Iron 2002 - Key Iron Deposits of the World

For information on the remainder of the tour, see the Deposit Descriptions for Module 2A, Module 2B

The Mount Whaleback orebody is located near the town of Newman and is 370 km SSW of its export port at Port Hedland. It is the largest known high grade hematite deposit in the Hamersley province and the largest known single, continuous iron ore deposit in Australia. The deposit is hosted by the Brockman Iron Formation which is part of a faulted outlier of Hamersley Group rocks. It had an original measured resource of 1700 Mt of low phosphorous, hard, micro-platy hematite ore. Proven + probable reserves at Mt Whaleback and its satellites in June 2000 were 1230 Mt @ approx. 64% Fe, generally with 0.053% P, 4.3% SiO2 and 1.7% Al2O3. Prior to mining the deposit extended from 240 m above, to more than 300 m below plain level, with much of the reserve now below the present water table. It is 5.5 km long, while the final pit will be 1.5 km wide. The production rate from Mt Whaleback and it satellites is around 33 Mtpa.

The host Brockman Iron Formation overlies the McRae Shale (also of the Hamersley Group), the uppermost member of which - the Colonial Chert - also contains some ore enrichment. The Brockman commences with the Dales Gorge Member,the main horizon containing extensive high grade hematite ore at Mt Whaleback. It is an alternating assemblage of 17 BIF and 16 shale macro-bands. The shale macro-bands are each 0.1 to 2 m thick, and the BIF bands 1 to 7 m. In the mine this member is approximately 65 m thick, but is up to 135 m thick where not enriched.

The Dales Gorge is overlain by the Whaleback Shale Member which forms the hangingwall of the main ore horizon. It is locally split into three zones, namely the 6 to 8 m thick basal shale, the 2 to 7.5 m central chert and the 12 to 25 m thick upper shale. The basal shale contains five alternating shale, cherty BIF macro-bands, the lower of which is commonly enriched and included in the orebody. The central chert is not usually enriched, but is strongly contorted, while the upper shale contains numerous chert bands and has a gradational contact with the overlying Joffre Member.

The Joffre Member is approximately 240 m thick and comprises mainly BIF with only minor thin shale interbeds, with regular macro-banding being absent. It is the host for some hematite enrichment, and at depth this ore is indistinguishable from that of the Dales Gorge Member. In general the ores in this member are more goethitic and softer.

The deposit is structurally very complex. Two major westerly plunging overturned synclines define the orebody, while a major fault with a 900 m throw terminates the orebody to the north-west by causing an associated major reversal of plunge. In the northern and western sections the ore is concealed by generally barren BIF of the Joffre Member.

Physically the hematite ore at Mt Whaleback is hard, blue, fine grained and weakly banded (reflecting the original BIF meso-banding) with a porosity of approximately 20%. At greater depths it is less porous where hematite has recrystallised into pore spaces, while some of the banded ore is also brecciated. The hematite ore contains minor primary magnetite and martite octahedra. Chemically it contains approximately 69% Fe, with shale interbeds also being enriched. The contacts between high grade hematite and barren BIF is gradational over 15 to 30 m of medium to low grade, friable, siliceous hematite.

Massive, occasionally banded, non-porous goethitic hematite occurs near the surface resulting from hydration of the hematite ore, and can extend to as deep as 30 m below the present topography. In addition large areas of non-ore BIF have also been enriched in the same manner as a 2 to 30 m skin below the present surface, particularly over the Joffre Member.

Satellite deposits include Orebody 29 which is martite-goethite ore within the Marra Mamba Iron Formation with a measured resource (1990) of 160 Mt @ 61.9% Fe, 6.7% LOI, 2.3% SiO2, 1.9% Al2O3, 0.072% P; and Jimblebar 40 km to the east of Newman comprising a series of 3 to 50 Mt orebodies within the Joffre Member of the Brockman Iron Formation, totalling proven+probable reserves of 254 Mt of ore. Ore at Jimblebar is developed in the Marra Mamba and Brockman Iron Formations (in both the Dales Gorge and Joffre Members) and includes both martite-goethite and hematite ores.

