Image: The 5.5 km long Mt Whaleback pit in the wet season.
DESCRIPTIONS of ORE DEPOSITS & PROGRAM
The program for this module of the tour included:
MODULE 1 - AUSTRALIA
Thursday 12 to Wednesday 18 September 2002,
For information on the remainder of the tour, see the Deposit Descriptions for Module 2A, Module 2B
New & Recent International |
Click on image for details.
MODULE 1 - AUSTRALIA, The Iron Deposits of the Pilbara
The Pilbara hosts the largest iron ore producers and reserves in Australia, and many of the world's leading iron mines. This tour visited a selection of the most important deposits and each of the main styles of ore within the province. In addition, an expert field workshop allowed the tour group to study the host succession and the banded iron formations of the Hamersley Group, including those that host ore and those that dont.
The main iron ores of the Hamersley province are hosted within the Archaean to Palaeoproterozoic volcanic and sedimentary sequence the Mount Bruce Supergroup which overlies the granites and greenstones of the Archaean Pilbara Block in the far north-west of the state of Western Australia. The Mount Bruce Supergroup is sub-divided into three Groups. The lowermost of these, the Fortescue Group, is composed of clastic sediments, mafic volcanics and sills.
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 carbonate, shale and minor chert. This passive sequence is followed by the third episode (the Weeli Wolli Iron Formation ) which was accompanied by intense 2450 Ma bimodal volcanism and mafic sills, overlain by a suite of felsic volcanics. Thickness variations in the Hamersley Group are only minor.
The 3000 to 5000 m thick Turee Creek Group is the youngest unit of the Mt Bruce Supergroup, and is basically a coarsening upwards clastic sequence in a choked basin - marking a major change from the starved basin of the Hamersley Group.
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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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 (#Location: 23° 21' 47"N, 119° 40' 30"E).
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It had an original measured resource of 1700 Mt of low phosphorous, hard, micro-platy hematite ore. 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 main Mt Whaleback and associated satellite deposits are hosted by both the Marra Mamba and Brockman Iron Formations, within a faulted outlier of Hamersley Group rocks. These rocks are part of the Archaean to Palaeoproterozoic volcanic and sedimentary sequence of the Mount Bruce Supergroup, which spans a time interval of over 400 m.y., from >2.77 to near 2.35 Ga. The ~2500 m thick Hamersley Group contains three main iron formation units, the basal Marra Mamba, medial Brockman and upper Weeli Wolli. It is underlain by the mafic and bimodal volcanic rocks and clastic sediments of the up to 5000 m thick Fortescue Group, and overlain by the basal Boolgeeda Iron Formation and coarsening upwards sequence of clastic sedimentary rocks that make up the 3000 to 5000 m thick Turee Creek Group.
See the Hamersley Basin Iron Province record for the regional setting.
The Marra Mamba Iron Formation is exposed to the south of the Brockman ore. It is composed of three units, commencing with the basal 135 m thick Nammuldi Member, comprising cherty BIF interbedded with thin shales; the overlying 35 m thick MacLeod Member, made up of BIF, chert and shale interbeds; and the uppermost Mount Newman Member, 60 m of interbedded BIF with carbonate and shale. High-grade martite-goethite ore (~45 to 50 m thick), is primarily found in the upper Mount Newman Member, with some enrichment in the upper 10 to 15 m of the MacLeod Member in some areas. Ore extends for 200 to 400 m down dip from the outcrop to depths of 100 to 150 m, locally to 300 m. The footwall contact with un-enriched BIF is sharp, although the transition along strike is over an interval of >100 m. An 18 m thick hard cap covers the elevated outcrops. The Marra Mamba ore occurs over a broad area within 2 km to the south and SE of the main Mount Whaleback Brockman ore deposit. It is developed over a strike length of >3 km within a shallowly WNW-plunging synform with shallow limbs (Orebody 29), and and antiform to the west, with a steep northern (Orebody 30) and shallow southern limb (Orebody 35).
The Marra Mamba Iron Formation is followed by the Wittenoom Dolomite, West Angelas Member shale, BIF and chert, and the Mt Sylvia Shale.
