Koolyanobbing, Mt Jackson, Windarling, Deception
Western Australia, WA, Australia
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The Koolyanobbing iron deposit is developed within the Southern Cross Province of the Archaean Yilgarn Craton, some 175 km west of Kalgoorlie and 350 km ENE of Perth. The Mt Jackson deposit is 80 km NNW of Koolyanobbing, Windarling 20 km north of Mt Jackson, and Deception, a further 20 km north of Windarling. Windarling is 120 km by haul road from the rail head at Koolyanobbing (#Location: 30° 48' 52"S, 119° 31' 8"E).
Koolyanobbing is localised in a small in-folded, arcuate, NNW trending, ~3.0 Ga greenstone belt which is ~35 km in length and 8 km wide, truncated to the west, north and south by the regional scale Koolyanobbing Fault zone, an ~6 to 14 km wide shear zone of mostly ductile deformation and mylonites. The adjacent rocks to the SW are gneisses of the Ghooli and Lake Deborah Domes, dated at 2775±10 to 2691±7 Ma, and to the NE by banded gneisses. To the south, the greenstone belt succession, Koolyanobbing shear zone and gneisses are intruded by the ~2656 Ma Lake Seabrook granite, one of a series of post-orogenic and post-metamorphic plutons (mostly monzogranites) that intruded gneiss domes and greenstone belts throughout the region between 2.66 and 2.60 Ga. Undifferentiated Proterozoic porphyritic dykes crosscut the entire greenstone belt (Angereret al., 2012 and sources quoted therein).
The Koolyanobbing greenstone belt (minimum age 3023±10 Ma) is composed of a lower greenstone succession, which is mainly composed of discontinuous basal, clastic quartzites, followed by thick tholeiitic pillowed basalt flows and intrusive rocks, high-magnesium basalts, minor basic tuffs and komatiites, several banded iron formation (BIF) units and minor clastic sedimentary rocks (quartzites and pelites). The estimated thickness of the tholeiitic volcanic sequence in the greenstone belt is 6 km, which contains less acid to intermediate lithologic units than most other greenstone belts in the Yilgarn craton. Ultramafic layers, originally peridotitic komatiite flows, are abundant in the lowermost and in the upper section of the lithostratigraphic column and have a total thickness of about 800 m. The basalts and tuffites have been metamorphosed to hornblende- and actinolite-dominated amphibolites and chlorite schists (locally talc bearing), while the ultramafic rocks are now carbonate-bearing talc-, chlorite-, tremolite-, antigorite-rich schists (Angereret al., 2010; 2012 and sources quoted therein).
Three BIF units within the Koolyanobbing greenstone belt, are expressed as topographical ridges. These BIFs strike NNW, parallel to the overall greenstone belt trend, and have thicknesses generally varying between 50 and 180 m, locally up to 260 m. The middle of these is the most prominent, and is continuous throughout the entire length of the greenstone belt. It mainly comprises layered quartz-magnetite rock, weathered to a quartz-martite±goethite BIF from the surface to depths of ~70 m. Laminated or massive metacherts, containing recrystallised quartz, are intercalated within the BIF, particularly in the north. Within the middle BIF unit, rocks are locally magnesium-rich, occurring as talc schist, layered talc-magnetite BIF, talc-martite BIF or dolomite-magnetite BIF. Amphibole±carbonate-rich BIF is found in the lower and middle BIF unit. Siderite-magnetite BIF occurs in many of the deposits, while an ~10 m thick, boudinaged layer of chlorite schist, probably a metamorphosed tuff, is intercalated in the middle BIF unit. Massive pyrite bodies with thicknesses between 5 and 70 m are locally present at the stratigraphic footwall of the BIF (Angereret al., 2010; 2012 and sources quoted therein).
A large part of the upper crust throughout the Southern Cross and Murchison provinces comprises 2.80 and 2.67 Ga multiphase granitoid batholiths, emplaced and deformed as pre- to synorogenic plutonic phases, resulting in separation of the greenstone belts. Peak metamorphism of most greenstone belts in the Southern Cross domain was associated with the intrusion of these granitoid batholiths. An outward increase in metamorphic grade from subgreenschist to greenschist in the core, to amphibolite facies at the boundary of greenstone belts is interpreted to be the result of regional-scale contact metamorphism (Angereret al., 2010; 2012 and sources quoted therein). Angerer and Hagemann (2010) established a five-stage structural history of the South Range of the Koolyanobbing greenstone belt. The D1 structures formed in a N-S to NW-SE compressional regime, whereas the D2 to D4 deformation events were generated during E-W compression and were expressions of the main orogenic event in the central Yilgarn craton (Gee, 1979; Dalstra et al., 1999; Chen et al., 2001a, b, 2004). Peak metamorphism (greenschist to amphibolite facies) occurred latest during D2 in the Koolyanobbing greenstone belt and was associated with the synorogenic intrusion of granitic batholiths (Ahmat, 1986).
