In-situ antimony (Sb) isotope variations as geochemical tracers of hydrothermal fluid evolution in the Harz Mountains, Germany

The St. Andreasberg deposit in the Upper Harz region (Germany) is one of the most famous historical silver deposits worldwide. According to [1], the deposit was formed during four distinct mineralization stages, each characterized by differences in mineralogy, temperature conditions, and fluid composition. Antimony is one of the main components of each mineralization stage occurring as various sulfides (tetrahedrite, pyragyrite, and stibnite) and antimonides (breithaupite, and dyscrasite). Given the variable mineral assemblages and mineralisation stages, this deposit serves as a model system to better understand Sb isotope behaviour during the hydrothermal evolution of the mineralization.

In-situ Sb isotope analysis following the method of [2] show significant Sb isotope variations: The earlier (oxidic-carbonatic) stage and the main sulfidic stage show relatively constant δ¹²³Sb values of 0.2–0.3‰, probably related to higher formation temperatures. In the main arsenic-antimonide stage and the last stage, however, significantly more variable δ¹²³Sb values between 0.1–1.1‰ are reported, indicative of a late-stage overprinting by hydrothermal fluids. Antimony minerals associated with the early oxidic/carbonatic and the last stage are isotopically heavier than later formed minerals. This trend of decreasing δ¹²³Sb values can be explained by a Rayleigh fractionation model, resulting in a depletion of ¹²³Sb in the residual hydrothermal fluid during progressive crystallization. Our results highlight the applicability of Sb isotopes as a valuable tool to reconstruct the genesis and fluid evolution in complex hydrothermal deposits such as the St. Andreasberg.

[1] Ließmann (1997). Historischer Bergbau im Harz, Springer

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