Stable isotope geochemistry of cold CO 2-bearing mineral spring waters, Daylesford, Victoria, Australia: sources of gas and water and links with waning volcanism

Abstract

Mineral springs in the Victorian Central Highlands, Australia, have high CO 2 contents and naturally effervesce. δ 13C values of CO 2 gas and dissolved inorganic carbon are −10.6‰ to −7.0‰ and −5.9‰ to −0.1‰, respectively, with a net δ 13C of −8‰ to −3‰. The carbon in these waters was derived from a mantle source associated with local Pliocene to Recent basaltic Newer Volcanic Province rocks. Previously reported 3He/ 4He (1.2–3.1 relative to air) and high He/Ne ratios are also consistent with the presence of magmatic volatiles. Silica contents imply that the waters were never heated above 130 °C and that the system is not hydrothermal. The occurrence of carbonated mineral springs in a relatively small region of the Newer Volcanics Province where volcanic activity ceased several thousands of years ago may be due to the presence of late intrusions combined with deep circulation of water through deeply weathered and fractured Ordovician basement. A region of low seismic velocity under the Daylesford area potentially images those intrusions. Most spring waters have δ 18O and δ 2H values of −8‰ to −6‰ and −45‰ to −35‰, respectively, and lie to the left of the local and global meteoric water lines. The anomalously low δ 18O values results from CO 2 exsolution at low temperatures which strongly partitions 18O into the gas. The lack of waters lying to the right of the local meteoric water line also implies that water–rock interaction at elevated temperatures did not occur. The δ 2H values are lower than contemporary meteoric water, suggesting that the waters may have recharged under colder climate conditions several thousand years ago. The local Ordovician rocks are gold bearing. The present spring system is cold and would not efficiently transport Au. However, volcanic is waning and the spring systems at the time of volcanism may have been hotter and able to redistribute Au.