Active geothermal systems occur widely in Sumatra and in the Taupo Volcanic Zone (TVZ) of New Zealand. There are
also many occurrences of epithermal gold mineralisation in Sumatra, some of which are economic. Minor sub-economic
occurrences of gold have been reported in the TVZ but none have yet been mined. There are both similarities and
differences in the nature of the active geothermal systems and mineral deposits in Sumatra and New Zealand, which can be
systematically related to the tectonic setting. The thicker crust and consequent predominance of silicic volcanism in New
Zealand leads to hydrothermal. systems which are little eroded and with limited lateral outflows. The systems in Sumatra
are of mixed character, with some of the characteristics of those in andesitic island arcs such as Java or the Philippines, eg a
higher proportion of systems with identifiable acid magmatic volatiles, a greater potential for deep erosion and lateral
outflows than in New Zealand; but modified by the more oblique component of subduction and thickened crust so that
some are localised in volcano-tectonic depressions and consequently have high piezometric levels of primary hydrothermal
fluid. The unusual occurrence of calcic zeolites in veins associated with gold mineralisation in Sumatra is thought to be
indicative of "satellite" boiling zones along de-gassed hydrothermal outflows.