Sporadic and waning hot spring activity in the Tokaanu Domain, Hipaua-Waihi-Tokaanu geothermal field, Taupo Volcanic Zone, New Zealand

Abstract

A substantial volume (c. 5000 m3) of siliceous hot spring deposits (sinter) has been deposited down to 1 m depth in and around the Tokaanu Domain, a 0.2 km2, thermally active central area of the Hipaua-Waihi-Tokaanu geothermal field. Associated thermal manifestations in the Domain included geysers, fumaroles, and hot pools with spring outflows depositing sinter from deeply derived NaCl-type waters as already described over 150 years ago. Later observations showed that flat flooded marshes hosting minor sinter deposits decreased in surface areal extent from at least 15,000 m2 in 1870 to less than 800 m2 in 2007. Decline in the area of flooded marshland reflects a decline in flow rate of feeding source pools. Depth and thickness (up to 0.5 m) of concealed (i.e. vegetative overgrown or covered in soil) sinter aprons in the Domain were mapped by students of the 1983–1987 Geothermal Diploma (UoA) field classes. The information was used recently to locate sites for coring of two buried sinter aprons. Radiometric dating of the lower Matewai apron yielded a 0.44 kyr age for sinter at 0.2 m depth. Sinter cores from three sites along a c. 60 m long N–S profile, following the apron axis of the Takarea pool source, yielded radiometric ages between 1.2 kyr at 0.25 m depth to 6.2 kyr at 0.35 m depth. Sinter deposition was found to be highly irregular. Sub-rounded sinter clasts at >2 m depth at two drill sites show no genetic relation to sinter found at the top of the cored section. The topmost sinter cap exhibits only minor diagenetic changes; some acidic hydrothermal alteration, however, is visible in the bottom section at >2.5 m depth of the cored sites pointing to a separate geothermal discharge phase. The irregular deposition of sinter in the Domain points to: 1. tortuous fluid flow paths in the shallow subsurface, and/or 2. shifting discharge channel flow directions on top of the developing sinter apron. The present and last sinter depositing episode appears to have come to an end; it probably lasted, with major interruptions, over 6 kyr.