In-situ Sb isotope variations of the orogenic Sb-Au deposits at Pezinok (Slovakia)

Orogenic Sb-Au ore bodies of deposits near Pezinok (Malé Karpaty Mts., Slovakia) are hosted in metamorphosed black shales in amphibolites. The ore bodies contain parageneses with various Sb sulfides, sulfoxides, and oxides. Given the variable mineral assemblages, the deposit serves as a model to understand the behaviour of Sb isotopes from initially reducing to an evolving oxidizing system at µm-scales. Here we determined the stable isotope composition (δ123Sb) of antimony minerals in-situ using a deep UV-femtosecond laser ablation system coupled with MC-ICP-MS [1]. The precision of analyses was <0.1 ‰ and all analysis were reported relative to NIST 3102a.

Polished sections, containing assemblages of successively precipitated antimony minerals, show an isotopic range from -0.2 to +0.8 ‰. Stibnite, which is the most abundant mineral, shows an isotopic variation of around 0.35 ‰ (median = -0.2 ‰) that is generally lighter than δ123Sb of kermesite (0.1 ‰), valentinite-senarmontite (~0.5‰), and schafarzikite (~0.8 ‰). This isotopic relationship is consistent with textural and thermodynamic observations, indicating earlier formation of stibnite, followed by kermesite, valentinite-senarmontite, and schafarzikite formation during decreasing temperatures and increasing fO2. The large isotopic variation within and between different minerals can be explained by a Rayleigh fractionation model with enrichment of 123Sb in the residual fluid and in successively later formed minerals. Therefore, the results of this study suggest that in-situ δ123Sb values could be a promising tool to understand the evolution of orogenic Sb-Au deposits in space and time.

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