Immobile element geochemistry of altered volcanics and exhalites at the Thalanga deposit, North Queensland

Thalanga is a deformed and metamorphosed volcanic hosted massive sulphide deposit consisting of several thin, stratiform semi-connected lenses located at a sub-vertically dipping contact between rhyolite and dacite formations. Rocks composed of quartzmagnetite- barite and chlorite-tremolite-carbonate, which have previously been interpreted to be exhalites, exist in intimate stratabound association with sulphides, particularly in the western lenses. Rhyolites stratigraphically beneath the deposit are extensively hydrothermally altered with a stratabound stringer zone of intense silicification containing 4-20% pyrite in the immediate footwall of the sulphide lenses, grading outwards and downwards through progressively weaker zones of qu~rtzchlorite- sericite and quartz-sericite alteration. Titanium, aluminium and zirconium remained chemically immobile during alteration, and subsequent metamorphism. These immobile elements permit identification of the volcanic precursors of altered rocks and quantitative estimation of the chemical changes due to alteration. Large gains of silica, iron, sulphur and loss of sodium are indicated for the zone of most intense footwall alteration. Chlorite-tremolite-carbonate rocks associated with sulphides have immobile element contents and ratios identical to those of altered footwall rhyolites and their chemistry is consistent with a derivation from rhyolite involving large gains of calcium, magnesium, C02 and losses of silica and sodium. They are re-interpreted to be metamorphosed chlorite-carbonate alteration assemblages which probably formed in permeable rhyolitic volcaniclastics by hydrothermal fluid and sea water mixing at the upper and outer parts of a mineralising sub-marine hydrothermal system. Magnetite-quartzites have very low immobile element contents and may be the only true exhalites at Thalanga, other than massive barite and base metal sulphide assemblages.