Intrusive and basement rock sources of lead in hydrothermal systems of the Taupo Volcanic Zone, New Zealand

Abstract

The Taupo Volcanic Zone is a complex volcanic-tectonic region over 250 km long, corresponding to the volcanic arc of the Taupo-Hikurangi subduction system. Volcanism is dominantly rhyolitic, with minor amounts of andesite, dacite and basalt. The volcanics overlie a downfaulted basement of Mesozoic metasediment (Waipapa and Torlesse terranes). Over twenty major geothermal systems are active, related to the young volcanism. Samples of sulfides (galena, sphalerite, chalcopyrite and pyrite) from five of the geothermal systems were collected from drill core and their Pb-isotopic compositions measured. Representative fresh samples from all volcanic and basement rock types of the region were also analyzed and show a range of values by which they may be distinguished. Sulfides from the Broadlands and Waiotapu geothermal systems are homogeneous in their Pb-isotopic compositions, with values similar to the average for the fresh rhyolites and andesites. The results indicate that the source of Pb (and possibly other metals) in the Broadlands and Waiotapu hydrothermal fluids is related to the parent magma of the volcanics. A magmatic fluid could be the source, consistent with independent geochemical and isotopic evidence for other magmatic fluid-derived components at Broadlands and to a lesser extent at Waiotapu, which are both anomalously mineralized in precious and base metals. In contrast, pyrite Pb from the unmineralized Kawerau geothermal system has isotopic compositions which vary over a range similar to that of the underlying Waipapa metasedimentary basement, present at depths of only 1000 m. This observation suggests that the Pb at Kawerau was mainly leached from the basement rocks, with a component of isotopically homogenous, magma-related Pb not present in the hydrothermal fluids. Isotopic compositions of Pb from the unmineralized Wairakei and Mokai systems are intermediate between the Broadlands and Kawerau examples. These data are interpreted to result from leaching of Pb from both the basement and volcanic rocks, consistent with the thick (at least 3 km) sequence of volcanics in the center of the zone, where Wairakei and Mokai are located. These results suggest that epithermal gold prospects may be distinguished on the basis of their Pbisotopic signature; those exhibiting a homogeneous and magmatic signature have a higher potential to contain ore, all else being equal, because of the potential for higher, magmatic-derived metal concentrations in the hydrothermal fluid. This suggestion appears to be applicable only to dilute hydrothermal systems, perhaps because the low salinity precludes leaching as an effective process to solubilize metals.