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 brecciahosted 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 the Late Jurassic, is unsupported by microstructural evidence and the fact that, in southeast Queensland, there are no known hydrothermal systems of those ages, nor are there any known igneous or deformation events that could have been responsible. A possible explanation for the apparent ages is by partial resetting of the K-Ar and Rb-Sr systems during emplacement of the latest Jurassic-Early Cretaceous (˜ 145 Ma) Noosa intrusive suite, dioritic members of which occur within 5 km of North Arm. It could be expected that during mild thermal metamorphism, hydrothermal sericite was reset to a greater degree than the whole-rock apparent ages, possibly reflecting the more retentive nature of the Late Triassic relict magmatic mineralogy. Alternatively, it is possible that similar K-Ar apparent ages (179-187 Ma) reported for Late Triassic ignimbrite from the Matjara No. 1 well southeast of Moreton Is. and 180 Ma for Late Triassic rhyolite from North Stradbroke Is. (both of which are possible correlatives of the North Arm Volcanics), are the result of a pervasive low-temperature alteration event, occurring perhaps during a period of high heat flow that may not be related to igneous activity.
- Hydrothermal alteration zones
- K-Ar and Rb-Sr ages
- North Arm Volcanics
- Southeast Queensland