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Regolith profile, mineralogy and geochemistry of the Weipa Bauxite, northern Australia

The regolith profile at Weipa is of two types. At Andoom, to the north of Weipa township, a 30 m profile overprints and overlies sediments of the Rolling Downs Group. At a depth of 23 m saprolite gives way to a Plasmic (pallid) Zone of kaolinite + quartz, which becomes mottled above 12 m. The degree of induration of the Mottled Zone increases upward to form a discontinuous and irregular Transition Zone about 1 m thick at the top of the wet season water-table and at a depth of about 4 m. In many places the Transition Zone is sufficiently indurated by goethite and hematite to be termed a Ferricrete. Above this is pisolithic bauxite and soil. On the Weipa Peninsula, Rolling Downs Group saprolite and Plasmic Zone terminate in a weak paleosol at 18 m, and this is unconformably overlain by sands and clays of the Paleogene Bulimba Formation. A quartz–kaolin Plasmic Zone developed in the clay and sandy clay units of the Bulimba Formation becomes mottled at about 5 m, with a Ferricrete or Transition Zone at around 4 m. Above this is pisolithic bauxite and soil. At both East Weipa and Andoom the Ferricrete is highly irregular, having an undulating surface at a wavelength of the order of 50 m or more and an amplitude of 2–3 m. Locally the surface is interrupted by depressions and holes ranging from 50 cm to 2 m in diameter and penetrating through the Ferricrete, and in places metre high pinnacles extend into the bauxite. The chemical and mineralogical changes that ultimately produce the bauxite pisoliths occur in the Transition Zone. At the base of the Transition Zone, kaolinite comprises 80% of the regolith. At the top of the Transition Zone, the composition is more than 75% Al(OH)3 in the form of gibbsite and ‘amorphous’ gibbsite. In the Transition Zone, goethite and hematite increase from 1–2% below to 15–25%. Over this 1 m interval, the fabric changes from 30–40 mm peds and nodules of generally massive kaolinite to 15 mm nodules of oolithic and sporadically pisolithic bauxite. The bauxites of Weipa and Andoom can be distinguished on the basis of higher Fe at Andoom, and higher quartz + kaolinite at East Weipa. Titanium, Zr, As, Nb and Sc also provide a basis for discrimination. Geochemical profiling of samples from two 30 m drillholes through each deposit shows strong coherence from the fresh Rolling Downs Group rocks to the bauxite at Andoom, and up to the unconformity at East Weipa, indicating these are in situ weathering profiles. The 30 m Andoom profile formed from an initial thickness of about 50 m of sediment. The variable substrate of the Bulimba Formation does not allow such conclusions to be reached for the East Weipa profile. These profiles have been developing, probably, over the last 50 Ma, and they are still forming, and/or changing, today.