Summary:
The bauxites have developed on the exposed Achaean basement of the Western Gneiss Terrane, which is located on the western edge of the Yilgarn Block. In the project area, the Western Gneiss Terrane is predominantly composed of granitic gneisses and migmatites, as well as the greenstones of the Saddleback Greenstone Belt. The northerly trending greenstone belt is approximately 43 km long and 5 - 12 km wide, and consists of sequences of mafic to felsic volcanics, pyroclastics, and sediments that have undergone greenschist facies metamorphism. The gneisses and greenstones have been intruded by relatively undeformed granites, and subsequently by numerous northerly trending doleritic dykes that range in thickness from 1 to 200 m. The dominant structural feature in the region is the Darling Fault, which forms the western boundary of the Western Gneiss Terrane.
The bauxites are thought to have formed from the lateritisation of the peneplained surface of the Western Gneiss Terrane rocks. Lateritisation is thought to have commenced during the Cretaceous and continued through to the Eocene.
In the project area, bauxite exists as irregular lenses, locally referred to as pods, within the remnant laterite. The pods are generally elongated in the NNW-SSE direction and vary in size from 1 to 200 ha. The majority of the pods contain approximately 2 million tonnes (Mt) of bauxite. The pods are generally confined to slopes where the gradient is between 5° and 10°. In steeper areas, the lateritic cover has often been removed by erosion. In areas where the slope gradients are less than 5°, sub-surface water flow is usually insufficient to promote the removal for the soluble silicate materials. The material in these areas often has a high clay content and is usually not economically viable.
Summary of bauxite deposits:
Brookton. Discontinuous mineralisation over an area covering 20 km northwest / southeast and 12 km northeast / southwest. The total profile depth from surface, varies from 1 m to a maximum of 14 m, but is generally only a few metres thick.
Collie East. Discontinuous mineralisation over an area covering 20 km north / south and 30 km east / west. Total thickness ranges from <1 m up to 16 m with an average thickness of 2.3 m.
Collie (Worsley). Discontinuous mineralisation over an area covering 15 km north / south and 17 km east / west. Total thickness ranges from <1 m up to 10 m with an average thickness of 1.9 m.
Collie South. Discontinuous mineralisation over an area covering 17 km north / south and 15 km east / west. Total thickness ranges from <1m up to 12 m with an average thickness of 2.2 m.
Hotham North. Discontinuous mineralisation over an area covering 20 km north / south and 14 km east / west Total thickness ranges from <1 m up to 27 m with an average thickness of 5.4 m.
Hotham West. Discontinuous mineralisation over an area covering 6 km north / south and 3 km east / west. Total thickness is up to 25 m but generally is roughly 5-6 m.
Marradong. Discontinuous mineralisation over an area covering 8 km north / south and 8 km east / west. Total thickness ranges from <1 m up to 30 m with an average thickness of 7.4 m.
Mid-Central. Discontinuous mineralisation over an area covering 45 km north / south and 30 km east / west. Total thickness ranges from <1 m up to 13 m with an average thickness of 3 m.
Saddleback. Discontinuous mineralisation over an area covering 19 km northwest / southeast and 10 km northeast / southwest. Total thickness ranges from <1 m up to 32 m with an average thickness of 7.3 m.
Southern MLA258SA. Hotham South covers and area is approximately 14 km north / south and between 5 km and 10 km east /west although mineralisation is discontinuous over this area. Williams South strikes roughly northwest over a distance of approximately 7 km. In the northeast direction, mineralisation stretched up to 2 km. Morgan covers to w areas with dimensions 6 km north / south and 4 km east / west as well as 6 km northwest / southeast and 2 km northeast / southwest. Mineralisation occurs from surface to depths <1 m and up to 11 m but general is only a few metres thick.
Bauxite characteristics
In the project area, bauxites have developed on the meta-basalts that comprise the Saddleback Greenstone Belt as well as on the surrounding granites. The bauxites that have formed on greenstone are usually thicker, higher in iron and sulphate, but lower in silica and organic carbon compared to the bauxites that have formed on granite. However, in general, the bauxite profile is similar across the project area, and typically comprises the horizons described below. Dolerite is not part of the lateritic profile, but it cuts the sequence and is included in the list below for completeness.
• Topsoil. A mix of humus, sandy loam and gravel, which is typically 0.1 m thick.
• Lateritic Gravel. Unconsolidated iron rich pisoliths and gravels in a sandy or silty matrix generally between 0 and 2 m thick.
• Hardcap. A ferruginous and bauxitic indurated laterite consisting of cemented angular fragments and / or pisoliths. The dominant minerals are gibbsite, hematite and goethite, with lesser amounts of quartz, maghemite and kaolinite also present. The hardcap is typically 3 m thick over greenstone and 1 m thick over granite.
• Bauxite Zone. (B zone). A friable, unconsolidated, yellow-brown to red-brown bauxitic layer in which the original rock textures have been destroyed. The dominant minerals are gibbsite, goethite and hematite, with minor kaolinite and trace illite. The horizon is often referred to as the B-zone, and is generally between 1 and 8 m thick.
• Clay Zone. Kaolinite clays with variable degrees of ferruginisation, silicification and kaolinisation. Relict bedrock textures and quartz veins are common. The dominant minerals are kaolinite, mica or illite, kaolinite-halloysite, quartz and phyllosillicates. The Clay Zone is generally 10 - 30 m thick.
• Lower Saprolite. A transitional zone between fresh bedrock and the overlying clay zone. This zone is typified by partially weathered bedrock fragments in a clay matrix. It is often referred to as “basement” in the geological logs.
• Dolerite. The dykes range in breadth from several to over 100 m wide. These are not bauxitised, and in places have acted as impediments to drainage. As a consequence, proximal bauxite on the upslope side of dykes often reports elevated reactive silica concentrations.
With the exception of the top of the Hardcap, most contacts are transitional, although the thickness of the transition zone can vary markedly. The dominant minerals are gibbsite, kaolinite, quartz, hematite and goethite. Elevated concentrations of organic carbon occur in the upper parts of the profile. These are monitored in the feed (expressed as oxalate) because of the adverse effect on refining. In general, boehmite occurs in trace concentrations only. It is thought to have formed from the dehydration of gibbsite during periodic wildfire activity and is typically confined to the upper parts of the profile. Some available alumina is also thought to occur as alumino-goethite, and some partially reactive silica is thought to exist in an amorphous form.
The deposits are described as lateritic bauxites. Compared to many mined lateritic bauxites, they report relatively low total and available alumina, low reactive silica, high quartz grades and, apart from organic carbon, few contaminant minerals.