Middleback Ranges - Iron Knob, Iron Monarch, Iron Baron, Iron Prince, Iron Duke, Iron Magnet

South Australia, SA, Australia

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The Middleback Range iron ore deposits on the Gawler Craton of South Australia include Iron Knob, Iron Monarch, Iron Prince, Iron Princess, Iron Baron, Iron Queen, Cavalier, Iron Chieftain, Iron Duke, Iron Duchess and Iron Magnet.   These deposits are located in the Middleback Ranges, some 25 to 40 km to the west of the steel furnaces of the port of Whyalla on the east coast of Eyre Peninsular, ~230 km NNW of Adelaide.

The Middleback Range comprises an ~40 km long, north-south trending set of hills situated in the central-eastern Gawler Craton. They lie within a broad region of high magnetic intensity that reflects the ~3.24 to 3.15 Ga Mesoarchaean units (Fraser et al., 2010; Reid and Jagodzinski 2011) that are the basement to the Palaeo- and Mesoproterozoic metasedimentary, volcanic and intrusive rocks that contain the Middleback Ranges iron deposits (Parker 1993; Fanning, Reid and Teale 2007; Fraser and Neumann 2010). This part of the Gawler craton was variably deformed during the 1.73 to 1.69 Ga Kimban Orogeny (e.g. Ramsay and Oliver 1980; Parker and Lemon 1982; Parker 1993; Yeates 1990; Vassalo and Wilson 2001; Leevers, Gaughan and Bubner 2005; Szpunar et al., 2011).

The host succession to the Middleback Ranges iron deposits comprises the 5000 m thick Palaeoproterozoic Hutchison Group metasediments which are composed of a mixed sequence of chemical and clastic sediments and extrusive basic and acid volcanics, which regionally include the Warrow Quartzite (up to 500 m thick), which is overlain by the Middleback Subgroup and the Bosanquet Formation (up to 1000 m of rhyodacite, interbedded with calcsilicate gneiss) (GeoScience Australia dforms database).

This succession extends discontinuously for some 600 km, from southern Eyre Peninsula to the Olympic Dam region (Cowley 2007) and possibly to the Mount Woods Inlier, a further 300 km to the north, and has been considered to have been deposited between ~2000 and ~1865 Ma (Parker and Lemon 1982; Parker 1993). However, Szpunar et al. (2011) report isotopic and geochemical data collected from this succession, suggest the apparently unified package may consist of three separate subgroups, deposited between ~2500 and ~1730 Ma. One of these, the Middleback Subgroup, which includes the Banded Iron Formation (BIF) that hosts iron ore deposits in the Middleback Range, is suggested to be deposited at ~2500 Ma (Szpunar et al., 2011).

BIFs of the Middleback Subgroup have a strong magnetic signature, particularly so in the Middleback Range, where a discontinuous series of strike ridges of BIF extend north-south for 60 km. The source of the magnetic anomaly has been identified as magnetite-rich BIF beneath an upper zone of supergene modified haematitic BIF that averages 90 m in thickness.

The basal metasedimentary unit of the Middleback Subgroup in the Middleback Range is a the Yadnarie Schist (up to 1000 m of pelitic quartz-sericite schist and metasiltstone), which is overlain in turn by a succession of chemical and clastic units (Parker and Lemon 1982; Parker 1993; Yeates 1990). The chemical and clastic succession comprises in ascending order, the Katunga Dolomite (an ~20 m thick dolostone), Lower Middleback Jaspilite (BIF), Burrawing Amphibolite, Cook Gap Schist (up to 1500 m of quartz-biotite-muscovite-sillimanite-garnet-tourmaline schist) and Upper Middleback Jaspilite (BIF) (Parker and Lemon 1982; Parker 1993; GeoScience Australia dforms database). The Lower Middleback Jaspilite contains the extensive BIF which predominantly hosts the iron ore deposits in the Middleback Range (Parker 1993; Davies 2000; Szpunar et al., 2011).

The Lower Middleback Jaspilite is a carbonate facies iron formation of iron carbonate, silica and iron oxides which weathers to porous goethite-limonite rocks at surface. Sulphide facies and graphite are intercalated at depth. This unit becomes more siliceous and iron oxide rich higher in the sequence with prominent iron bearing silicates, including iron rich talc and cummingtonite-grunerite series amphiboles. The Lower Middleback Jaspilite is found across the Gawler Craton, but is thickest and best developed in the Middleback Ranges.

The Lower Middleback Jaspilite/BIF is overlain by the Cook Gap Schist, a poorly outcropping quartz-biotite-muscovite-sillimanite-garnet-tourmaline schist which is in turn locally followed by the thick Corunna Conglomerate.

The ore deposits are developed within the carbonate facies ore formation where these are localised, thickened and upgraded in synclinal keels.

The Iron Magnet deposit, which lies down-dip and lateral to the Iron Duke hematite deposit, represents a hypogene enriched magnetite BIF that has not undergone supergene modification, and is an example of the magnetite resources of the Middleback ranges. Magnetite bearing units within the Lower Middleback Jaspilite/Iron Formation can be divided into a 'mixed' basal sequence, a lower iron carbonate unit, a middle iron talc unit and an upper iron silica unit, with all units grading up sequence and laterally into each another. The iron carbonate unit coincides with the strongest consistent mineralisation, with the Iron Duke hematite orebodies interpreted to represent up-dip supergene enrichment of the same unit. The magnetite mineralisation is hydrothermal in origin, with the bulk of the ore occurring as pervasive and selective replacement of layers at all scales from laminae to beds, with pervasive replacement often leading to intervals of massive magnetite. Magnetite breccias contribute much less volume than replacement-style ore but are generally of higher grade, often having a matrix of massive magnetite. The Iron Magnet deposit occurs within a large-scale parasitic syncline-anticline pair on the western limb of a larger syncline. Major north-south shear zones bound the east and west sides of the deposit, while cross-cutting WNW-, NW-, NE- and ENE-trending faults displace mineralisation (Leevers et al., 2005).

