The Broadlands–Ohaaki geothermal system, New Zealand Part 1. Strontium isotope distribution in well BrO-29

Well BrO-29 at the Broadlands-Ohaaki geothermal system, New Zealand, penetrates a mixed volcanic/sedimentary rock sequence whose 87Sr/86Sr ratios display linear distributions when plotted against 1/Sr values. These linear distributions do not indicate binary mixing between unaltered lithological units and a single fluid endmember. The linear distribution in the volcanic/pyroclastic sequences is related to the permeability of the host rocks, the degree of leaching of 86Sr rich primary plagioclase laths, and secondary crystallisation of the groundmass by a recharge fluid represented by modified Taupo rainwater. The greywacke/argillite basement from the same well displays a separate linear isotopic trend when their 87Sr/86Sr ratios are plotted against 1/Sr values, in addition to yielding a Sr/ Sr vs. Rb/Sr errorchron with an age of 88± 6 Ma BP (MSWD = 9.48). Based upon supporting petrographic observations, the two isotopic trends in the greywacke/argillite basement are believed to be an inherited metamorphic feature, from the Rangitata Orogeny (141 ± 3 Ma), which has been modified by geothermal fluids preferentially leaching 87Sr and 87Rb from primary micas.