An isotopic study of surficial alunite in Australia

1. Hydrogen and sulphur isotopes

Michael I. Bird*, Anita S. Andrew, Allan R. Chivas, David E. Lock

*Corresponding author for this work

Research output: Contribution to journalArticle

38 Citations (Scopus)

Abstract

A suite of alunite-group minerals from a variety of surficial environments in Australia has been analysed for δD and δ34S. 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 αhy of 1.004 is proposed for alunite-water and natroalunite-water at surficial temperatures, and while no numerical estimate for the αhy for jarosite-water is given, the αhy 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 δ34S 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 δ34S 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.

Original languageEnglish
Pages (from-to)3223-3237
Number of pages15
JournalGeochimica et Cosmochimica Acta
Volume53
Issue number12
DOIs
Publication statusPublished - 1989
Externally publishedYes

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