The mt ninderry acid sulphate alteration zone and its relation to epithermal mineralization in the north arm Volcanics, Southeast Queensland

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 δ³⁴S 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 in¹⁸O (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 oxygen isotopic compositions of fluids in equilibrium with the acid sulphate altered rocks are similar to those responsible for development of the nearby adularia-sericite type North Arm epithermal deposit. Fluid isotopic compositions (D < -79%o) indicate a dominant meteoric component, but one derived from a relatively high latitude source consistent with the position of southeast Queensland in the Triassic; it is thus unlikely that alteration is due to Tertiary-Recent weathering. Data suggest that the Mt Ninderry acid sulphate altered rocks formed above a fossil boiling zone, in a manner analogous to modern geothermal systems. A steam-heated (‘primary supergene’) origin at 100-150°C, pH < 4 and relatively oxidizing conditions is favoured. The acid sulphate altered rocks may have formed close to a Late Triassic palaeosurface and potentially overlie an epithermal precious metal system at a depth of 100-400 m, a concept yet to be tested by detailed exploration.