A low temperature oxygen isotope thermometer for cerussite, with applications at Broken Hill, New South Wales, Australia

Abstract

Slow precipitation experiments have been used to determine the oxygen isotope fractionation between cerussite (PbCO3) and water over the temperature range 20 to 65°C. The temperature dependent fractionation for oxygen can be expressed as: 1000 ln α(cerussiteCO3-water)=2.63 (106/T2)−3.58 Independent determinations of the oxygen isotope fractionation factor for recent historical cerussite samples from the Bwlch-Glas Mine, Wales, UK, and the Pinnacles Mine, Broken Hill, NSW, Australia, plot close to our experimentally determined fractionation curve. Similarly, earlier high temperature cerussite experiments are in agreement with our thermometer. This new oxygen isotope thermometer was used to examine data for 7 cerussite samples from the Block 14 and 3 from the Kintore open cuts on the main Broken Hill lode, NSW, Australia. Broken Hill cerussite formed at temperatures significantly greater than modern ambient air temperatures. This is consistent with malachite thermometry results at Broken Hill. It is suggested that these higher apparent temperatures result from the exothermic oxidation of concentrated sulphides. Results indicate that oxidation of sulphides in the Block 14 Mine produced shallow subsurface temperatures up to at least 50°C. Carbon isotope data suggest incorporation of significant amounts of organic carbon, possibly from a subsurface bacterial carbon source associated with sulphide oxidation.