Geophysical and geochemical investigations of the Horohoro geothermal prospect

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Allis, R.G. (comp.) 1987 Geophysical and geochemical investigations of the Horohoro geothermal prospect. Wellington: DSIR, Geophysics Division. Research report / Geophysics Division 213. 78 p.

Abstract :                                                                                                                                                                                                                                                                                                                                                                                                                                  The Horohoro thermal features comprise an overflowing pool (surface temperature 51 degrees C) and an area of seepage (maximum temperature 90 degrees C) which are situated approximately 2 km east of the Horohoro cliffs near the centre of the Guthrie graben. The thermal waters rise to the surface through more than 100 m of Huka Falls mudstone from an underlying aquifer. A nearby resistivity sounding suggests more resistive, presumably cooler conditions exist beneath the aquifer at more than 350 m depth. The surface convective heat loss from the thermal area is 0.3 MW, with an additional 4 MW possibly flowing conductively from a 5 km2 area overlying the thermal aquifer. The thermal waters are very dilute (150-170 m/kg Cl) with geothermometry suggesting a subsurface temperature in the aquifer of 150-160o C. Consideration of chemical dilution models and gas isotope data suggests distant geothermal component in the water with a source temperature possibly around 200o C.The location of the deep heat source for the water is uncertain but the near-surface aquifer is inferred to e charged by hot water rising on the Horohoro Fault, adjacent to the Horohoro cliffs. A very large low resistivity anomaly which lies west of the Horohoro rhyolite dome and extends beneath the Mamaku Plateau is not directly attributable to high temperature. The low resistivity zone generally has its upper surface 300-500 m below the ground surface. It is characterised by a resistivity as low as 10 qm in the Matahina Basin, and by resistivities typically between 20-30 qm beneath the Mamaku Plateau. Coinciding magnetic low and gravity high anomalies within the resistivity low near the northwestern corner of the Matahina Basin are attributed to the effects of intense hydrothermal alteration at some earlier time. Measurements on core, as well as downhole measurements in the 600 m depth drillhole HH1 in the southern Matahina Basin indicate that the low resistivity zone coincides with low rank hydrothermal alteration. However, a temperature gradient of 130o C/km (heat flow of 170 mW/m2 ) in this hole suggests that elevated temperature below about 500 m depth is also a factor contributing to the low resistivity near here. A temperature gradient of only 60o C/km occurs about 1 km south of the Horohoro dome, so this implies that a present day heat flow anomaly exists in the direction of the Matahina Basin. It is unclear whether this anomaly is also the deep heat source for the Horohoro thermal waters. Several temperature gradient holes of at least 200 m depth are needed to delineate this heat flow anomaly. Subsequently one hole to at least 1500 m depth wold be necessary to determine the thermal regime at depth (auth)