The Brockman Iron Formation overlies the next unit, the McRae Shale, 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 ~65 m thick, but is up to 135 m thick where not enriched.
The Dales Gorge Member is overlain by the Whaleback Shale Member, which forms the hanging wall to 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 and 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 complex. The Brockman orebody occurs on the limbs of two major, westerly plunging, overturned synclines, the South and East synclines, separated by an overturned anticline with a shallowly south-dipping axial normal faiult. A major, steeply south-dipping normal fault zone with a 900 m throw, the Whaleback Fault terminates the orebody to the north, while to the east it is limited by a major reversal of plunge on the East syncline. In the northern and western sections the ore is concealed by generally barren BIF of the Joffre Member. The Marra Mamba Orebody 29 syncline is up-plunge from the South Syncline, where the axis plunges at 10 to 20°NW.
The Marra Mamba martite-goethite ore is essentially friable martite and ochreous yellow goethite.
Physically the Brockman 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.
The information in this summary is mainly sourced from Harmsworth et al., 1990.
Proven + probable reserves at Mt Whaleback and its satellites in June 2000 were:
1.23 Gt @ ~64% Fe, 0.053% P, 4.3% SiO2, 1.7% Al2O3.
Resources and reserves at Mt Whaleback and its satellites at 30 June 2012 (BHP Billiton, 2012) were:
Measured + indicated + inferred Brockman resources - 3.152 Gt @ 60.6% Fe, 0.12% P, 5.0% SiO2, 2.6% Al2O3, 4.9% LOI;
Measured + indicated + inferred Mara Mamba resources - 1.257 Gt @ 59.4% Fe, 0.07% P, 4.2% SiO2, 2.5% Al2O3, 7.3% LOI;
Proved + probable Brockman reserves - 1.133 Gt @ 62.9% Fe, 0.10% P, 4.1% SiO2, 2.0% Al2O3, 3.2% LOI;
Proved + probable Mara Mamba reserves - 78 Mt @ 61.6% Fe, 0.06% P, 3.1% SiO2, 1.8% Al2O3, 6.4% LOI.
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%).
West Angelas is located some 110 km west of Newman, 110 km south-east of Mt Tom Price and 330 km from the coast in the Hamersley Basin of Western Australia.
West Angelas is one of a number of large Marra Mamba Iron Formation resources and is composed of a group of deposits that 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 was planned or the first year of operation in 2002, and had increased to 54 Mtpa by 2009.
See the Hamersley Basin Iron Province record for the regional setting.
The two larger orebodies are found within the Marra Mamba Iron Formation, 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.
In March 2000, prior to the commencement of operations in 2002, probable reserves totalled 440 Mt @ 62% Fe, with a further 585 Mt @ 61.5% Fe in the indicated + inferred category.
Reserves and resources at the end of 2011 (Rio Tinto 2012) were:
Total proved + probable reserves - 297 Mt @ 61.6% Fe, plus
Total measured + indicated + inferred resources - 218 Mt @ 61.9% Fe (in addition to reserves).
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%).
The Yandicoogina deposits of BHP Billiton and Rio Tinto 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 within the Marillana palaeochannel network. The deposit averages 500 to 650 m in width and is around 70 m thick in the channel centre.
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 3.50 to 2.80 Ga granitoids and greenstones that occupy the northern half of the Pilbara craton, and is overlain by the thick Wyloo Group sediments which comprise the remainder of the Hamersley province sequence continuing to near 1800 Ma.
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: i). a 0 to 20 m thick basal zone around 300 m wide with 45 to 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 ii). 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 to 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 to 58% Fe which is transitional with the underlying high grade (>58% Fe) ore.
The deposit is basically made up 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 i). cemented sub-rounded to rounded iron oxide pisoliths, with some void space; or more commonly ii). 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.
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 Rio Tinto had a proven reserve of 310 Mt @ 58.5% Fe plus a further 870 Mt @ 58% Fe in the indicated and inferred categories.
