The Mt Ninderry acid sulphate alteration zone and its relation to epithermal mineralization in the North Arm Volcanics, southeast Queensland.
Titel:
The Mt Ninderry acid sulphate alteration zone and its relation to epithermal mineralization in the North Arm Volcanics, southeast Queensland.
Auteur:
Ashley, P. M. Andrew, A. S.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 39 (1992) nr. 1 pagina's 79-98
Jaar:
1992-02
Inhoud:
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 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.
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