Variations in Chemical and Isotopic Compositions of Mineral Spring Systems in South Island, New Zealand

Abstract

There are about 60 mineral springs in the South island, about 42 of which are thermal. Thermal discharges range from 16.7oC to 66oC with the highest temperatures found in springs along the northern half of the Alpine Fault Zone. Estimated maximum subsurface temperatures range from <60oC in Canterbury and the FAFZ to >250oC in the NAFZ and Taeieri basin near Dunedin. Discharge temperatures in the NAFZ springs are lower than in the MFS but the calculated subsurface temperatures are higher due to differences in permeability, degree of conductive heating and possibly depth of fluid source. The highest HCO3/Cl ratios are confined in springs located along the MFS. In contrast spring waters along the NAFZ and FAFZ, with lower HCO3/Cl ratios, are farther away from the equilibrium line, suggesting differences in permeability, meteoric water throughput, storage and circulation among the MFS NAFZ and FAFZ and to a certain extent, reflecting (1) variations in water-rock interaction in the fluids circulating through the faults and (2) changes in bedrock composition. Along the Alpine Fault Zone meteoric waters gain solutes from interaction with fault comminuted rock, contributions from the subducted slabs in the NE and SW of the South Island and metamorphic fluids at depth. Anomalous heat along the Alpine Fault Zone is generally believed to be due to rapid uplift and exhumation. Hot waters at depth in the Taieri basin however, are probably due to heat generated from hot mantle at depth.