Iron Bridge - North Star, Glacier Valley
Western Australia, WA, Australia
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The North Star and Glacier Valley magnetite iron deposits, which together comprise the Iron Bridge Project, are located ~100 km south of Port Hedland and 25 km east of the FMGL railway line, in north-western Western Australia (#Location: 21° 14'S, 119° 3'E).
The ore is hosted within the Palaeoarchaean Pincunah Banded-iron Member of the 3235±3 Ma Sulphur Springs Group in the granite-greenstone terrane of the Archaean core of the Pilbara craton.
The oldest rocks within the Pilbara Craton represent 3.80 to 3.53 Ga crust (seen in rare exposures of gneissic granite and gabbroic anorthosite), which was widely exposed throughout the evolution of the craton. These rocks are intruded and overlain by a granite-greenstone terrane, characterised by a regional outcrop pattern of ovoid granitic domes separated by arcuate belts of volcanosedimentary rocks that define the greenstone belts, representing at least eight successive volcanic cycles. The oldest sequence in the greenstone belt stratigraphy is the 15 to 20 km thick Pilbara Supergroup, which has been subdivided into the lower, dominantly volcanic, 3.53 to 3.42 Ga Warrawoona Group, comprising up to four sub-groups separated by disconformities, and intruded by 3.53 to 3.46 and 3.45 to 3.42 Ga granitoid supersuites. The Warrawoona Group is unconformably overlain, after a 75 m.y gap, by the 3.35 to 3.31 Ga Kelly and 3.25 to 3.24 Ga Sulphur Springs Groups, represented in the area by intermediate to felsic volcanic and sedimentary rocks. The Kelly and Sulphur Springs groups are separated by an ~60 m.y. gap. Each of the gaps was preceded by deformation and metamorphism, and accompanied by subaerial erosion and deposition of shallow water sediments, e.g., the Strelley Pool stromatolitic dolomies at the base of the Kelly Group (Hickman and Van Kranendonh, 2012).
The Kelly Group was cut by the 3.32 to 3.29 Ga Emu Pool Supersuite granitoids, while the granitoids of the 3.27 to 3.23 Ga Cleland Supersuite accompanied the Sulphur Springs Group (Hickman and Van Kranendonh, 2012).
These rocks are unconformably overlain to the north and west, by the De Grey Supergroup, composed of a thick sequence of shale, wackes, feldspathic sandstone, arkose, banded iron formation and conglomerate, with interspersed significant basalt, high-Mg basalt, siltstone, chert, and lesser felsic tuffs and volcaniclastic rocks. The lower section of the supergroup includes the widespread iron formation and clastic meta-sediments of the 3.05 to 3.02 Ga Gorge Creek Group which in most areas consists of basal conglomerate and sandstone overlain by the 1000 m thick Cleaverville Formation comprising BIF, chert and black shale (Hickman and Van Kranendonh, 2012).
The Sulphur Springs and Kelly groups thin and pinch-out northward, where the Warawoona Group is unconformaby overlain by the Gorge Creek Group following an interval of deformation at 3.15 Ga. The Gorge Creek Group hosts significant iron ore deposits throughout the northern Pilbara craton (see the Yarrie, Mount Goldsworthy and Ridley records).
The Warrawoona, Sulphurs Springs and Gorge Creek groups suffered their most intense tectonic imprint together as a result of subsequent granitic diapirism producing batholiths between 3.0 and 2.8 Ga, which dominate the structural regime and separate the sequence into isolated greenstone belts.
The Sulphur Springs Group, which hosts the North Star and Glacier Valley magnetite iron deposits, comprise from the base, the:
Leilira Formation, composed of metamoprhosed quartz sandstone and minor layered quartz carbonate rock and siltstone; overlain by the
Kunagunarrina Formation, greenschist facies, dominantly of komatiitic, high-Mg and tholeiitic metabasalt; and the
Kangaroo Caves Formation, representing a differentiated volcanic-volcaniclastic pile of mainly tholeiitic magmatic affinity, varying from basalt to rhyolite, with comagmatic granite. It includes chert, local polymictic mega breccia and iron-formation, and calc-alkaline rhyodacite. The Pincunah Banded-iron Member occurs at the top of this unit and comprises metamorphosed jaspilitic banded-iron formation with minor layered chert and shale.
The Pincunah Banded-iron Member varies in stratigraphic thickness from 350 to 450 m, forming a north-south oriented flat-top strike-ridge with an elevation of 80 to 120 m above the surrounding countryside. The host BIF dips at between 70° E and 80°W, and is continuous to at least 500 m below the surface without diminution of grade. Small scale folding is evident, although no macrofolds have been recognised. The sequence is cut by a number of faults which are in general sub-parallel to strike (Fortescue Metals Group, 2010).
At North Star, the Pincunah BIF has been divided into two stratabound magnetite-rich domains, overprinted by and upper oxidised zone, which extends to a depth of ~50 m below the ridge top. The deposits have been tested to a depth of 500m, and are 200 to 400 m wide, with a strike length of more than 12 km (Fortescue Metals Group, 2010).
Magnetic hematite, referred to as mHematite, is magnetite that has been partly oxidised into hematite, but retains its magnetic properties and is therefore is amenable to magnetic
separation (Fortescue Metals Group, 2010).
The JORC compliant magnetite resource (Fortescue Metals Group Limited, 2012) in December 2012, was:
mHematite indicated resource - 44 Mt @ 34.2% Fe, 16.4% (DTR), 2.1% Al2O3, 43.7% SiO2, 0.065% P, 3.4% LOI.
mHematite inferred resource - 46 Mt @ 33.2% Fe, 13.6% (DTR), 2.7% Al2O3, 44.3% SiO2, 0.068% P, 3.5% LOI.
Magnetite indicated resource - 676 Mt @ 29.0% Fe, 25.6% (DTR), 2.0% Al2O3, 39.7% SiO2, 0.101% P, 7.4% LOI.
Magnetite inferred resource - 2801 Mt @ 29.0% Fe, 19.2% (DTR), 2.9% Al2O3, 41.8% SiO2, 0.098% P, 8.1% LOI.
TOTAL resource - 3.568 Gt @ 29.6% Fe, 20.3% (DTR), 2.7% Al2O3, 41.8% SiO2, 0.098% P, 7.6% LOI.
Magnetite inferred resource - 1637 Mt @ 32.2% Fe, NA (DTR), 1.7% Al2O3, 38.9% SiO2, 0.104% P, 8.8% LOI.
TOTAL resource, Iron Bridge - 5.205 Gt @ 30.4% Fe, NA (DTR), 2.4% Al2O3, 40.7% SiO2, 0.100% P, 8.2% LOI.
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.
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