Isotope diffusion and re-equilibration of copper and evaporation of mercury during weathering of tetrahedrite in an oxidation zone

Abstract

To understand the mobility of heavy metals during oxidative weathering of sulfides, we investigated weathering processes of tetrahedrite [(Cu,Fe,Zn,Hg)₁₂(Sb,As)₄S₁₃] in an oxidation zone with abundant siderite (FeCO₃) and baryte (BaSO₄) at Rudňany (Slovakia). The focus of this work lied in the isotopic (δ⁶⁵Cu, δ²⁰²Hg, δ³⁴S) variations of the minerals during weathering and the interpretation of such changes. In the studied oxidation zone, Hg-rich tetrahedrite converts in situ to pockets of powdery cinnabar (HgS) and an X-ray amorphous mixture rich in Sb, Fe, and Cu that slowly re-crystallizes to Cu-rich tripuhyite (FeSbO₄). Copper is mobile and precipitates as malachite [Cu₂(OH)₂(CO₃)], azurite [Cu₃(OH)₂(CO₃)₂], or less abundant clinoclase [Cu₃(AsO₄)(OH)₃]. The isotopic composition (δ⁶⁵Cu) of tetrahedrite correlates well with the degree of weathering and varies between 0.0 ‰ and −4.0 ‰. This correlation is caused by isotopic changes during dissolution and subsequent rapid equilibration of δ⁶⁵Cu values in the tetrahedrite relics. Simple diffusion models showed that equilibration of Cu isotopic values in the tetrahedrite relics proceeds rapidly, on the order of hundreds or thousands of years. Abundant secondary iron oxides draw light Cu isotopes from the aqueous solutions and shift the isotopic composition of malachite and azurite to higher δ⁶⁵Cu values as the distance to the primary tetrahedrite increases. Clinoclase and tripuhyite have lower δ⁶⁵Cu values and are spatially restricted near to the weathering tetrahedrite. The Hg and S isotopic composition of tetrahedrite is δ²⁰²Hg = −1.27 ‰, δ³⁴S = −1.89 ‰, that of the powdery secondary cinnabar is δ²⁰²Hg = +0.07 ‰, δ³⁴S = −5.50 ‰. The Hg isotopic difference can be explained by partial reduction of Hg(II) to Hg(0) by siderite and the following evaporation of Hg(0). The S isotopic changes indicate no involvement of biotic reactions in the oxidation zone, probably because of its hostility owing to high concentrations of toxic elements. This work shows that the Cu isotopic composition of the primary sulfides minerals changes during weathering through self-diffusion of Cu in those minerals. This finding is important for the use of Cu isotopes as tracers of geochemical cycling of metals in the environment. Another important finding is the Hg in the oxidation zones evaporates and contributes to the global cycling of this element through atmospheric emission.