Properties of regolith beneath lateritic bauxite in the Darling Range of south Western Australia

Abstract

A morphological key has been developed for regolith that is exposed during mining of lateritic bauxite in the Darling Range of south Western Australia. The key distinguishes materials with different mineralogical and chemical properties. Iron oxide cemented (Zh) regolith has a gibbsitic matrix, quartz-rich (Zm) regolith has a gibbsite and kaolin matrix, and clay-rich (Zp) regolith has a kaolin matrix. An Si affinity element map (Si, Hf, Th) and a K affinity element group (K, Ba, Rb) are associated with granitic quartz-rich regolith and an Al/Fe element affinity group (Al, Fe, Ti, P, Ni, Co, Cu, Mn, Zn, Ga, Cr, V) is associated with clay and iron rich regolith. Doleritic regolith is generally associated with the Al/Fe affinity group. Although granite and granitic regolith exhibit similar element affinity groups, the abundance of elements within each is highly variable, which reflects the diversity in composition of granite within the region. The degree of euhedral character of clay-size platy crystals (kaolinite/gibbsite) does not differ for materials distinguished by the key, as both quartz-rich (Zm) and clay-rich (Zp) regolith and both granitic and doleritic saprolite contain subhedral kaolin crystals. The crystal size of platy kaolin (approximately 0.5 µm) is similar for different mine pits and for different regolith materials (Zm and Zp) within mine pits. There is a difference in halloysite tube length (0.52–1.18 µm) between mine pits, which may be related to the presence of weathered mica or to the alteration of halloysite in gibbsite-rich regolith. The internal and external diameters of halloysite tubes (about 0.11 and 0.24 µm) are similar for different mine pits and different regolith types within mine pits. The resin used during thin section preparation contains chlorine, so that determination of chlorine by EMPA provides a measure of the porosity of regolith material. A systematic negative relationship exists between chlorine concentration and total oxide weight % of porous regolith matrix determined by EMPA; both measurements provide an indication of the porosity of the clay matrix in regolith.