CHANGES IN SILICA CHEMISTRY AND HYDROLOGY ACROSS THE ROTORUA GEOTHERMAL-FIELD, NEW-ZEALAND

A comprehensive review of field data from Rotorua geothermal held, New Zealand, was undertaken to evaluate the mixing relationships of coole r groundwaters with the primary fluid. Unlike previous studies the res ervoir 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 degrees C parent by simple boiling, a mechanism propo sed by previous authors. Mixing relationships showed conclusively that across the field from southeast to northwest, all the fluids were rel ated along a mixing line with low-chloride water at 150 degrees C. Bet ween 158 and 175 degrees C the diluting fluid changed to a low-chlorid e groundwater at 15 degrees C. An SiO2-temperature plot of all the wel l data showed that silica values for the hot wells fall around the qua rtz solubility line. The data for the majority of the cooler wells sca tter about the a-cristobalite solubility line. Plots of the Cl/SiO2 ra tio were used to evaluate changes in silica chemistry across the reser voir by comparing measured data with theoretical mixing lines, assumin g partial equilibrium with respect to the three silica polymorphs, qua rtz, a-cristobalite and amorphous silica. The only assumptions made in the calculation were that the 150 and 15 degrees C diluting fluids we re in equilibrium with cristobalite and amorphous silica, respectively . Surprisingly, the trends may be explained by mixing with cooler sili ca-rich diluting fluids without requiring equilibration with respect t o any of the silica polymorphs. (C) 1998 CNR. Published by Elsevier Sc ience Ltd.