The Moschellandsberg mercury deposit (SW Germany) is hosted in a Permo-Carboniferous volcanic caldera and has formed at shallow depths (100–300 m) and over a wide temperature range from about 200°C to less than 65°C. The deposit shows vertical zonation and the mineral paragenesis is unusually complex and contains significant amounts of Sb, As, Cu, and Ag. Fahlores are Hg-and Fe-rich Sb (Bi) and As members displaying a mixing gap. Vesicular ore textures and limited fluid inclusion evidence prove the existence of an immiscible gas phase during ore deposition. The fluid composition can be constrained to pH4, H₂S10^–5 molal, CO₂10^–1 molal, and f_H210^–2–10^–3 bar by comparing the observed mineral associations with stable phase assemblages calculated from thermodynamic data. Calculated metal solubilities indicate that Hg, Ag, and Sb can be transported very efficiently in such a fluid, but that the solubility of gold is very low.