APPARENT AGES OF HYDROTHERMAL ALTERATION ZONES IN THE NORTH ARM VOLCANICS, SOUTHEAST QUEENSLAND

The North Arm Volcanics in southeast Queensland contain two large zone s of hydrothermal alteration imposed on rhyolitic and dacitic rocks. A t the North Arm prospect, alteration encloses epithermal vein- and bre ccia-hosted Ag--Au mineralization and has characteristics of the adula ria-sericite type. The other zone, at Mt Ninderry, is of acid sulphate type. Both zones are considered to be temporally related on geologica l, 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 t he Late Triassic age of the felsic phase of the North Arm Volcanics, a s determined by published radiometric and palaeobotanical methods. How ever, at North Arm prospect, K Ar and Rb--Sr data for sericite ( K fel dspar)-altered whole-rocks yield apparent ages in the range 182 186 Ma (Middle Jurassic) and for separated sericite, ages in the range 152 1 66 Ma (Late Jurassic). The probability of hydrothermal episodes occurr ing within the Middle Jurassic, and within the Late Jurassic, is unsup ported by microstructural evidence and the fact that, in southeast Que ensland, there are no known hydrothermal systems of those ages, nor ar e there any known igneous or deformation events that could have been r esponsible. A possible explanation for the apparent ages is by partial resetting of the K Ar and Rb - Sr systems during emplacement of the l atest Jurassic Early Cretaceous (approximately 145 Ma) Noosa intrusive suite, dioritic members of which occur within 5 km of North Arm. It c ould be expected that during mild thermal metamorphism, hydrothermal s ericite was reset to a greater degree than the whole-rock apparent age s, 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 L ate Triassic rhyolite from North Stradbroke Is. (both of which are pos sible correlatives of the North Arm Volcanics), are the result of a pe rvasive low-temperature alteration event, occurring perhaps during a p eriod of high heat flow that may not be related to igneous activity.