Reserve and resource figures (Rio Tinto, 2012; BHP Billiton 2012) in 2011 were:
Rio Tinto - Total proved + probable reserves
HG Pisolites, 220 Mt @ 58.7% Fe + Process Ore, 161 Mt @ 58.7% Fe,
Rio Tinto - Total measured + indicated + inferred resource - 471 Mt @ 57.7% Fe (in addition to reserves),
BHP Billiton - Total proved + probable reserves - 867 Mt @ 57.2% Fe, 0.04% P, 5.7% SiO2, 1.5% Al2O3, 10.7% LOI, (included in resource),
BHP Billiton - Total measured + indicated + inferred resource - 2538 Mt @ 55.8% Fe, 0.05% P, 6.4% SiO2, 2.3% Al2O3, 10.9% LOI,
In 2002, the ores assayed approximately 0.05% P, 5% SiO2, 1.4% Al2O3, 10% LOI, with around 65% calcined Fe. BHP Billiton and Rio Tinto were each mining more than 50 Mtpa in 2012 from their respective leases to produce low alumina pisolitic goethite-hematite fines ore. Some 90% of the Rio Tinto reserves are below the water table and require draining before mining.
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 sedimentary rocks and mafic volcanism (the ~2.77 Ga Bellary Formation), deposited in localised grabens, with associated doleritic intrusions. These were followed by extensive sandstones and conglomerates with interbedded rhyolitic to dacitic flows, tuffs and epiclastic rocks (the 500 to 2000 m thick, 2.75 Ga Hardy Formation) which thicken markedly from north to south, with near 50% of the thickness in the south being mafic sills. These rocks are overlain by the 2.69 to 2.63 Ga, Jeerinah Formation, which commences with a sequence of bimodal basaltic to andesitic and rhyolitic volcanic rocks, with mafic sills increasing southwards, followed by 100 to 150 m thick organic and sulphide rich fine clastic rocks, lesser dolomites and sandstones. These are, in turn locally followed by several thousand metres of basaltic to andesitic volcanic rocks with interbedded stromatolitic dolomites, shales, sandstones and conglomerates, and a similar thickness and mafic sill, the percentage of which increases from north to south.
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 base of the Hamersley Group represents a change from a volcanic to chemical sedimentary environment, and is marked by the base of the Marra Mamba Iron Formation.
The basal, ~200 m thick Marra Mamba (2.60 Ga) and ~600 m thick medial Brockman Iron Formations are separated by ~750 m of carbonate, shale and minor chert of the Wittenoom, Mount Sylvia and Mount McRea Shale Formations (2.60 to 2.48 Ga). This passive sequence is followed, after the Brockman Iron Formation, by the third phase of iron formation deposition (the ~600 m thick Weeli Wolli Iron Formation) which was accompanied by intense 2.45 Ga bimodal volcanism and mafic sills (which locally account for up to 80% of the sequence), overlain by up to 800 m of felsic volcanics of the Woongarra Formation and the uppermost ~450 m thick Boolgeeda Iron 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 from the starved basin of the Hamersley Group.
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.
The southern half of the Hamersley Group was deformed by the north-south compressive, ~2.45 to 2.2 Ga Ophthalmia orogeny, that formed an east-west trending, northward-verging, fold and thrust belt, characterised by south-dipping thrust faults and asymmetric to overturned folds, which decrease in intensity to the north. A generation of NW trending folds were developed during the ~1.80 to 1.65 Ga Ashburton Orogeny, at the close of the Lower Wyloo Group, interacting with the Ophthalmia fold belt structures to form a series of domes and basins.
Ores mined in the Hamersley province may be divided into (1) enriched, bedded ores and (2) channel iron deposits (CID) within extensive palaeo-channels tens of kilometres in length, now largely preserved as mesas.
