An active amagmatic hydrothermal system: The Paralana hot springs, Northern Flinders Ranges, South Australia

Abstract

The Mesoproterozoic Mt. Painter Inlier (Northern Flinders Ranges, South Australia) is located in a zone of anomalously high heat flow attributed to high concentrations of radioactive elements in the Inlier. Paleozoic hydrothermal activity produced large volumes of uraniferous breccias and siliceous sinter, and secondary uranium deposits are mined in Tertiary sandstones east of the Inlier (Beverley mine). The Paralana hot springs (PHS) are the only remaining hot spring along the Paralana fault, the locus of long-standing hydrothermal activity, as shown by epithermal precious metal, Cu–Fe and Fe–U deposits. This study investigates the chemistry of the PHS water and other groundwaters in the Mt. Painter Inlier, aiming to constrain the primary source of the water and heat in PHS, and to explore the relationships between fossil ore deposits and modern groundwaters in the province. PHS discharges 16 L/s of water at a temperature of 57 °C. The water is neutral (pH 7–8) and has total dissolved solids of 1144 mg/L, towards of the lower end of the range for nearby cold springs and groundwater bores (1000 to 3041 mg/L TDS). Fluorine (5 ppm), Mo (33 ppb), W (11 ppb), Cs (16 ppb) and Rb (200 ppb) concentrations are comparatively high in the spring water. δ 1 8O and δD values show that the PHS water is of meteoric origin, and δ 1 3C values of CO 2 (g) emanating from the springs and dissolved HCO 3 − suggest the carbon source is organic matter, e.g., soil or plants. Rn concentrations are very high at the springs (radiation of 10,952 Bq/m 3) implying a localized radiogenic source at shallow depth. According to geothermometric calculations, the most recent water–rock interaction temperature is 95 ± 5 °C. High geothermal gradients, attributed to high concentrations of radiogenic elements, suggest that circulation depths between 1.4 and 2.4 km are required to produce this temperature. The PHS result from meteoric water circulating through hot rocks, and are the surface expression of a cyclic, low-temperature, non-volcanic hydrothermal system. Two possible sources of meteoric water for the springs have been identified: Mt. Painter Domain and a local Great Artesian Basin aquifer. The variability of the flow rate, the low temperature of last equilibrium fluid–rock interaction, and the geochemistry of the springs indicate that the water is sourced from the Mt. Painter Domain. The PHS contain negligible U (0.07 ppb), but some shallow groundwaters in the basement are enriched in U (300–600 ppb). Reactive transport modelling shows that these waters could produce Beverley-style mineralisation upon reaction with reduced carbon and/or sulphides. The PHS water, on the other hand, could produce upon cooling the jasperoidal mineralisation with traces of sulphides that outcrops along the Paralana fault. Hence, the chemistry of present-day groundwater may be related to that of the paleo-water responsible for major mineralisation events in the region, although particular circumstances affecting fluid flow and fluid chemistry (e.g., climate, tectonism, or magmatism) must have been instrumental in the formation of the major deposits.