The Mount Whaleback mine and its satellites are operated by the Mount Newman Mining Co., a partnership controlled by BHP Billiton (85%), with Mitsui & Co (Australia) Ltd (10%) and Itochu Corp (5%).

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West Angelas

West Angelas is located some 110 km west of Newman, 110 km south-east of Mt Tom Price and 330 km from the coast.

It is one of a number of large Marra Mamba Iron Formation and is composed of a number of deposits which in March 2000 comprised probable reserves totalling 440 Mt @ 62% Fe, with a further 585 Mt @ 61.5% Fe in the indicated + inferred category. These are mainly martite - ochreous goethite ores which carry around 0.065% P, 3.5% SiO2, 2.2% Al2O3 and 6,5% LOI. The lump to fines ratio of the reserve is 33:67. A production of 7.9 Mt is planned or the first year of operation, increasing to around 20 Mtpa in year 8.

The two larger orebodies are found in association with synclinal structures on the flanks of the west plunging Wanna Munna Anticline, although other minor deposits located around the anticline do not appear to be related to synclines. Mineralisation is mainly developed in the Mt Newman Member, the uppermost of the Marra Mamba Iron Formation, with minor occurrences in the West Angelas Member at the base of the overlying Wittenoom Dolomite.

The Marra Mamba Iron Formation has been sub-divided into three units, commencing with the basal 135 m thick Nammuldi Member comprising cherty BIF interbedded with thin shales, followed by the medial 35 m thick MacLeod Member made up of BIF, chert and carbonate with numerous shale interbeds. The uppermost Mount Newman Member hosts most of the mineralisation and is 60 m of interbedded BIF with carbonate and shale.

These are conformably overlain by the West Angelas Member, the basal of three recognised packages that make up the Wittenoom Dolomite. This member is 40 m thick, composed of shale (often manganiferous), chert and dolomite with minor BIF near its base. The other succeeding members comprise a 150 m thick package of crystalline dolomite with minor chert, capped by an alternating shale, dolomite and minor chert package.

The mine is owned and operated by Robe River Iron Associates, an unincorporated joint venture of Rio Tinto Ltd (53%), Mitsui Iron Ore Development Pty Ltd (33%), Nippon Steel Australia Pty Ltd (10.5%) and Sumitomo Metal Australia Pty Ltd (3.5%).

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Yandicoogina

The Yandicoogina deposits of BHP Billiton and Hamersley Iron are located some 90 km north-west of Newman and around 150 km east of Marandoo. They are part of a single, continuous, high grade, low phosphorous pisolitic goethite body which is over 80 km in length. The deposit averages 500 to 650 m in width and is around 70 m thick in the channel centre.

Together resources of some 4700 Mt have been indicated. In 2000 proven + probable reserves at the BHP Billiton Yandi deposits totalled 817 Mt @ 58.4% Fe, while Hamersley Iron had a proven reserve of 310 Mt @ 58.5% Fe plus a further 870 Mt @ 58% Fe in the indicated and inferred categories. The ores assay approximately 0.05% P, 5% SiO2, 1.4% Al2O3, 10% LOI, with around 65% calcined Fe. BHP Billiton and Hamersley Iron are each mining approximately 15 Mtpa from their respective leases to produce low alumina pisolitic goethite-hematite fines ore. Some 90% of the Hamersley Iron reserves are below the water table and require draining before mining.

The deposits infill east-west trending Tertiary palaeo-channels that were incised into shale, dolerite and BIF of the Weeli Wolli Formation in the core of a regional easterly trending syncline which plunges to the east, and exposes broad widths of shallow dipping Brockman Iron Formation on both of its flanks and around the western closure.

The main pisolitic zone is up to 80 m thick and overlies a basal gravel bed. The basal gravel is irregular and comprises a cemented gravel of 1 to 2 cm hematite pebbles rimmed by black goethite and siliceous cement, varying from 1 to 2, up to 12 m in thickness. The pisolitic unit is composed in turn of: (1) a 0 to 20 m thick basal zone around 300 m wide with 45-57% Fe, comprising a pisolitic goethite-ochreous goethite claystone in the channel centres averaging 15 m thick, and a massive goethitic clay unit on the lateral margins; and (2) the overlying main pisolitic zone, or ore zone, which is 40 to 70 m thick and from 400 to 1100 m wide with average grades of 56-59% Fe.

