Secondary Fe–As–Tl mineralization in soils near Buus in the Swiss Jura Mountains

Soils at the locality Erzmatt near the village of Buus in the Swiss Jura Mountains are part of the oxidation zone of a small hydrothermal Fe–As–Tl ore occurrence in dolomite rock. The soils are characterized mostly by near-neutral pH values, buffered by the dolomite and limestone bedrock. The concentrations of As and Tl in the soil profiles increase with depth and attain maximum values of ∼20 000 mg/kg for As and ∼10 000 mg/kg for Tl. Using a combination of mineralogical, geochemical, and spectroscopic techniques, we investigated soil samples from the Erzmatt with respect to the characterization of As- and Tl-bearing secondary minerals using total digestions, powder X-ray diffraction, X-ray absorption spectroscopy (XAS), polarized light microscopy, electron microprobe analysis (EMPA), scanning and transmission electron microscopy (SEM, TEM). Arsenic K edge XAS data reveal little variation in bulk soil As speciation and suggest that most As is arsenate bound to iron oxides. Samples with As contents exceeding ∼1 wt% As₂O₅ contain abundant crystals of bariopharmacosiderite [Ba_0.5Fe₄(OH)₄(AsO₄)₃·nH₂O] in the voids in the soil matrix. Less commonly, Ca–Fe arsenates (arseniosiderite) were identified. An earlier study also identified Tl(I)-containing jarosite and avicennite (Tl₂O₃) as secondary Tl-bearing minerals. The EMP analyses reveal the degree of jarosite–dorallcharite [(K,Tl)Fe₃(SO₄)₂(OH)₆] solid solution formation, and indicate the presence of Tl₂O₃ with variable contents of As and P. Using electron diffraction, Tl₂O₃ with low As content was positively identified as avicennite. The platy morphology of the Tl₂O₃ aggregates is not compatible with the cubic morphology of avicennite, indicating that they may be pseudomorphs after an unknown primary mineral, possibly carlinite, Tl₂S. The identification of bariopharmacosiderite, jarosite and avicennite in near-neutral soils points to their stability at near-neutral soil pH (in contrast to scorodite) and indicates their potential for the storage and retention of toxic As and Tl in mine wastes and contaminated environments. The mineralization on the Erzmatt could be an example of a geochemical anomaly similar to the famous Lengenbach deposit (Switzerland) before the metamorphic overprint at Lengenbach.