The enriched bedded ores of the Hamersley Group are sub-divided into:
i). extensive flat lying martite-goethite ores developed from both Marra Mamba and Brockman Iron Formations by deep supergene enrichment of precursor banded iron formations, mostly during the Mesozoic to Tertiary (e.g., Marandoo - 390 Mt @ 63% Fe; West Angelas - 515 Mt @ 61.8% Fe, Area C - 3.294 Gt @ 60.0% Fe). Ores developed from Marra Mamba Iron Formation, tend to have a higher proportion of ochreous goethite, which is more friable with a marked yellow colour, while those over the Brockman Iron Formation are generally brown and less friable;
ii). high grade hematite, mostly martite and microplaty hematite, but little goethite, predominantly developed within the Brockman Iron Formation within the main WNW-ESE trending Ophthalmia fold belt of the Hamersley Range. These ores commonly occur to much greater depths (to more than 400 m) and account for the largest high grade deposits of the province (e.g., Mount Whaleback - >3.5 Gt @ >60% Fe; Mount Tom Price - 900 Mt @ 64% Fe). Enrichment of the primary iron formation is interpreted to be predominantly hypogene, and to have taken place in three stages (Taylor et al., 2001, Hagerman et al., various and others). The first involved low to moderate temperature (110 to 280°C), highly saline, bicarbonate-saturated brines from the underlying sedimentary successions (e.g., Wittenoom dolomite and evaporites), transported via faults to the BIFs to migrated laterally in large folds, between shale aquitards. These fluids removed silica, leaving a thinned residue, enriched in iron oxides (mainly magnetite), carbonates (mainly siderite), magnesium silicates and apatite. The second stage involved deeply circulating, low temperature (<110°C), oxidised, Na-rich meteoric waters that interacted with evaporites, prior to their interaction with the BIF, to oxidise the mainly magnetite-siderite assemblage to hematite-ankerite, and then stripped all carbonate, leaving highly porous and permeable iron ore bands composed of martite-microplaty hematite-apatite, interbedded with magnesium-rich shale bands. A third, purely supergene stage, most likely in the Mesozoic to Tertiary, coincident with the formation of the martite-goethite ores described above, converted magnesium silicates to a kaolinitic residue, greatly thinning the shale bands, destroyed apatite, and leached calcium and phosphorus from the ore. This three stage process produced a highly porous hematite ore with a characteristic microplaty texture, interbedded with kaolinitic shale containing significant aluminium and titanium, which retained their relative proportions throughout the upgrading process. The Marra Mamba martite-goethite ore sheet in the Chichester Range, north of the Ophthalmia fold belt, but associated with structures on the northern margin of the Hamersley Basin, overprints and is underlain by microplaty hematite mineralisation (e.g., Cloud Break and Christmas Creek);
iii). magnetite-rich ores, mostly within the Brockman Iron Formation, occurring as a laminated, metamorphosed oxide-facies iron formation in which the original chert or jasper bands have been recrystallised into distinguishable grains of quartz and the iron is present as thin layers of hematite, magnetite or martite (e.g., Cape Preston - 2.185 Gt @ 22.1% MagFe and 31.3% Total Fe). At Cape Preston/Balmoral, the main host Joffre Member of the Brockman iron formation, has a condensed thickness of 300 m, compared to the regional 360 m. This suggests removal of components of the unit, possibly by similar hypogene fluids that leached silica and produced a magnetite-rich assemblage from BIFs as the first stage in the production of microplaty hematite ore, as described above.
The channel iron deposits (CID) mined in the Hamersley Province account for up to 40% of the total iron ore mined from the Hamersley Province of Western Australia. These deposits are of Miocene age and occupy meandering palaeochannels in a mature surface composed mainly of Archaean rocks containing iron formations. These palaeochannels are generally less than 1 km but can range to several kms in width and from 1 to more than 100 m thick. The Robe and Marillana/Yandi palaeochannels in the western and eastern Hamersley Province respectively contain the principal CID resources currently being mined. These two major CID channels extend over 100 to 150 km lengths, with the Robe system being up to 5 km wide. Known economic CID resources within the province exceed 10 Gt with grades of 56 to 58% Fe. The CIDs are dominated by goethitic granular facies, which are typically composed of ooids and lesser pisoids with hematite nuclei and goethite cortices, abundant goethitised wood/charcoal fragments and goethitic peloids, all cemented by goethite (Morris and Ramanaidou, 2007). The goethite was produced by chemically precipitated iron hydroxyoxides, derived from leaching of iron-rich soils in an organic environment. Common post depositional weathering produced secondary facies (Morris, 2007). In contrast, the often associated younger detrital ores, which are predominantly of Pliocene age, comprise colluvial/alluvial deposits of modified clasts of older proximal BIF mineralisation. These deposits are generally much more limited in total tonnage than the CID of BIF hosted hematite deposits.
