Abstract

Petrographic, microthermometric, and laser Raman microprobe analyses of fluid inclusions from the Zn-Pb-Ag sulfide deposit at Dugald River, northwest Queensland, indicate that fluids with a wide range of compositions in the CO 2 -CH 4 -H 2 O-NaCl system were involved in the formation of the deposit. Early primary inclusions are characterized by higher homogenization temperatures centered on 360 degrees C, an average salinity of 13 wt percent NaCl equiv, and they tend to be relatively CO 2 -rich. Such a fluid migrating through the thick sequence of the middle Proterozoic metasediments of the Corella Formation would acquire metals and reduced sulfur as well as CH 4 due to its interaction with the country rocks at deep levels. Ore deposition is interpreted to have resulted from Syntectonic boiling of CH 4 -rich volatile and aqueous chloride solutions, and consequent filling and replacement of brecciated and foliated black slate by metal-bearing hydrothermal fluids. The pressure and temperature conditions of mineralization show a narrow pressure range of 1.1 to 1.5 kbar and a wide temperature range of 315 degrees to <450 degrees C, but mainly operated at about 315 degrees C and 1.1 kbar.The close resemblance of fluid inclusion data between three different ore types favors the view that they were all subjected to a similar evolutionary history and probably have the same origin. The fluid inclusion data provide further evidence supporting an epigenetic formation model for the Dugald River deposit and this matches well with previous microstructural investigations.

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