The mining rights and assets involved with bauxite mining and alumina refining in Australia are 100% owned by Alcoa of Australia Limited (AofA), an affiliate of Alcoa owned by Alcoa World Alumina and Chemicals (AWAC).
AWAC is an unincorporated global joint venture between Alcoa and Alumina Limited. AWAC consists of a number of affiliated entities that own, operate or have an interest in bauxite mines and alumina refineries, as well as an aluminum smelter, in seven countries.
On August 1, 2024, Alcoa Corporation announced the successful completion of its acquisition of Alumina Limited (“Alumina”). Alcoa completed the acquisition of all ordinary shares of Alumina, through a wholly owned subsidiary, AAC Investments Australia 2 Pty Ltd.
With Alcoa’s acquisition of Alumina, the Alcoa World Alumina and Chemicals (AWAC) joint venture is now fully owned and controlled by Alcoa.
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Summary:
The bauxite deposits of the Huntly and Willowdale operations are located in the Darling Range region of southwest Western Australia. The predominant topographic feature of the region is the Darling Range Fault, a north-south trending scarp which extends approximately 220 km from Bindoon to Collie.
The Darling Range Fault is the structural boundary between two geological terranes: the Pinjarra Orogen to the west, now the sedimentary Swan Coastal Plain, and the Yilgarn Craton to the east, a gneissic granite complex with greenstones. To the east of the Darling Range Fault intense weathering and erosion of exposed Archean basement rocks of the Western Gneiss Terrane, the western portion of the Yilgarn Craton, formed widespread lateritic bauxite deposits by the intense weathering, accumulation and leaching of the aluminosilicate rich material of the bedrock granites (Hickman et al, 1992).
Alcoa’s current bauxite mining areas of Huntly and Willowdale are on the eastern side of the Darling Range Fault, as low-lying plateaus separated by valleys in which alluvial deposits have accumulated. Figure 6 1 shows the regional geology of the southwest region of Western Australia and Alcoa’s ML1SA lease boundary in relation to Perth, while Figure 6 2 shows the distribution of surficial deposits across the region.
The Jarrahdale, Del Park, Huntly and Willowdale areas that have been mined by Alcoa are on laterite within the Western Gneiss Terrane (Figure 6 2), formed over granites that have been intruded by numerous north trending tholeiitic, quartz dolerite dykes, of early to late Proterozoic age, with thicknesses ranging from 1 m to 200 m.
Lateritic bauxite developed from the Late Cretaceous (65 million years ago, Ma) to the Eocene (40 Ma), with several periods of erosion and intense weathering of the basement granites and dolerites. Subsequent reactivation of the Darling Fault combined with periods of erosion led to the establishment of plateaus and incised valleys, trending to wider valleys and low hills to the east which now characterize the physiography of the region.
Mineralization
Mineralization Weathering, alteration and leaching of the granite bedrock has developed the bauxite mineralization which principally occurs as 65% microcrystalline gibbsite Al(OH)3 with minor to rare boehmite AlO(OH), and accessory minerals of 18% goethite FeO(OH), 7% hematite Fe2O3, 9% quartz SiO2, 1% kaolinite/halloysite Al2Si2O5(OH)4, and 0.5% anatase/rutile TiO2.
Other minerals within the bauxite that may influence the alumina refinery performance include:
• Boehmite: generally occurring below 1%, this can cause premature precipitation of dissolved gibbsite resulting in alumina being lost to the red mud residues.
• Organic Carbon: as oxalate, typically less than 0.2%, (2.0 kg/t, measured as Na2C2O4) this can result in reduced digestion efficiencies and cause crystal growth issues during precipitation.
Property Geology
Most of the bauxites display a typical profile comprising the following sequence, from the top down:
• Overburden: A mix of soils, clays, rock fragments and humus that is typically 0.5 m deep, but deeper pockets are common.
• Hardcap: An indurated iron-rich layer that is usually 1 m to 2 m thick. It is generally high in available alumina (A.Al2O3) and low in reactive silica (R.SiO2).
• Friable Zone: A partially leached horizon that usually contains a mix of caprock fragments, clasts, nodules, pisolites, and clays. It is usually a few meters thick but can exceed several meters in places. It is generally high in A.Al2O3 and low in R.SiO2.
• Basal Clay: A kaolinitic clay horizon that represents the transition zone between the Friable Zone and the underlying saprolitic material. It is generally high in R.SiO2 and low in A.Al2O3.
The Hardcap and Friable Zone are targeted as the ore horizon. Selective mining practices are applied to minimize the inclusion of Overburden, because of its elevated organic carbon levels, and Basal Clay because of its elevated R.SiO2 concentrations. Within the Hardcap and Friable Zone, the dominant minerals, in order of abundance, are gibbsite, quartz, goethite, kaolinite, and hematite, with lesser amounts of anatase and muscovite.