Active geothermal systems occur
widely in Sumatra and in the Taupo Volcanic Zone (TVZ) ofNew
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, e.g. 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.