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Abstract

Some of Australia’s most significant gold deposits are associated with late Paleozoic high-level felsic intrusions in northeast Queensland. The gold deposit at Red Dome near Chillagoe is hosted by a complex skarn related to Permo-Carboniferous rhyolites intruded into Siluro-Devonian limestone of the Chillagoe Formation. Native gold and minor electrum are associated with a wollastonite-bearing skarn phase related to an early rhyolite. Gold was also deposited during subsequent retrograde alteration and late veining and appears to have been locally reworked with the intrusion of a late rhyolite. Whole-rock geochemistry has indicated that during skarn formation, fluids introduced appreciable amounts of Fe, Mn, Cu, Mo, Sn, Zn, W, and As and leached Na from all lithologies.

Fluid inclusion studies yield pressure-corrected homogenization temperatures in the range 300° to 380°C for garnet with fluorite forming at 200° to 350°C in the skarns. The fluids have a very low CO2 content (<0.2 mole %), with salinities of 2 to 24 equiv wt percent NaCl in the skarns rising to 30 to 50 equiv wt percent NaCl in the rhyolites. Stable isotope data indicate the progressive interaction of a magmatic fluid with the country rocks during skarn formation and suggest a water/rock ratio much greater than 1, probably>10. Although the oxygen isotope data do not indicate the involvement of meteoric water with low δ18O values, it is possible that connate or surface water which has been extensively modified by interaction with the sedimentary rocks may have been involved in the later stages of skarn growth, thereby causing a decrease with time in the salinity of the skarn fluids. Sulfur isotope data suggest a mixing of two sulfur sources: a magmatic sulfur and a country-rock source with more positive δ34S values.

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