Geologic, fluid inclusion, and stable isotope studies of the gold-bearing breccia pipe at Kidston, Queensland, Australia

Abstract

The Kidston mine, which is situated approximately 280 km west-northwest of Townsville, North Queensland, is currently Australia's second largest gold producer. The mineralization is hosted within a trapezoid-shaped breccia pipe with surface dimensions of 1,100 X 900 m. Brecciation and gold mineralization are spatially and temporally related to a swarm of Permo-Carboniferous rhyolite dikes which intrude middle Proterozoic metamorphic and Siluro-De-vonian granitoid host rocks. The rhyolite dikes are interpreted as being underlain by a Permo-Carboniferous batholith.Three phases of brecciation have been distinguished within the breccia pipe, all being associated with magmatic and or magmatic hydrothermal processes. Fluid inclusion data indicate that the present level of exposure was approximately 3,500 m below the contemporary land surface at the time of mineralization. The lack of any significant input of meteoric fluid into the hydrothermal system suggests that the breccia pipe, the bulk of which formed by collapse, failed to breach the contemporary land surface. Volatile-rich fluids, envisaged as being an integral part of breccia pipe formation, escaped fractures now occupied by breccia dikes.Prebreccia mineralization is uneconomic, consisting of stockwork vein mineralization in the carapaces of small prebreccia rhyolite stocks. Stockwork veining and localized brecciation resulted from the multiple buildup and escape of a high-temperature (>500 degrees C), highly saline (>40 wt % NaCl equiv) magmatic fluid associated with a number of crystallizing rhyolite stocks. This exsolved magmatic fluid comprised a liquid with a delta 18 O value of between 9.4 and 9.8 per mil. The pressure drop associated with fracture propagation and brecciation produced a vapor phase with a salinity of 3 to 12 wt percent NaCl equiv, which condensed at temperatures between 380 degrees and 460 degrees C due to adiabatic expansion to produce a liquid of similar salinity.Postbreccia mineralization was dominated by magmatic fluids (calculated delta 18 O value of 3-8ppm and delta D value of -50 to -20ppm), which in turn were dominated by a liquid with a salinity of 2 to 10 wt percent NaCl equiv. This liquid resulted from condensation of a vapor produced by the boiling of a highly saline magmatic fluid at a deeper level within the breccia pipe. Fluid inclusion studies indicate trapping temperatures in the ranges of 400 degrees to 540 degrees C for early-stage cavity infilling to as low as 170 degrees to 300 degrees C during the deposition of the sulfides and carbonate in the late-stage quartz veins and cavities. The economic-grade gold mineralization was deposited during late-stage mineralization. The lateral and vertical decrease in the grade of gold mineralization within the late-stage sheeted veins and cavities is associated with an increase in ratio of pyrrhotite to pyrite, which is interpreted as reflecting increasing temperature.The structural control on the distribution of postbreccia mineralization was an inverted funnel-shaped zone of enhanced permeability produced by the forceful emplacement of the postbreccia rhyolite into the lower portion of the breccia pipe. The persistence of this zone throughout the postbreccia mineralizing event is further evidence of the close genetic relationship between rhyolite magmatism and gold mineralization at Kidston.