Abstract:
The Li content and fractionation systematics of Taupo Volcanic Zone (TVZ) geothermal fluids reflect processes occurring within geothermal reservoirs and the deeper crust. Understanding Li cycles and isotopic fractionation behaviour in TVZ geothermal systems contributes to a broader understanding of physicochemical processes affecting New Zealand’s geothermal resources. A comprehensive dataset of seventy-three samples was compiled. Samples were collected from geothermal surface features and production wells of eighteen geothermal fields of the TVZ and elsewhere in the North Island of New Zealand. Ion chromatography and quadrupole inductively coupled plasma mass spectrometric techniques were used to analyse for Li, Cl–, SiO2, SO4 2- K, Na, Ca, Mg, B, Sr and Pb concentrations. Solute geothermometry indicates reservoir temperatures for TVZ geothermal systems ranges between ~190 ˚C and ~370 ˚C. Lithium concentrations for geothermal fluids from the TVZ have an average of 5.9 mg/L and range between 4 μg/L and 29 mg/L. There is no systematic relationship between Li concentrations and geographical location within the TVZ. Rather, Li content and Li/Cl ratios vary as a result of boiling, mixing and water/rock reaction processes. Lithium concentrations and Li/Cl ratios are, therefore, indicators of shallow (above 2.5 km) and locally variable reservoir processes. Lithium isotopic systematics are used to determine fluid dynamics in geothermal systems of the TVZ. δ7Li analysis was performed by the author at the University of Maryland, U.S.A. The average δ7Li value for TVZ geothermal fluids is –0.8‰. Most δ7Li values for geothermal fluids fall within a small range of approximately –3‰ to +2‰. This indicates similar processes are causing similar isotopic fractionation. Lithium isotopic systematics in TVZ geothermal fluids are dependent on reservoir temperature but also on the nature of reservoir lithology. Through the combined use of Li concentrations and δ7Li values, numerical models reveal possible evolution pathways of water/rock reactions in TVZ geothermal systems. Models based on rocks and surface waters indicate Li cycles and isotopic fractionation behaviour is affected by shallow surface processes. Geothermal well fluids give a better indication of water/rock reaction pathways. Mass balance calculations indicate that an additional source as well as reservoir rock leaching of Li is required to account for the entire 3.01 x 1011 mg/kg of Li present in young (340,000 year) TVZ geothermal systems. This amount of Li would take approximately 397,200 years to be removed from the TVZ by water/rock reactions typical of processes operating today. An alternative source of Li could be deep magmatic brine entering shallow hydrologic systems. As a resource, $US64.1 million of Li is cycled through TVZ geothermal reservoirs every year. Economically, TVZ geothermal systems are a potential Li source for the global commodities market.