Immobile element geochemistry of altered volcanics and exhalites at the Thalanga deposit, North Queensland
Hermann, W (1994) Immobile element geochemistry of altered volcanics and exhalites at the Thalanga deposit, North Queensland. Research Master thesis, University of Tasmania.
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.
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