The mineralogy of weathering products of Bi-bearing tennantite – clues for the process and the mobilisation of heavy metals and toxic elements

Since the complex sulfosalts of the tennantite-tetrahedrite solid-solution series [(Cu,Ag)6Cu4(Fe,Zn,Cu,Hg,Cd)2(Sb,As,Bi,Te)4(S,Se)13] are widespread in many geological environments and accommodate heavy metals and toxic elements, a better understanding of the general weathering process and mobility of elements are important to evaluate environmental risks. In this study, the weathering of Bi-rich members of this mineral group were investigated in detail using microscopy, EMPA, SEM, TEM, LA-ICP-MS, Raman, µXRD and MLA. Observation reveal a subdivision in four temporally distinguishable weathering stages, spatially attributed to weathering environments of different chemical potentials. During the first stage, weathering occurs as tubes within the fahlore producing a phase assemblage of nm-sized roméite group minerals, tripuhyite, a crystalline Cu-oxide phase and crystalline Cu-sulfides. The textural appearance and the occurrence of these secondary sulfide indicates a low oxidation potential during this stage, typical for cementation zones. Mass balance calculations show that during this stage As, Zn and partially S are released to the weathering fluids. In contrast, Sb, Bi, Fe, S and Cu are stored in the roméite group minerals, tripuhyite, the copper oxide phase, and sulfides. During weathering stage 2, amorphous and nano-crystalline arsenates were formed replacing fahlore in most cases as weathering fronts. Their textures indicate a fluid with higher oxidation potential than in stage 1, typical for oxidation zones. The occurrence of arsenates shows that arsenic in contrast to stage 1 behaves immobile during this stage. Bismuth behaves immobile and is stored in the amorphous nano-crystalline phase. Mass balance calculations reveal that Zn, Sb, and S and partially Cu are lost, whereas Fe is added. Weathering stage 3 occurs only locally and reflects processes in micro compartments that are different for each locality and not characteristic for the general weathering process including the dissolution of former phases but also precipitation of new ones such as amorphous Cu-silicates. Stage 4 is characterized by the formation of crystalline Cu-Ba-Ca-Al-arsenates and Cu-carbonates mostly along cracks and in voids, spatially independent of the precursor fahlore. This stage reflects the increasing importance of the local geology, host rock and gangue mineralogy on weathering, typical in near surface environments of oxidation zones, where elements are highly mobile and a high fluid rock ratio is realized.