The Iron Knob orebody consists almost entirely of hematite in fine-grained, dense and generally massive form. A later generation of hematite in the form of bands or veins of coarsely crystalline, specular hematite is present near the western wall of the pit. These veinlets up to 50 mm in width occur in fractures and joint planes in massive, fine-grained hematite. The ore appears to have been of a uniformly high grade, higher than Iron Monarch ore due to the absence of manganese minerals and less gangue minerals. Typical values are 95 to 97% Fe2O3, with assays of 68.8% Fe (98.3% Fe2O3) being common (Quast, 2012 and sources quoted therein).

The Iron Monarch orebody occupies what was originally one of the highest hills in the range. Ore is predominantly "hard", essentially composed of hematite in a dense, fine-grained massive form. Much of the hematite in the massive orebodies is derived from and retains cores of magnetite. Manganese minerals, including psilomelane and pyrolusite, associated with this iron ore are well developed. Gangue minerals consist primarily of quartz in minute grains enclosed by hematite and averaging about 2% of the ore, with minor gypsum and calcite (Quast, 2012 and sources quoted therein).

The Iron Prince ore comprises high-grade soft hematite in the northern area, showing the original banded structure of the parent BIF, and harder, more massive ore in the southern area. The hematite varies from massive to loosely coherent. The high-grade ore in this pit is particularly rich in Fe, averaging 65% Fe for the whole orebody (Quast, 2012 and sources quoted therein).

The Iron Baron deposits are predominantly composed of medium to fine granular hematite or martite, either massive and banded or schistose and powdery. Like the Iron Prince ore, the Iron Baron ore is characteristically high grade, being low in manganese, silica, alumina and phosphorus, with a weighted average of samples from the various lenses of 64.03% Fe and 0.19% Mn (Quast, 2012 and sources quoted therein).

Between 1901 and 1990 approximately 126 Mt of >60% Fe had been mined from the Iron Knob Mining Area (including Iron Monarch) and 52 Mt from the Iron Baron Mining Area (including Iron Prince), the two largest resource groups, with possible resources of 66 Mt remaining.   Grades mined from the Iron Knob Mining Area in 1986 were 66.8% Fe, 1.95% SiO
2, 0.76% Al2O3, and 60.2% Fe, 3.8% SiO2, 1.8% Al2O3 from the Iron Baron Mining Area.

In summary, Szpunar et al. (2011) note that from 1899, >200 Mt of ore has been extracted from the Middleback Range (e.g. Yeates 1990; Parker 1993; Davies 2000), with a total mineral resource (combined high and low-grade ore) estimated to be ~350 Mt (Leevers 2009).

JORC compliant mineral resources and ore reserves at 30 June 2012 (Arrium Mining and Materials, 2012) were:
    Measured + indicated + inferred resources - hematite = 147.8 Mt @ 58.8% Fe, including,
    Proved + probable reserves - hematite = 44.4 Mt @ 59.5% Fe;
    Measured + indicated + inferred resources - magnetite = 228.2 Mt @ 38.8% Fe, including,
    Proved + probable reserves - magnetite = 77.2 Mt @ 40.8% Fe DTR grade;

The most recent source geological information used to prepare this summary was dated: 2011.     Record last updated: 10/10/2012
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
 References to this deposit in the PGC Literature Collection:
Edwards A B,  1953 - Mineralogy of the Middleback iron ores: in Edwards A B (Ed.), 1953 Geology of Australian Ore Deposits Fifth Empire Mining and Metallurgical Congress, Australia and New Zealand, 1953, The AusIMM, Melbourne   v1 pp 464-472
Furber D V, Cook J N  1975 - Middleback Ranges iron ores: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 5 pp 945-951
Leevers P, Gaughan C and Bubner G,  2005 - The Iron Magnet Deposit: in   Iron Ore 2005 Conference, Perth, WA, September 19-20, 2005 The AusIMM, Melbourne   Publication Series 8 pp. 85-97
Owen H B and Whitehead S,  1965 - Iron ore deposits of Iron Knob and the Middleback Ranges: in McAndrew J and Madigan R T (Eds.),  1965 Geology of Australian Ore Deposits Eighth Commonwealth Mining and Metallurgical Congress, Australia and New Zealand, The AusIMM, Melbourne,   v1 pp 301-311
Quast K,  2012 - Some Surface Characteristics of Six Hematite Ores from the Middleback Range Area, South Australia: in    International Journal of Mining Engineering and Mineral Processing   v.1 pp. 73-83
Rudd E A and Miles K R  1953 - Iron ores of the Middleback Ranges: in Edwards A B (Ed.), 1953 Geology of Australian Ore Deposits Fifth Empire Mining and Metallurgical Congress, Australia and New Zealand, 1953, The AusIMM, Melbourne   v1 pp 449-463
Yeates G  1990 - Middleback Ranges Iron Ore Deposits: in Hughes FE (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   v2 pp 1045-1048

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