Mount Gibson, Extension Hill

Western Australia, WA, Australia

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The Gibson Range group of deposits, which include the Mount Gibson and Extension Hill magnetite and smaller hematite direct shipping ore (DSO) deposits, are ~250 km NE of the Pacific Ocean port of Geraldton, and ~300 km NNE of Perth, in Western Australia (#Location: 29° 34' 35"S, 117° 9' 39"E).

These deposits are ~1.5 km apart, and are situated in the Retaliation greenstone belt in the Murchison province of the Archaean Yilgarn craton. The supracrustal rocks of the greenstone belt are the Murchison Supergroup which forms an arcuate bands in an enclave open to the north, between large regions of predominantly older granitoids. Younger granitoids, which are exposed 10 km to the NE, intrude the lower portions of the greenstone belts, so the base of the Murchison Supergroup and the rocks on which it was deposited have not been observed (Watkins and Hickman, 1990).

The Murchison Supergroup is divided into two groups:
Luke Creek Group - the older of the two, is about 10 km thick, and occurs within greenstone belts throughout the Murchison and possibly the Southern Cross province also. It comprises four lithostratigraphic formations, from the base,
Murrouli Basalt, a thick pile of mafic and ultramafic rocks, with associated gabbro and dolerite sills;
Golconda Formation, a sequence of BIF units interlayered with mafic and ultramafic extrusive and intrusive rocks;
Gabanintha Formation, dated at 2.95 to 3.05 Ga, consists of ultramafic, mafic, and felsic volcanic rocks and sediments;
Windanning Formation, which hosts the Mount Gibson BIF, is the uppermost formation of the Luke Creek Group, comprising almost half the thickness of the Luke Creek Group, and forms a substantial part of every greenstone belt in the Murchison province. It is made up of a succession of BIF and chert, interlayered with felsic volcaniclastic and volcanogenic rocks and minor basalt; overlain by the
Mount Farmer Group - which is dominated by a series of broadly contemporaneous volcanic centres that overlie the Windanning Formation.

Five phases of regional deformation are defined in the Murchison province: D1, responsible for recumbent folds and thrusts within of the Luke Creek Group and the development of gneissosity in older granitoids; D2 that initially produced upright, easterly trending, tight folds affecting all of the Murchison Supergroup, and underlying gneisses, followed by NNE to NNW trending, intense, upright, tight to isoclinal folds that define the dominant tectonic grain of the province. D3 refolded D2 structures to form dome and basin fold patterns. D4, with the same compressional direction to D3 produced large-scale shear zones and sheath folds and was probably related to the emplacement of post-folding granitoids. D5 resulted in easterly to southeasterly trending shear zones and faults confined to the NW Murchison province (Watkins and Hickman, 1990).

The Retaliation greenstone belt has been subjected to a regional lower greenschist facies metamorphic grade (Lipple et al., 1983) with foliated amphibolites on the south and western margins. At Mount Gibson, the regional metamorphism has transformed the primary sedimentary components of BIF, with iron hydroxyoxides converteded to magnetite, chert to quartz, and early-formed greenalite and chamosite to minnesotaite and stilpnomelane, respectively.

The host banded iron formation typically comprises alternating bands of magnetite and microcrystalline quartz (chert) with rare carbonaceous and iron silicate-rich shale partings and layers, and thin fine-grained tuff bands. Repetition by isoclinal folding and attenuation by faulting is characteristic. As a consequence, the true thickness of the Mount Gibson BIF is unknown but appears to be on the order of 100 m. Although the Mount Gibson BIF is typical of many so-called Algoma-type iron formations, situated within a greenstone belt on an Archean craton, it is also similar to Hamersley-type BIF in petrology, areal extent and ore genesis.

The Mount Gibson magnetite deposit typically consists of unweathered oxide-facies BIF comprising mesobands of magnetite and chert, with abundant interstitial grains and fine laminae of iron silicates and carbonates. It has an oxidised, weathered upper zone up to 100 m deep (averaging 70 m), outcropping as a steep sided ridge over a strike length of more than 8 km.

