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Location: 55 km NW from Tom Price, Western Australia, Australia
152-158 St Georges TerracePerthWestern Australia, Australia6000
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Deposit types Bedded Iron Deposits (BID)The Bedded Iron Deposits (BIDs) of the Hamersley Province are typically classified as either martite microplaty hematite or martite-goethite and are hosted within BIF sequences of the Brockman, Marra Mamba and Boolgeeda Iron Formations of the Hamersley Group.Within these formations, Dales George, Joffre and Newman stratigraphy typically contain the most continuous high grade mineralisation with strike lengths up to 15 km and depths up to 200 m.There is an industry consensus that the martite-goethite ores of the Hamersley province formed as a result of supergene enrichment of BIF.Channel Iron Deposits (CID) Channel Iron Deposits (CIDs) are sub-divided into ‘mesa’ and ‘gorge’ type deposits. CIDs occurring in synclines and on mild dip slopes on the margin of paleochannels are ‘gorge’ deposits, and CIDs formed by relief inversion in the central zones of paleochannels are ‘mesa’ deposits. Such deposits are dominated by pisolitic goethite-hematite iron mineralisation. CID deposits of this type and quality are unique to Western Australia.The Yandicoogina paleochannel was the ancient course of the Marillana, Yandicoogina and Weeli Wolli Creeks. It is cut into the centre of the Yandicoogina Syncline. The Robe Formation occurs as mesas formed by topographic inversion in the central zones of the paleochannels.The pisoliths for these CIDs were sourced from the wide soil profile that developed over the adjacent lateritised iron-rich BIF and dolerite basement rocks of the Weeli Wolli Formation at Yandicoogina or the Marra Mamba Iron formation at Pannawonica. Pisolith particles were transported to and deposited in this pre-existing meandering river channel by natural processes. Once deposited in the channel, the pisoliths were progressively cemented with a goethitic matrix derived by periodic drying out of the ferruginous channel waters. Irregularly shaped aluminous clay bands and pods were locally deposited. When the channel was filled with CID, weathering processes downgraded and altered the upper few metres of the deposit through introduction of clays and goethitic infillings into joints. The mineralised Rio Tinto section of the Marillana-Yandicoogina-Weeli Wolli paleochannel system is approximately 50 km long. It averages 500 to 600 m wide, locally reaching 800 m. The main ore zone is 40 to 50 m thick in the centre of the channel and thins towards the channel margins. Although the quality of the CID is relatively consistent, there are some significant quality trends: the centre of the channel has lower levels of SiO2 and Al2O3 than at the edges, and the chemistry is more homogeneous than at the edges. The main ore zone is sub-divided on the basis of mineralogy and chemistry into Upper and Lower ore zones, with the Upper zone having slightly higher Fe grade and lower Loss on Ignition (LOI) compared to the Lower ore zone.Tertiary Detrital Iron Deposits (DID) Two detrital sub-groups in the Hamersley Province are relevant to the DIDs as follows:Marra Mamba-sourced detritals, also described as Ochre-Rich Detritals due to their distinctive red ochreous hematite and/or yellow ochreous goethite matrix, typically occur in channels or gorges cut into the Wittenoom Formation or associated Tertiary sediments. Mineralisation within these deposits can be up to 150 m deep and several km in length. They are comprised of layers of colluvial material, calcrete, Red Ochre Detritals (ROD), lignite, siderite, and clay. The ROD have angular to sub rounded hematite goethite fragments and low to high Fe grades within an ochreous hematitic matrix. The composition of these detritals is highly variable, both in the ratio of clasts to matrix and the type of clasts and matrix.The Brockman-sourced detritals primarily originate from the hard 1 to 2 m hardcap that forms on the surface of bedded iron ore outcrops. The eroded ore particles migrated downhill, and soil-derived rims deposited on the particles during their transportation to pre-existing drainage channels. The ferruginous clay-rich matrix of the initial accumulations dehydrated and formed a naturally cemented hard rock-hematite conglomerate. More detritals accumulated, but the loose clay/soil matrix remained un-cemented forming a hematite detrital. Finally the BIF-rich siliceous detrital (waste overburden) was deposited. The Brockman Detritals contain significantly less phosphorus (~0.06% P) than their bedded source rocks (~0.12% P) as a result of goethite dehydration to hematite. This style of detritals typically form relatively discontinuous zones of mineralisation frequently less than 200 m in length and typically up to 50 m thick.Hydrated ore zone The Hamersley Province accommodates a regolith (blanket of weathered rock) that occurs across a diverse range of rock types in a number of landform settings. A surficial component of this regolith is the Hydrated Zone. Named after the hydration process implicit in the conversion of hematite to goethite, this zone is differentiated from the underlying strata due to its characteristic high variability, weathering and lack of bedding.Mineralisation within the hydrated zone is characterised by high variability in terms of tonnes and grade, with relatively small and discontinuous high-grade parcels. Hydrated ore is typically harder and coarser than non-hydrated ore.Marra Mamba Iron Formation The unmineralised Marra Mamba Iron Formation is approximately 230 m thick, with mineralised sections of the same strata comprising approximately 50 to 60% of this thickness. This thinning effect also applies to orebodies formed in the Brockman Iron Formation and can be explained by the changes to the BIF during the ore forming process.The Mt Newman Member is the host rock for all the major Marra Mamba deposits in the Province, including Marandoo, Nammuldi, Silvergrass, West Angelas and the Hope Downs 1 deposits. Minor lower grade mineralisation occurs in the Nammuldi and MacLeod Members.Brockman Iron Formation The Brockman Iron Formation has an unmineralised thickness of approximately 620 m. It is divided from the base upward into four members: Dales Gorge, Whaleback Shale, Joffre and Yandicoogina Shale. The high grade iron deposits at Tom Price, Western Turner Syncline, Paraburdoo, Channar, Eastern Range, Brockman 2, Brockman 4, Hope Downs 4 and Gudai-Darri are hosted predominantly by the Dales Gorge and Joffre Members, with only very minor mineralisation in the Whaleback and Yandicoogina Shales. The Dales Gorge Member has a thickness of approximately 140 m and consists of an alternating sequence of 17 BIF and 16 shale macrobands. BIF macrobands are comprised of mesobands of chert and iron-rich material in a chert matrix (Trendall, 1983) Mesobands commonly consist of millimetre alternations of chert, shale and iron-rich bands, termed microbands.The Yandicoogina Shale is approximately 60 m thick and consists of interbedded chert and shale which have been intruded by a number of dolerite sills. Boolgeeda Iron Formation The Boolgeeda Iron Formation is characterised by a green chert at its base, which coarsens to a finegrained flaggy sandstone, which in turn transitions into a BIF. The Boolgeeda Iron Formation conformably overlies the Woongarra Rhyolite and is divided into three members. The lowermost B1 Member consists of a chertier base, sometimes jaspilitic and sandstone, with an upper shaley and more traditional BIF-like sequence. The middle B2 Member is divided into three parts, with a BIFdominant centre, which is often well-mineralised and grades upward and downward into thick shales. The Poonda Deposit was the first example of a declared Mineral Resource for a Boolgeeda Iron Formation deposit by Rio Tinto.
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