layers of alternating alumina minerals and kaolinite.
The bauxite is formed from weathering of feldspathic bedrock on the flat surface that has persisted since the Mid-Cretaceous regression in a monsoonal climate. The feldspars weathered to kaolin, quartz and iron minerals, followed by reaction of groundwater with kaolinite to produce silica and bauxite minerals as follows:
Al2Si2(OH)4O5 (kaolinite) + 5H2O ⇔ 2Al(OH)3 (gibbsite) + 2Si(OH)4.
During this reaction, the silica must be constantly removed, as its presence will lead to the process being reversed with the gibbsite reverting to kaolinite. Consequently good drainage is essential to bauxite formation (Clements et al., 2017). Heavy monsoonal rain in the wet season results in a seasonal rise in the water table. The influx of fresh water reacts with kaolinite and the resultant silica-rich groundwater is flushed away in the porous sediments and pisolitic profile with continued heavy rainfall and strong vertical and lateral flow. This results in bauxite being formed above the water table and ironstone interface. The presence of organic matter affects the groundwater chemistry, Eh and pH, mobilising Fe and preferentially enriching Al (Clements et al., 2017).
• ironstone, 1 to 5 m of lateritic duricrust, made up of pisolitic nodules of goethite and hematite, with varying quantities of kaolinite and quartz. There is generally a sharp contact between bauxite and ironstone, although this interface can be transitional over a 0.5 m interval. The ironstone represents the top of the water table in the monsoonal wet season and forms an indurated cap to the mottled zone.
Mottled zone - which is 5 to 15 m thick and occurs below the ironstone layer at the base of the laterite profile. The quantity of iron drops off into red and yellow iron oxide mottling in the weathered Bulimba Formation.
Pallid of Plasmic Zone - 10 to 20 m thick kaolin-rich layer, with relatively little iron oxide, resulting in the Bulimba Formation being white, with quartz grains showing evidence of strong corrosion.
Saprolite zone - 5 to 20 m thick, used locally to define the transition from the pallid zone to the fresh sedimentary host, represented by a gradation from white to yellow to the greenish-grey of the unweathered Bulimba Formation and/or underlying Rolling Downs Group.
The kaolin resource was developed within clay lenses of the Bulimba Formation (Weipa Beds) where they fall within the pallid zone. These lenses are elongated with a south-westerly trend, and are generally around 300 m wide and 2 to 3 km long with an average 4.5 m thickness. The top and bottom contacts with sands are sharp. The kaolin is generally white and massive with a dry bulk SG of 1.5 and a pre-beneficiation composition of 51.6% silica, 33.05% alumina, 1.36% TiO2, 1.57% Fe2O3, 0.18% K2O, <0.05% of each of the other major elements and 12% LOI.
In 1975 detailed drilling within a 30 km radius of Port Weipa had outlined (pre JORC) 'reserves' of >500 Mt of commercial grade bauxite, while an estimate of the resource between Vrilya Point and and Archer Bay (Aurukun) was >2 Gt of commercial bauxite (Evans, 1975).
In 1990 detailed drilling within a 40 km radius of Port Weipa had outlined (pre JORC) 'proved' reserves of >350 Mt of commercial grade bauxite, while an estimate of the resource between Vrilya Point and Archer Bay (Aurukun) was >3 Gt of commercial bauxite (Schaap, 1990).
Proven+probable (pre JORC) in situ reserves of kaolin in 1990 were 17.3 Mt, with potential for a much larger tonnage.
Total proven+probable bauxite reserves in 2002 were: 730 Mt (Rio Tinto Annual Rept., 2002)
Total resources in 2002 were: 3600 Mt of mineable bauxite (Rio Tinto Annual Rept., 2002)
Production in 2002 totalled 11.2 Mt of bauxite. (Rio Tinto Annual Rept., 2002)
The Weipa bauxite deposit averaged around 55% Al2O3 and 5 to 5.5% SiO2 in 2002.
The 500 millionth tonne of bauxite was despatched from Weipa in 2013, 50 years after the first shipment left the port.
Total proved + probable bauxite reserves in 2006 were (Rio Tinto Annual Rept., 2006): 1193 Mt @ 53.7% Al2O3
Total measured + indicated + inferred resources in 2006 were: 2114 Mt @ 51% Al2O3
Total proven+probable bauxite reserves at 31 December 2018 were (Rio Tinto Annual Report, 2018):
East Weipa and Andoom - 163 Mt @ 50.5% Al2O3
Amrun - 1104 Mt @ 53.2% Al2O3
Total measured + indicated + inferred resources at 31 December 2018 were:
East Weipa and Andoom - 15 Mt @ 49.5% Al2O3
North of Weipa - 1330 Mt @ 52.0% Al2O3
Amrun - 635 Mt @ 49.9% Al2O3
NOTE: Mineral Resources are additional to Ore Reserves, whilst those quoted above post-1990 are JORC compliant.
