Stable antimony isotope fractionation during surface alteration of primary ores

The orogenic Sb-Au deposits near Pezinok, located in the Malé Karpaty Mts. (Slovakia), were mined until 1992. Ores were extracted by flotation with low recovery rates, resulting in mining waste with large amounts of Sb. Exposed to surface conditions primary Sb(III) minerals got oxidized and formed secondary Sb(V) phases. This change in Sb oxidation state is associated with Sb isotope fractionation indicating oxidative weathering conditions during their formation. δ123Sb values of weathering products and precursor minerals were determined in-situ using a deep UV-femtosecond laser ablation system coupled with MC-ICP-MS and are reported relative to NIST 3102a using a protocol as described in [1].

Supergene products occur in two forms at Pezinok with different precipitation kinetics. Brandholzite (MgSb25+(OH)12·6H2O), the occurrence of which is restricted to samples with massive stibnite, formed large crystals with abundant stibnite inclusions. Its textural close association with stibnite and their similar δ123Sb (median = 0.31 ‰stibnite vs. 0.25 ‰brandholzite) indicates quantitative dissolution of stibnite and quantitative, likely rapid, precipitation as brandholzite. Contrary, tripuhyite (FeSb5+O4), that does not show a close association with potential precursors, displays a ~0.8 ‰ isotopic range and likely formed over decades in the mine tailings. This is consistent with Rayleigh fractionation in an open system, which depends strongly on (1) amount of leached stibnite and (2) redox changes during transport and formation. Thus, δ123Sb could be used as tool to understand the evolution of antimony deposits from initially reduced to more oxidizing supergene conditions.

[1] Kaufmann et al. (2021), JAAS. 36, 1554-1567.