The basal and main ore zones are separated by a 1 to 5 m thick band that varies from clay to clay matrix conglomerate to a massive banded vitreous goethite. At surface the main ore zone has an up to 12 m thick interval of sub-ore grade (<56% Fe) material, underlain from 12 to 20 m in depth by 56-58% Fe which is transitional with the underlying high grade (>58% Fe) ore. The deposit is basically composed of masses of cemented concretionary iron oxides occurring as irregular, sub-rounded goethitic clasts (up to 3 mm in diameter) separated by either a loose matrix, or a subsequent brown to grey sub-vitreous to vitreous goethite cement, or are just densely packed. The ore is composed of either (1) cemented sub-rounded to rounded iron oxide pisoliths, with some void space; or more commonly (2) pseudo pisoliths of non-iron oxides coated by goethite from iron charged ground waters.

The pisoliths are composed of concentric shells of limonite and vitreous goethite, generally with a core of goethite. Replacement of angular to sub-angular BIF and shale cores by limonite is discernible in some hand specimens. Hematite is subordinate to goethite in these ores.

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Hamersley Basin Field Workshop & Overview

Tony Harding, a leading expert from the Rio Tinto Iron Group provided an afternoon class room workshop and discussion on the Hamersley Basin and its iron deposits, their occurrence and genesis supported by images and plans, followed by a full day traverse of the host sequence and iron formations of the Mount Bruce Supergroup.

The main iron ores of the Hamersley province are hosted with in the Archaean to Palaeoproterozoic volcanic and sedimentary sequence the Mount Bruce Supergroup which spans a time interval of over 400 Ma, from greater than 2770 Ma to near 2350 Ma. It rests unconformably on pre-2800 Ma granitoids and greenstones of the Archaean Pilbara Block in the far north-west of the state of Western Australia, and is overlain by the Wyloo Group sediments which comprise the remainder of the Hamersley province sequence continuing to near 1800 Ma.

The Mount Bruce Supergroup is in turn sub-divided into three Groups. The lowermost of these, the Fortescue Group, commences with an early phase of clastic sediments and mafic volcanism in localised grabens (the around 2770 Ma Bellary Formation), followed by extensive sandstones and conglomerates (the 500 to 2000 m thick, 2750 Ma Hardy Sandstone) which thicken markedly from north to south, with near 50% of the thickness in the south being mafic sills. These sediments are unconformably overlain by the volcanics and sediments of the Tumbiana Formation, with similar thickness and mafic sill percentage increases from north to south. The uppermost unit is the 2690-2630 Ma, 100 to 150 m thick organic and sulphide rich fine clastics of the Jeerinah Formation, with mafic volcanics and sills increasing southwards. The Fortescue Group is conformably overlain by the 2500 m thick Hamersley Group which hosts most of the main iron ore deposits of Western Australia. It is characterised by around 1000 m of laterally extensive banded iron formation representing three major episodes.

The basal Marra Mamba (2600 Ma) and the medial Brockman Iron Formations are separated by the carbonate, shale and minor chert of the Wittenoom, Mount Sylvia and Mount McRea Shale Formations (2600 to 2480 Ma). This passive sequence is followed after the Brockman Iron Formation by the third phase of iron formation deposition (the Weeli Wolli Iron Formation) which was accompanied by intense 2450 Ma bimodal volcanism and mafic sills (which locally account for up to 80% of the sequence), overlain by the felsic volcanics of the Woongarra Formation . Thickness variations in the Hamersley Group are only minor.

