Geology, Hydrothermal Alteration and Geochemistry of the Iamalele Geothermal System, D’Entrecasteaux Islands, Papua New Guinea

The geothermal system at Iamalele in the D’Entrecasteaux Islands of southeastern Papua is hosted by late Quaternary high-silica ignimbrite, air-fall tuff and associated volcaniclastic rocks together with lava flows and domes. These volcanic rocks are part of the subduction-related Papuan Arc and are interpreted as the eruption products from a caldera. Whole rock chemical compositions range from andesite to rhyolite; the occurrence of rhyolitic rocks is a unique feature within in the Papuan Arc. Geothermal activity occurs over 30 km3 of the Iamalele area. Chemical analyses of hot spring discharges indicate a near-surface acid sulphate fluid and a deeper reservoir that probably has a significant seawater component. Hydrothermal alteration identified in a ~200 m drill core indicates that the geothermal reservoir is well-zoned and contains trace element signatures characteristic ofshallow epithermal precious metal deposits. With increasing depth there are mineral assemblages characteristic of advanced argillic, intermediate argillic and potassic alteration. The presence of an active geothermal system within a caldera structure raises the possibility of large-scale explosive volcanic activity in southeastern Papua which needs to be factored into future hazard assessment in the area.