METALLOGENY IN A MODERN SUBDUCTION ZONE SETTING, NORTH ISLAND, NEW ZEALAND

The modern subduction zone setting of the North Island, New Zealand is associated with magmatism and hydrothermal activity that is analogous to both epithermal and porphyry style mineralization. Most of this activity is concentrated in the Taupo Volcanic Zone, a 250 km stretch of arc which hosts ~20 discrete geothermal systems that convect ~4000 MW of thermal energy. Epithermal environments (<1 km depth) are hosted by silicic volcanic rocks, but on the basis of gas chemistry are linked to andesitic and basaltic heat sources. From deep fluid compositions, conservative estimates of H2S flux (100 g/sec) suggest a minimum gold flux potential of 7 mg/sec (~7 million oz/1000 yr), assuming deep fluids (250-300°C) are saturated with gold. A few direct measurements of deep fluids obtained with a titanium downhole sampler indicate these deep waters are undersaturated in gold and contain 10-15% of their gold transporting capacity. High concentrations of gold nonetheless deposit in flashing environments as indicated by well precipitates. None of the deep geothermal wells (2000-2700 m depth) have intersected porphyry style mineralization, but the evidence of its potential is reflected in Cu-Au bearing fluids from White Island and Ruapehu andesite volcanoes. 250 km behind the Taupo Volcanic Zone, localized basalt-peralkaline rhyolite volcanism is associated with a high temperature geothermal system at Ngawha, where the epithermal environment is entirely hosted by sedimentary rocks and associated with Au, Hg and Sb mineralization. The overall evidence from the subduction zone activity suggests that metal availability is controlled by magma composition and local stratigraphy, and that metal flux is controlled by ligand and metal availability, and rates of heat and mass transfer. Despite favorable fluid conditions for metal transport and deposition, ore bodies have apparently failed to form, raising questions about regional and local controls at shallow levels on epithermal and porphyry ore formation.