An isotopic study of surficial alunite in Australia: 1. Hydrogen and sulphur isotopes

Abstract

A suite of alunite-group minerals from a variety of surficial environments in Australia has been analysed for δD and δ 34 S. The δD values of the samples suggest that there is a strong dependence of the fractionation factor for hydrogen in alunite minerals on the nature of the ‘B’-site cation (Fe, Al), but relatively little dependence on the nature of the ‘A’-site cation (K, Na). An α hy of 1.004 is proposed for alunite-water and natroalunite-water at surficial temperatures, and while no numerical estimate for the α hy for jarosite-water is given, the α hy can be shown to be considerably lower than that of alunite. Evaporative modification of the isotopic composition of meteoric waters in equilibrium with alunite minerals in regolith profiles is commonly indicated by the high δD values of the samples. This suggests that the formation of alunite group minerals commonly occurs during the gradual ‘drying out’ of regolith profiles after periods of active weathering. Modification of the isotopic composition of meteoric waters by evaporation may play a greater part in controlling the isotopic composition of authigenic regolith minerals in arid terrains than has been previously recognized. The δ 34 S value of sulphate minerals in southern Australian inland salt-lakes (playas) indicates that the sulphate is derived predominantly from marine ‘cyclic’ sulphate, rather than from weathering of the continental landmass. Much of this ‘cyclic’ sulphate has been blown onto the continent in particulate form by the prevailing southwesterly wind-stream, rather than introduced as dissolved sulphate in precipitation. Wind-blown ‘cyclic’ sulphate also constitutes a major source of sulphate in many regolith profiles in southern Australia. Depletion of 34S in sulphates in the regolith compared with the widespread cyclic sulphate isotopic signature can indicate proximity to large bedrock sulphur-sources (such as an oxidizing sulphide orebody) in some cratonic areas. Therefore, the δ 34 S value of sulphate in groundwaters and regolith minerals may provide a useful exploration tool in deeply weathered parts of the Australian continent such as the Yilgarn Block in Western Australia.