BIF-Hosted Iron Ore Deposits-Hamersley Style R.C
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AGSO lournal of Australian Geology & Geophysics, 17(4),207- 2 11 © Commonwealth of Australia 1998 BIF-hosted iron ore deposits-Hamersley style R.C. Morris CSIRO Exploration and Mining, Private Mail Bag, PO Wembley, Western Australia 6014, Australia EXPLORATION MODEL Examples Local geological criteria Metamorphosed supergene: Mt Tom Price, Mt Whaleback, • Simple hydrodynamic (sub)-artesian systems with ElF as Paraburdoo, Channar, Giles (Western Australia); the aquifer; exposed to the atmosphere along strike (kms), Carajas (Brazil); Sishen (South Africa). with exits for dissolved gangue. Non-metamorphosed supergene: (Dominant Hamersley • Surface-expressed faults or fractures needed to initiate the resources.) Marandoo, BHP OB 29, Area C, Hope Downs, replacement process at depth (ores grow upward). West Angela, Ophthalmias • Other pre-ore faulting and thrusting can add considerable Target structural complexity, but are not essential for enrichment. • New finds of outcropping ores are rare. Mineralisation features • Exploration now includes search for buried ores suitable for • Ores are almost entirely ferric iron. open pit mining. • Lamination and texture are preserved. • 5-300 Mt, depending on proximity to established • Magnetite oxidises to martite (hematite). infrastructure. • Pseudomorphs of goethite after gangue minerals (includes P, • >62% Fe < 0.08% P. from BIF apatite) are characteristic of non-metamorphosed Mining and treatment ores. • BIF residues include chert, with kaolinite from silicate • Stratabound tabular bodies-mining is limited only by open• horizons. pit design needs. • Burial metamorphism converts goethite to microplaty • Grade within narrow limits, by on-site and port blending of hematite, destroying internal texture, but beddingllamination lump (6- 30 mm >64% Fe); fines «6 mm >62% Fe). remains to high metamorphic grades. • Typical export specification of::; 0.075% P. Section 6 Deposit Marra Mamba (,hr Water table .., o 500 m ! Figure I. Generalised section at MtTom Price, showing metamorphosed iron ore deposits (lIlp/ H) in the Brockman IF and non• metamorphosed deposits in Brockman and Marra Mamba I Fs. Regional geological criteria Alteration • Synforms in BIF sequences of any age. • Weathering of ore produces 3 tiers: • Enrichment occurs at unconformities exposed to an oxic I. 1- 2 m of hematite-dominated outcrop, texture/bedding! atmosphere (post-2000 Ma). lamination preserved. • Two major periods of ore forming worldwide (Precambrian 2. Deep hydrated zone (hematite forms AI-goethite) with and Mesozoic) with possible Paleozoic (North America). destruction of texture and bedding, a type of lateritic • Burial metamorphism (goethite to hematite) is needed for ferricrete. premium ores. 3. Ore zone, with post-enrichment leaching of goethite increasing friability. 208 R.c. MORRIS BANDED IRON FORMATION I I- HEMATITE CARBONATES SILICATES ~ BLUE DUST leaching + + ORES MAGNETITE SILICA, ~ supergene enrichment - I oxidation I metasomatic replacement by goethite I ~ , I burial I HEMATITE-GOETHITE ORE .. regional metamorphism J modi~ by leaching re-exposure~r1 HEMATITE-RICH+ ORES l or ehydration I contact+ [ metamorphism ] , , I VARIOUS ORE TYPES I -l MAGNETITE-RICH ORES I [ surficial processes •- eluvial concentration J , Figure 2. Schematic of genetic • : .!.1:" I .. ;i'U :r'u::( .....{.UI . ' . modelling for the major BIF• .... hosted iron ores of the world. A. System viewed in 3D B. Electrochemical Cathode cell 4e- + 02 +2H20 ..... 4(OHr c. Transfer of Fe to anode D.Trans~rofFetoanode H4SiO 4 Drainage E. Transfer of Fe to anode F. Final supergene ore body now subject to leaching by groundwater Figure 3. Model of supergene iron ore growth from below, as erosion removes the surface. BIF-HOSTED IRON ORE DEPOSITS- HAMERSLEY STYLE 209 Introduction M artite-microplaty hematite Much of the following text is summarised from Morris (1980, The martite- mieroplaty hematite group includes the premium, 1985) and Harmsworth et al. (1990), which should be consulted low-P hematite-rich ores (often inaccurately called specularite for detailed citation. Though based mainly on research in the ores), the origin of which has attracted much debate and which Hamersley Province ('Hamersleys'), experience has shown that are here attributed to metamorphism of post-2000 Ma super• the basic concepts apply to BIF-hosted ores world-wide. gene ores. The group is characterised by the presence of well• preserved primary banding, but with loss of the internal texture Major ore types found in the non-metamorphosed ores, as a result of the growth of abundant secondary microplaty hematite (mpl H) from the Although the dominant world resources of iron ore are associated goethite-pseudomorphed