The Turee Creek Group is the youngest unit of the Mt Bruce Supergroup. The uppermost unit of the Hamersley Group, the Boolgeeda Iron Formation passes conformably upwards into the thick basal Kungarra Shale of the 3000 to 5000 m thick Turee Creek Group which is basically a coarsening upwards clastic sequence in a choked basin - marking a major change fro the starved basin of the Hamersley Group. The southern half of the Hamersley Group was deformed by the Ophthalmia orogeny into an east west trending fold belt that decreases in intensity to the north and records north-south compression. The top of the Mount Bruce Supergroup is separated from the overlying Lower Wyloo Group Beasley River Quartzites by a first order unconformity. The basal conglomerate includes clasts of Hamersley Group banded iron formations and very rare hematite. These coarse sediments pass upwards into finer clastics and mafic volcanics to the 2000 m thick 2209 Ma Cheela Springs Basalt which are followed by dolomites to the west, but are cut by the major unconformity that separates the Lower and Upper Wyloo Groups which cuts down as far as the Fortescue Group. A generation of NW trending folds developed at the close of the Lower Wyloo Group interacted with the Ophthalmia orogeny structures to form a series of domes and basins.

The Upper Wyloo Group was deposited above a major unconformity It was formed in an extensional basin and comprise up to 12 km of sediments which are overlain to the south by the poorly sorted clastics of the Ashburton Formation which includes bimodal volcanics dated at 1842 to 1828 Ma. The Upper Wyloo Group was terminated at the time of the intrusion of the 1790 Ma Boolaloo Granite.

Ores mined in the Hamersley province may be divided into (1) enriched, bedded ores and (2) goethitic pisolitic accumulations within extensive palaeo-channels tens of kilometres in length, now largely preserved as mesas. The bedded ores are sub-divided into (a) extensive flat lying martite-goethite ores developed from both Marra Mamba and Brockman Iron Formations by deep supergene enrichment of precursor banded iron formations, and (b) high grade hematite, often with martite and microplaty hematite, but little goethite, and almost invariably developed within the Brockman Iron Formation. The latter commonly occur to much greater depths (to more than 400 m) and account for the largest high grade deposits of the province, including Mt Tom Price and Mt Whaleback.

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Mt Tom Price

The Mount Tom Price mine, which commenced production in 1965 is located some 210 km WNW of Newman and 260 km SSW of its export port of Dampier. It is owned and operated by the Rio Tinto Group company Hamersley Iron Pty Ltd. The orebody at Mount Tom Price originally contained the second largest known accumulation of high grade hematite in the Hamersley province and occurs near the keel of the large Turner Syncline, close to its eastern extremity. The initial reserve totalled around 900 Mt @ 64% Fe with a high lump to fines ratio, and low impurities. The impurity content of the high grade ore reserve in 1990 was 0.053% P, 3.5% SiO2 and 1.9% Al2O3. The deposit is some 7.5 km long and up to 1.6 km wide, but averages 0.6 km, occupying two local synclines and part of the intervening anticline. These early folds have been subjected to later cross folding producing an en echelon pattern, while two south dipping normal faults parallel and in part limit the ore. The base of the northern syncline is higher than that of the southern giving an overall southerly dip and apparent thickness of 150 m, extending to a depth of 250 m below surface.

The orebody is composed mainly of hematite within the Brockman Iron Formation, with the majority of the ore associated with the Dales Gorge and underlying Colonial Chert Members. The deepest drilling at the mine is generally to the top of the Marra Mamba Iron Formation which is overlain by the 150 m thick Paraburdoo Member (the carbonate unit of the Wittenoom Formation), that passes up into the shaly 157 m thick Bee Gorge Member, followed by the 30 to 40 m thick cherty Mount Sylvia Formation. The overlying 50 m thick Mount McRae Shale is composed of black pyritic shale, capped by the uppermost unit of the Wittenoom Formation, a 12 m thick chert band, the Colonial Chert Member.

The Colonial Chert is followed by the 150 m thick basal Dales Gorge Member of the Brockman Iron Formation, comprising 17 alternations of BIF and shale. These have been grouped into 3 sub-units on the basis of shale content. The lowest, DG1, and uppermost DG3 have 6% and 7% shale respectively, while the intervening DG2 has 31%. The Dales Gorge is overlain by the 50 m thick Whaleback Shale Member composed of green to black shale and chert, which is in turn followed by a 360 m thick BIF unit with only minor shale, the Joffre Member.

