Hydrothermal water–rock interaction and the redistribution of Li, B and Cl in the Taupo Volcanic Zone, New Zealand

Lithium, B and Cl occur as trace elements in the dominant protoliths (rhyolite, andesite and greywacke) underlying high temperature hydrothermal systems in the Taupo Volcanic Zone (TVZ). The fluid mobility of Li decreases substantially at > 200 °C ± 20 °C with the pervasive deposition of quartz ± chlorite, regardless of protolith and altering fluid compositions. Boron and Cl are more fluid mobile than Li at all temperatures with mobility affected by the rock type, composition of circulating aqueous solutions and to a limited extent, by incorporation of the trace elements in hydrothermal minerals. Uptake of B by clays decreases its mobility at < 180 °C. The relative Cl contents of altered rocks increase where zeolites (180 °C to 270 °C) and other calc-silicate minerals (>320 °C) occur. Compared to volcanic rocks the mobility of B and Cl in greywackes is very low and thus the B/Cl signature of greywacke in hydrothermal aqueous solutions of the TVZ is overwhelmed by the effects of rhyolite and andesite dissolution. Temperature-dependent transitions from Mg–Fe clays and phyllosilicates to K-bearing illitic clays tend to decrease the Mg/Li ratios in aqueous solutions at < 200 °C ± 20 °C. The decrease in Na/Li ratios with temperature can be attributed to the counterbalancing effects of a wide number of alteration processes involving dissolution of major aluminosilicate rock phases containing Na and Li, followed by deposition of secondary minerals that either incorporate (clays and phyllosilicates) or reject Li (calcite) in their structures, with the effects of alteration on Na/Li further modified by alteration intensity and the composition of the original phase being replaced. The substitution of Li for Na during albitization is a function of alteration intensity in the TVZ. The Li geothermometers are best used in low temperature systems (< 200 °C ± 20 °C) with low permeability where clay and phyllosilicate deposition prevail and quartz deposition is at a minimum. It is possible that kinetic rates of reaction in sedimentary basins, over millions of years, may be more efficient in enabling Na–Li and Mg–Li ion-exchange equilibration in clays and phyllosilicates than the faster water–rock interaction processes in high temperature active hydrothermal systems, and hence the viability of Li geothermometers in assessing low temperature aqueous solutions in sedimentary basins but not in active hydrothermal systems where temperatures are > 200 °C.