Synergies in elemental mobility during weathering of tetrahedrite [(Cu,Fe,Zn)12(Sb,As)4S13]: Field observations, electron microscopy, isotopes of Cu, C, O, radiometric dating, and water geochemistry

Tetrahedrite [(Cu,Fe,Zn)12(Sb,As)4S13] is commonly present in many ore deposits and may be transferred to mining waste. Here, we describe the behavior of all elements in tetrahedrite during oxidative dissolution in a natural oxidation zone at Piesky (Slovakia), this being a long-term analogue of weathering of tetrahedrite-rich mining wastes. Electron microprobe work identified initial, early, advanced, and mature stages of weathering. The initial stages include formation of secondary covellite and chalcocite, the following stages only oxidic products, divided into greenish masses and brown veinlets. Both of them are nanocrystalline mixtures of minerals. The masses are Cu-Sb-As-rich and consist of a nanocrystalline pyrochlore phase. The veinlets are Fe-Sb-As rich and contain tripuhyite, goethite, and pyrochlores. Quantitative elemental budgets show that some elements (Zn, S, Ni, Co) are rapidly lost whereas others (especially Cu and As) are retained and form copper arsenates in the mature stages of weathering. About 10% As and almost 50% of Sb are lost during weathering, likely released into water; some Sb is stored in secondary minerals such as camérolaite, cualstibite, or tripuhyite. Light carbon isotopic composition (δ13C down to −11.1‰) document significant biological contribution for C in the secondary minerals. Copper isotopes become progressively heavier during weathering (from initial δ65Cu of −2.45‰ in tetrahedrite up to 4.3‰ in some azurite samples but −6.3‰ in covellite and chalcocite), documenting cyclic removal of light copper isotopes into covellite and chalcocite. The observation of elemental synergies during weathering of tetrahedrite shows that the solubility and mobility of As is controlled by copper arsenates, not by iron oxides, in an environment rich in tetrahedrite but poor in pyrite. Antimony, another element of environmental concern, is mostly released into water and creates low-concentration anomalies in stream sediments and soils.