Abstract: Banded sphalerite from Bleiberg, Austria, contains ubiquitous relics of Fe-thiosulphates and Fe-sulphites up to 50 µm in size. Zn, Pb, Ni and As form admixtures ranging from a few to 12 wt.% in these compounds. It is suggested that banded sphalerite is formed by a replacement of banded thiosulphate and thiosulphite precursors. This suggestion is supported by a significant admixture of Fe 0.41–8.20 wt.%) and the presence of goethite inclusions in banded sphalerite. In contrast, crystalline ZnS intergrown with the banded variety is Ni-free and contains only 0.06–0.14 wt.% Fe.
Banded pyrite contains inclusions of Fe-thiosulphate up to 200 µm in size. These thiosulphates contain (wt.%): 0.09–1.92 Zn, and 2.67–4.02 Pb. Pyrite formed by replacement (breakdown) of Fe-thiosulphate precursors contains 3.69–3.83 wt.% Pb.
It is suggested that banded sulphides precipitate from solutions carrying sulphur with mixed valances, and crystalline sulphides grow from solutions where the dominant dissolved sulphur is in the form of sulphidic anions.