The chemical compositions of acanthite, naumannite, and associated ore minerals have been studied from the samples of polychronous Au–Ag ores at the Rogovik deposit. The following admixtures have been detected: S in naumannite (0–2.9 wt %), Se in acanthite (0–7.45 wt %), argyrodite (~4.8 wt %), and galena (~3.1 wt %), and Fe in sphalerite (~1.2 wt %). The physicochemical parameters of ore formation have been reconstructed on the basis of mineralogical and geochemical data and thermodynamic calculations. Eh–pH (25°C, 1 bar), logfO2–pH, logfS2–T, logfSe2–T, and logfS2–logfSe2 (100–300°C, 1–300 bars) diagrams for the Ag–S–Se–H2O system with the stability fields of Ag sulfoselenides Ag2S1–xSex of various composition (step x = 0.25, where 0 ≤ x ≤ 1) have been calculated for the first time. It has been established that Ag sulfoselenides of the naumannite series from polychronous ores of the Rogovik deposit precipitated below 70–133°C under reductive conditions (logfO2 =–65…–50) from near-neutral solutions containing elevated Se and relatively lowered S. It has been established that Ag sulfoselenides of acanthite series were formed later then naumannite but in the same range of logfO2 values at temperatures below 110–177°C from solutions with high S concentration and relatively lowered concentration of Se. The complex composition of the studied Au–Ag ores, whose characteristic feature is extremely variable mineralogy, is confirmed.