Some 90% of the ore at Mount Tom Price is within the Dales Gorge Member, with local enrichment in the Joffre Member where it is in fault contact with mineralised Dales Gorge. The remainder of the ore is in the Colonial Chert and Whaleback Shale Members. Primary Dales Gorge Member BIFs away from any enrichment are dominated by chert and magnetite, accompanied by variable, but lesser hematite, carbonate and Fe-silicates. The high grade mineable reserves at Mount Tom Price are present as hematite ore which preserves the meso- and micro-banding of the original BIF, is characteristically porous (averaging 30% porosity), has a high lump yield and low contaminants. In places the porous ore alternates with dense bright metallic lustre hematite with only around 4% porosity to produce a defined banding. It is essentially composed of randomly oriented fine grained platy hematite and martite with individual plates being 0.001 to 0.25 mm across. Fusing of these micro-plates, gives the lump ore its character. Ultra-fine earthy hematite filling the voids is generally less than 5%. Shale macro-bands within the orebody have been partially replaced by iron oxides and at times may exceed 50% Fe.

The orebody was capped by a variable layer of low grade hydrated material, predominantly goethite, averaging 18 m in thickness, but down to 50 m in synclinal troughs. It has an irregular and patchy distribution controlled by fractures and joints, etc.. Minor deposits of 'canga' - 1 to 20 cm fragments of hematite and/or BIF cemented by goethite - occur as scree deposits and hillside wash in streams.

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Marandoo

The Marandoo deposit is located some 35 km to the east of Tom Price and is owned and operated by the Rio Tinto Group company, Hamersley Iron Pty Ltd. The mine was opened in 1994 and produces martite-goethite ore ranging to martite-ochreous goethite from a pre-mining measured resource of 360 Mt @ 62.6% Fe with 0.053% P, 2.9% SiO2, 1.7% Al2O3, 0.7% Mn and 4.8% LOI. Manganese oxides, mainly pyrolusite and cryptomelane, occur sporadically through the ore and shales.

The mine produces ore that is mostly confined to the upper 25 to 28 m of the 50 m thick Mt Newman Member of the Marra Mamba Iron Formation, although at surface the entire member is enriched. The Mt Newman Member is usually composed of interbedded BIF with carbonate and shale. The deposit extends over a length of more than 7 km along strike, with a width of 1.6 km. The ore is composed of an upper hard band and a lower soft material that must be mined in equal quantities and blended. The orebody is found on the drag folded northern limb of the Milli Milli Anticline, associated with early folds modified by later, open north trending cross-folds. It is intensely deformed in outcrop, with folds becoming more open down dip where strata dip at 3 to 5 degrees north, and plunge gently east with no major faults or shears.

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Paraburdoo

The Paraburdoo operation, which commenced production in 1972 is located some 65 km south of Mt Tom Price and around 320 km SSW of its export port of Dampier. The mine is owned and operated by the Rio Tinto Group company Hamersley Iron Pty Ltd.

Enrichment to ore grade is developed in both the Dales Gorge and Joffre Member of the Brockman Iron Formation (see the Hamersley Basin Iron Province and Mt Tom Price descriptions for more stratigraphic detail) forming two lenticular hematite masses seperated by a thinner low grade zone corresponding to the intervening Whaleback Shale Member. About half of the ore is within the Joffre Member. The orebodies now lie entirely within the Tertiary weathering profile.

The main deposit is divided by a creek valley into two sections, 4 West and 4 East. It lies within a locally flattened near surface part of the steeply south dipping Brockman Iron Formation sequence.

Hematite is the dominant iron mineral in the high grade zones with subsidiary goethite and limonite. This ore is very similar to that at Mt Tom Price. The hematite ore is blue-grey, massive to porous and mostly micro-platy. Goethite associated with the hematite is dense, while massive, vitreous and porous cellular types are found closer to the surface. The amount of geothite decreases with depth and the hematite becomes more porous suggesting the goethite is a Tertiary weathering product.

Pre-mining in 1972, the reserve was 300 Mt @ 63% Fe, 0.088% P, 3.8% Silica, 2.1% Alumina.

