Aspects of the distribution and movement of aluminium in the surface of the Te Kopia geothermal field, Taupo Volcanic Zone, New Zealand

Abstract

The principal Al-bearing components of two surface quadrats in the central Te Kopia geothermal field are the atmosphere, substrate (∼10 wt% Al in ignimbrite, clay and protosoils, 0.3–0.6 Al wt% in sinter), vegetation (4–5 g Al/m 2) and waters (1–4 μg/g Al in semi-permanent acid surface waters, 6–9 μg/g in acid pools, 10–14 μg/g in post-rain, ephemeral streams and pools). About 0.7 g/ha/a of Al is received from the atmosphere. Water transports Al in and out of each quadrat and distributes it between the different components. During initial alteration of the parent ignimbrite by alkali chloride water in the deep reservoir, Al either remained within the quadrat boundaries or transfers out were balanced by contemporaneous gains. Subsequently, alteration by acid sulfate fluids redistributes elements into new mineral assemblages but again with no net movement of Al in or out of either quadrat. The latest, surface alteration event involves interaction of all the previously and variously altered rocks by steam, gases and steam condensate. A primary product of this process is transient, hydrated, Al-rich, water-soluble sulfate efflorescences whose persistence indicates a steady flux of Al at the surface. The magnitude of this flux depends on available moisture and the activities of H +, SiO 4− 4, SO 2− 4 and K + such that variations in the rate of discharge of Al alone may be used to detect changes in surface conditions as may result from exploitation of a geothermal field.