Hydrothermal alteration mineralogy as an indicator of hydrology at the Ngawha geothermal field, New Zealand

Abstract

Interaction between geothermal fluids and the rocks through which they migrate alters many earlier formed minerals and produces others. The minerals thus formed preserved evidence of hydrological conditions prevailing within an active geothermal system; in particular, they can reflect the range of temperatures under which they formed. This feature was tested at the Ngawha geothermal system, which is different from others in New Zealand in that its reservoir comprises fractured basement rocks covered by a 500–600m thick sequence of sedimentary rocks. Petrographic examination of cores and cuttings recovered from drillholes at Ngawha shows that the secondary minerals present within the rock matrices and veins are of different ages. The thermally sensitive minerals include epidote, titanite, biotite and clays, including some that are interlayered. Comparison of the measured downwell temperatures with those deduced from the secondary mineralogy and by homogenizing fluid inclusions, shows that the central part of the field has remained thermally stable since the youngest secondary minerals deposited there but its southern margin has cooled by 20–40°C or perhaps more. A likely cause of this is an inflow of cooler water from the east, which also causes the temperature inversion clearly evident in hole Ng8. By contrast, some fluid inclusion geothermometry results suggest that the northern part of the drilled field has heated since their host hydrothermal quartz crystals formed.