Epithermal metal deposits along convergent plate margins are of large economic value, however, the origin of the typical metal spectrum (Au, Ag, Sb, As, Hg) in these deposits remains debated. Here, we study the Hg isotope geochemistry of the vein-style Erdaokan Ag-Pb-Zn deposit of epithermal type in the eastern part of the Central Asian Orogenic Belt in NE China, to provide direct constraints on its metal source. Bulk ore samples and early-stage hydrothermal minerals (magnetite, pyrite, and quartz) show negative δ²⁰²Hg (−1.52 to −0.18‰) and positive Δ¹⁹⁹Hg (−0.03 to 0.17‰) values similar to those of previously reported epithermal Au deposits in NE China, and also of marine systems in general. However, late-stage calcite shows weak negative to positive δ²⁰²Hg (-0.61 to 0.90‰) and significant negative Δ¹⁹⁹Hg (−0.18 to −0.02‰), similar to the Late Paleozoic sandstone country rocks (δ²⁰²Hg, −1.09 to 0.76‰; Δ¹⁹⁹Hg, −0.09 to 0.04‰). Hydrothermally altered gabbro and diorite show large variations in δ²⁰²Hg (−1.83 to 0.50‰) and Δ¹⁹⁹Hg (−0.14 to 0.15‰) which overlap with those of the ore and country rocks, indicating a mixed source. The Δ¹⁹⁹Hg signature in the bulk ore and early-stage hydrothermal minerals likely derives from dehydration of the subducted oceanic slab, whereas Hg in late-stage calcite is likely derived from the sedimentary country rocks. Hydrothermal deposits in different tectonic settings have distinct Δ¹⁹⁹Hg signals, and our study suggests that Hg isotopes are a robust tool to distinguish metal sources of hydrothermal ore deposits.