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Chichester Range - Cloudbreak, Christmas Creek, Mt Nicholas, Mt Lewin, Mindy Mindy |
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Western Australia, WA, Australia |
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Fe
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Cloud Break, Christmas Creek, Mount Nicholas, Mount Lewin and Mindy Mindy iron deposits are located in the ESE elongated Chichester Range in the Pilbara of Western Australia, ~110 km north of Newman and ~250 km south to SSE of Port Hedland.
Mineralisation in the Chichester Range is distributed over a strike length of 200 km, while the more significant Cloud Break and Christmas Creek deposits, whose centres are approximately 50 km apart, are in an 80 km strike length of the range.
See the Hamersley Basin Iron Province record for the regional setting and stratigraphy.
The mineralisation at these deposits is developed within and over the Marra Mamba Formation of the Archaean to Palaeoproterozoic Hamersley Group, which hosts most of the major iron ores of the Pilbara.
The Hamersley Group is, in turn, 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 greater than 2.77 to near 2.35 Ga. It rests unconformably on 3.50 to 2.80 Ga granitoids 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, commences with an early phase of clastic sedimentary rocks and mafic volcanism, followed by extensive sandstones and conglomerates 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 volcanic and sedimentary rocks, with a similar thickness of mafic sills, increasing from north to south. The uppermost unit is the 2.69 to 2.63 Ga, 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 is characterised by around 1000 m of laterally extensive banded iron formation, representing three major episodes. The basal Marra Mamba (2.60 Ga), 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 (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 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 the felsic volcanics of the Woongarra Formation. Thickness variations in the Hamersley Group are only minor. For more detail on the regional setting see the "Hamersley Basin Iron Province" record.
The mineralisation of the Chichester Range is confined to the Nammuldi Member, the lowermost unit of the Marra Mamba Formation, overlying the black shales of the Jeerinah Formation at the top of the Fortescue Group.
The Marra Mamba Formation has been sub-divided into three units, commencing with the basal 135 m thick Nammuldi Member, which is locally characterised by extensive, thick and podded iron enriched Banded Iron Formations (BIF), separated by equally extensive units of siliceous and carbonate rich chert and shale. The Nammuldi Member tends to form low, flat topped ridges, with relief generally of <30 m, and a deep weathering profile, with no fresh rock evident at surface. The weathering profile comprises Tertiary colluvium containing generally uncemented detrital material derived from BIF, chert and shale, with a matrix of fine sediments, that has allowed percolation of groundwater and the precipitation of both calcrete and ferricrete forming local hardcaps.
These Nammuldi Member sedimentary rocks are 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, comprises 60 m of interbedded BIF with carbonate and shale.
Regionally, the Chichester Range has gentle dips, usually of <5°S, marking the onlap of the Hamersley Basin onto the Pilbara Craton. However, this gentle regional southerly dip of the Nammuldi Member at Christmas Creek, is offset by north-south to NE-SW trending faults and is overprinted by low amplitude (<20 m), long wavelength (200 to 800 m), north-south to NE-SW trending and SSW plunging folds, with bedding dips locally ranging from 2 to 88°, averaging 21°. This folding is interpreted to be the result of waning phases of deformation in the underlying Pilbara Craton (Hannon et al., 2005; Thorne et al., 2008).
Small amplitude folds developed at Christmas Creek, and elsewhere along the Chichester Range, have influenced both the development and preservation of mineralisation. The formation of high grade ore is controlled by NE-SW trending faults and folds. Synclines appear to have focused supergene fluids resulting in their preferential mineralisation compared to adjacent anticlines. Subsequent erosion, controlled in part by the same structures, led to the broad stripping of anticlines and also the local removal of synclines along drainage channels. Poorly mineralised, large, high-relief mesa tops (e.g., Mt Lewin) are commonly anticlinal hinge zones (Hannon et al., 2005; Thorne et al., 2008).
Pods of fine- to medium-grained martite-microplaty hematite ore are closely associated with NE-SW trending brittle shear zones. The microplaty hematite has a variable shape and size, ranging from 100 to 200 µm euhedral crystals at Christmas Creek and Cloud Break, to very fine 10 to 60 µm blades at Mount Nicholas. Disseminated, fine-grained martite-microplaty hematite is also found in the supergene ore, below the base of the modern weathering zone. The martite-microplaty hematite ore is interpreted to be the product of structurally controlled Palaeoproterozoic hypogene alteration of the Nammuldi Member, which was subsequently overprinted by supergene processes (Thorne et al., 2008).