In 1975 the total potenmtial was quoted at 700 Mt of +60% Fe, composed of:

411 Mt of proven ore at 63.6% Fe, 0.076% P, 3.1% Silica, 2.5% Alumina, 2.8% LOI,
108 Mt of drill indicated high grade ore in 3 deposits @ 62.9% Fe, 0.097% P, 3.5% Silica, 2.7% Alumina, 3.6% LOI,
181 Mt of indicated high grade ore in 13 deposits with 60 to 62% Fe.

The reserve grade in 2001 was 62.42% Fe, 3.77% Si, 2.08% Al, 0.113% P, 0.095% Mn, 3.97% LOI.
The developed resource in the currently operating Paraburdoo orebodies and Eastern Ranges is 185 Mt @ 63% Fe.
The Western Range deposits to be developed in coming years has a reported undeveloped resource of 245 Mt @ 62.5% Fe.

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Robe River Mesa 'J'

Pisolitic ores have been mined from a number of deposits near Pannawonica by Robe River Associates since 1972. The current long term operation in the district is the Mesa 'J' deposit. Pannawonica is some 190 km NW of Tom Price and 140 km SW of its export port, at Cape Lambert.

Mineralisation at Robe River consists of a series of mesas, mantled by hard goethitic pisolitic deposits of Tertiary age which occur on either side of the Robe River from Pannawonica Hill in an ESE direction for more than 35 km. In general the mesas are from 46 to 62 m above the current flood plains of the river and have steep walls from recent erosion. Most of the deposits are unconformably developed above the middle to upper Fortescue Group, particularly the basalts of the Mount Jope Member, although rocks of the Marra Mamba Iron Formation are found in the extreme south-west. The unconformity between the basalts and pisolitic deposits is usually marked by a zone of white to grey kaolinitic clay.

Mesa 'J' is the largest of the deposits worked in the district, and is a pisolitic goethite-hematite ore with a grade of 57.2% Fe over a thickness of up to 50 m. Overburden consists of thin soil horizons, clay and weathered goethite and sometimes calcrete, colluvium and alluvium which are usually thin but may be up to 15 m thick. These are underlain by the main ore zone which is generally 5 to 40 m thick. Typically the goethite-hematite pisolitic ore yields grades of 55-59% Fe, 0.04% P, 5-6% SiO2 and 2.5-3% Al2O3. Discontinuous horizontal lenses of clay and claystone occur within the main ore horizon, while clay (alumina contaminant) occurs as an alteration product around joints and fracture. Solution cavities up to several metres across are common, particularly below the water table. The ore zone is usually stratified with a porous pisolitic texture and a dark brown metallic lustre. Lower grade material is usually more friable with a high content of orange/yellow ochreous clay.

The pisolitic ores have a pisolitic to oolitic character. Generally spherulites of oolitic dimensions (ie. less than 2 mm in diameter) tend to be of higher grade and more indurated. Those with pisolitic sized concretions and up to 10 mm in diameter are of lower grade and higher in diluent and porosity.

The iron oxides goethite, limonite, hematite and maghemite are mixed in both the pisolites and ground mass. In general pisolites have a hematitic core surrounded by thin concentric concretionary spheres of goethite, hematite and maghemite. Diluents are usually minute particles of silica, generally more abundant in the outer shells. The groundmass consists of colloform isotropic yellow to brown limonite or brownish-black goethite. Minute cavities in more friable ores are often lined with opaline silica.

The Robe River operation currently produces more than 30 Mt of ore per annum for export, with a total cumulative production since 1972 of more than 500 Mt of sinter fines and lump ore. The mine is owned and operated by Robe River Iron Associates, an unincorporated joint venture of Rio Tinto Ltd (53%), Mitsui Iron Ore Development Pty Ltd (33%), Nippon Steel Australia Pty Ltd (10.5%) and Sumitomo Metal Australia Pty Ltd (3.5%).

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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. These are listed in the Literature Collections pages for this tour.

		Another PGC International Study Tour Developed & Managed by Porter GeoConsultancy Iron 2002-03 Key Iron Deposits of the World September 2002 & March-April 2003
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