Spatial Variation of High Temperature Hydrothermal Activity at Mokai Geothermal Field, Taupo Volcanic Zone, New Zealand

Abstract

The Mokai geothermal system is located 25 km north-west of the Wairakei field in the western part of the Taupo Volcanic Zone, North Island, New Zealand. Its deep production reservoir is defined by a microgravity anomaly of about 4-5 km2 extent, with north and north-easterly lateral outflows toward the Waikato River. The Mokai stratigraphy is dominated by a sequence of variably welded ignimbrites, rhyolite lavas and breccias, pumiceous fall deposits and volcaniclastic sediments (inferred basement Torlesse greywacke has not been encountered by drilling), cross-cut by NE-trending normal faults. Production wells are located above the deep production reservoir, where encountered temperatures are largely greater than 240°C, but also above 300°C. Injection wells are located in N, NW and NNE sectors of the field. Petrographic examination of core and drill cuttings, combined with fluid inclusion microthermometry and qualitative clay fraction XRD analyses of three injection wells, provide mineralogical and textural evidence for the coexistence of a “high” temperature hydrothermal mineral assemblage (e.g., actinolite, epidote; >300°C) with a “low” temperature mineral assemblage (e.g., smectite, laumontite; <220°C) at the same depths. The inference is that the outflow region of the Mokai reservoir is now cooler than at some time in the past and that this temperature change has occurred without any substantial change in fluid chemistry. It is likely to have been caused either by thermal contraction of a slowly cooling geothermal reservoir, or by a heat source that has migrated towards the S and SE to where the present geothermal upflow is situated.