Fractionation of Zn isotopes in terrestrial ferromanganese crusts and implications for tracing isotopically-heterogeneous metal sources

Abstract

Zinc liberated from geological sources during surface weathering is the main source of Zn to the surface environment, and is ultimately sequestered by sinks such as Mn oxides and oxyhydroxides. This behaviour has been applied to geochemical mineral exploration studies and terrestrial ferromanganese crusts have been shown to have anomalously-high Zn/Mn ratios around known Zn-Pb ore deposits. This poses the question: if Zn isotopic compositions of these Fe-Mn crusts can also discriminate the underlying anomalous metal sources from the surrounding non-mineralized lithologies. Isotopic fractionation during fractionation onto Mn oxides in well-studied marine Fe-Mn crusts has shown that isotopically-heavy Zn is preferentially-adsorbed in that environment, which has a comparatively isotopically-homogeneous Zn reservoir. However, isotopic fractionation behaviour in terrestrial Fe-Mn crusts that may have more heterogeneous Zn sources is unknown. Here, we present the first Zn and Pb isotope study of terrestrial Fe-Mn crusts to test if Zn isotopic compositions can be used to delineate isotopically-different lithological metal sources.Our data from Fe-Mn crusts that formed overlying isotopically-variable lithologies; hydrothermal polymetallic (Zn-Pb) deposits, and non-mineralized Archean rocks show that, while adsorption of Zn onto Mn oxyhydroxides in terrestrial Fe-Mn crusts generally favours isotopically-heavy Zn, the magnitudes of isotopic fractionation varies in Fe-Mn crusts overlying mineralized and non-mineralized areas. Fractionation factors (Δ66/64Zncrust – source up to 0.54‰) in crusts overlying non-mineralized lithological sources with a δ66Zn value of 0.30‰ are comparable with the predicted equilibrium fractionation onto birnessite. Conversely, magnitudes of Δ66/64Zncrust – source up to 0.37‰ in crusts overlying isotopically-light Zn-Pb mineralization, that has a mean δ66Zn value of -0.15‰, are much lighter than non-mineralized counterparts with similar Zn/Mn ratios. This difference may be explained by changes in hydrogeochemical complexation of Zn, with localized chloro-complexes that carry isotopically-light Zn from the isotopically-light mineralization.These findings demonstrate that different lithological Zn sources that are isotopically-variable give rise to isotopic heterogeneity in the surface Zn reservoir. This can be reflected by differing fractionation patterns following adsorption onto Mn oxyhydroxides in terrestrial Fe-Mn crusts that overly different Zn sources. Furthermore, Pb isotope compositions of the Fe-Mn crusts also closely reflect that of the underlying geology. This highlights that the use of Zn isotopes may be a valuable means of vectoring towards anomalous metal sources such as ore deposits.