Abstract
At Mt Ninderry, in the Triassic North Arm Volcanics, southeast Queensland, an acid sulphate alteration zone contains hydrothermal breccia and quartz‐, alunite‐ and kaolinite‐bearing assemblages with peripheral propylitic alteration. An alunite‐bearing sample has yielded a whole‐rock K‐Ar age of 217 ± 2 Ma (Late Triassic). Acid sulphate alteration is exposed over 200 m vertically and is superimposed on felsic volcanic rocks which have experienced substantial volume loss related to leaching of mobile elements. Rocks are enriched in S, H2O and to a minor degree in As, Sb, Hg, Bi, Mo and Au. Sulphur isotope results show that ?34S values of alunite (1.8 ‐5.6%o) and trace pyrite (2.1%o) overlap; lack of equilibrium fractionation between these minerals implies that the acid sulphate altered rocks are not magmatic hydrothermal in origin. Altered rocks are enriched in 18O (6.9–12.2%o) compared to least altered North Arm Volcanics, and hydrothermal clays are D‐depleted (‐107 to ‐103%o). Calculated hydrogen and oxyg...
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