Boron in Thermal Spring Systems in the Greater Auckland Area, (New Zealand)

Abstract

Most thermal spring systems in the greater Auckland area contain a rather high amount of boron (10-20 mg/kg). There is no significant difference in the B concentration of thermal waters discharging from outcropping greywacke basement and those discharging through Tertiary sediments. Thermodynamic Na-K and K-Mg equilibria point to resource temperatures between 50 -and 120?C and almost adiabatic conditions with in the fracture zones along which the fluids ascend to the surface. A small sorbtion/leaching experiment was conducted which showed that some B can be leached from sedimentary and volcanic rocks at low temperatures (50?C). However, B is re-adsorbed quickly. by most rocks; greywacke rocks did not adsorb B once it had been leached. Porefluids of deeper greywacke basement rocks are significantly mineralized and are probably also enriched in B. Boron in thermal spring systems in the study area could therefore originate from B enriched deep pore fluids and from leaching of greywacke rocks.