Apparent ages of hydrothermal alteration zones in the North Arm Volcanics, southeast Queensland

Abstract

The North Arm Volcanics in southeast Queensland contain two large zones of hydrothermal alteration imposed on rhyolitic and dacitic rocks. At the North Arm prospect, alteration encloses epithermal vein‐ and breccia‐hosted Ag‐Au mineralization and has characteristics of the adularia‐sericite type. The other zone, at Mt Ninderry, is of acid sulphate type. Both zones are considered to be temporally related on geological, alteration and stable isotopic criteria. A K‐Ar age of 217 ±2 Ma for an alunite‐bearing whole‐rock at Mt Ninderry is consistent with the Late Triassic age of the felsic phase of the North Arm Volcanics, as determined by published radiometric and palaeobotanical methods. However, at North Arm prospect, K‐Ar and Rb‐Sr data for sericite (‐K feldspar)‐altered whole‐rocks yield apparent ages in the range 182–186 Ma (Middle Jurassic) and for separated sericite, ages in the range 152–166 Ma (Late Jurassic). The probability of hydrothermal episodes occurring within the Middle Jurassic, and within th...