Lead isotopes in ground and surface waters: fingerprinting heavy metal sources in mineral exploration

We collected ground and surface water samples, as well as samples of host felsic and mafic volcanic lithologies and massive sulphide ore from the Restigouche deposit in the Bathurst Mining Camp, New Brunswick, Canada. These water and rock samples were analysed for their Pb isotope composition in order to investigate the utility of Pb isotope analyses of waters for mineral exploration and environmental tracing. Waters proximal to the near-surface Zn–Pb Restigouche deposit have Pb isotope compositions indistinguishable from galena from a number of volcanogenic massive sulphide deposits in the camp (²⁰⁶Pb/²⁰⁴Pb = 18.18 – 18.34; ²⁰⁷Pb/²⁰⁴Pb = 15.63 – 15.68; ²⁰⁸Pb/²⁰⁴Pb = 38.10 – 38.21). These deposit proximal waters are interpreted to have derived the dissolved (< 0.45 μm) Pb primarily from oxidation of massive sulphide minerals (galena, sphalerite). Groundwaters more distal from the Restigouche deposit have more radiogenic isotopic compositions, with Pb derived by weathering of U- and Th-rich minerals in the felsic volcanic and volcaniclastic country rocks that host the deposit. There is no correlation between dissolved Pb concentration and isotopic signature owing to the low solubility of Pb in water. The Pb isotopic composition of the dissolved loads of waters is therefore a powerful tool in fingerprinting Pb sources and has important implications for mineral exploration and environmental baseline studies.