Nature of the Weipa Bauxite deposit, northern Australia

Abstract

The extensive bauxite deposits of the Weipa region, northern Queensland, are a loose agglomeration of nodules, pisoliths, ooliths and fine material, with a mean particle size of 5 mm. Only about 5% of the deposit is finer than 0.1 mm, about 5% is coarser than 10 mm, and the bulk of the deposit is made up of pisoliths and ooliths. The Bauxite is overlain by soil that in most places is mixed with the upper portion of the Bauxite over about 30 cm. There is little more than 20 m of relief across the deposit; in some drainages the bauxite is overlain by or contains within it unconformably ‘redsoil', a fine-grained oolitic facies, rich in Al2O3. The Bauxite overlies a ferricrete with a sharp or gradual contact. In the latter case the Bauxite contains soft earthy pisoliths mixed with pisoliths similar to those in the body of the deposit. Pisoliths have a core which may be a lump made of any of the materials of the deposit; earthy pieces, accumulations of ooliths, a fragment of a broken pisolith, or a small pisolith or oolith. The cores are surrounded by up to seven cortices distinguished by various shades of red-brown-yellow from almost white to dark red. The cortices may be continuous around the core, or discontinuous. Pisoliths have been classified on the basis of these characteristics, and the features interpreted to indicate that the cortices form largely by accretion and that many have undergone transport and erosion during their formation. The pisoliths are composed of gibbsite, boehmite, nano-particulate anhydrous aluminium minerals, kaolinite, quartz, hematite and/or goethite in widely varying proportions and about 2% anatase. The deposit at East Weipa sits on Paleogene sands and clays of the Bulimba Formation; this bauxite contains up to 30% quartz and of the order of 5% Fe2O3. The deposit at Andoom, north of Weipa overlies Rolling Down Group smectitic/vermiculitic sandstones; the bauxite contains little to no quartz but up to 25% Fe2O3. In both deposits the gibbsite:boehmite ratio increases with depth through the bauxite, and kaolinite and quartz decrease in amount down the profile until the base. Both deposits contain, on an anhydrous basis, about 75% Al2O3, 10% SiO2, from 10 to 20% Fe2O3 and 3% TiO2. Distinguishing the bauxite from the two deposits is easiest using the abundance of quartz and less easily by the Al:Fe ratio which is lower at Andoom than at East Weipa.