Hope Downs

Western Australia, WA, Australia

Main commodities: Fe
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Hope Downs is a major iron ore deposit within the Marra Mamba Iron Formation of the Hamersley Group of the Hamersley Basin in Western Australia. It is located some 75 km NW of Mt Whaleback on the Pilbara Craton (#Location: 22° 57' 29"S, 119° 6' 58"E).

The Hamersley Group is part of the Archaean to Palaeoproterozoic volcanic and sedimentary sequence of 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 3500 to 2800 Ma 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 sediments 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 the volcanics and sediments, with a similar thickness with mafic sill percentages increasing from north to south. The uppermost unit is the 2690 to 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. 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.

See the Hamersley Basin Iron Province record for the regional setting and stratigraphy.

The Marra Mamba Iron Formation has been sub-divided into three units, commencing with the basal Nammuldi Member which is 109 m thick at Hope Downs and comprises BIF and chert with 18 internal thin shale macrobands, followed by the medial 52 m thick MacLeod Member made up of 13 shale macrobands, each of <1 m in thickness within BIF and chert which are podiform in appearance.   The uppermost Mount Newman Member hosts most of the mineralisation and is 65 m thick at Hope Downs, essentially comprising BIF with 8 thin (<200 mm to 650 mm) shale macrobands.

These are conformably overlain by the West Angelas Member, the basal of three recognised packages that make up the Wittenoom Dolomite.   This member is 45 m thick at Hope Downs, composed of shale (often manganiferous), chert and dolomite and 2 to 10 m thick BIF macrobands 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 high-grade BIF derived hematite ore deposits are hosted within the Marra Mamba Iron Formation, with subsidiary deposits in the Brockman Iron Formation. The bulk of the high-grade Marra Mamba hematite ore at Hope Downs is within the Newman Member but also occurs in parts of the Nammuldi Member, and at the base of the lower West Angelas Member of the overlying Wittenoom Formation. Overall, the ore consists of low phosphorous martite-limonite-goethite derived from chert-free BIF by supergene weathering.

In addition to the high grade mineralisation, low-grade ore (50 to 60 wt.% Fe) in the West Angelas and MacLeod Members consists mainly of limonite and goethite, typically containing high levels of alumina and silica (>10 wt %) from interbedded clay weathered shale-carbonate-chert bands. The low-grade section of the Nammuldi Member ore comprises limonite with minor goethite that yields calcined grades of up to 67 wt.% Fe and low alumina and silica (~2 wt.%) contents.

The main high grade orebody is unusually rich in martite (pseudomorphous hematite after magnetite) and poor in limonite and goethite compared to other ore deposits of the Marra Mamba Iron Formation.

Although there is a marked variability in mineralogy, thickness and texture of individual bands, the Marra Mamba Iron Formation it is typified by coarser banding than most of the other BIF of the Hamersley Group, with mesobands commonly between 2 to 10 cm, and rarely ranging up to 30 cm thick. The magnetite and iron silicate-carbonate mesobands typically have fine microscopic microbanding, with of microbanding also evident in chert layers in the form of fine iron oxide, amorphous carbon and silicate laminae.

Karst erosion of the Wittenoom Formation that overlies the Marra Mamba Iron Formation, has produced steep-sided buried valleys adjacent to the in situ orebodies that contain thick deposits (<160 m) of goethitic and sideritic detrital sediments, and remnants of the Tertiary CID deposits of bedded siderite, hematite gravels, red ochreous detrital material, and enriched scree deposits that are additional sources of ore.

The Hope Downs deposits are located on the eastern end of the east-west trending, doubly plunging D3 Weeli Wolli Anticline which has been eroded to expose the Jeerinah Formation at its core. The Marra Mamba beds dip at 15 to 30°S on its southern limb, but have a steeper northerly dip on the northern limb.  At both the North and South Hope Downs deposits, a broad, parasitic D3 anticline forms the main ridge to the south of each orebody.  On the northern side of each anticline the bedding is buckled into a series of open folds and folded to the north.

The Hope Downs deposits occur as banded hematite-goethite ores that extend to depths of 270 m below surface.   Hope North is developed over a length of 5.5 km with a width of 250 m, which widens to 1 km over 1 km strike interval.   The vertical interval between the highest and lowest points on the orebody is 310 m.   Hope South extends over a 5 km length and a width of 200 to 1200 m with a similar vertical extent.

The total JORC compliant Mineral Resources and Ore Reserves in 1998 at a 58% Fe cut-off were stated at:
    Total Mineral Resources - 551 Mt, which included proved Ore Reserves of:
    Hope North - 337 Mt @ 61.6% Fe, 2.9% SiO
2, 1.5% Al2O3, 0.06% P, 7.1% LOI,
    Hope South - 185 Mt @ 61.6% Fe, 3.5% SiO
2, 1.8% Al2O3, 0.06% P, 6.2% LOI.

JORC compliant Mineral Resources + Ore reserves at the end of 2011 (Rio Tinto Annual Report, 2012) were:
  Operating Mines and Development Projects:
      Hope Downs 1 Marra Mamba - 295 Mt @ 61.6% Fe Mineral Resource + 96 Mt @ 61.5% Fe Ore Reserve
      Hope Downs 1 Detritals - 9 Mt @ 59.4% Fe Mineral Resource
      Hope Downs 4 Brockman - 244 Mt @ 62.2% Fe Mineral Resource + 136 Mt @ 63.1% Fe Ore Reserve
  Un-developed Mineral Resources:
      Hope Downs Brockman - 116 Mt @ 61.9% Fe
      Hope Downs Brockman Process ore - 244 Mt @ 64.8% Fe
      Marra Mamba and Detritals - 210 Mt @ 61.6% Fe.
  The total Ore Reserve + Mineral Resource is ~1.35 Gt @ >61% Fe.

The most recent source geological information used to prepare this summary was dated: 2012.    
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.

  References & Additional Information
   Selected References:
Harmsworth R A, Kneeshaw M, Morris R C, Robinson C J, Shrivastava P K  1990 - BIF Derived Iron Ores of the Hamersley Province: in Hughes FE (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   v1 pp 617-642
Lascelles D F,  2006 - The Genesis of the Hope Downs Iron Ore Deposit, Hamersley Province, Western Australia: in    Econ. Geol.   v101 pp 1359-1376
Paquay R D, Ness P K  1998 - Hope Downs Iron Ore Deposits: in Berkman DA, Mackenzie DH (Ed.s), 1998 Geology of Australian and Papua New Guinean Mineral Deposits The AusIMM, Melbourne    pp 381-386
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. 197221

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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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