The
Taupo Volcanic Zone inNew
Zealand and the Philippine Mobile Belt in the
Philippines are characterized by
active subduction, active volcanism and high seismicity. The main factors
affecting characteristics of geothermal systems in both countries are: tectonic
setting, source magmas, maturity of the systems, and depth to the degassing
magma body. Within New
Zealand, geothermal systems are grouped into
arc- and rift-type. Fluids in arc-type systems contain high amounts of volatiles
(<20%), with high co2 cl,>
CO2/He, and N2/Ar ratios. Mantle-type volatiles
predominate in rift-type systems. Philippine geothermal systems are largely
arc-type, the topography is rugged, leading to the separation of water and
vapour upflows, as opposed to the overlap found in New
Zealand. The relatively shallow
depths of degassing magma bodies in the Philippines results in steeper
geochemical gradients, leading to higher proportions of arc-type magmatic water
(>40%) and higher Cl contents (3000 - 10 000 mg/kg),
Maximum temperatures are also higher, reaching up to >400C. Arc-type alteration
assemblages in New
Zealand
and the Philippines are similar. Because
Philippine geothermal systems are less mature, have higher temperatures and
steeper geochemical gradients than New Zealand ones, advanced argillization is
more widespread occurring down to depths of 2000 m. Mineral phases are more
varied due to the active input of magmatic waters and the formation of vapor
conditions at depth, forming P04-, B-, Li-, Ce-, F-, and Cl-bearing
minerals at high temperatures. In the Philippines, high temperature
magmatic-hydrothermal to contact metamorphic assemblages containing biotite,
amphiboles and pyroxenes are relatively common, but rare in New Zealand.