gangue. With increasing metamorphism with the major late Archean- Proterozoic BIF depositories, ores they range to granular or 'micaceous' hematite. The immense are known from the whole gamut of these chemical sediments. size of some of these deposits (Mt Whaleback, Western Australia, Because export specifications for Australian bedded ores > 1500 Mt; N4E deposit, Carajas, Brazil, >3000 Mt) makes typically call for :0; 0.075% P, iron ores in Australia tend to be them the largest and most concentrated secondary accumula• considered in terms of' low phos or high ph as '. tions of any single metalliferous element in the Earth's crust, BIF-hosted iron ores can be divided broadly into three approached in scale only by some aluminium orebodies. groups: A subgroup comprises small deposits of magnetite-rich ore, i.e. residual concentrates of Fe oxides I. Leached BIF: rarely exceeding a few million tonnes and generally associated ('blue dust ores'). Very low P. with mpl H ores. These magnetite segregations can be attributed moderate-high typically 0.07- 2. Martite- goethite ores: P, to thermal metamorphism of normal ores by basic intrusives. 0.17% P. 3. Martite- microplaty hematite ores (± residual goethite): Low P, < 0.07%. Ore genesis Deposits of group 2 and 3 ores are shown in Figure I. There are five features of the BIF-hosted Fe-enrichment Weathering affects all three ore types in various ways, orebodies that merit special attention when considering their particularly in tropical areas, and early exploration was typically ong1l1. confined to the resulting complex profiles. This, compounded Scale, Individual deposits range from a few hundred to over by limited communication between rival groups, led to the con• 3 billion tonnes of >64°A, Fe. Thcy may extend along strike for flicting genctic modcls of the past. five or more kilometres and more than two kilometres down The mature three-ticred wcathering profile of ore in the structure. Typically, thcy grade abruptly from ore grade Hamcrslcys includcs: (>55'Yo Fc) to altered BIF «30% Fe) over distances that are ({II outcroppillg :;0111' of 12m of hematite-rich ore, the negligible compared with the size of the deposits- sometimes 'carapacc', with preserved BIF features resulting from less than a metrc. dchydration of the goethite-replaced matrix. underlain by: Purity ofthe ores, The orcs are dominated by ferric Fe oxides a highly modified Izl'lll'Uted :;Olle, up to 70 m, dominated by and only extremely rarely contain exotic components that cannot aluminous goethite, and marked by destruction of parental be attributed to residues from the parent rocks. features. Oxidatioll state o/'the ores. i\ fe\\ rclati\'ely small depo"its are mainly magnetite: others contain magnetite and hematite, These two tiers, comprising the 'hardcap', overlie but, quantitatively, it is the largely oxidised deposits, comprising the ore pmper, which retains the original BI F features, but hematite or hematite goethite ores, that dominate the world is typically affected by moderate scvcre post-enrichment scene. Despite their oxidised character, these e"tcnd to depths groundwater leaching of gocthitc and, Icss onen, ofhcmatitc. well below the normal influcnce of the atmosphere. but there arc no signi ficant mineralogical di fTerences bet\\ecn shallow Leached BIF (below hardcap) and deep ore. The leached BIF group is derived from BIF by direct dissolution Wall-rock alteration, It is generally agreed, even by advocates of the gangue minerals by groundwater to give residual concen• of hypogene models, that none of the major iron deposits con• trates of Fe oxides. The ore quality ranges from high-grade residues tains evidence for the upward passagc of fluids through the host ofmartite (oxidised magnctite = hematite) ± primary hematite rocks or associated faults. This is ofconsiderablc importance in (so-called 'blue dust orcs'), to highly siliceous but very friable view of the immense amount of solution necessary for enrich• BIF, readily upgraded by magnetic or density methods. Ores of ment, whatcver mechanism is advocated. this derivation, often associated with enrichment ores, are found Stratigraphic situation, Most iron ores are strata-bound and in high-rainfall areas such as Brazil and India, but not in the arid there is a total absence of genetically related ' mineralisation' in Hamersleys. A small deposit of this type is known from the the associated rocks. wetter southern coastal area ofWestcrn Australia (South Downs deposit). SYI/gel/etic models These hypotheses have received minimal endorsement. Modelling Martite-goethite was based originally on a clastic origin for BIF, and on the The martite-goethite group includes a wide range of ores, in strata-bound character of the ore zoncs, 'Which werc thus deposits up to hundreds of millions of