HYDROCHEMISTRY ON THE YILGARN BLOCK, WESTERN AUSTRALIA - FERROLYSIS AND MINERALIZATION IN ACIDIC BRINES

In the southeastern part of Western Australia internal drainage and a semi-arid climate have resulted in extensive development of playas. The brines beneath the playas wedge outwards and force local infiltration to crop out at playa margins as discharge zones. The discharging water contains dissolved Fe2+, oxidation and hydrolysis (ferrolysis) of which generates very acidic water (pH greater-than-or-equal-to 2.8). At least some of the alkalinity generated by the original reduction of Fe3+ has been sequestered as subsurface calcrete within local soils. The acidity causes the dissolution of kaolinite, which is abundant in playa sediments and as a thick capping on weathered bedrock, giving water with a low pH and high concentrations of dissolved Al and Si. Evaporation, and mixing with playa brines containing high concentrations of K and SO4, causes alunite to precipitate as pH rises from a minimum of 2.8, at chlorinities of 1 to 2 M, to > 6 at chlorinities of 4.8 M. The alunite deposits formed by this process are many orders of magnitude larger than occurrences elsewhere in the world. The large-scale mobilisation of Si during this process may be one mechanism for the formation of silica-cemented rocks (silcrete) that are widespread in the Australian regolith.