Tamie R. Weaver

Stable isotope geochemistry of cold CO2-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 13 C 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 13 C 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 3 He/ 4 He (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 18 O and δ 2 H 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 18 O values results from CO 2 exsolution at low temperatures which strongly partitions 18 O 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 2 H 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.

Constraining flow paths of saline groundwater at basin margins using hydrochemistry and environmental isotopes: Lake Cooper, Murray Basin, Australia

Solutes in saline groundwater (total dissolved solids up to 37 000 mg/L) in the Lake Cooper region in the southern margin of the Riverine Province of the Murray Basin are derived by evapotranspiration of rainfall with minor silicate, carbonate and halite dissolution. The distribution of hydraulic heads, salinity, percentage modern carbon (pmc) contents, and Cl/Br ratios imply that the groundwater system is complex with vertical flow superimposed on lateral flow away from the basin margins. Similarities in major ion composition, stable (O, H, and C) isotope, and 87Sr/86Sr ratios between groundwater from the shallower Shepparton Formation and the deeper Calivil – Renmark aquifer also imply that these aquifers are hydraulically interconnected. Groundwater in the deeper Calivil – Renmark aquifer in the Lake Cooper region has residence times of up to 25 000 years, implying that pre-land-clearing recharge rates were <1 mm/y. As in other regions of the Murray Basin, the low recharge rates account for the occurrence of high-salinity groundwater. Shallow (<20 m) groundwater yields exclusively modern 14C ages and shows a greater influence of evaporation over transpiration. Both these observations reflect the rise of the regional water-table following land clearing over the last 200 years and a subsequent increase in recharge to 10 – 20 mm/y. The rise of the regional water-table also has increased vertical and horizontal hydraulic gradients that may ultimately lead to the export of salt from the Lake Cooper embayment into the adjacent fresher groundwater resources.

O, H, C isotope geochemistry of carbonated mineral springs in central Victoria, Australia: sources of gas and water–rock interaction during dying basaltic volcanism

Waters discharging from carbonated cold mineral springs in the central Victorian Highlands of Australia have δ2H and δ18O values of −45 to −35‰ and −8 to −5.5‰, respectively, that suggest that they were derived from local meteoric water recharged under cooler climatic conditions than present. δ13C values of entrained CO2 (−10.4 to −7.0‰) and dissolved inorganic carbon (−3.9 to −0.1‰) imply that the C was probably derived from outgassing of Recent igneous rocks within the Newer Volcanic Province. The waters lie to the left of both local and global meteoric water lines due to CO2 exsolution from waters that were never involved in high-temperature water–rock interaction.

Patterns of fluid flow in Proterozoic calc‐silicates: Fluid channelling and variations in fluid fluxes and intrinsic permeabilities

Calc‐silicates in three Proterozoic terrains (Mt Isa, the Reynolds Range and the Grenville of Canada) record the influx of water‐rich fluids through the resetting of mineral assemblages, stable‐isotope ratios and major elements. In all cases time‐integrated fluid fluxes are inferred to have varied by over an order of magnitude on the millimetre to metre scale, and fluids were often channelled across strike. These results indicate that intrinsic permeabilities during metamorphism varied considerably over a small scale. It is suggested that fluid flow was within microfractures, and that the variations in intrinsic permeability reflect variable microfracture densities.