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

The principal Al-bearing components of two surface quadrats in the central Te Kopia geothermal field are the atmosphere, substrate (similar to 10 wt% Al in ignimbrite, clay and protosoils, 0.3-0.6 Al wt% in sinter), vegetatio n (4-5 g Al/m(2)) and waters (1-4 mu g/g Al in semi-permanent acid surface waters, 6-9 mu g/g in acid pools, 10-14 mu g/g in postrain, ephemeral strea ms and pools). About 0.7 g/ha/a of Al is received from the atmosphere. Wate r transports Al in and out of each quadrat and distributes it between the d ifferent 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 int o 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-r ich, water-soluble sulfate efflorescences whose persistence indicates a ste ady flux of Al at the surface. The magnitude of this flux depends on availa ble moisture and the activities of H+, SiO44-, SO42- and K+ such that varia tions 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. (C) 2000 Elsevier Science Ltd. All rights reserved.