Groundwater geochemistry and composition of hardpans in southwestern Australian regolith

The mineralogy and geochemistry of pedogenic and groundwater silcretes and ferricretes, and associated groundwater geochemistry, were investigated for deeply weathered, highly kaolinised lateritic regolith in southwestern Australia. The silcretes and ferricretes form subsurface hardpans and impede movement of the highly saline, acidic, shallow groundwaters in lower valleys of the subdued, ancient landscape. These valleys are associated with the discharge of groundwater and the formation of saline seeps, which host a variety of secondary minerals including amorphous silica [opal-CT], goethite [α-FeOOH], hematite [α-Fe2O3], akaganeite [β-FeOOH], jarosite [KFe3(SO4)2(OH)6], barite [BaSO4], gypsum [CaSO4.2H2O], and halite [NaCl]. Deep groundwater silcretes are commonly composed of silicified in situ lateritic pallid zone materials and saprolite that have developed from granitic rocks. The matrix consists of amorphous silica diffused through a porous kaolin groundmass. These materials were initially highly porous but the voids are now mostly occupied by amorphous silica and secondary anatase. Acid groundwater weathering has resulted in the dissolution of most primary silicates including feldspars and micas, which has greatly elevated the concentration of dissolved silicon in groundwater. Isovolumetric weathering of feldspar initially produced highly porous assemblages of fine-grained kaolin and minor halloysite, which now contain high concentrations of amorphous silica (up to about 50 wt.%) occupying most of the pores. Alkali feldspars alter to kaolin pseudomorphs that are much more porous than vermicular kaolin pseudomorphs replacing mica minerals; consequently, these pseudomorphs contain much less amorphous silica. Geochemical modeling using PHREEQC enabled prediction of the saturation state of minerals and indicated the dissolution and precipitation reactions occurring in the regolith-groundwater environment. Acid, moderately saline groundwaters were in near equilibrium with respect to amorphous silica, while near-neutral pH, saline groundwaters corresponded to undersaturated conditions. For extremely acid groundwaters (pH<3.5), PHREEQC predicted dissolution of kaolin, goethite and hematite, which is reflected in the elevated concentrations of iron (up to 169 mg/l) and aluminium (up to 389 mg/l) in the groundwater. This study is important for the understanding of how groundwater geochemistry influences the precipitation of cementing minerals that form various hardpans in the southwest Australian landscape.