The BIFs in the lower part of the Koolyanobbing greenstone belt are predominantly composed of little altered quartz-magnetite BIF, weathered quartz-martite-goethite BIF and locally hydrothermally oxidised quartz-martite BIF. Iron oxides are mainly anhedral, <0.05 mm magnetite grains. Quartz occurs as recrystallised ~0.025 mm grains. The texture of these BIFs is characterised by micro- to meso-layers of iron oxide, intercalated with quartz meso-layers. The average meso-layer of iron oxide average 2 to 5 mm in thickness, which increases from the NW to SE within the middle BIF unit. The layer are typically anastomosing, with common centimetre- to microscale structures, including boudinage, shear zones, harmonic isoclinal or tight folds, disharmonic folds, and discrete faults and veins that crosscut layering. The Mg:Fe ratio of amphiboles is high (i.e., cummingtonite composition) in BIF to the south and low (i.e., grunerite) in the north (Angereret al., 2010).
Laminated magnetite ore is present as medium- to locally high-grade 45 to 63 wt.% Fe, with high Mg, Si, S, P contaminated magnetite, which is usually laminated, microfolded and characterised by primary magnetite mesolayer and layers of fine-grained, microporous, anhedral magnetite and by abundant iron oxide-rich pods or veins several tens of cm to several metres in thickness. Magnetite ore breccias are mineralogically similar, but have brecciated layers in a fine-grained, cataclastic, magnetite matrix. Common gangue minerals in magnetite ore are quartz, ferroan talc and Fe carbonate. Up to 10 vol.% pyrite is present as disseminated, coarse, euhedral crystals, clusters, or bands that are tens of cm thick. Locally, medium grade mineralised BIF is overprinted by hydrothermal specularite (Angereret al., 2010).
Martite ore, containing 58 to 67 wt % Fe, with medium to high P and low S, is either massive or vuggy laminated, and in the weathering zone and along faults, typically shows goethite replacement to form goethite-martite ore. It occurs as massive laminated and martite ore breccia, with similar characteristics to the magnetite ores. The vuggy laminated martite ore is characterised by martite layers with interstitial layer-parallel voids, suggesting recent mineral leaching (Angereret al., 2010).
Specularite ore, occurs as zones of monomineralic hydrothermal specularite with grades of 65 to 68 wt.% Fe, low P and low S, and is only encountered in some fault zones in some deposits, where it is locally preserved as large, pseudohexagonal crystal blades with basal planes of up to 10 cm diameter, and more commonly as friable masses of fine-grained, brecciated specularite. Specularite (commonly with >5 mm grain sizes) occur as disseminated crystals, veins, or pods in some martite ore and goethite-martite ores (Angereret al., 2010).
High-grade goethite-martite ore, with 58 to 63 wt.% Fe, medium P and low S, is locally present in the weathering zone, i.e., to ~70 m below the present surface in all iron ore deposits. Goethite typically replaces martite, specularite or some gangue minerals. The ore may be massive layered goethite-martite, or massive goethite-martite breccia (Angereret al., 2010).
Goethite ore occurs as either: i). massive and vuggy, vitreous goethite with 58 to 60 wt.% Fe, high Al, high Si, medium to high P and low S, localised at the present surface, typically above goethite-martite ore; ii). gossanous goethite, occurring near the surface, above deep-seated massive pyrite bodies in the basal sections of the middle BIF unit, with 55 to 59 wt.% Fe, high Al, high Si, low P and low S; iii). unconsolidated ochreous goethite with 50 to 58 wt.% Fe, high Al, high Si, low P and low S, occurring as the weathering product of massive, vitreous, or gossanous goethite weathering (Angereret al., 2010).
Angereret al., 2012 interpret four hypogene ore-forming/alteration stages and one supergene upgrading event, as follows:
i). Stage 1, involved the formation of LREE-depleted, transition metal-enriched, Mg-Fe (±Ca) carbonates replacing quartz in BIFs, induced by devolatilisation of sea floor altered, Ca-Si depleted mafic rocks during early regional, syn-D1, very low to low-grade metamorphism, most strongly developed at reactivated BIF-basalt contacts;
ii). Stage 2, resulting in the formation of patchy magnetite ore by syn-D2 to D4 dissolution of early carbonate, enriching the total Fe2O3 in magnetite ore by a factor of between 2 and 2.4;
iii). Stage 3 magnetite growth, forming granular magnetite-martite ore, related to a subsequent hydrothermal event that occurred locally throughout the belt, especially in D2b faults;
iv). Stage 4 was associated with Fe-Ca-P-(L)REE-Y enriched hydrothermal fluids, possibly from a magmatic source such as the post-metamorphic Lake Seabrook granite that
outcrops ~10 km west of the Koolyanobbing deposits and at the southern margin of the greenstone belt. These Ca-enriched fluids, which were channelled through regional D4 faults, interacted with distal metamorphosed mafic rock and influenced the BIF-ore system in a small number of deposits. They produced specularite-dolomite-quartz alteration, resulting in Fe grades of up to 68%;
v). Supergene upgrade by (further) gangue leaching in the weathering zone was most effective in carbonate-altered BIFs and magnetite ore.