Aurukun
The Aurukun project area comprises eleven clustered deposits distributed within a NW-SE elongated 75 x 25 km area between Weipa and Aurukun, inland from the west coast of Cape York Peninsula and the Amrun project area. The area, which is on the SE margin of the Weipa Bauxite Plateau, had been the subject of exploration between 1957 and 1968 and was further tested by the Queensland State Government during 2004-05. In 2008, the Chinese aluminium company Chalco entered into a Development Agreement with the State of Queensland over the Aurukun Bauxite Project. Subsequently, a drilling program delineated a total JORC compliant Measured + Indicated + Inferred Mineral Resources of 357 Mt @ 51.4% Al2O3, 11.2% SiO2. However, in 2010, Chalco withdrew from the
agreement with the Queensland Government and in 2012 the project area was put up for tender. In late 2017, Glencore was granted a mineral development licence over the area.
Bauxite within the Aurukun project area is developed over the Bulimba Formation quartzose sandstone, siltstone and coal claystone which in turn overlies the Normanton Formation glauconitic sandstone with carbonaceous zones of the Normanton Formation in the upper Rolling Downs Group. The lateritic and underlying saprolitic profile is similar to that described previously for Weipa, and may be summarised as follows, from the surface down (Mining Associates presentation, Mines and Money Conference, Bali, 2013):
• Zone 1 - 0 to 0.25 m depth - Topsoil - grey, dry fines and fine dark nodules;
• Zone 2/3 - 0 to 3 m thick - Reworked bauxite, with higher fines and lower recoveries, grading from earthy (Zone 2) to pisolitic (Zone 3);
• Zone 4 - 1 to 3 m thick - High boehmite-gibbsite content with higher hematite and a dry, reddish rusty colour and a range of pisolite/nodule sizes;
• Zone 5 - 3 to 5 m thick - High gibbsite, low (0 to 7%) boehmite. Moist and red-brown colour. Very uniform with smaller pisolites;
• Zone 6 - transition zone of variable thickness;
• Zone 7 - 0.25 to 0.5 m thick - Hard, iron-stained kaolinite;
• Zone 8 - Weakly mottled to pallid kaolinite.
Mineral Resources at 31 December, 2018 (Glencore Reserve and Resource Statement, 2018) were:
Measured Mineral Resource - 94 Mt @ 53.4% Al2O3;
Indicated Mineral Resource - 322 Mt @ 50% Al2O3;
Measured + Indicated Mineral Resource - 416 Mt @ 50.7% Al2O3;
Inferred Mineral Resource - 3 Mt @ 49.5% Al2O3.
Bauxite Hills
The Bauxite Hills Project is located 95 kms north of Port Weipa and 5 kms SE of the port at Skardon River on the west coast of Cape York Peninsula. The operation is half way between Weipa and Vrilya Point. Mining commenced in 2018. The geology of these deposits is similar to that described above. Within the mine leases, the plateau is mostly composed of bauxite which comprises a single 0.5 to 3 m thick flat-lying layer, generally composed of 55 to 80% high quality bauxitic pisoliths with the remainder being sand, silt and clay. Within the area of the estimated resources the average bauxite thickness is 1.6 m. The bauxite is generally covered by only a thin layer of soil, but in the western parts of the project area bauxite is sometimes found beneath sand dunes at depths of up to 6 m. The pisoliths are well rounded, and normally 5 to 20 mm in diameter, although larger pisoliths of up to 30 mm across do occur in the bauxite horizon. The bauxite passes down into an iron rich ferricrete horizon and then into mottled, bleached Bulimba Formation sandy clays. Larger, irregular shaped pisoliths and concretions are typical of the underlying ferricrete horizon and form a visual marker of the base of the bauxite (Metro Mining JORC Report, March, 2017).
Estimated Ore Reserve at the end of 2018 were 92.2 Mt included within a total Mineral Resources of 144.8 Mt @ 49.2% Al2O3, 13.2% SiO2 direct shipping ore (Metro Mining Limited web site, viewed June, 2019).
Urquhart
The Urquhart Bauxite Project is located 5 km SW of Port Weipa on the west coast of Cape York Peninsula, and consists of two bauxite plateaux containing extensive high-quality pisolitic bauxite, known as Area A and Area B. The geology of these deposits is similar to that described above.
Total Measured + Indicated + Inferred Mineral Resources are (Metallica Minerals Annual Report, 2018) at a 48% Al2O3 cut-off were:
9.5 Mt @ 52.8% Al2O3, 13.7% SiO2.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2017.
Record last updated: 12/6/2019
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Clements, A., Bower, J., Mills, M. and Rees, I., 2017 - Weipa bauxite plateau, Cape York: in Phillips, G.N., (Ed.), 2017 Australian Ore Deposits, The Australasian Institute of Mining and Metallurgy, Mono 32, pp. 855-856.
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Evans H J 1975 - Weipa bauxite deposit, Q: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne Mono 5 pp 959-964
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Schaap A D 1990 - Weipa Kaolin and Bauxite deposits: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1669-1673
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Taylor, G., Eggleton, R.A., Foster, L.D. and Morgan, C.M., 2008 - Landscapes and regolith of Weipa, northern Australia: in Australian J. of Earth Sciences v.55, pp S3-S16.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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