ABSTRACT

Isotopic measurements have been made on lead extracted from samples of andesite, propylitized andesite and rhyolite, and from galenas present in quartz-calcite veins occurring in the Miocene Hauraki Goldfield region, New Zealand. Measurement bias due to isotopic fractionation has been removed by the double-spike technique. All samples show varying degrees of J-type anomaly. Although there is some overlap, the galena samples generally appear more anomalous than the andesites; lead in the veins is isotopically different from lead in adjacent andesite. There is no evidence of contamination by ancient, low radiogenic rock systems, or of isotopic fractionation during vein formation. Although the vein galenas may be slightly enriched in preferentially-extracted radiogenic lead, the results are thought to represent isotopic inhomogeneity in the sources. With one exception the vein galena, rhyolite, propylitized andesite, and hydrothermally altered rocks form a linearly-distributed set on the ²⁰⁸Pb/²⁰⁴Pb - ²⁰⁶Pb/²⁰⁴Pb diagram, which appears to be distinct from the unaltered andesite. A similar but less distinct trend is present on the ²⁰⁷Pb/²⁰⁴Pb - ²⁰⁶Pb/²⁰⁴Pb diagram. It is suggested that the vein galenas and rhyolites, together with the propylitization and hydrothermal fluids, could all have originated from the one bulk source-material, the thick Mesozoic greywacke-argillite sequence, and that the andesites have come from another, necessarily deeper source. Propylitization processes appear to have introduced the hydrothermal-rhyolite-galena-type lead into the altered andesites. A criticism is made of the use of lead isotope results from hydrothermal material as input data for the calculation of single-stage model growth-curves.