The majority of mineralisation developed in the Chichester Range occurs as a sub-horizontal sheet of typical supergene martite-goethite and martite-ocherous goethite enrichment, overprinting hypogene microplaty hematite, which locally persists below the martite-goethite sheet. Hypogene enriched microplaty hematite mineralisation is structurally controlled, while supergene enriched mineralisation is very extensively developed as a sheet continuing for kilometres under recent cover. The majority of the mineralisation is typically a mixture of goethite, martite and hematite in varying amounts, similar to other Marra Mamba ores of the Hamersley Basin.
At Christmas Creek and Cloud Break, in contrast to Mt Nicholas and Mt Lewin, the dip of the Nammuldi Member is more gentle, typically at 2 to 5°, exposing the ore over a broader width at shallow depths. Much of the ore is covered by shallow, 0 to 50 m thick, free digging Cenozoic sediments, with the Nammuldi Member only having limited direct outcrop. The ore zone varies from 3 to 20 m in thickness, and is at shallow depths over widths of as much as 4 km and a strike length of 80 km.
The Mindy Mindy deposit is a Channel Iron Deposit, developed within an ancient riverbed which follows, and passes through the Weeli Wolli Formation higher in the Hamersley Group, although the northern section passes through part of the Brockman Formation. Mindy Mindy is preserved in topographic lows rather than occurring as mesas.
The resources in the Chichester Range in 2005 totalled more than 2.3 Gt and included:
- Cloud Break - 730 Mt @ 58.5% Fe, including 293 Mt @ 60.4% Fe,
- Christmas Creek - 1410 Mt @ 58.1% Fe, including 465 Mt @ 60.3% Fe,
- Mt Lewin - 198 Mt @ 56.5% Fe, including 48 Mt @ 60.5% Fe,
The higher grade mineralisation within this resource comprised:
- Indicated - 322 Mt @ 60.2% Fe, 3.39% SiO2, 2.01% Al2O3, 0.051% P, 7.40% LOI.
- Inferred - 484 Mt @ 60.4% Fe, 3.16% SiO2, 1.92% Al2O3, 0.056% P, 7.42% LOI.
Mineral resources and ore reserves for the Chichester Hub deposits at June 30, 2012 (Fortescue Metals Group, 2012) were:
- Measured resources, 420 Mt @ 56.7% Fe, 6.15% SiO2, 3.05% Al2O3, 0.045% P, 8.2% LOI.
- Indicated resources, 1891 Mt @ 56.6% Fe, 5.94% SiO2, 3.31% Al2O3, 0.051% P, 8.0% LOI.
- Inferred resources, 1068 Mt @ 56.4% Fe, 6.29% SiO2, 3.28% Al2O3, 0.057% P, 7.8% LOI.
- TOTAL resources, 3.379 Gt @ 56.6% Fe, 6.08% SiO2, 3.27% Al2O3, 0.052% P, 8.0% LOI.
which included
- Proved reserves, 31 Mt @ 59.9% Fe, 3.41% SiO2, 1.85% Al2O3, 0.061% P, 8.1% LOI.
- Probable reserves, 1464 Mt @ 58.2% Fe, 5.13% SiO2, 2.24% Al2O3, 0.052% P, 7.6% LOI.
- TOTAL reserves, 1.495 Gt @ 58.3% Fe, 5.09% SiO2, 2.24% Al2O3, 0.052% P, 7.6% LOI.
These deposits were developed by Fortescue Metals Group Ltd, who shipped the first ore in May 2008.
The most recent source geological information used to prepare this decription was dated: 2006.
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.
Cloudbreak Christmas Creek
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Selected References:
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Clout, J.M.F., Counsell, C. and Simpson, C., 2017 - Chichester and Solomon bedded and channel iron deposits: in Phillips, G.N., (Ed.), 2017 Australian Ore Deposits, The Australasian Institute of Mining and Metallurgy, Mono 32, pp. 351-358.
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Hannon E, Kepert D A and Clark D, 2005 - From Target Generation to Two Billion Tonnes in 18 Months - The Re-Invention of the Chichester Range: in Iron Ore 2005 Conference, Perth, WA, September 19-20, 2005 The Australasian Institute of Mining and Metallurgy, Melbourne, pp. 73-77
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Thorne, W., Hagemann, S., Webb, A. and Clout, J., 2008 - Banded iron formation-related iron ore deposits of the Hamersley Province, Western Australia: in Hagemann, S., Rosiere, C., Gutzmer, J. and Beukes, N.J., (eds.), 2008 Banded Iron Formation-Related High-Grade Iron Ore Reviews in Economic Geology v.15, pp. 197-221.
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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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