Tetrahedrite-(Hg) from the Litija deposit, Central Slovenia: Mineral chemistry insights into fluid evolution processes

For over a century, the main carrier of Silver in the Litija mineralisation has been debated, with fine-grained galena often identified as the primary host. The in-situ quantitative microanalysis of a silver-bearing ore from the Alma orebody reveals that silver is not hosted within the galena but occurs instead as inclusions within tetrahedrite-(Hg). The mechanism of silver incorporation in tetrahedrite is complex and may occur through atom-to-atom substitution, where monovalent silver replaces monovalent copper (Ag⁺ ↔ Cu⁺). Additionally, silver can be present as a separate phase, either as nanoscale inclusions of acanthite or through the replacement of pre-existing silver-rich chalcopyrite. Elemental correlations and minor variations in the element distribution within the studied tetrahedrite provide insights into the chemistry of the mineralizing fluid. These findings suggest an initial reducing, near-neutral, low-chlorinity fluid promoting incorporation of Sb³⁺ and Hg²⁺ into tetrahedrite. This fluid subsequently mixed with high-salinity, Cl-rich, near-neutral fluid transporting Zn²⁺. Sulfide precipitation introduced additional acidity in the mixed fluids, altering the pH and promoting As³⁺ and Zn²⁺ incorporation into tetrahedrite. We suggest that Hg²⁺ ↔ Zn²⁺ substitution in tetrahedrite is influenced by pH fluctuations and fluid mixing. These findings provide new insights into the geochemical processes governing trace element incorporation in sulfosalt minerals and offer valuable framework for understanding mineralization in similar hydrothermal systems worldwide.