ABSTRACT
The geochemical cycle of mercury in Earth's surface environment (atmosphere, hydrosphere, biosphere) has been extensively studied; however, the deep geological cycling of this element is less well known. Here we document distinct mass-independent mercury isotope fractionation (expressed as Δ199Hg) in island arc basalts and mid-ocean ridge basalts. Both rock groups show positive Δ199Hg values up to 0.34 and 0.22, respectively, which deviate from recent estimates of the primitive mantle (Δ199Hg: 0.00 ± 0.10, 2 SD)1. The positive Δ199Hg values indicate recycling of marine Hg into the asthenospheric mantle. Such a crustal Hg isotope signature was not observed in our samples of ocean island basalts and continental flood basalts, but has recently been identified in canonical end-member samples of the deep mantle1, therefore demonstrating that recycling of mercury can affect both the upper and lower mantle. Our study reveals large-scale translithospheric Hg recycling via plate tectonics.
ABSTRACT
Trace elements and Hg isotopic composition were investigated in mineralized rocks, barren rocks, and mineral soils in the Xianfeng prospect, a shallow buried epithermal gold deposit in northeastern China, to understand whether this deposit has left a diagnostic geochemical fingerprint to its weathered horizon. All the rocks and soils display congruent patterns for immobile elements (large ion lithophile elements, high field strength elements, and rare earth elements), which reflect the subduction-related tectonic setting. Both mineralized rocks and soils showed common enrichment of elemental suite As-Ag-Sb-Hg, suggesting that the Xianfeng gold deposit has released these elements into its weathered horizon. Similar mercury isotopic composition was observed between mineralized rocks (δ202Hg: -0.21 ± 0.70; Δ199Hg: -0.02 ± 0.12; 2SD) and barren rocks (δ202Hg: -0.46 ± 0.48; Δ199Hg: 0.00 ± 0.10; 2SD), suggesting that mercury in the Xianfeng deposit is mainly derived from the magmatic rocks. Mineralized soils (δ202Hg: -0.44 ± 0.60; -0.03 ± 0.14; 2SD) and barren soils (δ202Hg: -0.54 ± 0.68; Δ199Hg: -0.05 ± 0.14; 2SD) displayed congruent Hg isotopic signals to the underlying rocks, suggesting limited Hg isotope fractionation during the release of Hg from ore deposit to soils via weathering. This study reveals evidence of a simple and direct geochemical link between this shallow buried hydrothermal deposit and its weathered horizon, and highlights that the weathering of shallow-buried hydrothermal gold deposits can release a substantial amount of heavy metals (e.g. Hg, As and Sb) to surface soil.
