ANTIFRAC – Antimony Isotope Fractionation

The programme, which has been running since 2020, was extended for a further three years in 2023 following a successful evaluation, during which 26 new individual research projects were launched. One such research project, which is being conducted at CEZA in Mannheim, is investigating antimony isotope fractionation in stibnites from various antimony (Sb) sources (DFG research project ‘Antifrac’).

Ore deposits have always been the basis of technical achievements, but their availability is a recurring problem. There is currently an acute supply-demand gap, especially for so-called ‘critical’ metals, which are essential for cutting-edge technologies aimed at combating global CO2 emissions. To address this, new ore deposits must be found and developed. DOME will help improve the understanding of ore-forming processes and the properties of ore deposits, as well as the associated models. In this way, exploration strategies can be developed and refined, and recycling and treatment processes can be further advanced.

The DOME programme brings together researchers from different fields who are working collaboratively to decipher the complex processes of ore formation, making multidisciplinary contributions through advanced experimental, analytical, or numerical methods. The aim is to gain new insights into the complexity of ore formation and to achieve a better understanding of the formation processes.

Antimony Isotope Fractionation in Stibnites from Different Antimony (Sb) Sources

The research project at CEZA focuses on the isotope systematics of ¹²³Sb/¹²¹Sb in stibnite (antimonite) and runs from 1 January to 31 December 2024. Antimony isotopes could not be measured before the development of new high-resolution multi-collector ICP-MS, and therefore represent a new field of research. Some previous studies on Sb isotope composition in antimonite and Sb-containing minerals from ore deposits in China, Romania, and the USA have shown that δ¹²³Sb values cover a wide range from negative to positive.

The reason for this large variation is still unknown. Possible explanations for Sb isotope fractionation include redox reactions, evaporation, biological and precipitation processes, adsorption, or Rayleigh distillation mechanisms (e.g., Wang et al. 2021 and sources cited therein).

The working hypothesis of this research project is based on the combination of IR-microthermometric measurements of fluid inclusions in antimonite and ¹²³Sb/¹²¹Sb isotope ratios in samples from different types of Sb deposits. Samples for Sb isotope analyses are selected based on IR microthermometric data (formation temperature, salinity of aqueous inclusion fills) to investigate possible temperature-dependent Sb isotope fractionation. Furthermore, samples with inclusions of different salinities but constant homogenisation temperatures will be analysed to detect possible fractionation of Sb isotopes due to different complexation (i.e., chloride vs. OH- or HS-). In addition, Sb fractionation between different types of Sb reservoirs (e.g., epithermal vs. hydrothermal sediment-bound reservoirs) will be tested.

The above discussion suggests that the release, transport, and deposition of Sb are probably controlled by temperature and the speciation of Sb in ore-forming fluids, which is very likely to influence Sb isotope fractionation.