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 (#Location: 22° 46' 11"S, 117° 46' 27"E).
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.
See the Hamersley Basin Iron Province record for the regional setting.
The Mount Tom Price orebody is composed mainly of micoplaty 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.
The initial reserve totalled around 900 Mt @ 64% Fe with a high lump to fines ratio, and low impurities (Harmsworth et al., 1990);
Mining to the end of 1972 had produced 134 Mt @ 61.8% Fe (Gilhome, 1975);
Remaining reserves (and resources) in 1974 were 610 Mt @ 62.6% Fe (Gilhome, 1975);
The impurity content of the high grade ore reserve in 1990 was 0.053% P, 3.5% SiO2 and 1.9% Al2O3 (Harmsworth et al., 1990).
Remaining reserves and resources in 2007 (Rio Tinto, 2008 Annual Report; reserves are additional to resources) were:
Proven + probable Brockman reserves - 125 Mt @ 64.4% Fe;
Proven + probable Marra Mamba reserves - 33 Mt @ 61.2% Fe;
Measured + indicated + inferred high grade Brockman resources - 100 Mt @ 63.9% Fe;
Measured + indicated + inferred low grade Brockman resources - 60 Mt @ 55.9% Fe;
Measured + indicated + inferred Marra Mamba resources - 15 Mt @ 61.8% Fe;
Remaining reserves and resources in 2010 (Rio Tinto 2010 Annual Report; reserves are additional to resources) were:
Proven + probable Brockman reserves - 83 Mt @ 63.6% Fe;
Proven + probable Marra Mamba reserves - 20 Mt @ 61.2% Fe;
Measured resources - 35 Mt @ 63.3% Fe;
Indicated resources - 67 Mt @ 63.8% Fe
Inferred resources - 9 Mt @ 64.4% Fe.
Measured resources - 26 Mt @ 56.9% Fe;
Indicated resources - 25 Mt @ 55.6% Fe
Inferred resources - 3 Mt @ 55.0% Fe.
Measured resources - 14 Mt @ 61.9% Fe;
Indicated resources - 3 Mt @ 61.8% Fe.
The Marandoo deposit is located some 35 km to the east of Tom Price in the Hamersly Basin and 260 km SSW of its export port of Dampier on the Indian Ocean in north western Western Australia.
See the Hamersley Basin Iron Province record for the regional setting.
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.
The deposit comprises mainly martite-goethite ore ranging to martite-ochreous goethite. Manganese oxides, mainly pyrolusite and cryptomelane, occur sporadically through the ore and shales.
Prior to the commencement of operations in 1994, the measured resource was 360 Mt @ 62.6% Fe with 0.053% P, 2.9% SiO2, 1.7% Al2O3, 0.7% Mn and 4.8% LOI.
Reserves and resources at the end of 2011 (Rio Tinto 2012) were:
Total proved + probable reserves - 225 Mt @ 63.1% Fe, plus
Total measured + indicated + inferred resources - 167 Mt @ 62.0% Fe (in addition to reserves).
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.
Robe River Mesa 'J'
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.
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 'A' and Mesa 'J' deposit. Pannawonica is some 190 km NW of Tom Price and 140 km SW of its export port, at Cape Lambert (#Location: 21° 40' 50"S, 116° 19' 10"E).
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 to 6% SiO2 and 2.5 to 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.
Channel Iron Deposit mineral resources and ore reserves at 31 December, 2012 (Rio Tinto 2012 annual report), were:
Measured + indicated + inferred resources - 2785 Mt @ 57.1% Fe, plus
Proved + probable reserves - 274 Mt @ 57.2% Fe
TOTAL reserves + resources - 3.059 Gt @ 57.1% Fe
Note: Rio Tinto does not include reserves in resources.
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 800 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%). Ore is transport by private rail, 200 km to the Indian Ocean port of Cape Lambert, 40 km east of Karratha.
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.
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T M (Mike) Porter, of Porter GeoConsultancy
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T M (Mike) Porter of Porter GeoConsultancy Pty Ltd.
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