The Mount Gibson deposit is located within an isolated, south-plunging, faulted, anticline-syncline pair on a steeply east dipping limb of the Mount Gibson BIF, that has been structurally thickened by folding and strike-slip faulting. The Extension Hill deposit is to the north of this structure, where the BIF is sub-vertical and isoclinally folded, and repeated along oblique strike-slip faulting, increasing the thickness of the formation locally to a maximum of 450 m. Plunges of isoclinal folding within the Extension Hill deposit are both to the north and the south, and varied from horizontal to vertical.

Banding in the BIF is highly variable in thickness and mineralogy and is characteristically strongly layer parallel, except where disrupted by slumping or folding. The bands typically range in thickness from a few microns to several centimetres with rare bands of iron-rich carbonaceous black shale up to 30 cm. Chert mesobands account for 30 to 80 vol % of the rock measured over 1 m intervals, except for the chert-free BIF intervals where chert is completely absent. Chert bands are typically dark grey to black in color, due to the abundance of fine-grained magnetite, commonly oriented in band-parallel layers (Lascelles, 2006).

Magnetite bands range from <1 mm thick laminae to >10 cm mesobands and are characteristic throughout the BIF with little mineralogical or morphological variations apart from thickness and grain size, which are typically proportional. Coarse-grained magnetite (50 to 200 µm) in the mesobands is commonly fringed by fine-grained dusty magnetite (<1 to 10 µm) at the contacts with chert and other mesobands. Magnetite grains are typically granoblastic to idiomorphic in the oxide mesobands, and small isolated laminar aggregates of magnetite in other mesobands show similar textures (Lascelles, 2006).

Hydrous iron silicates such as greenalite, minnesotaite, grunerite, stilpnomelane, and rare chlorite typically fill the interstices between magnetite grains.

High grade hematite deposits within the BIF are thought to have formed by the supergene leaching of chert from typical cherty BIF, although recent evidence suggests that at least some of these deposits are formed by hypogene replacement of chert by carbonates with subsequent supergene leaching of the carbonate and accessory minerals and oxidation of magnetite to hematite. Magnetite-carbonate BIF, in which there is clear evidence of hydrothermal replacement of chert by carbonate, forms distinctive magnetite-goethite ore with magnetite locally persisting to the surface. High-grade hematite occurrences, up to 1 km in strike length, are found within the weathered zone overlying the magnetite BIF at Mount Gibson and continue into unweathered chert-free BIF at depth that show no evidence of hydrothermal carbonate or supergene enrichment. The cherty BIF has sharp contacts with chert-free BIF and high-grade ore, even when strongly weathered (Lascelles, 2006).

The Extension Hill deposit has reported JORC compliant resources (Asia Iron Australia presentation, December, 2012) of:
    Measured resource - 298 Mt @ 43.1 wt.% DTR magnetic fraction;
    Indicated resource - 422 Mt @ 32.4 wt.% DTR magnetic fraction;
    Inferred resource - 960 Mt @ 34.1 wt.% DTR magnetic fraction;
    TOTAL resource - 1.68 Gt @ 35.1 wt.% DTR magnetic fraction;
  including an initial high grade starter resource of - 896 Mt @ 44.6 wt.% DTR magnetic fraction.

The 1.7 Gt resource is estimated to be able to produce ~600 Mt of magnetite concentrate at >67.5% Fe.

The most recent source geological information used to prepare this summary was dated: 2012.     Record last updated: 8/5/2013
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 & Additional Information
 References to this deposit in the PGC Literature Collection:
Lascelles D F,  2006 - The Mount Gibson Banded Iron Formation-Hosted Magnetite Deposit: Two Distinct Processes for the Origin of High-Grade Iron Ore: in    Econ. Geol.   v101 pp 651-666

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