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Controls on the genesis of a high-fluoride thermal spring: Innot Hot Springs, north Queensland

This study reports on the source, evolution, reactions and environmental impacts of F-rich thermal water at Innot Hot Springs, north Queensland. Thermal water of the Innot Hot Springs has a surface temperature of 71°C, alkaline pH (8.1), low dissolved oxygen (0.61 mg/L) and low total dissolved solids (652 mg/L). The main chemical composition is Na – Cl, with F concentrations (16 mg/L) being comparatively high. Concentrations of alkali and alkali-earth metals (Cs, Li, Rb, Sr) are elevated, while those of other trace elements (Ag, Al, As, Ba, Be, Cr, Cu, Ga, Mn, Mo, U, Zn) are significantly less. Hydrochemical and stable isotope data of hot spring water show that the fluid is meteoric in origin and has undergone significant water – granite interaction. Common geothermometers suggest temperatures of water – rock interaction at depth in the 119 – 158°C range (corresponding to a depth of <3.9 – 5.2 km). Solubility modelling of the thermal fluid demonstrates that the evolution of F concentrations in spring waters at the discharge site can be accounted for by fluid – rock interaction of a H2O – NaCl solution with fluorite – calcite-bearing granite assemblages between 150 and 200°C and subsequent granite-buffered cooling. Modelling also indicates that the F concentration in the hydrothermal system is largely controlled by interactions with fluorite, with less evidence for the significant involvement of F-topaz. Speciation calculations demonstrate that F speciation in the fluid is dominated by F− (99.4%), followed by minor CaF+ (0.5%) and NaF(aq) (0.1%), and traces of other F complexes. Thus, the F-rich Innot Hot Springs result from meteoric water circulating through fluorite-bearing granitic rocks and are the surface expression of a low-temperature, non-volcanic geothermal system. Discharge of the hot spring water occurs into an ephemeral stream located in a seasonally wet – dry tropical climate. As a result, the F content of local surface waters is distinctly elevated (max. 18 mg/L) during the dry season, making them unsuitable for stock water supplies.