The deposits at Koolyanobbing were initially mined by the Western Australian Government in 1948 to supply the charcoal iron industry at Wundowie near Perth. Additional deposits were developed in the mid 1960s by BHP Ltd, who operated the mine until 1983. Portman resumed mining from 1994. The operation was subsequently acquired by Cliffs Natural Resources in 2007.
Iron mineralisation occurs over a 14 km length of the Koolyanobbing Range and has been mined at a number of localities along this interval. The oldest and largest is the K mine, in the centre of the greenstone belt, while pits A to E are distributed over an interval of ~6 km in the southern half of the belt.
The Dowd's Hill (or K deposit) at Koolyanobbing, had a surface length of 900 m, 50 to 300 m in width and extended for 50 to 80 m below the current surface. The ore comprised hard massive goethite, coarse grained friable specular hematite, some massive fine grained hematite and ochreous yellow limonite, and minor magnetite. Bands of chlorite schist and friable iron-leached jaspilite occur within the ore zone. It has been suggested that the oxidation and enrichment may have a Proterozoic age.
The Mt Jackson and Windarling deposits are located within the Marda greenstone belt, near its western margin where the greenstone belt is truncated by the Koolyanobbing Fault. They have a similar geologic setting to Koolyanobbing.
These deposits are currently being mined as a single operation by Cliffs Natural Resources Pty Ltd. Ore is trucked from Mt Jackson and Windarling to Koolyanobbing to be railed south for 575 km to the port of Esperance on the southern Ocean. Production exported in 2012 was ~11 Mt of crushed and screened ore, mined from eight separate open pit mines.
Angereret al., 2012 suggest the Koolyanobbing deposits contained an original resource of ~200 Mt @ >58% Fe.
The average grade of the ore at Kooyanobbing from 1967 to 1972, when 8 Mt was mined, was 61.4% Fe, 0.13% P, 6% LOI.
Total indicated + inferred resources at the end of 2004 were:
Koolyanobbing - 37.4 Mt @ 61.52% Fe, 0.077% P, 3.38% Silica, 0.71% Alumina, 0.091% S, 6.63% LOI
Mt Jackson - 47.7 Mt @ 60.92% Fe, 0.079% P, 2.25% Silica, 1.02% Alumina, 0.108% S, 9.16% LOI
Windarling - 55.4 Mt @ 63.73% Fe, 0.138% P, 1.89% Silica, 1.29% Alumina, 0.019% S, 4.67% LOI
TOTAL - 149.5 Mt @ 62.13% Fe, 0.110% P, 2.45% Silica, 1.04% Alumina, 0.067% S, 6.71% LOI
Proved + probable ore reserves at the Koolyanobbing operation were:
93.7 Mt @ 62.21% Fe, 0.094% P, 2.45% Silica, 1.06% Alumina - at the end of 2004,
95 Mt at the end of 2008.
89.1 Mt @ 60.9% Fe at the end of 2011 (Cliffs Natural Resources Annual Report, 2012).
The most recent source geological information used to prepare this summary was dated: 2012.
Record last updated: 29/4/2013
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 to this deposit in the PGC Literature Collection:
Angerer T and Hagemann S G, 2010 - The BIF-Hosted High-Grade Iron Ore Deposits in the Archean Koolyanobbing Greenstone Belt, Western Australia: Structural Control on Synorogenic- and Weathering-Related Magnetite-, Hematite-, and Goethite-rich Iron Ore : in Econ. Geol., v.105 pp. 917-945|
Angerer T, Hagemann S G and Danyushevsky L V, 2012 - Geochemical Evolution of the Banded Iron Formation-Hosted High-Grade Iron Ore System in the Koolyanobbing Greenstone Belt, Western Australia : in Econ. Geol. v.107 pp. 599-644|
BHP Staff 1975 - Koolyanobbing Iron Ore Deposits, WA: in Knight CL (Ed.), 1975 Economic Geology of Australia & Papua New Guinea, Monograph 5 The AusIMM, Melbourne v1 - Metals pp 940-942|
Lascelles D F, 2007 - Genesis of the Koolyanobbing iron ore deposits, Yilgarn Province, WA, Australia: in Trans. IMM (incorp. AusIMM Proc.), Section B, Appl. Earth Sc. v116 pp 86-93|
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