New Pb isotopic data from Japanese hydrothermal deposits for tracing heavy metal sources

Abstract

Lead isotopes are useful in determining the source of metals in environment, and studies on lead isotopic characteristics of mineral deposits have provided important insights for economic geologists, archaeologists and environmental scientists over the past 50 years. This study reports new Pb isotope and trace element data of sulfide minerals from 25 mineral deposits in Japan in order to partly update the data set reported in the 1980s, which have long been used for provenance studies. The analytical precision of the Pb isotope ratios, measured by multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) in this study, is an order of magnitude higher than the previous data set, allowing regional- and deposit-level discrimination of isotopic signatures. The Pb isotope ratios of vein-type or intrusion-centered Japanese deposits from this study (²⁰⁶Pb/²⁰⁴Pb = 18.151–18.545, ²⁰⁷Pb/²⁰⁴Pb = 15.552–15.642, ²⁰⁸Pb/²⁰⁴Pb = 38.389–38.918) are mostly within the range of data from Japanese deposits of the same type measured in previous studies but tend to have slightly lower ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios. This is likely be due to the differences in analytical instrumentation and isotopic variations within the deposit, which should carefully be considered in future regional-scale provenance studies. Data from the Sai, Ohizumi, Budo and Shiraita deposits in Northeast Japan as well as the Kishu and Tokoo deposits in Southwest Japan indicate a two-component mixing presumably involving magma and meteoric fluid. The isotopic variation within each of these deposit is much smaller compared to previously reported variations within volcanogenic massive sulfide deposits in Japan, indicating a relatively uniform metal source with only a small contribution from basement rocks. In contrast, Daira and Ani deposits in Northeast Japan have highly variable isotopic characteristics similar to volcanic rocks with lower crustal assimilation, indicating an additional input of material from the lower crust.