Changes in silica chemistry and hydrology across the Rotorua geothermal field, New Zealand

A comprehensive review of field data from Rotorua geothermal field, New Zealand, was undertaken to evaluate the mixing relationships of cooler groundwaters with the primary fluid. Unlike previous studies the reservoir well compositions were calculated using only the most reliable measured downhole fluid temperatures. Inconsistencies in gas chemistry showed that the hot fluid in the southeast could not be derived from a postulated 250°C parent by simple boiling, a mechanism proposed by previous authors. Mixing relationships showed conclusively that across the field from southeast to northwest, all the fluids were related along a mixing line with low-chloride water at 150°C. Between 158 and 175°C the diluting fluid changed to a low-chloride groundwater at 15°C. An SiO2-temperature plot of all the well data showed that silica values for the hot wells fall around the quartz solubility line. The data for the majority of the cooler wells scatter about the c~-cristobalite solubility line. Plots of the C1/SiO2 ratio were used to evaluate changes in silica chemistry across the reservoir by comparing measured data with theoretical mixing lines, assuming partial equilibrium with respect to the three silica polymorphs, quartz, e-cristobalite and amorphous silica. The only assumptions made in the calculation were that the 150 and 15°C diluting fluids were in equilibrium with cristobalite and amorphous silica, respectively. Surprisingly, the trends may be explained by mixing with cooler silica-rich diluting fluids without requiring equilibration with respect to any of the silica polymorphs. © 1998 CNR. Published by Elsevier Science Ltd