Sedimentary Facies and Geochemistry of the products of Alkali Chloride Spring Discharge and Hydrothermal Alteration at Te Kopia, Taupo Volcanic Zone, New Zealand

Abstract

The Te Kopia geothermal field is one of the undeveloped high enthalpy sites in the Taupo Volcanic Zone (TVZ), New Zealand. The thermal field extends over an area of about 3 x 1.2 km along the scarp of the 30km long Paeroa Fault, which is a normal fault that strikes 040-050° and is downthrown to the NW (Berryman et al., 2008). The study area is located at the foot of the fault scarp which steeply rises for about 220m (Bignall et al, 1994). The combination of acidic steaming fumaroles, acid springs and meteoric water flow have transformed most of the area into a ‘rotten’ landscape and barren ground, with mostly altered volcaniclastic sediment in this high-temperature and low-pH environment. The remnants of sinter blocks found associated with a steaming landslide in this area shows that they were originally formed in near neutral alkali chloride surface hot springs and are now broken up and undergoing hydrothermal alteration. This means the water table dropped, whereby chloride waters have been replaced by steam acid condensate (Simpson & Bignall, 2016). Previous studies have focused on the fault displacement, geothermal changes, and element movement in the target area. The aim of this study is to systematically map and distinguish the different sedimentary facies and determine the texture and the geochemical features of the altered rocks and sediments to record the products of acidic overprinting on a once extensive geothermal system of near neutral pH in the area. Based on field occurrences in the target area, a facies map has been made to demonstrate geographical features and how the sediments as well as local altered volcanic host rocks are distributed in the area. The lithology of the local sediments was further observed from thin sections. X-Ray powder diffraction was used to identify mineral phases, and element contents in the surface soil were determined by the ICP-MS. I attempt to separate potential older alteration effects (such as disseminated pyrite in ignimbrite) from the more recent acidic alteration and deposition of silica residue. Except for the sinter buttress, a large possibly intact remnant of a sinter apron terrace margin that is currently under study by another student, the facies distribution of the sinter blocks in the Te Kopia study area near the steaming landslide deposit matches the general facies distribution from vent to distal apron as the elevation declines in the area. One possible hypothesis is that these discrete sinter blocks belong to a single geothermal system more or less in its present position, and the former geyser discharged with chloride fluid was located at or topographically above the present steaming landslide. To explain the phase transformation in silica sinter and residue, I use the source-to-sink model raised by Landmesser (1995), which utilizes chemical potential of the silica component (μ(SiO2)) to interpret the behaviour of metastable silica movement in different phases. The variation of phase maturation in different sinter blocks is seen in their fabrics. The maturation of the silica phase in the silica residue is mainly because of relatively high kinetic energy between opaline silica and quartz. Clay minerals may be another agent that speeds the maturation process, although clays are in low abundance in the studied samples. Finally, depletion of elements observed would be attributed to the acidic fluid leaching out these elements and leaving the acid resistant silica compound, especially on the arid surface terraces which is occasionally washed by ephemeral rain runoff. The enrichment of Au, Ag, Hg, and S may come from direct precipitation of sulfides in the acidic environment. Finally enrichment of As in the area is deduced to relate to the underground reservoir that is also likely rich in arsenic.