Geology, hydrothermal alteration and geochemistry of the Iamalele (D'Entrecasteaux Islands, Papua New Guinea) and Wairakei (North Island, New Zealand) geothermal areas (1989)
Type of ContentTheses / Dissertations
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury. Geology
AuthorsMitchell, Peter Ashleyshow all
The geothermal system at Iamalele is hosted by a series of late Quaternary high-silica dacite to rhyolite ignimbrite, air-fall tuff and related volcaniclastic rocks. The ignimbrite flows are intercalated with calc-alkalic andesite and low-silica dacite lavas, some of which are high-Mg varieties. The Iamalele Volcanics may be related to caldera collapse and post-caldera volcanism. Geothermal activity occurs over 30 km2 of the Iamalele area. Chemical analyses of water from hot springs indicate that the near-surface reservoir is dominated by an acid-sulphate fluid, and that the deeper reservoir fluid probably has a significant seawater component. Analyses of rock and soil samples within the limits of geothermal activity identified several areas of above background values in Au, Hg, As and Sb. A diamond drill hole was completed to a depth of ~200m in one of these areas. Hydrothermal alteration identified in the drill core indicates that the upper 200 m of the geothermal reservoir is well-zoned and contains a trace element signature characteristic of high-level, epithermal precious metal deposits. With increasing depth mineral assemblages indicative of advanced argillic, intermediate argillic and potassic alteration were observed in the recovered core. The Wairakei geothermal system is hosted by a voluminous sequence of late Quaternary rhyolitic ignimbrite, air fall tuff and related volcaniclastic rocks intercalated with andesite to rhyolite lavas. The volcanic sequence was deposited during formation of the Maroa and Taupo caldera volcanoes, and geothermal activity is localized within a diffuse border zone between these two volcanic centres. The high-temperature reservoir at Wairakei is primarily restricted to porous pyroclastic rocks of the Waiora Formation. Geothermal activity is exposed over ~25 km2 of the Wairakei area. Chemical analyses of well discharge indicate that the fluid is a low salinity, low total sulphur, near-neutral pH chloride water with a local meteoric source. Temperature profiles for ~60% of the Wairakei wells were used to construct a c. 1950 view of the thermal zoning of the reservoir. When compared to the estimated preproduction isotherms, reconnaissance fluid inclusion homogenization temperatures indicated that the deeper portion of the reservoir had cooled by ~45ºC prior to production discharge. Hydrothermal rock alteration within the reservoir is systematically zoned and may be separated into four principal assemblages: propylitic, potassic, intermediate argillic and advanced argillic. Calcium zeolites, mainly wairakite, mordenite and laumontite, occur throughout the reservoir and, with the exception of laumontite, form an integral part of either the propylitic or potassic assemblage. Intermediate argillic alteration is widespread but is not strongly developed. The distribution of advanced argillic alteration is sporadic and restricted to depths less than 65 m. Below a depth of ~500 m potassic alteration commonly overprints propylitic alteration. The location of the "average" Wairakei fluid on several activity diagrams drawn for 100°, 200°, 250° and 300°C indicates that propylitic and potassic alteration probably formed in equilibrium with a hydrothermal fluid chemically equivalent to the modern reservoir fluid at temperatures between ~275° and ~210°C. Assays of drill samples indicate that trace amounts of gold (<0.04 g/t) and other metals permeate the reservoir. Samples of siliceous sinter collected from wellhead production equipment contain significant quantities of precious metals and also platinum group and base metals. Metal-rich scale from a back pressure plate (well 66) was analysed by optical microscopy and by electron microprobe analysis. The scale is composed of several discrete mineral phases which show a distinct paragenesis. Hydrothermal alteration and metallization identified within the reservoirs at Iamalele and Wairakei are similar to hydrothermal alteration and metallization identified within the epithermal precious metal deposits of Rawhide and Round Mountain (Nevada, U.S.A.). The major difference between these systems is the much greater abundance of gold and silver at Rawhide and Round Mountain. Conclusions drawn from these comparisons include: (1) within high-temperature active systems gold remains in solution or is dispersed at low grades; (2) boiling does not appear to be a viable means of producing a gold ore deposit within deep (>500 m) hydrothermal reservoirs and (3) the formation of a major precious metal ore deposit may require the superposition of a structural event on a waning geothermal system to initiate an extended period of fluid mixing. High-Mg lavas similar to ones identified at Iamalele occur elsewhere in the late Cenozoic arc-type volcanic associations of south-eastern Papua New Guinea. Detailed geochemical studies of these rocks have revealed the presence of relatively aphyric lavas which are high in MgO, Cr, and Ni and form an integral part of the arc-type association. The high concentrations of these elements relative to typical arc-related rocks are thought to reflect the chemical composition of the initial melt. High-Mg lavas occur in other volcanic arcs of Papua New Guinea as well as in several other circum-Pacific volcanic arcs, and it is likely that high-Mg lavas form a fundamental component of most, if not all, volcanic arcs.