Antimony isotope fractionation in stibnite – constraints from δ123Sb values and infrared microthermometric data of stibnite-hosted fluid inclusions “Antifrac”

• Applicants: Dr. Michael Brauns; Dr. Volker Lüders
• Subject Area: Geology
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 521623684

Antimony (Sb) is a “critical” element. As Stibnite (Sb2S3), Sb is highly concentrated in different types of deposits: - Low-sulfidation epithermal vein-type - hot spring deposits, some porphyry and molybdenum - parts of orogenic gold or sediment-hosted Carlin-type gold - sediment-hosted hydrothermal quartz-stibnite veins - some SEDEX-type - or massive metasomatic carbonate replacement In some deposits, stibnite mineralization is also associated with considerable concentrations of gold, mercury, silver, tin, tungsten, or base metals (e.g., Gumiel and Arribas 1987; Dill 1998; Peng et al. 2002; Wagner and Boyce 2003; Hagemann and Lüders 2003; Yang et al. 2006; Buchholz et al. 2007; Bortnikov et al. 2010; Voudouris et al. 2019; Pohl, 2020). The source(s) of Sb in hydrothermal ore deposits are often not well constrained and still a matter of debate. The average abundance of Sb in the Earth´s crust is 0.2 ppm (Taylor and McLennan 1995). Higher average Sb concentrations of about 5 ppm are reported from black shales (Ketris and Yudovich 2009) and thus, alteration of black shales may constitute a potential source for Sb in hydrothermal quartz-stibnite deposits (e.g. Wagner and Boyes 2003, Sośnicka et al. 2021). Stibnite deposits are also associated with felsic magmatic rocks. In this case is not always clear whether Sb enrichment is directly related to magmatic fluids or hydrothermal alteration processes (Krolop et al. 2018). Previous studies of isotope compositions of stibnite have shown that the 123Sb/121Sb ratios in stibnite from various occurrences may provide a means to decipher the source(s) of stibnite deposits (Zhai et al. 2021 and references therein). However, most of the conclusions about Sb isotope fractionation that are drawn so far are based on observed shifts of some δ123Sb values within a deposit or ore types, and experimentally data on Sb(III) Sb(V) redox reactions (e.g. Lobo et al. 2012; Dillis et al. 2019). The transport of Sb (III) in hydrothermal fluids at high temperatures is dependent on ph and the sulfide concentration in the fluid and occurs in from of HS- or OH- complexes (e.g. Krupp 1988; Olsen et al. 2019) whilst Sb chloride complexes are only stable in strong saline acidic fluids (e.g. Obolensky et al. 2007). However, the dependence of Sb isotope fractionation in stibnites that precipitated from either brines or low-salinity fluids at different temperatures has not been studied so far. Since stibnite shows extremely good transparency in near IR light (Lüders 2017) fluid inclusions hosted in stibnite can be studied routinely when using IR microthermometry and information about T-x of the stibnite-forming fluids can be obtained. Studying Sb isotopic systematics in stibnite in detail in selected ore deposits in combination with microthermometric data of stibnite-hosted fluid inclusions will provide new insights for the applicability of the Sb isotope system for questions related to ore deposit research.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2238: Dynamics of Ore-Metals Enrichment - DOME